Nonlinear Langmuir Wave Interactions inside the Foreshock of Saturn Observed By the Cassini Spacecraft

Abstract:

In the planetary foreshock, solar wind electrons reflected from the bow shock form upstream electron beams, which can generate electrostatic Langmuir waves at frequencies close to the local plasma frequency f_p. Langmuir waves can be subsequently converted to radio waves at f_p and 2f_p. Observed spectra often have the form of a superposition of two or more spectral peaks. It is believed that these spectra might be the result of the electrostatic (ES) decay and the coalescence of two Langmuir waves. To identify a possible signature of these nonlinear processes, bicoherence analysis can be applied. Bicoherence analysis can be used to estimate the phase coupling between interacting waves. However, it has been recently shown that electric fields produced by nonlinear currents and sheath rectification can be phase coherent with observed Langmuir waves that can result in multi-peak spectra similar to those expected for ES decay or coalescence. We present a systematic study using data from the Radio and Plasma Wave Science (RPWS) instrument onboard the Cassini spacecraft. We analyze all Saturn's foreshock crossings from 2004 to 2013 using an automated method of identification of Langmuir waves. Bicoherence analysis is used to study possible nonlinear wave interaction. Statistical significance of the results and robustness of the technique was evaluated using synthetic signals.