Solar wind electron temperature depressions following some interplanetary shock waves: Evidence for magnetic merging?

Abstract

An investigation of periods of anomalously low solar wind electron temperatures using 14 months of Vela 5, Vela 6, and Imp 6 measurements has shown a very strong tendency for unusually low temperatures to follow interplanetary shock waves by 10–20 hours. Of 12 low-temperature regions observed, 10 occurred in direct association with a well-defined shock-associated ssc. A detailed investigation of these events has shown that (1) the periods persist for 10 to >40 hours; (2) a distinct depression in proton temperature occurs simultaneously; (3) the electron heat flux tends to be significantly reduced, but the velocity distributions are less Maxwellian; (4) for one event the He/H concentration is found to rise significantly at about the time of the initial decrease in temperature; for most of the events the data are not continuous enough to check this association; and (5) about half of the observed shocks were followed by temperature depressions. The existence of regions with abnormally low electron temperatures is consistent with the formation of closed or nearly closed magnetic field configurations that more or less thermally isolate the electrons from the relatively hot corona. The electron component of the plasma within these regions thus cools in relation to plasma electrons embedded in open field regions as the closed or constricted regions expand away from the sun.