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

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Read Full Article (file size: 399424 bytes) Cited by JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, A04213, doi:10.1029/2004JA010449, 2005 Storm-time convection electric field in the near-Earth plasma sheet T. Hori Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA A. T. Y. Lui Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA S. Ohtani Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA P. C:son Brandt Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA B. H. Mauk Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA R. W. McEntire Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA K. Maezawa Institute of Space and Astronautical Science, Sagamihara, Japan T. Mukai Institute of Space and Astronautical Science, Sagamihara, Japan Y. Kasaba Institute of Space and Astronautical Science, Sagamihara, Japan H. Hayakawa Institute of Space and Astronautical Science, Sagamihara, Japan Abstract 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. Received 25 February 2004; accepted 4 February 2005; published 27 April 2005. Keywords: convection; electric field; storm; plasma sheet; magnetosphere. Index Terms: 2712 Magnetospheric Physics: Electric fields (2411); 2731 Magnetospheric Physics: Magnetosphere: outer; 2744 Magnetospheric Physics: Magnetotail; 2760 Magnetospheric Physics: Plasma convection (2463); 2788 Magnetospheric Physics: Magnetic storms and substorms (7954). Read Full Article (file size: 399424 bytes) Cited by Citation: Hori, T., A. T. Y. Lui, S. Ohtani, P. C:son Brandt, B. H. Mauk, R. W. McEntire, K. Maezawa, T. Mukai, Y. Kasaba, and H. Hayakawa (2005), Storm-time convection electric field in the near-Earth plasma sheet, J. Geophys. Res., 110, A04213, doi:10.1029/2004JA010449. Copyright 2005 by the American Geophysical Union.

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