On the location of the Solar Alfvén Point and the imminent crossing by Parker Solar Probe

One of the driving objectives behind the Parker Solar Probe (PSP) mission is to cross into the magnetically closed corona and to observe the coronal plasma in situ. The transition from open to closed coronal phenomena can be a nuanced distinction, but in gross terms the key boundary is the "Alfvén Point." This is the surface at which the bulk flow speed, u , of the solar wind is equal to the phase speed of the transverse Alfvén wave, c_A . In encounter 2, the Fields and SWEAP experiments measured dimensionless bulk flow speeds as low as u/c_A = 1.3 ± 0.1. Based on inner heliospheric scalings with distance from the first two encounters, lower estimates for the slow wind Alfvén Point were produced, ranging from 14 to 26 R_⊙ , and the likelihood that PSP would first cross the Alfvén point some time in encounters 3-5 was estimated to be 25-50%. In this paper, we repeat the analysis for encounters 3-5 and report the following findings: (1) the Alfvén point has not yet been crossed for any extended (~10 minute) period prior to encounter 6, (2) speeds as low as u/c_A = 1.15 ± 0.10 were observed over an extended period during encounter 4, (3) the Alfvén point associated with certain well-measured slow streams in encounters 3-5 was estimated from 5 to 18 R_⊙ , and (4) it is highly likely that PSP will spend extended parts of perihelion 8 in the sub-Alfvénic wind, thus achieving a major mission milestone in early 2021 if not before.