The effect of foreshock on the motion of the dayside magnetopause

The role of the foreshock or quasi parallel shock in causing motions of the magnetopause is studied using both the number of crossings of the magnetopause found as ISEE 1 and 2 passed through the expected region of the magnetopause on each orbit and by determining the minimum and maximum distances to the magnetopause on each pass. Downstream of the afternoon magnetosheath and bow shock, a region free from foreshock effects for typical interplanetary magnetic field (IMF) directions, the north-south component of the IMF controls these motions. On the morning side additional oscillation in the magnetopause position is found for both northward and southward IMF. We interpret these increased motions as driven by the pressure variations in and behind the foreshock region or quasi-parallel shock. These results are consistent with other studies of foreshock effects on the magnetosphere which show little effect on erosion of the magnetopause or flux transfer event occurrence.