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Deceleration of Alpha Particles in the Solar Wind by Instabilities and the Rotational Force: Implications for Heating, Azimuthal Flow, and the Parker Spiral Magnetic Field

Verscharen, D; Chandran, BDG; Bourouaine, S; Hollweg, JV; (2015) Deceleration of Alpha Particles in the Solar Wind by Instabilities and the Rotational Force: Implications for Heating, Azimuthal Flow, and the Parker Spiral Magnetic Field. The Astrophysical Journal , 806 (2) , Article 157. 10.1088/0004-637X/806/2/157. Green open access

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Abstract

Protons and alpha particles in the fast solar wind are only weakly collisional and exhibit a number of nonequilibrium features, including relative drifts between particle species. Two non-collisional mechanisms have been proposed for limiting differential flow between alpha particles and protons: plasma instabilities and the rotational force. Both mechanisms decelerate the alpha particles. In this paper, we derive an analytic expression for the rate Qflow at which energy is released by alpha-particle deceleration, accounting for azimuthal flow and conservation of total momentum. We show that instabilities control the deceleration of alpha particles at r < rcrit, and the rotational force controls the deceleration of alpha particles at r > rcrit, where rcrit ≃ 2.5 AU in the fast solar wind in the ecliptic plane. We find that Qflow is positive at r < rcrit and Qflow = 0 at r ≥ rcrit, consistent with the previous finding that the rotational force does not lead to a release of energy. We compare the value of Qflow at r < rcrit with empirical heating rates for protons and alpha particles, denoted Qp and Qα , deduced from in situ measurements of fast-wind streams from the Helios and Ulysses spacecraft. We find that Qflow exceeds Qα at r < 1 AU, and that Qflow/Qp decreases with increasing distance from the Sun from a value of about one at r = 0.29-0.42 AU to about 1/4 at 1 AU. We conclude that the continuous energy input from alpha-particle deceleration at r < rcrit makes an important contribution to the heating of the fast solar wind. We also discuss the implications of the alpha-particle drift for the azimuthal flow velocities of the ions and for the Parker spiral magnetic field.

Type: Article
Title: Deceleration of Alpha Particles in the Solar Wind by Instabilities and the Rotational Force: Implications for Heating, Azimuthal Flow, and the Parker Spiral Magnetic Field
Open access status: An open access version is available from UCL Discovery
DOI: 10.1088/0004-637X/806/2/157
Publisher version: https://doi.org/10.1088/0004-637X/806/2/157
Language: English
Additional information: This version is the version of record. For information on re-use, please refer to the publisher’s terms and conditions.
Keywords: instabilities; plasmas; solar wind; Sun: corona; turbulence; waves
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Space and Climate Physics
URI: https://discovery.ucl.ac.uk/id/eprint/10081146
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