The polytropic behaviour and turbulence properties of sub-Alfvénic intervals

ABSTRACT The solar wind at 1 au is primarily super-Alfvénic, but rare instances occur when the Alfvén speed surpasses the solar wind speed, resulting in occurrences of sub-Alfvénic intervals. During sub-Alfvénic intervals, the solar wind no longer creates a bow shock around Earth, allowing for a direct interaction between the solar wind and Earth’s magnetopause. The thermal properties and turbulence characteristics of such rarely observed sub-Alfvénic regions at 1 au are not well understood. Our study comprehensively investigates sub-Alfvénic intervals observed from 1995 to 2023 and identifies a total of eleven such distinct regions embedded within solar wind streams and interplanetary coronal mass ejections (ICMEs). We focus on examining the thermodynamic properties of these regions by using the polytropic index and compare them with nearby super-Alfvénic regions. We find a transition in polytropic behaviour from super-Alfvénic to sub-Alfvénic intervals. Interestingly, we observed a bimodal distribution in the polytropic index, characterized by two distinct states: (i) sub- to super-adiabatic ($\alpha \gt 1$) and (ii) non-adiabatic ($\alpha \lt 1$). The presence of a non-adiabatic state at 1 au strongly suggests that heating dynamics is a significant factor influencing the thermodynamic state of the solar wind at this distance. We also investigated turbulence properties, and it indicates that turbulence is lower in sub-Alfvénic flow compared to neighboring super-Alfvénic regions, suggesting different processes are involved in these regions.