The mapping of field lines by the Tsyganenko 1987 model suggests that the footpoints of field lines from the plasma sheet lie along the auroral oval: midnight maps to midnight in the oval, and as one progresses towards the flanks of the tail the footpoints advance towards the cusps. The oval also matches or parallels regions of Birkeland currents, the major electrodynamic link between the magnetosphere and the ionosphere. Region 2 currents are due to the interaction of the electric field E and trapped plasma, and are fairly well understood, but the primary system is that of region 1 and its theory is still incomplete. Near noon region 1 currents may extend to the solar wind or to boundary layers adjoining the magnetopause, but at night they map to the plasma sheet. A simple mechanism accounts for the latter currents: if the sheet contains a dawn-to-dusk E, it will try to impose its potentials on the ionosphere, a process which requires currents. The removal of such currents from the tail, however, must leave charge neutrality intact, and must also satisfy the Vasyliunas equation. The latter requires the pressure p₂ in the midnight sector of the plasma sheet to be less than the pressure p₁ near the flanks. A mechanism for establishing such a pressure profile is discussed, but the problem is essentially still unsolved.

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The mapping of the nightside auroral oval into the magnetotail is investigated using DE-1 and -2 plasma data in combination with the Tsyganenko '89 and the Hilmer-Voigt'91 magnetic field models. In an attempt to clarify the relationship between magnetospheric regions and the BPS and CPS, data from the Low Altitude Plasma Instrument (LAPI) are used to determine the invariant latitude of the polar cap boundary, the poleward edge of the discrete aurora, the discrete-diffuse (BPS/CPS) auroral boundary, and the equatorward edge of the diffuse aurora. When these field lines are traced into the magnetotail, both models indicate that the region of discrete aurora (BPS) maps to an extended region (usually>20 R_E in width) of the main portion of the plasma sheet outside the more dipolar, inner plasma sheet (CPS). Since BPS field lines map to such a wide range of neutral sheet crossing distances, we propose a less confusing and more descriptive terminology in which the term “BPS” is divided into two regions: the Plasma Sheet Boundary Layer (PSBL) and the “IPS”, for Isotropic (Boundary) Plasma Sheet.
Auroral zone-magnetotail topology is further examined by using data from the High Altitude Plasma Instrument (HAPI) to determine values of electron number density and temperatures at 1° intervals across the auroral zone. The corresponding field lines are traced into the magnetotail to a distance of-15R_E and compared with magnetotail (10-25R_E) mean plasma sheet ion densities and temperatures for low- and mid-activity levels from HUANG and FRANK (1986). We find a very good correspondence between the density and temperature measurements mapped from DE-1 altitudes and those measured in the mid-tail, suggesting that both models successfully reproduce the average magnetospheric configuration at this distance. The mean of the ion temperatures mapped with the Hilmer-Voigt model were slightly higher than those reported from the ISEE data, indicating that the model may be slightly too stretched. A significant region (between-5-7 R_E) of overlap between the hotter ring current and the cooler, fresher plasma sheet is observable at DE-1; these hotter electrons are so anisotropic (with a trapped pancake distribution) that they are not observable at the 1000km altitude of DE-2.

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We apply the magnetosphere model of TSYGANENKO (1989) to project into the ionosphere the tail plasma domains (with density >0.1cm^-3) as measured by PROMICS-2 on Prognoz-8. The Plasma Sheet Boundary Layer (the separatrix layer) in our data is easily recognizable as a region distinct from the Central Plasma Sheet only when the plasma sheet expands at the beginning of substorm recovery. During weakly disturbed and quiet periods and at flanks the separatrix layer is not seen by our plasma instrument. However, when both the electron temperature and ion mass composition are taken into account we usually see three distinctly different plasma domains in the tail: the region A (we call it tentatively the warm envelope of the plasma sheet) dominated by the solar wind originated plasma with cold electrons (T_e≤100eV), the region B (called here the high-latitude plasma sheet) with the electron temperature less than 1keV, density fluctuations, irregular spectra and presence of cold oxygen ion beams (field aligned and/or conical) and the region C (called here the hot core of the plasma sheet) with electron energy density≈1keV/cm³ where the hot oxygen ions dominate over the doubly ionized helium. This classification of plasma sheet subregions is similar to the one discussed by FELDSTEIN and GALPERIN (1985). We suggest that the warm envelope originates from the stagnating Low Latitude Boundary Layer. The warm envelope of the plasma sheet maps in most cases poleward of the statistical auroral oval of Feldstein-Starkov. The hot core of the plasma sheet maps into the equatorward half of the statistical oval and is connected to R<15 R_E in the equatorial plane. The high latitude plasma sheet with the field aligned and/or conical O⁺ beams maps immediately poleward and/or into the poleward half of the F-S oval and is mostly on the field lines which close at R<60R_E.

