Magnetospheric Wave Injection by Modulated Hf Heating of the Auroral Electrojet

8/4/2019 Magnetospheric Wave Injection by Modulated Hf Heating of the Auroral Electrojet

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M A G N E T O S P H E R I C W A V E I N J E C T I O N B YM O D U L A T E D H F H E A T I N G

O F T H E A U R O R A L E L E C T R O J E T

A DISSERTATIONSUBMITTED TO THE DEPARTMENT OF ELECTRICAL

ENGINEERINGAND THE COMMITTEE ON GRADUATE STUDIES

OF STANFORD UNIVERSITYIN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OFDOCTOR OF PHILOSOPHY

Mark GolkowskiDecember 2008


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UMI Number: 3343955Copyright 2009 byGolkowski, Mark

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Copyright by Mark Golkowski 2009All Rights Reserved


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I certify that I have read this dissertation and that, in my opinion, itis fully adequate in scope and quality as a dissertation for the degreeof Doctor of Philosophy.

(Um ran/S. Inan) Principal Adviser


(Donald L. Carpenter)


(Mark A. Kasevich)

Approved for the University Committee on Graduate Studies.

^ . / . AfcyVin


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This thesis is dedicated to my grandparents.

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A b s t r a c tModulated High Frequency (HF, 330 MHz) heating of the auroral electrojet to generate electromagnetic waves in the Extremely Low Frequency (ELF, 33000 Hz) andVery Low Frequency (VLF, 330 kHz) bands is investigated in the context of mag-netospheric wave injection experiments. The ionospheric heating facility of the HighFrequency Active Auroral Research Program (HAARP) is used to excite non-linearamplification of whistler mode waves in the Ea rth 's ma gnetosp here. Exp erim entalevidence is presented of the first HF heater generated signals experiencing 'ducted'inter-hemispheric propagation and wave-particle interactions resulting in amplification and triggering of free running emissions. The roles of transmitter parameters aswell as na tur al backgrou nd cond itions of the observations are characterized. Dispersion of observed signals is used to determine the magnetospheric propagation pathsand associated cold plasm a densities. It is found th at H AA RP induced triggeredemissions occur primarily inside the plasmapause and the availability and couplinginto mag netospheric 'du cts' is likely one of the limiting factors for observations. Pha seand a mp litude changes in the observed signals are used to resolve the te m por al b ehavior of the non-linear resonant current vector that drives amplification. The observedresonant current behavior is discussed in the context of numerical models and usedto make inferences about the magnetospheric hot plasma distribution. Ground basedcapabilities of detection of energetic particle precipitation from the Earth's radiationbelts induced by HA AR P generated E LF /V LF waves are assessed experimentally andtheoretically. A phenomenon of cross-modulation between whistler-mode signals andHF ionospheric heating is observed and investigated as a new method to generateELF/VLF radiation using an HF ionospheric heater.

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A c k n o w l e d g e m e n t sThere are many individuals who have made this work possible and have aided methro ugh m y grad uate stud ent career. First and foremost, I would like to than k myadviser, Professor Umran Inan for his support, guidance, and scientific vision. I amhonored to have been a member of Umran's research group and appreciate the manyopportunities for scholarship, teaching, and adventure that this has involved. Second,I am grateful for my opportunities to interact with Professor Don Carpenter who hasbeen a great source of advice on analyzing and interpreting data as well providingvaluable perspective and historical context for my work.

Several students in the VLF group have been instrumental in my research effortsand intellectua l develo pm ent. I th an k Morris Cohen for m any years of successfulcollaboration in deployment of hardware, design of experiments, analysis of data,and publishing of results. My discussions with Andrew Gibby on triggered emissiontheory solidified key concepts and helped me make significant progress in my last year.Prajwal Kulkarni has served as a mentor, co-instructor, tennis partner, mutual soccerenthusiast, and above all a friend always willing to lend an ear in matters of scienceand life. I am grateful to Ryan Said for many fruitful discussions particularly on thenuances of signal processing. Ben C otts h as always been a truste d source of solidarityand supp ort. Ro bb Moore introduc ed me to the world of HA AR P wave injectionexperiments; Joe Payne will always be an example of dedication and persistencein engineering an d field work. I com men d Noah Redd ell, Max Klein, an d JeffreyChang for their efforts in the realization of the Buoy receivers which provided mewith valuable observations. I tha nk D enys Piddyachiy, George Jin and othe rs formany hours of monitoring the HA AR P experiments and helping me obtain im porta nt

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results.Shaolan Min and Helen Niu are responsible for the VLF group being not only

an efficient research enterprise but also an organization founded on camaraderie andfriendship. D ata collection from th e HA AR P experiment has been greatly aided bythe work and organizational skills of Dan M usetescu. My research also would no thave been possible without the cooperation and commitment of the receiver hosts inAlaska and the operato rs of the H AA RP facility. Special tha nk s to Norm a and DoyleTraw, Mike McCarrick, Helio Zwi, and David Seafolk-Kopp.

Finally, I would like to thank my family and my wife Patrycja for the love andsupport that has guided me to this accomplishment.

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C o n t e n t sA b s t r a c t vA c k n o w l e d g e m e n t s v i1 I n t r o d u c t i o n 1

1.1 Th e Iono sphere 21.2 Th e M agnetosphere 41.3 W histler Mode Wave Injection Exp erim ents 9

1.3.1 Early Exp erim ents and Observations 101.3.2 Siple St ati on 111.3.3 HF Ionosp heric He ating 121.3.4 Th e HA AR P Facility 16

1.4 Thes is Org aniza tion 171.5 Scientific Co ntrib utio ns 18

2 E x p e r i m e n t S e t u p a n d O b s e r v a t i o n s 2 02.1 Exp erimen t Setup 20

2.1.1 No rthern Hem isphere 202.1.2 South ern Hemisphere 212.1.3 Spacecraft Ob servatio ns 242.1.4 Transmission Form ats and Op eration 24

2.2 Observa tions 252.2.1 Firs t Ob serva tions: 20 Ap ril 2004 262.2.2 Full Power HA AR P 26

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2.3 Sum ma ry of Observations 373 O c c u r r e n c e S t a t i s t i c s a n d D e p e n d e n c i e s 3 9

3.1 HA AR P EL F/V LF Generation Strength 403.2 Dete rmin ation of Mag netospheric Pa th 443.3 Mo dulation Technique and Beam Geom etry 513.4 Com parison with Siple Statio n Results 553.5 Sum ma ry 59

4 T h e o r y a n d O b s e r v a t i o n s 6 14.1 Theo retical Background 62

4.1.1 Cyclotron Resonance 624.1.2 Single Par ticle Dy nam ics 644.1.3 Resonan t Curr ents 674.1.4 Non -linear Am plification Mec hanism 69

4.2 Observations in Theore tical Contex t 724.2.1 Resonance and Trapp ing Conditions 734.2.2 Cha racterization of Non-linear Resonan t Cur rent 74

5 C r o s s M o d u l a t i o n 8 25.1 Ob serva tion 825.2 Theo retical Modeling 835.3 Cross Mo dulation for Genera tion 86

6 H A A R P I n d u c e d E l e c tr o n P r e c i p i t a ti o n 9 16.1 Exp erime nt Setup 926.2 Theo retical Predictions 956.3 Effect of HF He ating 986.4 Mitigation of HF Heating 1006.5 Ionospheric Effect of E LF /V LF Mo dulation Frequency 1026.6 Sum mary and Futu re Work 103

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7 Su m m ary and Sug ges t ion s for Future Work 1057.1 Sum mary of Con tributions 1057.2 Suggestions for Fu tur e Work 107

A D e r i v a t i o n o f t h e W a v e U p d a t e 1 0 8

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Lis t o f Tables2.1 Sum mary of Observation Occurrence 38

3.1 Pa ram ete rs of Diffusive Equ ilibrium Models 47

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I.

L i s t o f F i gu r e s1.1 Typ ical Ionosph eric Profiles 31.2 Th e Ou ter Mag netosphere 51.3 The Inner Mag netosphere 61.4 W histler Mode Prop agatio n in the Ma gnetosphere 91.5 Mag netospheric Echoes from Siple Statio n Experim ent 131.6 HA AR P Wave Injection Experim ent 182.1 Expe rimen t Hardw are and Locations 222.2 First Observations 252.3 27 Febru ary 2007 Observations 272.4 4 March 2007 Observations 282.5 Non -linear Am plification 292.6 Active Frequency Range 302.7 Stro ng 2-Hop Echoes . .' 312.8 One-hop Echoes on DE M ET ER 322.9 One-ho p Echoes on Buoy 2.0 342.10 Two -hop Echoes from Ram ps Only 352.11 One-ho p Echoes on Tang aroa 363.1 HA AR P Signal Am plitudes Yielding Echoes 423.2 Echo Am plitude vs. Tim e I 433.3 Echo Am plitude vs. Tim e II 443.4 Magnetospheric Pa th Determination 483.5 Eq uat ori al Den sity Profiles 50

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3.6 Echoes Excited by Geom etric Mo dulation 523.7 Sca tter Plo t for Different M odu lation Techniqu es 533.8 Investigation of HF Beam Position 553.9 Sp atial Effects of H F Beam Position 563.10 Siple Sta tion Sta tistics 574.1 Cyclotron Resonance 634.2 Co ordina te System for Single Particle Dynam ics 644.3 Resona nt Cu rrent Form ation 654.4 Resona nt Cu rrent Configuration 674.5 Non -linear Am plification Dy nam ics 704.6 Trapp ing Am plitude and Resonan t Energies 744.7 Track ing of Non -linear Cu rren t Vector 764.8 Different In pu t Am plitud es 774.9 Different In pu t Am plitud es 784.10 Same Inp ut Am plitudes 795.1 Echo Heating Cross M odulation 835.2 Effect of M od ulate d H F He ating on Ionosp heric Me dium 855.3 Spectral Analysis of Cross M odulation 875.4 Cross Mo dulation Gen eration Concept 885.5 Gen eration Using Cross Mo dulation 895.6 Frequency Response of Cross Mo dulation Based Gen eration 906.1 VL F Rem ote Sensing 946.2 VL F Transm itter Path s 956.3 Pred icted Non -ducted Precip itation 976.4 Scale of H F He ated Region 996.5 Effect of H F He ating on V LF Re mo te Sensing 1016.6 M itigatio n of H F He ating Effect 1026.7 Effect of E L F /V L F Frequen cy 104

