Recent Results concerning Interplanetary Plasma

The MIT plasma probe, installed aboard the first IMP, has shown that the space around the earth is sharply divided into three regions. At large distances from the earth, we find a region (region 1) occupied by a plasma moving with a velocity from about 300 to 400 km/sec, in a direction nearly radial away from the sun; its thermal energy density is small compared with its kinetic energy density of bulk motion. Presumably we see in this region the "solar wind," unperturbed by the presence of the earth. As we approach the earth, we pass from region 1 to region 2 where the velocity of the plasma is often below the limit of detectability and in any case much smaller than in region 1. The thermal energy density is large, corresponding to temperatures of the order of millions of degrees, but a rigorous definition of the temperature is impossible because the velocity distribution does not appear to be Maxwellian. Approaching the earth further, we meet a second boundary, separating region 2 from region 3, where the plasma probe does not detect any plasma. Both boundaries have a characteristic comet-like shape, with a blunt nose facing the sun and a long diverging tail in the opposite direction. Presumably the boundary between regions 1 and 2 represents a shock front, across which the plasma flow changes from supersonic to subsonic.