Investigating The Cross-section of Coronal Mass Ejections Through the Study of Non-Radial Flows with STEREO/PLASTIC

The solar wind, when measured close to 1 au, is found to flow mostly radially outward. There are, however, periods when the flow makes angles up to 15$^\circ$ away from the radial direction, both in the east-west and north-south directions. Stream interaction regions (SIRs) are a common cause of east-west flow deflections. Coronal mass ejections (CMEs) may be associated with non-radial flows in at least two different ways: 1) the deflection of the solar wind in the sheath region, especially close to the magnetic ejecta front boundary, may result in large non-radial flows, 2) the expansion of the magnetic ejecta may include a non-radial component which should be easily measured when the ejecta is crossed away from its central axis. In this work, we first present general statistics of non-radial solar wind flows as measured by STEREO/PLASTIC throughout the first 13 years of the mission, focusing on solar cycle variation. We then focus on the larger deflection flow angles and determine that most of these are associated with SIRs near solar minimum and with CMEs near solar maximum. However, we find no clear evidence of strongly deflected flows, as would be expected if large deflections around the magnetic ejecta or ejecta with elliptical cross-sections with large eccentricities are common. We use these results to develop a better understanding of CME expansion and the nature of magnetic ejecta, and point to shortcomings in our understanding of CMEs.
Comment: Accepted to ApJ