MEASUREMENTS OF PLASMA-FLOW AT THE DAWN MAGNETOPAUSE BY VOYAGER-1

Measurements are presented showing strong tailward flow of ions along the dawn magnetopause as the Voyager I spacecraft crossed the earth's magnetosphere boundary following launch on September 5, 1977. With one exception all of the observed flows occur outside the magnetopause. The particle flux measurements at E approximately greater than 30 keV, together with the observed magnetic field signatures of the boundary crossing, are consistent with a minimum tailward ion energy flow of approximately (2 - 7) multiplied by 10**1**7 ergs/s at the time of observation. High-time resolution particle data indicate that the ion flow can vary on a time scale of approximately 400 ms. These results, together with recent results from several other spacecraft, show conclusively that a source of energetic particles exists sunward of the dawn-dusk meridian.