Authors: Samuel T. Badman (Center for Astrophysics | Harvard & Smithsonian) , Stuart D. Bale (UC Berkeley), Michael L. Stevens (Center for Astrophysics | Harvard & Smithsonian)

The solar alfvén surface is the boundary at which the solar wind bulk velocity exceeds the local alfvén speed and is a key parameter describing the atmosphere of the Sun and other stars. We study its structure and temporal evolution over the first half of solar cycle 25, using in situ data from Parker Solar Probe and Solar Orbiter and performing radial scaling. Combining the two spacecraft data sets we identify structural features which are robust and long-lived.  We assess the average height over a half-solar cycle timescale, the typical roughness implied by direct crossings, and its relationship to solar activity and different solar wind stream types.