Abstract
Abstract. During the 14 November 2012 geomagnetic storm, the Van Allen Probes spacecraft observed a number of sharp decreases (“dropouts”) in particle fluxes for ions and electrons of different energies. In this paper, we investigate the global magnetosphere dynamics and magnetosphere–ionosphere (M–I) coupling during the dropout events using multipoint measurements by Van Allen Probes, TWINS, and AMPERE together with the output of the two-way coupled global BATS-R-US–CRCM model. We find different behavior for two pairs of dropouts. For one pair, the same pattern was repeated: (1) weak nightside Region 1 and 2 Birkeland currents before and during the dropout; (2) intensification of Region 2 currents after the dropout; and (3) a particle injection detected by TWINS after the dropout. The model predicted similar behavior of Birkeland currents. TWINS low-altitude emissions demonstrated high variability during these intervals, indicating high geomagnetic activity in the near-Earth tail region. For the second pair of dropouts, the structure of both Birkeland currents and ENA emissions was relatively stable. The model also showed quasi-stationary behavior of Birkeland currents and simulated ENA emissions with gradual ring current buildup. We confirm that the first pair of dropouts was caused by large-scale motions of the OCB (open–closed boundary) during substorm activity. We show the new result that this OCB motion was associated with global changes in Birkeland (M–I coupling) currents and strong modulation of low-altitude ion precipitation. The second pair of dropouts is the result of smaller OCB disturbances not related to magnetospheric substorms. The local observations of the first pair of dropouts result from a global magnetospheric reconfiguration, which is manifested by ion injections and enhanced ion precipitation detected by TWINS and changes in the structure of Birkeland currents detected by AMPERE. This study demonstrates that multipoint measurements along with the global model results enable the reconstruction of a more complete system-level picture of the dropout events and provides insight into M–I coupling aspects that have not previously been investigated. Keywords. Magnetospheric physics (magnetosphere–ionosphere interactions; magnetospheric configuration and dynamics); space plasma physics (numerical simulation studies)
Highlights
Lobe encounters and associated particle dropouts are considered to be an important indicator of magnetospheric reconfiguration during active times (Sauvaud and Winckler, 1980; Moldwin et al, 1995; Kopanyi and Korth, 1995)
We speculate that the absence of the clear bipolar signature and the layering of currents in the AMPERE data may be a manifestation of Hall reconnection, which is absent in the MHD equations used in BATS-R-US–Comprehensive Ring Current Model (CRCM)
Buildup and no signatures of injection or Birkeland current reconfiguration. These later two dropouts are interpreted as resulting from smaller motions of the open–closed boundary (OCB) boundary not connected with global reconfigurations of field-aligned currents or particle injections
Summary
Lobe encounters and associated particle dropouts are considered to be an important indicator of magnetospheric reconfiguration during active times (Sauvaud and Winckler, 1980; Moldwin et al, 1995; Kopanyi and Korth, 1995). The dropouts were observed in the dawn sector, 04:00 to 06:00 MLT, at radial distances of 4.6 to 5.8 RE near the GSM equator (ZGSM ≈ −1.8 to −0.5 RE) and contained lobe plasma (Dixon et al, 2015) This storm is of particular interest since multipoint measurements from Van Allen Probes, geostationary satellites, TWINS, Geotail, THEMIS, and AMPERE were available, making possible the reconstruction of a global picture of magnetospheric dynamics during the event. An alternate interpretation by Dixon et al (2015), hereinafter called Di15, suggested that the dropouts on 14 November 2012 were caused by crossing the boundary between open and closed magnetic field lines (the open– closed boundary or OCB) and lobe encounters Their analysis of Van Allen Probe data found that low-energy plasma traveling from the southern ionosphere was in the dropout regions. Appendix A contains a more complete review of the approach for constructing the coupled models
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