Anisotropic Proton Heating by Kinetic Instabilities in Corotating Interaction Regions

Corotating interaction regions (CIRs), formed by the interaction of fast and slow solar wind streams, are characterized by strong compression, enhanced turbulence, and elevated proton temperatures. We investigate how temperature anisotropy and kinetic instabilities regulate proton temperatures using 463 Earth-directed CIRs observed by the Wind spacecraft during 1995–2009 at 1 au. The plasma distribution in ( β p ∥ , R p ) space, where R p = T p ⊥ T p ∥ , is largely constrained by the mirror-mode (MM) and oblique firehose (OFH) instabilities. Proton temperatures are enhanced near the MM boundary for β p ∥ < 1 and near the OFH boundary for β p ∥ > 1. The enhancement near the MM boundary is dominated by perpendicular proton temperature ( T p ⊥ ), whereas that near the OFH boundary is dominated by parallel proton temperature ( T p ∥ ). Intervals unstable to proton cyclotron modes show temperatures up to 5.2–6.5 times higher than stable plasma, while parallel firehose-unstable intervals exhibit enhancements of ∼2–3 times, indicating distinct preferential heating channels. Temperature enhancements are strongest for low- β p ∥ plasma, whereas high- β p ∥ intervals exhibit limited heating despite elevated magnetic fluctuations and compressibility. An anticorrelation between proton temperature and collisional age indicates that hotter plasma is less collisional and remains closer to instability thresholds. Magnetic fluctuation power decreases weakly with increasing collisional age, suggesting stronger wave activity in collisionally young plasma. These results demonstrate that proton heating in CIRs is anisotropic and self-regulated by kinetic instabilities, with plasma beta, turbulence, compressibility, and collisionality jointly governing CIR thermodynamics.