Collisional thermalization of hydrogen and helium in solar-wind plasma

B A Maruca; S D Bale; L Sorriso-Valvo; J C Kasper; M L Stevens

doi:10.1103/PhysRevLett.111.241101

Collisional thermalization of hydrogen and helium in solar-wind plasma

Phys Rev Lett. 2013 Dec 13;111(24):241101. doi: 10.1103/PhysRevLett.111.241101. Epub 2013 Dec 9.

Authors

B A Maruca¹, S D Bale², L Sorriso-Valvo³, J C Kasper⁴, M L Stevens⁵

Affiliations

¹ Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley, California 94720, USA.
² Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley, California 94720, USA and Department of Physics, University of California, LeConte Hall, Berkeley, California 94720, USA.
³ Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley, California 94720, USA and IPCF-CNR, U.O. Cosenza, Ponte P. Bucci, Cubo 31C, 87036 Rende, Italy.
⁴ Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA and Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA.
⁵ Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA.

PMID: 24483639
DOI: 10.1103/PhysRevLett.111.241101

Abstract

In situ observations of the solar wind frequently show the temperature of α particles (fully ionized helium) Tα to significantly differ from that of protons (ionized hydrogen) Tp. Many heating processes in the plasma act preferentially on α particles, even as collisions among ions act to gradually establish thermal equilibrium. Measurements from the Wind spacecraft's Faraday cups reveal that, at r=1.0 AU from the Sun, the observed values of the α-proton temperature ratio, θαp≡Tα/Tp, has a complex, bimodal distribution. This study applied a simple model for the radial evolution of θαp to these data to compute expected values of θαp at r=0.1 AU. These inferred θαp values have no trace of the bimodality seen in the θαp values measured at r=1.0 AU but are instead consistent with the actions of the known mechanisms for α-particle preferential heating. This result underscores the importance of collisional processes in the dynamics of the solar wind and suggests that similar mechanisms may lead to preferential α-particle heating in both slow and fast wind.