The Onset of a Substorm and the Mating Instability

Abstract

During a substorm, the energy accumulated in the tail is released and injected into the magnetosphere. The energy accumulation phase, the growth phase, begins with magnetic reconnection on the dayside leading to the transport of magnetic flux across the polar cap into the tail. Stretching and compressing of the tail field by the solar wind leads to thinning of the central plasma and current sheets. This process is accompanied by the transport of closed magnetic field from the near-Earth tail and nightside magnetosphere toward the morning and evening magnetopause. The equatorward motion of the southernmost auroral arc, the growth phase arc (GPA), is a consequence of that. At substorm onset, this or a nearby arc starts brightening and is followed within a few minutes by a poleward expansion of auroral activity proceeding in successive steps (Akasofu, 1964;Oguti, 1973;Sergeev et al., 2000). This definition of the substorm onset is identical with the one given by Akasofu (1964) under the designation "expansive phase". He distinguished two stages of the onset, (a) sudden brightening and (b) rapid poleward expansion. He actually defined a third stage for the

Space Plasma Physics: A Review

“…There are other scenarios for substorms, possibly hundreds of them. We mention just a few: [180], [181], [182], [183], [184], [185], [186], [187]. There may be many different types of substorms fitting these different scenarios (Akasofu, personal communication, 2018).…”

How Energy Enters the Magnetosphere During a Substorm: A Perspective After 60 Years of Working in the Field

“…Unfortunately, they are preceded by the so-called auroral beads and for a long time not recognized as owed to a different process (e.g., Haerendel, 2015b;Liang et al, 2008;Sakaguchi et al, 2009). Recently, we could clearly demonstrate in Haerendel and Frey (2021) that the auroral beads are caused by an instability of the high-beta plasma layer in the outer layers of the quasi-dipolar magnetosphere forming during the growth phase. We ascribed their origin to a "mating instability" enabled by a contact with an auroral streamer.…”

“…A paper on the breakup arc followed (Haerendel, 2009), one on the equatorward moving arcs before onset (Haerendel, 2010), a joint work with Harald Frey on the poleward Alfvénic arcs (Haerendel & Frey, 2014), and two papers on the substorm onset (Haerendel, 2015a(Haerendel, , 2015b. A last publication in this context (Haerendel & Frey, 2021) contributed only indirectly to this theme by clarifying that the appearance of auroral beads shortly before breakup was a side effect, namely an instability of the high-beta plasma assembled in the outer magnetosphere during the growth phase.…”

Statistical Characterization of the Dynamic Near‐Earth Plasma Sheet Relative to Ultra‐Low Frequency (ULF) Wave Growth at Substorm Onset

“…There have been several candidate plasma instabilities proposed to generate such ULF waves in the near‐Earth magnetotail (A. Lui, 2004). These include but are not limited to: the cross‐field current instability (A. T. Y. Lui et al., 1991); ballooning instabilities such as the shear flow (Voronkov et al., 1997), pressure anisotropy (Oberhagemann & Mann, 2020) and kinetic variants (Cheng, 2004; Panov et al., 2012); the current‐driven Alfvénic instability (Perraut et al., 2000); or other non‐magnetohydrodynamic (non‐MHD) processes (Haerendel & Frey, 2021; Nishimura et al., 2022). However, ULF waves in the period band of interest may also be generated by convective flows in the magnetotail (e.g., Horvath & Lovell, 2019; Keiling & Takahashi, 2011; Kim et al., 2007; Ream et al., 2015).…”