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Dynamics of the auroral electrojets and their mapping to the magnetosphere
Radiation Measurements 30 (1999) 579±587
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Dynamics of the auroral electrojets and their mapping to the magnetosphere Y.I. Feldstein a,*, L.I. Gromova a, A. Grafe b, C.-I. Meng c, V.V. Kalegaev d, I.I. Alexeev d, Yu.P. Sumaruk e a IZMIRAN, 142092 Troitsk, Moscow Region, Russia GeoResearch Centre Potsdam, Adolf Schmidt Observatory, D-14823 Niemegk, Germany c John Hopkins University, Applied Physics Laboratory, Laurel, MD 20723-6099, USA d Institute of Nuclear Physics, Moscow State University, Moscow 119899, Russia e Lvov Magnetic Observatory, 292196 Lvov Region, Ukraine
b
Received 7 May 1998; accepted 18 December 1998
Abstract Data of the EISCAT and IMAGE magnetic observatories chains in combination with data of three Russian observatories (St. Petersburg, Borok and Moscow) were used to determine the eastward and westward electrojet dynamics in the course of magnetic storms. During the storm main phase and maximum substorm intensity the eastward electrojet is located at latitudes lower than usual. During intervals between substorms the westward electrojet centre shifts equatorwards as Dst increases. At a substorm maximum the westward electrojet widens polewards. The spectrograms of precipitating electrons and ions of auroral energies obtained onboard the DMSP F8, F10 and F11 satellites allow to connect the regions of the electrojet location with characteristic plasma structures at ionospheric altitudes. The eastward electrojet in the evening sector is located in the region of diuse electron precipitations. The electrojet centre coincides with the latitude of an energy ¯ux maximum of auroral protons. In the course of substorms the westward electrojet at the nightside is located at latitudes of both diuse and discrete electron precipitations. The electrojets and plasma region boundaries are mapped to the magnetosphere. The paraboloid model of the magnetosphere is used here. The in¯uence of paraboloid model input parameters on the dayside cusp latitude, on the ionospheric boundaries between open and closed as well as dipole-like and tail-like ®eld lines is considered. It is shown that tail currents in¯uence magnetic ®eld line con®guration in the nightside magnetosphere stronger than the ring current. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Geophysical phenomena accompanying the occurence of magnetic storms are well observed equatorwards of auroral latitudes 658 < F < 678. During very strong magnetic storms the aurora was observed at the geomagnetic latitudes lower than F 0 508 (Chapman
* Corresponding author.
and Akasofu, 1964). The auroral electrojet location as a function of Dst index describing the symmetrical part of magnetic ®eld variation at low latitudes was investigated by Khorosheva (1986, 1987), Weimer et al. (1990) and Feldstein (1992). Below we continue documenting the electrojet positions based on observational data of meridional magnetic observatory chains, EISCAT and IMAGE (Luehr et al., 1984; Viljanen and Hakkinen, 1996), as well as on measurements of middle latitude and subauroral
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Russian magnetic observatories near 110 geomagnetic meridian. Special attention is paid to investigation of the relationship between electrojet locations and spectral features of auroral plasma precipitating particles. Dronov et al. (1988), using the Kosmos-900 satellite measurements, showed that the eastward electrojet is located near the ring current main peak region, characterized by precipitation of protons with energy 50 KeV < Ep < 80 KeV. The latitudinal maxima of electron and proton precipitation lies within the westward electrojet region. At the magnetic storm recovery phase betatron acceleration of radiation belt electrons associated with ring current magnetic ®eld decrease occurs. The largest change of the geomagnetic ®eld takes place in the region of ring current maximum. The maximum of accelerated electrons with energies Ee > 1MeV is located in this region. Tverskaya (1986, 1996) obtained the nonlinear relation between vDstv and Lmax of relativistic electron penetration from the external radiation belt in the form vDstvmax=2.75 10/L4max. This empirical relation can be used to predict the location of the L-shell of the ring current in the magnetosphere depending on the largest vDstv value in the course of magnetic storm. Data of the meridional magnetic observatory chain allows more precise determination of the auroral electrojet boundary locations, while DMSP satellite spectrograms with 1 s temporal resolution for electrons and ions with energies between 10 eV and 20 keV make possible a state-of-the-art identi®cation of plasma precipitation regions with various spectral and structural characteristics. Mapping of the electrojet boundaries from ionospheric altitude to the magnetosphere reveals locations of plasma structures connected with electrojets by magnetic ®eld lines. The paraboloid model of magnetospheric magnetic ®eld (Alexeev et al., 1996) is used for the mapping. Compared with other models this one allows, in more detail, to describe the magnetospheric magnetic ®eld dynamics depending on several physically important magnetospheric parameters. In the framework of this model the in¯uence of the model input parameters on the location of characteristic magnetic ®eld lines in the magnetosphere is considered. 2. Auroral electrojet dynamics in the course of magnetic storms Variations of the northern (X ) component of the geomagnetic ®eld along the IMAGE magnetometers chain were used to calculate the auroral electrojet dynamics during the magnetic storm on 5±7 February 1994. Consideration of dynamics of the westward elec-
Fig. 1. The position of the eastward (a) and westward (b) electrojet centres as a function of Dst intensity. Line 1 for the westward electrojet was obtained by the least-squares method. The lines marked by 2 present the position of maximum of storm-time injected (>1 MeV) electrons (Tverskaya, 1986).
trojet for quiet intervals between substorms and that of the eastward electrojet shows that the electrojet centres shift equatorwards during magnetic storms. Figs. 1(a) and (b) show the dependencies of such shifts on the Dst index. When vDstv increases, the electrojet centres (marked by crosses) shift equatorwards. The line marked by 1 in Fig. 1(b) obtained by the leastsquares method characterizes the linear dependence between the geomagnetic latitude of westward electrojet centre and the Dst intensity. The eastward electrojet centre shifts equatorwards faster when Dst is small. For both electrojets equatorward shift begins from auroral zone latitudes (F 0658±678) and with Dst 0 ÿ250 nT it ends at F 0 558. The electrojet centre locations depending on Dst obtained from the relation by Tverskaya (1986) are shown in Figs. 1(a) and (b) by lines marked by 2. The westward electrojet in our data is located slightly to
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Fig. 2. The latitudes of centre (C) and boundaries (E is equatorial and P is polar) of eastward electrojets in the evening sector and the location of plasma domain boundaries obtained using DMSP F08 and F10 measurements according to Newell et al. (1996) classi®cation in the course of magnetic storms on 6±7 February, 21 February 1994 and 10 May 1992. Here b1e, b1i are boundaries of the `zero-energy', usually determined by the softness channels; b2e is a point where the electron average energy remains constant with latitude; b2i is the boundary of precipitating ion with energy ¯ux maximum; b3a is the equatorwar boundary of the region where the electron acceleration events are identi®ed (the equatorial boundary of the structural auroral forms); b3b is the poleward boundary of the region where the electron acceleration events are identi®ed (the polar boundary of the discrete auroral forms). Hourly mean Dst values correspond to UT of the passes.