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C h a p t e r 1I n t r o d u c t i o nI t is on ly wi t h in the las t 75 years th a t a com prehens ive des c r ip t ion o f th e im m edia teenv i ro nm ent su r roun d ing our p lane t has been ach ieved . For hund red s o f years sc ient i s t s were more versed in the o rb i t s o f d is tan t p lane ts and s ta rs than in the na tu re o fmat te r a few hundred k i lomete rs above the Ear th ' s su r face . I t was a rguab ly the d is covery o f e lec t ro ma gne t ic w aves and th e dawn of th e Space Age tha t f inal ly p rov ide dth e too ls to exp lo re th e su r rou nd ing s of our p lan e t . Mos t genera l ly , the ne ar - Ea r thspace env i ronment can be d iv ided in to two over -a rch ing reg ions known as the magne-to s p h e r e a n d t h e i o n o s p h e r e . A l th o u g h a t t h e t im e of t h i s wr i t i n g t h e b a s i c s t r u c tu r e sand boun dar ies of th e ma gne t osph ere and ionosphere have been success fu l ly m ap pe d ,the spa t ia l and tempora l va r ia t ions , dynamics , and coup l ing o f the phys ica l p rocessesin these reg ions con t inue to be the sub jec ts o f ac t ive exper imenta l and theore t ica lresearch . Th e top ic of th i s d is se r ta t ion i s th e assessment and ana lys is of resu l t s of ap io n e e r in g e x p e r im e n t i n v o lv in g p r o b in g t h e E a r th ' s m a g n e to s p h e r e w i th E x t r e m e lyLow Frequ ency (E LF) an d Very Low Frequen cy (VL F) e lec t rom agne t ic waves gene ra te d v ia High Frequen cy (H F) hea t in g o f the auro ra l ionosphe re . In th is c hap te rwe p resen t a b r ie f background and mot iva t ion fo r the top ic and desc r ibe the spec i f iccon t r ibu t ion s o f th e p resen t work .

1


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CHAPTER 1. INTRODU CTION 2

1.1 T h e Ion os p he reAlthough the dominant states of matter on the Earth's surface are the familiar solid,liquid, or gas, the surrounding space environment is distinguished by the presence ofvarying densities of ionized gases, also known as plasma or the fourth stat e of m atte r.Plasma densities become appreciable at altitudes of about 50 km, where photoioniza-tion caused by solar radiation generates significant densities of free electrons and ionsto affect the propagation of radio waves. This altitude marks the approximate lowerboundary of the ionosphere, the region of the atmosphere that is electrically charged[Ratcliffe, 1959; Tascione, 1994, Sec. 7.0]. The vertical structure of the ionosphereshows considerable variation as a function of the time of day, latitude, season, andsolar activity. However, the essential features of the ionosphere can usually be identified and manifest themselves as horizontal layers organized by altitude and electrondensity. Dis tinct ion ospheric layers develop because th e deposition of solar energyand physics of recombination depend on the atmospheric density and composition,which both vary predictably with altitude. Figure 1.1 shows the typical ionosphericnighttime and daytime profiles with identified layers D, E, Fl, F2 [Tascione, 1994,Sec. 7.1]. At nighttime the overall electron density decreases since photoionizationceases, but drainage form the overlying region and cosmic rays ensure that the ionospheric plasma persists through the night. In addition to the identifiable ionosphericlayers, the ionosphere is known to regularly host transient and laterally inhomoge-neous structures. These variations include dense slabs of ionization on scales of tensto hundreds of kilometers known as sporadic E or spread F depending on the altitude[Calvert and Warnock, 1969] and also much smaller, even meter scale, density irregularities [Dyson, 1969; Clark and Raitt, 1976; Gross and Muldrew, 1984; Sonwalkarand H arikumar, 2000].

Th e ionosphere is generally characterized as a collisional plasma. For altitude s below 200 km, in the D and E regions, collisions between electrons and neu tral moleculesare dominant with collision frequencies reaching 10 6107 Hz in the D-region. Above200 km, in the F region, the collision frequency is typically 2 to 3 orders of magnitude lower and collisions are predominantly coulomb collisions between charged


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CHAPTER 1. INTRODUC TION 3

species [Helliwell, 1965, Sec. 3.8]. Particles in the ionosphere have significant thermal motions, with temperatures about 200 K in the D region and 1000 K2000 K inthe F region. Nevertheless, in the context of electromagnetic wave propagation, thethermal motions can be neglected and the propagation of waves within and throughthe ionosphere can be accurately modeled assuming a 'cold' (0 K) plasma [Budden,1988, p. 4-5].

Since the ionosphere contains free charges, it can support electrical currents andin fact many kilo-Amperes of current regularly flow around the Earth at ionosphericaltitudes. Certain ionospheric currents are purely local or global but devoid of identifiable structure, such as those associated with the global electric circuit [Rycroft et al,2000]. However, two distinct global current structures exist, the auroral electrojetand the equatorial electrojet. Th e equatorial electrojet is driven by neu tral windsand partly results from the unique geometry of a horizontal magnetic field found inthe equatorial region [Forbes, 1981]. The so-called auroral electrojet, on the otherhand, is driven by the electrodynamic coupling of magnetospheric plasma dynamicsto the ionosphere via geomagnetic field lines [Baumjohann, 1983].

Day/Night Electron Concentrations1000800600400

| 200< 1501008060 ,10 102 10 10 105 106Electron C oncentration (cm~3)

Figure 1.1: Typical daytime and nigh ttime electron density profiles in th e ionosphere withidentified layers. Adapted from [Tascione, 1994, p. 90].


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1.2 T h e M a g n e t o s p h e r eAt altitudes of ~2000 km the ionosphere gives way to what is known as the magneto-sphere. Although the boundary between the neutral atmosphere and the ionosphereis abrupt, the higher altitude transition from the ionosphere to the magnetospheredoes not have easily identifiable features and the definition of this boundary is largelya matt er of convention. The ma gnetosphere is so nam ed because the E ar th 's sta ticma gnetic field dom inates the particle and wave dynamics. To first order, the geomagnetic field is that of a simple magnetic dipole, tilted and off-center relative to therotat ion axis of th e Eart h. At distances greater tha n 5 Ea rth radii from th e centerof the Earth (1 Earth radius RE 6370 km), the geomagnetic dipole geometry issignificantly distorted by the solar wind, a hot plasma (10 5 K) composed mainly ofproton s and electrons traveling outward from the sun at speeds of ~5 00 km /s . Abow shock is formed at ~11RE with the solar wind dragging the geomagnetic fieldlines away from the sun , forming a large ma gneto tail. Th us, as is shown in Figure1.2, the magnetosphere extends to about 10 12RE toward the sun and about 100i?Eaway from the sun. In this research we are primarily concerned w ith regions of themagnetosphere with 'closed' magnetic field lines, meaning lines of force that beginand end at the Earth's surface and are not grossly distorted from the dipole geometry. This region extends to about 67R& and is known as the inner magnetosphere,shown schematically in Figure 1.3,

The magnetosphere is entirely filled with plasma that is fully ionized and canbe regarded as collisionless. The magnetospheric plasma is usually divided into twodistinct populations. The background 'cold' plasma has low energies typically ~0.5eV (~5000 K), and densities in the range of 101000 cm - 3 . One characteristic of thiscold plasm a distribu tion is a sharp drop in density from 1 to 2 orders of mag nitud eat a boundary known as the plasmapause [Carpenter, 1963]. The region of the innermagnetosphere within the plasmapause is known as the plasmasphere. The locationof the plasmapause varies from ~ 2 / ? E to 7R& and is believed to be determined by thebalance of the electric field arising from the Earth's rotation in its own magnetic field(co-rotation field) and the cross-tail electric field in the outer magnetosphere which


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>

Solar Wind>-

>

>->

Figure 1.2: The general outline of the magnetosphere. Adapted from [Inan, 1977].

is driven by the solar wind [Tascione, 1994, Sec. 5.6]. The effects of these fields andtheir associated currents, as well as other magnetospheric processes, are observablein ground measurements of variations of the geomagnetic field. The level of activityin these measurements is often generally referred to as the geomagnetic conditions.Disturbed conditions correspond to an erosion of the plasmasphere, while under quietconditions the plasmasphere expands radially outward as depleted regions are refilledfrom the dense underlying ionosphere.

In addition to the background cold plasma, the magnetosphere contains small


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CHAPTER 1. INTRODUCTION 6

Figure 1 .3: Rep resenta tion of the inner mag netosph ere. Th e black lines represent t hegeoma gnetic field lines. Th e gray scale colors represent cold plasm a density inside andbeyond the plasmapause. The red lines represent trajectories of trapped energetic electronsthat make up the hot plasma of the radiat ion belts . Adapted from [Gibby, 2008].

bu t s ign i f ican t popu la t ions o f 'ho t ' p lasma charac te r ized by dens i t ie s be low 1 cm - 3bu t w i th h igh energ ies rang ing f rom 1 keV to severa l MeV. T he energe t ic pa r t ic lesth a t c o n s t i t u t e t h e h o t p l a s m a a r e t r a p p e d i n t h e m i r r o r g e o m e t r y of t h e m a g n e t i cf ie ld where they execu te a he l ica l gyro-mot ion a round the magne t ic f ie ld l ines and'bounce ' be tween the oppos i te hemispheres as shown in F igure 1 .3 . Co l lec t ive ly , theho t p lasm a pop u la t ion s in the ma gne tosp here a re re fe r red to as the 'V an Al len be l t s 'o r s im p ly t h e r a d i a t i o n b e l t s [Tascione, 1994, Sec. 4.8].

