资源环境科技发展态势分析平台

We study the effects of the east-west (y) component of the interplanetary magnetic field (IMF) on the occurrence of substorms by analyzing 16,743 magnetic substorm events identified with the SuperMAG SML index from 1995 to 2016. It is found, surprisingly, that substorm occurrence rates depend highly on the sign of IMF B-y, with, on average, similar to 1/3 more substorms for IMF B-y > 0 than for IMF B-y < 0. We attribute this asymmetry to the enhanced convection (e.g., more energy in the tail) under IMF B-y > 0 conditions. A superposed epoch analysis of the IMF indicates that the average IMF B-y prior to onset is positive but becomes less positive similar to 15 min prior to the onset, indicating that the release of the stress associated with a clockwise twisted magnetotail may be an important onset trigger. We conjecture that an asymmetry in the dayside merging efficiency may be the cause.