Abstract
The plasma experiment on board of the ESRO satellite HEOS-1 detects solar wind positive ions in the energy range from 200 eV/Z–16000 eV/Z. α-particles have been observed for 10% of the time, from December 1968 to March 1969, when in the interplanetary medium α-particle and proton bulk velocities are on average equal. The average relative abundance of α-particles is 5.5 %.
Although in general the two species, hydrogen and helium, have a similar behaviour, very often α-particles have an independent time history. Strong discontinuities may be observed in one species, without occurring in the other, or having different characteristics. At certain times, α-particles seem to behave as an independent fluid.
Well defined fluxes of α-particles are observed within the magnetosheath: in several cases α-particles, present in the unperturbed solar wind, are found with unchanged energy spectrum through a certain layer behind the proton shock front inside the magnetosheath. This suggests that thermalization of α-particles does not occur at the same place as that of protons.
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Formisano, V., Palmiotto, F. & Moreno, G. α-particle observations in the solar wind. Sol Phys 15, 479–498 (1970). https://doi.org/10.1007/BF00151853
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DOI: https://doi.org/10.1007/BF00151853