On the relationships between double‐onset substorm, pseudobreakup, and IMF variation: The 4 September 1999 event

Abstract

[1] The relationships between double-onset substorm, pseudobreakup, and IMF variation were investigated with magnetic, auroral, and particle observations from space to the ground during 0200-0600 UT on 4 September 1999. There were five consecutive bursts of Pi2 pulsations on the ground during the time of interest. The onset time of ground Pi2s maps to the same variation sequence in the IMF structure seen propagating to the Earth in multiple satellite observations in the upstream region. The comparison of auroral and energetic particle data with IMF observations shows that a sequence of two doubleonset substorms intervened by a pseudobreakup appears in two distinct cycles of southward IMF followed by a northward interval. For the first substorm, the first onset begins when the B y magnitude declines after the IMF turns southward for about 90 min, and the second onset occurs after northward turning of the IMF accompanied by an increasing B y magnitude. The pseudobreakup appears while the IMF turns southward and the B y magnitude slightly decreases. For the second substorm, the first onset commences while the IMF remains southward with a steady B y magnitude, and the second onset occurs after the IMF becomes strongly northward and the B y magnitude decreases instead. These observations can be explained with the two-neutral-point model. The first onset occurs when the IMF turns southward. Reconnection at the near-Earth neutral point first begins on closed field lines within the plasma sheet, and the second onset occurs when the IMF turns northward and reconnection at the distant neutral point ceases and reconnection at the near-Earth neutral point may reach the open flux of the tail lobes. In addition, a decrease in the B y magnitude may help reduce magnetotail convection and release all the built-up flux to allow the onset to commence after northward turning of the IMF. If the IMF remains southward, the reduction of magnetotail convection due to a decreasing B y would lead to a pseudobreakup instead. In contrast, an increasing B y magnitude would increase magnetotail convection and weaken magnetospheric substorm after the IMF turns northward. Consequently, for the occurrence of double-onset substorms and pseudobreakups, not only the first onset begins spontaneously during steady southward IMF and the second onset appears after northward turning of the IMF but the B y change also affects magnetotail convection which may evoke (or abate) the substorm-related activation while the IMF turns southward (or northward).

THEMIS observations of double-onset substorms and their association with IMF variations

“…7 for the aurora-related westward electrojet at the first onset to appear at lower latitude than that at the second onset for each activation period. This feature is consistent with the findings by Cheng et al (2005) with the meridian scanning photometer data. Namely, the above tendency in the INTERMAG-NET magnetograms justifies the inference on the source sites 600 C.-C. Cheng et al: THEMIS observations of double-onset substorms of two Pi2 bursts for each substorm through the spectral analysis in the aforementioned subsection.…”

“…More ground and space observations (e.g. Mishin et al, 2001;Cheng et al, 2005) demonstrated that nightside disturbances can have two or more onsets during a substorm sequence. However, these early studies did not have well-coordinated measurements for further verifications of the link of ground Pi2s with other substorm signatures in the nightside magnetosphere.…”

“…Saito, 1969;Clauer and McPherron, 1974) in association with the variations of the interplanetary magnetic field (IMF) (e.g. Cheng et al, 2005Cheng et al, , 2009a. Determining how these successive bursts are triggered and their relation to the onset of auroral activations is still an important and unresolved topic.…”

Comparison of chaotic aspects of magnetosphere under various physical conditions using AE index time series

“…It is still unclear to what extent the fluctuations in the solar wind are reflected in the fluctuations of magnetosphere-ionosphere system. Numerous studies have tried to approach this problem by using both deterministic and stochastic paradigms (Tsurutani et al, 1990;Hnat et al, 2005;Cheng et al, 2005;Pulkkinen et al, 2006).…”

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

“…The onset times of the two have only 3 min difference. In some previous studies (e.g., Cheng et al, 2005Cheng et al, , 2011, this kind of events was named as a double-onset substorm, which was explained as the effect of the reconnection at the near-earth neutral point on closed field line within the plasma sheet and on the open flux of tail lobes. However, if that is really the case, the second onset should be at latitudes higher than the first one, which is not the case in our event.…”