Theoretical Dependences of Type II Solar Radio Emission on Electron and Density Turbulence Parameters

Abstract:

Type II solar radio bursts involve a shock reflecting and accelerating ambient electrons back upstream, these electrons developing beam distribution functions unstable to the growth of Langmuir waves, generation of Langmuir waves, transformation of some Langmuir wave energy into radio emission near the electron plasma frequency f_p and near 2f_p in an inhomogeneous plasma, and propagation of the radio emission to observers. If the ambient electrons have Kappa rather than Maxwellian distribution functions, then the number of fast electrons (and so energy available) increases substantially. In recent years we developed a quantitative theory for type II bursts that combines MHD simulations of a specific CME-driven shock with an analytic theory for the accelerated electrons, Langmuir waves, and radio emission. Here we present detailed theoretical predictions for the dynamic spectrum and source regions of the split-band type II burst of 7 September 2014, where we vary κ, the electron thermal speed V_e , and the relative level and characteristic length scale of ambient density irregularities, and compare the predictions with observations by the Murchison Widefield Array. We find that the intensity of the dynamic spectrum decreases when κ increases and V_e decreases, consistent with the presence of fewer fast electrons and with analytic predictions. Larger relative amplitudes ΔN / N and scale lengths L for the ambient density turbulence lead to weaker escaping fundamental radiation and a smaller frequency ratio between fundamental and harmonic bands. This frequency ratio constrains the combination of ΔN / N and L. Emission levels, frequency-time fine structures (including the split-bands), and source regions are close to the observations when the ambient electrons have kappa distributions with κ ≈ 2 - 4 and V_e and the density irregularities have nominal characteristics. This theory for type II bursts thus appears viable.