No abstract

Abstract.Simultaneous observations by the Cluster spacecraft and SuperDARN radars are presented of magnetotail flux transport during northward, but B Y -dominated IMF. Two events are discussed, which occurred on 14 August 2004 and 17 September 2005, during intervals of negative and positive IMF B Y , respectively. During both intervals the Cluster spacecraft observed isolated bursts of Earthward plasma convection in the central plasma sheet. During the first event, the flows observed by Cluster also had a significant V ⊥Y component in the duskward direction, consistent with westward azimuthal flows observed in the midnight sector by the Northern Hemisphere SuperDARN radars. During the second event, Cluster 4 observed a significant dawnward V ⊥Y component, again consistent with the Northern Hemisphere SuperDARN observations which revealed eastward azimuthal flow. In this instance, however, Cluster 3 observed a duskward V ⊥Y component which was more consistent with the duskward sense of the convection observed by the Southern Hemisphere SuperDARN radars. This implies that Cluster 3 and Cluster 4 were located on different field lines which experienced opposite net azimuthal forces and hence observed oppositely directed convection. These observations are consistent with previous SuperDARN studies of nightside flows under northward IMF and, more importantly, provide the first simultaneous in-situ evidence for a mode of tail reconnection occurring during non-substorm intervals in an asymmetric tail.

[1] Magnetic reconnection in the Earth's magnetotail converts open magnetic flux to closed, producing fast flows in the plasma sheet. Earthward fast flows can thus be used as a proxy for energy and momentum transfer into the closed magnetosphere during the course of a substorm. Characteristics of flows observed before substorm onset can provide information on the role of the flows in substorm onset triggering as well as the sequence of substorm-related processes. We use 15 years of data obtained by Geotail, Cluster, and THEMIS to study statistically the distribution of earthward plasma sheet flows during substorms. Substorm phases are determined from time series of the AL index and the z-component of the interplanetary magnetic field. We find that the occurrence frequency of medium-speed (100 km/s < V < 500 km/s) flows increases slightly toward the end of the substorm growth phase. At substorm onset there is a sharp increase of medium-speed flows, and during substorm recovery there is a gradual decrease of such flows. The occurrence frequency of high-speed (V > 500 km/s) flows, on the other hand, does not clearly increase during the substorm growth phase. The increase at substorm onset is more gradual than for the medium-speed flows. The occurrence frequency of high-speed flows peaks at the beginning of substorm recovery. These results indicate that any effect of the flows on substorm onset triggering would be caused by medium rather than very high speed flows.

Abstract. Substorm properties during different storm phases have been studied using an automated recognition of substorm and storm phases in the auroral electrojet (AL) and ring current (Dst) index data from 1995–2009. The large number of events (about 500 storms and 15 000 substorms) provides statistically reliable distributions, average behaviour and long time series of simple parameters, such as durations and intensities. The phases of storms and substorms have been examined independently. Substorm phases have been further combined to single and multi-cycle events. The former consist of one growth, one expansion and one recovery phase, while the latter include multiple expansion and recovery phases after one growth phase. Our findings show that most substorms take place during non-storm times, and substorms during storm initial phases resemble isolated non-storm time substorms. Both during storm initial phases and non-storm times, the substorm growth phases may last longer than the other substorm phases. Substorm recovery phase is typically the longest phase but its duration also varies most. The longest substorm recovery phase duration was observed during multi-cycle substorms. The longest substorm expansion and storm main phases were found during the years close to the solar maximum. The shortest substorm events (the shortest phase durations) are the single-cycle substorms. The period of expansion onsets during multi-cycle substorms varied hugely for events with a small number of expansion phases. For events with a larger number of expansions, a clearer periodicity of about one hour (median value) was suggested.