Bursty Mode Conversion in Inhomogeneous Plasmas

The process of linear mode conversion of electrostatic Langmuir waves into transverse electromagnetic waves in the presence of density inhomogeneities in an unmagnetized plasma is investigated numerically. In inhomogeneous plasmas, Langmuir waves often tunnel into density wells near the mode conversion region and it is shown that these density wells can act as resonators for the mode conversion process. Specifically, changes in the size of the density cavity on scales of order the Langmuir wavelength can alter the mode conversion efficiency from zero to its maximum value (up to 100% in some instances). Thus, with density inhomogeneities evolving on MHD timescales, the production of radio emission by mode conversion of Langmuir waves should be a bursty process. The dependence of the degree of burstiness on the inhomogeneity scales is discussed, together with potential applications to mode conversion phenomena in magnetized space plasmas, including interplanetary and magnetospheric radio emissions and pulsations in auroral roar emissions.