Electron density images of the middle- and

high-latitude magnetosphere in response to

the solar wind

Jiannan Tu,1 Paul Song,1 Bodo W. Reinisch,1 Xueqin Huang,1 James L. Green,2

Harald U. Frey,3 and Patricia H. Reiff4

Received 20 July 2005; revised 31 August 2005; accepted 5 October 2005; published 15 December 2005.

[1] Electron density distributions and plasma dynamics in the middle- and high-latitudedayside magnetosphere are studied with observations from the IMAGE radio plasmaimager. Remote measurements of the electron densities along magnetic field lines weremade before and during a magnetic storm when the solar wind and interplanetary magneticfield (IMF) impinging on the magnetopause varied considerably. Several regions ofdifferent density distribution characteristics, including plasmasphere, plasma trough,subauroral/auroral density depletion, density enhancements in the aurora/cusp, and polarcap, are identified in ‘‘two-dimensional images,’’ i.e., along the satellite orbit and fieldlines. The plasma dynamics, such as the plasma refilling in the outer plasmasphere and theplasma acceleration in the aurora/cusp region, are inferred from density gradients alongthe field lines. It is shown that the densities and locations of the plasma regions vary inaccordance with the solar wind, particularly with the IMF variations for the caseexamined. In the partial recovery phase of the magnetic storm when the IMF wasnorthward, the density depletion region expanded to wider latitude range and extended tolower altitudes, with much lower densities than those in the density depletion regionsof the other RPI measurement periods. The density enhancements associated with theaurora/cusp region were not visible, possibly because of the diffusive nature of the daysideaurora and higher-latitude location of the cusp during this period. At the peak of the storm,characterized by a persistent southward IMF Bz, all plasma regions moved to lower Lshells. The results imply that the solar wind/IMF effects should be included in anystatistical study of the electron density distributions in these regions.

Citation: Tu, J., P. Song, B. W. Reinisch, X. Huang, J. L. Green, H. U. Frey, and P. H. Reiff (2005), Electron density images of the

middle- and high-latitude magnetosphere in response to the solar wind, J. Geophys. Res., 110, A12210, doi:10.1029/2005JA011328.

1. Introduction

[2] The plasma density distributions in the Earth’s mag-netosphere are primarily determined by the plasma transportas well as the plasma sources (solar wind and ionosphere).One of fundamental factors that influences the plasmatransport is the solar wind/magnetosphere interaction. Theplasmapause location and the plasma density distributionsbeyond the plasmapause are sensitive to variations of thesolar wind and interplanetary magnetic field (IMF) becauseof the magnetospheric convection produced by the solarwind/magnetosphere interactions. In order to better under-

stand the magnetospheric electron plasma distributions, it isimportant to examine their dynamical relationship with thesolar wind. This can best be accomplished from remotelyimaging large regions of electron densities in the middle- tohigh-latitude magnetosphere in which the location of openand closed field lines can change rapidly.[3] The density distributions in the plasmasphere have

been extensively studied and the responses of the plasma-sphere and its boundary, the plasmapause, to the solar wind/IMF variations, in terms of geomagnetic activities, havebeen documented in numerous publications [e.g., Lemaireand Gringauz, 1998, and references therein]. Nevertheless,the plasma density distributions in the plasma trough,auroral zone, cusp, and polar cap, as well as their variationsas functions of magnetospheric and upstream solar windconditions, are not well understood. The empirical relationof the equatorial density as function of L value and magneticlocal time (MLT) presented by Carpenter and Anderson[1992] has been often cited in connection with the densitiesin the plasma trough. Several similar empirical relationshave been developed in recent years for the equatorialelectron densities [Gallagher et al., 2000; Goldstein et al.,

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, A12210, doi:10.1029/2005JA011328, 2005

1Center for Atmospheric Research, University of Massachusetts-Lowell, Lowell, Massachusetts, USA.

2NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.3Space Sciences Laboratory, University of California, Berkeley,

Berkeley, California, USA.4Department of Physics and Astronomy, Rice University, Houston,

Texas, USA.

Copyright 2005 by the American Geophysical Union.0148-0227/05/2005JA011328$09.00

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2001; Sheeley et al., 2001; Denton et al., 2002, 2004].Using a large data set from the Polar spacecraft plasmawave measurements, Denton et al. [2002] found that thelatitudinal dependence of both the plasmaspheric and plasmatrough density could be represented by a power law formwith a single formula for the power law index. Their latitu-dinal dependence of the densities is quite similar to thatderived from the active remote sounding by Reinisch et al.[2004] and Huang et al. [2004] for plasmaspheric densities.However, the dependence of the plasma trough densities onthe solar wind/IMF conditions has not been explicitlyaddressed until now.[4] Electron densities at high latitudes have been sampled

using in situ measurement techniques for decades butinstantaneous measurements of spatially extended regionsby radio sounding were not possible until recently. Persoonet al. [1983] developed a statistical empirical model ofelectron densities in the polar cap for the radial distancerange of 2.1–4.66 RE. They found that the radial distancedependence of the electron density followed a power lawwith an exponent of �3.85, implying an outward flowvelocity of ions increasing linearly with radial distance.Chandler et al. [1991] studied ion densities for O+, H+,and He+ in the altitude range of 1000–4000 km measured bythe retarding ion mass spectrometer on the DE-1 satellite.Their results illustrated that O+ is the dominant ion speciesup to 4000 km altitude in the polar cap. Johnson et al. [2001]presented maps of thermal electron densities in the auroralzone and polar cap at �1 RE altitude, covering all magneticlocal time. The density variations associated with solarillumination and hence the ionospheric ionization rate var-iations, along with the auroral density cavities, are clearlyseen in those maps. Johnson and Wygant [2003] furthershowed that the effects of the solar illumination on the polarcap densities extend up to 3.5 RE in altitude. Laakso andGrard [2002] studied the statistical distributions of theelectron densities in the auroral zone and polar cap in theradial distance range of 2–9 RE. Their results demonstratedthat the electron densities at high latitudes increase at alldistances with increasing geomagnetic activity, with largerincreases during local summer than during local winter.They also showed examples of enhanced polar cap densitiesin response to the sudden storm commencements (SSC).[5] Recently, the radio plasma imager (RPI) [Reinisch et

