Ages in the solar wind from global MHD simulation

Abstract:

The question of the strength of turbulence compared with other physical processes is relevant to natural systems such as the solar wind, where different processes compete to influence the dynamics. One such process is that of collisions between the constituent particles that serve to thermalize the plasma. An evaluation of the strength of these processes is possible through comparison of characteristic timescales. The "Age" of Coulomb collisions is a measure of the extent of collisional thermalization, and has been used to study non-thermal features of the solar wind (Maruca et al 2013). However, such observational estimates of the Collisional Age have been forced to employ simplified formulae, given the limitations of spacecraft observations. In this work, we use a global 3-D MHD simulation of the solar wind that includes turbulence modeling to evaluate the Turbulence and Collsional Ages through the inner heliosphere. Our 3-D data-set enables us to use the complete analytical expressions for the ages without using approximations. We also evaluate the Ages using the approximate formulae employed in previous work, and compare these approximate results with our exact calculations to examine the validity of the simplified formulae, which necessarily must be used when working with spacecraft data, including that from Solar Probe Plus.