Near Subsonic Solar Wind Outflow from an Active Region

Abstract

During Parker Solar Probe (Parker) Encounter 15 (E15), we observe an 18 hr period of near-subsonic (M S ∼ 1) and sub-Alfvénic (SA), M A ⋘ 1, slow-speed solar wind from 22 to 15.6 R ⊙. As the most extreme SA interval measured to date and skirting the solar wind sonic point, it is the deepest Parker has probed into the formation and acceleration region of the solar wind in the corona. The stream is also measured by Wind and the Magnetosonic Multiscale mission near 1 au at times consistent with ballistic propagation of this slow stream. We investigate the stream source, properties, and potential coronal heating consequences via combining these observations with coronal modeling and turbulence analysis. Through source mapping, in situ evidence, and multipoint arrival time considerations of a candidate coronal mass ejection, we determine the stream is a steady (nontransient), long-lived, and approximately Parker spiral aligned and arises from overexpanded field lines mapping back to an active region. Turbulence analysis of the Elsässer variables shows the inertial range scaling of the z + mode (f ∼ −3/2) to be dominated by the slab component. We discuss the spectral flattening and difficulties associated with measuring the z − spectra, cautioning against making definitive conclusions from the z − mode. Despite being more extreme than prior SA intervals, its turbulent nature does not appear to be qualitatively different from previously observed streams. We conclude that this extreme low-dynamic-pressure solar wind interval (which has the potential for extreme space-weather conditions) is a large, steady structure spanning at least to 1 au.