W h i l e t h e c o ld p l a s m a g o v e r n s t h e p r o p a g a t io n c h a r a c t e r i s t i c s o f e l e c t r o m a g n e t i cwaves in the ma gne tosp here , i t i s the ho t p lasm a th a t d r ives ins tab i l i t ie s and su pp or t swave-par t ic le in te rac t ions tha t can lead to wave ampl i f ica t ion , pa r t ic le acce le ra t ion ,


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and particle loss thro ugh p itch angle scattering . Interactio ns of waves with ho t plasm apopulations in the magnetosphere constitute a rich field of study and continue to bethe subject of both experimental and theoretical work over a broad range of frequencies and interaction modes [Platino et al, 2004; Omura and Summers, 2006; Omuraet al, 1991; O'Brien et al, 2003; Hui and Seyler, 1992]. The specific wave-particleinteraction considered here is the whistler mode instability, also variously referredto as the coherent wave instability, non-linear magnetospheric amplification, or theVLF triggered emission phenomenon. The whistler mode instability occurs for wavespropagating in the whistler mode, which describes electromagnetic waves propagatingin a magnetized plasma with frequencies below the electron cyclotron frequency andelectron plasma frequency bu t above the proto n cyclotron frequency. In the Ea rth 'smagnetosphere such waves can undergo resonant interactions with energetic electronsin the keV energy range.

Even without the inclusion of hot plasma interactions, the propagation of whistlermode waves in the magnetosphere is complicated by the anisotropic and inhomoge-neous natu re of the m edium . Generally, whistler mode waves in the ma gnetosp herepropagate along complex trajectories that are governed by the evolution of the angle between the wave normal vector (k) and the geomagnetic field (B 0 ). This anglechanges dynamically during propagation through the magnetosphere, which is in-homogeneous in terms of both the cold plasma distribution and geomagnetic fieldgeometry. [Helliwell, 1965, Sec. 3.4]. Furthermore, the direction of propagation ofwave energy is not along the k-vector direction, due to the anisotropy of the medium.An example of a whistler mode ray trajectory originating from a terrestrial source isshown in Figure 1.4 by the blue ray pat h. Typical of such a ray pa th is the abr up treversal of direction known as a magnetospheric reflection (MR) [Lyons and Thome,1970]. The MR effect prevents many whistler mode waves, whether terrestrial ormagnetospher ic in o r ig in , f rom reach ing the lower ionosphere , thus render ing themnot observable on the Earth. Furthermore, even for the subset of trajectories that doreach the lower ionosphere, the sharp plasma gradients therein lead to total internalreflection of most wave energy. Only waves with k-vectors oriented near no rm al t othe lower ionospheric surface can penetrate into the Earth-ionosphere waveguide and


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be observed on the ground [Sonwalkar and Harikumar, 2000].However, th e magn etosphere regularly contains field-aligned den sity enhan cem ents

which can efficiently guide waves and th us gre atly simplify the ir trajec torie s [Angerami,1970]. These density enhancements are commonly referred to as 'ducts' and guidewhistler mode waves in the same manner as optical fibers guide waves in the visible band, namely via confinement through an index of refraction gradient transverseto the direction of propagation [Helliwell, 1965, Sec. 3.6]. The effect of a duct onwhistler mode propagation can be qualitatively depicted as in the red ray path inFigure 1.4, which shows a trajectory confined to propagation along the geomagneticfield line and therefore intersecting the lower ionospheric bound ary. A ducted raypath intrinsically implies a k-vector parallel, or close to parallel, to the geomagneticfield line. Consequently, waves along such a pa th impinge onto the ionospheric bo und ary near normal to the density gradients and penetrate to the ground. In contrastto non-ducted waves, waves in ducts can readily be observed on the ground at predictable locations, namely near points where a magnetic field line associated with aduct intersects the Earth's surface.

A convenient coordinate for describing positions in the magnetosphere with reference to the geomagnetic field is the Mcllwain //-param eter. Th e L-param eter isused to specify the surfaces along which particles trapped in the geomagnetic fieldmove. The L-value of a given drift shell in the real geomagnetic field corresponds tothe equatorial crossing in Earth radii, of the equivalent drift shell (i.e., the particlesthat have the same adiabatic invariants) in a titled, off-centered dipole model of theEarth's magnetic field. Thus if a magnetic field line or position in the magnetosphereis specified using its L-value, it corresponds roughly to a position on a dipole field linethat crosses the magnetic equator at L Earth radii [W^aft, 1994, Chap. 4]. Two locations on the Earth's surface in opposite hemispheres but along the same geomagneticfield line, or L-shell, are referred to as magnetic conjugate points.


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Figure 1 .4: A representation of propagation trajectories of whistler mode waves in thema gnetosp here. Bo th the blue ray path and the red ray pa th are for signals initially launchedat the Ea rth ' s surface. The blue ray path is not ducted and undergoes m agnetosphericreflections (MR). The red ray path represents ducted propagation along the geomagneticfield line. T he ducted p ath re turn s to the ionosphere in the south ern hem isphere and be causeit arrives with near-vertical incidence it will penetrate the ionosphere and be observed onthe ground.

1.3 W h i s t l e r M o d e W a v e I n je c ti o n E x p e r i m e n t sDu cted p ropa ga t io n of whis t le r mo de waves in the ma gne tosp here fac i l i ta tes th e rea l iza t ion o f an exper imenta l concep t in which s igna ls o f known te r res t r ia l o r ig in can beo b s e r ve d o n t h e g r o u n d a f te r p r o p a g a t io n t h r o u g h th e m a g n e to s p h e r i c p l a s m a a lo n ga known pa th . In the s imples t case , such a p ropaga t ion pa th invo lves a s ing le t raversebe tw een con juga te po in ts by way o f the m agne tosp here . In fac t th e fi rs t know ledgeof th e p lasm a con ten t o f the mag ne to sph ere cam e f rom the cor rec t in te rp re ta t ion o fthe o r ig in o f na tu ra l ly occur r ing rad io s igna ls known as ' a tmospher ic whis t le rs , ' sonam ed becau se of the i r charac te r i s t ic whis t l ing sound when p layed th r ou gh a speaker .


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These signals were shown by Storey [1953] to originate from impulsive lightning induced radiation that had been dispersed by propagation along a ducted path throughthe magnetospheric plasma.1 Carpenter [1963] used observations of whistlers to mapthe cold plasma distribution in the magnetosphere and discover the plasmapause.The successful usage of whistler data led to the use of signals injected by man-madesources for magnetospheric probing and observation of hot plasma effects in additionto the dispersion which is governed predominantly by the cold plasma population.

1.3 .1 Ea r l y Ex p e r i m en t s an d O b se r v a t i o n sThe VLF triggered emission phenomenon induced by a man-made source was firstidentified in 1964 in data originally recorded in 1959 [Helliwell et al., 1964]. Thesignal from the 18.6 kHz US Navy NPG communication transmitter in Jim Creek,Washington was observed, after ducted magnetospheric propagation, in Wellington,New Zealand. Th e main features of the instability were identified to be tem por alamplification as observed at a stationary receiver and the triggering of free runningemissions such th at ad ditional frequency com ponen ts are generated. Furth er exam plesof the phenomenon were found in VLF recordings made on board the USNS Eltaninin 1961, showing tha t amplified signals and triggered em issions were induc ed by th e14.7 kHz NAA transmitter in Cutler, Maine [Helliwell, 1965, Sec. 7.2].

Upon identification of this unique and dynamic but yet highly repeatable phenomenon, efforts were undertaken to investigate it with a controlled experiment employing dedicated hardware. In 19661969 a VLF transmitter was established nearByrd Station, Antarctica (80.02 S, 119.53 W). In a setup that came to be knownas Byrd Longwire, a long antenna was laid out on the Antarctic ice for generatingVLF waves for magnetospheric wave injection. The Byrd configuration was unfortunately far from ideal as the proximity of the snow, made worse by immediate snowaccumulation over the antenna, led to substantial near field losses and reduction ofrad iatio n efficiency. Fu rthe rm ore , movem ent of th e ice crea ted frequent b reak s in thewire. Despite successful ionospheric sounding and reception of signals on the OGO 4

1 This is the origin of the name whistler mode.


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spacecraft, signals from the Byrd transmitter were never observed at the conjugatepoint [Helliwell and K atsufrakis, 1974, 1978].

Another experiment worthy of mention is the transportable VLF transmitter witha 1200 m balloon suspended antenna that transmitted 6.6 kHz signals from Port Hei-den, Alaska in 1973. Several cases of th e VLF trigger ed emission instab ility weresuccessfully observed at the conjugate point in Dunedin, New Zealand even thoughthe vertically oriented antenna was not optimal for launching waves into space [Dow-den et al., 1978]. Nevertheless, observations and transmissions were limited as theexperiment was intrinsically temporary in nature.

1.3.2 Siple S ta t io nThe first long term, and to this day most referenced, magnetospheric wave injectionexperiment was that operated by Stanford University at Siple Station, Antarcticafrom 1973 to 1988. Th e site of th e Siple Sta tion facility (75.93 S, 84.25 W) waschosen because of the accessibility of its conjugate point (located near the city Rober-val, Quebec), its magnetic latitude offering access to the plasmapause, and becausethe site was known to be an excellent location for observations of naturally occurringmagnetospheric emissions and whistlers [Helliwell, 1970]. Perhaps most important inthe selection criteria was that the site was atop a 2 km thick ice sheet that elevatedthe horizontal antenna above the conducting ground plane a significant fraction of awavelength. This elevation combined with the many kilometer length of the antennaallowed the transmitter to have a reasonable radiation efficiency of 13 % [Raghuramet al, 1974].

