Plasma observations on Explorer 34

This paper discusses results of the Explorer 34 plasma detector obtained during three magnetic storms, May 30, June 5, and June 25, 1967. Detailed comparisons with magnetic field observations on the same satellite are used to interpret the observations and to identify shocks and tangential discontinuities. A relation between the magnitude of such sudden changes in the earth's magnetic field and the solar wind parameters has been derived by Siscoe. Our observations are consistent with this relationship. The local shock speeds are derived from data obtained at the time of the sudden commencements. An example of an si+ si− pair is discussed, and in this case the negative impulse is shown to be due to a tangential discontinuity rather than a shock, whereas the positive impulse is associated with a shock. Helium observations are described that show that during the main phase of some geomagnetic storms the abundance ratio na/np shows a marked increase. In the May 30 storm it attained a value of 0.15 ± 0.02, 5½ hours after the sc, an increase of seven times over the value observed earlier.