Y. I. Feldstein, L. I. Gromova
IZMIRAN, Troitsk, Moscow Region, Russia
A.Grafe
Geo ForschungZentrum Potsdam, Adolf Schmidt Observatorium, Niemegk, Germany
C. -I. Meng
Johns Hopkins University, Applied Physics Laboratory, Laurel, MD 20723-6099, USA
V. V. Kalegaev, I. I. Alexeev
Institute of Nuclear Physics, Moscow State University, Moscow, Russia
Yu. P. Sumaruk
Lvov Magnetic Observatory, 292193 Lvov Region, Ukraine
Effect of the equatorward shift of the eastward and westwardelectrojets during magnetic storms main phase is analysed based on the meridional chains of magnetic observatories EISCAT and IMAGE and several Russian observatories (geomagnetic longitude ~110 deg. corrected geomagnetic latitudes 74 deg. < F < 51 deg.) DMSP (F08, F10 and F11) satellite observations of auroral energy plasma precipitations at upper atmosphere altitudes were used for the determination of precipitation region structure and location of boundaries of various plasma domains during magnetic storms on May 10-11, 1992, February 5-7 and February 21-22, 1994. Interrelationship between locations of the center, poleward and equatorward boundaries of electrojets and characteristic plasma regions is discussed. The electrojet center, poleward and equatorward boundaries along the magnetic observatories meridional chain were mapped to the magnetosphere using the geomagnetic field paraboloid model. In the framework of the magnetospheric magnetic field paraboloid model the influence of the ring current and magnetospheric tail plasma sheet currents on large-scale magnetosphere structure is considered.
Below we continue documented the electrojet positions and investigation of electrojet dynamics based on meridional magnetic observatories chains data, EISCAT and IMAGE [Luehr et al., 1984; Viljanen and Hakkinen, 1996] observations, attracting measurements of middle latitudinal and subauroral Russian magnetic observatories near 110 deg. geomagnetic meridian as well. Special attention was payed to investigation of the relationship between electrojets location structure and spectral features of auroral plasma precipitating particles. Meridional magnetic observatories chain data allow more precisely determine the auroral electrojet boundaries location, while DMSP satellite spectrograms with 1 s temporal resolution for electrons and ions of energy between 10 eV and 20 keV make it possible state-of-the-art identification of plasma precipitation regions with various spectral and structural characteristics. Mapping of electrojet boundaries from ionospheric altitude to the magnetosphere reveals locations of plasma structures, connected with the electrojets by magnetic field lines. The paraboloid magnetospheric magnetic field model [ Alexeev et al., 1996] is used for the mapping. This model allows in more detail, comparing with other models, to describe the magnetospheric magnetic field dynamics in dependence on several physically important magnetospheric parameters. In the framework of this model influence of the model input parameters on the location of characteristical magnetic field lines in the magnetosphere is considered.
Figure 1 presents variations of the northern (X) component of the geomagnetic field along the IMAGE chain during the magnetic storm on February 5-7, 1994 (at the top). The variations intensity was measured relative to the level at 0600-0800 UT on February 5, 1994. Dotted vertical lines correspond to UT's with latitudinal cross-sections of X and Z geomagnetic field components during maxima of eastward and westward electrojets and quiet intervals between substorms. The location of the MLT midnight in each station is marked by triangle. Dst and PC variations of the geomagnetic field for the same storm is shown at the bottom. Arrows directed downwards (upward) mark moments of latitudinal cross-sections through eastward (westward) electrojets.
The magnetic storm had begun with Dst variation decrease after 1100-1200 UT on February 5, 1994. Dst variation reached (80 nT at 2000-2100 UT and then became weaker. The magnetic storm maximum falls on 1600-2200 UT on February 6 (Dst ~-120 nT), on February 7 Dst variation decays (the recovery phase).
