Energization of the Solar Wind and Relativistic Jets by Alfven-Wave Turbulence
Beginning with Parker's 1958 paper predicting a supersonic outflow from the Sun, theoretical investigations and spacecraft measurements have led to considerable progress in our understanding of the solar wind. In this talk, I will review one of the leading models for the solar wind's origin and discuss what this model may tell us about more distant astrophysical outflows. In this model, the solar wind is powered primarily by Alfven waves, which are like waves on a string, where magnetic field lines play the role of the string. Photospheric motions, driven by convection, shake the magnetic field lines, launching Alfven waves into the corona and solar wind. As the waves propagate away from the Sun, they are partially reflected by the radial gradient in the wave phase velocity. Counter-propagating Alfven waves subsequently interact to produce Alfven-wave turbulence. Turbulence causes wave energy to cascade to small wavelengths, where it dissipates, heating the plasma. This heating increases the plasma pressure, which, in conjunction with the wave pressure, accelerates the solar wind to high speeds. A focus of the talk will be to explain the basic phenomenology of reflection-driven Alfven-wave turbulence in the solar wind in order to clarify the conditions under which the same type of turbulence could be important in other systems. The last part of the talk will focus on an analytic calculation within the framework of general relativistic magnetohydrodynamics. This calculation develops simple, compact expressions for the Alfven-wave amplitudes and turbulent heating rate as functions of position in statistically steady-state relativistic plasmas in curved or flat spacetime. The calculation also leads to an estimate of the Alfven-wave luminosity from turbulent accretion disks. I will conclude by discussing the possible role of Alfven-wave turbulence in generating accretion-disk coronae and energizing relativistic jets.