Abstract
Abstract. We report on THEMIS in-situ and ground-based observations during a substorm between 04:30~04:50 UT on 22 February 2008. The spacecraft (probes) were aligned along the tail between XGSM=−5 RE to −25 RE. The most distant probe P1 (X=−24.5 RE) detected two successive tailward moving bipolar magnetic structures. P2 (X=−18 RE), P3 (X=−11 RE), P4 (X=−10.5 RE) all captured signatures related to the Earthward movement of a magnetic structure. THEMIS ground stations and all-sky imagers also recorded Pi2 pulsations and a sudden brightening in a white-light auroral imager followed by poleward expansion. We perform a detailed timing analysis of probe and ground-based data and reconstruct the time sequence of phenomena during this substorm. The earliest sign of substorm onset was the bipolar perturbation in the northward component of the magnetic field (interpreted as the result of reconnection onset) at P1 at 04:35:16 UT and corresponding magnetic perturbation at P2 at 04:35:14 UT. Auroral onset was seen at or before 04:36:55 UT, consistent with the visual onset of high-latitude magnetic pulsations at around that time. Earthward flows at P3 and P4 seen at ~04:36:03 UT, and dipolarization onset at ~04:36:50 UT, were observed at almost the same time as the ground onset signature, implying that near-Earth dipolarization happened in the aftermath of tail reconnection but not significantly ahead of the auroral intensification. Reconnection in the tail preceded ground onset and near-Earth dipolarization (current disruption) by ~2 min. Two reconnection pulses (the first one weaker than the second one) accompanied by correlative increases of cumulative magnetic flux transfer into the reconnection region were observed. A direct association of the reconnection pulses with two auroral intensifications can be made, suggesting that tail reconnection, like the auroral expansion, advances in steps rather than continuously.
Highlights
Substorms are global reconfigurations of the magnetosphere involving solar wind energy storage in Earth’s magnetotail and abrupt conversion of energy to particle heating and kinetic energy (Akasofu, 1964; Axford, 1999)
The sense of the bipolar signature depends on the location of the satellites relative to the reconnection site: positive--negative Bz is observed for a tailward moving nightside flux transfer events (NFTEs), whereas negative--positive Bz is observed for an Earthward moving NFTE
Our observations indicate that tail reconnection onset happened at or prior to 04:35:14 UT, between Probe 1 (P1) and P3, and most likely tailward of P2
Summary
Substorms are global reconfigurations of the magnetosphere involving solar wind energy storage in Earth’s magnetotail and abrupt conversion of energy to particle heating and kinetic energy (Akasofu, 1964; Axford, 1999). The THEMIS mission (Angelopoulos, 2008; Sibeck and Angelopoulos, 2008) was designed to address the question of which mechanism of substorm onset is supported by the data and how the different parts of the magnetosphere are connected causally It employs five satellites (probes) on orbits enabling them to align parallel to the Sun-Earth line once per four days. The event was first briefly reported in the on-line materials and methods section of Angelopoulos et al (2008) and is similar to other events presented in that paper, as well as the event from Runov et al (2008) In all these events auroral intensification was observed in the aftermath of magnetic reconnection onset, and preceded or happened at approximately the same time as current disruption onset. The dipolarization onset is seen in the aftermath of the Earthward flow at 10 RE and is likely due to the reconnection process further downtail. (2) It shows tail reconnection proceeds in a step-like fashion that can have a one-to-one correspondence to auroral activations, strengthening the aforementioned recent findings of a direct association between mid-tail reconnection and auroral activations during the early stages of substorm expansion
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