Abstract
Abstract. Attempting to derive the field-aligned current (FAC) density using the EISCAT radar and to understand the role of the ionosphere on closing FACs, we conducted special radar experiments with the EISCAT radar on 9 October 1999. In order to derive the gradient of the ionospheric conductivity (grad S) and the divergence of the electric field (div E) nearly simultaneously, a special experiment employed an EISCAT radar mode which let the transmitting antenna sequentially point to four directions within 10 min; two pairs of the four directions formed two orthogonal diagonals of a square. Our analysis of the EISCAT radar data disclosed that SP div E and E · grad SP produced FACs with the same direction inside a stable broad arc around 05:00 MLT, when the EISCAT radar presumably crossed the boundary between the large-scale upward and downward current regions. In the most successfully observed case, in which the conductances and the electric field were spatially varying with little temporal variations, the contribution of SP div E was nearly twice as large as that of E · grad SP . On the other hand, the contribution of (b × E) · grad SH was small and not effective in closing FACs. The present EISCAT radar mode along with auroral images also enables us to focus on the temporal or spatial variation of high electric fields associated with auroral arcs. In the present experiment, the electric field associated with a stable arc was confined in a spatially restricted region, within ~ 100 km from the arc, with no distinct depletion of electron density. We also detected a region of the high arc-associated electric field, accompanied by the depletion of electron density above 110 km. Using auroral images, this region was identified as a dark spot with a spatial scale of over 150 × 150 km. The dark spot and the electron depletion were likely in existence for a limited time of a few minutes.Key words. Ionosphere (auroral ionosphere; electric fields and currents; particle precipitation)
Highlights
The field-aligned current (FAC) plays a crucial role in the transfer of momentum and energy between the solar wind, the magnetosphere, and the ionosphere
Several studies have reported the characteristics of a region with an enhanced electric field, its temporal development is still not clear. Based on both European Incoherent Scatter (EISCAT) radar measurements and auroral images, we focus on the temporal or spatial variation of the region with high electric fields associated with auroral arcs
The EISCAT radar observed a region around a relatively small-scale auroral arc. Both the ground-based magnetometer data in Fig. 3 and auroral images in Figs. 4b and c indicate that temporal variations are small during these time periods
Summary
The field-aligned current (FAC) plays a crucial role in the transfer of momentum and energy between the solar wind, the magnetosphere, and the ionosphere. Observations from the ground, especially using incoherent scatter (IS) radars, can provide the temporal and spatial variations of ionospheric currents connecting FACs. If spatial distributions of the ionospheric conductances and electric fields, i.e. the gradient of the ionospheric conductances and the divergence of the electric fields, are known, the FAC density is fully determined. In order to understand how the ionospheric current is connected as a closure current to the FAC, ground-based observations covering a wide area of the ionosphere are indispensable. Based on EISCAT radar experiments, the aim of this study is to better understand the ionospheric current closure of FACs and the relative contribution of the electric field and the ionospheric conductance to the FAC
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