Changes in the distant tail configuration during geomagnetic storms

Details Changes in the distant tail configuration during geomagnetic storms Changes in the distant tail configuration during geomagnetic storms

May 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997jgr...102.9587n&link_type=abstract

Journal of Geophysical Research, Volume 102, Issue A5, p. 9587-9602

Physics

17

Magnetospheric Physics: Magnetospheric Configuration And Dynamics, Magnetospheric Physics: Magnetotail, Magnetospheric Physics: Magnetotail Boundary Layers, Magnetospheric Physics: Storms And Substorms

Scientific paper

Changes in the structure of the distant tail associated with geomagnetic storms are studied by using plasma and magnetic field data obtained from Geotail. Thirteen storm intervals between October 1993 and October 1994 are examined when the satellite was located in the distant tail between X=-83RE and X=-210RE. Geotail observed the magnetosheath during all storms including those when the satellite was located near the nominal tail axis. Assuming the flow direction in the magnetosheath to be parallel to the magnetotail axis, we estimated the dimension and the flux of the tail for the magnetopause crossing events. Systematic changes of the distant tail structure are found in association with the development of the storms. Before the main phase onset of the storms, when the Dst shows positive excursion, the enhanced solar wind pressure reduces the average radius of the distant tail to ~23RE as compared with the quiet time value, ~31RE. During the storm main phase the dimension of the tail is comparable to the quiet time value in spite of the high solar wind pressure. This is attributed to the enhanced magnetic flux in the tail in association with the southward interplanetary magnetic field (IMF) Bz. An average energy of ~5×1015J is stored also in the distant tail during the storm main phase, which is a comparable value to that stored in the midtail during an intense substorm growth phase reported in the previous studies. During storm time, when the Bz component of the magnetosheath field was larger compared with the By component, the average tail cross section has a north-south elongated elliptical shape. The average tail for quiet time, however, was elongated toward the dawn-dusk direction. By component was larger than Bz component in our quiet time data set. These observations imply that the anisotropy in the magnetosheath magnetic field pressure for different IMF orientation changes the shape of the tail, which was also shown in global MHD models. Changes in the average magnetosheath parameters (density, speed, direction) observed during the storms are found to be consistent with solar wind fluctuations expected from corotating interaction regions.

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