Heavy ion differential streaming observed in small-scale flux ropes

Context. In the collisionless solar wind, different ion species propagate with different speeds. The respective nonzero differential streaming of an ion species (i) with speed vi relative to the proton speed (vp), i.e., vi − p = vi − vp, is associated with wave activity in a nonthermal plasma. Yet wave activity is expected to be weak in flux ropes where the magnetic field exhibits a smooth rotation. Aims. We evaluated wave activity within small-scale flux ropes (SFRs) with different scale sizes and background solar wind conditions, with differential streaming as a reliable tracer, at a 12-minute time resolution. Methods. Based on measurements from the Advanced Composition Explorer (ACE), we reconstructed the differential velocity between He2+ and protons ($ \boldsymbol{v}_{\mathrm{He}^{2+}- p} $) in over 18 000 SFRs detected at 1 au. We then conducted a statistical study to check the overall status of differential streaming therein. Results. The differential streaming is weak in SFRs inside interplanetary coronal mass ejections (ICMEs) and magnetic clouds (MCs) detected by ACE at 1 au. Regarding SFRs outside ICMEs and MCs, about 40% exhibit rather strong differential streaming, i.e., streaming exceeds 0.6 times the local Alfvén speed. Such SFRs also tend to have collisionally young and fast plasma. Conclusions. Our results suggest that the identification of wave activity might be underestimated by the criteria used to construct the available SFR list. Differential streaming appears to be a more reliable tracer of wave activity.