On the Statistical Properties of Turbulent Energy Transfer Rate in the Inner Heliosphere

The transfer of energy from large to small scales in solar wind turbulence is an important ingredient of the long-standing question of the mechanism of the interplanetary plasma heating. Previous studies have shown that magnetohydrodynamic (MHD) turbulence is statistically compatible with the observed solar wind heating as it expands in the helio-sphere. However, in order to understand which processes contribute to the plasma heating, it is necessary to have a local description of the energy flux across scales. To this aim, it is customary to use indicators such as the magnetic field partial variance of increments (PVI), which is associated with the local, relative, scale-dependent magnetic energy. A more complete evaluation of the energy transfer should also include other terms, related to velocity and cross-helicity. This is achieved here by introducing a proxy for the local, scale-dependent turbulent energy transfer rate epsilon(Delta t) (t), based on the third-order moment scaling law for MHD turbulence. Data from Helios 2 are used to determine the statistical properties of such a proxy in comparison with the magnetic and velocity fields PVI, and the correlation with local solar wind heating is computed. PVI and epsilon(Delta t) (t) are generally well correlated; however, epsilon(Delta t) (t) is a very sensitive proxy that can exhibit large amplitude values, both positive and negative, even for low amplitude peaks in the PVI. Furthermore, epsilon(Delta t) (t) is very well correlated with local increases of the temperature when large amplitude bursts of energy transfer are localized, thus suggesting an important role played by this proxy in the study of plasma energy dissipation.