THE INTERPLANETARY MASS EJECTIONS OBSERVED BY HELIOS

Using abnormally low proton temperature as the primary identification signature of interplanetary mass ejections （ICME）, about 160 probable ICME observed by Helios l and 2 between 0.3 AU and l AU have been identified. The resulting list of events enables us to study the radial evolution of ICME in a statistical sense. The occurrence rate of ICME approximately follows the solar activity cycle. ICME expand during propagation in the solar wind; the radial width increase with distance as R^0.64. The radial expansion speed of the ICME is of the order of the Alfven speed. The average temperature and magnetic field magnitude inside ICME decrease faster with distance than the ambient solar wind, as expected for an ex panding structure. The relatively slower decrease of temperature with distance than the adiabatic expansion solar wind implies a substantial heating of the ejecta. The average bulk speed of ICME remains constant with distance between 0.3 and 1 AU.