Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsm31b0325h&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #SM31B-0325
Physics
2431 Ionosphere/Magnetosphere Interactions (2736), 2451 Particle Acceleration, 2704 Auroral Phenomena (2407), 2721 Field-Aligned Currents And Current Systems (2409)
Scientific paper
We present a detailed case study of the plasma and fields measured at high-altitude (~ 3.5 R_E) by the Cluster spacecraft fleet across the plasma sheet boundary layer and into the polar cap during relatively quiet magnetospheric activity. This event is of particular interest in that Cluster provides measurements at key instances during the temporal development of a new discrete auroral arc system in association with the expansion of the plasma sheet to higher latitudes. The expansion process, which was quite sudden, is not due to a transverse poleward motion of the plasma sheet-polar cap boundary as one may expect from a dipolarization process, but appears to be due to the sudden injection of a new source population of plasma sheet electrons and ions onto magnetic flux tubes at higher latitudes adjacent to the preexisting plasma sheet- polar cap boundary, which may be the result of an impulsive spatial and/or temporal reconfiguration of the reconnection x-line. Associated with this reconfiguration, we observe the development of a density cavity in concert with a forming parallel potential drop below the spacecraft, as inferred from outflowing ion and electron signatures in the upward and downward current regions bounded by the cavity. From multiple spacecraft timings, the cavity was observed to be propagating equatorward at roughly the local convection speed. Though highly filamented during the initial stages, the current becomes less structured and intensifies into a well- defined up-down current system. The co-evolution of the upward and downward acceleration potentials within the cavity suggests that the formation of the potentials in the upward and downward current regions are linked. A principle driving factor of the size of the upward current region parallel potential drop is the temperature of the newly injected source electrons. Furthermore, the erosion of density is shown to be a consequence of the replacement of a dense cold ionospheric population with a rarefied hot energetic magnetospheric source population of electrons. These novel observations provide new physical insights into the behavior of the plasma and fields during the formation of auroral acceleration processes at lower altitudes.
Bonnell J. W.
Chaston Christopher. C.
Goldstein Michael
Hull Arthur J.
McFadden Johnjoe
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