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Fig. 3. The same as in Fig. 2, for the westward electrojets during magnetic storms on 6±7 February 1994 and 10 May 1992 in the morning sector. In addition to the plasma domain boundaries b1±b3, the position of the b6 boundary is shown (b6 is the poleward boundary of subvisual drizzle).
the pole, and the eastward one is by 18 equatorwards from the relativistic electron maximum. The location of the westward electrojet centres are in a good agreement with the latitudinal maximum of the relativistic electrons with energies Ee 0 1.5 MeV obtained for the magnetic storm recovery phase. This is apparently due
to the fact that we determine the westward current centre between substorms, i.e. during relatively quite conditions. The location of the eastward electrojet centre was determined at the moment of the maximum substorm development at the magnetometers chain and therefore it proved to be somewhat shifted equator-
Y.I. Feldstein et al. / Radiation Measurements 30 (1999) 579±587
wards relative to the westward current location at the same values of Dst. 3. Auroral electrojets and precipitating charged particles of auroral energy It has long been known that auroral electrojets are located in the region of the most frequent appearance of aurorae produced by precipitating plasma particles with energies between 10 eV and 10 KeV. As a rule, the eastward electrojet is observed at the evening (MLT) hours and westward ones Ð along the auroral oval with the highest intensity at near midnight and early morning hours of MLT. Structure of the auroral precipitation region in the auroral oval night sector has been discussed in detail by Feldstein and Galperin (1985). Boundaries of various structural regions with dierent physical characteristics of precipitating plasma ¯uxes and their energy spectra in the interval from 10 eV to 10 KeV (Galperin and Feldstein, 1991, 1996) are accepted below in accordance with classi®cation by Newell et al. (1996) and Feldstein and Galperin (1996). Newell et al. (1996) have proposed the following identi®cation of the plasma regions in the auroral oval night sector. The b1e boundary is a `zero-energy' electron boundary, usually determined by the 32 and 47 eV electron channels; the b2i boundary is a region of energy ¯ux maximum of precipitating ions, which is also the isotropy boundary for ions and the ionospheric projection of the geotail current sheet inner boundary; the b2e boundary is a point where the average electron energy is neither increasing, nor decreasing with latitude (the start of the main (central) plasma sheet precipitation); b3a is the equatorward boundary where electron acceleration events are identi®ed, the equatorial boundary of the structural auroral forms (the equatorward auroral oval boundary); b3b is the poleward boundary of the region where electron acceleration events are identi®ed, the polar boundary of discrete auroral forms (auroral oval poleward boundary); b6 is the poleward boundary of subvisual drizzle roughly adjacent to the auroral oval. At near midnight hours the b2e and b3a boundaries are very often practically identical but diverge by several degrees in latitude at evening and morning hours. Figure 2 shows the boundaries for the eastward electrojet obtained by the magnetometer chain, and location of plasma domain boundaries based on DMSP F08 and F10 satellite data during the magnetic storms on 6±7 February 1994, 10 May 1992 and 21 February 1994. The eastward electrojet in the evening sector is located either entirely equatorwards of the discrete auroral forms region (Fp < Fb3a), or its poleward boundary is placed between the discrete precipitation region
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boundaries (Fb3a < Fp < Fb3b). The electrojet centre falls on the b2i boundary latitude (or near this latitude), the equatorward boundary practically coincides with the boundary b1e of plasma precipitation, which is apparently the projection of the plasmapause to ionospheric altitudes. Thus, the eastward electrojet is mainly located in the region of diuse auroral luminosity (equatorwards of b3a ), but sometimes the eastward electrojet poleward part can cover the region of discrete precipitation between the b3a and b3b boundaries as well. The eastward electrojet poleward boundary, as a rule, is located equatorwards of b3b, i.e. structured auroral precipitation encompasses the region polewards of the eastward electrojet. Thus, the eastward electrojet is mainly located more equatorwards of the region with auroral discrete forms, i.e. equatorwards of the auroral oval. The eastward electrojet electric currents do not ¯ow in the auroral oval as it has been assumed by Rostoker et al. (1997). The eastward electrojet is mapped by magnetic ®eld lines in the ring current located in the inner magnetosphere earthwards from the central plasma sheet to which the auroral oval is mapped. In a wide interval of Dst intensity variations, it can be assumed in the ®rst approximation that the eastward electrojet equatorial boundary coincides with the plasmapause projection to ionospheric altitudes and the electrojet centre coincides with the latitude of ions with energy ¯ux maximum precipitating to the upper atmosphere. The latitude of this maximum lies equatorwards of the ionospheric projection of the magnetospheric tail plasma sheet inner boundary. The eastward electrojet in the evening sector of the magnetosphere is associated with processes in the inner magnetospheric regions adjacent to the plasma sheet inner boundary. During the storm on 5±7 February 1994 the DMSP F08 orbits intersected the westward electrojet at 005:00 MLT. Fig. 3 presents the westward electrojet centre, equatorward and poleward boundaries for the IMAGE meridian at the moment of the satellite intersection of the auroral region morning sector. The electrojets are located entirely in the region of auroral plasma precipitations covering both diuse and discrete precipitations. In this case precipitations extend to latitudes lower than the electrojet equatorward boundary. The electrojet centre coincides with or lies in the vicinity of the auroral discrete forms equatorial boundary (b3a ). The electrojet poleward boundary mainly coincides with the poleward boundary of the auroral discrete forms (b3b ). During periods of very intense magnetic storms the westward current in the late evening hours covers the whole auroral region. For the storm on 10 May 1992, the DMSP F08 satellite intersected the evening sector at 18.01 UT, i.e. near the moment of the substorm development maximum at 18.06 UT and for
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Fig. 4. Results of mapping the centre and boundaries of the eastward electrojets to the magnetospheric equator in the meridional cross-section using the paraboloid model of the magnetospheric magnetic ®eld for 21 February 1994, 14.9 UT (17.9 MLT, Dst=ÿ90 nT), (a), and 10 May 1992, 15 UT (18 MLT, Dst=ÿ290 nT), (b).
