Storm-time convection electric field in the near-Earth plasma sheet

T. Hori, A.T.Y. Lui, S. Ohtani, P.C. Brandt, B.H. Mauk, and R.W. McEntire, 
K. Maezawa, T. Mukai, Y. Kasaba, and H. Hayakawa


We have examined the electric field observations made by the Geotail spacecraft
in the near-Earth magnetotail during magnetic storms to study enhanced
convection and the associated electric field that is thought to be key to
causing the injection of particles into the ring current. Several recent
modeling studies of the storm-time ring current suggest that an enhanced
convection electric field in the magnetosphere, which is induced by a continuous
southward interplanetary magnetic field (IMF), is responsible for steady
particle transport into the inner magnetosphere during the storm main phase. The
enhanced convection is envisioned to weaken, or cease after the main phase in
response to the decrease of southward IMF, leading to the formation of a
relatively symmetric ring current around the Earth during the recovery phase.
However, surprisingly, our present study has not found clear evidence for the
existence of such a large, steady earthward convection during either the storm
main phase or the recovery phase. The observed electric field properties in the
near-Earth plasma sheet are basically classified into two categories: One is
characterized by intermittent bursts of fluctuating duskward electric fields
associated with substorm expansions, and the other is dominated by a relatively
steady, weak duskward electric field. The weak strength of the convection
electric field in the latter category is seen even during storm main phase. The
statistical study on this relatively steady, weak field shows that it has a weak
duskward component of ~0.3 mV/m on average during both the main and recovery
phases, which is almost comparable to that observed during quiet times. Their
comparison with the solar wind parameters and the polar cap potential drop
calculated using the Boyle model and Weimer model reveals that the weak duskward
electric field tends to show poor correlation with these parameters, suggesting
that storm-time convection electric field in the plasma sheet is not directly
driven by either of them. These results imply that in the near-Earth plasma
sheet beyond geosynchronous distance, particle injection for the storm-time ring
current is not governed by enhanced convection induced by the solar wind,
contrary to conclusions based on simulation studies of the storm-time ring
current. The present study suggests the importance of re-examining the
contribution to the ring current from the near-Earth plasma sheet for both
substorm and nonsubstorm time intervals on the basis of observations made in the
magnetosphere.

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Journal of Geophysical Research, 110, A04213, doi:10.1029/2004JA010449.