The original installation was a 80 kW transmitter and 21.2 km antenna with resonan t frequency of approx imately 5 kHz [Helliwell and K atsufrakis, 1974]. This facilitywas later upgraded to a 150 kW transmitter and two 42 km long antennas arranged ina crossed dipole geometry leading to a resonant frequency of 2.5 kHz and an estim atedradiated power of 1.5 kW [Carpenter and Bao, 1983; Helliwell, 1988; Gibby, 2008].The Siple transmitter excited magnetospheric wave amplification and triggering inthe 1.56 kHz frequency range and on magnetospheric paths ranging from L ~ 3 t o


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L ~ 5 . Th e facility itself was located at L ~4.3. An example of the observations fromthe Siple experiment can be seen in Figure 1.5, showing spectrogram records from areceiver at Siple Station in Antarctica and also from the conjugate point in Roberval,Quebec. A 2 second long pulse at 1.4 kHz is initially transmitted at 0.5 seconds inthe record. Approximately 1.5 seconds later the signal arrives at the northern hemisphere after propagation through the magnetosphere in the whistler mode, as shownin the lower right inset. Such a signal that has completed a single inter-hemispherictraverse through the magnetosphere is referred to as a '1-hop echo.' Subsequent tothe 1-hop echo observation in the north, the signal is seen to return to the southernhemisphere at which time it is termed to be a '2-hop echo.' Typical of the magne-tospheric wave-particle interaction, each traverse through the magnetosphere causesthe signal to be not only amplified but also results in the creation of free runningemissions at frequencies extending well away from that of the original transmission.

The Siple Station experiment produced many years of excellent observations ofwhistler mode wave amplification and triggering of emissions which continue to form afoun dation for curr ent expe rim ental and theo retica l efforts in th e field. Un fortun ately,the cost of the logistics support of the facility was very high. The station was closedin 1988 and the site was abandoned. Given that snow accumulation on the Antarcticpeninsula in that region is ~1.8 meters a year, the remnants of the Siple Stationfacility are now buried under tens of meters of snow and any resurrection of thestation is unlikely.2

1 .3 .3 H F Iono sphe r i c H ea t ingAs was indicated in Section 1.3.2, one of the challenges in operating a whistler modewave injection experiment is generating waves in the appropriate frequency band. Inthe magnetosphere whistler mode waves propagate at frequencies from a few hundredHz to tens of kHz, with corresponding free space wavelengths of tens to thousands ofkilometers. Since efficient generation requires both the antenna dimensions and theseparation from the ground plane to be on the order of a wavelength, construction of

2For a recently compiled histor y and assessment of the Siple Statio n facility see Cha pte r 2 ofGibby [2008].


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3~ >1 -

0 -

321-0-

Siple Station, Antarctica 26 July 1977Transmitted pulse?

2-Hop Echo

Roberval, Quebec

X '*>-Hop Echo

1026 July 1977

10

1137:00 UT

20 1137:00 UTVLF Wave Injection fromSiple Station, Antarctica

iRoberval

Siple 1 rs

dB

-1 0 8

-20

-30

F i g u r e 1.5: Observations from the Siple Station experiment showing a 1.4 kHz t ransmi t ted pulse (top panel) , a 1-hop echo observed at the conjugate point in northern hemisphere(bottom panel) and a subsequent 2-hop echo observed in hemisphere of origin . The phenomena repeats for the subsequently transmitted pulses . The echoes are amplified and containfree running emissions of changing frequency typical of the whistler mode instability. Theinset in the lower right illustrates the Siple experiment setup.

t h e a p p r o p r i a t e h a r d wa r e p o s e s a cons iderab le eng ineer ing cha l lenge . In fac t , apar tf rom un ique loca t ions l ike the A n t a r c t i c ice sheet used as a p la t f o r m for the SipleStat ion faci l i ty , achieving the necessa ry e leva t ion for a m a n - m a d e h o r i z o n t a l (s k y wa r dd i rec ted) d ipo le is near ly imposs ib le .

D u e to the cha l lenges of c o n s t r u c t i n g the necessa ry geometr ies for E L F / V L F w a v eg e n e r a t i o n , an a t t r a c t i v e a l t e r n a t i v e i n v o lv in g t a k in g a d v a n ta g e of n a tu r a l l y o c c u r r ing ionospher ic cu r ren ts has been pursued s ince the 1970 's [Getmantsev et al, 1974;Stubbe et al, 1977, 1982] . High Frequency (HF) waves in the 1-10 MHz b a n d h a v ewa v e l e n g th s on the scale of m e t e r s and can t h u s be eff ic ien tly genera te d . W it h sufficient power, HF r a d i a t i o n can be u s e d to c h a n g e the t e m p e r a t u r e of e l e c t r o n s int h e i o n o s p h e r e at a l t i t u d e s of 60100 km. Since the c o n d u c t iv i t y of the ionospher icp l a s m a d e p e n d s on the e l e c t r o n t e m p e r a t u r e , HF hea t ing a l lows for modif ica t ion of


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the natu ral currents flowing in the ionosphere. Periodic heating at an E LF /V LFfrequency can transform the largely DC (or very slowly varying) ionospheric currentsinto a giant radiating antenna in the sky, suitably elevated and oriented for radiatingwaves into the magnetosphere as well as into the Earth-ionosphere waveguide. Several facilities have been c onstructed around the world which are capable of performingsuch wave generation. Although the present work represents the first instance of thesuccessful use of HF heating for whistler mode wave injection experiments, a briefreview of the most important HF heater ELF/VLF wave generation facilities andresults provides valuable context and is provided below.

T r o m s 0 , N o r w a yThe European Incoherent Scatter (EISCAT) Association has extensively used the HFheating facility near Troms0, Norway to generate ELF/VLF signals by modulating theoverhead au roral electro jet c urren ts with 1 MW of radiated HF power in the 2.78.0MHz band [Barr and Stubbe, 1991]. ELF/VLF signals were typically produced usingsquare wave amplitude modulation in the 200 Hz to 6.5 kHz frequency range withobserved amplitudes of ~1 pT, ~0.1 pT and ~0.03 pT at respective ground distancesof ~20 km, ~200 km and ~500 km from the heating facility [Stubbe et al, 1982; Barret al, 1985]. Being the first facility to operate continuously for many years with ahigh radiated power, the Troms0 experiment brought to light many characteristics ofHF heater induced ELF/VLF generation that would later be confirmed with otherfacilities and are today considered conventional. At distances of less than ~100 km,the observed ELF/VLF amplitudes exhibited maxima in frequency near multiples of~2 kHz. These peaks were shown to be the result of vertical resonances in the Earth-ionosphere waveguide. Heating with X -mode polarization of the HF waves was shownto be more effective in generating ELF/VLF waves than heating with O-mode polarization, with corresponding amplitudes observed on the ground exhibiting a 3 dBdifference.3 The variations in ELF/VLF signal amplitude on time scales of hours and

3 In the context of HF heating, X-mode and O-mode polarization simply defines the sense of rotation of the wave fields. X-mode polarization is defined as a radio wave entering the ionosphere withright hand elliptical polarization in the northern magnetic hemisphere (or left hand in the southern hemisphere). O-mode polarizat ion involves rotat ion in the opposi te sense. These polarizat ions


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days have shown correlation with geomagnetic activity represented by the Kp index[Barr et al, 1985]. This correlation results from the fact that the auroral electrojet isdriven by dynam ics in the m agnetosp here. Electric fields in the mag netosph ere, whichcan be enhanced during disturbances, are mapped down to the ionosphere via geomagnetic field lines. Additionally, accompanying precipitation of energetic particlescauses enhancements of ionospheric conductivity [Baumjohann, 1983]. The electricfields drive currents via Ohm's Law, which for the anisotropic ionospheric plasmaneeds to be expressed as a 3-dimensional tensor equation. Generated ELF/VLF amplitudes were shown to be well correlated with the electrojet electric field (even onshort sub-hour timescales). Polarization measurements led to the conclusion that itis modulation of the ionospheric Hall conductivity that is the dominant radiator ofEL F/V LF waves observed in ground based measurements [Rietveld et al., 1983, 1987].

Unfortunately, although Troms0 generated signals were observed in space [Jameset al, 1984, 1990], th e facility is not useful for th e du cted wh istler m ode wave injectionexperim ents of the type performed at Siple Station . Located at L > 6, Troms0 ison sub-auroral/auroral field lines that are mostly 'open' and on which conditionsfor hemisphere-to-hemisphere ducting are much less favorable [Carpenter and Sulic,1988].

A r e c i b o , P u e r t o R i c oSeveral ELF/VLF wave generation experiments have been performed by modulatingthe equatorial electrojet current using a heater located at the Arecibo Observatoryin Pu erto Rico. During these experim ents, EL F/ V LF waves were produ ced overa frequency range of 500 Hz to 5 kHz using a heater frequency of approximately3 MHz and a total input power of 800 kW [Ferraro et al, 1982]. In addition toconfirming characteristics observed in the Troms0 experiments, the dependence of theELF/VLF amplitude on the duty cycle of the modulation was investigated. Ferraroet al. [1984] found tha t a 48% duty cycle yielded maxim um EL F/ V LF am plitudes.E LF /V L F wave generation can exhibit sensitivity to duty cycle because of differencesshould not be confused with ordinary and extraordinary modes of propagation in a cold magnetizedplasma.


