Microstructure at a fully formed Forward-Reverse Shock pair due to the interaction between two Coronal Mass Ejections observed at 0.5 AU.

Interplanetary (IP) shocks can be driven in the solar wind by fast coronal mass ejections, and by the interaction of fast solar wind with slow streams of plasma. These shocks can be preceded by extended wave and suprathermal ion foreshocks perturbing large extensions of the heliosphere. In a recent study (Trotta et al., 2024 it was shown that the interaction between two interplanetary coronal mass ejections (ICMEs) can drive a forward and a reverse shock with similarities to those bounding stream interaction regions (SIRs). In this work we analyse the microstructure of this event observed by Solar Orbiter on March 8th, 2022 at 0.5 AU. We find that wave characteristics change from one ICME to the other. Inside the first ICME waves have a broad band sprectrum. In contrast, there are regions in the second ICME with very monochromatic waves. In both cases, waves are associated with proton and alpha particle distributions that show a super-Alfvenic drift, suggesting local wave generation. Larger amplitude waves due to ion reflection are found upstream of the forward shock forming an extended foreshock. Although the shock was weak,  reflected populations include alpha particles, and O⁶⁺ ions. Of particular interest is the fact that monochromatic ion cyclotron waves associated with anisotropic (Tperp >Tpar) ion distributions are found in the region between the two ICMEs. Our results show how ICME-ICME interaction can result in regions with a variety of microstructure phenomena in the inner heliosphere.