Deflection Flows as Measured by STEREO/PLASTIC: Statistics and Causes

Abstract:

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 degrees away from the radial direction, both in the east-west and north-south directions. Stream interaction regions are a common cause of east-west flow deflection. Coronal mass ejections may be associated with radial flows in at least two different ways: 1) the deflection of the solar wind in the sheath region, especially ahead of the magnetic ejecta may result in large non-radial flows, 2) the expansion of the magnetic ejecta must include a non-radial component which should be easily measured when the ejecta is crossed away from its central axis. If the cross-section of the magnetic ejecta is elliptical, the expansion in the non-radial expansion should be several times larger than the expansion in the radial direction and should reach values of 100s of km/s. In this work, we first present general statistics of non-radial flows as measured by STEREO/PLASTIC throughout the 14 years of the mission, focusing on solar cycle variation. We then focus on the larger defection 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 extreme deflection flows, as would be expected with large deflection around the magnetic ejecta or elliptical cross-sections with large eccentricities. We use these results to develop a better understanding of the nature of magnetic ejecta.