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1 Introduction

The behavior of the plasma that surroundsthe Earth from an altitude of several tens ofkilometers in the ionosphere to the magnetos-phere is a major topic of interest for scientistsstudying Solar Terrestrial plasma physics.Particularly important is the response of plas-ma in the Earth's magnetosphere to solar windplasma and resulting phenomena such as mag-netic field line reconnection, generation of thefield-aligned current (FAC) system, accelera-tion of charged particles above the polarregions, and accompanying auroral illumina-tion. Over the past 30 years, plasma observa-tions seeking to clarify the mechanisms ofthese phenomena have primarily been madeby direct, or in-situ, measurement of plasmaby satellites deployed in the region wherethese phenomena occur. However, since theseobservations have only been able to provide

instantaneous data for single observationpoints, they have been inadequate for separat-ing the components of temporal variationsfrom spatial variations of magnetosphericresponse to the solar wind changes.

Alongside these in-situ observations, vari-ous efforts have sought to understand theglobal plasma distribution near Earth space byvisualization. One major success in theseefforts revealed the good correlation betweenthe temporal evolutions of auroral illuminationand magnetospheric activity. This discoverywas based on observation by a polar-orbitingsatellite passing above the polar region[1][2].Such remote-sensing observations have beenmade at visible to ultraviolet wavelengths, butnot in the extreme ultraviolet (EUV) region.The resonance-scattering emission lines vitalto plasma observation―the 30.4-nm (HeⅡ)and 58.4-nm (HeⅠ) Lymanαlines of heliumions and atoms and the 83.4-nm (OⅡ) line of

YAMAZAKI Atsushi et al.

3-5 Development of Instrument for the Reso-nance Scattering Emission of Oxygen Ion(OⅡ:83.4nm) ― Toward the Imagery ofMagnetosphere ―

YAMAZAKI Atsushi, MIYAKE Wataru, YOSHIKAWA Ichiro, NAKAMURA Masato, and TAKIZAWA Yoshiyuki

According to previous theories a large number of oxygen ions are not able to escapefrom the ionosphere to the magnetosphere due to its heavy mass and loss process in theupper atmosphere. Recent satellite observations, however, reveal that oxygen ions of theionospheric origin exist in the magnetosphere, and that the outflow flux from the polar iono-sphere is comparable to that of hydrogen ions, which have its light mass, during the highsolar activity and the high geomagnetic activity. The distribution of oxygen ions providesthe interpretation of the plasma transfer during the high activity, and gives us the effectiveinformation for monitoring the space weather. The remote-sensing method is useful for themeasurement of the distribution of oxygen ions all over the magnetosphere. We advancedevelopment of new optics for the resonance scattering emission of oxygen ions.

Keywords Oxygen ion, Resonance scattering, Extreme ultraviolet, Remote sensing, Imaging

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oxygen ions―are found in the EUV region.These emission lines correspond to energygaps with the highest transition probability forparticles in the ground state. The concept ofvisualizing the tenuous plasma environmentwas originally conceived with the detection ofthese scattered emissions[3]. The technicaldifficulties of developing optical equipmenthave restricted such efforts involving EUVobservations at the initial stages. However,developments in nanotechnology over the past10 years have enabled the production of amultilayer-coated mirror that can efficientlycollect EUV light, leading to discussions of thepossibility of or need for studying the behaviorof plasma by imaging the global plasma envi-ronment near Earth in EUV[4][5][6].

In resonance-scattering emission observa-tions, care must be taken to account forchanges in an emission rate factor due toDoppler shifts. The major light source for res-onance-scattering emission lines is solar radia-tion. Doppler shifts must be taken into con-sideration in a system that moves with plasmarelative to the Sun. If the speed of the plasma,the scattering material, is such that the result-ing Doppler shift exceeds the solar spectralwidth, the plasma cannot induce resonancescattering. This is apparent in the range ofsolar radiation spectrum that consists of isolat-ed emission lines, like that for HeⅡ. Howev-er, the spectrum in the OⅡ region consists ofseveral lines that appear as a single line, dueto degeneracy of multiple resonance states.Also present is a continuous spectrum ofhydrogen atom. Even when a Doppler shiftoccurs, a different emission line or a continu-ous band will overlap with the wavelength forresonance scattering, maintaining the reso-nance-scattering emission rate factor at a cer-tain level. Thus, unlike helium ions, oxygenions can be used as tracers for imaging plasmaoutflow from the polar ionosphere and theplasma distribution and transport in the mag-netotail[5][7].