al., 2000] on the IMAGE satellite [Burch et al., 2001] hasprovided nearly instantaneous measurements of the electrondensity distribution along a magnetic field line [Reinisch etal., 2001]. Such measurements were used to study thedependence of the polar cap electron densities on the radialdistance and geomagnetic activity [Nsumei et al., 2003] to ahigh degree of accuracy on a timescale never before accom-plished. Their study established a quantitative relation be-tween the polar cap electron densities and the geomagneticactivity, in the radial distance range of 1.4 to 5 RE.[6] In this study we take advantage of the radio sounding

technique implemented by IMAGE/RPI to combine multi-ple (nearly instantaneous) measurements of the field-alignedelectron density profiles to derive two-dimensional electrondensity images from the IMAGE orbit down to altitude of�3000 km. Such density images allow us, for the first time,to examine the various plasma regions (plasmasphere,plasma trough, aurora/cusp and polar cap) in ‘‘two dimen-

sions,’’ i.e., along the satellite orbit and the magnetic fieldlines. We show that the plasma densities in these regionsand the location of these plasma regions vary in accordancewith the solar wind, particularly IMF variations for the casestudied. The observations suggest that the solar wind/IMFeffects should be taken into account when interpretingstatistically averaged electron density distributions in themagnetosphere.

2. Measurement and Density Inversion Technique

[7] The radio plasma imager (RPI) [Reinisch et al., 2000]on the IMAGE satellite, in active sounding modes, transmitscoded signals with frequencies sweeping from 3 kHz to3 MHz and listens to the echoes. The received signals areplotted in the form of plasmagrams, a color-coded display ofsignal amplitude as function of frequency and echo delaytime. Echoes that experience the same dispersion during thepropagation form a distinct trace in the plasmagram. Undercertain conditions [see Reinisch et al., 2001; Fung andGreen, 2005] the traces represent the reflected signals thatpropagated along a magnetic field line from the satellite.Natural waves are received in a passive observation modeand are displayed as a dynamic spectrum [Reinisch et al.,2000; Green et al., 2000, 2004].[8] By scaling the traces in a plasmagram and using the

density inversion algorithm based on the ionospheric den-sity inversion technique of Huang and Reinisch [1982], wecan derive a density distribution along a field line almostinstantaneously [Huang et al., 2004]. For the case studiedhere, the duration for obtaining a trace is less than 46 s andthe typical duration is about 20 s. Figure 1 displays aplasmagram obtained by the RPI at 1845:49 UT on16 April 2002, with measurement duration of �40 s. Thereis a trace extending from �111 kHz to �819 kHz and avirtual range to �1.4 RE (the virtual range is the echo delaytime multiplied by one half of the vacuum speed of light).The local (at the satellite location) electron plasma, electroncyclotron, and upper hybrid resonances are seen as verticallines of enhanced signals. With the scaled trace (the thincurve on the trace), the electron density along the field lineis calculated using the inversion algorithm described byHuang et al. [2004], as shown in Figure 2. The density at1.4 RE radial distance is �3200 cm�3 and decreases alongthe field line to �35 cm�3 at 2.6 RE where the satellite waslocated. The typical uncertainty of the derived electrondensities due to the uncertainty of determining frequenciesalong the echoes is less than 6%. The radial distanceresolution of the derived electron density profiles dependson the altitude: At altitudes of 3–4 RE it is up to 0.075 RE

(about 480 km); at altitude of about 3000 km, it can be asgood as 0.003 RE (about 20 km). When the satellite fliesacross an extended region, a sequence of such densityprofiles is obtained and a ‘‘two-dimensional’’ density imagealong the satellite orbit and the magnetic field lines can beconstructed. Although the measurement of each individualfield-aligned density profile is nearly instantaneous, eachdensity image consists of multiple field-aligned densityprofiles along an orbit segment of about 1 hour duration.Therefore the density images derived from multiple field-aligned density profiles may not be purely spatial structures.Nevertheless, they provide useful information regarding the

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underlying physical processes from measurements on atimescale never before possible.

