Substorm‐like magnetospheric response to a discontinuity in the Bx component of interplanetary magnetic field

Abstract

We examined the magnetospheric magnetic field and plasma responses to an encounter of a discontinuity in the Bx component of interplanetary magnetic field (IMF). The striking variations of simultaneous solar wind dynamic pressure and IMF‐Bz were not observed. Furthermore, we found that this IMF‐Bx discontinuity was a heliospheric current sheet, separating two high‐speed solar wind streams with different velocity and magnetic polarity. In this study, the magnetic field and plasma data were obtained from Time History of Events and Macroscale Interactions during Substorms (THEMIS), Cluster, and GOES to investigate the magnetospheric responses, and those were taken from ACE and Geotail to monitor the solar wind conditions. Simultaneous geomagnetic field variations from the ground observatories and aurora activity from Polar were also examined. When the discontinuity encountered the magnetosphere, THEMIS‐D, ‐E, and THEMIS‐A observed abrupt and transient magnetic field and plasma variations in the dawnside near‐Earth magnetotail and tail‐flank magnetopause. Significant magnetic field perturbations were not observed by Cluster as located in the duskside magnetotail at this time interval. Although simultaneous dipolarization and negative bay variations with Pi2 waves were observed by GOES and the ground observatories, global auroral activities were not found. Around the dawnside tail‐flank magnetopause, THEMIS‐C and ‐A experienced the magnetopause crossings due to the magnetopause surface waves induced by Kelvin‐Helmholtz instability. These results suggest that the magnetic field and plasma variations in the near‐Earth magnetotail and tail‐flank magnetopause were caused by moderate substorm‐like phenomena and magnetopause surface waves. They also indicate that clear magnetospheric disturbances can be brought even without significant variations in the solar wind.