Substorm effects on the neutral sheet inside 10 Earth radii

In this paper evidence is provided 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 approximately 7 Rsub(E). ATS 1, in geostationary orbit at 6.6 Rsub(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 approximately 8 Rsub(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 8Rsub(E) from the Earth, just above the geomagnetic equator and the observations in this region strongly support their findings. (Auth.)