Corotating events in the energy range 4–13 MeV as observed on board Helios 1 and 2 in 1975 and 1976

Intensity increases of 4 – 13 MeV protons correlated with recurrent high speed solar wind streams were observed on board Helios 1 and 2 at heliocentric distances between 0.3 and 1.0 AU. Study of events which occurred in the time period 1975 – 1976 showed that recurrent fast solar wind streams were sometimes not accompanied by corotating events and small events were more frequently observed than large corotating events, which occurred only occasionally. The explanation for these phenomena seems to be the variation of the number of suprathermal particles injected into the acceleration process. Radial gradients of corotating events from February till April 1976 were investigated. The gradient changed its sign in successive solar rotations and exhibited strong time dependence inside 1 AU. Clearly negative gradients were measured in corotating events which occurred in February and April 1976. These observations showed that probably there were no stationary corotating structures in interplanetary space. We suggest that this effect is related to local disturbances of solar origin, e. g. flare initiated shocks.