Dst=ÿ200 nT. Fig. 3 shows the locations of plasma boundaries for this pass. Practically the whole electrojet, including its equatorward and poleward boundaries, is located at discrete auroral forms latitudes in the evening sector.
4. Electrojets mapping to magnetosphere Latitudinal intervals where electrojets are located were mapped to the magnetosphere. The paraboloid model of the magnetic ®eld in the magnetosphere (Alexeev et al., 1996) is used here for mapping. The input parameters of this model are physical magnetospheric characteristics: the geomagnetic dipole axis orientation (tilt angle c ); the magnetic ®eld ¯ux in tail lobes (F1); the maximum ring current magnetic ®eld intensity (Brc ); geocentric distances to the subsolar
point on the magnetopause (R1) and to the earthward boundary of the tail current (R2). These characteristics can be determined from observational data (solar wind parameters, geomagnetic indices, boundaries of auroral precipitation regions). To determine the midnight and noon auroral oval boundaries based on Newell et al. (1996) identi®cation, data of DMSP F08 and DMSP F10 are used. Figs. 4 and 5 show correspondingly mapping results in the meridional magnetospheric cross-sections for the eastward and westward electrojets. The eastward electrojet is mapped along quasidipole magnetic ®eld lines to the inner magnetosphere in the evening sector (MLT 1 18 h). The centres of source regions of precipitating ions with energy ¯ux maximum are located at geocentric distance of 03.2 RE for Dst=ÿ290 nT (Fig. 4(a)) and 03.8 RE for Dst=ÿ90 nT (Fig. 4(b)). The distance between projections of poleward and equatorward boundaries of the eastward electrojet is 01.2 RE independent of Dst intensity. Geocentric distances of 0(3±5.5)RE are typical for the ring current position in the magnetosphere during magnetic storms intervals. Such spatial coincidence of the ring current and projection of the eastward electrojet to the magnetosphere gives additional evidence in favour of coupling between physical processes leading to the eastward electrojet formation and the ring current generation in the magnetospheric evening sector. The westward electrojet is mapped to the nightside magnetosphere near midnight. For Dst=ÿ200 nT, the equatorward boundary is mapped to the magnetosphere along quasidipole magnetic ®eld lines at 04RE (see Fig. 5(a)). The ®eld lines threading the electrojet centre and poleward boundary are extended to the magnetospheric tail at distances >50 RE. At these distances the magnetic ®eld intensity on the ®eld lines intersecting the electrojet centre is 034 nT. The geomagnetic ®eld intensity in the ring current region at the equator is 01100 nT. At Dst=ÿ120 nT the westward electrojet equatorward boundary is extended to the magnetospheric tail at a distance of 07 RE (see Fig. 5(a)). Although the electrojet centre and the poleward boundary are extended to the distant tail the main part of the westward electrojet's related cross-tail currents ¯ows at distances close than 50 RE. The geomagnetic ®eld intensities on the ®eld lines intersecting the electrojet centre at the distances of 50 RE and at ring current centre are 027 nT and 0800 nT, correspondingly. The magnetic ®eld values are in a good agreement with GEOTAIL measurements in disturbed magnetosphere (Kokubun et al., 1996). The paraboloid model allows to separate the impact of dierent magnetospheric parameters on the largescale magnetospheric structure peculiarities. For 21 February 1995 event at 21.05 UT, Table 1 lists geo-
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Fig. 5. The same as in Fig. 4, for the westward electrojets for 6 February 1994, 21.4 UT (0.4 MLT, Dst=ÿ120 nT), (a), and 10 May 1992, 18.6 UT (21.6 MLT, Dst=ÿ200 nT), (b).
magnetic latitudes of the last (high-latitudinal) closed magnetic ®eld line at noon (1), the last quasidipolar ®eld line at midnight (2), and the last ®eld line, which is closed via the magnetospheric tail plasma sheet (3) calculated in the framework of the paraboloid model when the input parameters of the paraboloid model are varied. We can see that: 1. R1 variation, when it increases from 6.5 RE to 9.0 RE, causes poleward shifts of the closed/open ®eld lines boundary by 9.78, dayside cusp by 1.98 and the nightside dipole line by 6.28; 2. R2 variation, when it increases from 4.25 RE to 6.0 RE, causes poleward shifts of the last midnight dipole line by 3.48 and the dayside cusp by 0.58; the closed/open ®eld lines boundary shifts equatorwards by 2.78; 3. Brc variation, when the ring current ®eld intensity
decreases from ÿ40 nT to ÿ150 nT, in¯uences the boundary location weaker than geocentric distances R1 and R2. The region with dipole magnetic ®eld lines at the night and dayside cusp shift equatorwards by 18 and by 1.38, respectively. The auroral oval diameter increases by 2.38 when the ring current magnetic ®eld reaches a value of ÿ150 nT. This diameter enhancement is comparable with those obtained by Schulz (1997). The closed/open ®eld line boundary at the nightside magnetosphere shifts polewards by 2.28. 4. F1 variation leads to the most signi®cant shift of boundaries between regions with magnetic ®eld lines of dierent character, especially at the night side. The auroral oval diameter increases by 9.78. Thus, in the framework of the paraboloid model of the magnetospheric magnetic ®eld the in¯uence of the ring current on the boundaries of structural regions
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Table 1 The ionospheric projections of the near noon magnetopause, geotail current sheet inner boundary and the boundary between closed and open ®eld lines for 21 February 1995 event at 21.05 UT. The results of calculation in the framework of paraboloid model with dierent input parameters R1 (RE )
R2 (RE )
F1 (Wb)
Brc (nT)
Noon closed
Night dipole
Night last closed
Dier. R1 6.5 7.5 9.0
4.25 4.25 4.25
ÿ100 ÿ100 ÿ100
75.58 76.28 77.48
60.88 62.78 67.08
72.28 75.98 81.98
7.5 7.5 7.5
4.25 4.80 6.00
0.0786E+10 0.0524E+10 0.0210E+10 Dier. R2 0.1047E+10 0.1083E+10 0.1156E+10
76.68 74.88 75.18
58.78 59.98 62.18
68.78 67.68 66.08
7.5 7.5 7.5
4.25 4.25 4.25
ÿ100 ÿ100 ÿ100 Dier. Brc ÿ40 ÿ100 ÿ150
75.38 74.68 74.08
59.28 58.78 58.28
67.28 68.78 69.48
7.5 7.5 7.5
4.25 4.25 4.25
ÿ100 ÿ100 ÿ100
74.68 76.28 77.08
58.78 60.08 66.08
68.78 75.98 82.18
0.1047E+10 0.1047E+10 0.1047E+10 Dier. F1 0.1047E+10 0.0524E+10 0.0262E+10
in the magnetosphere is substantially weaker than the impact of magnetospheric tail current.
