Polytropic behavior in the structures of Interplanetary Coronal Mass Ejections

TL;DR

This study analyzes the thermodynamic behavior of interplanetary coronal mass ejections using Wind spacecraft data, revealing a cyclic pattern in the polytropic index that indicates turbulent heating rates in ICME plasma.

Contribution

It provides the first comprehensive analysis of the polytropic index distribution in ICME regions over multiple years, linking thermodynamic behavior to turbulent heating.

Findings

Sheath regions show longer recovery times to adiabatic behavior.

Polytropic index varies cyclically across ICME regions.

Lower index values correlate with higher turbulent heating rates.

Abstract

The polytropic process characterizes the thermodynamics of space plasma particle populations. The polytropic index, ${\gamma}$, is particularly important as it describes the thermodynamic behavior of the system by quantifying the changes in temperature as the system is compressed or expanded. Using Wind spacecraft plasma and magnetic field data during $01/1995 - 12/2018$, we investigate the thermodynamic evolution in 336 Interplanetary Coronal Mass Ejection (ICME) events. For each event, we derive the index ${\gamma}$ in the sheath and magnetic ejecta structures, along with the pre- and post- event regions. We then examine the distributions of all ${\gamma}$ indices in these four regions and derive the entropic gradient of each, which is indicative of the ambient heating. We find that in the ICME sheath region, where wave turbulence is expected to be highest, the thermodynamics takes longest to recover into the original quasi-adiabatic process, while it recovers faster in the quieter ejecta region. This pattern creates a thermodynamic cycle, featuring a near adiabatic value ${\gamma}$ ~ ${\gamma}$${_a}$ (=5/3) upstream of the ICMEs, ${\gamma}$${_a}$ - ${\gamma}$ ~ 0.26 in the sheaths, ${\gamma}$${_a}$ - ${\gamma}$ ~ 0.13 in the ICME ejecta, and recovers again to ${\gamma}$ ~ ${\gamma}$${_a}$ after the passage of the ICME. These results expose the turbulent heating rates in the ICME plasma: the lower the polytropic index from its adiabatic value and closer to its isothermal value, the larger the entropic gradient, and thus, the rate of turbulent heating that heats the ICME plasma.