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An attempt is made to construct a road map for translating various plasma domains in the magnetosphere to their counterparts in the auroral ionosphere. Results of previous work have allowed much to be inferred about where in the magnetosphere auroral processes are taking place. The main auroral oval appears to be associated with magnetospheric processes well separated from boundary layer processes with the exception of the dayside sector between about 8 and 16 MLT. This sector is apparently dominated by sources in the dayside boundary layer and by the region where the nightside cross-tail current intersects the boundary layer regions. The division between the stably trapped particles and isotropic distributions probably coincides with the most equatorward discrete arc system involved with the substorm onset. Thus the diffuse aurora equatorward of the discrete aurora would be a ring current related phenomena and the nightside discrete aurora would originate dominantly from the central plasma sheet. During magnetically active conditions the outer edge of the central plasma sheet plays a more important role in the production of the discrete aurora. Diffuse aurora poleward of the discrete system is probably linked to velocity dispersed ion signatures seen in the low altitude ionosphere.

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The characteristics of the ion and electron precipitation in the cusp region are examined using the particle measurements on board DMSP F7 spacecraft in January, July and December 1984. It is shown that the ion differential spectrum changes in its regular manner while crossing the auroral oval. The following structural zones in the dayside oval can be separated using the ion data: cusp region, where the ion flux is maximal at all energies with a wide peak at E_i=0.1-5keV; poleward edge of the cusp where the ion spectrum regularly softens poleward displaying a steady peak at E_i<0.1keV; equatorward edge of the cusp, where the spectrum softens preserving maximal number flux at E_i=0.1-2keV, this region corresponds to transition boundary in electron precipitation. The cusp-like ion precipitation is common in the region centered on the noon meridian at geomagnetic latitudes Φ=74-82°with a longitudinal extension±2MLT hours.
Using the ion data for the identification of the cusp boundaries the influence of magnetic activity and the IMF B_z component on the location of the cusp are studied. It is shown that effects of the northward B_z are not seen for periods with low magnetic activity (AE<250nT). On the contrary influence of magnetic activity on the location of both the poleward and equatorward boundaries of the cusp is evident especially for winter season. The cusp boundaries are displaced by 1.5-2° poleward in summer season relative to their position in winter season.

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We present a case study of spatially and temporally coincident DMSP-F7 electron and ion precipitation measurements and Sondrestrom incoherent scatter radar observations of ionospheric plasma parameters. The particle flux data allow us to determine the boundaries between the magnetospheric regions central plasma sheet (CPS), plasma sheet boundary layer (PSBL), cusp proper, and plasma mantle (PM), mapped down to the DMSP orbit (835km altitude). The PSBL, cusp, and the equatorward section of the PM are characterized by intense low energy (<400eV) electron precipitation. The radar measured enhanced plasma density and elevated electron temperature at 350km altitude at the same invariant latitude interval where DMSP-F7 detected the intense low-energy electron flux. The ion temperature was unaffected, as were plasma density and electron and ion temperatures at 200km altitude. The plasma convection pattern inferred from radar Doppler shift measurements suggests that the heated electron cloud observed in the F region was locally produced by the soft electron precipitation in and near the cusp proper rather than convected into the radar field of view. This study demonstrates that soft electron precipitation, which is typical for magnetosheath-like plasma entry observed in particular in the cusp, can under certain conditions be identified by its thermal ionospheric plasma signatures.

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The distribution of field-aligned currents (FAC) during quiet and disturbed periods is calculated with the Magnetogram Inversion Technique (MIT). The polar cap boundary is determined as the high-latitude boundary of the region 1 FAC. Statistical results on the location and size of the polar cap obtained from MIT are compared with those from two independent methods: one based on Viking auroral images and the other on mapping using the Tsyganenko 1987 magnetic field model. The three methods yield essentially the same results. The northern polar cap is shifted towards dawn or dusk, depending on the sign of the By component of the interplanetary magnetic field (IMF). The polar cap area grows by a factor of 2 to 3 during substorms.

