On the possibility of mode coupling for low frequency electromagnetic waves in plasmas with anisotropy of temperature

Abstract

We study the dispersion relation for low frequency electromagnetic waves propagating along the ambient magnetic field and investigate the possibility of occurrence of coupling between waves in the ion cyclotron branch and waves in the whistler branch. The results obtained show that the coupling may occur in the case of conditions leading to the ion cyclotron instability, for sufficiently high value of the ratio between perpendicular and parallel ion temperature, and does not occur in the case of conditions leading to the ion firehose instability. The results also show that the decrease in the value of the plasma beta may lead to the disappearance of the mode coupling conditions. Regarding the effect of the electron population, it is shown that the change in the shape of the electron velocity distribution, from Maxwellian to bi-Kappa form, does not change the results obtained, as long as the electron temperatures are isotropic, but the increase in anisotropy in the electron temperatures may lead to the disappearance of the coupling between the different waves. The consequences of the frequency dependency of the mode coupling conditions are discussed considering wave propagation in an inhomogeneous medium, leading to the conclusion that the energy of a packet of waves of a given mode can be absorbed or mode converted over an extended region of space. These findings can be of relevance for the analysis and understanding of processes related to the conversion between ion cyclotron waves and whistler waves.