Thinning and expansion of the substorm plasma sheet

More than 50,000 measurements with the AMPTE IRM satellite in the magnetotail are used for a superposed epoch analysis of the occurrence frequency of the near‐Earth central plasma sheet, which is directly related to its thickness, around 39 major substorm onsets. For all radial distances between 10 and 19 RE, the central plasma sheet gets thinner during the 30–45 min preceding substorm onset, but starts to expand immediately after the onset. The central plasma sheet regains its initial thickness around the beginning of the recovery phase. These observations fit into the framework of the near‐Earth neutral line model, if the neutral line is assumed to form beyond the IRM apogee.