Abstract
Abstract. Based on a comprehensive catalogue with more than 4000 magnetospheric substorm entries from the years 2000–2005, the spatial distribution of the substorm-related magnetic signatures at mid and low latitudes around local midnight was investigated. Superposed epoch analysis of a larger number of recent observatory data from mid and low latitudes revealed a field strength increase that is consistent with the results of earlier studies. For the first time, the magnetic signature of the substorm current wedge formation is studied also in near-Earth satellite data from CHAMP. The average maximal deflection measured on board the satellite is smaller by a factor of 2 than that determined from ground observations. The recurrence frequency of substorms as well as the amplitude of their magnetic signature depends strongly on the prevailing magnetic activity. The observed average substorm-related magnetic field signatures cannot be described adequately by a simple current wedge model. A satisfactory agreement between model results and observations at satellite height and on ground can be achieved only if the current reconfiguration scenario combines four elements: (1) the gradual decrease of the tail lobe field, (2) the re-routing of a part of the cross-tail current through the ionosphere, (3) eastward ionospheric currents at low and mid latitudes driven by Region-2 field-aligned currents, and (4) a partial ring current connected to these Region-2 FACs.
Highlights
Substorms are regarded as an important part of the magnetospheric activity cycle
In the following we present as an example the observations of a single substorm event at the IMAGE spacecraft, at high and low latitude ground stations and at the CHAMP satellite
In order to assess the significance of the single event results and to reproduce the observations at mid latitudes by Caan et al (1975) with data from today’s modern instrumentation and with the large number of substorm events available in the catalog of Frey and Mende (2006) we investigate the magnetic signature at the four observatories listed in Sect. 2.1 and at the near-Earth orbiting satellite CHAMP
Summary
Substorms are regarded as an important part of the magnetospheric activity cycle. The loading by the solar wind is fol-lowed by an impulsive unloading of energy previously stored in the magnetospheric tail. The cross-tail current instability model postulates that plasma instabilities in the near-Earth region (6–10 RE) start the process causing a cross-tail current disruption and a divergence into the ionosphere via field-aligned currents (Lui, 1996). The near-Earth neutral line model predicts that the substorm is initiated at a further distance downtail (>20 RE) by reconnection (Baker et al, 1996). The disruption of crosstail currents is an intrinsic part of reconnection. In this model the region of disrupted cross-tail currents expands earthward. Common to both models is the formation of a substorm current wedge. In a large-scale reconfiguration the disrupted cross-tail current is rerouted along the field lines towards the Earth and closed in the high-latitude ionosphere on the night side. A schematic presentation of this current system was published by Clauer and McPherron (1974)
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