Resonance scattering of helium ions (HeⅡ) is a strong candidate for imaging the plas-masphere that surrounds the Earth, since heli-

um ions comprise nearly a tenth of all ions inthe plasmasphere, and since their low kineticenergy provides immunity from negativeDoppler shift effects. The XUV (eXtremeUltraviolet Scanner) onboard the Planet-Bsatellite[8][9][10] and the EUV (Extreme Ultra-violet Imager) on the IMAGE (Imager forMagnetosphere and Aurora) satellite[11][12][13]have made successful imaging observationsbased on HeⅡ, providing information on real-time 2-D behavior in the plasmasphere.

On the other hand, the resonance scatter-ing of oxygen ions (OⅡ: 83.4 nm) is bettersuited for visualizing plasma with high kineticenergy, such as plasma escape from the iono-sphere or present within the magnetosphere.More significantly, contrary to theoretical pre-dictions, oxygen ions play a more importantrole in the magnetosphere than hydrogen orhelium ions. The escape of plasma from thepolar ionosphere is known as the polar wind.Theoretical studies during the 1960s had pre-dicted that plasma consisting of low-mass par-ticles such as protons and helium ions wouldprove to be the main constituent of the polarwind, and that the amount of escape flux ofheavier oxygen ions was only several percentof the proton and helium ion flux[14][15].However, observations by polar-orbiting satel-lites (Akebono, Dynamic Explorer-1 and 2)show large amounts of oxygen ions escapingfrom the polar ionosphere[16][17][18]. Thisphenomenon can be explained only if acceler-ation and heating of oxygen ions occur at alti-tudes below 1,000 km[19]. The precise mech-anism has yet to be clarified. The Geotailsatellite detected the presence of largeamounts of cold oxygen ions in the magneto-tail, especially in the lobe/mantle region,which were observed to flow towards the mag-netotail with the beam-like distribution[20][21][22]. Based on comparisons withαparticles ofsolar wind origin penetrating into the magne-tosphere, these oxygen ions have been identi-fied as originating from the Earth's iono-sphere. However, an as-yet unidentifiedmechanism of acceleration and heating mustaccount for their existence[23][24]. Further-

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more, the comparison of the density of theoxygen ions in the magnetosphere with thebalance between the escape to interplanetaryspace and the supply from the ionospherebased on in-situ measurement data haveshown that the number of oxygen ions enter-ing the magnetosphere is an order of a magni-tude greater than that exiting it, indicating anunidentified reservoir or transport route foroxygen ions[25]. The discovery of cold heli-um ions within the plasma sheet by the Planet-B satellite[26] also supports the hypothesis ofan unknown reservoir of plasma. These stud-ies indicate that cold plasma, the presence ofwhich were not previously confirmed by in-situ observation due to the satellite's potential,exists within the magnetosphere, and implythat the behavior of oxygen ions within themagnetosphere may serve as an effective trac-er for resolving the mechanism of plasmaacceleration and heating.

It must be noted that since imaging obser-vations can only provide information on col-umn density in the line-of-sight direction, theycomplement in situ measurements in the mag-netosphere (Fig.1). If the position and timingof satellites in the magnetosphere making in-situ observations of the magnetotail dynamicscan be determined by imaging, data obtainedby direct plasma observations could be inter-preted more effectively. Conversely, provid-ing information on plasma density and veloci-ty at each observation point of the satellites inthe magnetosphere as the boundary conditionsfor imaging will make it possible to create 3-D

images from the 2-D images obtained.

2 Resonance Scattering of Oxy-gen Ions

2.1 Oxygen Ion Emission Lines in SolarRadiation

The Sun is the major light source inducingresonance scattering of oxygen ions in spacenear the Earth. The solar radiation spectrumexhibits emission lines for monovalent anddivalent oxygen ions (OⅡ and OⅢ) near the83.4-nm wavelength. Since the energy levelsof each ion are degraded, multiple emissionlines exist in close proximity. A model of thedetailed spectrum of this bandwidth has beenproposed based on the results of rocket obser-vations in the 1960s[7][27]. The continuousLyman spectrum component of hydrogen isalso known to exist in this wavelength region[28]. Fig.2 shows an example of a spectrumduring a solar minimum―a sum of the emis-sion line spectrum model and values observedfor the continuous spectrum. Seven emissionlines are visible between 83.26 nm and 83.55nm. (There are in fact 9 emission lines, but 2are within the emission line width and cannotbe resolved.)

