Transpolar voltage and polar cap flux during the substorm cycle and steady convection events

Abstract

[1] Transpolar voltages observed during traversals of the polar cap by the Defense Meteorological Satellite Program (DMSP) F-13 spacecraft during 2001 are analyzed using the expanding-contracting polar cap model of ionospheric convection. Each of the 10,216 passes is classified by its substorm phase or as a steady convection event (SCE) by inspection of the AE indices. For all phases, we detect a contribution to the transpolar voltage by reconnection in both the dayside magnetopause and in the crosstail current sheet. Detection of the IMF influence is 97% certain during quiet intervals and >99% certain during substorm/SCE growth phases but falls to 75% in substorm expansion phases: It is only 27% during SCEs. Detection of the influence of the nightside voltage is only 19% certain during growth phases, rising during expansion phases to a peak of 96% in recovery phases: During SCEs, it is >99%. The voltage during SCEs is dominated by the nightside, not the dayside, reconnection. On average, substorm expansion phases halt the growth phase rise in polar cap flux rather than reversing it. The main destruction of the excess open flux takes place during the 6-to 10-hour interval after the recovery phase (as seen in AE) and at a rate which is relatively independent of polar cap flux because the NENL has by then retreated to the far tail. The best estimate of the voltage associated with viscous-like transfer of closed field lines into the tail is around 10 kV.

IMF effect on the polar cap contraction and expansion during a period of substorms

“…Hence, we suggest that at least locally, the nightside reconnection rate may be higher in the recovery than in the expansion phase. This is in line with the studies by Milan et al (2007), Milan et al (2008) and Lockwood et al (2009).…”

“…Geophys., 31, 1021-1034, 2013 www.ann-geophys.net/31/1021/2013/ mean value is larger than the typical duration of an expansion phase, so this suggests that reconnection may take place at a significant amount in the recovery phases. Recently, Lockwood et al (2009) have found that the nightside reconnection voltages maximise in the recovery phase. The MIRACLE magnetometers showed that during recovery phases of Substorms 2-4, the westward electrojets continued to be intense within the main oval, well separated from the PCB.…”

Dayside and nightside contributions to cross-polar cap potential variations: the 20 March 2001 ICME case

“…A significant increase (30-40 kV) in the CPCP-NH (173 kV) is observed at this time. We hypothesize that this enhancement is due to an active magnetotail source of polar cap convection (see also Provan et al, 2004;Lockwood et al, 2009;Kullen et al, 2010). (1995) and Ruohoniemi and Baker (1998) (see also Chisham et al (2007)).…”

The pulsed nature of the nightside contribution to polar cap convection: repetitive substorm activity under steady interplanetary driving

“…The relative contributions to the cross-polar cap potential (CPCP) from the dayside (CPCP/day) and nightside (CPCP/night) sources during intervals of substorm activity have been studied in recent years, applying different observational techniques (Bristow et al, 2004;Lockwood et al, 2009;Kullen et al, 2010;Gordeev et al, 2011;Andalsvik et al, 2011Andalsvik et al, , 2012. Fox et al (1999), Grocott et al (2002) and Provan et al (2004) estimated the convection response to isolated substorms by continuous observations, applying a ground-based radar technique.…”