IMAGE EUV observations of plasmaspheric erosion: Indications of 
plasmasphere/ring-current/ionosphere coupling

Jerry Goldstein, Bill R. Sandel, Pontus C:son Brandt, 
Stanislav Y. Sazykin


The IMAGE extreme ultraviolet (EUV) imager obtains global images
of the He+ plasmasphere as seen in 30.4 nm light. EUV directly
observes the effects of plasmaspheric erosion by capturing the
motion of the plasmapause. Several erosion events have been
identified in which the nightside plasmapause moves inward, while
at the same time the dayside fills up with (presumably sunward-
moving) plasma. This global transport of plasmaspheric material
from the nightside to the dayside is generally accepted to be
connected in some way to enhanced convection electric fields that
arise during periods of increased geomagnetic activity following
southward turnings of the interplanetary magnetic field (IMF).
However, the precise chain of events between a southward IMF
turning and the onset of plasmaspheric erosion has not been
established. EUV data consistently indicate a time delay of about
30 minutes between a southward IMF turning at the magnetopause,
and the onset of plasmaspheric erosion. One simple explanation
for this 30-minute delay is that the global convection field
requires some time after a southward IMF turning to reach a new
configuration with well-established flows, and thus the inner
magnetosphere would not experience the effects of enhanced
convection until that new global configuration is established.
There are strong indications that this simple picture is
incomplete and that coupling (between the inner magnetosphere and
other regions of the magnetosphere) plays a significant role.
Partial ring currents at the inner edge of the plasmasheet
participate in the erosion process (to an unknown extent) by
providing a dynamically-changing shielding layer and/or by
coupling to the ionosphere to produce regions of intense
azimuthal flows known as sub-auroral polarization streams (SAPS).
This raises the possibility that the 30-minute delay may be
explained as an interval of buildup of ring current pressure. We
shall present a picture of this plasmaspheric erosion process
from an observational and theoretical/simulational perspective.

_______________
Presented at the IUGG/IAGA meeting, Sapporo, Japan 
30 June - 11 July 2003