Of the 7 emission lines, the 3 lines at83.2762, 83.3332, and 83.4462nm correspondto those of OⅡ, and the others to OⅢ. If amonovalent oxygen ion were motionless rela-tive to the Sun, resonance scattering would

YAMAZAKI Atsushi et al.

Relationship between imaging obser-vation and in-situ particle observation

Fig.1

The solar radiation spectrum near 83.4nm

Fig.2

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only occur with the above 3 emission lines.However, if the ion is in motion relative to theSun, the above emission lines would be affect-ed by Doppler shift in a system moving withthe oxygen ion. As the original wavelength isλ0, the wavelengthλafter the shift can beexpressed as:

Here, is the velocity of the oxygen ionrelative to the Sun with a positive valuetowards the Sun, and is the speed of light.If, for example, the oxygen ions were movingaway from the Sun at 300 km/s, the OⅢ emis-sion line near 83.37 nm would shift to a wave-length 0.0834 nm longer in the system, over-lapping with the resonance-scattering emis-sion line at 83.4462 nm. In other words, theemission rate factor of resonance scattering, orthe g-factor, depends on the velocity relativeto the Sun, the radiation source. Additionally,since the oxygen ions in the magnetosphereand the ionosphere have a finite temperature,the effect of Doppler broadening will be pres-ent. Even if the central wavelength does notmatch precisely, scattering will occur in pho-tons with wavelengths within the Dopplerwidth, and emission rate factor will also dis-play temperature dependency. Thus, calcula-tions of the g-factor must take into account theeffects of both the Doppler shift and Dopplerwidth. The g-factor is represented by the fol-lowing equation:

Here, i represents the number of OⅡ emis-sion lines, vth the thermal velocity, and thesolar radiation photon flux per unit wave-length. Representing the scattering cross sec-tion at the central wavelength for each emis-sion line, can be expressed as:

where , , and , respectively, representthe mass of the electron, the elementary elec-tric charge, and the harmonic oscillatorstrength of the i th emission line.

Fig.3 shows the g-factor as a function ofvelocity relative to the Sun at oxygen ion tem-peratures of 1, 100, and 1000 eV. The velocityvector is positive towards the sun (i.e., theheading is towards the Sun). The dependenceof the g-factor on the temperature of the scat-tering particles is influenced by the relation-ship between the widths of the solar radiationspectra. The g-factor shows a complex varia-tion pattern at low temperatures, while chang-ing smoothly at higher temperatures. Notethat the g-factor is higher when the velocity is300 km/s, compared to stationary conditions,since resonance scattering occurs with the OⅢemission line of the solar radiation with highillumination intensity.

Given the g-factor calculated above, theintensity of the scattered light is given by:

Here, n is the oxygen ion density and g theg-factor. Both are functions of line-of-sightdistance z from the observer. The integrationis performed from the position of the observa-tion instrument to an infinite point.

2.2 Development of the ObservationInstrument

Journal of the Communications Research Laboratory Vol.49 No.4 2002

Emission rate factor (g-factor) withDoppler shift effects

Fig.3

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The difficulty of developing an imaginginstrument for measuring the resonance-scat-tering emission of oxygen ions arises from thenecessity of accounting for the effects of thegeocorona emitted by hydrogen gases sur-rounding the Earth. The geocorona consists ofthe hydrogen Lymanαline (Ly-α; wave-length of 121.57 nm) close to the 83.4-nm OⅡemission line. Furthermore, its maximumintensity has been reported to be 10 kRayleighs(kR)[29]. As will be explained later, the OⅡemission for observation is extremely weak.Thus, the required ratio of sensitivity to OⅡand Ly-αis extremely large, 105-106. Devel-opment is further hindered by the need foroptics that use a collector mirror and for anoptical element with a bandpass in the OⅡemissions due to weak emissions. But nomaterial exists that is transparent to or hashigh reflective efficiency for 83.4-nm light. Aprimary-focus reflective optical system, con-sisting of a main mirror, a bandpass filter, anda detector, is adopted to overcome this prob-lem. The optical elements that determine thesensitivity of the instrument are the coating ofthe main mirror and the filter material.

