The solar wind, especially the fast streams (speed greater than 600−700 KM/s), is permeated by a flux of almost incompressible, large amplitude fluctuations of plasma velocity and magnetic field,…

University of California, Los Angeles (UCLA) was awarded a $123,499.86 grant for research from the National Aeronautics and Space Administration (NASA) Shared Services Center. The grant is focused on investigating the role of nonlinear wave and wave-particle interactions in maintaining or generating the Alfvénic state in the solar wind. This aligns with NASA's Heliophysics Science Research program area on heliospheric plasma processes, turbulence, and waves. The research aims to provide new insights on the dynamics of Alfvén waves and their potential contributions to solar wind heating and acceleration. The grant does not have a set-aside designation, and UCLA will perform the work at its facilities in Los Angeles. As a public research university, UCLA leverages its expertise, facilities, and partnerships to support federal agencies like NASA on scientific research and technology development initiatives.

The solar wind, especially the fast streams (speed greater than 600−700 KM/s), is permeated by a flux of almost incompressible, large amplitude fluctuations of plasma velocity and magnetic field, known as alfv NIC turbulence. Though the spectrum is turbulent, the velocity-magnetic field correlation corresponds to that of waves propagating away from the sun. Another important and as yet unexplained property is that the total magnetic field magnitude remains almost constant, in the sense that the fluctuations in the magnitude of the field are much smaller than the magnitude of the fluctuations in the magnetic field. Despite the wealth of observational evidence covering distances from 0.3 out to many AU, how this turbulent yet coherent alfv NIC state is achieved in the solar wind as well as its potential dynamical role in solar wind heating and acceleration remain fundamental open questions of heliophysics. We propose to investigate theoretically the role of nonlinear wave and wave-particle interactions in maintaining or generating this alfv NIC state and to search for specific signatures of such interactions in the solar wind. A recent theoretical result has predicted that linearly polarized alfv n waves can generate sufficient pressure anisotropy to remove their own restoring force when plasma>1. This causes significant nonlinear modifications to the alfv n wave and excites the firehose instability, possibly leading to constant magnetic field steps. This places a stringent limit on the amplitude of alfv n waves, in terms of the plasma , or kinetic to magnetic pressure ratio, above which they can no longer oscillate or propagate in collisionless plasmas. An analysis of solar wind data from the NASA wind spacecraft shows clear evidence of this threshold as well as second, low threshold, that appears to be consistent with theoretical expectations of parametric decay. Indeed, though large amplitude alfv NIC fluctuations with constant (total) magnetic field magnitude are an exact solution in magnetohydrodynamics they are parametrically unstable at low and decay by coupling with compressible modes and secondary alfv n waves. Preliminary studies suggest that there is an absolute limit to alfv NIC fluctuation amplitudes as a function of plasma beta related to these instabilities. So far, parametric decay has been studied assuming a maxwellian background plasma. However, the weakly collisional solar wind displays several non-thermal features, e.g. temperature anisotropies and ION differential streaming, especially in the fast streams. This proposal investigates the role of non-linear wave and wave-particle interactions in maintaining or generating the alfv NIC state, with the aim of understanding how the shape of the distribution function contributes to the existence of such turbulence. The objectives are to (1) carry out a theoretical study of alfv n wave stability in non-maxwellian plasmas supported by hybrid simulations starting from anisotropic proton distributions of the type found in the solar wind. This will provide scalings of critical wave amplitudes with ; (2) complete data analysis of wind and ulysses measurements of the correlation between the state of solar wind turbulence (alfv NIC VS. Non-alfv NIC, wave-forms, amplitudes and joint probability distributions) and the proton distribution functions at different values. The proposal fits within the heliophysics decadal survey goal discover and characterize fundamental processes that occur both within the heliosphere and throughout the universe and falls under HS-r science area 5 heliospheric plasma processes, turbulences, waves, composition. The proposed study will provide new insights on the role of alfv n waves in solar wind dynamics as well as predictions that may be verified by the upcoming parker solar probe mission.