Abstract
It has previously been suggested that the very high relative abundances of helium occasionally observed in the solar wind mark the plasma accelerated by major solar flares. To confirm this hypothesis, we have studied the 43 spectra with He/H ⩾ 15% that were observed among 10300 spectra collected by Vela 3 between July 1965–July 1967. The 43 spectra were distributed among 16 distinct periods of helium enhancement, 12 of which (containing 75% of the spectra) were associated with solar flares. Six new flare-enhancement events are discussed in this paper. It is concluded that the association of helium enhancements with major flares is real, non-random and very strong.
With this study, there are 12 cases of reliable associations between helium enhancements (He/H ⩾ 15%) and flares reported in the literature. The general characteristics of these events are discussed. It is found that the flares are typically large and bright (2B or 3B), often they produce cosmic ray protons, and they are widely distributed in solar longitude. The average transit velocity of the pistons (i.e., flare accelerated driver gas) is in excellent agreement with earlier observations of flare shock velocities. The degree to which the pistons have been slowed in transit is in good agreement with theory. The average percentage of helium in the enhanced regions is 15%, but this number should not be considered more than an extremely rough estimate because of very arbitrary decisions that had to be made as to when we would consider an ‘enhancement’ had ended. The number of positively charged particles in the enhanced region is estimated to be of the order of 4 × 1039.
A qualitative discussion of some of the possibilities for the source of helium enhanced plasma is presented. It is suggested that the helium enriched plasma may be the piston producing the shock causing the Type II radio emission. The size of the Type II emission region and the number of particles in the helium enhancement permit an estimate to be made of the density of the corona at the origin of the piston. From this it is estimated further that the piston must come from below about 0.5 R ⊙, in agreement with the 0.2–0.3 R ⊙ often given for the initial height of the Type II emission source. Recent theoretical discussions have indicated that the corona as a whole can be expected to show helium enrichments at these levels.
It is pointed out that observations of solar wind helium enhancement can be expected to be a useful tool in studying the distribution and relative abundance of helium in different layers of the solar corona, as well as mechanisms for the acceleration of plasma by solar flares.
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Hirshberg, J., Bame, S.J. & Robbins, D.E. Solar flares and solar wind helium enrichments: July 1965–July 1967. Sol Phys 23, 467–486 (1972). https://doi.org/10.1007/BF00148109
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DOI: https://doi.org/10.1007/BF00148109