Abstract:
The solar wind at 1 au is primarily super-Alfvénic, butrare instances occur when the Alfvén speed surpassesthe 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 (α > 1) and (ii) non-adiabatic (α < 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.
