An alternative to the plasma emission model: Particle-In-Cell, self-consistent electromagnetic wave emission simulations of solar type III radio bursts

摘要

1.5D PIC, relativistic, fully electromagnetic (EM) simulations are used tomodel EM wave emission generation in the context of solar type III radiobursts. The model studies generation of EM waves by a super-thermal, hot beamof electrons injected into a plasma thread that contains uniform longitudinalmagnetic field and a parabolic density gradient. In effect, a single magneticline connecting Sun to earth is considered, for which several cases arestudied. (i) We find that the physical system without a beam is stable and onlylow amplitude level EM drift waves (noise) are excited. (ii) The beam injectiondirection is controlled by setting either longitudinal or oblique electroninitial drift speed, i.e. by setting the beam pitch angle. In the case of zeropitch angle, the beam excites only electrostatic, standing waves, oscillatingat plasma frequency, in the beam injection spatial location, and only low levelEM drift wave noise is also generated. (iii) In the case of oblique beam pitchangles, again electrostatic waves with same properties are excited. However,now the beam also generates EM waves with the properties commensurate to typeIII radio bursts. The latter is evidenced by the wavelet analysis of transverseelectric field component, which shows that as the beam moves to the regions oflower density, frequency of the EM waves drops accordingly. (iv) When thedensity gradient is removed, electron beam with an oblique pitch angle stillgenerates the EM radiation. However, in the latter case no frequency decreaseis seen. Within the limitations of the model, the study presents the firstattempt to produce simulated dynamical spectrum of type III radio bursts infully kinetic plasma model. The latter is based on 1.5D non-zero pitch angle(non-gyrotropic) electron beam, that is an alternative to the plasma emissionclassical mechanism.