East-West scattering level asymmetry of the solar corona Original Research Article

Coronal radio sounding experiments with spacecraft near solar conjunction show a steady tendency toward enhanced radio frequency fluctuations during the ingress phase with respect to the egress phase. This behavior can be explained by the higher mean relative velocity of the solar wind density inhomogeneities traversing the radio ray path. Indeed, the relative velocity is the sum of the velocity of the inhomogeneities (solar wind +/− possible wave velocity) plus the ray path velocity during ingress and the difference of these components during egress. There are isolated exceptions to this rule (e.g., the Galileo spacecraft conjunctions in 1994/95 and 1995/96). These anomalous cases, which were observed in the years preceeding solar activity minimum, can be explained by the strongly asymmetric structure of the rotating corona over the course of the conjunction.