Variations of Heavy Ions in Different Types of Interplanetary Shocks

Abstract Heavy ions upstream and downstream of interplanetary shocks are not only subject to acceleration, heating, and spatiotemporal modulation during their propagation in interplanetary space, but they can also, in turn, influence the dynamical properties of the shocks themselves. Using ACE/Solar Wind Ion Composition Spectrometer observations, we analyzed the heavy-ion characteristics associated with 163 interplanetary shocks and classified the shocks into four categories for separate examination. We find that most forward shocks are driven by ICMEs, and their occurrence exhibits an 11 yr modulation consistent with the solar cycle, whereas reverse shocks show no clear periodic behavior. Overall, both heavy-ion abundance ratios and average charge states are systematically higher in forward shocks than in reverse shocks. For forward shocks, weak and strong shocks exhibit broadly similar trends: the heavy-ion parameters remain relatively steady prior to shock arrival, followed by a pronounced enhancement immediately after the shock crossing. Events with larger Alfvén Mach numbers tend to show steeper and stronger increases. In contrast, both the heavy-ion abundance ratios and average charge states decrease to their minimum values before the arrival of reverse shocks and gradually recover afterward. This behavior suggests that reverse shocks reorganize the spatial distribution of heavy ions as they propagate through interplanetary space.