Hot Solar-Wind Helium: Direct Evidence for Local Heating by Alfv\'en-Cyclotron Dissipation

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Hot Solar-Wind Helium: Direct Evidence for Local Heating by Alfvén-Cyclotron Dissipation

J. C. Kasper, A. J. Lazarus, and S. P. Gary

Phys. Rev. Lett. 101, 261103 – Published 22 December 2008

See Viewpoint: What keeps the solar wind hot?

Abstract

A study of solar-wind hydrogen and helium temperature observations collected by the Wind spacecraft offers compelling evidence of heating by an Alfvén-cyclotron dissipation mechanism. Observations are sorted by the rate of Coulomb interactions, or collisional age, in the plasma and the differential flow between the two species. We show that helium is preferentially heated perpendicular to the magnetic field direction by more than a factor of 6 when the flow between the species is small relative to the Alfvén wave speed and collisions are infrequent. These signatures are consistent with predictions of dissipation in the presence of multiple ion species. We also report an unexpected result: observations of efficient heating of helium parallel to the magnetic field for large differential flow relative to the sound speed.

Received 21 April 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.261103

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What keeps the solar wind hot?

Published 22 December 2008

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Authors & Affiliations

J. C. Kasper^*

Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA

A. J. Lazarus

MIT Kavli Institute for Astrophysics and Space Research, Cambridge, Massachusetts 02139, USA

S. P. Gary

Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

^*jkasper@cfa.harvard.edu

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Vol. 101, Iss. 26 — 31 December 2008

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Figure 1
The relative occurrence of $T_{α} / T_{p}$ in the solar wind over the course of the Wind mission to date, illustrating the bimodal nature of the dominant components of the plasma, with peaks near equal temperature ( $T_{α} / T_{p} = 1$ ) and equal thermal speed ( $T_{α} / T_{p} = 4$ ). For 23% of the observations $T_{α} / T_{p} > 5$ , indicating anomalous heating beyond the already unusual equilibration of thermal speeds.Reuse & Permissions
Figure 2
The breakdown of a simple single-fluid description of the solar wind is strongly ordered by the collisional age $A_{c}$ of the plasma, as illustrated by the panels in this figure. In (a) we show the distribution of $T_{α} / T_{p}$ with solar-wind speed. Statistically, $T_{α} \sim T_{p}$ for low speeds, and $T_{α} / T_{p} \geq 4$ occurs more often a higher speeds, but the occurrence of nonthermal features is much more strongly ordered by $A_{c}$ , as can been seen in the distributions of (b) $T_{α} / T_{p}$ , (c) $T_{⊥ p} / T_{∥ p}$ , and (d) $Δ V_{α p} / C_{A}$ with age. For $A_{c} > 1$ , collisions produce thermal equilibrium, with $T_{α} = T_{p}$ , $T_{⊥ p} / T_{∥ p} = 1$ , and $Δ V_{α p} = 0$ . For $A_{c} < 1$ the plasma is collisionless and nonthermal features persisting from the corona or generated in situ are seen.Reuse & Permissions
Figure 3
The ratio $T_{⊥ α} / T_{⊥ p}$ of the perpendicular component of the ion temperatures as a function of the collisional age $A_{c}$ and $Δ V_{α p} / C_{A}$ , the differential flow between the two species normalized by the local Alfvén speed. The overall association of $T_{α} / T_{p}$ with $A_{c}$ seen in Fig. 2 is reproduced here. Additionally, there is a clear indication than in the collisionless regime $A_{c} < 0.5$ , $T_{⊥ α} / T_{⊥ p}$ is larger when $Δ V_{α p} / C_{A}$ is small. The region $0.03 < A_{c} < 0.1$ indicated by the dashed lines is the focus of the following figures.Reuse & Permissions
Figure 4
The average values of ion anisotropies $T_{⊥ α} / T_{∥ α}$ (red diamonds) and $T_{⊥ p} / T_{∥ p}$ (blue triangles) as a function of $Δ V_{α p} / C_{A}$ show that the region of anomalous $T_{⊥ α} / T_{⊥ p}$ at small $Δ V_{α p} / C_{A}$ is additionally associated with large alpha temperature anisotropy, a further indication of dissipation of Alfvén waves. In this figure and the next the vertical error bars are the uncertainty in the mean and the horizontal bars indicate the width of each interval.Reuse & Permissions
Figure 5
The median value of the ratio of the perpendicular (upper panel) and parallel (lower panel) temepratures as a function of normalized differential flow. The red diamonds are for $Δ V_{α p}$ normalized by the Alfvén speed $C_{A}$ , and the blue triangles use the sound speed $C_{S}$ . The data in the upper panel clearly demonstrate that the Alfvén speed more strongly organizes $T_{⊥ α} / T_{⊥ p}$ then the sound speed, supporting the theory that the anomalous heating is due to enhanced absorption of dissipated kinetic Alfvén waves by the helium.Reuse & Permissions

Physical Review Letters

Hot Solar-Wind Helium: Direct Evidence for Local Heating by Alfvén-Cyclotron Dissipation

J. C. Kasper, A. J. Lazarus, and S. P. Gary

Phys. Rev. Lett. 101, 261103 – Published 22 December 2008

See Viewpoint: What keeps the solar wind hot?

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What keeps the solar wind hot?

Published 22 December 2008

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