On the Properties of an Extended and Fragmented Alfven Zone in the Young Solar Wind - Locations, Scales, and Flows

A global magnetohydrodynamic model of the solar wind with turbulence transport is used to investigate the properties of the critical transition between sub-Alfvenic flow and super-Alfvenic flow in the solar atmosphere. Ubiquitous turbulent fluctuations in this region imply that the transition takes place in fragmented subvolumes within a general "Alfven zone" that extends over several solar radii. Model estimates of the probability of Alfvenic transitions as a function of heliocentric distance r are compared with Parker Solar Probe (PSP) observations, finding remarkable agreement. Maps are made of the characteristic size and duration of subAlfvenic blobs/intervals as a function of r and heliolatitude. The flow of Alfvenic signals in this zone is investigated, indicating the possibility of a prolonged period of stochastic motion wherein signals are trapped between subAlfvenic and superAlfvenic states before eventually flowing towards or away from the Sun. Flow speeds from the model are compared with those obtained from flow-tracking methods applied to STEREO observations. These results are relevant to PSP, which is expected to sample subAlfvenic intervals with increasing frequency during its primary mission phase, and also to NASA's planned PUNCH mission, which aims to map the Alfven zone using remote sensing.