A Statistical Study of the Compressible Energy Cascade Rate in Solar Wind Turbulence: Parker Solar Probe Observations

TL;DR

This study analyzes the energy cascade rates in solar wind turbulence at various distances from the Sun using Parker Solar Probe data, revealing increased cascade rates closer to the Sun and with higher plasma compressibility.

Contribution

It applies two recent exact relations for compressible MHD turbulence to observational data, providing new insights into how cascade rates vary with heliocentric distance and plasma compressibility.

Findings

Cascade rates increase closer to the Sun.

Compressibility correlates with higher cascade rates.

Similar cascade rates are observed regardless of the exact relation used.

Abstract

We investigated incompressible and compressible magnetohydrodynamic (MHD) energy cascade rates in the solar wind at different heliocentric distances. We used in situ magnetic field and plasma observations provided by the Parker Solar Probe (PSP) mission and exact relations in fully developed turbulence. To estimate the compressible cascade rate, we applied two recent exact relations for compressible isothermal and polytropic MHD turbulence, respectively. Our observational results show a clear increase of the absolute value of the compressible and incompressible cascade rates as we get closer to the Sun. Moreover, we obtained an increase in both isothermal and polytropic cascade rates with respect to the incompressible case as compressibility increases in the plasma. Further discussion about the relation between the compressibility and the heliocentric distance is carried out. Furthermore, we compared both exact relations as compressibility increases in the solar wind and although we note a slightly trend to observe larger cascades using a polytropic closure, we obtained essentially the same cascade rate in the range of compressibility observed. Finally, we investigated the signed incompressible and compressible energy cascade rates and its connection with the real cascade rate.