5. Conclusions 1. A prominent equatorward shift of eastward electrojets occurs during magnetic storms. In the course of substorms the westward electrojet widens polewards covering auroral latitudes. 2. Comparison with auroral plasma precipitations showed that the eastward electrojet in the course of magnetic storms is located predominantly at latitudes of diuse precipitations equatorwards of the discrete auroral forms region. This region is mapped along magnetic lines to the inner magnetosphere at geocentric distances of 2.5±4 RE, between the plasmapause and plasma sheet inner boundary in the magnetospheric tail. The eastward electrojet centre coincides with the region of maximum energy ¯ux of ions precipitating into the upper atmosphere. 3. The westward electrojet is located in the latitudinal range of auroral plasma precipitations covering regions with both diuse and discrete auroral forms. The electrojet centre coincides with or lies in the vicinity of the electron acceleration events boundary (the equatorial boundary of the auroral oval of discrete auroral forms, b3a ). 4. Mapping of the electrojet boundaries from ionospheric altitude to the magnetosphere using paraboloid model of the magnetic ®eld reveals locations of plasma structures connected with electrojets by magnetic ®eld lines. During magnetic storms in the sub-
storms intervals the eastward electrojet is mapped into the ring current region and the westward electrojet is mapped to the large part of the nightside magnetosphere. 5. The ring current and the magnetospheric tail currents growth leads to alterations of the magnetospheric magnetic ®eld lines con®guration. The ionospheric projections of the near noon magnetopause (boundary 1) and the tail current sheet inner boundary (boundary 2) shift equatorwards. A volume of the inner magnetosphere with quasidipole ®eld lines decreases. The tail currents in¯uence on the positions of boundaries 1 and 2 is substantially more eective than the ring current impact. In the nightside magnetosphere these currents in¯uence in opposite directions the location of the boundary between closed and open ®eld lines (boundary 3). 6. Variations of the solar wind, electric ®elds in the magnetospheric tail and plasma injections from the tail to the inner magnetosphere aect geocentric distances to the subsolar magnetopause point, R1, and to the magnetospheric tail current sheet inner boundary, R2. An increase of R1 leads to a poleward shift of all three characteristic magnetospheric boundaries. Moreover, the dayside cusp boundary shift is three times smaller than a similar shift of the inner magnetosphere boundary in the night sector. An increase of R2 leads to a poleward shift of both the dayside cusp boundary and the nightside boundary of inner magnetosphere. This shift at the nightside is substantially more prominent than in the day hours.
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Acknowledgements We appreciate very much fruitful discussions with H. Luehr. We are grateful to the EISCAT and IMAGE magnetometer chain teams. The IMAGE magnetometer data used in this paper were compiled in the scope of the German±Finnish±Norwegian± Polish project conducted by the Technical University of Braunschweig. This work has been supported by RFBR grants 96-05-66279 and 98-05-64784, and INTAS-RFBR-95-0932 grant. References Alexeev, I.I., Belenkaya, E.S., Kalegaev, V.V., Feldstein, Ya.I., Grafe, A., 1996. Magnetic storms and magnetotail currents. Journal of Geophysical Research 101, 7737±7747. Chapman, S., Akasofu, S.-I., 1964. The Aurora, Research in Geophysics, V.1: Sun, Apper Atmosphere, and Space. The MIT Press, pp. 367±400 Chapter 15. Dronov, A.V., Morozova, T.I., Sosnovetz, E.H., Tverskaya, L.V., Tulupov, V.I., Khorosheva, O.V., 1988. Relationship between the ring current, electron precipitation regions and aurora luminosity in the dusk-midnight sector of the magnetosphere. Geomagnetism and Aeronomy 28, 1011± 1013. Feldstein, Ya.I., 1992. Modelling of the magnetic ®eld of magnetospheric ring current as a function of interplanetary medium. Space Science Review 59, 83±165. Feldstein, Ya.I., Galperin, Yu.I., 1985. The auroral luminosity structure in the high-latitude upper atmosphere: its dynamics and relationship to the large-scale structure of the Earth's magnetosphere. Review of Geophysics and Space Physics 23 (3), 217±275. Feldstein, Ya.I., Galperin, Yu.I., 1996. The auroral precipitations structure in the magnetosphere night sector. Cosmic Research 34 (3), 227±247. Galperin, Yu.I., Feldstein, Ya.I., 1991. Auroral luminosity and its relationship to magnetospheric plasma domains. In: Meng, C.-I., Rycroft, M.J., Frank, L.A. (Eds.), Auroral Physics. Cambridge UP, pp. 207±222.
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Galperin, Yu.I., Feldstein, Ya.I., 1996. Mapping of the precipitation region to the plasma sheet. Journal of Geomagnetism and Geoelectricity 48 (56), 857±875. Khorosheva, O.V., 1986. Relation of the magnetospheric disturbances to the parameters of the interplanetary medium. Geomagnetism and Aeronomy 26, 447±453. Khorosheva, O.V., 1987. Magnetospheric disturbances and the associated dynamics of ionospheric electrojets, aurorae and plasmapause. Geomagnetism and Aeronomy 27, 804± 811. Kokubun, S., Frank, L.A., Hayashi, K., Kamide, Y., Lepping, R.P., Mukai, T., Nakamura, R., Paterson, W.R., Yamamoto, T., Yumoto, K., 1996. Large ®eld events in the distant magnetotail during magnetis storms. Journal of Geomagnetism and Geoelectricity 48 (56), 561±576. Luehr, H., Thurey, S., Klocker, N., 1984. The EISCAT-magnetometer cross, operational aspects Ð ®rst results. Geophysical Surveys 6, 305±315. Newell, P.T., Feldstein, Ya.I., Galperin, Yu.I., Meng, C.-I., 1996. The morphology of nightside precipitation. Journal of Geophysical Research 101, 10737±10748. Rostoker, G., Friedrich, E., Dobs, M., 1997. Physics of magnetic storms. In: Tsurutani, B.T., Gonzalez, W.D., Kamide, Y., Arballo, J.K. (Eds.), Geophysical Monograph, vol. 98. AGU, pp. 149±160. Schulz, M., 1997. Direct in¯uence of ring current on auroral oval diameter. Journal of Geophysical Research 102, 14149±14154. Tverskaya, L.V., 1986. On the boundary of electron injection to the Earth's magnetosphere. Geomagnetism and Aeronomy 26, 864±865. Tverskaya, L.V., 1996. The latitude position dependence of the relativistic electron maximum as a function of Dst. Advances in Space Research 18 (8), 135±138. Viljanen, A., Hakkinen, L., 1996. IMAGE magnetometer network. In: Proceedings of the 2nd International Workshop for the co-ordination of ground-based observations and Claster. Weimer, D.R., Reinleitner, L.A., Kan, J.R., Zhu, L., Akasofu, S.-I., 1990. Saturation of the auroral electrojet current and the polar cap potential. Journal of Geophysical Research 95, 18981±18988.
