A study of magnetic clouds observed by MESSENGER and Venus Express

Abstract:

We present an analysis of a recently compiled list of magnetic clouds observed by the MESSENGER and Venus Express spacecraft. Despite their planetary focus, both spacecraft spent significant amounts of time in the solar wind, allowing transient solar wind structures such as magnetic clouds to be observed. The clouds were observed at a range of heliocentric distances (at 0.72 AU by Venus Express, and from 1 to 0.3 AU by MESSENGER) over the course of a significant fraction of a solar cycle (2006 to 2013 by Venus Express, and 2007 to 2012 by MESSENGER). Neither spacecraft carried a dedicated solar wind plasma instrument, and so the clouds have been identified using magnetic field data only: a coherent rotation of the magnetic field direction lasting for at least four hours that coincides with a field strength above that of the ambient solar wind were the primary identification criteria. The approximate orientations of the magnetic clouds relative to the solar equatorial plane have been found. We consider the frequency distribution of these orientations, and their dependence on heliocentric distance and the solar magnetic field configuration. We also use these observations, in conjunction with observations at 1 AU, to estimate the likelihood that two spacecraft will encounter the same magnetic cloud as a function of the spacecraft’s azimuthal separation.