The characteristics of the hrge-scale electrodynamic parameters, field-aligned currents (FACs), electric fields, and electron prec/pitation, which are associated with auroral substorm events in the nighttime sector, have been obtained through a unique analysis which places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images. A generic bulgetype auroral emission region has been deduced from auroral images taken by the Dynamics Explorer I (DE 1) satellite during a number of isolated substorms, and the form has been divided into six sectors, based on the peculiar emission characteristics in each sector: west of bulge, surge horn, surge, middle surge, eastern bulge, and east of bulge. By comparing the location of passes of the Dynamics Explorer 2 (DE 2) satellite to the simultaneously obtained auroral images, each pass is placed onto the generic aurora. The organization of DE 2 data in this way has systematically clarified peculiar characteristics in the electrodynamic parameters. An upward net current mainly appears in the surge, with litfie net current in the surge horn and the west of bulge. The downward net current/s distributed over wide longitudinal regions from the eastern bulge to the east of bulge. Near the poleward boundary of the expanding auroral bulge, a pair of oppositely directed FAC sheets is observed, with the downward FAC on the poleward side. This downward FAC and most of the upward FAC in the surge and the middle surge are associated with narrow, intense antisunward convection, corresponding to an equatorward d/rected spikelike electric field. This pair of currents decreases in amplitude and latitudinal width toward dusk in the surge and the west of bulge, and the region 1 and 2 FACs become embedded in the sunward convection region. The upward FAC region associated with the spikelike field on the poleward edge of the bulge coincides well with intense electron precipitation and aurora appearing in this western and poleward portion of the bulge. The convection reversal is sharp in the west of bulge and surge horn sectors, and near the high-latitude boundary of the upward region I FAC. In the surge, the convection reversal is near the low-latitude boundary of the upward region 1, with a near stagnation region often extending over a large interval of latitude. In the eastern bulge and east of bulge sectors, the region I and 2 FACs are located in the sunward convection region, while a spikelike electric field occasionally appears poleward of the aurora but usually not associated with a pair of FAC sheets. In the eastern bulge, magnetic field data show complicated FAC distributions which correspond to current segments and filamentary currents. INTRODUCTION Auroral substorms are defined by a systematic sequence of auroral motions and magnetic disturbances [Akasofu, 1964]. Since field-aligned currents (FACs), ionospheric electric fields or convection, charged particle precipitation, and resulting ionospheric conductivity are closely...

The seasonal dependence of large‐scale Birkeland currents has been determined from the analysis of vector magnetic field data acquired by the TRIAD satellite in the northern hemisphere. Statistical characteristics of single sheet (i.e., net currents) and double sheet Birkeland currents were determined from 555 TRIAD passes during the summer, and 408 passes during the winter (more complicated multiple‐sheet current systems were not included in this study). The average Kp value for the summer events is 1.9 and for the winter events is 2.0. The principal results include the following: (1) The single sheet Birkeland currents are statistically observed more often than the double sheet currents in the dayside of the auroral zone during any season. The single sheet currents are also observed more often in the summer than in the winter (as much as 2 to 3 times as often depending upon the MLT sector). (2) The intensities of the single and double sheet Birkeland currents on the dayside, from approximately 1000 MLT to 1800 MLT, are larger during the summer (in comparison to winter) by a factor of about 2. (3) The intensities of the double sheet Birkeland currents in the nightside (the dominant system in this local time) do not show a significant difference from summer to winter. (4) The single and double sheet currents in the dayside (between 0600 and 1800 MLT) appear at higher latitudes (by about 1° to 3°) during the summer in comparison to the winter. These characteristics suggest that the Birkeland current intensities are controlled by the ionospheric conductivity in the polar region. The greater occurrence of single sheet Birkeland currents during the summertime supports the suggestion that these currents close via the polar cap when the conductivity there is sufficiently high to permit it. Since the intensities of Birkeland currents are larger during periods of greater ionospheric conductivity, an important source (but perhaps not the only source) of these currents must be a voltage generator in the magnetosphere, possibly related to the convective electric field.

