MHD-Scale Energy Transfer in the Inner Heliosphere from PSP observations

Observations at 1 AU have reported direct evidence of an inertial-range energy cascade [1]. The average value of energy cascade rate in 1 AU solar wind plasma is around 1000 J/kg/s, which is shown to be sufficient to account for the heating of the solar-wind [2]. Parker Solar Probe (PSP) offers the first opportunity to estimate a similar, fluid-scale energy decay rate closer to the solar corona. Using a Politano-Pouquet [3] third-order law, we provide estimates of fluid-range energy cascade rate at 50 to 36 solar radii, during the first perihelion first encounter of PSP. Despite the cross-helicity being high in these regions of the heliosphere, there is an inertial-range cascade occurring. The energy transfer rate is at least 100 times higher than the average value at 1AU. Further, we estimate the global energy decay rate at the energy-containing scales using a Taylor-Karman decay phenomenology [4]. The von Karman energy decay estimates agree reasonably well with the third-order-law estimates. We also compare the two estimates with the heating rate obtained from a turbulence-based, global solar wind simulation [5]. [1] Sorriso-Valvo et al., Phys. Rev. Lett., 99, 115001 (2007) [2] MacBride et al., ApJ, 679, 1644 (2008) [3] Politano & Pouquet, GRL, 25, 273 (1998) [4] Wan et al., JFM, 697, 296315 (2012) [5] Usmanov et al., ApJ, 865, 25 (2018)