The Storm Ring Current as an Ionospheric Phenomenon

T.E. Moore (NASA Goddard Space Flight Center, Greenbelt, MD 20771), 
M.-C. Fok, D.C. Delcourt, and Y. Ebihara

Recent IMAGE observations of the ring current indicate that O+ is directly
responsive to discrete substorms, while H+ in the ring current is more slowly
varying. Direct observations from the Polar (and Cluster) spacecraft suggest
that the midnight sector plasma sheet is created directly from lobal wind
entry through the lobes. We have explored H+ entry into the plasma sheet and
ring current using single particle motions in fields provide by Slinker and
Fedder from the Lyon-Fedder-Mobarry magnetohydrodynamic simulation, for
southward and northward interplanetary magnetic field. We consider the
transport paths to the ring current from sources in the solar wind and
ionosphere. We find that solar ion entry to the ring current occurs
principally through the dawn low latitude boundary layer, without passage
through the substorm-active plasma sheet. However, the light ion polar wind
contribution to the ring current occurs via the substorm-active plasma sheet,
but is at most com-parable with the solar wind contribution for SBz. We
propose that large storm ring currents consist mainly of ionospheric O+ that
also en-ter the ring current through the substorm-active plasma sheet and are
consequently responsive to discrete substorm dipolarizations. We also discuss
the role of ionospheric con-ductivity in modifying magnetospheric convection,
and the strength of the growth phase ring current. We find that ring current
strength scales closely with ionospheric conductivity, reflecting both solar
wind and solar radiation energy inputs to the auroral ionosphere. We argue
that large ring current increases stem directly from the resultant O+
outflows, operated on by the midnight sector plasma sheet substorm process.

___________
Presentation at the Yosemite Conference of Inner Magnetospheric Interactions,
3 - 6 February 2004, Yosemite, California, USA