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Dynamics of the auroral electrojets and their mapping to the magnetosphere Y.I. Feldstein a,*, L.I. Gromova a, A. Grafe b, C.-I. Meng c, V.V. Kalegaev d, I.I. Alexeev d, Yu.P. Sumaruk e a IZMIRAN, 142092 Troitsk, Moscow Region, Russia GeoResearch Centre Potsdam, Adolf Schmidt Observatory, D-14823 Niemegk, Germany c John Hopkins University, Applied Physics Laboratory, Laurel, MD 20723-6099, USA d Institute of Nuclear Physics, Moscow State University, Moscow 119899, Russia e Lvov Magnetic Observatory, 292196 Lvov Region, Ukraine
b
Received 7 May 1998; accepted 18 December 1998
Abstract Data of the EISCAT and IMAGE magnetic observatories chains in combination with data of three Russian observatories (St. Petersburg, Borok and Moscow) were used to determine the eastward and westward electrojet dynamics in the course of magnetic storms. During the storm main phase and maximum substorm intensity the eastward electrojet is located at latitudes lower than usual. During intervals between substorms the westward electrojet centre shifts equatorwards as Dst increases. At a substorm maximum the westward electrojet widens polewards. The spectrograms of precipitating electrons and ions of auroral energies obtained onboard the DMSP F8, F10 and F11 satellites allow to connect the regions of the electrojet location with characteristic plasma structures at ionospheric altitudes. The eastward electrojet in the evening sector is located in the region of diuse electron precipitations. The electrojet centre coincides with the latitude of an energy ¯ux maximum of auroral protons. In the course of substorms the westward electrojet at the nightside is located at latitudes of both diuse and discrete electron precipitations. The electrojets and plasma region boundaries are mapped to the magnetosphere. The paraboloid model of the magnetosphere is used here. The in¯uence of paraboloid model input parameters on the dayside cusp latitude, on the ionospheric boundaries between open and closed as well as dipole-like and tail-like ®eld lines is considered. It is shown that tail currents in¯uence magnetic ®eld line con®guration in the nightside magnetosphere stronger than the ring current. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Geophysical phenomena accompanying the occurence of magnetic storms are well observed equatorwards of auroral latitudes 658 < F < 678. During very strong magnetic storms the aurora was observed at the geomagnetic latitudes lower than F 0 508 (Chapman
* Corresponding author.
and Akasofu, 1964). The auroral electrojet location as a function of Dst index describing the symmetrical part of magnetic ®eld variation at low latitudes was investigated by Khorosheva (1986, 1987), Weimer et al. (1990) and Feldstein (1992). Below we continue documenting the electrojet positions based on observational data of meridional magnetic observatory chains, EISCAT and IMAGE (Luehr et al., 1984; Viljanen and Hakkinen, 1996), as well as on measurements of middle latitude and subauroral
1350-4487/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 4 4 8 7 ( 9 9 ) 0 0 2 1 9 - X
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Russian magnetic observatories near 110 geomagnetic meridian. Special attention is paid to investigation of the relationship between electrojet locations and spectral features of auroral plasma precipitating particles. Dronov et al. (1988), using the Kosmos-900 satellite measurements, showed that the eastward electrojet is located near the ring current main peak region, characterized by precipitation of protons with energy 50 KeV < Ep < 80 KeV. The latitudinal maxima of electron and proton precipitation lies within the westward electrojet region. At the magnetic storm recovery phase betatron acceleration of radiation belt electrons associated with ring current magnetic ®eld decrease occurs. The largest change of the geomagnetic ®eld takes place in the region of ring current maximum. The maximum of accelerated electrons with energies Ee > 1MeV is located in this region. Tverskaya (1986, 1996) obtained the nonlinear relation between vDstv and Lmax of relativistic electron penetration from the external radiation belt in the form vDstvmax=2.75 10/L4max. This empirical relation can be used to predict the location of the L-shell of the ring current in the magnetosphere depending on the largest vDstv value in the course of magnetic storm. Data of the meridional magnetic observatory chain allows more precise determination of the auroral electrojet boundary locations, while DMSP satellite spectrograms with 1 s temporal resolution for electrons and ions with energies between 10 eV and 20 keV make possible a state-of-the-art identi®cation of plasma precipitation regions with various spectral and structural characteristics. Mapping of the electrojet boundaries from ionospheric altitude to the magnetosphere reveals locations of plasma structures connected with electrojets by magnetic ®eld lines. The paraboloid model of magnetospheric magnetic ®eld (Alexeev et al., 1996) is used for the mapping. Compared with other models this one allows, in more detail, to describe the magnetospheric magnetic ®eld dynamics depending on several physically important magnetospheric parameters. In the framework of this model the in¯uence of the model input parameters on the location of characteristic magnetic ®eld lines in the magnetosphere is considered. 2. Auroral electrojet dynamics in the course of magnetic storms Variations of the northern (X ) component of the geomagnetic ®eld along the IMAGE magnetometers chain were used to calculate the auroral electrojet dynamics during the magnetic storm on 5±7 February 1994. Consideration of dynamics of the westward elec-
Fig. 1. The position of the eastward (a) and westward (b) electrojet centres as a function of Dst intensity. Line 1 for the westward electrojet was obtained by the least-squares method. The lines marked by 2 present the position of maximum of storm-time injected (>1 MeV) electrons (Tverskaya, 1986).
