Polytropic Behavior in Corotating Interaction Regions: Evidence of Alfvénic Heating

Corotating interaction regions (CIRs) are recurring structures in the solar wind, characterized by interactions between fast and slow solar wind streams that compress and heat plasma. This study investigates the polytropic behavior of distinct regions in and around CIRs: uncompressed slow solar wind, compressed slow solar wind, compressed fast solar wind, and uncompressed fast solar wind. Using Wind spacecraft data and an established methodology for calculating the polytropic index (γ), we analyze 117 CIR events. Results indicate varying γ values across regions, with heating observed in compressed regions driven by Alfvén wave dissipation originating from fast streams. In the uncompressed fast solar wind, γ exceeds adiabatic values the most and correlates well with strong Alfvénic wave activity.