02/09/2026
By Lynne Schaufenbil
Abstract: The geometry of a star's Alfvén surface determines stellar angular momentum loss, separates a causally distinct "corona" and stellar wind, and potentially affects exoplanetary habitability. The solar Alfvén surface is the only such structure that is directly measurable and, since 2021, has been routinely measured in situ by NASA's Parker Solar Probe (Parker). In this seminar, we review our recent work (doi:10.3847/2041-8213/ae0e5c) in which we use these unique measurements in concert with Solar Orbiter and L1 data further from the Sun, spanning the first half of solar cycle 25 in time and from 0.045 to 1 au in heliocentric distance. From this data, we develop a radial scaling technique to estimate the morphology of the Alfvén surface from measurements of the solar wind speed and local Alfvén speed. We produce continuous 2D equatorial cuts of the Alfvén surface over half a solar cycle (ascending phase and maximum). We analyze the average altitude, departure from spherical symmetry, and surface roughness, finding that all are positively correlated to solar activity. For the current modest solar cycle, the height varies up to 30%, which corresponds to a near doubling in angular momentum loss per unit mass loss. Lastly, we present our ongoing work on assessing and separating impacts on this geometry from steady flows and transients such as coronal mass ejections. We suggest that transient flows leave outwards spikes in the Alfvén surface which persist for extended periods of time and therefore temporarily enhance angular momentum loss.
Bio link: Sam is an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian (CfA) in Cambridge, MA. He received his undergraduate degree from the University of Oxford in 2016 and his doctorate from the University of California Berkeley in 2022 (both in physics) before moving to the CfA. He researches the global structure of the Sun's extended atmosphere combining numerical models, remote observations of solar structure and in situ measurements of the solar wind. He has a particular emphasis on using new measurements from the inner heliosphere from spacecraft such as Parker Solar Probe, and is a team member of the Solar Wind Electrons Alphas and Protons instrument. He is also a strong believer in the use of open source software to enable reproducible research, easy collaboration and allowing others to build on prior work without doing it again from scratch (see his github), and is a maintainer of the sunpy-affiliated sunkit-magex package for performing potential field extrapolations of the solar magnetic field.
If you are interested in attending either in-person or via Zoom, please email Lynne_Schaufenbil@uml.edu