Multi-Scale Coupling and Auroral Dynamics: Multi-Scale Coupling and Auroral Dynamics: Reporter ReviewReporter Review

Bob Lysak, University of Minnesota, USABob Lysak, University of Minnesota, USA

Purpose of the review: to examine the consequences of small-scale auroral processes on large-scale dynamics, and the coupling of different plasma regions with each other.

Significant advances in the last 2 years, due to new and continuing observations from FAST, Akebono, IMAGE, Polar, and Cluster as well as ground observations, theory, and numerical modeling.

Over 200 papers published in this area since the 2001 IAGA meeting.

Outline of the TalkOutline of the Talk

Optical aurora and relation to plasma parameters

Electrostatic acceleration and time-independent coupling

Time-dependent coupling by Alfvén waves

Alfvénic particle acceleration

Ion acceleration and outflow

Nonlinear plasma waves: Electron and ion phase space holes

Coupling between aurora and outer magnetospheric processes

Optical Auroral Features and Plasma Optical Auroral Features and Plasma ProcessesProcesses

Conjugacy of auroras: First simultaneous images of aurora with one camera, showing 40 min

MLT eastward displacement (Frank and Sigwarth, 2003). Conjugate observations of spirals, consistent with Hallinan theory,

rotation opposite to KH instability (Partamies et al., 2001). Latitude displacements of up to 5° due to IMF (Vorobjev et al., 2001)

Comparison with particle data FAST data compared with images from UVI (Peria et al., 2001) and

IMAGE (Mende et al., 2002, 2003) show poleward arcs associated with wave-accelerated electrons, equatorward with inverted V’s

Coordinated images for dynamic auroras, such as poleward moving auroral forms (Kozlovsky and Kangas, 2002; Drury et al., 2003) and black aurora (Peticolas et al., 2002)

Multi-spectral imagingUse of multiple wavelengths to determine precipitation

characteristics (Semeter et al., 2001; Janhunen, 2001; Galand et al., 2002)

Northern and Southern AuroraNorthern and Southern Aurora

First simultaneous images of both auroral ovalsDisplacement of auroral features by MLT of 40 minutes, with southern aurora displaced to the east.Frank and Sigwarth, JGR, 2003.

Crossing of “Double Oval” with FAST Crossing of “Double Oval” with FAST and IMAGE (Mende et al. 2003)and IMAGE (Mende et al. 2003)

IMAGE

11:20

11:26

IMAGE images of double oval compared with FAST (arrows give time of images)

LBH Lyman α 135.6 (OI)

Plasma density in auroral acceleration region larger in summer than in winter (Johnson et al., 2001, 2003)

Polar cap densities also show strong seasonal asymmetry (Laakso et al., 2002)

Auroral activity dependent on solar wind dynamic pressure more for southward IMF and winter (Shue et al., 2002)

N Hemis,

4-6 RE

S Hemis,

2 RE

Seasonal effects on auroraSeasonal effects on auroraWinter Dark

Summer Light

Three Regions of Auroral AccelerationThree Regions of Auroral Acceleration

Illustration of three regions of auroral acceleration: downward current regions, upward current regions, and the region near the polar cap boundary of Alfvénic acceleration (Courtesy C. Carlson, from Auroral Plasma Physics, International Space Science Institute, 2003)

Electrostatic Coupling and Acceleration Electrostatic Coupling and Acceleration of Auroral Electrons (Observations)of Auroral Electrons (Observations)

Direct measurements of parallel electric fieldsUpward current region:

FAST shows large E|| (~ 100 mV/m) at lower edge of auroral density cavity (Ergun et al., 2002)

parallel electric fields of 25-300 mV/m seen on Polar; scale sizes 10’s of km based on potential of energized particles (Hull et al., 2003)

Downward current region

Strong E|| (up to 1 V/m), consistent with oblique double layer model (Ergun et al., 2001; Andersson et al., 2002)

In both current regions, observation of E|| has been supported by in situ particle acceleration as well as double probe measurement.

Large Upward Parallel Electric Fields Large Upward Parallel Electric Fields from FAST and Polarfrom FAST and Polar

← FAST data showing large E|| at boundaries of auroral cavity (Ergun et al., 2002)

→ Polar data showing E||

associated with electron and ion acceleration (Hull et al., 2003)

Downward Parallel Electric Fields From Downward Parallel Electric Fields From FASTFAST

Observation of large downward field in region of strong turbulence (Ergun et al., 2001)

Downward E|| and particle data, together with inferred trajectory of spacecraft with respect to potential (Andersson et al., 2002)

Auroral Current-Voltage RelationAuroral Current-Voltage Relation

Old subject (e.g., Knight, 1973), but new interest, especially due to observations in downward current region

Borovsky and Bonnell (2001) consider linear Knight relation (j = KΦ) including dipole mapping with different constants in up and down current regions

Rönnmark (2002) considered case when source population varies so that quasi-neutrality is satisfied (with fixed ion density profile), found j ~ Φ1/2.

