Substorm Effects on the Neutral Sheet Inside 10 Earth Radii

Abstract

The major features of the Earth’s geomagnetic tail have been known for some time (Heppner et al., 1963; Cahill, 1965; Ness, 1965). On the night side of the Earth the field lines are stretched out away from the Sun. The solar and anti-solar directed field lines are separated by a neutral sheet in which the field magnitude is small and across which the field direction reverses. The neutral sheet region is inflated by a plasma sheet, several R _E thick, which diamagnetically reduces the field magnitude near to the neutral sheet and provides the pressure to maintain the solar and antisolar field separation. There is usually a small component of the field normal to the sheet indicating that the field lines pass through it (Speiser and Ness, 1967). However most sightings of a well defined neutral sheet have been at fairly large distances down the tail and there is therefore some controversy as to how close to the Earth the neutral sheet may be found. It is generally regarded that the Earth’s field is tail-like beyond 10 R _E behind the Earth and that the neutral sheet is parallel to the Earth-Sun line and stretches away behind the Earth from a point hinged at 10 or 11 R _E from the Earth on the geomagnetic equator. Away from the midnight meridian it is assumed to curve towards the geomagnetic equator with an approximately circular cross section (Russell and Brody, 1967). To our knowledge, there are no reports of neutral sheet encounters inside 10 R _E and it is generally regarded that the neutral sheet does not approach nearer to the Earth than this. In this paper we provide evidence that during the growth phase of a substorm or during very disturbed periods the neutral sheet may be observed as close to the Earth as ~7 R _E. ATS 1, in geostationary orbit at 6.6 R _E, observes more tail-like fields prior to a substorm onset but no neutral sheet observations have been reported. During the growth phase of a substorm the plasma sheet at ~8 R _E from the Earth has been observed to become very thin (McPherron, 1973; McPherron et al., 1973a; Kivelson et al.,1973). It has been proposed that coincident with the substorm onset the plasma sheet may neck to form a localized neutral point which then propagates down the tail (McPherron et al., 1973b). This hypothesis is deduced from OGO-5 observations at 8 R _E from the Earth, just above the geomagnetic equator and our own observations in this region strongly support their findings.