PC index of the magnetic activity developed by Troshichev et al. [1979], Vennerstrom et al., [1991] for description of the disturbance level at the polar cap center region due to Bz IMF component gives an idea on the level of disturbances in high latitude region. For the interval under consideration the index was calculated based on Thule station observations, near the geomagnetic pole in Greenland. Close connection between PC intensity increase and Dst is clearly seen(Figure 1).
Latitudinal cross-sections DX and DZ when magnetometer chain recorded the eastward electrojet for magnetic storm February 5-7, 1994 are presented Figure 2. The electrojet centers marked by arrows. Their location is determined by maximum DX value and change of sign of DZ (DZ < 0 poleward of the electrojet center latitude and DZ > 0 equatorward of it). Prior to the magnetic storm on February 5 onset the eastward electrojet centers are located at F = 66.5 deg. at 11.18 UT (during substorm maximum) and at F = 69.5 deg. at 12.31 UT (in the interval between adjacent substorms). Right after the magnetic storm commencement the electrojet center is located already at F = 64.3 deg. at 14.24 UT during substorm maximum (Dst = -42 nT). When Dst increases up to -68 nT, in approximately similar conditions of substorm maximum and the same MLT, the eastward electrojet center is located already at F = 61 deg. at 14.48 UT on February 6. The eastward electrojet shift equatorwards continues with |Dst| increase till F = 58.5 deg. at 16.28 UT for Dst = -114 nT. At the magnetic storm recovery phase the eastward electrojet center following Dst variation mitigation shifts to higher latitudes, up to F = 60 deg. at 14.21 UT on February 7, 1994. Dotted line in Figure 2e marks the location of the eastward electrojet center during an intensive magnetic storm on May 10, 1992 main phase, which is analysed in detail by Feldstein et al. [1997]. At 15.14 UT, when Dst = -282 nT, the electrojet center shifted to F = 55.0 deg.
Character of latitudinal cross-sections through the westward, presented in Figure 4, and the eastward electrojets are substantially different. Westward current at near midnight and early morning hours MLT covers practically the whole auroral zone in the latitudinal interval 60 deg. < F < 75 deg. During relatively quiet intervals between substorms, like time moment 20.14 UT February 6, the electrojet center was located at F = 60 deg. It is about 5 deg. equatorward than the usual location of the westward electrojet in the near midnight sector. At substorms maximum, like 19.14 UT and 21.26 UT on February 6, 19.40 UT on February 7, 1994 the electrojet center does not shifts equatorward but the electrojet sharply widens poleward. The electrojet latitudinal splitting with more than one peak in DX is often observed. Figure 4 contains such an example at 19.14 UT. In addition to main maximum of intensity at F ~ 71 deg., the latitudinal cross-section contains one more weaker maximum at F ~ 61 deg. Consideration of the westward auroral electrojets dynamics for quiet intervals between substorms showed reasonable agreement with statistical data compising other magnetic storms presented by Feldstein et al. [1997]. It is very peculiar that for both the eastward and westward electrojets their shift equatorward in the course of a magnetic storm development begins from auroral zone latitudes (65 deg. < F < 67 deg. and ends at F ~ 55 deg. with Dst ~ - 250 nT.
Boundaries of various structural regions below are adopted in accordance with Newell et al. [1996], Feldstein and Galperin [1996] classification. This classification is based on physically substantiated boundaries of regions with different physical characteristics as far as structure of precipitating plasma fluxes and their energy spectra in the interval from dozens eV to dozens thousands eV are concerned. Newell et al. [1996] proposed the following indentification of the plasma regions in the auroral oval night sector. Boundary b1e is the "zero-energy" electron boundary, usually determined by the 32 and 47 eV electron channels; boundary b2i -- precipitating ion energy flux maximum, which is also the isotropy boundary for ions and the current sheet inner boundary in the plasma sheet of the magnetospheric tail; boundary b2e -- the point where electron average energy is neither increasing nor decreasing with latitude: one interpretation is the start of the main (central) plasma sheet precipitation; boundary b3a -- the equatorwardmost electron acceleration event identified, equatorial boundary of the structural auroral forms (equatorial auroral oval boundary); boundary b3b -- the polewardmost such event, polar boundary of the discrete auroral forms (auroral oval polar boundary); boundary b6 -- the poleward boundary of subvisual drizzle roughly adjacent to the auroral oval.
