Plasma wave turbulence due to the wake of an ionospheric sounding rocket

In the ionosphere, a rarefied plasma region called "plasma wake" is formed behind a sounding rocket. Based on a one-dimensional Vlasov-Maxwell simulation, it was suggested that electron distribution functions in the plasma wake behind spacecraft are different from the Maxwell-Boltzmann distribution function [Singh et al., 1987]. Thus, plasma waves are expected to be generated in the wake of a sounding rocket. Some studies reported plasma waves around the wake of artificial satellites [Keller et al., 1997] and solar system bodies such as Moon [Nakagawa et al., 2003]. Yamamoto (2000) is the first study that focused on plasma waves induced by sounding rockets on the basis of the results of several rocket experiments. He compared the observed wave frequency with the electron number density in the wake and indicated plasma waves could be generated inside the rocket wake. In order to investigate the properties of the waves in more detail (e.g. spin-phase dependence, generation mechanism, etc.), we are now analyzing the data of electron number density and electric fields of plasma waves in mid-latitude ionosphere by an impedance probe and a plasma wave receiver, which were installed on the sounding rocket S-520-26. In the analysis, we have found plasma waves in a frequency range of 1.3-2.4 MHz (hereinafter called Group-A) as well as those in a frequency range between 0.02 MHz to about 0.6 fce (Group-B), and those in a frequency range from about 0.5 fce to 0.9 fce (Group-C), where fce is the electron cyclotron frequency deduced from the IGRF model. The Group-A emissions are similar to the waves observed in previous studies [Yamamoto, 2000]. Comparison with the data of the impedance probe has suggested the Group-A waves are short-wavelength electrostatic waves including upper-hybrid resonance (UHR) mode waves and electrostatic electron cyclotron harmonic (ESCH) waves. On the other hand, the Group-B and Group-C waves are whistler mode waves. Besides, the analysis with the rocket attitude data has clarified that the Group-A emissions are enhanced when the antenna element pointed in the directions of 320°-20° and 150°-250° in spin-phase angle while that the Group-B waves have been observed clearly when the antenna element pointed to 50°-110° and 200°-300°, and that the Group-C waves are found in 90°-160°. The spin-phase dependences suggest inhomogeneous distributions of the occurrence regions of plasma instabilities with respect to the wake structure, or anisotropy of the wave propagation in plasma. In order to discuss the generation mechanism of the observed plasma waves, we have performed numerical calculations of linear growth rate of plasma waves by assuming anisotropic velocity distribution functions with an electron beam or temperature anisotropy. As a result, we could confirm positive linear growth rates in wave numbers and frequency ranges of UHR mode waves, ESCH waves, and electrostatic whistler mode waves. Actual distribution functions around the rocket wake, however, might not be simple as we assumed. Therefore, further studies with using Vlasov-Maxwell simulation will be needed.