Stochastic Acceleration in Turbulence: L. A. Fisk University of Michigan

Evolution of low-energy ion energy spectrum across TS Increasing intensities j(40 keV) ≈103 f/u Spectral fluctuations markedly decreased by 2005.1 Spectrum of protons 40-4000 keV evolving to single power-law with index =1.5

Different Expressions for the Observed Power Law When expressed as a distribution function in velocity space: When expressed as differential intensity [T is kinetic energy per nucleon]: When expressed as differential number density:

The Transport Equations In the frame of the solar wind with random convective motions. Continuity equation is ST is the differential stream; term on right is the change in differential number density due to the turbulence doing work on the particles. The differential pressure is

Transport Equations Multiply equation by kinetic energy T and manipulate, The differential energy density is The spatial transport of energy is The third term is the flow of energy in energy space, with The term on the right is the work done on the particles by the turbulent motions.

Equilibrium In a steady state: Time derivative must be zero Flow of energy into and out of the core must be zero. The work done on the particles by the turbulence must be balanced by the flow of energy [heat flux] into and out of the volume. The particles receive energy from the turbulence and do an equal amount of work on the turbulence, and the heat flux distributes the energy.

Equilibrium Spectrum The flow of energy into and out of the core must be zero, which requires that Which is satisfied by U  T ‑2 as is observed

Summary Statements The tail is formed by a cascade in energy, in which the tail particles are in equilibrium receiving and performing an equal amount of work on the turbulence. The tail pressure is proportional to the pressure in the core particles; the proportionality constant depends on the amplitude of the fluctuations in pressure. The tails start where the particles are sufficiently mobile to undergo stochastic acceleration. The tails stop when the particle gyro-radii exceed the scale size of the turbulence. The proportionality of the tail and core particles can be used to specify the composition of the tail particles.