Структура и динамика полярных токовых систем : материалы международного симпозиума «Полярные геомагнитные явления», 25-31 мая, Суздаль, СССР / Акад. наук СССР, Кол. фил. им. С. М. Кирова, Поляр. геофиз. ин-т. – Апатиты : [б. и.], 1988.

For studying physical mechanisms involved in field-aligned currents, it is of course desirable to make a simultaneous in situ measurement for the electric and magnetic fields and the charged particle motions by one and the same satellite, even if the accuracy of the magnetic field measurement is slightly less due to the contamination from other instruments on board the satellite. A number of recent satellites have contributed to the study of the phenomena associated with field-aligned currents and various important conclu sions from their observations have been reported, for example, in the papers by ftlwnpar (1979), Maier et al. (1960), Klumpar and Heikkila (1982) with ISIS-2 data; by Bythrow et al.(1980) with AE-C data; by Sugiura et al.(1982), Burch et al.(1983), Sugiura (1984), Sugiura et al.(1984), Burch et al.(1985) with DE-1 and -2 data; by Burke et al.(1983), Burke (1984), Denig and Rich (1986) with S3-2 data; by Aggson et al.(1983), Maynard et al.(1983) with ISEE-1 data; by iihuzgov et al.(1983), Arshinkov et al. (1984,19&5), Bochev et al.(1984), Dolginov et al.(1984), Chmyrev et al.(1985), Bankov et al.(1986) with Intercosmos Bulgaria-1300 data; by Dronov and Tverskaya (1984) with Kosmos-900 data; by Bythrow et al.(1984) and Fremouw et al.(1985) with HILAT data. ELECTRIC CURRENT IN THE IONOSPHERE AT THE TIME OP SC. A comparison of the SC (sudden commencement of magnetic storms) magnitudes observed above and below the ionosphere will give us a clue to know whether or not a westward zonal electric current flows in the ionosphere to shield, to some extent, a sudden northward magnetic field due to a compression of the magnetosphere by an intensified solar wind at the time of SC. However, this study encounters a fundamental difficulty in deducing the SC magnitude observed by MAGSAT, which flies over the earth with location-dependent magnetic field, sometimes with magnetic anomalies. Furthermore, it will seldom happen that an SC occurs when the satellite is just over the vicinity of a magnetometer chain or even a magnetic observatory. Three papers have been hitherto published on this subject, i.e. by Araki et а1.(1982), Araki et al.(1984a), and Kane (1985). The conclusions in these papers differ slightly each other. Araki et al.(1984a) reported that the SC amplitude was usually greater at the MAGSAT level than on the ground (average ratio 1.3), whereas Kane (1985) recognized no significant difference in the total SC magnitudes observed above and below the ionosphere in the dawn and dusk regions. The disagreement in their conclusions seems to have been origin ated from the difference in their methods of data analysis. We may however safely conclude that the electromagnetic shielding of the ionosphere is not so great as to result in a considerable reduction of the SC magnitude through the ionosphere when the effect of magnetic field compression propagates from the magnetosphere to the ground on the dawn and dusk sides of the earth. It will be worth noting here that Araki et al. (1982,1984a) pointed out the first experimental evidence for SC* current in ionosphere. Fig.4 shows an example of the comparison between the magnetic field changes observed by MAGSAT and on the ground over Japan - Western Siberia on 19 March 1980. We see here that the D-component magnetic variation is in opposite directions in the MAGSAT and ground data, and this is attributable to a southward electric current flowing in the ionosphere. The example shown in Fig.4 is the important observational evidence for the electric current flowing in the ionosphere at the time of SC*, possibly as the Hall current produced by an imposed electric potential in the polar regions. 79