In order to investigate the characteristics of generation mechanisms of large‐scale Birkeland currents from the standpoint of relative importance of a voltage source and a current source, we have determined quantitatively the relationships between large‐scale Birkeland current intensities and ionospheric conductivities under geomagnetic quiet conditions. The large‐scale Birkeland currents are the well‐defined region 1 and region 2 systems that were determined from transverse magnetic disturbances acquired with the Magsat satellite over the entire polar regions except for the midnight sector. The ionospheric conductivities were determined by solar EUV ionization. The principal results include the following: (1) The region 1 and region 2 systems exhibit different dependence on ionospheric conductivities. The region 1 current intensities are proportional linearly to the conductivities at all MLTs. The region 2 currents also show a dependence on ionospheric conductivities, but they have in general poorer correlations at all MLTs than the region 1 currents have. (2) The correlations with conductivities are improved by using the region 1 current density and the region 2 current latitudinal width. (3) The degree of the coupling between the region 1 system and the region 2 system seems to have no correlation with conductivities. (4) The above‐mentioned characteristics are common both in the northern and southern polar regions. These findings suggest that the large‐scale region 1 system is primarily driven by voltage generators in the magnetosphere. On the other hand, the region 2 system seems to be driven by a combination of voltage and current generators.

Abstract. Two long runs of EISCAT Svalbard Radar (ESR), in February 2001 and October 2002, have been analysed with respect to variability in the F2 region peak density and altitude. The diurnal variation in the F2 peak density exhibits one maximum around 12:00 MLT and another around 23:00 MLT, consistent with solar wind controlled transport of EUV ionized plasma across the polar cap from day to night. High density plasma patch material is drawn in through the cusp inflow region independent of IMF B Y . There is no apparent IMF B Y asymmetry on the intake of high density plasma, but the trajectory of its motion is strongly B Y dependent. Comparison with the international reference ionosphere model (IRI2001) clearly demonstrates that the model does not take account of the cross-polar transport of F2-region plasma, and hence has limited applicability in polar cap regions.

[1] We have investigated how geomagnetic activity, the solar wind (SW), and the interplanetary magnetic field (IMF) influence the occurrence of the F-region/topside ionospheric ion upflow and downflow. Occurrence of dayside ion upflow observed with the European Incoherent Scatter Svalbard radar (ESR) at 75.2°magnetic latitude is highly correlated with the SW density, as well as with the strength of the IMF By component. We suggest that this correlation exists because the region where ion upflow occurs is enlarged owing to SW density and IMF By magnitude, but it does not move significantly in geomagnetic latitude. The occurrence frequency of dayside ion upflow displays peaks versus the geomagnetic activity index (Kp), SW velocity, and negative IMF Bz component; that is, ion upflow is less frequently seen at the highest values of these parameters. Dayside ion downflow in the F-region/topside ionosphere occurs only when the Kp index and/or SW velocity are high or when IMF Bz is largely negative. The ion downflow is likely due to ballistic return of the ion upflow. We suggest that the region of ion upflow not only becomes larger but also moves equatorward with increasing Kp, SW velocity, and negative IMF Bz. The ESR can so be poleward of the upflow region and observe ions convecting poleward and returning ballistically downward.

