Abstract
Abstract. The current-voltage relationship in the auroral particle acceleration region has been studied statistically by the Akebono (EXOS-D) satellite in terms of the charge carriers of the upward field-aligned current. The Akebono satellite often observed field-aligned currents which were significantly larger than the model value predicted by Knight (1973). We compared the upward field-aligned current estimated by three different methods, and found that low-energy electrons often play an important role as additional current carriers, together with the high-energy primary electrons which are expected from Knight's relation. Such additional currents have been observed especially at high and middle altitudes of the particle acceleration region. Some particular features of electron distribution functions, such as "cylindrical distribution functions" and "electron conics", have often been observed coinciding with the additional currents. They indicated time variability of the particle acceleration region. Therefore, we have concluded that the low-energy electrons within the "forbidden" region of electron phase space in the stationary model often contribute to charge carriers of the current because of the rapid time variability of the particle acceleration region. "Cylindrical distribution functions" are expected to be found below the time-varying potential difference. We statistically examined the locations of "cylindrical distribution function", and found that their altitudes are related to the location where the additional currents have been observed. This result is consistent with the idea that the low-energy electrons can also carry significant current when the acceleration region changes in time.
Highlights
The field-aligned currents play a key role in the magnetosphere-ionosphere coupling system
The purpose of this study is to examine the current-voltage relationship in the acceleration region statistically, using data obtained by Akebono, and discuss the results in terms of charge carriers of the upward field-aligned currents
On the basis of the LEP and MGF data obtained by Akebono, we have statistically examined the current-voltage relationship in the particle acceleration region quantitatively
Summary
The field-aligned currents play a key role in the magnetosphere-ionosphere coupling system. Frank and Ackerson, 1971; Bryant et al, 1978; Kaya et al, 1981) and they are believed to relate closely to generation mechanisms of the parallel electric field. Various mechanisms for the generation of parallel electric fields have been discussed for a few decades (Borovsky, 1993, and reference therein). A relationship between the field-aligned current density (J ) and the potential difference (V ) has been predicted by Knight (1973). In the limit of eV kTS, the Eq 1 becomes approximately linear.
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