Multi-satellite and Conjugate Ground-based Studies of Magnetosphere-Ionosphere Coupling at Substorm Expansion Phase Onset

Abstract:

The explosive release of energy within a substorm marks the beginning of one of the most dynamic and energetically significant processes in the coupled solar-terrestrial system. Stored magnetic energy is quickly converted to plasma kinetic energy, resulting in dramatic changes in the large-scale magnetic topology of the Earth’s night-side magnetic field and in increases of the flux of energetic particles in near-Earth space, generating an apparently repeatable time series of events in the dynamic aurora, spanning many degrees of latitude and hours of local time. Whilst the processes leading to energy storage in the magnetotail are well-understood, the conditions which lead to rapid energy release rather than a more gradual dissipation of stored energy remain very poorly so. Here we examine the potential role of magnetosphere-ionosphere coupling (MIC), facilitated by field-aligned currents, in triggering large scale morphological changes in the magnetotail across many hours of local time. We present ground-based magnetometer and all-sky imager observations combined with conjugate in-situ observations of the magnetic fields and temperature anisotropies of electrons and ions from GOES, as well as the NASA Van Allen Probes and THEMIS satellites. By utilising the extensive ground coverage available from the CARISMA magnetometer and THEMIS All-Sky Camera arrays we resolve longitudinal and relative timing uncertainties between the measurement platforms at onset. We seek to establish a causal sequence of events and thereby examine especially the potential role of near-Earth MIC processes in the substorm sequence, particularly that of the Akasofu auroral evolution at onset – independent of whether this precedes or follows the onset of magnetic reconnection at the near-Earth neutral line.