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The existing corrected geomagnetic coordinate system is based on internal sources of the Earth's magnetic field and describes observed phenomena in a time-independent way. Space experiments, however, use universal time to follow the dynamics of observed phenomena in 3-dimensional space and external sources play an important role there. A new corrected coordinate system based on the “realistic” geomagnetic equator is proposed to order observations of experiments at each moment of time. The system is based on constant B-minimum ovals at the 3-D geomagnetic equator plane. The magnetospheric model by TSYGANENKO (1989) is used and an algorithm has been developed to derive the lines of constant latitude at ionospheric heights for a specific universal time. Combining these latitudes with corrected geomagnetic longitudes provides an opportunity to order polar ionospheric phenomena in accordance with their “realistic” positions near footpoints of geomagnetic field lines. An attempt is made to interpret some results from the PACE radar experiment using the calculated latitude ovals.

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As shown from ground-based measurements and satellite-borne imagers, one type of global auroral pattern characteristic of quiet (usually northward IMF) intervals is that of a contracted but thickened emission region in which the dawn and dusk portions can spread poleward to very high latitudes. Because of its shape, such a pattern has been referred to as a “horse-collar” aurora (HONES et al., 1989). In this report we use the Dynamics Explorer data set to examine a case in which this “horse-collar” pattern was observed by the DE-1 auroral imager while at the same time DE-2, at lower altitude, measured precipitating particles, electric and magnetic fields, and plasma drifts. Our analysis shows that in general there is close agreement between the optical signatures and the particle precipitation patterns. In many instances, over scales ranging from tens to a few hundred kilometers, electron precipitation features and upward field-aligned currents are observed at locations where the plasma flow gradients indicate negative ∇·E. The particle, plasma, and field measurements made along the satellite track and the 2-D perspective of the imager provide a means of determining the configuration of convective flows in the high-latitude ionosphere during this interval of northward IMF. Recent mapping studies are used to relate the low-altitude observations to possible magnetospheric source regions.

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Relationships between average characteristics of particle precipitation and substorm activity are investigated in the low latitude region as well as in the high latitude region within the dusk-to-midnight sector. “Low latitude” is defined here as the region below the equatorward boundary of the most equatorward electron BPS, while “high latitude” is defined as the region higher than that boundary. DMSP particle data are divided according to the phase of the substorm and according to the longitude with respect to the onset region by referring to ground-based magnetograms and DMSP auroral images. At high latitude, not only electron precipitation but also ion precipitation significantly intensifies in accordance with substorm activity. The energization of these particles presumably takes place in the near-tail region where an unstable tail current develops and locally diverges associated with substorm onset. At low latitude, temporal and spatial changes of energetic precipitation would be caused by injected, drifting particles from the substorm onset regions, particularly in the case of electrons.

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In this paper we present evidence from the low-altitude DMSP F7 satellite that the poleward edge of auroral luminosity in the nightside auroral zone does not necessarily correspond to the boundary between plasma-filled flux tubes and flux tubes devoid of plasma. Assuming that that low-altitude boundary corresponds to the boundary between the lobe and the plasma sheet, this implies that the boundary between open and closed field lines may lie poleward of the most poleward auroral luminosity. Thus the assumption that the poleward boundary of auroral luminosity is a good indicator of the open-closed boundary may not always be correct. Furthermore, we show clear evidence that an auroral surge may also be located equatorward of the open-closed boundary. Therefore, tailward of the region of the plasma sheet to which the surge is connected there may exist undisturbed plasma sheet that has not yet been disconnected from the ionosphere. This means that substorm-associated reconnection does not necessarily begin to reconnect lobe field lines at the onset of a substorm. Moreover, available evidence strongly suggests that the arc that brightens at the onset of a substorm and that develops into a surge maps to the inner magnetotail, to that region at the inner edge of the plasma sheet where the magnetic field changes from a dipolar to a taillike configuration. This would be consistent with recent studies that connect auroral breakup to the near-Earth (<10R_E) plasma sheet.

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