Substorm‐Ring Current Coupling: A Comparison of Isolated and Compound Substorms

Abstract

Substorms are a highly variable process, which can occur as an isolated event or as part of a sequence of multiple substorms (compound substorms). In this study we identify how the low‐energy population of the ring current and subsequent energization varies for isolated substorms compared to the first substorm of a compound event. Using observations of H+ and O+ ions (1 eV to 50 keV) from the Helium Oxygen Proton Electron instrument onboard Van Allen Probe A, we determine the energy content of the ring current in L‐MLT space. We observe that the ring current energy content is significantly enhanced during compound substorms as compared to isolated substorms by ∼20–30%. Furthermore, we observe a significantly larger magnitude of energization (by ∼40–50%) following the onset of compound substorms relative to isolated substorms. Analysis suggests that the differences predominantly arise due to a sustained enhancement in dayside driving associated with compound substorms compared to isolated substorms. The strong solar wind driving prior to onset results in important differences in the time history of the magnetosphere, generating significantly different ring current conditions and responses to substorms. The observations reveal information about the substorm injected population and the transport of the plasma in the inner magnetosphere.

Electron precipitation characteristics during isolated, compound and multi-night substorm events

“…This criterion is different from earlier substorm studies, which mostly require a quiet time of about 3 hours prior to isolated substorms. As pointed out by Sandhu et al (2019), a stronger solar wind driving was often maintained for several days prior to the compound substorm onsets.…”

“…In fact, auroral emission in many of these events extended from Lapland to the southern part of the Svalbard archipelago, being observed in the ground-based auroral images there. The inner magnetospheric observations by Sandhu et al (2019) showed that the energy content in the ring current was azimuthally more localised during isolated than compound substorms, leading to smaller-scale substorms. However, they concluded that the latitude (L shell) extent of the enhancements varied less, suggesting that the same would apply to the high-energy part of the substorm precipitation.…”

“…Compound substorms are defined as consecutive expansion-recovery phase pairs that are not interrupted by quiet time or a growth phase. A similar definition was employed by Sandhu et al (2019). In case of compound substorms, the substorm onset is the beginning of the first expansion phase.…”

Average Ionospheric Electric Field Morphologies During Geomagnetic Storm Phases

“…Whilst the majority of storms are of similar length, it provides a good framework for studying the average storm-time responses; however, other time-dependent phenomena, such as substorms, are averaged out. It is well known that substorms frequently occur during geomagnetic storms and are important for the energization of the ring current (e.g., Daglis, 2006;Sandhu et al, 2019), but Grocott et al (2009) showed that substorms primarily produce a response in the high-latitude ionospheric convection pattern on the nightside and that ordering by onset location is important when trying to gain insight from the average convection pattern. It thus follows that although substorms commonly occur during geomagnetic storms, we do not see their signatures.…”

Electron precipitation characteristics during isolated, compound, and multi-night substorm events

“…Instead of the AL, Partamies et al (2015) used a regional electrojet index (an IL version they called IL ASC ) constructed from baselined data collected at five Lapland magnetometer stations of the IMAGE network (Tanskanen, 2009) E). The selected stations were co-located with the MIRACLE auroral all-sky cameras (Sangalli et al, 2011), allowing the auroral morphology to be analysed over the same area. They also concluded that the long-term median value of −50 nT was valid for this regional index as well.…”