Figure 5 shows boundaries for the eastward electrojet on evidence of magnetometer chain and location of plasma domains boundaries based on DMSP F08 and F10 satellite data during magnetic storms on February 6-7, 1994, May 10, 1992 and February 21, 1994. Poleward and equatorward boundaries of electrojets were defined on either side on an electrojet center as latitudes, where DZ reached extreme values and DX ~ DXmax/2. The eastward electrojet in the evening sector is located either fully equatorward of the discrete auroral forms region (Fp < Fb3a), or its poleward boundary is placed between discrete precipitation region boundaries Fb3a < Fp < Fb3b). The electrojet center falls either on b2i boundary latitude (or near this latitude), equatorward boundary practically coincides with the equatorward boundary of plasma precipitation b1e, which apparently is the projection of the plasmapause to the ionospheric altitudes. Thus, the eastward electrojet is located mainly in the region of diffuse auroral luminosity (equatorward of b3a), but the eastward electrojet poleward part in separate cases can cover the region of discrete precipitation between boundaries b3a and b3b as well. The eastward electrojet poleward boundary, as a rule, is located equatorward of b3b, i.e. structured auroral precipitation encompasses the region poleward of the eastward electrojet.
Figure 6 presents the westward electrojets center, equatorward and poleward boundaries for IMAGE meridian at the moment of the satellite intersection of the auroral region morning sector. The electrojets are fully located in the latitudinal range of auroral plasma precipitations covering both diffuse and discrete precipitations. In this case precipitations extend to lower latitudes, than the electrojet equatorward boundary. The electrojet center coincides with or lies in the vicinity of the auroral discrete forms equatorial boundary (b3a). The electrojet poleward boundary generally coincides with the poleward boundary of the auroral discrete forms (b3b). In the intervals of very intense magnetic storms the westward current in the late evening hours covers the whole auroral region. For the storm on May 10, 1992 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 Dst = -200 nT. Figure 6 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.
Latitudinal intervals where electrojets are located were mapped to the magnetosphere. The paraboloid model of the magnetic field in the magnetosphere [Alexeev et al., 1996] was used for mapping. Input parameters of this model are the physically meaning magnetospheric characteristics and therefore it has definite advantages in comparison with widely known Tsyganenko's model [Tsyganenko, 1987, 1989]. Tsyganenko's model belongs to empirical models, while Alexeev's model is a conceptual one. Input parameters in paraboloid model are the following magnetospheric characteristics: the geomagnetic dipole axis orientation (angle psi); the magnetic field flux in tail lobes (Fƒ); the ring current magnetic field intensity (BRC); geocentric distances to subsolar magnetopause point (R1), to the earthward boundary of the tail current (R2). These characteristics may be determined from observational data on the solar wind parameters, boundaries of auroral precipitation regions. For determination of the midnight and noon auroral oval boundaries based on Newell et al. [1996] identification data of DMSP F08 and DMSP F10 were used. Figure 8 and Figure 9 show mapping results to the equatorial and meridional magnetospheric cross-sections for the eastward and westward electrojets, correspondingly.
