Radial Evolution of Solar Wind Turbulence, a Multi-Spacecraft Study in the Inner Heliosphere

The solar wind is a constant stream of magnetized plasma propagating from the Sun into the heliosphere. The presence of waves and turbulence make it a highly complex environment. It has previously been shown that the temperature of the solar wind does not follow the trend of an adiabatic expansion. Instead, one or more additional sources must heat the plasma as the wind expands. This work investigates the role of turbulence in transferring the large-scale energy to smaller scales, where kinetic processes can convert it to plasma heating. To this aim two alignments of two spacecraft are used (ACE and Ulysses in 2007, Solar Orbiter and Wind in 2022) that sampled the same plasma parcel on its way outwards from the Sun. The turbulence of each measurement is characterized, the energy transfer rate is estimated and compared to a heating model. These parameters are compared to investigate the radial evolution. All intervals show nonlinear interactions and intermittency typical of Kolmogorov turbulence, supporting the existence of a turbulent cascade. The estimation of the energy transfer rate provides sufficient energy for the ion heating at all instances. Additionally, a good agreement between the power law decay of the heating rate and the energy cascade rate was found. The results demonstrate the importance of turbulence in the energy budget of the solar wind.