Citation: Colestock, P. L., and S. Close (2013), Reply to comment by I. Katz on "Electromagnetic pulses generated by meteoroid impacts on spacecraft," J. Geophys. Res. Space Physics, 118, 5806, doi:10.1002/jgra.50535.[1] We note that I. Katz in his critique of our paper has focused on the question of coherent plasma oscillations that might occur during the expansion of a plasma plume caused by a hypervelocity impact on a spacecraft. In so doing we feel he has somewhat misconstrued the intent of our paper and failed to recognize our primary conclusion, namely, that there is a clear mechanism for EMP generation associated with such an impact.[2] In fact our paper was intended to establish a scaling to the empirically established fact that such an EMP signal does occur and to shed light on radiation mechanisms that might exist. In particular, our primary contribution was to find the radiation spectrum associated with the plasma expansion, a result which had not appeared in any of the previous work on this subject. The radiated spectrum in our admittedly simplified model contains a component due to plasma oscillations, as well as a low-frequency component associated with the plasma expansion itself. We clearly stated in our paper that the expansion, i.e., the low-frequency component, is the only aspect of the radiated spectrum that can likely be observed experimentally.[3] We note further that the subject of plasma expansion is not new and has been studied for at least 40 years in various publications. While Katz has correctly referenced some of that material [Crow et. al., 1975; Denavit, 1979;Nedelea and Urbassek, 2002], we refer to an early, but definitive, work by Hiroshige and Yang [1969], in which an expansion of a collisionless plasma is treated in a fully kinetic form. In addition to an analytical treatment of the linearized Vlasov equation, they use the well-known sheet model in a fully nonlinear computer simulation as pioneered by Dawson [1962]. These authors find that coherent oscillations can occur around the sheath expanding at the sound speed, which is the basis of our simplified model. These oscillations persist even when there is a spread in velocities, as shown by the results of Hiroshige and Yang.[4] Katz does correctly point to the difficulty of specifying the initial conditions, which becomes problematic at frequencies approaching the instantaneous plasma frequency. Moreover, instabilities such as Rayleigh-Taylor [Sharp, 1984] or Richtmyer-Meshkov [Richtmyer, 1960] may occur, which limit the time scale over which plasma oscillations can remain coherent. Once the oscillations are randomized, the plasma can only radiate via bremsstrahlung, which is very small at high frequencies. However, at sufficiently low frequencies, such as those associated with the expansion itself, the electron-rich sheath carries a net charge imbalance which will indeed radiate. This is the main point of our work, which we believe Katz has failed to recognize.