[1] In this paper we study reversed flow events (RFEs) that seem regulated by Birkeland current arcs in the winter cusp ionosphere above Svalbard. An RFE is a longitudinally elongated, 100-200 km wide channel, in which the flow direction is opposite to the background convection, persisting for 10-20 min. The RFE onset occurs with the brightening of a discrete arc near the open-closed boundary. The auroral arc is situated exactly at a sharp clockwise flow reversal, consistent with a converging electric field and an upward field-aligned current. One category of RFEs propagates into the polar cap in tandem with poleward moving auroral forms, while another category of RFEs moves with the cusp/cleft boundary. The RFE phenomenon is addressed to a region void of electron precipitation, and in lack of direct sunlight the E-region conductivity will be very low. We propose two possible explanations: (1) the RFE channel may be a region where two MI current loops, forced by independent voltage generators, couple through a poorly conducting ionosphere and (2) the reversed flow channel may be the ionospheric footprint of an inverted V-type coupling region. Electron beams of <1 keV will not give rise to significant conductivity gradients, and the form of a discontinuity in the magnetospheric electric field will be conserved when mapped down to the ionosphere, although reduced in amplitude. These two explanations may be related in the sense that the boundary discontinuity in the magnetospheric electric field in (1) may be the driver for the inverted V in (2).

Abstract. We have determined, based on 28 days of European Incoherent Scatter Common Program 1 mode I data obtained between 1989 and 1991, statistical characteristics of the energycoupling processes between the lower thermosphere, ionosphere, and magnetosphere through an analysis of the electromagnetic energy transfer rate J.E, the Joule heating rate J.E', and the mechanical energy transfer rate U.(JxB) at altitudes of 125, 117, 109, and 101 km. At all altitudes the input electromagnetic energy is distributed to both Joule heating and mechanical energy. The energy distributed to Joule heating is larger than that to mechanical energy, but the latter is generally not negligible. All three rates respectively have two maxima, not in the midnight region but in the dawn and dusk. The enhancements of these rates have positive correlations with the increase of geomagnetic activity represented by the Kp index. The electromagnetic energy transfer rate is greatest at 117 km, becoming smaller with decreasing altitude. It is mostly positive but can be negative. At 117 km the mechanical energy transfer rate is considerably smaller than the electromagnetic energy transfer rate, suggesting that most of the electromagnetic energy at this altitude is converted to Joule heating and a small portion of the electromagnetic energy goes to mechanical energy. At 125 km the mechanical energy transfer rate is larger than that at 117 km. On average, 65% of the input electromagnetic energy is converted to Joule heating and 35% is converted to neutral mechanical energy. At 109 and 101 km altitude the mechanical energy transfer rate becomes negative, hence the Joule heating rate is greater than the electromagnetic energy transfer rate, suggesting that not only electromagnetic energy but also mechanical energy contribute to Joule heating. IntroductionThe magnetosphere and ionosphere exchange energy, in the form of electromagnetic energy flux accompanied by fieldaligned currents j//and electric fields E, particle fluxes associated with plasma precipitation and outflow, or plasma waves. The energy transferred from the magnetosphere to the ionosphere can be a major energy source for driving ionospheric currents J. Most of this energy is eventually absorbed by the neutral gas, but under some conditions it can go partly back to the ionosphere and then to the magnetosphere. The rate of the electromagnetic

[1] Regions where dayside field-aligned (FA) ion upflows occur are identified, and the relative occurrences and characteristics are compared. The study is based on $170 simultaneous events observed with the European Incoherent Scatter (EISCAT) Svalbard radar (ESR) and spacecraft from the DMSP. We found that ion upflows occur not only in the cusp and cleft (the low-altitude portion of the low-latitude boundary layer), which traditionally have been regarded as regions of ion upflow, but also in the region connected to the mantle. Ion upflows are less frequently seen in the boundary plasma sheet (BPS) and are very rarely seen in the central plasma sheet at high latitude on the dayside. Almost all of the events in which the average FA ion velocity is >100 m s À1 are associated with relatively high soft electron precipitation (differential energy flux of electrons at 100 eV > 10 7 eV cm À2 s À1 sr À1 eV À1), although soft electron precipitation with similarly high flux exists also in the BPS, where the ion velocities are mostly <100 m s À1 . These results indicate that soft particle precipitation is the predominant energy source driving ion upflow in the topside ionosphere, but it triggers ion upflow effectively not in the BPS, only in the other high-latitude regions on the dayside.