Evidence for Mass‐per‐Charge–dependent Acceleration of a Multiple‐Component Seed Population by CME‐driven Interplanetary Shocks Near 1 AU

Abstract

Using measurements from the ULEIS and SEPICA instruments on board the Advanced Composition Explorer between 1997 November and 2000 October, we have surveyed the abundances of 0.25-0.8 MeV nucleon−1 He+,3He, and heavy ions from C-Fe during 18 CME-driven interplanetary (IP) shocks observed near 1 AU. Our results show that each of the 18 IP shocks is accompanied by enhancements in the intensities of both 3He and He+ ions. In addition we find that, on a case-by-case basis, the abundances of He+ and the heavier elements such as C-Fe (but not 3He) are depleted systematically as a function of the ion's M/Q ratio when compared with those measured in the ambient suprathermal ion population upstream of the IP shocks. These results show for the first time that individual IP shocks routinely accelerate ions from multiple seed populations, such as multiple solar energetic particle events, corotating interaction regions, pickup ions, etc., via systematic rigidity-dependent acceleration processes where ions with higher rigidity or M/Q ratios are accelerated less efficiently than those with lower M/Q ratios. We also compare the M/Q-dependent depletion of these abundances with the locally measured shock parameters and explore why the 3He abundance does not fit into the systematic M/Q-charge-dependent fractionation processes.