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in the heatin g an d cooling rates of ionospheric electrons. This sensitivity intrinsicallydepends on the E LF /V LF frequency in question and also on the heating/cooling ratesthemselves which vary with altitude.H I P A S , A l a s k aThe High Power Auroral Stimulation (HIPAS) facility, near Fairbanks, Alaska, hasbeen used in a number of experiments to modulate the auroral electrojet to produceELF/VLF waves. The HIPAS facility radiates 800 kW of power at 2.85 MHz. Mostnotably, the HF heater was used to create ELF/VLF waves through three differentmodulation techniques, amplitude modulation, phase modulation, and beat-frequencymo dulation . It was generally found tha t amplitude mod ulation yielded the bestresults [Villasenor et al, 1996]. EL F/ V LF signals generated by HIPAS were alsoobserved in space aboard the Akebono satellite [Kimura et al, 1991].

1 .3 .4 T he H A A R P Faci l it yThe High Frequency Active Auroral Research Program (HAARP) began the construction of an ionospheric heating facility in 1990 near the town of Gakona, Alaska.The HAARP heater was originally a 96 element crossed dipole array capable of radiating 960 kW of power. In 2007 a planned multi-year up grade of the facility wascompleted, bringing the number of elements to 180 and the radiated power to 3.6MW, making HAARP the most powerful ionospheric heating facility in the world.ELF/VLF waves generated by HAARP were first observed by Milikh et al. [1999] andin subsequent years observations were reported at distances of up to 4400 km [Mooreet al, 2007] and in space [Platino et al, 2004, 2006]. Cohen et al [2008b] showedthat in the far field, the effective radiation pattern of the HAARP induced ELF/VLFis that of a dipole. Besides an unprecedented level of radiated power, the HAARPfacility also provid es for increased flexibility in bea m ph asing and steerin g. H A A R P'sunique capabilities have allowed for new methods of ELF/VLF generation other thanthe conventional amplitude modulation [Cohen et al, 2008a].

Unlike other hea ting facilities, H AA RP 's location at L~ 4.94 (at 85 km altitu de) is


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favorable for conducting ducted whistler mode wave injection experiments, a projectwhich has been led by Stanford U niversity. Figure 1.6 shows a schem atic of theHAARP wave injection experiment, the results of which are the subject of the presentwork. ELF/VLF waves generated in the ionosphere above HAARP are injected intothe Earth-ionosphere waveguide and also into the magnetosphere. Waves in the mag-netosphere travel in ducts to the equatorial region where they undergo a wave-particleinteraction w ith radiation be lt electrons. Th e amplified waves are observed at th eHAARP conjugate point as 1-hop echoes and subsequently in the northern hemisphere as 2-hop echoes. In addition to wave amplification and triggering, the waveparticle interaction can cause pitch angle scattering and precipitation of energeticparticles onto the ionosphere.

1.4 Th es i s O rga niz a t io nThe present work is organized into 7 chapters.

Chapter 1, the present chapter, introduces the relevant background and motivationfor this work.

Chapter 2 describes the experimental setup and presents observations of magne-tospherically amplified signals resulting from several years of experimental investigations.

In Ch apter 3 we analyze the sensitivities and occurrence pro perties of the observations.

In Chapter 4 we present the theoretical treatment of VLF triggered emissions andput our observations in the context of numerical simulations.

Chapter 5 presents observations and analysis of a new phenomenon of cross modulation between whistler mode signals and HF ionospheric heating. The concept isinvestigated as a new technique for generating signals with HF heating.

In Chapter 6 we address the detection of energetic particle precipitation inducedby HAARP ELF/VLF transmissions using both computer modeling and qualifyingobservations.

Chapter 7 is a summary and includes suggestions for future work.


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Pitch anglescatteredelectrons

Reflectedwaves _.>,

- - ' " '

, ' ' 'y'

Precipitatingelectrons

_- ----,..---

7"-*v~^ .

InjectedwavesV Heatedv \ _ region

Wave-particleInteractionregion

- Energetic. V electrons

Amplifiedand/'ortriggered waves

Reflectedwaves

Figure 1 .6: A schematic of the HAARP whist ler mode wave inject ion experiment.ELF/VLF waves generated in the ionosphere above HAARP are in jected into the Earth-ionosphere waveguide and into the magn etosphere. Waves in the m agnetosphere propag atein duc ts to the wave-particle interaction region at the equator . Amplified waves can beobserved at the conjugate point or, if reflected from the lower ionospheric boundary andcoupled back into the duct, also in the northern hemisphere in the vicinity of the HAARPfacility. Energ etic electrons are scattered in pitch angle and pre cipitat e onto the ion osphere.

1.5 Scient if ic C o n tr i b u t io n sThe spec i f ic con t r ibu t ions o f the p resen t work can be summar ized as fo l lows :

1. We presen t the f ir st observa t ions of ma gne to sphe r ic ampl i f ica t ion an d t r igge r ingof emiss ions by s igna ls genera ted by HF ionospher ic hea t ing the reby p rov id ing anew p la t fo rm fo r con t ro l led wave in jec t ion exp er im ents . R epe a te d ob serva t io ns


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of amplified signals, and associated triggered emissions were made in conjugate hemispheres using custom designed receiver equipment for ocean basedmeasurements in the southern hemisphere.

2. We characterize the occurrence of observations of the mag netospherically amplified signals with respect to transmitter parameters and methods of excitation, aswell as background geomagnetic conditions. The magnetospheric propagationpaths and associated cold plasma densities traversed by the HAARP inducedechoes are determined. We identify magnetospheric propagation conditions asa likely limiting factor in HF heater induced echo observations

3. The observations are analyzed in the context of theoretical and numerical models. Phase and amplitude of the observed signals are used to determine thetem por al behavior of the non-linear resonant cu rrent vector. We use groundbased observations to infer hot plasma distribution dynamics.

4. We present the first observations of a new phenomena of cross modulation between whistler mode signals and HF ionospheric heating and use the conceptas a new method to generate ELF/VLF radiation using an HF heater.

5. We provide an assessment of capabilities for ground based detection of electronprecipitation induced by HAARP generated whistler mode signals. We evaluatethe effects of HF heating on the VLF remote sensing technique. We predict ofprecipitation for non-ducted HAARP ELF/VLF signals using a ray tracing andparticle scattering simulation code.


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C h a p t e r 2E x p e r i m e n t S e t u p a n dO b s e r v a t i o n sMagnetospheric wave injection experiments with the HAARP facility were first performed in late 2000 and continue to be actively pursued at the time of this writing.The global scales of the experiment and specifically the location of the conjugatepoint pose man y logistical and opera tional challenges. Consequently, although thescientific premise of the experiment has not changed significantly, the hardware andoperational approach have evolved over a period of several years. In this chapter weoutline the experiment setup and present the accumulated observations.

2 .1 E x p e r i m e n t S e t u p2 .1 .1 N o r t h e r n H e m i s p h e r eThe HAARP facility is located at 62.4N and 145.2W geographic (63.1N and 92.4Wgeom agnetic). Since February 2007, the H AA RP a rray is capable of radia ting 3.6 MWof HF power at frequencies in the 2.759.8 MHz band. The array can be phased totilt the beam a maximum of 15 from zenith in any direction. For observation of 2-hopechoes and HAARP generated ELF/VLF signals, an array of radio receiver stations

20


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CHAPTER 2. EXPERIMENT SETUP AND OBSERVATIONS 21

has been established across the state of Alaska. The site nearest to the HAARP facility is Chistochina, located 36 km to the n orthe ast. Initial receiver site installationsbegan in 2000, and were followed by many upgrades and subsequent deployments. Akey requirement for successful observations is lack of local electromagnetic interference. In the E LF /V LF b and , the most common source of interference is radiationfrom pow er lines whose harm onics can exten d up to 6 kHz. De spite best efforts todeploy equipment in remote locations, preferably not connected to the commercialpower grid, finding suitab le locatio ns is difficult, especially with th e expa nsion of commercial power in Alaska. The m ap in the to p row of Figure 2.1 shows the locationof the most important Stanford receiver sites in Alaska. The ELF/VLF receivers uselarge square (4.8 m by 4.8 m) or triangular (4.2 m high with 8.4 m base) air coreantennas, with terminal resistive and inductive impedances respectively of 1-Q and1-mH, matched to a low-noise preamplifier with a flat frequency response ~300 Hz to~47 -kH z. Recordings are made with 16 bit 100 kHz digital sampling, synchronizedto GPS timing signals allowing for 200 nanosecond timing accuracy at all stations.Since 2005 all receivers have been of the AW ESOM E ty pe design described by Cohenet al. [2009].

2 .1 .2 So u t h e r n H e m i sp h e r eThe conjugate point of the HAARP array is located in the South Pacific Ocean(56.67S and 174.48E) about 1000 km southeast from New Zealand and ~500 kmfrom the nearest landmass of Cam pbell Island. Consequently, radio observationsat the conjugate point can only be made using ships or floating buoy platforms.Deployment of a buoy based system is made all the more difficult by the fact thatthe ocean depth of 5400 meters is at the upper limit for tested mooring systems andthe area is known to host extreme weather conditions with 1015 meter wave heightsregularly observed. Nevertheless, Stanford U niversity manag ed to deploy a tot al ofthree autonomous buoy receivers in addition to making numerous observations withship borne receivers.

The first Stanford buoy receiver, Buoy 1.0, was deployed in April 2004 at the


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70 Ni

60 N

.1=7

' "* ^ H LHAARP " #H(

*

VLF R eceiving AntennaHAARP HF Dipole Array

165 WC H : ChistochinaHL: HealyHO: Homer

150 WVZ: ValdezYK:Ya k u ta tJU: Juneau

135 WKO: Kodiak

40 S

50S

60S150 E 165 E 180E

B1.0: Buoy 1.0B1.5 Buoy 1.5B2.0 Buoy 2.0T A 04 : Tangaroa Apr-2004TA07: Tangaroa Feb-2007TA08: Tangaroa May-2008

Buoy 1.0, 1.5 Buoy 2.0

M l RVTa ngaroa

F i g u r e 2 . 1 : Maps showing receiver locations in the northern and southern hemisphere andphotographs of selected hardware.

