Rigidity‐independent coronal propagation and escape of solar protons and α particles

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Abstract
A statistical study of 42 solar proton and α particle events has been performed in the energy range of 9–36 MeV/nucleon (nuc) as measured by the Space Science Department/European Space Agency experiment on Heos 2 during 1972–1974. From one event to another there is a high variability of the p/α ratio at equal energy per nucleon ranging from about 4 to 1000 at 10 MeV/nuc. It is found that the lower value of the p/α ratios increases with azimuthal distance from the flare site. We show that this is not the result of rigidity‐dependent coronal processes but that the increase follows rather as a consequence of two observations: (1) the p/α ratio is correlated with the size of the particle event (Van Hollebeke, 1975) and (2) the number of observed particle events decreases to the east as is found by numerous authors. A model calculation based on these observations is in good agreement with the data. The conclusion of rigidity‐independent coronal propagation and escape also follows from other observations. First, the p/α ratios simultaneously measured by the University of Chicago experiment on board Pioneer 10 and 11, which have different connection longitudes, are generally the same as the Heos 2 values. Second, no systematic variation of the p/α ratio is found for the few cases of large individual events, when the p/α ratio can be followed over an extended time period, i.e., over an extended range of connection longitudes. Third, the proton and α particle spectral exponents show no systematic variation with heliolongitude. It is concluded that the observed large variations of the p/α ratio from event to event are essentially due to different acceleration or storage conditions at the flare site.