Testing the flux tube expansion factor -- solar wind speed relation with Solar Orbiter data

Abstract

The properties of the solar wind measured in-situ in the heliosphere are largely controlled by energy deposition in the solar. Previous studies have shown that long duration and large scale magnetic structures show an inverse relation between the solar wind velocity measured in situ near 1 au and the expansion factor of the magnetic flux tubes in the solar atmosphere. We exploit Solar Orbiter data in conjunction with the Potential Field Source Surface (PFSS) coronal model, and the solar wind trajectory to evaluate the flux expansion factor - speed relation at the solar "source surface" at rss. We find a statistically weak anti-correlation between the in-situ bulk velocity and the coronal expansion factor, for about 1.5 years of solar data. Classification of the data by source latitude reveals different levels of anticorrelation. We show the existence of fast solar wind that originates in strong magnetic field regions at low latitudes and undergoes large expansion factor. We provide evidence that such winds become supersonic during the super radial expansion (below rss), and are theoretically governed by a positive correlation v-f. We find that faster winds on average have a flux tube expansion at a larger radius than slower winds. An anticorrelation between solar wind speed and expansion factor is present for solar winds originating in high latitude structures in solar minimum activity, typically associated with coronal hole-like structures, but this cannot be generalized to lower latitude sources. Therefore, the value of the expansion factor alone cannot be used to predict the solar wind speed. Other parameters, such as the height at which the expansion gradient is the strongest must also be taken into account.

18 pages, 10 figures, 1 table

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