1 Institute of Geophysics, University of Tehran
2 The Bradley Department of Electrical and Computer Engineering,Virginia Tech, USA
عنوان مقاله [English]
On the source of plasma density and electric field perturbations in PMSE and PMWE regions
Polar mesospheric summer/winter echoes (PMSE/PMWE) are very strong radar echoes produced by ionospheric electron density fluctuations at half the radar wavelength. The main focus of this paper is the altitude range 80 to 90 km in summer and altitude range 65-86 km in winter in the polar mesosphere . This paper studies the formation of electron density fluctuations in the PMSE/PMWE source region. Using a computational model, the current paper investigates the coupling of the neutral air turbulence with mesospheric dusty plasma as a generation source of fluctuations in plasma and dust densities as well as electric field. The impact of spectrum of irregularity wavelengths in neutral air turbulence including the presence of charged dust particles is investigated and extension of diffusion timescale for electron density fluctuations in smaller wavelength is studied. A comparison of the numerical results with VHF radar observations and in-situ rocket measurement of plasma density perturbations in the mesopause region is presented. The effect of dust density and dust-neutral collision frequencies on the coupling of neutral turbulence and the dust layer in PMSE source region is studied. The computational results are compared with the past theoretical predictions of impact of heavy charged dust on the wavenumber spectrum of electron irregularities. Formation of fluctuations in plasma and electric field in PMWE is considered. The required plasma and dust parameters for neutral turbulence coupling in winter mesosphere is determined.
Numerical simulations for coupling of neutral air turbulence with the dusty plasma in the mesosphere are presented to study the fluctuations in the electric field, dust and plasma densities. The consistency of fluctuation amplitude of dusty-plasma densities and electric field with the theory of neutral air turbulence developed by Robertson (2009) is discussed. The effect of dust density and dust-neutral collision frequency on the strength of fluctuations in plasma density and electric field as a result of neutral air coupling in the summer polar mesosphere is investigated. It has been shown minimum dust density to background plasma density of the order of 30 percent and dust-neutral collision frequency about 10$^5$ Hz are required for the coupling to be efficient. The fluctuation amplitude of electric field estimated by computational model shows a good agreement with the theoretical model and in-situ rocket measurements in the vicinity of PMSE source region. Enhancement of electron density fluctuations with smaller wavelengths is observed which validates the VHF and UHF PMSE observations. Variation of electron density was studied as the main requirement for coupling in the winter polar mesosphere. A maximum fluctuation amplitude in the PMWE source region is determined when the electron density is equal to the dust density.
These results and the computational model presented in this work can be applied to other problems such as the effect of shock waves due to the passage of rockets through mesosphere on the generated plasma turbulence and associated radar echoes. Other applications such as laboratory applications will be considered in the future work.