Ionospheric response to oscillatory flow braking in the magnetotail

Abstract

We study the ionospheric response to oscillatory braking of bursty bulk flow observed by THEMIS on 17 March 2008 between 10:22 and 10:36 UT. By calculating different current components generated in the plasma sheet and correlating the space and ground observations, we discriminate the ionospheric current relevant to the large‐scale substorm wedge currents produced by the general reconfiguration of the magnetotail pressure gradient from the currents that appeared as a result of the flow oscillation. While the former currents are large and quasi‐stable, the latter (oscillating) currents are substantially (2–3 times) weaker and flow in opposite directions during earthward and tailward flow bursts. The oscillating currents include the polarization current and the current generated by the oscillating part of the pressure gradient. The two oscillating currents appear to produce modulation of the ionospheric currents (with about 2.5 min period) that was seen as Pi2 pulsations in the ground magnetometer observations. Our estimates of the ionospheric conductance suggest that the damping of the plasma sheet flow oscillation is due to heating the ionosphere through Pedersen currents. We also found that the all‐sky imager at Fort Yukon observed four auroral forms during the first two periods of the oscillatory flow braking: two auroral forms related to the earthward plasma sheet flows and the other two auroral forms related to the tailward rebounds of the earthward flow. The auroral forms evolve in accordance with the appearance and motion of the upward field‐aligned current spot of the modulated part of the ionospheric field‐aligned current.