SH049-0002
On the location of the Solar Alfvén Point and the imminent crossing by Parker Solar Probe

Wednesday, 16 December 2020
Poster
Michael Louis Stevens1, Anthony W Case1, Kelly E Korreck1, Tatiana Niembro Hernandez1, Kristoff W Paulson2, Justin Christophe Kasper3, Davin Larson4, Roberto Livi4, Phyllis L Whittlesey4, Keith Goetz5, Peter Harvey4, Robert J MacDowall6, Kristopher G. Klein7, Stuart Bale8, Marc Pulupa4, David Malaspina9, John W Bonnell10, Thierry Dudok de Wit11, Michel Moncuquet12, Robert Colby Allen13 and Samuel Timothy Badman14, (1)Smithsonian Astrophysical Observatory, Cambridge, MA, United States, (2)Harvard-Smithsonian Center for Astrophysics, Cambridge, United States, (3)University of Michigan, Climate and Space Sciences and Engineering, Ann Arbor, MI, United States, (4)Space Sciences Laboratory, University of California Berkeley, Berkeley, CA, United States, (5)University of Minnesota, School of Physics and Astronomy, Minneapolis, MN, United States, (6)NASA/Goddard Space Flight Center, Greenbelt, United States, (7)University of Arizona, Lunar and Planetary Laboratory, Tucson, AZ, United States, (8)University of California, Berkeley, Berkeley, CA, United States, (9)University of Colorado, Astrophysical and Planetary Sciences Department, Boulder, CO, United States, (10)University of California, Berkeley, Space Sciences Laboratory, Berkeley, CA, United States, (11)LPC2E, CNRS and University of Orléans, Orléans, France, (12)LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, Meudon, France, (13)Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States, (14)University of Oxford, Oxford, United Kingdom
Abstract:
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, cA. In encounter 2, the Fields and SWEAP experiments measured dimensionless bulk flow speeds as low as u/cA = 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/cA = 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.
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