Physics Dept Seminar

September 15, Monday

Beyond growth rates: Explaining the energy cascade during ionospheric instabilities

Dr. William Longley

CSTR, NJIT

(Terrestrial Physics, Host: Goodwin)

Time: 11:45 am - 12:45 pm with 11:30 am teatime

Room: ECE 202

A fundamental problem in plasma physics is understanding the cascade of energy across multiple scales during turbulence. The long-standing paradigm in plasma physics is to predict the onset of an instability by calculating a growth rate, and then simulate the instability to show its nonlinear evolution and turbulence. In this presentation, a new weak turbulence theory is developed to bridge the gap between growth rate analysis and simulations. This approach is derived by Fourier-Laplace transforming the kinetic plasma equations, and retaining the nonlinear convolution integral that explicitly couples different wave modes together. The weak turbulence theory is applied to the problem of 150-km echoes observed in the ionosphere. 150-km echoes are strong, coherent backscatter observed by radars since the 1960’s, but their generation mechanism has been unknown until now. We show that 150-km echoes are generated by peaks in the suprathermal electron velocity distribution. These peaks drive a bump-on-tail instability that excites Langmuir (or Upper Hybrid) waves, which then couple to ion-acoustic waves through weak turbulence. A unique feature of this approach is the ability to calculate power spectra of the ion-acoustic waves, showing a direct and testable comparison between the theory and observations.