3. Observations

[9] In this section we present observations by the IMAGE/RPI in the northern hemisphere made before and during amoderate magnetic storm, which started at about 1200 UTon17 April 2002. The purpose of the present study is toexamine the relation of the electron density distributionswith the solar wind/IMF conditions. Therefore we selectobservations in prestorm and storm times when the solarwind/IMF conditions varied significantly. There are threeperiods during which multiple field-aligned density profilesare available to derive ‘‘two-dimensional’’ density images.These time periods are 1809–1906 UT on 16 April, 2221–2332 UT on 17 April, and 0732–0827 UT on 20 April 2002.Between these periods when the satellite flew through thesame regions, the RPI provided too few density profiles toconstruct robust ‘‘two-dimensional’’ density images becausethe RPI often operated in other measurement programs. Thefootprints of the IMAGE orbit for these periods are shown inFigure 3 in corrected geomagnetic (CGM) coordinates[Hakura, 1965], in a dial plot of CGM latitude–magneticlocal time (MLT) format. Throughout the paper, the latitudesand MLT are defined in CGM coordinates on the surface ofthe Earth unless otherwise indicated. The Tsyganenko 96

(T96) [Tsyganenko, 1995] magnetospheric magnetic fieldmodel is used to map the satellite orbit to the Earth’s surface.The passes discussed here are all close to noon sector andacross the nominal dayside aurora/cusp latitudes, the regionsof the boundary between open and closed field lines. Allfootprints of the three orbit segments were under sunlight(solar zenith angle <72�) and the satellite flew from high tolow latitudes. Figure 4 displays the variation of hourly Dstindex from 15 April to 27 April 2002. Each vertical lineroughly indicates the times of the IMAGE/RPI measure-ments used to construct a ‘‘two-dimensional’’ density image.The first period of the IMAGE/RPI observations was beforethe storm, the second one was in the temporal recover of theDst, and the last one was at the peak of the storm when thehourly Dst dropped to about �148 nT.

3.1. Solar Wind/IMF Conditions

[10] Figure 5 displays the Dst index and solar wind/IMFparameters measured by the ACE satellite in the upstreamsolar wind on 16–17 April and on 20 April 2002. Plottedfrom top to bottom are Dst, IMF strength, IMF By and Bz

components in GSM coordinates, solar wind proton density,solar wind velocity, and solar wind dynamic pressure,respectively. Using the solar wind velocity componentalong the Sun-Earth line, the three periods of IMAGE/RPImeasurements, marked by three pairs of vertical lines in Dstpanels, are backward shifted by the transit time for the solar

Figure 1. A plasmagram from IMAGE/RPI, showing signal amplitude dBnV/m as function offrequency and time delay expressed in terms of virtual range. A trace extending from �111 kHz to�819 kHz is formed from X mode echoes. Local electron plasma, electron cyclotron, and upper hybridresonances are seen as vertical lines. fx is the local X mode cutoff frequency.

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wind to travel from the ACE location to the expectedmagnetopause (10.5 RE in front of the Earth). The IMF Bz

was southward for the first period of IMAGE/RPI measure-ments (1809–1906 UT of 16 April), with Bz of �3.3 to�0.8 nT. The By was negative (eastward), in the range of�5 to �2 nT. There is a data gap in the solar wind protondensity during which the dynamic pressure cannot becalculated. If the proton density did not change much inthe data gap, which was a reasonable assumption since therewere no significant changes in the solar wind speed andIMF orientation, it would be about 7 cm�3 as inferred fromlater measurements. The solar wind dynamic pressure thenwould be about 2 nPa, typical for the solar wind dynamicpressure [Hundhausen, 1995].[11] At about 1020 UT on 17 April, the IMF strength,

proton density, solar wind velocity, and dynamic pressureincreased steeply, indicating a shock passed the ACEsatellite. The second pair of vertical lines mark the back-ward shifted start and end times of the second period ofIMAGE/RPI measurements (2221–2332 UT of 17 April).The IMF Bz was slightly northward during the period andthe dynamic pressure was moderately high, about 4–5 nPa.Before this period, the IMF Bz had been primarily positivefor about 2 hours. In this period the IMF By was positive(westward), up to 10 nT.[12] The third period of the RPI measurements (0732–

0827 UT of 20 April) was under westward IMF By andsouthward IMF Bz (see the period within the third pair ofvertical lines). The solar wind dynamic pressure was againmoderately high, about 4 nPa. Except several short north-ward excursions, the IMF Bz had been strongly negative, aslow as �20 nT, for more than 6 hours. Owing to thepersistent strong southward IMF Bz, the Dst experienced

Figure 4. Dst variation during 15–27 April 2002. Eachvertical line indicates the central time of the IMAGEfootprint segment shown in Figure 3.

Figure 3. Footprints of the IMAGE orbits during 1809–1906 UT on 16 April (solid line), 2221–2332 UT on17 April (dotted line), and 0732–0827 UT on 20 April(dashed line), in CGMLAT-MLT coordinates. The satelliteorbits were from high to low latitudes.

Figure 2. Field-aligned electron density profile calculatedfrom the trace in the plasmagram in Figure 1.

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another decrease starting at about 0000 UT on 20 April anddropped to below �148 nT at about 0700 UT on 20 April.

3.2. Two-Dimensional Density Images

[13] In Figure 6 we show the ‘‘two-dimensional’’ densityimages projected onto the solar magnetic (SM) XSM-ZSMplane for the three periods mentioned previously. The leftpanel shows the electron density image from 84.3� to 49.3�CGM latitude, derived from individual field-aligned densityprofiles measured by the RPI from 1809 to 1906 UT on16 April 2002. Several regions of different plasma densitycharacteristics can be identified from this density distribu-tion. High electron densities are seen in the region below�54� (L < 2.9, region of red color), with densities greaterthan 2600 cm�3 at the satellite locations (�1 RE altitude).This region is inside the plasmasphere. The densitiesdecrease rapidly and keep above this latitude decreasingalong the satellite orbit because of the density decrease dueto the increase in radial distance and/or latitude, as will beseen in Figure 7. Such continuous density decrease makesthe identification of the plasmapause difficulty. Neverthe-

