THEMIS and ground-based observations of successive substorm onsets following a super-long growth phase

Abstract. We present four successive substorm events, which followed a super-long, as long as 9 h, growth phase on 5 December 2008, observed by the Time History of Events and Macroscale Interaction during Substorms (THEMIS) and the GOES 11 satellite with simultaneous coverage by the Alaska and THEMIS ground magnetometers. Several interesting and unique features were found for these cases. The interplanetary magnetic field was steadily southward and the solar wind speed was slow, less than 450 km s⁻¹, which are thought to drive the long growth phase for the following onsets. At least four substorm expansion onsets occurred, including a double-onset event, which appears to be a challenge to the reconnection hypothesis for double-onset substorm and favored an instability mechanism for the onsets and could not be explained by the two neutral line models. For the onsets at 09:32 UT and 09:42 UT, the dipolarization signature was observed by GOES 11, which was located earthward of THEMIS C and THEMIS B. THEMIS C satellite caught a delayed and much weaker signature 1–3 min after GOES 11. THEMIS B observed no relating signature. These observations provide us with direct evidence that these events initiated at the near-earth region. The observations of THEMIS C and THEMIS B around the onsets favor the near-earth instabilities model for substorm onset.