Magnetic-field observations in the low-latitude magnetotail during substorms

Explorer 34 observations of the low-latitude tail field beyond 25 R_E are critically examined to see if the signature of the neutral-line formation is always visible during substorm expansion phases. Cases are found where a clear signature cannot be recognized. However, comparison of the simultaneous tail observations by multiple satellites suggests that the absence of a clear signature can largely be due to the spatial effect, namely due to the presence of the satellite outside the region where the local magnetic field condition is influenced by the neutral-line formation. On the other hand, evidences supporting the close association between the neutral-line and the expansion phase are found for substorm events having double expansion-phase onsets.