Control of Plasmaspheric Dynamics by Both Convection and Sub-Auroral
Polarization Stream (SAPS): Observations by IMAGE EUV Compared to Results of
Simple Models. 

J. Goldstein (Space Science and Engineering Division, Southwest Research
Institute, P.O. Drawer 28510, San Antonio, TX 78228-0510 USA), B. R. Sandel, 
P. C:son Brandt


In the classic picture, plasmaspheric dynamics is controlled by an inner
magnetospheric electric (E) field that is composed of two components, (1) a
corotation E-field that encourages cold plasma to move eastward with the
rotating Earth, and (2) a convection E-field that encourages cold plasma to flow
sunward. Whereas the corotation field is strongest near the Earth, with
increasing radial distance the convection field becomes more important. As this
convection field is driven by the flow of the solar wind and the dayside
reconnection rate, the strength of convection varies with geomagnetic
disturbance levels. A simple E-field model that fits with the classic picture,
but also contains a primitive treatment of inner magnetospheric shielding, is
that of Volland-Stern (VS). We have constructed a very basic plasmapause
evolution model that is driven by a suitably normalized Volland-Stern E-field
whose strength is proportional to the product of the solar wind speed and the
southward component of the interplanetary magnetic field. We show the results of
applying this basic VS-driven model to three events: (a) 2 June 2001, (b) 25-27
June 2000, and (c) 17 April, 2002. We compare the VS-driven model output to
observations of the IMAGE extreme ultraviolet (EUV) imager. For these three
events, we find that the VS electric field yields quantitatively good agreement
with EUV data on the morning side, but only qualitative agreement in the
afternoon sector. More specifically stated, the VS-driven model plasmapause
occurs within 0.5 earth radii (RE) of the EUV-observed plasmapause on the
morning side, but is 1-2 RE further out than the EUV plasmapause on the
afternoon side. The afternoon disagreement suggests that the V-S electric field
model is not strong enough on the afternoon side, and that a better E-field
model can be constructed by superposing the V-S convection field plus another
component that is strongly localized to the afternoon sector. The most obvious
candidate for this afternoon component is the sub-auroral polarization stream
(SAPS) electric field that has been shown to produce a strong azimuthal flow
channel at the radial location of the inner edge of the plasma sheet. We show
that a quantitatively better E-field model must include both a convection field
and a SAPS field. We show that a SAPS-like radially-localized flow channel can
explain the details of the observed duskside plasmapause evolution as seen by
EUV, especially on 17 April 2002, when a duskside plasmapause "undulation" was
observed in which a thin strip of the outermost plasmasphere was rapidly
removed. To corroborate the hypothesis that a SAPS-like E-field can account for
the afternoon discrepancy, we present observations of the DMSP satellites that
show strong sub-auroral duskside flows during the proposed SAPS intervals. This
talk will be complemented by a second talk by Brandt et al, that presents the
ring-current perspective using data from the IMAGE high-energy neutral atom
(HENA) imager.

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Presented at the August 2003 AGU Chapman Conference, Helsinki, Finland