less, we will show later that the field line of CGM latitude65.2� (L = 5.3) may be used to delimit two adjacent regionsof different radial distance dependence of the densities anddistinct plasma flow characteristics. Therefore we tentativelydefine L = 5.3 as the boundary, or the plasmapause, sepa-rating the plasmasphere from the plasma trough. The densitydistributions in the plasmasphere display two layers: onehigh-density region mentioned above, and a high L-valueregion with lower densities. It will be also shown that theregion of greater L value is the outer plasmasphere withinwhich the flux tubes are still experiencing refilling of theplasma from the underlying ionosphere.[14] The density variations along the satellite orbit can

also be determined by the upper hybrid resonance (UHR)noise band observed in RPI passive mode measurements.The UHR noise band is a narrow band of signal intensityenhancements bounded by the local hybrid resonance fre-quency (fuh) and electron plasma frequency (fpe) [e.g.,Mosier et al., 1973]. It is a typical feature of RPI passivespectrum measurements when the electron cyclotron fre-quency (fce) is smaller than the fpe. Figure 7 shows threedynamic spectra, for 1730–1930 UT on 16 April, 2200–2400 UT on 17 April, and 0700–0900 UT on 20 April2002. In each dynamic spectrum, the UHR noise signatureis visible and decreases dramatically in frequency (fromright to left, i.e., from low to high latitude) at the timeindicated by a vertical arrow. The sharp decrease of thefrequency of the UHR band indicates a steep gradient ofthe plasma density since Ne / fpe

2 . For the period of 1730–1930 UT on 16 April 2002, the sharp decrease occurred atabout 1902UTwhen the IMAGEcrossed�54�CGMlatitude.The UHR band continued to drop quickly in frequency fromlow to high latitude beyond�54�, consistent with the densityvariations seen in the ‘‘two-dimensional’’ density image (leftpanel of Figure 6).[15] Beyond the plasmapause marked by the field line of

65.2� in the left panel of Figure 6, the in situ densitiesdecrease to lower than 100 cm�3. This is the low-densityplasma trough. It is remarkable that there is a V-shapeddensity depletion extending from about 73.8� to 78� inlatitude. The density is below �20 cm�3 in the depletionregion and the minimum density is about 7 cm�3. Thepoleward boundary of this region is along the field line of�78� CGM latitude and extends down to about 2.5 RE

radial distance. The low-density structure in the plasmatrough is possibly in the region close to or even partiallyoverlapped with the auroral oval. For this period, the auroralimages obtained by the far ultraviolet (FUV) imager [Mendeet al., 2000] on IMAGE show faint auroral emissions(images not shown) on the dayside so that it is difficult tolocate the boundaries of the dayside auroral oval. However,there is a density enhancement in a narrow latitude range(78�–79.2�), just poleward of the density depletion. Thisdensity enhancement may be the signature of the cuspwhere both precipitating magnetosheath plasma and en-hanced ion outflows can cause relative high densities inthe cusp proper [e.g., Lockwood et al., 1985; Horwitz andLockwood, 1985; Smith and Lockwood, 1996; Andre andYau, 1997]. We associate the density enhancement observedin the present period with the cusp because of its narrowlatitude range, �1�, typical of the latitudinal width of thecusp, and the distinct plasma flow characteristics as will be

Figure 5. IMF strength, IMF By and Bz components, solarwind proton density, solar wind velocity, and dynamicpressure measured by the ACE satellite at GSM-X =�223RE

during (left) 16–17 April 2002 and (right) 20 April 2002.Hourly Dst index is also plotted for comparison. Three pairsof vertical lines in Dst panels mark three periods of the RPImeasurements shown in Figure 6. In solar wind/IMF panels,those periods are backward shifted using the estimated transittime for the solar wind to travel from the ACE location to themagnetopause.

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Figure 7. The IMAGE/RPI dynamic spectral image showing the plasma wave environment along thesatellite orbits for the periods of 1730–1930 UT on 16 April, 2200–2400 UT on 17 April, and 0700–0900 UT on 20 April 2002. The auroral hiss noise and upper hybrid resonance (UHR) noise band areindicated in each spectrum. Down-pointing vertical arrows indicate the starting time (location) of thesharp frequency change of the UHR band. Red line is local electron cyclotron frequency derived from theRPI active sounding. Universal time (UT), CGM latitude, and radial distance (R) along the orbits arelabeled under the abscissa.

Figure 6. Two-dimensional (2-D) electron density (Ne) images projected onto the solar magneticXSM-ZSM plane. The 2-D images are derived from the field-aligned density profiles measured by theRPI from 1809 to 1906 UT on 16 April, 2221 to 2332 UT on 17 April, and 0732 to 0827 UTon 20 April, respectively. The stars on each orbit segment indicate the locations from which the field-aligned density profiles were measured. Also plotted are three field lines (solid) with the CGMlatitude labeled. The field line of lowest latitude indicates the plasmapause, while the other two fieldlines limit density depletion region. See text for details.

