Abstract
AbstractA multiple auroral onset substorm on 28 March 2010 provides an opportunity to understand the physical mechanism in generating auroral intensifications during a substorm expansion phase. Conjugate observations of magnetic fields and plasma from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, of field‐aligned currents (FACs) from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellites, and from ground‐based magnetometers and aurora are all available. The comprehensive measurements allow us to further our understanding of the complicated causalities among dipolarization, FAC generation, particle acceleration, and auroral intensification. During the substorm expansion phase, the plasma sheet expanded and was perturbed leading to the generation of a slow mode wave, which modulated electron flux in the outer plasma sheet. During this current sheet expansion, field‐aligned currents formed, and geomagnetic perturbations were simultaneously detected by ground‐based instruments. However, a magnetic dipolarization did not occur until about 3 min later in the outer plasma sheet observed by THEMIS‐A spacecraft (THA). We believe that this dipolarization led to an efficient Fermi acceleration to electrons and consequently the cause of a significant auroral intensification during the expansion phase as observed by the All‐Sky Imagers (ASIs). This Fermi acceleration mechanism operating efficiently in the outer plasma sheet during the expansion phase could be a common explanation of the poleward auroral development after substorm onset. These results also show a good agreement between the upward FAC derived from AMPERE measurements and the auroral brightening observed by the ASIs.
Highlights
A magnetospheric substorm is a major space weather phenomenon, which includes significant disturbances in the magnetosphere, ionosphere, and upper atmosphere
These results show a good agreement between the upward field-aligned currents (FACs) derived from AMPERE measurements and the auroral brightening observed by the All-Sky Imagers (ASIs)
Conjugate measurements from THEMIS spacecraft, AMPERE satellite, geomagnetic stations, and all-sky imagers have been presented for a substorm event on 28 March 2010 with four near-identical auroral intensifications
Summary
A magnetospheric substorm is a major space weather phenomenon, which includes significant disturbances in the magnetosphere, ionosphere, and upper atmosphere. During the substorm expansion phase, the near-Earth magnetotail is highly dynamic, with the magnetic field becoming strongly perturbed [Lui, 1996] and the cross-tail currents being diverted into the ionosphere from the magnetosphere [Boström, 1964]. Hsu and McPherron [2003] indicate that approximately 60% of all substorms appear to be triggered by a number of events including interplanetary magnetic field transitions, a reduction in By or a sudden change in the solar wind dynamic pressure, and the remaining 40% are nontriggered events Their results imply that substorm expansions may be triggered by multiple mechanisms. Auroral intensifications during the expansion phase may be associated with relatively limited perturbation in the magnetic field and plasmas since these are not directly related to any major energy release process and are ideal to investigate fundamental processes related to substorm dynamics. The coordinated FACs in the magnetotail and ionosphere are analyzed, as well as the electron acceleration process that directly causes the auroral intensifications
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