Causes of variability in plasmasphere rotation rate: IMAGE EUV observations (Invited)

Details Causes of variability in plasmasphere rotation rate: IMAGE EUV observations (Invited) Causes of variability in plasmasphere rotation rate: IMAGE EUV observations (Invited)

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsa33c..03g&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #SA33C-03

Physics

[2431] Ionosphere / Ionosphere/Magnetosphere Interactions, [2730] Magnetospheric Physics / Magnetosphere: Inner, [2736] Magnetospheric Physics / Magnetosphere/Ionosphere Interactions, [2768] Magnetospheric Physics / Plasmasphere

Scientific paper

IMAGE EUV observations demonstrate that the plasmasphere usually does not corotate as assumed in simple convection models, even at low L shells. The prevailing hypothesis states that plasmaspheric subcorotation is due to enhanced auroral zone Joule heating which drives equatorward thermospheric winds. As the neutral thermospheric material moves to lower latitudes, it grows farther from the Earth’s spin axis and turns westward to conserve angular momentum. This induces a westward motion in the ionosphere (a subcorotation), which produces a change in the corotation electric field that maps out to the plasmasphere, causing a subcorotation there as well. We test this hypothesis by searching for a correlation between plasmaspheric rotation rates and several geomagnetic indices (used as proxies for enhanced Joule heating in the auroral zone). We carry out a statistical survey of plasmaspheric rotation rates over several months of IMAGE EUV data in 2001, using two different measurement techniques. Azimuthal features such as “notches” are tracked in local time over a single pass of the IMAGE satellite, both visually and using an automated cross-correlation routine. Each technique provides an estimate of the plasmasphere’s rotation rate. We find a weak correlation between rotation rate and Dst, Kp, AE, the midnight boundary index (MBI), and Joule heating estimates from assimilative mapping of ionospheric electrodynamics (AMIE) at L = 2.5, but not at L = 3.5. In general, lower rotation rates correspond to higher auroral and geomagnetic activity. We also make the first direct observation of plasmaspheric superrotation. The rotation rate is found to be highly variable on multi-day timescales, but the typical state of the plasmasphere is subcorotation, with inferred mean values ranging from 88% to 95% of corotation, depending on L shell. In addition, a statistical analysis shows that rotation rates near dusk are generally lower than those at dawn, suggesting that local time and magnetospheric convection contribute to the variation in rotation rate as well. We conclude that the cause of variability in plasmaspheric rotation rate is a combination of storm phase, local-time-dependent convection, and westward ionospheric drift.

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