Collisional thermalization of hydrogen and helium in solar-wind plasma.
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.
Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley, California 94720, USA.
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BA
Maruca
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.
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SD
Bale
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.
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L
Sorriso-Valvo
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.
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JC
Kasper
Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA.
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Stevens