Authors: W. H. Matthaeus (U Delaware), S. Adhikari (U Delaware), R. Bandyopadhyay (Princeton U) , D. Ruffolo (Mahidol U), P. Thepthong (Kasetsart U) , P. Pongkitiwanichakul (Kasetsart U), S. Roy (U Delaware), F. Pecora (U Delaware), R Chhiber (U Delaare, GSFC), A Usmanov (U Delaare, GSFC) , M. Stevens (CFA), S. Badman (CFA), O. Romeo (SSL Berkeley), J.Wang (U Delaware), J. Goodwill (U Delaware), M. L. Goldstein (Space Science Institute)

The point or surface at which the solar wind speed exceeds the Alfv’en speed has long been viewed as of significance in understanding the physics of the corona and solar wind. Recent results suggest that this transition occurs not on a simple surface but over a complex region (e.g., Chhiber+ 2024). It is sometimes suggested that the subAlfv’enic region should be associated with the corona, while the superAlfv’enic region is thought of as the domain of the solar wind, even if this partitioning of the solar atmosphere might be viewed as arbitrary in some respects. In any case the transition separates the atmosphere into regions that may be expected to exhibit distinctive physical and plasma properties. Here a number of features of the transition and the regions it separates, are described, based on pioneering measurements by Parker Solar Probe. First, the nature of the propagation of low frequency plasma fluctuations is examined, asking whether WKB theory (the classic description) is valid when the basic features of the fluctuations are organized appropriately by Alfv’en Mach number. We find that WKB fails dramatically in the subAlfv’enic region (Ruffolo et al, 2024), presumably due to strong turbulence effects. Next we contrast distributions and means of several plasma properties in subAlfv’enic and in superAlfv’enic regions, extending the work of Bandyopadhyay et al (2022). Quantities considered are: turbulence amplitude, variance anisotropy, PVI and switchback parameter. The results help to clarify the nature of plasma dynamics and turbulence in the “coronal” region in comparison with the “solar wind” region.

Chhiber et al, MNRAS. 533, L70 (2024)
Ruffolo et al, ApJ Lett, 977, L19 (2024)
Bandyopadhyay et al, ApJ Lett, 926, L1 (2022)