Near-Earth substorm features from multiple satellite observations
ABSTRACT This paper was published as Journal of Geophysical Research, 2008, 113, A07S26. Copyright 2008 American Geophysical Union. It is available from http://www.agu.org/pubs/crossref/2008/2007JA012738.shtml. Doi: 10.1029/2007JA012738 Metadata only entry We investigate a substorm on 3 October 2004 during which 11 satellites were located in near-Earth magnetotail (X GSM > −10 R E). Double Star 1 (TC-1), Cluster, and LANL-97 satellites were closely aligned in the dawn-dusk direction (<1 R E apart) for this conjunction. After substorm expansion onset, TC-1 observed plasma sheet thinning at X ≈ −5.5 R E and later detected signature of plasma flow shear that may be associated with an auroral arc. Analysis of the dawn-dusk magnetic perturbations from GOES-10 and Polar suggests that these could be caused by a substorm current system consisting of not only the azimuthal closure of field-aligned currents (the substorm current wedge) but also the meridional closure of field-aligned currents. The temporal sequence of substorm activity (particle injection, current disruption, and dipolarization) revealed by these satellites indicates that the substorm expansion activity was initiated close to the Earth and spread later to further downstream distances. Furthermore, TC-1 and Cluster data show that there is no close relationship between some dipolarizations and Earthward plasma flows in the near-Earth region. The overall development of substorm activity is in agreement with the near-Earth initiation model for substorms. A temporal evolution of the magnetic field reconfiguration and plasma boundary motion during this substorm is constructed from these observations.
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ABSTRACT: A close conjunction of several satellites (LANL, GOES, Polar, Geotail, and Cluster) distributed from the geostationary altitude to about 16 R<sub>E</sub> downstream in the tail occurred during substorm activity as indicated by global auroral imaging and ground-based magnetometer data. This constellation of satellites resembles what is planned for the THEMIS (Time History of Events and Macroscopic Interactions during Substorms) mission to resolve the substorm controversy on the location of the substorm expansion onset region. In this article, we show in detail the dipolarization and dynamic changes seen by these satellites associated with two onsets of substorm intensification activity. In particular, we find that dipolarization at ~16 R<sub>E</sub> downstream in the tail can occur with dawnward electric field and without plasma flow, just like some near-Earth dipolarization events reported previously. The spreading of substorm disturbances in the tail coupled with complementary ground observations indicates that the observed time sequence on the onsets of substorm disturbances favors initiation in the near-Earth region for this THEMIS-like conjunction.01/2007;
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ABSTRACT: We have investigated a near-Earth depolarization event in the midnight sector using simultaneous observations of Polar and Geotail. We have found evidence for near-Earth depolarization to be a non-MHD process: depolarization occurring without significant plasma flow or with tailward flow and during dawnward electric field different from that inferred based on the frozen-in condition. These observations are inconsistent with the idea that depolarization is an MHD process of magnetic flux pileup from braking of sunward plasma flow. Possible variances of the flow braking scenario are considered but none is satisfactory in accounting for the observed features. On the other hand, these findings are quite consistent with the expectations from the current disruption scenario.Geophysical Research Letters - GEOPHYS RES LETT. 01/1999; 26(19):2905-2908.
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ABSTRACT: On board the four Cluster spacecraft, the Cluster Ion Spectrometry (CIS) experiment measures the full, three-dimensional ion distribution of the major magnetospheric ions (H<sup>+</sup>, He<sup>+</sup>, He<sup>++</sup>, and O<sup>+</sup>) from the thermal energies to about 40 keV/e. The experiment consists of two different instruments: a COmposition and DIstribution Function analyser (CIS1/CODIF), giving the mass per charge composition with medium (22.5°) angular resolution, and a Hot Ion Analyser (CIS2/HIA), which does not offer mass resolution but has a better angular resolution (5.6°) that is adequate for ion beam and solar wind measurements. Each analyser has two different sensitivities in order to increase the dynamic range. First tests of the instruments (commissioning activities) were achieved from early September 2000 to mid January 2001, and the operation phase began on 1 February 2001. In this paper, first results of the CIS instruments are presented showing the high level performances and capabilities of the instruments. Good examples of data were obtained in the central plasma sheet, magnetopause crossings, magnetosheath, solar wind and cusp measurements. Observations in the auroral regions could also be obtained with the Cluster spacecraft at radial distances of 4–6 Earth radii. These results show the tremendous interest of multispacecraft measurements with identical instruments and open a new area in magnetospheric and solar wind-magnetosphere interaction physics.01/2001;