INTERPLANETARY SUPRATHERMAL He + AND He ++ OBSERVATIONS DURING QUIET PERIODS FROM 1 TO 9 AU AND IMPLICATIONS FOR PARTICLE ACCELERATION

Abstract

We measured quiet-time differential intensities of ~2-60 keV nucleon–1 He+ and He++ during the 1999-2004, 1-9 AU portion of Cassini's interplanetary cruise to Saturn and found that the He+/He++ composition ratio grows as the distance from the Sun r increases. An increase in the ratio is expected from the theoretical pickup ion and solar wind intensities, but the absolute He+ intensity, counter to the predicted falling r –1 dependence of the density, is actually slightly increasing, and He++ falls off much more slowly than the r –2 dependence one might expect from a population with a solar source. With an approximately r 2.2 radial dependence, our rigorous numerical transport and acceleration model (with stochastic acceleration) matches the higher-energy (>13 keV nucleon–1) measured He+/He++ composition profiles well, as does our analytical theory. Two acceleration processes are likely needed: the composition ratios are explainable by stochastic acceleration while a velocity-dependent mechanism that acts beyond 1 AU equally on He+ and He++ is required to explain the spatial intensity profiles.