Zakharov simulations of Langmuir turbulence: Effects on the ion-acoustic waves in incoherent scattering

Abstract

This paper presents a numerical study of the effect of Langmuir turbulence on incoherent scatter spectra. The Langmuir turbulence is driven by low energy beams of electrons in the Earth’s upper ionosphere above 300km. The nonlinear coupling between Langmuir waves and ion-acoustic waves is governed by the Zakharov system of equations. The model is enhanced with stochastic forcing in order to estimate by how much over the thermal level the spectrum seen by an incoherent scatter radar will be enhanced. This also allows us to directly compare the modeled spectra to the observed spectra collected by the incoherent scattering technique, as well as to statistically investigate the signature of the modeled spectra through an exploratory data analysis. Results for different beam energies are presented, covering the regimes of weak as well as strong turbulence. The incoherent scatter spectra signature is discussed in light of these regimes. It is shown that incoherent scatter radar observations of enhanced ion-acoustic and/or Langmuir waves compared to thermal level can provide good estimates of the beam parameters and of the type of turbulent regime. The cascade regime leads to strongly asymmetric spectra with enhancements over a limited range of wave numbers. The cavitation regime leads to marginally asymmetric spectra, with enhancement over a wide range of wave numbers, and features a central peak for a limited range of wave numbers. Finally, it is shown that the Langmuir turbulence should be preferentially observed for scattering wavelengths large compared to the Debye length.

Enhanced EISCAT UHF backscatter during high-energy auroral electron precipitation

“…A distinct zero Doppler shift feature is only accounted for in LT models. In the LT model the spatially averaged wave energy density necessary to create cavitons is proportional to 1/L 2 (Robinson, 1997; Guio and Forme, 2006). To observe enhanced radar power backscattered from cavitons, the spacing L of the cavitons needs to be close to half the radar wavelength.…”

“…For ISRs to measure the coherent backscatter from cavitons, the quasiperiodic cavitons must have a separation (L) of about the radar Bragg wavelength. The spatially averaged wave energy density necessary to create the cavitons is proportional to 1/L 2 (Robinson, 1997;Guio and Forme, 2006).…”

“…In the WLT approach waves are described by their kinetic equations and random wave phases are assumed. Guio and Forme (2006) use a SLT approach and simulate the full Zakharov (Zakharov, 1972) set of equations with a low-energy electron beam driver. In the SLT approach density cavities (cavitons) are formed by a ponderomotive force due to the Langmuir oscillations.…”

Nonlinearly generated plasma waves as a model for enhanced ion acoustic lines in the ionosphere

“…[17] We have presented results from full numerical solutions of the coupled electron-ion Vlasov equations, using parameters that are relevant also for ionospheric conditions, but nonetheless differing from those that can be modeled by standard studies based on for instance Zakharov-type equations with energy sources and sinks [Guio and Forme, 2006].…”

“…[4] It seems that a wave-decay model may be the most promising for explaining the observations, at least concerning the of features up-and down-shifted ion lines [Sedgemore-Schulthess and St.-Maurice, 2001;Grydeland et al, 2003;Guio and Forme, 2006]. In these models it is assumed, seemingly with no exceptions, that the basic dispersion relation for the Langmuir waves is the one obtained for a thermal plasma, i.e., w 2 = w pe 2 + 3u Te 2 k 2 , in terms of the plasma frequency w pe and the electron thermal velocity u Te = ffiffiffiffiffiffiffiffiffiffiffiffiffi kT e =m p .…”