Local Fluctuations and Correlations in Turbulent Cascade Rates in the Solar Wind

English Abstract:

Politano and Pouquet showed in two 1998 papers that the turbulent heating rate in stationary homogeneous plasmas is proportional to the slope of certain signed third moments of fluctuations in Elsasser variables with scale distance. Using plasma data from the ACE spacecraft at L₁, we found that the required third moments are highly variable, so that months of data were needed to get the third moments at each scale to converge, so we could get a meaningful heating rate. These heating rates compared well with the in-situ heating of the solar wind deduced from the non-adiabatic decay of proton temperature from the corona to 1 AU. We thought the need for large amounts of data was due to intermittency. Lately, however, we discovered that in more local 1 to 12 hour ACE data sets, the third moment of fluctuations in Elsasser variables are in practically all cases proportional to scale, with correlation coefficients close to ±1. However, the slopes and inferred local cascade rates (we hesitate to call these heating rates) are highly variable among these shorter data sets. Combining many such local cascade rates converges on meaningful heating rates as before. Furthermore, the cascade rates for outward- and inward-travelling pseudo-energies (in each Elsasser variable) in these smaller data sets tend to be anti-correlated according to the local cross-helicity. Evidence of this will be presented in this talk. We are left with the questions: Is this variation in local behavior of the third moments in solar wind another aspect of intermittency? and What do the observed anti-correlation, its dependence on cross-helicity, and the negative local cascade rates mean?