Energetic Particles in the Heliosphere: from the Sun to Planetary Atmospheres
Self-consistent PIC simulations of beam-plasma instabilities in strongly inhomogeneous plasmas: effects on relaxation time-scales, Langmiur wave excitation and subsequent electrostatic decay.
Jonathan Thurgood
David Tsiklauri (QMUL)
Queen Mary University of London
The classical model of type III solar radio bursts at double plasma frequency invokes a three-stage process; namely the excitation of Langmiur waves via beam-plasma instabilities, subsequent electrostatic decay (ESD) producing counter-propagating waves, and finally the coalescence of the wave population to produce electromagnetic waves.

Observations show that electron beams propagate to around 1AU, in excess of typical instability relaxation scales. Recently, Krafft et al. (2013) demonstrated that random density fluctuations can act to slow the relaxation of the instability. This significantly alters the power, localisation and spectrum of Langmiur waves excited by the instability, raising the question; "how are the subsequent stages of plasma emission affected?"

Here we present results of self-consistent particle-in-cell simulations of beam-plasma systems of variable beam density, velocity, and background inhomogeneity. Our analysis focuses on particle-wave interaction at the instability stage and subsequent effects on ESD and coalescence of Langmiur waves. We confirm that inhomogeneity increases the relaxation time-scales and consequently reduces the amplitude of excited Langmiur waves. We show that power in the excited waves must exceed a threshold for ESD to proceed, and so certain high-inhomogeneity setups can limit/prohibit ESD (hence plasma emission). We also confirm the localisation of Langmiur turbulence and broadening in k-space due to inhomogeneity, placing further restrictions on the efficacy of ESD due to kinematic restrictions. Despite a sensitive parameter space, the three-stage model is consistent with an inhomogeneous solar wind for certain parameter ranges.
13:30 - 15:00