The EXOS‐D fluxgate magnetometer data obtained at 3000–10000 km altitude have shown that small‐scale field‐aligned currents always exist in large‐scale region 1, region 2, cusp and polar cap current systems. Assuming that these small‐scale field‐aligned currents have current sheet structure, the width of current sheet is estimated to be 5–20 km at ionospheric altitude. By comparing the magnetometer data with charged particle and high frequency plasma wave data simultaneously obtained from EXOS‐D, it is found that small‐scale currents have one‐to‐one correspondence with localized electron precipitation events characterized by flux enhancement over a wide energy range from 10 eV to several keV and broadband electrostatic bursts occasionally extending above local plasma frequencies or electron cyclotron frequencies.

Characteristics of field‐aligned currents (FACs) in the evening and morning regions during the March 13–14, 1989, great magnetic storm have been determined using magnetic and electric field data obtained from the EXOS D spacecraft. This storm began with an SSC at 0128 UT on March 13, and the second SSC occurred at 0747 UT on the same day. The storm continued until March 14. The equatorward boundary of the FAC region began to move equatorward right after the first SSC in both the evening and morning sectors, but the poleward boundary did not immediately respond to the SSC. The equatorward boundary of the FAC system reached as low as below 48° invariant latitude, which corresponds to L = 2.2, and the latitudinal width of the FAC region increased greatly, particularly in the morning sector (∼33° in invariant latitude). In the evening sector the conventional current system characterized by a pair of upward region 1 and downward region 2 FACs changed into complicated patterns consisting of many pairs of upward and downward FACs with the development of the storm, particularly around 22 UT on March 13 when an intense eastward electrojet was observed as low as 50° invariant latitude on the ground. In the morning sector an additional large‐scale upward FAC was observed poleward of the conventional downward region 1 and upward region 2 FAC system throughout the storm. In addition, a pair of FACs with a narrow latitudinal width (∼1.5°) was observed at the poleward boundary of the extra upward FAC.

Magnetosphere-ionosphere coupling processes in inverted-V electron precipitation regions have been investigated ing particle and magnetic field data obtained from two antarctic sounding rockets S-310JA-11, and S-310JA-12, launched into quiet and active auroral arcs, respectively. These data have suggested the presence of a liltear relation between field-aligned current and fieldaligned potential difference, i.e., Jll = KVII , given by Fridman and Lemaire [1980] and Lyons [1980]. The density n and thermal energy E0of primary electrons have been obtMned by fitting accelerated Ma.xwellian distribution functions to tile observed electron energy spectra. It is found that the thermal energy E0 is larger illside the arcs than outside tile arcs for both the quiet and active arcs observed, suggesting that precipitating electrons are heated during the field-aligned acceleration. In contrast, tile electron density n is enhanced outside tile arcs for the quiet arc eVelit, while it is enhanced inside tile arcs for the active arc event. !,'r(,•l nn investigation of the relation between E0 and Vii and also the relation between Eo and ill, it is suggested that tile heating occurs effectively wheia tile current intensity exceeds a certain threshold and that the electron heating rate is 5-40% of the energy gain due to the field-aligned acceleration. Assuming that this heating is due to Jo,le heating thro,gh anomalous resistivity, it is also SUggest, e([ that tile total ano•alous resistivity along tile magnetic field line decreases with an increase of R.

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Magnetic field observations of a planetary magnetosphere and interplanetary space are fundamental in clarifying the structure of the planetary magnetosphere and its interaction with solar wind. The inherent magnetic dipole of Mercury was discovered by flyby observations performed by Mariner 10; however, the structure of the magnetic field and its interaction with solar wind have not been sufficiently investigated. A Japanese fluxgate magnetometer is to be installed on the BepiColombo Mercury magnetospheric orbiter. During the interplanetary space flight and in orbit around Mercury, the sensor of the magnetometer, which is placed on an extension mast, will be exposed to temperatures ranging from −100 to +175 °C. However, the sensor characteristics—sensitivity, magnetic offset and noise—are temperature dependent. In particular, data pertaining to the sensor characteristics over a wide temperature range from −160 to +200 °C are not available. We developed a wide-range temperature (from −160 to +200 °C) testing system with a magnetic shield box and tested two types of sensor core supports (Inconel and ceramic core bobbins). As a result, we obtained the characteristics of the sensor sensitivity, offset and noise and found that the noise performance of the ceramic core bobbin was superior to the Inconel core bobbin.