A STATISTICAL-SURVEY OF 5-MEV PROTON EVENTS AT TRANSIENT INTERPLANETARY SHOCKS

Between 1974 and 1985 the two Hellos spacecraft observed 351 transient interplanetary shocks. For 5-MeV protons the particle events associat ed with these shocks can be divided into three groups: (1) events with out intensity increase above quiet time or increased background (47%), (2) solar and interplanetary particle (SLP) events consisting of part icles accelerated on or close to the Sun (solar or near-Sun component) as well as at the interplanetary shock (24%), and (3) pure interplane tary particle (PIP) events (29%) which consist of particles accelerate d at the shock in interplanetary space but do not show evidence for si gnificant or even excess particle acceleration on the Sun. This classi fication shows that (1) only about half of the shocks accelerate MeV p rotons in interplanetary space and (2) MeV protons accelerated on the Sun are neither a necessary nor a sufficient condition for the acceler ation of MeV protons in interplanetary space, Shock parameters such as speed or shock strength alone do not give an indication for the class of the associated particle event, because in the parameter range whic h covers most of the shocks, all three classes are distributed rather evenly, However, the shocks strongest in these parameters tend to acce lerate particles, The intensity at the time of shock passage, which ca n be used as a crude measure for the local acceleration efficiency, is correlated with the local shock speed and the magnetic compression, T he correlation coefficients are small but statistically significant, i ndicating that (1) the correlations are real and (2) the intensity is influenced by additional parameters, which are not necessarily shock i nherent. As an example I will show that the local acceleration at the shock decreases roughly symmetrically with increasing distance from th e nose of the shock with a median e-folding angle of 10 degrees. Occas ionally, larger e-folding angles are observed close to the nose of the shock. The question of how the shock accelerates protons in the MeV r ange could not be answered here, but I will suggest future studies tha t could shed a new Light on this problem.