trojet for quiet intervals between substorms and that of the eastward electrojet shows that the electrojet centres shift equatorwards during magnetic storms. Figs. 1(a) and (b) show the dependencies of such shifts on the Dst index. When vDstv increases, the electrojet centres (marked by crosses) shift equatorwards. The line marked by 1 in Fig. 1(b) obtained by the leastsquares method characterizes the linear dependence between the geomagnetic latitude of westward electrojet centre and the Dst intensity. The eastward electrojet centre shifts equatorwards faster when Dst is small. For both electrojets equatorward shift begins from auroral zone latitudes (F 0658±678) and with Dst 0 ÿ250 nT it ends at F 0 558. The electrojet centre locations depending on Dst obtained from the relation by Tverskaya (1986) are shown in Figs. 1(a) and (b) by lines marked by 2. The westward electrojet in our data is located slightly to
Y.I. Feldstein et al. / Radiation Measurements 30 (1999) 579±587
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Fig. 2. The latitudes of centre (C) and boundaries (E is equatorial and P is polar) of eastward electrojets in the evening sector and the location of plasma domain boundaries obtained using DMSP F08 and F10 measurements according to Newell et al. (1996) classi®cation in the course of magnetic storms on 6±7 February, 21 February 1994 and 10 May 1992. Here b1e, b1i are boundaries of the `zero-energy', usually determined by the softness channels; b2e is a point where the electron average energy remains constant with latitude; b2i is the boundary of precipitating ion with energy ¯ux maximum; b3a is the equatorwar boundary of the region where the electron acceleration events are identi®ed (the equatorial boundary of the structural auroral forms); b3b is the poleward boundary of the region where the electron acceleration events are identi®ed (the polar boundary of the discrete auroral forms). Hourly mean Dst values correspond to UT of the passes.
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Fig. 3. The same as in Fig. 2, for the westward electrojets during magnetic storms on 6±7 February 1994 and 10 May 1992 in the morning sector. In addition to the plasma domain boundaries b1±b3, the position of the b6 boundary is shown (b6 is the poleward boundary of subvisual drizzle).
the pole, and the eastward one is by 18 equatorwards from the relativistic electron maximum. The location of the westward electrojet centres are in a good agreement with the latitudinal maximum of the relativistic electrons with energies Ee 0 1.5 MeV obtained for the magnetic storm recovery phase. This is apparently due
to the fact that we determine the westward current centre between substorms, i.e. during relatively quite conditions. The location of the eastward electrojet centre was determined at the moment of the maximum substorm development at the magnetometers chain and therefore it proved to be somewhat shifted equator-
Y.I. Feldstein et al. / Radiation Measurements 30 (1999) 579±587
wards relative to the westward current location at the same values of Dst. 3. Auroral electrojets and precipitating charged particles of auroral energy It has long been known that auroral electrojets are located in the region of the most frequent appearance of aurorae produced by precipitating plasma particles with energies between 10 eV and 10 KeV. As a rule, the eastward electrojet is observed at the evening (MLT) hours and westward ones Ð along the auroral oval with the highest intensity at near midnight and early morning hours of MLT. Structure of the auroral precipitation region in the auroral oval night sector has been discussed in detail by Feldstein and Galperin (1985). Boundaries of various structural regions with dierent physical characteristics of precipitating plasma ¯uxes and their energy spectra in the interval from 10 eV to 10 KeV (Galperin and Feldstein, 1991, 1996) are accepted below in accordance with classi®cation by Newell et al. (1996) and Feldstein and Galperin (1996). Newell et al. (1996) have proposed the following identi®cation of the plasma regions in the auroral oval night sector. The b1e boundary is a `zero-energy' electron boundary, usually determined by the 32 and 47 eV electron channels; the b2i boundary is a region of energy ¯ux maximum of precipitating ions, which is also the isotropy boundary for ions and the ionospheric projection of the geotail current sheet inner boundary; the b2e boundary is a point where the average electron energy is neither increasing, nor decreasing with latitude (the start of the main (central) plasma sheet precipitation); b3a is the equatorward boundary where electron acceleration events are identi®ed, the equatorial boundary of the structural auroral forms (the equatorward auroral oval boundary); b3b is the poleward boundary of the region where electron acceleration events are identi®ed, the polar boundary of discrete auroral forms (auroral oval poleward boundary); b6 is the poleward boundary of subvisual drizzle roughly adjacent to the auroral oval. At near midnight hours the b2e and b3a boundaries are very often practically identical but diverge by several degrees in latitude at evening and morning hours. Figure 2 shows the boundaries for the eastward electrojet obtained by the magnetometer chain, and location of plasma domain boundaries based on DMSP F08 and F10 satellite data during the magnetic storms on 6±7 February 1994, 10 May 1992 and 21 February 1994. The eastward electrojet in the evening sector is located either entirely equatorwards of the discrete auroral forms region (Fp < Fb3a), or its poleward boundary is placed between the discrete precipitation region
583
boundaries (Fb3a < Fp < Fb3b). The electrojet centre falls on the b2i boundary latitude (or near this latitude), the equatorward boundary practically coincides with the boundary b1e of plasma precipitation, which is apparently the projection of the plasmapause to ionospheric altitudes. Thus, the eastward electrojet is mainly located in the region of diuse auroral luminosity (equatorwards of b3a ), but sometimes the eastward electrojet poleward part can cover the region of discrete precipitation between the b3a and b3b boundaries as well. The eastward electrojet poleward boundary, as a rule, is located equatorwards of b3b, i.e. structured auroral precipitation encompasses the region polewards of the eastward electrojet. Thus, the eastward electrojet is mainly located more equatorwards of the region with auroral discrete forms, i.e. equatorwards of the auroral oval. The eastward electrojet electric currents do not ¯ow in the auroral oval as it has been assumed by Rostoker et al. (1997). The eastward electrojet is mapped by magnetic ®eld lines in the ring current located in the inner magnetosphere earthwards from the central plasma sheet to which the auroral oval is mapped. In a wide interval of Dst intensity variations, it can be assumed in the ®rst approximation that the eastward electrojet equatorial boundary coincides with the plasmapause projection to ionospheric altitudes and the electrojet centre coincides with the latitude of ions with energy ¯ux maximum precipitating to the upper atmosphere. The latitude of this maximum lies equatorwards of the ionospheric projection of the magnetospheric tail plasma sheet inner boundary. The eastward electrojet in the evening sector of the magnetosphere is associated with processes in the inner magnetospheric regions adjacent to the plasma sheet inner boundary. During the storm on 5±7 February 1994 the DMSP F08 orbits intersected the westward electrojet at 005:00 MLT. Fig. 3 presents the westward electrojet centre, equatorward and poleward boundaries for the IMAGE meridian at the moment of the satellite intersection of the auroral region morning sector. The electrojets are located entirely in the region of auroral plasma precipitations covering both diuse and discrete precipitations. In this case precipitations extend to latitudes lower than the electrojet equatorward boundary. The electrojet centre coincides with or lies in the vicinity of the auroral discrete forms equatorial boundary (b3a ). The electrojet poleward boundary mainly coincides with the poleward boundary of the auroral discrete forms (b3b ). During periods of very intense magnetic storms the westward current in the late evening hours covers the whole auroral region. For the storm on 10 May 1992, the DMSP F08 satellite intersected the evening sector at 18.01 UT, i.e. near the moment of the substorm development maximum at 18.06 UT and for
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Fig. 4. Results of mapping the centre and boundaries of the eastward electrojets to the magnetospheric equator in the meridional cross-section using the paraboloid model of the magnetospheric magnetic ®eld for 21 February 1994, 14.9 UT (17.9 MLT, Dst=ÿ90 nT), (a), and 10 May 1992, 15 UT (18 MLT, Dst=ÿ290 nT), (b).
