ON THE STRUCTURE OF THE DISTANT MAGNETOTAIL - ISEE-3

ISEE 3 magnetic field and electron plasma data have been used to determine the relative frequency of observation of the magnetoschcath and magnetotail in the region where a nominal magnetotail is expected. These observations have been compared with how frequently a tail of a given radius would be expected to be seen, assuming typical variations in the direction of the solar wind flow relative to the radial. Observations match expectations if the average radius of the distant magnetotail is 24 R(E). This relatively small radius is consistent with an open magnetotail where field lines are lost both through the magnetopause and also by closing across the equatorial current sheet. Further evidence suggests that the distant tail is seen somewhat less frequently during geomagnetically quiet periods which may mean that the radius is smaller during such conditions. The average solar magnetospheric B(x) component of the field in the distant plasma sheet is 0.6 nT during quiet times but zero during disturbed times. The observation suggests that when the polar cap becomes smaller during quiet times, many of the field lines that previously formed the distant tail lobes are converted into closed field lines that cross the equatorial plane earthward of 240 R(E). Further indications are that the east-west dimension may exceed the north-south dimension. The lobe is observed 4.5 times as often as the plasma sheet, implying an average half thickness of the distant plasma sheet of 3.4 R(E).