Based on a consideration of optical prop-erties[30][31], molybdenum was selected as themost appropriate coating material for the mainmirror. An indium thin film was selected forthe bandpass filter. Both materials are rela-tively unaffected by aging and have highreflectivity and transmissivity to the OⅡ emis-sion region. We have built an instrument thathas to date achieved a sensitivity ratio of OⅡto Ly-αof 106. The optical system has a mir-ror reflector with a 100-nm thick molybdenumcoating at its center, a 275-nm thick indiumfilter, and a channel electron multiplier (CEM)as a detector. Fig.4 is a cross-section of theoptical system. Figs.5 and 6 present theresults of measurements of mirror reflectivityand filter transmissivity. The wavelengthdependence of reflectivity and transmissivityis determined by the optical constant specificto the metals. The general sensitivity charac-teristic of the instrument is shown in Fig.7.This instrument, onboard a SS-520-2 sounding

rocket which was launched from the northernpolar region and with an apex at an altitude of1,000 km, successfully detected resonance-scattering emission of oxygen ions from theupper ionosphere. Marking the first step inimaging observation of oxygen ions escapingfrom the ionosphere, the observation resultsindicate that oxygen ions are distributed abovethe polar ionosphere[32].

YAMAZAKI Atsushi et al.

Cross-section of the observation instru-ment on the SS-520-2 rocket

Fig.4

Reflectivity of molybdenum mirrorFig.5

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Studies have also sought to improve sensi-tivity to OⅡ. As shown in Fig.8, a Ni/MgF2

coating on an aluminum substrate providesreflectivity at H Ly-αtwo orders of magnitudelower than that of the earlier mirror[33]. If thismirror can be successfully applied to practicalapplications, the removal of the H Ly-αlinepresently performed by the indium filter mayalso be performed simultaneously at the mir-ror, making it possible to reduce the removalrate at the filter. In other words, it will be pos-sible to reduce the thickness of the indium fil-ter while retaining the sensitivity ratio of OⅡto H Ly-αat the onboard instrument level,thereby improving absolute sensitivity to OⅡ.Table 1 shows the expected sensitivity to OⅡin this case, comparing it to that of the instru-

ment aboard the SS-520-2 rocket. The mirrorreflectivity and quantum efficiency of thedetector are equal, but the thinner filterincreases transmissivity, with sensitivityexpected to increase by a factor of around 5.The problem of interfacial diffusion in thecoating over time has been reported as apotential problem for practical applications[34]. This problem represents another topic forfuture group investigation.

3 Polar Wind Imaging

3.1 Escape from the Polar IonosphereIn the 1960s, the escape flux of oxygen ion

was predicted to be only several percent ofthat of protons and helium ions[14]. This pre-diction was based on the large mass of theoxygen ion and the ease with which oxygenions become neutral through charge exchange[15]. However, observations by polar-orbitingsatellites in the 1980s indicated that these pre-dictions were not always correct. The obser-vation results by Dynamic Explorer-1 and 2revealed that during high solar activity, or

Journal of the Communications Research Laboratory Vol.49 No.4 2002

Transmissivity of indium filterFig.6

Sensitivity of the observation instru-ment on the SS-520-2 rocket

Fig.7

The reflective characteristics of mirrorwith a Ni 70Å/MgF2 105Å coating onan aluminum substrate

Fig.8

Sensitivity comparison of the mirroraboard the SS-520-2 rocket and thenew mirror

Table 1

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when the magnetosphere has been activelydisturbed, the oxygen ion flux increases[16].The velocity of oxygen ions parallel to mag-netic field lines calculated from observationdata of the Akebono satellite showed that oxy-gen ions were accelerated below an altitude of4,000 km[18]. Rocket observations revealedthe presence of an acceleration regime near analtitude of 1,000 km. The acceleration mecha-nism of oxygen ions was found to be the keyto determining plasma flux from the iono-sphere[19]. Fig.9 shows the distribution of theoxygen ion escape flux determined from sta-tistical analysis of Akebono observation data[17]. Escape flux is high near the dayside cuspregion and the duskside, reaching 108 /cm2/s,and is low at midnight or in high-latituderegions.

