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, ε C , 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 ε C with respect to the incompressible cascade rate, ε I . 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, ε C . Finally, the observational results are connected with the local ion temperature and the solar wind heating problem.