The eastward electrojet is mapped along quasidipole magnetic field lines to the inner magnetosphere in the evening sector. Its center and source region of precipitating to the upper atmosphere ion energy flux maximum are located at geocentric distance ~3.2 RE for Dst = -290 nT and ~3.8 RE for Dst = -90 nT. The interval between maximum and minimum geocentric distances of projections of poleward and equatorward boundaries of the eastward electrojet is ~1.2 RE independently on Dst intensity. The westward electrojet (Figure 9) is mapped to the nightside magnetosphere near midnight. For Dst = -200 nT equatorward boundary is mapped to the magnetosphere along quasidipole magnetic field lines at ~4 RE. The field lines threading the electrojet center and poleward boundary are elongated to the magnetospheric tail at distances > 50 RE. The magnetic field intensity at this distance on the field lines intersecting the electrojet center is ~34 nT. The geomagnetic field intensity in the ring current region at the equator is ~1100 nT. At the Dst = -120 nT the westward electrojet equatorward boundary is elongated to the magnetospheric tail at the distance ~7 RE and the electrojet center and the poleward boundary are elongated to the distance more than 50 RE. The geomagnetic field intensity on the field lines intersecting the electrojet center and ring current center is ~27 nT and ~800 nT, correspondingly.
The paraboloid model allows to separate influences of physically meanIng magnetospheric characteristics on the large-scale magnetospheric structure peculiarities. Table 1 contains geomagnetic latitudes of the last (high-latitudinal) closed magnetic field line at noon (1), the last quasidipolar field line at midnight (2), and the last field line, which is closed via the magnetoshperic tail plasma sheet (3), when the input parameters of the paraboloid model vary. Table 1 contains mapping results for February 21, 1995 event at 21.05 UT, when the dipole axis was tilted in the antisunward direction at 7.42 deg. R1, R2, BRC and Fƒ changed consequently from top to bottom. When R1 changes, it was supposed that the value R1 = 6.5 RE characterizes the disturbed magnetosphere conditions, the value R1 = 7.5 RE - the middle disturbed level and the value R1 = 9.0 RE - the quiet magnetosphere conditions. The intensity of the magnetic field at the plasma sheet inner boundary bt was accepted as -150 nT, -100 nT and -40 nT, correspondingly. Values of Fƒ for these three magnetosphere cases are shown in the upper part of the Table 1.They were obtained using the relation between Fƒ, bt, R1 and R2 for the paraboloid model. Under changes of R2 and BRC the intensity of magnetic field at the plasma sheet inner boundary is bt = -200 nT. Fƒ variation in the Table 1 lower part is selected in such a way, that bt equals consequently -200 nT, -100 nT and -50 nT.
1. Prominent equatorward shift of eastward electrojets during magnetic storms occurs. In the course of substorms the westward electrojet widens poleward covering auroral latitudes.
2. Comparison with auroral plasma precipitations has shown that the eastward electrojet pending magnetic storms is located predominantly at latitudes of diffuse precipitations equatorward of the discrete auroral forms region. This region is mapped along magnetic field lines to the inner magnetosphere at geocentric distance 2.5-4 RE, between the plasmapause and plasma sheet inner boundary in the magnetospheric tail. The eastward electrojet center coincides with the region of maximum precipitating to the upper atmosphere ion energy flux.
3. The westward electrojet is located in the latitudinal range of auroral plasma precipitations covering regions with both diffuse and discrete auroral forms. The electrojet center coincides with or lies in the vicinity of the electron acceleration events boundary (equatorial boundary of the auroral oval of the discrete auroral forms - b3a). During magnetic storms in the substorms intervals the westward electrojet is mapped to bigger part of the night magnetosphere, from its inner regions to the plasma sheet periphery.
4. Change of the ground magnetic field variations of the RC current (from -40 nT to -150 nT) and the magnetospheric tail currents (from -50 nT to -200 nT) leads to magnetospheric magnetic field lines configuration alterations. The ionospheric projection of the near noon magnetopause (boundary 1) and the tail current sheet inner boundary (boundary 2) shift equatorward. The volume of the inner magnetosphere with quasidipole field lines decreases. Influence of the tail currents on boundaries 1 and 2 positions is substantially more effective than the ring current impact. In the nightside magnetosphere these currents influence in opposite directions on the location of the boundary between closed via the tail plasma sheet and opened field lines (boundary 3). Four times increase of RC and TC magnetic fields leads to poleward shift of boundary 3 by 2.2 deg. (RC impact) and to equatorward shift by 13.4 deg. (TC influence via the magnetic flux Fƒ increase).