HAARP c o n ju g a t e p o in t . T h e ~ 3 6 0 0 k g ( ~ 8 0 0 0 lb ) Bu o y 1 .0 s t r u c tu r e c o n s i s t e d o fa 2 .5 m ete r cylindrical f iberglass do me ato p an alum inu m hull enclosed in f loatablefoam (see F igu re 2 .1 ) . Th e moor in g sys tem ha d a record 7 km le ng th cons is t ing o fsegm ents o f cha in , meta l wi re , ny lon rope , and po lypro pe lene rope . Th e rece iv inga n t e n n a s a n d E L F /VL F d a t a c o l l e c t i o n we r e s im i l a r t o t h e h a r d wa r e u s e d i n t h eno r th ern hem isphere excep t th a t th e sampl ing f requency was lowered to 12 kHz s ince


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da ta storage and transfer w ere very limited. The air core loop antenn as were embedded into the structure of the fiberglass dome. The unique challenge in the buoyreceiver was that the system had to be fully autonomous, with scheduling and datatransfe r contro lled remotely with no plan ned recovery of th e system . In an effortthat involved considerable custom hardware and software development, a system wasimplemented with communication via IRIDIUM satell i te modem and data s torageon Compact Flash cards. Power was supplied through a massive battery bank supplemen ted with photovo ltaic panels. Due to the extremely low telem etry rat es ofthe IRIDIUM system (2.4 kbps), recordings were made synoptically and data wereprocessed into compressed spectrogram records in JP G form at for transmission. Rawda ta could also be transm itted if necessary. The Buoy 1.0 system oper ated untilJanuary 2005 when an unexpected break of the mooring line required the launch ofa rescue mission to recover the hardware.

Subsequent to the recovery of Buoy 1.0, two new buoy systems were developed anddeployed. Buoy 1.5 utilized the same hull and dome as Buoy 1.0 but had upgradedelectronics with considerable improvements to data storage and communication capabilities. Th e Buoy 2.0 system was housed in a newly acquired dome and hulland included more battery capacity and an expanded photovoltaic array. To achievegreater geographic spread in the conjugate point observations, Buoy 1.5 and Buoy2.0 were deployed as shown in the map in Figure 2.1 in February and March of 2007.Both systems were operational several months after deployment and made valuableobservations of HAARP induced 1-hop echoes. Unfortunately, data from Buoy 2.0was compromised by impulsive noise, tentativ ely believed to origin ate from a crackedbattery terminal or intermittent cable connection.

Additional observations in the conjugate region were made on board RV Tangaroaoperated by New Zealand's National Institute of Water and Atmospheric Research( NI W A) a n d o n Nathaniel B. Palmer o p e r a t e d b y t h e Na t io n a l S c ie n ce F o u n d a t io n(NSF). Tangaroa was the ship used in the deployments of all the buoy receivers andalso the recovery of Buoy 1.0. The ship-borne receivers and anten nas were the sameas those used in Alaska.


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CHAPTER 2 . EXPERIMENT SETUP AND OBSERVATIONS 24

2 .1 .3 Spacecraf t Ob serv a t ionsEL F/V LF waves generated by HAA RP have been observed on the CLU STER satell i teat ~26,000 km alti tude [Platino et al, 2004] and on the DEMETER satellite at ~700km alti tude [Platino et al, 2004]. The low altitude DEMETER satellite is particularlyuseful for observing HAARP induced echoes since it makes daily passes over theheating facility and its conjugate point. A description of the D EM ET ER satelliteand its instruments, which include electric and magnetic field antennas as well as aparticle detector, is given by Parrot [2006]. Recent observations of HAARP-inducedELF/VLF signals on DEMETER are presented by Piddyachiy et al. [2008].

2 .1 .4 T r an sm i s s i o n Fo r m a t s an d O p e r a t i o nOperational logistics associated with the HAARP facility necessitate that wave injection experiments are performed in campaigns, meaning periods of usually two weeksin dur ation during which transmissions are carried out daily for several hou rs. Th eHAARP facility can be used to modulate the electrojet currents with a wide variety of ELF/VLF frequency time-formats including pulses, frequency-time ramps,and chirps. In the initial experim ents, transmission form ats were rigidly scheduledin advance of a campaign and ELF/VLF data recordings were only analyzed afterfull recovery of raw da ta, u sually a few weeks after th e transm ission s. W hile sucha system was perhaps the most straightforward in implementation, it did not allowfor modifications of the transmission format in response to changing ionospheric andmagnetospheric conditions. As is shown in Section 2.2, the magnetospheric responseis sensitive to both frequency and frequency-time signature of the injected waves.Beginning in 2007, in conjunction with the HAARP facility power upgrade, all of thereceiver sites in Alaska were equipped with Internet connectivity and currently send1-minute spectrog ram s in real time to a server at Stanford University. The onlinespectrograms allow for a near-real-time assessment of the ELF/VLF generation conditions and the magnetospheric amplification response. Using an Internet chat-roomto communicate with the HAARP operator, transmissions can then be modified torespond to changing conditions. Such an arrangement has proven to be very effective


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in exc i t ing rnagne to spher ic echoes by tak i ng ful l advan tag e o f favorab le geom agne t icc o n d i t i o n s . S im i l ar n e a r - r e a l - t im e a d ju s tm e n t of t r a n s m i t t e r p a r a m e te r s b a s e d o nobserva t ions o f na tu ra l s igna ls and echoes was a l so used wi th success dur ing the S ip lee x p e r i m e n t s [Helliwell, 1988].

2 .2 Obse rva t i onsTh e cur ren t co l lec t ion of whis t le r mo de echo observa t ion s exc i ted by the H A A R Phea te r i s qu ick ly exp and ing wi th the con t inu ing opera t io n of H A A R P wave in jec t ionex per im en ts . I t is l ikely th at suffic ient s ta t is t ics wil l be com piled in th e futur e tow arra n t the iden t i f ica t ion of typ ica l observa t ions o r s ta nd ar d resu l t s . However , a t thet ime o f th i s wr i t ing , the d ivers i ty and var ie ty o f the observa t ions make i t wor thwhi leto b rie fly touch up on the m ajo r i ty o f no te d cases .

Chistochina 20-Apr-2004

I 4.03.02.01.0

2 4 6 8Tanqaroa 20-Apr-2004

1-HopEcho \4 6 8 10

Time (seconds) after 03:20 UT12

dB -pT -1 0-2 0-30-40-50

F i g u r e 2 . 2 : Spectrograms showing 1-hop echoes observed at the conjugate point on boardTangaroa (bottom panel) triggered by a 500 Hz/sec frequency-time ramp observed at Chistochina (top panel). The 2-hop echo is also visible in the Chistochina record.


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2 .2 .1 F i r s t Ob serv a t ions : 20 Ap r i l 2004The first observations of HAARP induced whistler mode echoes occurred on 20 April2004 du ring tran sm ission s with th e original 960 kW facility. On e-hop echoes wereobserved on board Tangaroa, which was en route to deploy Buoy 1.0, and 2-hopechoes were observed simultaneously at Chistochina. The location of the ship at thetime of the observations can be seen on the map in Figure 2.1. Figure 2.2 showssimultaneous spectrograms from Chistochina and Tangaroa 1. One-hop and 2-hopechoes are observed to be triggered by the 500 Hz/sec frequency-time ram p. Onlythe 500 Hz/sec ramp shows amplification even though steeper ramps (1 kHz/sec) andpulses were tran sm itted in the same frequency ran ge. Th e echoes continued to beobserved for a period of ~30 minutes during which multiple reflected hops, up toten th order (i.e., 10-hop) were observed. Th e 20 April 2004 observations are repo rtedby Inan et al. [2004].

2 .2 .2 Fu ll Po w er H A A R P27 February and 4 March 2007The upgrade of the HAARP facility to its full power and the inception of near-realtime monitoring of the experiment, as described in Section 2.1.4, led to an accompanied increase in the number of echo observations. In the first campaign at full power,which coincided with the deployment of Buoy 1.5 and Buoy 2.0, echoes were observedon two separate days. Figure 2.3 shows 1-hop echoes observed on Tangaroa, whoseposition at the time is denoted in Figure 2.1. The observed echoes are triggered primarily by a unique frequency-time format known as a 'snake' ramp which involves acontinually increasing, but sinusoidally varying frequency-time slope. As is evidentfrom the lower four panels of Figure 2.3, showing Tangaroa data observed at different

x The majority of spectrograms in this thesis are scaled by frequency resolution so that the intensity shown corresponds to an equivalent amplitude of a narrowband (purely sinusoidal) signal. Thecolor axis is therefore appropriately labeled in dB-pT. It should be noted that traditionally, spectrograms have been presented as power spectral densi ty using the corresponding units of power perbandwidth. Since local ly observed HAARP generated ELF/VLF signals have negligible bandwidth,we find the narrowband approach to be more appropriate as i t al lows for reading the generatedsignal ampli tude direct ly from the spectrogram.


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m inute s , the echo am pl i tu de and f requency con ten t show cons iderab le va r ia t io n . No2-hop echoes were observed a t Ch is toch ina , nor a t any o f the s i te s in the nor thernhemisphere , even though 1 -hop echoes were observed fo r approx imate ly 30 minu teso n b o a r d Tangaroa.

Chistochina Chistochina

Seconds after 06:00:00 UT (27-Feb-2007)

F i g u r e 2 . 3 : The top two records are spectrograms from Chistochina, while the bottomfour are from Tangaroa at various min utes. One-hop echoes are observed to be triggeredby a transmitted 'snake' ramp. The echoes exhibit considerable fading in amplitude andfrequency content on the scale of minutes.

