Alfvénic Slow Solar Wind Observed in the Inner Heliosphere by Parker Solar Probe

• Published in: arXiv.org
• Main Authors: Huang, Jia, Kasper, J C, Stevens, M, Vech, D, Klein, K G, Martinović, Mihailo M, Alterman, B L, Jian, Lan K, Hu, Qiang, Velli, Marco, Horbury, Timothy S, Lavraud, B, Parashar, T N, Ďurovcová, Tereza, Niembro, Tatiana, Paulson, Kristoff, Hegedus, A, Bert, C M, Holmes, J, Case, A W, Korreck, K E, Bale, Stuart D, Larson, Davin E, Livi, Roberto, Whittlesey, P, Pulupa, Marc, Thierry Dudok de Wit, Malaspina, David M, MacDowall, Robert J, Bonnell, John W, Harvey, Peter R, Goetz, Keith
• Format: Paper
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 25.05.2020
• Subjects: Abundance; Anisotropy; Heliosphere; Helium; High temperature; Intervals; Solar cycle; Solar maximum; Solar probes; Solar wind; Spacecraft tracking; Wind; Wind spacecraft

The slow solar wind is typically characterized as having low Alfvénicity. However, Parker Solar Probe (PSP) observed predominately Alfvénic slow solar wind during several of its initial encounters. From its first encounter observations, about 55.3\% of the slow solar wind inside 0.25 au is highly Alfvénic (\(|\sigma_C| > 0.7\)) at current solar minimum, which is much higher than the fraction of quiet-Sun-associated highly Alfvénic slow wind observed at solar maximum at 1 au. Intervals of slow solar wind with different Alfvénicities seem to show similar plasma characteristics and temperature anisotropy distributions. Some low Alfvénicity slow wind intervals even show high temperature anisotropies, because the slow wind may experience perpendicular heating as fast wind does when close to the Sun. This signature is confirmed by Wind spacecraft measurements as we track PSP observations to 1 au. Further, with nearly 15 years of Wind measurements, we find that the distributions of plasma characteristics, temperature anisotropy and helium abundance ratio (\(N_\alpha/N_p\)) are similar in slow winds with different Alfvénicities, but the distributions are different from those in the fast solar wind. Highly Alfvénic slow solar wind contains both helium-rich (\(N_\alpha/N_p\sim0.045\)) and helium-poor (\(N_\alpha/N_p\sim0.015\)) populations, implying it may originate from multiple source regions. These results suggest that highly Alfvénic slow solar wind shares similar temperature anisotropy and helium abundance properties with regular slow solar winds, and they thus should have multiple origins.