5. Variations of the solar wind, electric fields in the magnetospheric tail and plasma injections from the tail to the inner magnetosphere affect geocentic distances to the subsolar magnetopause point R1 and the magnetospheric tail current sheet inner boundary R2. Increase of R1 leads to the poleward shift of all three characteristic magnetospheric boundaries. Moreover, the interval of the dayside cusp boundary shift is three times less, than similar shift of the inner magnetosphere boundary in the night sector. Increase of R2 leads to the poleward shift of both the dayside cusp boundary and the night inner magnetosphere boundary. This shift at night is substantially more prominent, than in day time.
Figure 1. Variations of the magnetic field X component along the IMAGE chain during storm main phase on February 05-07, 1994(at the top). Dotted lines (vertical) correspond to ten UT's with latitudinal cross-sections of the DX and DZ components presented further in the text. The Dst and the PC indices of the magnetic field variations for the same storm are shown at the bottom. Arrows directed downwards mark UT's of latitudinal cross-section through the eastward electrojet, and arrows directed upwards mark cross-section through the westward electrojet. The location of the magnetic midnight in each station is marked by triangle.
Figure 2. The latitudinal cross-sections DX (dotted line) and DZ (solid line) through the eastward electrojet on February 05, 1994 at 1118 UT, 1231 UT and 1424 UT, February 06 at 1448 UT and 1628 UT, February 07 at 1421 UT. Arrows mark the latitudes of the eastward electrojet center. The dotted arrow on February 06 at 1628 UT marks the position of the eastward electrojet on May 10, 1992 at 1514 UT, when Dst = -282 nT.
Figure 4. The latitudinal cross-sections through the westward electrojet on February 06, 1994 at 1914 UT and 2126 UT, February 07 at 1940 UT during substorms maxima, on February 06, 1994 at 2014 UT during quiet interval between substorms. Arrows mark the latitudes of the westward electrojet center.
Figure 5. The center (C) and boundaries (E -- equatorial and P -- polar) of eastward electrojets in the course of magnetic storms on February 06-07, February 21, 1994 and May 10 1992 in the evening sector. For DMSP F08 and DMSP F10 passes with close to electrojets crosses UT and MLT the location of plasma domain boundaries according to Newell et al. [1996] is presented: b1e,i - boundaries of the "zero - energy", usually determined by the softnes channels; b2e - the point where electron average energy is neither increasing nor decreasing with latitude; b2i - precipitating ion energy flux maximum; b3a - the equatorwardmost electron acceleration event identified, equatorial boundary of the structural auroral forms; b3b - the polewardmost such event, polar boundary of the discrete auroral forms. Hourly mean Dst values correspond to UT of passes.
Figure 6. Similar to Figure 5 for westward electrojets during magnetic storms of February 06-07, 1994 and May 10, 1992 in the morning sector. In addition to plasma domain boundaries b1-b3 the position of b6 boundary is shown; b6 -- the poleward boundary of subvisual drizzle.
Figure 8. The mapping of the center and boundaries of eastward electrojets to the magnetospheric meridional and equatorial cross-sections using the paraboloid model of the magnetospheric magnetic field. a) February 21, 1994, 14.9 UT, 17.9 MLT; Dst (((90 nT; b) May 10, 1992, 15 UT , 18 MLT; Dst (((290 nT.}
Figure 9. Similar to Figure 8 for westward electrojets: a) February 6, 1994, 21.4 UT, 0.4 MLT; Dst = -120 nT; b) May 10, 1992, 18.6 UT, 21.6 MLT, Dst = -200 nT.