Boström (2003) considered general description of current carried by various types of distributions, found Knight-type relations, but variations in details.

Ergun et al. (2002) consider more local situation and found double layer solution with realistic populations.

A gallery of current-A gallery of current-voltage relations voltage relations (Bostr(Boström, 2003)öm, 2003)

Response of plasma to parallel potential drops depends on form of particle distribution at top of field lines. Figure gives a variety of distributions, with voltage as a function of current at right.

AlfvAlfvénic Coupling: énic Coupling: ObservationsObservations

New Cluster observations show time-variations in auroral electric fields and currents between passage of 4 spacecraft (Marklund et al., 2001)

Alfvén wave Poynting flux correlated with aurora and with electron energy flux (Keiling et al., 2002, 2003)

Observations consistent with field line resonance found (Milan et al., 2001)

Laboratory experiments confirm kinetic Alfvén wave dispersion relation (Kletzing et al., 2003)

E

J||

Vsc

Effects of parallel resistivity on small-scale structures (~ 1 km) emphasized: enhanced damping of ionospheric resonator (Lessard and Knutsen, 2001)

Energy budget of field line resonance with anomalous resistivity included shows maximum dissipation at intermediate scales (~ 10 km) (Streltsov and Lotko, 2003)

Role of E|| in reflection of Alfvén waves trapped in ionospheric Alfvén resonator emphasized (Pilipenko et al., 2002)

Model coupling the ionosphere to outer magnetosphere developed showing role of auroral acceleration in modifying tail pressure (Blixt and Vogt, 2002)

Effects of ionospheric Joule heating on production of tall auroral rays modeled (Otto et al., 2003)

AlfvAlfvénic Coupling: Theory and Modelingénic Coupling: Theory and Modeling

Great deal of new work on ionospheric feedback: self-consistent conductivity variations due to Alfvén wave currentsCompressible effects included in theory: effect not critical to

instability (O. Pokhotelov et al., 2001)

Energetics of the instability considered: supported by reduction of ionospheric Joule heating (Lysak and Song, 2002)

New modeling of field line resonances including ionospheric conductivity variations (D. Pokhotelov et al., 2002a,b; Prakash et al., 2003)

Alfvénic Coupling: Feedback InstabilityAlfvénic Coupling: Feedback Instability

AlfvAlfvén Feedback Modelingén Feedback Modeling

Seasonal asymmetry between winter (top) and summer (bottom) ionospheres: feedback instability stronger in winter (Pokhotelov et al., 2002)

Simultaneous excitation of feedback in ionospheric resonator (upper band near 0.1 Hz) and for field line resonance (lower band at 0.01 Hz) (Pokhotelov et al., 2003)

AlfvAlfvénic Acceleration of Electronsénic Acceleration of Electrons

In addition to “classical” quasi-static electron acceleration, recent observations and theories have emphasized time-dependent acceleration, giving broad energy distribution and narrowly field-aligned pitch anglesAkebono shows field-aligned electrons on edge of auroral

acceleration region (Miyake et al., 2001)

Freja observations show field-aligned distributions with energy dispersion: particles arrive before wave, accelerated above spacecraft (Andersson et al., 2002)

Polar observations see field-aligned acceleration of electrons at ~ 4 RE at plasma sheet boundary layer (Wygant et al., 2002)

FAST observations and modeling of FAST observations and modeling of AlfvAlfvéénic accelerationnic acceleration

Chaston et al. (2002ab, 2003abc) has looked at field-aligned electrons and done test particle models to compare with data.

FAST data showing field-aligned bursts (Chaston et al., 2002a)

Simulated (left column) and observed (right) energy-pitch angle distributions (Chaston et al., 2002b)

Kinetic Theory of AlfvKinetic Theory of Alfvén Wavesén Waves

Tikhonchuk and Rankin (2002) developed kinetic theory of field line resonances, found enhancement of E|| due to bouncing electrons

Wright et al. (2002) and Wright and Hood (2003) emphasized nonlinear convective term in two-fluid and kinetic models.

Lysak and Song (2003a,b) considered nonlocal kinetic theory in ionospheric resonator, also found enhancement of E|| and strong absorption near IAR resonances.

Génot et al. (2001) performed electromagnetic particle simulations with Alfvén wave fields; found formation of localized electric fields in density cavities

New work has emphasized kinetic effects on Alfvén waves:

Ion OutflowsIon Outflows

Recent work has emphasized correlations with external parameters and auroral arcsAkebono shows good correlations with solar F10.7, solar wind dynamic pressure, solar wind E, and magnetic fluctuations (Cully et al., 2003)

Direct response of ions to solar wind pressure pulse seen by IMAGE (Fuselier et al., 2002)

However, short-scale (5 minute) AE fluctuations do not correlate with ion outflow (Peterson et al., 2002)

AEOut

flow

(io

ns/m

2 s)

More ion outflowMore ion outflow

Ion outflow found to correlate well with auroral forms (Wilson et al., 2001)

Outflows follow convective motions of arcs (Kistler et al., 2002) and concentrate on arc boundaries (Stevenson et al., 2001)

