The solar wind is turbulent, exhibiting a power spectrum extending over several decades in scale. The solar wind is weakly collisional, as evidenced by the wide variety of non maxwellian particle…

This is a federal grant award to The Trustees of Princeton University, acting through its Office of Research and Project Administration, for a research project focused on understanding the turbulent nature of the solar wind and its impact on plasma physics. The $427,593.66 grant, awarded by the National Aeronautics and Space Administration (NASA), aims to accomplish several science goals, including determining the mechanisms responsible for perpendicular heating of solar wind ions, examining processes that set the observed ion-scale spectral break in solar wind turbulence, making predictions on the effects of violating the Taylor hypothesis on the inferred magnetic energy power spectrum, and diagnosing the effects of velocity-space anisotropy and micro-scale plasma instabilities on the observed turbulent cascade. The research leverages cutting-edge kinetic simulations and analysis of in-situ spacecraft data, with the goal of informing the interpretation of anticipated data from NASA's upcoming Solar Probe Plus mission. No set-aside designation was specified for this award.

The solar wind is turbulent, exhibiting a power spectrum extending over several decades in scale. The solar wind is weakly collisional, as evidenced by the wide variety of non maxwellian particle velocity distributions amongst the proton, electron, and minor ION populations. This proposal treats these defining characteristics of solar wind plasma on equal footing, by coupling start of the art kinetic simulations of driven alfvenic turbulence in a collisionless low beta plasma with analyses of in situ measurements of the fluctuating magnetic field and particle distribution functions in the solar wind. The science goals of this proposal are i to determine the mechanisms responsible for the observed perpendicular heating of solarwind ions II to examine what physical processes set the observed ionscale spectral break in solarwind turbulence III to make quantitative predictions concerning the effects of violating the taylor hypothesis on the inferred magneticenergy power spectrum in the nearsun environment and IV to diagnose the effects of velocityspace anisotropy and microscale plasma instabilities on the observed turbulent cascade. These science goals are not only relevant to one of the decadel survey goals to discover and characterize fundamental processes that occur both within the heliosphere and throughout the universe, but also are particularly timely, as solar probe plus SPP and solar orbiter are scheduled to launch in 2018 and as advances in computer hardware have made first principles kinetic simulations possible. These goals represent necessary building blocks in formulating an accurate model for how the solar wind is launched and evolves through interplanetary space. To accomplish our research objectives, we plan to use the new hybrid kinetic particle in cell code, pegasus, developed by the PI. The hybrid kinetic approach, in which the ions are treated kinetically while the electrons constitute a neutralizing magnetohydrodynamic fluid, is an attractive compromise between capturing as much kinetic physics as possible using first principles techniques and reducing computational costs to a feasible level given modern resources. Three dimensional simulations of driven alfvenic turbulence in a collisionless plasma similar to the solar wind will be performed and analyzed, with a predictive focus on non adiabatic ION heating, electromagnetic power spectra, and the effects of plasma microphysics on global dynamics. These simulations will be complimented by analyses of wind and ulysses data, which will focus on characterizing magnetic field fluctuations in data intervals that exhibit significant velocity space anisotropy in their particle distribution functions. Finally, the two approaches numerical and observational will be combined to aid the interpretation of anticipated data from SPP. Synthetic spacecraft fly throughs of our turbulence simulations at different speeds and angles with respect to the mean magnetic field will be performed to formulate a modified taylor hypothesis a necessary tool to gauge how much aliasing will occur in SPP measurements and thereby maximize the return from the SPP mission. Many of the science goals and anticipated returns of the proposed heliophysics research will have broad implications for our understanding of other weakly collisional astrophysical plasmas, such as the intracluster medium of galaxy clusters and certain classes of black hole accretion flows. These systems, the focus of x RAY missions like the NASA funded chandra x RAY observatory, are purported to be turbulent, non maxwellian, non adiabatic plasmas whose velocity space anisotropy and thus transport properties are regulated by microscale kinetic instabilities. The fundamental physical processes at play in these systems have much in common with those that can be directly measured in the heliosphere.