The Dynamics of Plasma Convection in the Polar Cap Ionosphere under Northward IMF Conditions

Abstract:

During periods of northward IMF the site for anti-parallel merging on the dayside magnetopause is known to shift tailward of the cusp and to drive ‘reverse connection’, i.e., sunward plasma flows, deep in the polar cap ionosphere. These flows reach to auroral latitudes on the dayside and then rotate to return along the dawn and dusk flanks, forming relatively small reverse convection cells. The manner in which the flows in the polar cap respond to northward turnings of the IMF and to variations under predominantly northward IMF is not known. For example, how quickly does plasma convection respond to northward turnings and to their reversals, how closely coupled is the convection response to variations in IMF magnitude, and do the flow velocities saturate if the driving conditions become extreme? The first direct images of a reverse convection cell were generated in the mid-1990s using the Kapuskasing and Saskatoon SuperDARN radar pair. SuperDARN radars have since been built at higher latitudes in both hemispheres to provide deep coverage within the polar cap, namely, three PolarDARN radars in the northern hemisphere and a radar at McMurdo in the southern hemisphere. A solar coronal mass ejection (CME) in late September 2014 brought initially large (> 20 nT) and variable northward IMF to Earth that was followed by days of more moderate but sustained northward IMF. The polar cap was observed during this time by a suite of instruments including the Resolute Bay Incoherent Scatter Radar (RISR), PolarDARN, and DMSP satellites. Dramatic effects were observed in the polar cap flow which indicate, that, at times, the coupling between solar wind IMF and polar cap flow is very direct, even quasi-linear. In this talk we describe the dynamics of the polar cap flow observed under both time-varying and steady northward IMF and examine the connections to solar wind drivers and to the physics of dayside reconnection.