On the energy cascade rate of solar wind turbulence in high cross helicity flows

摘要

Improved observations and analysis of solar wind fluctuations at 1 AU show that the total energy spectrum, kinetic plus magnetic, is in better agreement with the Kraichnan 3/2 scaling than with the Kolmogorov 5/3 scaling, consistent with simulations of incompressible MHD turbulence with a strong ambient magnetic field which exhibit a perpendicular energy spectrum proportional to k_⊥~(3/2). The Kraichnan scaling is especially clear in solar wind flows having large values of the normalized cross helicity σ_c. In this study, a generalization of Boldyrev's theory that applies to turbulence with nonvanishing cross helicity is used to estimate the energy cascade rate for high cross helicity flows in the solar wind at 1 AU. The analysis of 85 intervals of solar wind data from the Wind spacecraft characterized by normalized cross helicities in the range |σ_c| > 0.88 yields typical values of the energy dissipation rate E ranging from 100 to 2400 J/kg/s. This is roughly a factor of 4 or 5 smaller than estimates of the proton heating rate at 1 AU needed to account for the nonadiabatic solar wind expansion. The results suggest that the reduction in turbulent heating in high cross helicity flows creates a tangible deficit in the heating rate required by empirical models based on average solar wind properties.