Dst=ÿ200 nT. Fig. 3 shows the locations of plasma boundaries for this pass. Practically the whole electrojet, including its equatorward and poleward boundaries, is located at discrete auroral forms latitudes in the evening sector.
4. Electrojets mapping to magnetosphere Latitudinal intervals where electrojets are located were mapped to the magnetosphere. The paraboloid model of the magnetic ®eld in the magnetosphere (Alexeev et al., 1996) is used here for mapping. The input parameters of this model are physical magnetospheric characteristics: the geomagnetic dipole axis orientation (tilt angle c ); the magnetic ®eld ¯ux in tail lobes (F1); the maximum ring current magnetic ®eld intensity (Brc ); geocentric distances to the subsolar
point on the magnetopause (R1) and to the earthward boundary of the tail current (R2). These characteristics can be determined from observational data (solar wind parameters, geomagnetic indices, boundaries of auroral precipitation regions). To determine the midnight and noon auroral oval boundaries based on Newell et al. (1996) identi®cation, data of DMSP F08 and DMSP F10 are used. Figs. 4 and 5 show correspondingly mapping results in the meridional magnetospheric cross-sections for the eastward and westward electrojets. The eastward electrojet is mapped along quasidipole magnetic ®eld lines to the inner magnetosphere in the evening sector (MLT 1 18 h). The centres of source regions of precipitating ions with energy ¯ux maximum are located at geocentric distance of 03.2 RE for Dst=ÿ290 nT (Fig. 4(a)) and 03.8 RE for Dst=ÿ90 nT (Fig. 4(b)). The distance between projections of poleward and equatorward boundaries of the eastward electrojet is 01.2 RE independent of Dst intensity. Geocentric distances of 0(3±5.5)RE are typical for the ring current position in the magnetosphere during magnetic storms intervals. Such spatial coincidence of the ring current and projection of the eastward electrojet to the magnetosphere gives additional evidence in favour of coupling between physical processes leading to the eastward electrojet formation and the ring current generation in the magnetospheric evening sector. The westward electrojet is mapped to the nightside magnetosphere near midnight. For Dst=ÿ200 nT, the equatorward boundary is mapped to the magnetosphere along quasidipole magnetic ®eld lines at 04RE (see Fig. 5(a)). The ®eld lines threading the electrojet centre and poleward boundary are extended to the magnetospheric tail at distances >50 RE. At these distances the magnetic ®eld intensity on the ®eld lines intersecting the electrojet centre is 034 nT. The geomagnetic ®eld intensity in the ring current region at the equator is 01100 nT. At Dst=ÿ120 nT the westward electrojet equatorward boundary is extended to the magnetospheric tail at a distance of 07 RE (see Fig. 5(a)). Although the electrojet centre and the poleward boundary are extended to the distant tail the main part of the westward electrojet's related cross-tail currents ¯ows at distances close than 50 RE. The geomagnetic ®eld intensities on the ®eld lines intersecting the electrojet centre at the distances of 50 RE and at ring current centre are 027 nT and 0800 nT, correspondingly. The magnetic ®eld values are in a good agreement with GEOTAIL measurements in disturbed magnetosphere (Kokubun et al., 1996). The paraboloid model allows to separate the impact of dierent magnetospheric parameters on the largescale magnetospheric structure peculiarities. For 21 February 1995 event at 21.05 UT, Table 1 lists geo-
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Fig. 5. The same as in Fig. 4, for the westward electrojets for 6 February 1994, 21.4 UT (0.4 MLT, Dst=ÿ120 nT), (a), and 10 May 1992, 18.6 UT (21.6 MLT, Dst=ÿ200 nT), (b).
