Synoptic survey for the neutral line in the magnetotail during the substorm expansion phase

The magnetic structure of the magnetotail during the expansion phase of the substorm is examined by summarizing observations made at more than fifty different locations scattered over the (solar magnetospheric) range of 0 ≳ x ≳ −65 RE, 20 ≳ y ≳ −20 RE, and 30 ≳ |z| ≥ 0 RE. It is found that all across the local-time sector of |y| ≲ 15 RE, the neutral line is formed somewhere between x = −10 and −30 RE within ∼10 min of the onset of the substorm expansion phase. This is due to the severe weakening or disruption of the tail current inside x ≃ −20 RE. The neutral line continues to exist for about 1 hour, and its disappearance occurs almost as quickly as its formation. The role of the neutral line on the substorm process is discussed in the light of the above observation.