RESPONSE OF THE NEAR-EARTH MAGNETOTAIL TO A NORTHWARD TURNING OF THE IMF

The response of the near-earth magnetotail to a sharp and isolated nor thward turning of the IMF observed at the WIND spacecraft is studied i n the near-Earth plasma sheet at x=-19 Re. Because of the possible tim e coincidence this northward turning could be causally related to an o nset of the substorm expansion phase where Bz increased and the earthw ard convection intensified in the plasma sheet. The changes in the flo w velocity and Bz to lower values occurred about 30 min later but they were not associated with an expansion phase onset. Thus the observati ons do not support the suggestion that the northward IMF turning produ ces a tailward sweep of the reduced dawn-to-dusk electric field and th ereby triggers the substorm onset.