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The Electron Temperature and Anisotropy in the Solar Wind. Comparison of the Core and Halo Populations

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Abstract

Estimating the temperature of solar wind particles and their anisotropies is particularly important for understanding the origin of their deviations from thermal equilibrium and the effects this has. In the absence of energetic events, the velocity distribution of electrons reveals a dual structure with a thermal (Maxwellian) core and a suprathermal (kappa) halo. This article presents a detailed observational analysis of these two components, providing estimations of their temperatures and temperature anisotropies, and decoding any potential interdependence that their properties may indicate. The dataset used in this study includes more than 120 000 of the distributions measured by three missions in the ecliptic within an extended range of heliocentric distances from 0.3 to over 4 AU. The core temperature is found to decrease with the radial distance, while the halo temperature slightly increases, clarifying an apparent contradiction in previous observational analyses and providing valuable clues about the temperature of the kappa-distributed populations. For low values of the power-index kappa, these two components manifest a clear tendency to deviate from isotropy in the same direction, which seems to confirm the existence of mechanisms with similar effects on both components, e.g., the solar wind expansion, or the particle heating by the fluctuations. However, the existence of plasma states with anticorrelated anisotropies of the core and halo populations and the increase in their number for high values of the power-index kappa suggest a dynamic interplay of these components, mediated, most probably, by the anisotropy-driven instabilities.

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Acknowledgements

The research leading to these results has received funding from the Scientific Federal Policy in the framework of the program Interuniversity Attraction Pole for the project P7/08 CHARM. The authors acknowledge support from the Katholieke Universiteit Leuven, the Ruhr-University Bochum, and Alexander von Humboldt Foundation. These results were obtained in the framework of the projects GOA/2015-014 (KU Leuven), G0A2316N (FWO-Vlaanderen), and C 90347 (ESA Prodex 9). The research leading to these results has also received funding from the European Commission’s Seventh Framework Programme FP7-PEOPLE-2010-IRSES-269299 project-SOLSPANET ( www.solspanet.eu ). The authors further acknowledge the grant 15-17490S of the Czech Science Foundation. The data can be obtained from S. Štverák on request.

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Authors and Affiliations

  1. Space Physics and Solar-Terrestrial Center of Excellence, Royal Belgian Institute for Space Aeronomy, 3 av. Circulaire, 1180, Brussels, Belgium

    V. Pierrard

  2. Center for Space Radiations (CSR) and Georges Lemaître Centre for Earth and Climate Research (TECLIM), Earth and Life Institute (ELI), Université Catholique de Louvain (UCL), Place Louis Pasteur 3 bte L4.03.08, 1348, Louvain-La-Neuve, Belgium

    V. Pierrard

  3. Institut für Theoretische Physik, Lehrstuhl IV: Weltraum- und Astrophysik, Ruhr-Universität, Bochum, Germany

    M. Lazar

  4. Center for Mathematical Plasma Astrophysics, K.U. Leuven, Celestijnenlaan 200B, 3001, Leuven, Belgium

    M. Lazar & S. Poedts

  5. Institute of Atmospheric Physics, Czech Academy of Sciences, Prague, Czech Republic

    Š. Štverák & P. M. Trávníček

  6. Astronomical Institute, Czech Academy of Sciences, Ondrejov, Czech Republic

    Š. Štverák & P. M. Trávníček

  7. LESIA and CNRS, Observatoire de Paris-Meudon, Meudon, France

    M. Maksimovic

  8. Space Science Laboratory, University of California, Berkeley, Berkeley, CA, United States

    P. M. Trávníček

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  1. V. Pierrard
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  2. M. Lazar
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  3. S. Poedts
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Correspondence to V. Pierrard.

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Pierrard, V., Lazar, M., Poedts, S. et al. The Electron Temperature and Anisotropy in the Solar Wind. Comparison of the Core and Halo Populations. Sol Phys 291, 2165–2179 (2016). https://doi.org/10.1007/s11207-016-0961-7

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