Parker Solar Probe approaches the Alfvén Point

International audienceOne of the driving objectives behind the Parker Solar Probe 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 clearly the "Alfvén Point"—the point where the magnetic and kinetic energy of the plasma are equal. This is not a point, of course, but a surface or layer—and almost certainly a fuzzy, highly corrugated one. It is inside of this surface that Alfvén waves propagate faster than the bulk flow of the wind, and it is thus inside of this surface that Alfvén waves can transport energy sunward. The presence of sunward Alfvén waves in the plasma unlocks modes of wave-particle energy exchange that may be key to the heating and acceleration of the solar wind, but are not accessible elsewhere in the heliosphere. In the years (and decades) leading up to the Parker Solar Probe mission, the location of the Alfvén point has been estimated from remote observations and from the scaling of in situ observables to lie anywhere from 10-30 solar radii above the surface of the sun. In this paper, we use inner heliospheric scalings from the first Parker Solar Probe encounters to estimate the Alfven Point in the streamer belt and coronal hole wind… and we reveal just how close the Parker Solar Probe has already come to skimming the Alfvén surface