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seen later in Figure 11. If this is the case, then the densitydepletion must be at least partially over the auroral ovalbecause the cusp is immediately poleward of the daysideauroral oval under southward IMF Bz condition [e.g., Smithand Lockwood, 1996].[16] The density depletion that occurred from 1831 to

1841 UT (78� to 73.8�), coincides with the equatorward partof the intense broadband emissions below the electroncyclotron frequency (see left panel of Figure 7). Previousobservations have established that such emissions at highlatitudes are whistler mode auroral hiss [e.g., Labelle andTreumann, 2002, and references therein]. It is possible thatthe auroral hiss is generated at radial distance of 2.7–2.9 RE

in the auroral current region and propagates both polewardand equatorward from its source region [e.g., Gurnett et al.,1983]. However, our data set does not contain enoughinformation for us to further study the possible relationshipbetween the density depletion and the auroral hiss.[17] During the partial recovery phase of the Dst when the

IMF Bz has been northward for 2 hours, the electrondensities in the IMAGE orbit plane demonstrate quite adifferent picture. The middle panel of Figure 6 displays thedensity image between 81.3� and 46� CGM latitude, derivedfrom the RPI measurements from 2221 to 2332 UT on17 April 2002. A remarkable change compared to thedensity image in the left panel is that the density depletionexpanded to wider latitude range and extended to loweraltitude, as marked by the field lines of 69.1� and 76.6�. Thedensity depletion corresponds again to the equatorward partof intense auroral hiss emissions displayed in the middlepanel of Figure 7. The auroral images taken by the IMAGE/FUV during this period (images not shown) do not displayclear auroral oval on the dayside, preventing us fromidentifying the locations of the boundaries of the daysideauroral oval. The densities in the depletion region are verylow, with the lowest density �2 cm�3. Like the previousperiod of the RPI measurements, the plasmapause, markedby the 62.3� CGM latitude field line (L = 4.57), is identifiedas the boundary between adjacent regions of different radialdistance dependence of the densities and distinct plasmaflow characteristics. The plasmasphere also consists of twolayers: the inner plasmasphere (believed to be in diffusiveequilibrium) and the outer plasmasphere, in which the fluxtubes were refilling with the plasma from the ionosphere, aswill be discussed later.[18] In this satellite pass, no localized density enhance-

ment appears to be associated with the aurora/cusp pole-ward of the density depletion region. Instead, we will seefrom the middle panel of Figure 10 that the polar cap ispossibly immediately poleward of the density depletion,i.e., at latitudes higher than 76.6�. The cusp was notobserved because it might be located further polewardunder northward IMF Bz [e.g., Song et al., 2003]. Inaddition, the cusp is much more localized during north-ward IMF Bz than during southward IMF Bz [Smith andLockwood, 1996]. Thus it is possible that the IMAGE didnot cross the cusp, especially as the positive IMF By shiftedthe cusp position postnoon while the IMAGE orbit wasprenoon (see Figure 3).[19] The third period of the RPI measurements from 0732

to 0827 UT on 20 April 2002 occurred in the period ofpersistent southward IMF Bz, which caused the Dst mini-

mum of �148 nT (see Figure 4). It is seen from the rightpanel of Figure 6 that the electron density image during thisperiod is to some degrees similar to that in the first period ofthe RPI measurements shown in the left panel of Figure 6.The plasmasphere, however, was compressed by the in-creased magnetospheric convection associated with thestrong southward IMF Bz and the plasmapause moved tolower latitude (lower L value). Again, the plasmapause,marked by the field line of CGM latitude 56.5� (L = 3.07), isnot clearly visible from the electron density image, but itwill become identifiable in later plots showing the radialdependence of the densities and the inferred plasma flows.The density depletion (between �63.4� and �67�) and thelocalized density enhancements (between �67� and �70�)are still visible but shifted equatorward as well. In this case,the localized density enhancements are largely coincidentwith the auroral oval as identified from a series of IMAGE/FUV images taken from 0746 to 0800 UT on 20 April 2002when the IMAGE flew through �73� to 67� CGM latitude.Therefore the auroral oval was poleward of the densitydepletion region for this period. As an example, a FUVimage taken at 0758:30 UT is displayed in Figure 8. Thecoordinates in the FUV image are APEX geomagneticcoordinates [Richmond, 1995]. As noted by Gasda andRichmond [1998], at auroral to polar cap latitudes the APEXlatitudes are essentially the same as the CGM latitudes: themaximum difference between the APEX latitude at 110 kmaltitude and CGM latitude at ground level is 0.17�. Thesatellite footprints for the period of 0732–0827 UT areoverplotted on the FUV image with the satellite location atthat time (at 68.3� and 1140 MLT) marked by a short bar. Itis shown that the auroral oval was located roughly between67.5� and 69� APEX latitude, poleward of the densitydepletion (between 63.4� and 67�) and covered by the

Figure 8. An auroral image from the IMAGE/FUV at0758:30 UT on 20 April 2002. The footprints of the satelliteorbit for the period of 0732–0827 UT on 20 April 2002 isoverplotted as a red line. The auroral oval on the dayside islocated roughly between 67.5� and 70� latitude. Thesatellite location at this time was at �68.3� latitude and1144 MLT (marked by a short bar).

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density enhancements (67�–70�). The most poleward partof the density enhancements (69�–70�) probably corre-sponds to the cusp.[20] It can be seen in Figure 6 that the densities in

different regions have distinct radial distance dependencealong the field lines. This is clearly demonstrated byFigure 9, in which we plot the sample field-aligned densityprofiles from various plasma regions for the first period ofthe RPI measurements. It is shown that the plasmasphericdensities show the smallest variation with the radial distancealong the field line while the density gradient in the densitydepletion region is largest. The statistical study of Nsumei etal. [2003] showed that the polar cap densities have a powerlaw relation with an exponent index of about �5 with theradial distance. Since the densities in different plasmaregions have distinct radial distance dependence, normaliz-ing the densities with r�5, where r is the radial distancealong individual field lines, helps better differentiate theplasma regions relative to the polar cap. In Figure 10 wepresent distributions of the densities divided by r�5. It isclear that different plasma regions are indeed betterdelimited. In Figure 6 we have marked the plasmapausewith the field line of CGM latitude 65.2�, 62.3�, and 56.5�,respectively, from left to right. In that figure the plasma-pause is not clearly identifiable. Figure 10 demonstrates thatthese field lines are roughly the boundaries that separateadjacent regions of different radial distance dependence ofthe densities along the field lines. The normalized densitiesin the plasma trough region decrease with radial distancealong the field lines, indicating a density decrease fasterthan r�5. The normalized densities in the plasmasphere aremore smoothly distributed along the field lines, particularly

Figure 9. Sample field-aligned electron density profilesfrom (a) polar cap, (b) cusp, (c) density depletion region,(d) outer plasmasphere, and (e) inner plasmasphere. Thedensity profiles were from the first period of the RPImeasurements.

