Abstract
Abstract. Space and time variations of equivalent currents during morning-sector Pc5 pulsations (T ∼ 2–8 min) on 2 days (18 January and 19 February 2008) are studied in the context of substorm activity with THEMIS and MIRACLE ground-based instruments and THEMIS P3, P5, and P2 probes. These instruments covered the 22:00–07:00 magnetic local time during the analyzed events. In these cases abrupt changes in the Pc5 amplitudes, intensifications and/or weakenings, were recorded some minutes after auroral breakups in the midnight sector. We analyze three examples of Pc5 changes with the goal to resolve whether substorm activity can have an effect on Pc5 amplitude or not. In two cases (on 19 February) the most likely explanation for Pc5 amplitude changes comes from the solar wind (changes in the sign of interplanetary magnetic field Bz). In the third case (on 18 January) equivalent current patterns in the morning sector show an antisunward-propagating vortex which replaced the Pc5-related smaller vortices and consequently the pulsations weakened. We associate the large vortex with a field-aligned current system due to a sudden, although small, drop in solar wind pressure (from 1 to 0.2 nPa). However, the potential impact of midnight substorm activity cannot be totally excluded in this case, because enhanced fluxes of electrons with high enough energies (∼ 280 keV) to reach the region of Pc5 within the observed delay were observed by THEMIS P2 at longitudes between the midnight and morning-sector instrumentation.
Highlights
During auroral substorms the strongest geomagnetic activity and auroras appear typically in the midnight sector of the auroral oval (McPherron, 1979; Akasofu, 1964)
We have studied the spatial and temporal distribution of equivalent currents in the morning sector of the auroral oval during two sequences of Pc5 pulsations with FLR characteristics
A disturbance in the over-reflection mechanism would be a natural explanation for the temporal weakenings in Pc5 intensity which we describe in this study
Summary
During auroral substorms the strongest geomagnetic activity and auroras appear typically in the midnight sector of the auroral oval (McPherron, 1979; Akasofu, 1964). That sudden changes in the conditions in other sectors of the auroral oval may happen simultaneously with the substorm breakups at midnight. In the case of substorm–electrojet linkage Borälv et al (2000) come to the conclusion that the cause–effect relation exists, and they attribute the transfer of information from midnight sector to dawn and dusk to rapid changes in the convection electric field. Ps6 is a fluctuation with a period of 10–40 min They are seen primarily in the east component of magnetic field in the post-midnight sector during the recovery phase of substorms
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