Abstract
Abstract. The expansion-contraction model of Dungey cell plasma convection has two different convection sources, i.e. reconnections at the magnetopause and in the magnetotail. The spatial-temporal structure of the nightside source is not yet well understood. In this study we shall identify temporal variations in the winter polar cap convection structure during substorm activity under steady interplanetary conditions. Substorm activity (electrojets and particle precipitations) is monitored by excellent ground-satellite DMSP F15 conjunctions in the dusk-premidnight sector. We take advantage of the wide latitudinal coverage of the IMAGE chain of ground magnetometers in Svalbard – Scandinavia – Russia for the purpose of monitoring magnetic deflections associated with polar cap convection and substorm electrojets. These are augmented by direct observations of polar cap convection derived from SuperDARN radars and cross-track ion drift observations during traversals of polar cap along the dusk-dawn meridian by spacecraft DMSP F13. The interval we study is characterized by moderate, stable forcing of the magnetosphere-ionosphere system (EKL = 4.0–4.5 mV m−1; cross polar cap potential (CPCP), Φ (Boyle) = 115 kV) during Earth passage of an interplanetary CME (ICME), choosing an 4-h interval where the magnetic field pointed continuously south-west (Bz < 0; By < 0). The combination of continuous monitoring of ground magnetic deflections and the F13 cross-track ion drift observations in the polar cap allows us to infer the temporal CPCP structure on time scales less than the ~10 min duration of F13 polar cap transits. We arrived at the following estimates of the dayside and nightside contributions to the CPCP (CPCP = CPCP/day + CPCP/night) under two intervals of substorm activity: CPCP/day ~110 kV; CPCP/night ~50 kV (45% CPCP increase during substorms). The temporal CPCP structure during one of the substorm cases resulted in a dawn-dusk convection asymmetry measured by DMSP F13 which is opposite to that expected from the prevailing negative By polarity of the ICME magnetic field, a clear indication of a nightside source.
Highlights
The temporal structure in polar cap convection and the associated cross polar cap potential (CPCP) has been the subject of much study over the years
The position of eight magnetometer stations in the IMAGE chain which are central in this study are marked by solid dots. These stations are representative of the three latitude regimes we study, i.e. (i) KAR, NUR and DOB lie within the subauroral polarization streams (SAPS) regime, (ii) RVK and OUJ lie within the EEJ, and (iii) SOR, AND and LOZ lie within the latitude regime of the westward electrojet (WEJ)
We reported cases of polar cap convection enhancements associated with two intervals of substorm activity (15:50– 16:40 UT and 17:40–18:30 UT) during a 4-h long interval of very steady interplanetary conditions on 10 January 2004 (15:00–19:00 UT)
Summary
The temporal structure in polar cap convection and the associated cross polar cap potential (CPCP) has been the subject of much study over the years. The presence of an underlying temporal structure was indicated by the large scatter appearing in plots of CPCP, as measured by low-altitude spacecraft, versus the geoeffective interplanetary electric field, EKL (Kan and Lee, 1979) (see Cowley, 1984, his Fig. 3) An interpretation of this scatter in terms of the influence from the closure of open flux during substorm activity, i.e. satellite polar cap traversals under different substorm phases, was given by e.g. The temporal resolution of CPCP values derived from such cross-track ion drift observations during dawn-dusk traversals of the polar cap by low-altitude satellites is limited. The combination of the present data sets allow us to document a non-traditional dawn-dusk convection asymmetry, as measured during a DMSP F13 polar cap transit, which may appear during intervals of substorm activity. We hypothesize that the reversed asymmetry we observe is further evidence of a nightside contribution
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