Figure 10. Same format as Figure 6 but for electron density images divided by r�5, where r is the radialdistance (in RE) along individual field lines.

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in the inner plasmasphere. However, the plasmasphericdensity variations along the field lines result from thecombined effects of the radial distance and latitude changes.We thus cannot conclude that the plasmaspheric density hasa r�5 dependence on the radial distance from the smoothvariations of the normalized densities.[21] In the middle panel of Figure 10, the polar cap is

identified as the region immediately poleward of the densitydepletion because of the uniform radial distance variation ofthe normalized densities. Note that at high latitudes the fieldlines are close to aligned with the radial direction. In allthree panels the normalized densities are more uniformlydistributed with the radial distance in the polar cap than inthe plasma trough since the electron densities in the polarcap vary with the radial distance in a way closer to r�5.

3.3. Inferred Plasma Flow Characteristics

[22] The instantaneously measured field-aligned densityprofiles by the IMAGE/RPI provide the information of thedensity gradient along the field lines, which is not availablefrom in situ density measurements. In this section we usethe density gradient to infer the characteristics of the plasmaflows in various plasma regions and thus get insight into theplasma dynamics in response to the solar wind.[23] In the altitude range of the present RPI observations,

there is essentially no production and loss of the chargedparticles. If neglecting the plasma transport perpendicular tomagnetic field lines, the continuity equation can be writtenas

@Ne

@tþ B

@

@s

Nev

B

� �¼ 0; ð1Þ

where Ne is the electron density, B is the magnetic field andv is the field-aligned electron velocity, and s is the arc lengthalong a field line. When there is no or weak field-alignedcurrent, v is the average field-aligned velocity of all ionspecies since v = (SiviNi � J/e)/Ne, where Ni and vi is theion density and field-aligned velocity, respectively, J is thefield-aligned current density, and e is the elementary charge.In the case of quasi-steady state or slow time variation of thedensity, we have Nev/B = const. In Figure 11 we plot thedistributions of v/v0 = Ne0B/NeB0, the normalized field-aligned velocities. The variables with subscript 0 are thoseat the bottom of the individual density profiles measured bythe IMAGE/RPI. Although the steady-state approximationmay not be perfect and perpendicular plasma transport maynot be completely ignorable, we can obtain usefulinformation regarding the plasma dynamics by examiningsuch normalized velocities.[24] Before we proceed with our discussion, we point out

that because v0 is different for each individual densityprofile, only the change of the normalized velocities isrelevant: the larger values of the normalized velocities onlymean the larger change of the velocities along the individualfield lines. In addition, the variation of the normalizedvelocities is along the field lines but it is often referred toas the altitudinal variation in the high-latitude regionsbecause of the nearly vertical field lines. We also pointout that the velocities along the field lines may be down-ward (negative v0). However, previous observations haveshown that the plasma flows are generally upward along thefield lines beyond the plasmapause, particularly during daytime when the ionospheric ions are driven upward by thepressure gradient [e.g., Sojka et al., 1983]. The observed

Figure 11. Same format as Figure 6 but for Ne0B/NeB0, where Ne0 and B0 are the electron density andmagnetic field at the bottom of the individual field-aligned density profiles.

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flow velocity increases along the field lines are larger thanvelocity increases associated with the adiabatic decrease ofthe plasma pitch angle, and the flow velocities are oftensupersonic (polar wind characteristic) [Singh and Horwitz,1992; Moore et al., 1999, and references therein].[25] We first examine the left panel of Figure 11. There

are two regions of slightly different flow characteristics inthe plasmasphere. At lower latitudes or lower L shells, thenormalized velocities are almost constant along the fieldlines suggesting that this is the inner plasmasphere withplasma in diffusive equilibrium. Meanwhile, the change inthe normalized velocities along the field lines is significantin the outer region. Although the variations of the normal-ized velocities in the outer region may be the decrease of thedownward flow velocities, it is reasonable to interpret suchvariations as the upward flowing of the ionospheric plasma,as discussed in, for example, the review paper of Singh andHorwitz [1992]. Therefore the flux tubes in the outer regionare still experiencing refilling of the plasma from theionosphere.[26] Beyond the plasmapause, in the trough region, the

normalized velocities rapidly increase along the field linesat altitudes above about 1 RE, indicating there is possibleplasma acceleration above this altitude. The largest altitudeincrease in the normalized velocities is seen in the densitydepletion region, which possibly overlaps partially with theauroral oval for this period of measurements, as statedpreviously. The altitude variations of the normalized veloc-ities display more structures in the aurora/cusp and polarcap. The normalized velocities in the region correspondingto the density enhancement seen in the left panel of Figure 6vary relatively smoothly. If it is the cusp, as argued before,we have a simple explanation for such smooth variations ofthe normalized velocities: Strong heating and upward ac-celeration of plasma are expected in the cusp [e.g., Moore etal., 1999, and references therein]. Meanwhile, the precipi-tating magnetosheath plasma balances the velocities ofupward flowing ionospheric plasma in the normalizedvelocities. The large altitude increase of the normalizedvelocities in the region close to the poleward boundary ofthe cusp is due to the spread of the energized ions from theaurora zone/cusp to the polar cap by the antisunwardconvection for southward IMF [e.g., Lockwood et al.,1985; Andre and Yau, 1997; Tu et al., 2005].[27] In the middle panel of Figure 11, the normalized

