Estimates of alpha particle heating in the solar wind inside 0.3 AU

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Abstract
Helios observations are used to determine and compare the average temperature gradients of protons and alpha particles in the solar wind between 0.3 and 1 AU. The He2+ ions are found to cool off more slowly than adiabatically. The radial profiles of their temperature are somewhat steeper in fast than in slow solar wind, a result which is in contrast to the proton results. These in situ measurements are extrapolated to inside 0.3 AU in order to estimate the radial gradients of Tα there and the required heating rates. Thereby, the temperature profiles are adjusted to match coronal boundary values. Clear evidence is found for continuous interplanetary alpha particle heating, which supports theories invoking extended ion heating by waves in the outer solar corona.

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