The evolution of compressible solar wind turbulence in the inner heliosphere: PSP, THEMIS, and MAVEN Observations

The first computations of the compressible energy transfer rate from ~0.2 up to ~1.7 au is obtained using Parker Solar Probe (PSP), Time History of Events and Macroscale Interactions during Substorms (THEMIS), and Mars Atmosphere and Volatile EvolutioN (MAVEN) observations. Using a recently derived exact relation for isothermal magnetohydrodynamics turbulence, the compressible energy cascade rate, ϵC, is computed for hundred of events at different heliocentric distances, for time intervals when the spacecrafts were in the pristine solar wind. The observational results show moderate increases of ϵC with respect to the incompressible cascade rate, ϵI. Depending on the level of compressibility in the plasma, which reaches up to 25% at PSP's perihelion, the different terms in the exact compressible relation are shown to have different impacts on the total cascade rate, ϵC. Finally, the observational results are connected with the local ion temperature and the solar wind heating problem.