velocities slightly decrease with altitude along the field linesin the inner plasmasphere where the densities are high, alsosuggesting that the inner plasmasphere was in diffusiveequilibrium. The velocity appreciably changes along thefield lines in the outer plasmasphere, again indicatingpossible plasma refilling from the ionosphere. The largestaltitude increase in the normalized velocities is seen in theplasma trough for this period of the RPI measurements butthe normalized velocities display more structures. Under thenorthward IMF Bz, the convection and auroral emissionpatterns are much more complicated than those during thesouthward IMF Bz. This is possibly the reason that thespread of the energized plasma from the aurora oval tothe polar cap is not seen and the altitudinal increase of thenormalized velocities is smaller in the polar cap.[28] The right panel is for the last period of the RPI

measurements. In the inner plasmasphere, variation in the

normalized velocities along the field lines is small, implyingthis is the region with the plasma in diffusive equilibrium.Note that the outer region of the plasmasphere where thenormalized velocities display visible changes along the fieldlines is in a narrow region because of the overall compressionof the plasmasphere. The largest increase of the normalizedvelocities with the radial distance is again seen in the densitydepletion region, which is to large extent at subaurorallatitudes (see discussion in section 3.2). The cusp is possiblyjust poleward of the auroral oval with a weaker increase ofthe normalized velocities along the field lines. Furtherpoleward, the increase in the normalized velocities alongthe field lines become even weaker, due to the spread of theenergized ions from the aurora/cusp [e.g., Tu et al., 2005].

4. Summary and Discussion

[29] Using multiple field-aligned electron density profilesmeasured by the IMAGE/RPI, we have constructed ‘‘two-dimensional’’ density images in the middle- and high-latitude dayside inner magnetosphere for different solarwind/IMF conditions. This allows regions of differentplasma characteristics, namely the plasmasphere, plasmatrough, density depletion, aurora/cusp, and polar cap, to beidentified along satellite orbit and the field lines. It is shownthat the densities in various regions and the location of thoseplasma regions vary in accordance with the solar wind/IMFconditions. During the partial recovery of the Dst when theIMF Bz was northward, the density depletion region widensin latitude and extends to lower altitudes. Also, the plasmatrough densities are lower during northward IMF and thereis no signature of the cusp. When the IMF Bz had beenstrongly southward for several hours, the Dst reached aminimum and all the plasma regions moved to lowerlatitudes. Table 1 lists solar wind/IMF parameters duringthe periods of the RPI measurements (backward shifted withthe solar wind transient time) and latitude range of thoseplasma regions for three periods of the RPI measurements.This table helps to clarify the relation between the locationsof the plasma regions and the solar wind/IMF conditions forthe cases considered.[30] We have also used the density gradients along the

field lines to examine the plasma flows. Although theassumption of steady state and neglecting the perpendicularplasma transport may not perfectly apply, the inferredcharacteristics of the plasma flows and underlying dynam-ics for each region under different solar wind/IMF con-ditions are reasonable. For instance, the weak variation ofthe flow velocities in the inner plasmasphere is understand-able since there is diffusive equilibrium established. Theinner plasmasphere in the diffusive equilibrium and theouter plasmasphere, possibly still experiencing refilling ofthe plasma, can be distinguished in the density images. Thelarge increase of the plasma velocities in the auroral oval isalso expected because of the strong auroral acceleration andheating of the plasma.[31] In the past, electron density distributions were typi-

cally studied using a very large set of in situ densitymeasurements that are averaged in specified spatial bins.The large variation in the location of the plasma regionsunder different solar wind/IMF conditions implies that wemust be cautious in interpreting these types of average

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electron density distributions. For example, Palmroth et al.[2001] noted that the variation in the cusp positions underdifferent solar wind conditions likely spreads out the cuspsignature in average density distributions. Therefore it isnecessary to include the solar wind/IMF effects in futurestatistical studies using a large set of density measurements.[32] The variations of the density distributions, seen in

our images, appear to be primarily associated with thechange of the IMF orientation and correspondingly differentstates of the magnetosphere. For instance, the densityimages in the second and last period of the RPI measure-ments were quite different from one another, most likelybecause of the intervening storm emptying the outer plas-masphere. However, the solar wind velocities and protondensities, and thus the dynamic pressures as well as IMF By

were similar in the two periods except that the IMF Bz hadopposite orientations, as can be seen from Table 1. Thedensity images in the first and last period were similar eventhough the solar wind conditions and IMF By orientationwere different. The shift of the various plasma regions tolower L shells in the last period density image was primarilydue to strong and persistent negative IMF Bz, which alsocaused the drop of the Dst from �94 nT to �148 nT inabout 7 hours. These observations suggest that the control-ling factor for density variations imaged by the RPI is theIMF Bz orientation for the case considered here. However,more cases are needed to examine before we reach definiteconclusion on how the solar wind and IMF conditions affectthe density distributions.[33] In previous studies the in situ plasma density mea-

