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 magnetoshe ath encounters by GEOTAIL near the center of the tail axis at a down t ail distance of X = -94 R(E) during the main phase of a weak geomagnet ic storm on January 25, 1993. The magnetosheath entries of the satelli te 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 p ulses caused these brief encounters. Assuming the flow direction in th e magnetosheath to be parallel to the magnetotail boundary, we estimat e the tail magnetic flux and the dimension of the tail for the encount er events. It is shown that solar wind variations: which generate geom agnetic storms, alter significantly the orientation as well as the siz e of the distant tail, such that the magnetosheath can be observed eve n near the center of the ''average'' tail.