Plasma Turbulence and observational effects

Plasma Turbulence is present in many astronomical settings, and it plays an important role in releasing the magnetic and/or kinetic energy into accelerating particles and heating the plasma. With the diffusion approximation, I study the cascade and damping of Alfven-cyclotron turbulence in solar plasmas numerically. Motivated by wave-wave couplings and nonlinear effects, I test several forms of the diffusion tensor. For a general locally anisotropic and inhomogeneous diffusion tensor in the wave vector space, the turbulence spectrum in the inertial range can be fitted with power-laws with the index varying with the wave propagation direction. For several locally isotropic but inhomogeneous diffusion coefficients, the steady-state turbulence spectra are nearly isotropic in the absence of damping and can be fitted by a single power-law function. However, the energy flux is strongly polarized due to the inhomogeneity that leads to an anisotropic cascade. Including the anisotropic thermal damping, the turbulence spectrum cuts off at the wave numbers, where the damping rates become comparable to the cascade rates. The combined anisotropic effects of cascade and damping make this cutoff wave number dependent on the wave propagation direction, and the propagation direction integrated turbulence spectrum resembles a broken power-law, which cuts off at the maximum of the cutoff wave numbers or the 4He cyclotron frequency. Taking into account the Doppler effects, the model can naturally reproduce the broken power-law wave spectra observed in the solar wind and predicts that a higher break frequency is always accompanied with a greater spectral index change that may be caused by the increase of the Alfven Mach number, the reciprocal of the plasma beta, and/or the angle between the solar wind velocity and the mean magnetic field. These predictions can be tested by future observations.;Solar flare is the most energetic process in solar system and becomes the natural laboratory for studying plasma turbluence and wave-particle interactions. I focus on the observation of solar flare spatial and spectral evolution to study the effects of turbulence on solar flare energetics. I collect a sample of 6 limb flares observed by RHESSI. A distinct coronal source, which I identify as the loop top (LT) source, was seen in each of these flares from the early "pre-heating" phase through the late decay phase. Spectral analyses reveal an evident steep power-law component in the pre-heating and impulsive phases, suggesting that the particle acceleration starts upon the onset of the flares. In the late decay phase the LT source has a thermal spectrum and appears to be confined within a small region near the top of the flare loop, and does not spread throughout the loop, as is observed at lower energies. The total energy of this source decreases usually faster than expected from the radiative cooling but much slower than that due to the classical Spitzer conductive cooling along the flare loop. These results indicate the presence of a distinct LT region, where the thermal conductivity is suppressed significantly and/or there is a continuous energy input. I suggest that plasma wave turbulence could play important roles in both heating the plasma and suppressing the conduction during the decay phase of solar flares. With a simple quasi-steady loop model I show that the energy input in the gradual phase can be comparable to that in the impulsive phase and demonstrate how the observed cooling and confinement of the LT source can be used to constrain the wave-particle interaction.