3.2 Intensity of Scattered Light ofEscaping Oxygen Ions

A model will be devised for the densitydistribution and g-factor required for Eq. (2.4)to calculate O-Ⅱ scattering intensity. Thedensity distribution was determined based onthe pressure gradient, polarization electricfield, and gravity, using the results of statisti-cal analysis of Akebono observation datashown in Fig.9 as boundary conditions. Theplasma condition is assumed to be quasi-neu-

tral. The escaping oxygen ions from the iono-sphere are assumed to have temperatures of4,000 K and energy of 10 eV. Escape velocityand temperature at an arbitrary position can bedetermined simultaneously, and the g-factorcan be estimated from Eq. (2.2). Fig.10 showsthe intensity distribution of scattered emis-sions calculated based on these assumptions.The half-circle in the middle of the lower partof each plot represents Earth, while the whitelines stretching from the half-circle show themagnetic field lines of the Earth, assuming adipole. The plot ranges are ±5 RE (Earth radii)in the equatorial plane and 5 Re in the direc-tion of the pole. The top and bottom panelsshow the intensity of scattered emissions ofoxygen ions for observation when the line-of-sight is in the day-night and morning-duskdirections, respectively. This result indicatesthat the maximum intensity is 1 R. Thus, ithas been concluded that in order to makeobservations of regions several RE away fromthe Earth in order to identify the escape routeof oxygen ions, instruments must be able tomeasure intensities around 0.1 R. Each mag-

YAMAZAKI Atsushi et al.

Distribution of oxygen ion escape fluxfrom the polar ionosphere based onAkebono satellite observation data

Fig.9

The predicted intensity distribution ofscattered light

Fig.10

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netic force tube was found to display a differ-ent illumination pattern. This may help toidentify the spatial structure of the oxygen ionescape mechanism.

4 Magnetosphere Imaging

4.1 Oxygen Ions Distributed in theMagnetotail

Oxygen ions with large mass are not ableto attain sufficient velocity to escape Earth'sgravity only by accelerating from the pressuregradient along open magnetic field linesextending from the Earth to interplanetaryspace. Previously, it had been believed thatthey would be unable to escape from Earthand would instead enter the plasma sheet byExB drift[35][36]. However, the Geotail satel-lite observation has revealed the presence ofoxygen ions of ionospheric origin in thelobe/mantle region of the magnetotail[22] [23].Figs.11, 12, and 13 show the temperature,density, and velocity of the observed oxygenions[37]. The horizontal axes of all three plotsare distant from the Earth and show the resultsof observation in the range of 0-220 RE. Fig.11shows that the oxygen ion temperature is low.While the data is mostly plotted within the 10-100 eV range, results of linear fitting shownby a line in the plot indicate that the tempera-

ture displays a rising trend away from theEarth. Fig.12 shows that oxygen ion densitydecreases with increasing distance from Earth.The average densities at 50 RE and 200 RE are3×10-3/cm3 and 3×10-4 /cm3, respectively. Itcan be seen from Fig.13 that the averagevelocity from the Earth towards the tailchanges significantly, from 120 km/s nearEarth at 20 RE to 280 km/s deep in the magne-totail at 200 RE. However, deviations are

Journal of the Communications Research Laboratory Vol.49 No.4 2002

Temperature distribution of oxygenions in the magnetotail

Fig.11

Density distribution of oxygen ions inthe magnetotail

Fig.12

Velocity distribution of oxygen ions inthe magnetotail

Fig.13

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large, and velocity may even exceed 400 km/sin the tail.

The shape of the magnetosphere and theoxygen ion distribution were modeled to cal-culate the emission intensity. As shown inFig.14, the magnetosphere is assumed to havean oval shape of X×Y = 30×20 RE in theregion beyond X =－50 RE. In the region sun-ward of X =－50 RE, the dayside magne-topause is set at X = 10 RE, with a parabolicsurface that connects smoothly with the mag-netotail at X =－50 RE. The GSM coordinatesare used here (X axis: sunward; Z axis: pointsnorth in the plane containing the X axis andthe geomagnetic axis).

Fig.15 shows the g-factors calculatedusing, as model values, the fitting lines ofapproximation for temperature, density, andbulk velocities of oxygen ions in the magne-tosphere shown in Figs.11, 12, and 13. Thehorizontal and vertical axes represent the X-coordinate in the GSM system and the g-fac-tor, respectively. Calculations were made byvarying the oxygen ion bulk velocity parame-

ter, for the model value Vb, for values 0.5Vb

and 2Vb, and for constant velocity of 200km/s. When velocity is constant, the g-factorbecomes a function only of temperature,remaining nearly constant regardless of X.However, if velocity is not constant, the g-fac-tor changes significantly.

