The ion-proton differential streaming observed in Small-scale Flux Ropes

Heavy ion composition and charge-state distributions provide valuable information about the source region of the solar wind due to the 'freeze-in' effect, making them valuable diagnostics for understanding the conditions of their source regions. Small-scale flux ropes (SFRs) have been studied for decades, but their source regions and formation mechanisms are still under debate. While heavy ion signatures in relatively large-scale flux rope structures, known as magnetic clouds (MCs), have been well studied, those signatures are still unclear in SFRs that last only couple of minutes. More importantly, heavy ions do not necessarily travel at the same speed as protons in the solar wind. A potential ion-proton differential velocity could cause a temporal lag between the heavy ion signal and the boundaries of SFRs, which introduces deviations when heavy ion signatures in SFRs are investigated.

In this study, we review ten years of in-situ solar wind heavy ion data obtained from the Solar Wind Ion Composition Spectrometer (SWICS) on board the Advanced Composition Explorer (ACE). The data set is derived from the Pulse Height Analysis (PHA) data, at 12-min resolution. By investigating every energy per charge step of each SWICS measurement interval, more SFRs with short duration, even shorter than 12 minutes, are included. We conduct a statistical study on the ion-proton differential streaming in over 6300 SFRs that are heavy ion abundant, as well as in the surrounding solar wind.

Positive ion-proton differential streaming is found common in SFRs but less common in SFRs that are located in recorded Interplanetary Coronal Mass Ejections (ICMEs) . About 50% heavy-ion-dense SFRs show ion-proton differential velocity larger than 0.2 times the local Alfvén speed. Positive ion-proton differential streaming has also been observed in the background solar wind near SFRs. However, some cases show strong positive ion-proton differential streaming exclusively within SFRs. Ion-proton differential streaming is crucial for understanding heavy-ion signatures in small-scale structures, with their acceleration mechanisms being of particular interest. A further study shows that SFRs detected at 1 AU are unlikely to be the interplanetary manifestations of nanoflare- or microflare-associated small CMEs, or at least not solely so.