Solid: O+ ouflow; dashed: LBH luminosityModeling:

Dynamic fluid/kinetic hybrid model of ion transport shows transverse heating, “pressure cooker” effect of downward E|| (Wu et al., 2002)

3-d electrostatic PIC simulations show electron beam excitation of low frequency (LH) waves, producing ion heating (Singh et al., 2001)

Global modeling shows heavy ion outflow mass-loads the outer magnetosphere, lowering polar cap potential (Winglee et al., 2002)

Nonlinear Plasma Waves: Electron HolesNonlinear Plasma Waves: Electron Holes

Electron holes form as a result of beam-plasma interaction (Newman et al., 2002)

Polar observations show width increases with amplitude (Cattell et al., 2001)

Consistent with allowed regions in BGK models (Chen and Parks, 2002)

3-d hole structures stable (Jovanovic et al., 2002; Roth et al., 2002) especially when strongly magnetized (Singh et al., 2001) AmplitudePe

rp w

idth

Electron Holes as Source of VLF saucersElectron Holes as Source of VLF saucers

VLF saucers well-known for many years (James, 1976); new FAST data shows association with electron holes (Ergun et al., 2001)

Theoretical model shows electrons on trapped orbits in hole can radiate VLF (Berthomier et al., 2002)

Simulations show that whistler mode wave packets can be radiated from holes (Singh, 2002)

Nonlinear Plasma Waves: Ion HolesNonlinear Plasma Waves: Ion Holes

Ion holes observed by Polar between H+ and O+ velocities, with larger amplitudes for holes near H+ speed (Dombeck et al., 2001), supporting ion-ion two-stream mechanism.

Ion holes observed by FAST show acceleration of electrons by hole potential (McFadden et al., 2003)

Outer magnetospheric coupling: shocks Outer magnetospheric coupling: shocks and pressure pulses and pressure pulses

Interplanetary shock produces anti-sunward propagating auroras (Zhou et al., 2003)

Solar wind pressure pulses increase auroral oval width and decreases polar cap area (Boudouridis et al., 2003)

WIND observations IMAGE UV images

Southward turning of IMF leads to equatorward expansion of auroral oval, propagating anti-sunward at convection speed (McWilliams et al., 2001)

Outer magnetospheric coupling: Outer magnetospheric coupling: expansion phase onsetexpansion phase onset

Onset arc forms a few minutes before onset, rather than being long-lasting growth phase arc (Lyons et al., 2002)

Pseudo-breakup associated with localized current system in post-midnight oval (Partamies et al., 2003).

Two types of auroral development during onset: arc “jumping” to higher latitudes (associated with reconnection); general expansion of bright patch in all directions (development of instability) (Kornilova et al., 2003)

Outer magnetospheric coupling: bursty Outer magnetospheric coupling: bursty bulk flows (BBF) and polar boundary bulk flows (BBF) and polar boundary

intensifications (PBI)intensifications (PBI)Comparison of MSP and All-sky cameras show north-south extent of aurora associated with PBI (Zesta et al., 2002)

PBI/BBF associated with large-scale ULF wave power in the tail (Lyons et al., 2002). Association with Pi2 packets also seen (Sutcliffe and Lyons, 2002)

BBF is diverted azimuthally in inner magnetosphere, as seen from equatorward extension of PBI (Kauristie et al., 2003)

Series of PBI’s moving equatorward in late expansion phase indicate series of BBF’s (Sandholt et al., 2002)

SummarySummary

Auroral plasma physics remains a quickly developing field combining aspects of microscropic plasma physics, MHD waves and energy flows, ionospheric structure and chemistry, optical observations, etc.

Importance of parallel electric fields has been well established; evolution and structure of the auroral acceleration region remain important questions.

Auroral features are a diagnostic of both the local plasma conditions and the outer magnetospheric conditions along the field lines.

Combination of in situ plasma measurements and optical features in the aurora are providing a wealth of new information.

Some outstanding questionsSome outstanding questionsDevelopment and evolution of parallel potential drops and their relation to external conditions is still not well understood: our suspicion is that Alfvenic processes pre-condition auroral field lines to form quasi-static potentials (see, e.g., Song and Lysak, 2001).

Relation of ion outflows to driving forces (e.g., Poynting flux into auroral zone) is still not clear; role of these ions in magnetospheric dynamics is still speculative.

Formation of narrow (< 1 km) auroral forms remains a mystery; probably due to interactions between the acceleration region and the ionosphere.

Role of ionospheric processes (ionization, recombination, heating, chemistry) on auroral formation is just beginning to be investigated

For further information…For further information…

Special section of JGR-Space Physics, “Causes of the

Aurora” published in April, 2003.

New monograph Auroral Plasma Physics by team of

> 30 authors published by International Space Science

Institute (Kluwer, 2003).

Set of 219 articles published in 2001-2003 period

were compiled for this review: list will be available at

http://ham.space.umn.edu/spacephys/~bob after this

meeting (as will this presentation).