surements were used to identify various plasma regions[e.g., Marklund et al., 1990; Carpenter and Anderson,1992; Laakso and Grard, 2002]. The IMAGE/RPI obser-vations provide the ability to identify plasma regions morereliably because ‘‘two-dimensional’’ spatial extension ofthem can be seen. Furthermore, the difference in the radialdistance dependence of the densities along the field lines inthese regions can be determined using the near instanta-neously measured field-aligned density profiles. Thereforedifferent plasma regions are better differentiated, as hasbeen shown in the normalized density images of Figure 10.[34] It has been shown that the density depletion is the

region with strongest plasma acceleration along the fieldlines. Figure 11 shows that the strong plasma accelerationprimarily occurs at altitudes above 1 RE. We may conjec-ture that the density depletion resulted from the plasmaescape to high altitudes caused by the strong plasmaacceleration above 1 RE altitude and the insufficientsupply of the plasma from ionospheric altitudes to com-pensate the plasma escape. Note that the density depletion

region is beyond the plasmapause. The escaped plasma isconvected to the outer magnetosphere and eventually tothe solar wind at the magnetopause. Most likely, thedensity depletion region partially overlapped the daysideauroral oval in the first period of measurements, as arguedbefore, while it was at subauroral latitudes in the lastperiod of measurements. In the auroral oval, the strongplasma acceleration at high altitudes can be caused by thewave heating and/or a large potential drop along the fieldlines [e.g., Andre and Yau, 1997; Moore et al., 1999]. Atsubauroral latitudes, the density depletion observed by theRPI in the last period of measurements must have beenproduced by mechanisms different from those responsiblefor the formation of the subauroral ionospheric trough atlow altitudes [e.g., Roger et al., 1992]. It has also beennoted that the density depletion is coincident with theequatorward part of the auroral hiss emissions, suggestingthe possible link of the auroral hiss and the densitydepletion on the dayside.[35] The present observations also demonstrate that the

densities in the polar cap vary with the radial distance in away close to r�5, implying that there is rapid increase of theplasma velocities upward along the field lines in the polarcap [Nsumei et al., 2003]. In the region of the polewardboundary of the dayside aurora/cusp, such a velocityincrease may be accomplished through the spread of theenergized ions, particularly O+, from its acceleration regionin the aurora/cusp when the convection is antisunward [e.g.,Lockwood et al., 1985; Tsunoda et al., 1989; Tu et al.,2005]. The progressively weaker increase of the normalizedvelocities poleward of the aurora/cusp displayed in the leftand right panels of Figure 11 may be the signature of thespread of the energized ions from the aurora/cusp to thepolar cap. Note that in the polar cap the electron velocity isthe average velocity of all ion species because there areessentially no field-aligned currents. In the polar cap theions are also locally accelerated upward by the ambipolarelectric fields (most effective for light ions like H+ and He+

ions) [e.g., Axford, 1968]. Nevertheless, it is necessary toperform more detailed observations, combined with simu-lations, to quantitatively explain how such radial distancedependence is produced in the polar cap.

[36] Acknowledgments. This work was supported by NASA undersubcontract 83822 from Southwest Research Institute and by NSF grantNSF-ATM 0518227 to the University of Massachusetts Lowell. We thankthe efforts of ACE team (magnetic field experiment PI: N. Ness at BartolResearch Institute and solar wind experiment PI: D. J. McComas atSouthwest Research Institute) for making ACE level 2 IMF and solar winddata available at Coordinate Data Analysis Website (CDAWeb). The WorldData Center for Geomagnetism (WDC-C2) at Kyoto University, Japan isacknowledged for providing the Dst data. We acknowledge I. A. Galkin and

Table 1. Solar Wind/IMF Parameters During the RPI Measurements (With Solar Wind Transient Time Considered), and Latitude Range

of Plasma Regions for Three Periods Shown in Figure 6a

Date

Solar Wind/IMF Latitude Range

NSWb VSW

b Pdynb By Bz PS PT DP Cusp/PC

04/16/2002 �7c �420 �2c �5 – �2 �3.3 – �0.8 <65.2� 65.2��73.8� 73.8��78.0� >78.0�04/17/2002 �8 �600 �4.5 2 – 10 0.2 – 3 <62.3� 62.3��69.1� 69.1��76.6 >76.6�04/20/2002 �6 �635 �4 1.2 – 6 �6 – �2 <56.5� 56.5��63.4� 63.4��67.0� >67.0�

aPS is plasmasphere; PT is plasma trough; DP is density depletion; PC is polar cap.bSolar wind proton density NSW in cm�3, velocity VSW in km/s, and dynamic pressure Pdyn in nPa.cInferred value. See section 3.1 for detail.

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G. Khmyrov for the BinBrowser analysis support. J.-N Tu thanks V. M.Vasyliu�nas for his suggestions and comments.[37] Arthur Richmond thanks the reviewers for their assistance in

evaluating this paper.

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�����������������������H. U. Frey, Space Sciences Laboratory, University of California,

Berkeley, 7 Gauss Way, Berkeley, CA 94720, USA.J. L. Green, NASA Goddard Space Flight Center, MC 630, Bldg 26,

Greenbelt, MD 20771, USA.X. Huang, B. W. Reinisch, P. Song, and J. Tu, Center for Atmospheric

Research, University of Massachusetts-Lowell, 600 Suffolk Street, Lowell,MA 01854, USA. (jiannan_tu@uml.edu)P. H. Reiff, Department of Physics and Astronomy, Rice University, 6100

Main Street, Houston, TX 77005, USA.

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