Multiscale views of an Alfvénic slow solar wind: 3D velocity distribution functions observed by the Proton-Alpha Sensor of Solar Orbiter
Year: 2021
Type: article
Abstract: Context.The Alfvénic slow solar wind is of particular interest, as it is often characterized by intense magnetic turbulence, complex proton 3D velocity distribution functions (VDF), and an ensuing richness of kinetic and dynamic processes. Aims.We take advantage of the fast time cadence of measurements taken by the Proton-Alpha Sensor (PAS) on board Solar Orbiter to analyze the kinetic properties of the proton population, the variability of their VDFs, and the possible link with propagating magnetic structures. We also study the magnetic (B) and velocity (V) correlation that characterizes this type of wind down to the ion gyroperiod. Methods.We analyzed the VDFs measured by PAS, a novelty that take advantages of the capability of 3D measurements at a 4 Hz cadence. In addition, we considered MAG observations. Results.We first show that there is a remarkable correlation between theBandVcomponents observed down to timescales approaching the ion gyrofrequency. This concerns a wide variety of fluctuations, such as waves, isolated peaks, and discontinuities. The great variability of the proton VDFs is also documented. The juxtaposition of a core and a field-aligned beam is the norm but the relative density of the beam, drift speed, and temperatures can considerably change on scales as short as as a few seconds. The characteristics of the core are comparatively more stable. These variations in the beam characteristics mostly explain the variations in the total parallel temperature and, therefore, in the total anisotropy of the proton VDFs. Two magnetic structures that are associated with significant changes in the shape of VDFs, one corresponding to relaxation of total anisotropy and the other to its strong increase, are analyzed here. Our statistical analysis shows a clear link between total anisotropy (and, thus, beam characteristics) and the direction ofBwith respect to the Parker spiral. In the present case, flux tubes aligned with Parker spiral contain an average proton VDF with a much more developed beam (thus, with larger total anisotropy) than those that are inclined, perpendicular, or even reverse with regard to the outward direction. Conclusions.These observations document the variability of the proton VDF shape in relation to the propagation of magnetic structures. This is a key area of interest for understanding of the effect of turbulence on solar wind dynamics.
Source: Astronomy and Astrophysics
Authors: P. Louarn, A. Fedorov, L. Prech, C. J. Owen, R. Bruno, S. Livi, B. Lavraud, A. P. Rouillard, V. Génot, N. André, G. Fruit, V. Réville, R. Kieokaew, I. Plotnikov, E. Penou, A. Barthe, D. Khataria, M. Berthomier, R. D’Amicis, L. Sorriso-Valvo, F. Allegrini, J. Raines, D. Verscharen, V. Fortunato, G. Mele, T. S. Horbury, H. O’brien, V. Evans, V. Angelini, M. Maksimovic, J. C. Kasper, S. D. Bale
Institutions: Centre National de la Recherche Scientifique, Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, Charles University, University College London, Institute for Space Astrophysics and Planetology, National Institute for Astrophysics, Southwest Research Institute, Laboratoire d'Astrophysique de Bordeaux, École Polytechnique, Laboratoire de Physique des Plasmas, Swedish Institute of Space Physics, Institute for the Science and Technology of Plasmas, National Research Council, University of Michigan, University of New Hampshire, Planetek Italia, Imperial College London, Université Paris Sciences et Lettres, Observatoire de Paris, Sorbonne Université, Laboratoire d’études spatiales et d’instrumentation en astrophysique, Space Sciences Laboratory, University of California, Berkeley
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $3,550
Awards: G1256297496, Solar System Consolidated Grant 2019-22, MSSL Solar & Planetary Physics Consolidated Grant 2016-2019, Transport of energy of solar wind variations from larger to smaller scales, G542031838, Thermodynamics of Astrophysical Plasmas: Macroscopic Effects of Collisionless Microphysics
OpenAlex ID: W3208739657