4.2 Intensity of Resonance-scatteringemission from the Magnetotail

The distribution of OⅡ emission intensityin the magnetosphere was estimated using g-factors calculated in the previous section andthe density distribution shown in Fig.12.However, since the spatial distribution of theoxygen ion beam has not been confirmed byobservation, the temperature, density, and bulkvelocity of oxygen ions were assumed to be afunction only of X, being uniform in the Y andZ directions. The intensity of scattered emis-sion in the magnetotail estimated here is themaximum expected value. Fig.16 shows theresults of calculations, assuming a constantvelocity of 200 km/s, regardless of X. In thetwo panels on the left, the Earth is located inthe center on the left-hand side (X = 0, Y = 0).The upper left and lower left panels show theintensity of emission in the magnetotail asobserved from within the equatorial plane(from the +Y direction) and from the northpole (from the +Z direction) of the magnetos-phere, respectively. The right panel shows thedistribution of the scattered emission intensitywhen the magnetotail is observed from theEarth (from the +X direction). Since the g-factor is nearly constant, the scattered emis-sion intensity is proportional to the columndensity along the line-of-sight. The strongest

YAMAZAKI Atsushi et al.

The shape of the modeled magne-tosphere

Fig.14

The g-factors for oxygen ions in themagnetotail

Fig.15Distribution of resonance-scatteringemission intensity, assuming constantplasma velocity

Fig.16

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emission of 50μR is seen in the region alongthe plasma sheet approximately 10-30 RE fromEarth. Even in the deepest region of the mag-netotail located 200 RE from Earth, intensity isstill around 1μR.

Fig.17 shows the results of calculationsaccording to the bulk velocity model based onthe Geotail satellite observation. The panelsare arranged as in Fig.16. Since the g-factorin this model is a function of the distance fromEarth, the maximum resonance-scatteringemission intensity of 50μR is seen near X =－50 RE, where the product of the column den-sity and g-factor is a maximum value. Mini-mum intensity is observed near X =－180 RE

in the plasma sheet. But even in this region,emission intensity exceeds 5μR.

5 Observation Performance

Simple calculations in Sections 3 and 4indicate that detection levels of 0.1 R for polarwind imaging and 5μR for magnetotail imag-ing are sufficient for effective imaging obser-vations. This section examines the feasibilityof producing such an optical system. For thesake of simplicity, the measured count isassumed simply to be the sum of the signaland the noise of the detector. A value fornoise is assumed based on calibration experi-ments performed on the ground. If both signal(S) and noise (N) are assumed to have normaldistributions, the signal count must be greaterthan the statistical error of the measurementcount (S+N), satisfying the conditional expres-sion in Eq. (5.1), to distinguish the signal from

noise (Fig.18). Here, the S/N ratio is definedas the ratio between and .

The intensities of signal (S) and noise (N)are expressed as follows:

Here,αis the telescope aperture,δtheangle of field-of-view (FOV),ηthe efficiencyof the optical system, T the exposure time, Ithe intensity of the scattered emission, and Nd

the detector noise count per unit area and time.Fig.19 shows the S/N ratio as a function of

scattered emission intensity (vertical axis) andexposure time (horizontal axis) under the fol-lowing conditions. Micro Channel Plates(MCPs) are used as the detector; Nd is 0.5/cm2/s. The efficiency of the optical system is0.0004 for the instrument aboard the SS-520-2rocket and 0.002 for the instrument with thenew mirror. The angle of FOV, aperture, andF value are assumed to be 1˚, 10 cm, and 1.0,respectively.

