Abstract
<p>We study the role of substorms and steady magnetospheric convection (SMC) in magnetic flux transport in the magnetosphere, using observations of field-aligned currents (FACs) by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE).  We identify two classes of substorm, with onsets above and below 65<sup>o</sup> magnetic latitude, which display different nightside FAC 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.  We show that the high-latitude substorms, which do not experience braking, can evolve into SMC events if the interplanetary magnetic field (IMF) remains southwards for a prolonged period following onset.  Our results provide a new explanation for the differing modes of response of the terrestrial system to solar wind-magnetosphere-ionosphere coupling, as understood in the context of the expanding/contracting polar cap paradigm, by invoking friction between the ionosphere and atmosphere.</p>
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 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 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
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 antisunward transport of open flux and sunward 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., Cowley, 2000; Iijima & Potemra, 1976), resulting in an ionospheric twin-cell convection pattern (e.g., Heppner & Maynard, 1987, and references therein), which comprises antisunward plasma drift in the footprint of open field lines (known as the polar cap) and sunward plasma drift at lower latitudes. Grocott, et al (2009) and Grocott et al (2009) studied the auroral intensity and the convection response of substorms with different onset latitudes, that is, substorms that accumulated different amounts of open magnetic flux prior to onset.
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