Upstream shocks and interplanetary magnetic cloud speed and expansion: Sun, WIND, and Earth observations

Abstract

Identifications have been made of the probable sources on the Sun (from SOHO data), where possible, of the magnetic clouds observed by WIND near Earth over the span of about 4 years (1995–1998), i.e., mainly during the quiet solar phase. The timing of the probable source transients at the Sun allow the estimations of transit times from the Sun to Earth for a set of 20 magnetic clouds with the result that acceleration effects in the interplanetary medium appear to be small. For a larger set of magnetic clouds (N=27) it is found that the relative speed of the front part of a cloud at 1 AU (its ramming speed) correlates better than cloud expansion with the existence of an upstream interplanetary shock. Hence, cloud expansion is not likely the primary agent responsible for clouds driving shocks. Approximately 1/2 of the magnetic clouds (i.e., 14 cases) were accompanied by upstream shocks, and for an additional 1/4 of the cases pressure pulses ahead of the magnetic clouds were present. It is shown that these upstream shocks (or pressure pulses) are well correlated with SSC's at Earth. We also examine the effects of the engulfing of the magnetosphere by the shocked solar wind-magnetic cloud complex, by comparing the integrated solar wind Poynting flux input (Σ εΔt = energy, where ε is the Akasofu ε-function) with the associated minimum Dst, and we develop a formal relationship between them.