Journal of the Communications Research Laboratory Vol.49 No.4 2002

Distribution of resonance-scatteringemission intensity assuming bulkvelocity, determined from results ofGeotail satellite observation

Fig.17

Concept of separating signal fromnoise

Fig.18

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When the required observation conditionis assumed to be an S/N ratio greater than 1,the instrument onboard the SS-520-2 rocketrequires an exposure of approximately 1 hourto perform imaging observations of the polarwind given illumination levels of 0.1 R. Incontrast, with the new mirror, imaging obser-vations would require only 10 minutes. Formagnetosphere imaging observations withmaximum light intensity of 50μR, an expo-sure of 1 hour using the new mirror wouldresult in a S/N ratio of about 0.01. Since theS/N ratio is proportional to the telescope aper-ture and the square root of exposure, an aper-ture 30 times larger (or 500 cm) and an expo-sure 10 times longer (10 hours) would berequired (for example) to achieve a S/N ratioof 1. However, such apertures are unrealistic,and the temporal resolution for such longexposures would be excessive relative to theresponse time of magnetospheric plasma vari-ations. The resulting observations would havelittle significance. This has lead to heightenedanticipation of the highly sensitive supercon-ducting tunnel junction (STJ) device[38]. Thefeatures of the STJ detector are high quantumefficiency of 90%, generally noise-free counts,superior energy resolution, and capacity forcooling to 0.3 K. The STJ detector wouldachieve a 10-fold improvement in the sensitiv-ity of the observation instrument. Since noisecan be ignored, S/N ratio can be expected toimprove by several orders of magnitude,enabling significant magnetosphere imaging

observations with telescopes with apertures ofseveral 10 cm, and at exposures of around 1hour.

6 Conclusions

Both the polar wind and magnetotail havebeen found to be suitable as subjects of imag-ing observation replying on the resonance-scattering emission of oxygen ions containedtherein. The optical system of the observationinstrument must be capable of distinguishinglight of 0.1 R and 5μR from noise. Polarwind imaging has been demonstrated to bepossible with a new coating for the mirrorreflector. Magnetotail imaging observations,on the other hand, will require a new detectorof STJ, in addition to the new coating. Thisrepresents a major line of inquiry for futureresearch.

Imaging observations provide a global per-spective on plasma distribution. This methodwill contribute to our understanding of thetransport route and make it possible to distin-guish between temporal and spatial variations.Information on the bulk velocity and tempera-ture of oxygen ions may be obtained if satel-lites are deployed to make simultaneous in-situmeasurements of plasma in the imagingregion. This will enable more detailed discus-sions of density distribution. Additionally, thedeployment of multiple imaging satellites forsimultaneous observations from differentangles will make it possible to perform real-time measurements of the 3-D density distri-bution of oxygen ions via tomography.

Numerous problems remain unresolved fornear-Earth plasma, particularly the density andenergy distributions of oxygen ions－such asthe route and process of escape from the iono-sphere, the mechanism of its supply to themagnetosphere, and the distribution of coldplasma in the plasma sheet. Remote-sensingmethods have provided information on 2-Ddistributions of the plasma environment cru-cial for resolving such problems. They havealso enabled us to view the unfolding of spaceenvironment disturbances such as geomagnet-

YAMAZAKI Atsushi et al.

S/N ratio shown as a function of scat-tered emission intensity and exposuretime

Fig.19

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ic storms through variations in the general dis-tribution of oxygen ions, thereby demonstrat-ing the effectiveness of remote-sensing tech-niques as space weather monitoring systems.

We expect these remote-sensing methods tomake significant contributions to our under-standing of planetary atmosphere and plasmaescape and transport mechanisms.

Journal of the Communications Research Laboratory Vol.49 No.4 2002

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120 Journal of the Communications Research Laboratory Vol.49 No.4 2002

YAMAZAKI Atsushi, Ph. D.

Research Fellow, Space WeatherGroup, Applied Research and Stand-ards Division

Earth and Planetary Science

TAKIZAWA Yoshiyuki, Ph. D.

Contract Researcher, Institute of Physi-cal and Chemical Research

YOSHIKAWA Ichiro, Ph. D.

Associate Professor, Institute of Spaceand Astronautical Science

Earth and Planetary Science

NAKAMURA Masato, Ph. D.

Professor, Institute of Space and Astro-nautical Science

Earth and Planetary Science

38 Zehnder, A., C. W. Hagen, and W. Rothmund, "Superconducting tunneling junction detectors", EUV, X-ray,

and Gamma-ray instrumentation for astronomy; H. S. Hudson., O. H. Siegmund, Editors, Proc. SPIE, 1,344,

286, 1990.

MIYAKE Wataru, Dr. Sci.

Senior Researcher, Space WeatherGroup, Applied Research and Stand-ards Division

Space Weather