The “Alfvénic surge” at substorm onset/expansion and the formation of “Inverted Vs”: Cluster and IMAGE observations

Abstract

From multipoint, in situ observations and imaging, we reveal the injection-powered, Alfvénic nature of auroral acceleration during onset and expansion of a substorm. It is shown how Alfvénic variations over time dissipate to form large-scale, inverted-V structures characteristic of quasistatic aurora. This characterization is made possible through the fortuitous occurrence of a substorm onset and expansion phase on field lines traversed by Cluster in the high-altitude acceleration region. Substorm onset was preceded by the occurrence of multiple poleward boundary intensifications (PBIs) and subsequent development/progression of a streamer toward the growth phase arc indicating that this is of the PBI-/streamer-triggered class of substorms. Onset on Cluster is marked by the injection of hot, dense magnetospheric plasma in a region tied to one of the preexisting PBI current systems. This was accompanied by a surge of Alfvénic activity and enhanced inverted-V acceleration, as the PBI current system intensified and striated to dispersive scale Alfvén waves. The growth of Alfvén wave activity was significant (up to a factor of 300 increase in magnetic field power spectral density at frequencies 20 mHz urn:x-wiley:jgra:media:jgra52576:jgra52576-math-0001 few hertz) and coincided with moderate growth (factor 3–5) in the background PBI current. This sequence is indicative of a cascade process whereby small-scale/dispersive Alfvén waves are generated from large-scale Alfvén waves or current destabilization. It also demonstrates that the initial PBIs and their subsequent evolution are an intrinsic part of the global auroral substorm response to injection and accompanying wave energy input from the magnetotail. Alfvénic activity persisted poleward of the PBI currents composing a broad Alfvén wave-dominated region extending to the polar cap edge. These waves have transverse scales ranging from a few tens of kilometers to below the ion gyroradius and are associated with large electric fields (up to 200 mV/m) and Poynting fluxes (up to 200 mW/m2 mapped at ionospheric altitudes). The fluctuations show mixtures of traveling and/or reflected (including standing) wave signatures, depending on frequency and location. A transition from incoming traveling wave to standing wave signatures is also seen near onset. Depending on location, electron distributions show signatures of Alfvén acceleration, inverted-V acceleration, or evidence of both. Poleward expansion of substorm emissions coincided with the poleward expansion of the hot energetic plasma, and the formation of a large-scale inverted-V current system with concurrent attenuation of Alfvénic fluctuations within the Alfvén-dominated region. We suggest that the attenuation is due to a dissipative effect owing to changes in the dispersive properties of these waves via the injection process and/or due to a transient magnetotail source. These findings suggest that in addition to playing active roles in driving substorm aurora, inverted-V and Alfvénic acceleration processes are causally linked.