Kinetic instabilities constraining proton temperature anisotropy in corotating interaction regions at 1 AU

Abstract

Corotating Interaction Regions (CIRs), formed by the interaction between slow and fast solar wind streams, provide an ideal environment for studying plasma waves and instabilities. In this study, we analyze 290 CIR events using Wind spacecraft data at 1 AU during Solar Cycle 24 (2008–2019) to explore the relationship between proton temperature anisotropy and kinetic instabilities. Our results show that the Mirror Mode (MM) and Oblique Firehose (OFH) instabilities primarily regulate temperature anisotropy within CIRs for $T_{p\perp }/T_{p\Vert }>1$ and $T_{p\perp }/T_{p\Vert }<1$ respectively, even at ${\beta }_{p\Vert }⩽2$. Although Proton Cyclotron Instability (PCI) is theoretically expected to operate at low ${\beta }_{p\Vert }$, our observations and linear instability analysis suggest that for a growth rate of $\gamma ={10}^{-3}{\Omega }_p$ PCI does not significantly constrain or limit the anisotropy in CIRs, potentially due to inefficient energy extraction or the stabilizing effects of minor ions. However, the present study does not account for the effects of collision age and the presence of heavy ions, which may also influence anisotropy constraints in CIRs. Moreover, for ${\beta }_{p\Vert }⩾2$, the anisotropy is constrained by a combination of MM, PCI, and firehose instabilities. These findings highlight that the mechanisms constraining temperature anisotropy in CIRs are consistent with those in the ambient solar wind and interplanetary coronal mass ejection’s sheath, underscoring the robustness of these regulatory processes across different heliospheric environments.