The Stability of the Electron Strahl against the Oblique Fast-magnetosonic/Whistler Instability in the Inner Heliosphere

Jeong, Seong-Yeop, Abraham, Joel B., Verscharen, Daniel, Berčič, Laura, Stansby, David, Nicolaou, Georgios, Owen, Christopher J., Wicks, Robert, Fazakerley, Andrew N., Agudelo Rueda, Jeffersson A. and Bakrania, Mayur (2022) The Stability of the Electron Strahl against the Oblique Fast-magnetosonic/Whistler Instability in the Inner Heliosphere. The Astrophysical Journal Letters, 926 (2). L26. ISSN 2041-8205

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We analyze the micro-kinetic stability of the electron strahl in the solar wind depending on heliocentric distance. The oblique fast-magnetosonic/whistler (FM/W) instability has emerged in the literature as a key candidate mechanism for the effective scattering of the electron strahl into the electron halo population. Using data from the Parker Solar Probe (PSP) and Helios, we compare the measured strahl properties with the analytical thresholds for the oblique FM/W instability in the low- and high-β ∥c regimes, where β ∥c is the ratio of the core parallel thermal pressure to the magnetic pressure. Our PSP and Helios data show that the electron strahl is on average stable against the oblique FM/W instability in the inner heliosphere. Our analysis suggests that the instability, if at all, can only be excited sporadically and on short timescales. We discuss the caveats of our analysis and potential alternative explanations for the observed scattering of the electron strahl in the solar wind. Furthermore, we recommend the numerical evaluation of the stability of individual distributions in the future to account for any uncertainties in the validity of the analytical expressions for the instability thresholds.

Item Type: Article
Additional Information: Funding information: D.V. is supported by STFC Ernest Rutherford Fellowship ST/P003826/1. J.B.A. is supported by STFC grant ST/ T506485/1. D.V., C.J.O., A.N.F., D.S., and L.B. are supported by STFC Consolidated Grant ST/S000240/1. R.T.W. is funded by STFC consolidated Grant ST/V006320/1. J.A.A. R. is supported by the ESA Networking/Partnering Initiative (NPI) contract 4000127929/19/NL/MH/mg and the Colombian program Pasaporte a la Ciencia, Foco Sociedad—Reto 3, ICETEX grant 3933061. M.B. is supported by a UCL Impact Studentship, jointly funded by the ESA NPI contract 4000125082/18/NL/MH/ic. This work was discussed at the “Joint Electron Project (JEP)” at MSSL. The authors acknowledge insightful discussions within the International Team “Heliospheric Energy Budget: From Kinetic Scales to Global Solar Wind Dynamics” at the International Space Science Institute (ISSI) in Bern led by M. E. Innocenti and A. Tenerani.
Uncontrolled Keywords: Solar wind, Space plasmas, Heliosphere
Subjects: F300 Physics
F500 Astronomy
Department: Faculties > Engineering and Environment > Mathematics, Physics and Electrical Engineering
Depositing User: John Coen
Date Deposited: 22 Feb 2022 15:51
Last Modified: 22 Feb 2022 16:00

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