Observations of the magnetosheath near the nominal tail axis during the geomagnetic storm of January 25, 1993

This paper reports plasma and field changes associated with magnetosheath encounters by GEOTAIL near the center of the tail axis at a down tail distance of X = -94 R(E) during the main phase of a weak geomagnetic storm on January 25, 1993. The magnetosheath entries of the satellite occurred in conjunction with an enhancement in the total pressure, having a temporal scale between 2 and 18 minutes. It is suggested that temporal and/or local structures in the solar wind such as pressure pulses caused these brief encounters. Assuming the flow direction in the magnetosheath to be parallel to the magnetotail boundary, we estimate the tail magnetic flux and the dimension of the tail for the encounter events. It is shown that solar wind variations: which generate geomagnetic storms, alter significantly the orientation as well as the size of the distant tail, such that the magnetosheath can be observed even near the center of the ''average'' tail.