A few days la ter , on 4 March 2007, echoes of 3-second pulses a t 1100 Hz wereobserved in bo th hemisp heres on Buoy 1 .5 , Buoy 2 .0 , and a t Ch is toch in a . Obser va t ion of th e 2 -hop echoes abr up t ly com men ced when the t ransm iss ion fo rm at w aschanged f rom one wi th parabo l ic ch i rps and 1-second 1225 Hz pulses , to a formatwith 3-second 1100 Hz pulses , 1-second 900 Hz and 1600 Hz pulses , and a frequency-t im e ra m p f rom 500 Hz to 3 kHz . F igu re 2 .4 shows spe c t ra s im ul taneo us ly observeda t Ch is toch ina ( top pane l ) , on Buoy 1 .5 (midd le pane l ) , and f rom a la te r minu te on


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Buoy 2 .0 (bo t t om pa ne l ) . Th e da ta from Buoy 2 .0 a re com prom ised by impu ls ivein te r fe rence ra d ia ted f rom th e sys tem e lec t ron ics and m ani fes ted as ve r t ica l l ines inthe spec t rum . Th e in te r fe rence was mi t iga te d us ing no ise sub t rac t ion an d echoes andt r iggered emiss ions a re v is ib le in be twee n the rem nan t impuls ive in te r fe rence . Th e

Chistochina 4-Mar-2007

0 10 20 30 40 50Seconds after 06:25:00 UTBuoy 2.0 4-Mar-20 07

0 10 20 30 40 50Seconds after 06:20:00 UTFigure 2 .4: HAARP transmission and faint 2-hop echoes at Chistochina (top panel) andclear 1-hop echoes observed simultaneously on Buoy 1.5 (middle panel). The bottom panelshows echoes observed on Buoy 2.0 five minutes earlier when the transmission format wasthe same. The 2-hop echoes at Chistochina (top panel) appear superimposed on consecutiveHAARP transmissions of the same frequency since the 2-hop propagation time and formatrepetitio n p eriod are com parable, being 8.4 and 10 seconds, respectively.

observation of s ingle frequency echoes on 4 March 2007 al lows for quantif icat ion oftempora l ampl i f ica t ion resu l t ing f rom the magne tospher ic wave-par t ic le in te rac t ion .Figure 2 .5 shows temporal amplif icat ion of a 1-hop echo tr iggered by a 1 .1 kHz pulseobserved on Buoy 1 .5 . In th is examp le the ampl i f ica tion r a te i s observed to be ~ 10


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dB /sec . In other cases, amplification r ates as high as 3050 dB /sec have been observed. In the Siple Station experiments amplification rates were reported to rangefrom 20150 dB/sec [Helliwell, 1988, and references therein]. The observations of 27February 2007 and 4 March 2007 are reported by Golkowski et al. [2008]

HAARP Transmission Format

c< u

c(U

20

l -Q .m2 .< uT3

JaE w"Az 11 rAz0obs = / -i-dz = / 7^-dz (4.16)J-Az az J-Az ^ A v

The integrals in Equations (4.15) and (4.16) constitute average values of the resonantcurrents over the interaction region, i.e.,

7obs = - / i o / - ^ W (4.17)

0obs = - M o / - ^ - W (4.18)The observed phase and amplitude can thus be used to estimate the average magnitude ( ( | J R | ) ) and orientation ((\P)) of the non-linear resonant current vector asillustrated in Figure 4.4.

(\M) oc V702bs + & . (4-19)t an = - ^ . ~ ^ ^ ( 4 . 2 0 )JB


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CHAPTER 4. THEORY AND OBSERVATIONS 76

Chistochina 23-Aug-2007 40 20

38.5

o> 140(VQ 100

36.5

(e) ^=atan(yobs /(|)0bs)^^UA.c

37 37.5 38 38.5Seconds after 23:16:00 UT

38.5

F i g u r e 4.7: Spectrogram of t ransmi t ted 2.25 kHz pulse and triggered 2-hop echo (panel(a)). Corresponding ampli tude in a narrow band around transmission frequency is shownin panel (b). Ampli tude and phase for the 2-hop echo only (demarked by red dotted lines)are shown in panels (c) and (d). Panels (e) and (f) show the derived magnitude and phaseof the non-linear current vector.

g r o u n d o b s e r v a t i o n s to theore t ica l fo rmu la t ions . F igu re 4.7 s h o ws an e x a m p l e m e a s u r e m e n t in which the average non- l inear cu r ren t vec to r has been decom posed f roma 2 -hop echo observa t io n . Pane l (a) in F ig u r e 4.7 shows a s p e c t r o g r a m f r o m Ch i s t o c h in a wh e r e a 2-hop echo is seen to be t r iggered by a 2.25 kHz, 3-second pulse .P a n e l (b) shows the a m p l i t u d e in a 30 Hz b a n d w i d t h a r o u n d the pulse frequency.In pane ls (c) and (d) the p h a s e and a m p l i t u d e of the 2-hop echo have been i so la tedc o r r e s p o n d in g to the red do t ted l ines in the u p p e r p a n e l s . We have l imi ted our a n a l ysis to the in i t ia l g rowth phase of the 2-hop echoes before any t r i g g e r in g of emiss ionswi th rap id ly chang ing f requenc ies . In the c o n te x t of the g e n e r a t i o n m e c h a n i s m th i sp o r t i o n of the i n t e r a c t i o n c o r r e s p o n d s to r a d i a t i n g c u r r e n t s g e n e r a t e d by e l e c t r o n s


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within the particle trap or just outside of the trap but close to the equator as presented in Gibby et at. [2008]. Panels (e) and (f) show the calculated average phaseand magnitude of the non-linear current vector as derived from the 2-hop echo phaseand amplitude using Equations (4.19) and (4.20). It is seen that as the 2-hop echoamplitude increases, the magnitude of the resonant current vector also increases andthe phase angle approaches 180. This result is not surprising since after its growthphase the 2-hop echo is seen to saturate and attenuate, which is manifested by thereso nan t curre nt vector surpassin g 180 in phas e (see Figu re 4.4). After th e 2-hopecho becomes dominated by off frequency components (after 38.3 seconds, second redline) the measurement of its relative phase is no longer possible. The general calculated behavior of the resonant current vector is as predicted by theoretical models,especially the notion of saturation as described by Gibby et al. [2008]. More specifically, the amplitude and phase during initial non-linear growth are both observedto increase but phase advance outpaces amplitude growth. According to Equations(4.10) (4.11) this m ore rapid phas e advance relative to am plitu de grow th im pliesrotation (clockwise in Figure 4.4) of the non-linear current vector and is in agreementwith numerical models [Omura and Summers, 2006; Gibby, 2008].

Effect of Input Amplitude Effect of Linear Growth Rate

).2 Time [s ] Time [s]

F i g u r e 4 . 8 : Pr ed ic t e d echo am pl i tu de s as a func t ion o f t im e fo r d i f feren t inp u t waveamplitude (left panel) and different linear growth rates (right panel). The red dotted lineshows a realistic noise floor that would limit observations and indicates that for a groundbased receiver the effect of lower input amplitude or lower linear growth would both yielddelayed onset of observations. Adapted from Gibby [2008].


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CHAPTER 4 . THEORY AND OBSERVATIONS 78

2.22 kHz (Harmonic) and Echo dB-pT 2.25 kHz (Fundamental) and Echo

o> 10"6.5 7 7.5 8 8.5 9 9.5

Seconds after 23:13:00 UT

1401009.5 36.5

20h10

& .5 37 37.5 38 38.5 39 39.5Seconds after 23:16:00 UT

Figure 4 .9: The panels on the left hand side show the observed amplitude and phase, andderived non-linear current vector parameters for a 2-hop echo excited by a 2.22 kHz signalthat is a harmonic of a 1.11 kHz transmission. The panels on the right hand side show thesame parameters for a 2-hop echo excited by a 2.25 kHz fundamental transmission. Blackand w hite dotte d lines are reference lines showing constan t delay from trans mission . Reddotted lines demark echo onset and coherent growth phase. The harmonic signal is ~14 dBweaker than the fundamental transmission and the 2-hop echo excited by the harmonic isobserved to arrive with a ~ 1 second delay in relation to the 2-hop echo from the fundam entaltransmission.

Th e t rack in g of th e vec to r decomp os i t ion of the non - l inear cu r re n t a l lows fo r amo re de ta i led in t e rp re ta t ion o f observa t ion s in a theore t ica l con tex t . F igu re 4 .8 showsthe resu l t s o f a sens i t iv i ty s tudy per fo rmed us ing the numer ica l mode l o f Gibby [2008].The two pane ls in F igure 4 .8 show the p red ic ted echo ampl i tudes as a func t ion o ft ime for different input wave ampli tudes ( lef t panel) and different linear g r o wth r a t e s(r ight panel) . The red dotted l ine shows a noise f loor that would exis t for any real is t icobserva t ions o f the phenomena , l imi t ing observa t ions to ampl i tudes above the l ine .


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2.25 kHz (Fundamental) and Echo dB-pT 2.25 kHz (Fundamental) and Echo

"6.5 7 7.5 8 8.5Time (seconds) after 23:16:00 UT

37 37.5 38

38.5

318 014 010 0

5.5 37 37.5 38 3i

| ,' vj^rS^

38.5

6.5 37 37.5 38 38.5Time (seconds) af ter 23:18:00 UT

F i g u r e 4 . 1 0 : Same as Figure 4.9 except th at now both echoes are triggered by signals of thesame am plitude and th e difference in onset delay is attribu ted to lower linear amplification.

