Measurements are presented showing strong tailward flow of ions along the dawn magnetopause as the Voyager 1 spacecraft crossed the earth's magnetosphere boundary following launch on September 5, 1977. With one exception all the observed flows occur outside the magnetopause. The particle flux measurements at E>30keV, together with the observed magnetic field signatures of the boundary crossing, are consistent with a minimum tailward ion energy flow of ~ (2--7) ¿1017ergs/s at the time of observation. High-time resolution particle data indicate that the ion flow can vary on a time scar of ~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.