magnetic latitudes of the last (high-latitudinal) closed magnetic ®eld line at noon (1), the last quasidipolar ®eld line at midnight (2), and the last ®eld line, which is closed via the magnetospheric tail plasma sheet (3) calculated in the framework of the paraboloid model when the input parameters of the paraboloid model are varied. We can see that: 1. R1 variation, when it increases from 6.5 RE to 9.0 RE, causes poleward shifts of the closed/open ®eld lines boundary by 9.78, dayside cusp by 1.98 and the nightside dipole line by 6.28; 2. R2 variation, when it increases from 4.25 RE to 6.0 RE, causes poleward shifts of the last midnight dipole line by 3.48 and the dayside cusp by 0.58; the closed/open ®eld lines boundary shifts equatorwards by 2.78; 3. Brc variation, when the ring current ®eld intensity
decreases from ÿ40 nT to ÿ150 nT, in¯uences the boundary location weaker than geocentric distances R1 and R2. The region with dipole magnetic ®eld lines at the night and dayside cusp shift equatorwards by 18 and by 1.38, respectively. The auroral oval diameter increases by 2.38 when the ring current magnetic ®eld reaches a value of ÿ150 nT. This diameter enhancement is comparable with those obtained by Schulz (1997). The closed/open ®eld line boundary at the nightside magnetosphere shifts polewards by 2.28. 4. F1 variation leads to the most signi®cant shift of boundaries between regions with magnetic ®eld lines of dierent character, especially at the night side. The auroral oval diameter increases by 9.78. Thus, in the framework of the paraboloid model of the magnetospheric magnetic ®eld the in¯uence of the ring current on the boundaries of structural regions
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Table 1 The ionospheric projections of the near noon magnetopause, geotail current sheet inner boundary and the boundary between closed and open ®eld lines for 21 February 1995 event at 21.05 UT. The results of calculation in the framework of paraboloid model with dierent input parameters R1 (RE )
R2 (RE )
F1 (Wb)
Brc (nT)
Noon closed
Night dipole
Night last closed
Dier. R1 6.5 7.5 9.0
4.25 4.25 4.25
ÿ100 ÿ100 ÿ100
75.58 76.28 77.48
60.88 62.78 67.08
72.28 75.98 81.98
7.5 7.5 7.5
4.25 4.80 6.00
0.0786E+10 0.0524E+10 0.0210E+10 Dier. R2 0.1047E+10 0.1083E+10 0.1156E+10
76.68 74.88 75.18
58.78 59.98 62.18
68.78 67.68 66.08
7.5 7.5 7.5
4.25 4.25 4.25
ÿ100 ÿ100 ÿ100 Dier. Brc ÿ40 ÿ100 ÿ150
75.38 74.68 74.08
59.28 58.78 58.28
67.28 68.78 69.48
7.5 7.5 7.5
4.25 4.25 4.25
ÿ100 ÿ100 ÿ100
74.68 76.28 77.08
58.78 60.08 66.08
68.78 75.98 82.18
0.1047E+10 0.1047E+10 0.1047E+10 Dier. F1 0.1047E+10 0.0524E+10 0.0262E+10
in the magnetosphere is substantially weaker than the impact of magnetospheric tail current.
5. Conclusions 1. A prominent equatorward shift of eastward electrojets occurs during magnetic storms. In the course of substorms the westward electrojet widens polewards covering auroral latitudes. 2. Comparison with auroral plasma precipitations showed that the eastward electrojet in the course of magnetic storms is located predominantly at latitudes of diuse precipitations equatorwards of the discrete auroral forms region. This region is mapped along magnetic lines to the inner magnetosphere at geocentric distances of 2.5±4 RE, between the plasmapause and plasma sheet inner boundary in the magnetospheric tail. The eastward electrojet centre coincides with the region of maximum energy ¯ux of ions precipitating into the upper atmosphere. 3. The westward electrojet is located in the latitudinal range of auroral plasma precipitations covering regions with both diuse and discrete auroral forms. The electrojet centre coincides with or lies in the vicinity of the electron acceleration events boundary (the equatorial boundary of the auroral oval of discrete auroral forms, b3a ). 4. Mapping of the electrojet boundaries from ionospheric altitude to the magnetosphere using paraboloid model of the magnetic ®eld reveals locations of plasma structures connected with electrojets by magnetic ®eld lines. During magnetic storms in the sub-
storms intervals the eastward electrojet is mapped into the ring current region and the westward electrojet is mapped to the large part of the nightside magnetosphere. 5. The ring current and the magnetospheric tail currents growth leads to alterations of the magnetospheric magnetic ®eld lines con®guration. The ionospheric projections of the near noon magnetopause (boundary 1) and the tail current sheet inner boundary (boundary 2) shift equatorwards. A volume of the inner magnetosphere with quasidipole ®eld lines decreases. The tail currents in¯uence on the positions of boundaries 1 and 2 is substantially more eective than the ring current impact. In the nightside magnetosphere these currents in¯uence in opposite directions the location of the boundary between closed and open ®eld lines (boundary 3). 6. Variations of the solar wind, electric ®elds in the magnetospheric tail and plasma injections from the tail to the inner magnetosphere aect geocentric distances to the subsolar magnetopause point, R1, and to the magnetospheric tail current sheet inner boundary, R2. An increase of R1 leads to a poleward shift of all three characteristic magnetospheric boundaries. Moreover, the dayside cusp boundary shift is three times smaller than a similar shift of the inner magnetosphere boundary in the night sector. An increase of R2 leads to a poleward shift of both the dayside cusp boundary and the nightside boundary of inner magnetosphere. This shift at the nightside is substantially more prominent than in the day hours.
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Acknowledgements We appreciate very much fruitful discussions with H. Luehr. We are grateful to the EISCAT and IMAGE magnetometer chain teams. The IMAGE magnetometer data used in this paper were compiled in the scope of the German±Finnish±Norwegian± Polish project conducted by the Technical University of Braunschweig. This work has been supported by RFBR grants 96-05-66279 and 98-05-64784, and INTAS-RFBR-95-0932 grant. References Alexeev, I.I., Belenkaya, E.S., Kalegaev, V.V., Feldstein, Ya.I., Grafe, A., 1996. Magnetic storms and magnetotail currents. Journal of Geophysical Research 101, 7737±7747. Chapman, S., Akasofu, S.-I., 1964. The Aurora, Research in Geophysics, V.1: Sun, Apper Atmosphere, and Space. The MIT Press, pp. 367±400 Chapter 15. Dronov, A.V., Morozova, T.I., Sosnovetz, E.H., Tverskaya, L.V., Tulupov, V.I., Khorosheva, O.V., 1988. Relationship between the ring current, electron precipitation regions and aurora luminosity in the dusk-midnight sector of the magnetosphere. Geomagnetism and Aeronomy 28, 1011± 1013. Feldstein, Ya.I., 1992. Modelling of the magnetic ®eld of magnetospheric ring current as a function of interplanetary medium. Space Science Review 59, 83±165. Feldstein, Ya.I., Galperin, Yu.I., 1985. The auroral luminosity structure in the high-latitude upper atmosphere: its dynamics and relationship to the large-scale structure of the Earth's magnetosphere. Review of Geophysics and Space Physics 23 (3), 217±275. Feldstein, Ya.I., Galperin, Yu.I., 1996. The auroral precipitations structure in the magnetosphere night sector. Cosmic Research 34 (3), 227±247. Galperin, Yu.I., Feldstein, Ya.I., 1991. Auroral luminosity and its relationship to magnetospheric plasma domains. In: Meng, C.-I., Rycroft, M.J., Frank, L.A. (Eds.), Auroral Physics. Cambridge UP, pp. 207±222.
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