The level of the noise f loor above the non-l inear growth threshold is appropriate forth e HA A R P exper im ent s ince a l l recorded observa t ions of echoes exh ib i t t em po ra lg rowth , mean ing tha t un l ike in the S ip le S ta t ion exper iment [Paschal and Helliwell,1984] th e noise f loor is above am pli t ude s tha t result solely from l inear grow th. Th enum erica l results indi cate th at for a gro un d based receiver th e effect of th e lowerinpu t ampl i tude o r lower l inear g rowth ra te would be the same, namely a de layedecho onset t im e. A s discussed in Section 4 .1 .4 , the dyn am ics of th e amplif icat ion arede te r min ed by th e p resence o f wave am pl i tu de g rad ien ts wi th respec t to the t ra pp ingam pl i tu de th resho ld . Pa r t o f the reason fo r the s imi la r i ty be tw een var ia t ions o f theinpu t am pl i tu de and l inear g row th ra te is th a t b o t h lead to lower wave a mp l i tud esin th e in te rac t io n reg ion and cause the p r ima ry t rap p in g of e lec t rons to occur fu r the r


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up the f ie ld l ine, leading to a delayed observation onset (see Figure 4 .5) .Th e de layed onse t of echo observa t ions resu l t ing f rom d i f ferent inp u t am pl i tu des

can be seen in th e HA A R P observa t ions . F igure 4 .9 show the sam e ana lys is fo rtwo cases of s imilar frequency and form at, bu t s ignif icantly different in pu t am pli tud eobserved on ly 3 min u tes a par t . Th e le ft ha nd pane ls in F igure 4 .9 show the an a lys isof an echo tr iggered at 23:16 UT by a pulse a t 2 .22 kHz that is a harmonic of afunda me nta l t rans mis s ion a t 1.11 kHz . Th e harm onic i s mu ch weaker (abou t 14dB ) th an the fundam enta l a s i s norm al ly the case . Th e pane ls on th e r igh t h an dside of Figure 4 .9 show analysis of an echo excited by a fundamental tone at 2 .25kHz a t 23 :16 UT a f te r a t ransmiss ion fo rmat change a t 23 :14 UT. The whi te dashedl ine in the spec t rograms in the top row cor responds to the b lack do t ted l ines in thelower panels and al l of these l ines are reference l ines representing constant delay fromtransmiss ion ac ross bo th cases . The two red dashed l ines respec t ive ly mark the onse tof th e observed 2 -hop echoes and th e end o f the coh eren t g ro wth phase o f those echoes .The echo in the low ampl i tude (harmonic ) case i s seen to a r r ive much la te r than theh igher ampl i tud e ( fundam enta l ) t ra nsm iss ion . Th e s ign if icant ly c lose r p rox im i ty o fthe white and black l ines to the f irs t red l ine in the second case shows that the higherinput ampli tude leads to a delayed echo onset of about a ful l second in th is case. I t isim po r ta n t to no te tha t ap ar t f rom th is onse t de lay , the dyna mic s of the ampl i f ica t ionfor the two cases is s tr ik ingly s imilar , as can be seen in the s imilar i ty of the averagem agn i tud e and pha se of the non- l inear cu r re n t vec to r in th e bo t to m four pane ls o fF igure 4 .9 . Bo th cases show an inc rease in ampl i tude and advance o f phase to 180 ,a t a b o u t t h e s a m e r a t e s .

Th e cor responden ce of onse t de lay to inpu t am pl i tu de var ia t ion i l lus t ra te d in F ig u re 4 .9 i s fu r the r ev idence o f the degree to which the observa t ions agree wi th thenumer ica l resu l t s o f Gibby et al. [2008] and Gibby [2008]. Moreover, having exposedth is c lose connec t ion i t i s a l so poss ib le to use the observa t ions to compara t ive ly in ves t iga te the ho t p lasm a d is t r ib u t ion in the magn e tosp here . F igur e 4 .10 shows apresen ta t ion iden t ica l to F igure 4 .9 excep t tha t now the 2 -hop echoes a re t r iggeredby pu lses th a t a re bo th fundam enta l to nes and consequ en t ly have the same am pl i tude . The d ispar i ty in spac ing be tween the re fe rence l ines and red l ines o f echo onse t


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across the two cases in Figure 4.10 shows that for the right hand case the echo onset isdelayed. Given that the two input amplitudes are the same, based on the predictionsof the Gibby [2008] model, the difference in arrival onset can be inferred to resultfrom a lower linear amplification. A lower linear amplification resul ts from e ithera lower electron flux or a lower pitch angle anisotropy of the electron distribution.The observed amplitude and phase of ground based observations can thus be used totrack the evolution of the hot plasma distribution in the mag netosphere. Althou ghobservations of phase and amplitude of whistler mode echoes have been reported before [Dowden et al, 1978; Paschal and Helliwell, 1984], the analysis presented hereinconstitutes the first interpretation of the observed joint variation of amplitude andphase in the context of a rigorous numerical model.


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C h a p t e r 5C r o s s M o d u l a t i o n b e t w e e nW h i s t l e r M o d e E c h o e s a n d H FH e a t i n gIn th is chap te r we p resen t a new phenomenon se rend ip i tous ly d iscovered in the courseof H A A R P wave in jec t ion exper im ents . Th e observa t ions a re in te r p re te d in a theore t ica l con tex t and the observed concep t i s inves t iga ted exper imenta l ly as a new methodf o r E L F /VL F wa v e g e n e r a t i o n w i th t h e HAARP h e a t i n g f a c i l i t y .

5 .1 Obse rva t i onTh e f ir s t and mos t rep res en ta t ive obse rva t ion of c ross m odu la t io n be twe en w his t le rmo de echoes and H F hea t in g occur red on 23 Aug us t 2007 . F igure 5 .1 shows a spect r o g r a m r e c o r d f ro m C h i s to c h in a in wh ic h H A A RP t r a n s m is s io n s a n d c o r r e s p o n d in gt r iggered 2 -hop echoes a re ma rked by the o range a r rows . In add i t ion to the 2 -hopechoes excited by th e ram ps an d pulses , ' replic as ' of th e 2-hop echoes shif ted u p infrequency are vis ible in th e recor d as show n by th e black arrow s. T he frequency shif to f the echo rep l icas co r respo nds exac t ly to the EL F f requency o f th e s imu l taneou sH A A R P t ransm iss ion . Th e rep l icas thu s app ear to be a d i rec t resu l t of non- l inearc ross modula t ion be tween the incoming 2 -hop echoes and the concur ren t modula t ion

82


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CHAPTER 5. CROSS MOD ULATION 83

of th e H F hea te r . One o f the f i rs t imp l ica t ions o f the observ a t ions o f such c ross mo dula t ion i s th a t t he 2 -hop echoes on the i r re tu rn to the nor t her n hem isphe re mus t haveb e e n p r o p a g a t in g t h r o u g h th e i o n o s p h e r e i n t h e h e a t e d r e g io n a b o v e t h e HAARPfaci li ty . Th is is in d i rec t ag reem ent wi th the whis t le r mo de pa t h de te r mi na t ion re su l t s fo r th i s day and o thers d iscussed in Ch ap t e r 3 and su pp or ts th e accuracy o f thed i s p e r s io n a n a ly s i s a n d p a th d e t e r m in a t i o n t e c h n iq u e s t h e r e in .

8 10 12 14 16 18 20 22 24 26Seconds after 23:05:00 UTTmyjced^Hnpa-rK ie'} r Cros* Mcdjtation Pradwte

F i g u r e 5 . 1 : Echo heating cross modu lat ion observed at Chistochina. HA AR P transm ittedramps and pulses excite 2-hop echoes. Modulation products of 2-hop echoes with simultaneous HAARP transmissions are also seen in the record.

5.2 T he o re t i c a l M ode l i ngTh e c ross mo dul a t io n e ffec t is exh ib i ted by echoes p ro pag a t in g th r ou gh the H F h ea t ing reg ion . Th e HF mod ula ted hea t ing modifies the cond uc t iv i ty o f the ionosphe reand the reby changes the p ropaga t ion charac te r i s t ic s fo r e lec t romagne t ic waves p ropaga t ing in th is med ium. To s tudy the e f fec ts o f HF waves on the ionosphere we usea numer ica l hea t ing mode l which takes in to accoun t the p ropaga t ion o f and energydepos i t ion b y the H F waves as a func t ion o f t im e and a l t i tud e . Here we re ly on a


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CHAPTER 5. CROSS MOD ULATION 84

numerical heating model based on the work of Moore [2007]. Figure 5.2 shows theeffect of a single 1.110 kHz AM modulation cycle on typical daytime and nighttimeionospheres and the corresponding induced propagation characteristics for a 2 kHzwave. The primary effect of the HF heating is to change the electron temperatureas shown in panel (b) of Figure 5.2. The electron temperature change manifests itself almost exclusively as a change in wave attenuation (panel (d)), leaving the realpart of the refraction index (wavenumber in panel (c)) virtually unchanged. It is thismodulating attenuation in sync with the ELF/VLF AM modulation of the HF beamth at generates the cross modu lation products shown in Figure 5.1. This p henomenonis analogous to the Luxembourg effect [Bailey and Martyn, 1934] in that one signalmodifies the ionosphere and so imposes its modulation on another signal that propagates through or in the vicinity of the same medium. Manifestation of this effect inthe VLF band was reported by Inan [1990]. It is clear from panel (d) in Figure 5.2that the change in attenuation is more dramatic for a daytime ionospheric profile inagreement with the observations on 23 August 2007 which took place during 12:00MLT.

A quantitative comparison of the observed cross modulation and that predictedby the numerical heating model can be achieved by examining the frequency spectra.Panel (a) in Figure 5.3 shows a spectrogram of an observed 2-hop echo and its crossmodulation products from a simultaneous 1110 Hz HAARP transmission. Panel (c)shows the frequency spectrum at the time corresponding to the white dot

Magnetospheric Wave Injection by Modulated Hf Heating of the Auroral Electrojet

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