Temporal evolutions of the solar wind conditions at 1 AU prior to the near-Earth X lines in the tail: Superposed epoch analysis

Abstract : Utilizing conjunction observations of the Geotail and ACE satellites from 1998 to 2005, we investigated the temporal evolutions of the solar wind conditions prior to the formation of X lines in the near-Earth magnetotail. We first show the statistical properties of B_z, B_y, density, and velocity of the solar wind related to the 374 tail X line events. A superposed epoch analysis is performed to study the temporal evolutions of the solar wind conditions 5 h prior to the tail X lines. The solar wind conditions for tail X lines during southward interplanetary magnetic field (IMF) (SW-IMF) and northward IMF (NW-IMF) are analyzed. The main results are as follows: (1) For events classified as SW-IMF, near-Earth X line observations in the magnetosphere are preceded by ~2 h intervals of southward IMF; (2) for events classified as NW-IMF, the northward IMF orientation preceding near-Earth X line observations lasts ~ 40 min.