Abstract
AbstractWe study the role of substorms and steady magnetospheric convection (SMC) in magnetic flux transport in the magnetosphere, using observations of field‐aligned currents by the Active Magnetosphere and Planetary Electrodynamics Response Experiment. We identify two classes of substorm, with onsets above and below 65° magnetic latitude, which display different nightside field‐aligned current morphologies. We show that the low‐latitude onsets develop a poleward‐expanding auroral bulge, and identify these as substorms that manifest ionospheric convection‐braking in the auroral bulge region as suggested by Grocott et al. (2009, https://doi.org/10.5194/angeo-27-591-2009). We show that the high‐latitude substorms, which do not experience braking, can evolve into SMC events if the interplanetary magnetic field remains southward for a prolonged period following onset. We conclude that during periods of ongoing driving, the magnetosphere displays repeated substorm activity or SMC depending on the rate of driving and the open magnetic flux content of the magnetosphere prior to onset. We speculate that sawtooth events are an extreme case of repeated onsets and that substorms triggered by northward‐turnings of the interplanetary magnetic field mark the cessation of periods of SMC. Our results provide a new explanation for the differing modes of response of the terrestrial system to solar wind‐magnetosphere‐ionosphere coupling by invoking friction between the ionosphere and atmosphere.
Highlights
The dynamics of the magnetosphere are driven primarily by the interaction of the solar wind and embedded interplanetary magnetic field (IMF) with the terrestrial field through the process of magnetic reconnection
We show that the high-latitude substorms, which do not experience braking, can evolve into steady magnetospheric convection (SMC) events if the interplanetary magnetic field (IMF) remains southwards for a prolonged period following onset
Observations and Discussion This paper focusses on three aspects of solar wind-magnetosphere coupling, substorms, and substorm-related fieldaligned currents (FACs): how does the FAC response vary with substorm onset lati154 tude?; what is the relationship between substorms and steady magnetospheric convection?; and what do the FAC systems tell us about magnetosphere-ionosphere coupling during substorms? We investigate these three themes in turn
Summary
The dynamics of the magnetosphere are driven primarily by the interaction of the solar wind and embedded interplanetary magnetic field (IMF) with the terrestrial field through the process of magnetic reconnection. During periods of southward-directed IMF this excites the Dungey cycle of circulation – or convection – of the field and plasma within the magnetosphere, in which reconnection at the subsolar magnetopause creates open magnetic flux and reconnection in the magnetotail closes flux again, with a general antisunwards transport of open flux and sunwards return flow of closed flux [Dungey, 1961] This transport is communicated to the polar ionosphere by an electrical current system linking the magnetopause, ionosphere, and ring current [e.g., Iijima and Potemra, 1976; Cowley, 2000], resulting in an ionospheric twin-cell convection pattern [e.g., Heppner and Maynard , 1987, and references therein], which comprises antisunwards plasma drift in the footprint of open field lines (known as the polar cap) and sunwards plasma drift at lower latitudes. Dayside and nightside reconnection can occur independently of one another, leading to changes in the open magnetic flux content of the magnetosphere, with attendant changes in the size of the ionospheric polar caps; the flux transport and convection associated with these changes is described by the expanding/contracting polar cap (ECPC) model [e.g., Siscoe and Huang, 1985; Cowley and Lockwood , 1992; Hubert et al, 2006a; Milan, 2015]
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