Nonlinear Equatorial Spread F: Dependence on Altitude of the F Peak and Bottomside Background Electron Density Gradient Scale Length.
NAVAL RESEARCH LAB WASHINGTON D C
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Four different two dimensional perpendicular to the ambient magnetic field, plasma fluid-type, numerical simulations following the nonlinear evolution of the collisional Rayleigh-Taylor instability in the nighttime equatorial F region ionosphere have been performed. Realistic altitude dependent ion-neutral collision frequencies, recombination rates, and ambient electron density profiles were used. In three cases ESF 0, 1, 3 the electron density profile was kept constant, with a minimum bottomside background electron density gradient scale length L about 10 km, but the altitude of the F peak was changed, with F peak altitudes at 340, 350, and 430 km. All cases resulted in bottomside growth of the instability Spread F with dramatically different time scales for development. Plasma density depletions were produced on the bottomside with rise velocities, produced by nonlinear polarization E x B forces, of 2.5, 12, and 160 msec, and percentage depletions of 16, 40, and 85, respectively. High altitude of the F peak, small bottomside electron density gradient scale lengths, and large percentage depletions yield large vertical bubble rise velocities, with the first two conditions favoring bottomside linear growth of the instability. The numerical simulation results are in good agreement with rocket and satellite in situ measurements and radar backscatter measurements, including some of the recent results from the August 1977 coordinated ground based measurement campaign conducted by DNA at Kwajalein.
- Atmospheric Physics
- Plasma Physics and Magnetohydrodynamics