Fully multidimensional flux-corrected transport algorithms for fluids

[1] The Naval Research Laboratory three-dimensional simulation code SAMI3/ESF is used to study the response of the postsunset ionosphere to circular gravity waves. We model the coupling of both circular (local) and plane wave (nonlocal) gravity waves to the bottomside F layer as a mechanism for triggering equatorial plasma bubbles. Results support the hypothesis that nonplane gravity waves can more strongly couple to the F layer than plane gravity waves. Results also show that the coupling of the seed wave to the F layer depends on the (nonlocal) growth rate and the local electron density at the position of the seed wave.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2012GL054022

Simulation of the seeding of equatorial spread F by circular gravity waves

We use radar observations from the Jicamarca Observatory from 1968 to 1992 to study the effects of the F region vertical plasma drift velocity on the generation and evolution of equatorial spread F The dependence of these irregularities on season, solar cycle, and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts In the early night sector, the bottomside of the F layer is almost always unstable The evolution of the unstable layer is controlled by the history of the vertical drift velocity When the drift velocities are large enough, the necessary seeding mechanisms for the generation of strong spread F always appear to be present The threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux The geomagnetic control on the generation of spread F is season, solar cycle, and longitude dependent These effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields The occurrence of early night spread F decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset The generation of late night spread F requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour These irregularities occur most often at ∼0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes The occurrence of late night spread F is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux

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Effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread F

Flux-corrected transport II: Generalizations of the method

In a test of the generally accepted Rayleigh-Taylor (R-T) instability mechanism for equatorial spread F (ESF) a linear instability growth rate γ RT is derived following the formalism of Haerendel (preprint, 1973) which takes into account the variations of physical parameters along geomagnetic flux tubes. The resulting form of γ RT extends the results of previous work by including direct dependencies on transequatorial neutral winds, zonal electric fields, vertical and horizontal ionospheric density gradients, the presence of an E region, and chemical recombination. Realistic atmospheric and ionospheric density model inputs are used for the first time to make quantitative calculations of R-T growth rates for a range of geophysical conditions. The key result of this study is that time/altitude domains having positive calculated instability growth rates are found to coincide with observed time/altitude patterns of ESF occurrence over both a monthly and a yearly time frame. This success in being able to model the climatological occurrence of ESF lends support to the physical model adopted for the instability mechanism and opens up new avenues of research into ESF predictability on a night-to-night and even an hour-to-hour basis.

Linear theory and modeling of the Rayleigh‐Taylor instability leading to the occurrence of equatorial spread F

Spread-F theories—a review

A simple model of the interaction of the equatorial ionosphere with the eastward F region neutral wind in the presence of evolving spread F bubbles is given. A consequence of the model is that the upper portions of bubbles will take on a westward tilt, while the lower portions will tilt eastward, giving rise to the 'fishtails' and 'C's' observed by coherent backscatter radar measurements of field-aligned small-scale irregularities. The essence of the model is that the plasma away from the equatorial plane (e.g., a background nighttime E region at higher latitudes) makes a finite contribution to the magnetic field line-integrated Pedersen conductivity, causing an incomplete coupling of the plasma motion to the neutral wind. The degree of coupling is then a function of the Pedersen conductivities both near the equator in the F region and in the higher-latitude E region, giving rise to vertical shears of east-west plasma motion having opposing signs on either side of the equatorial Pedersen conductivity maximum. Evolving spread F bubbles are caught up in this shear as they rise vertically, resulting in the characteristic 'C' shape seen by backscatter radar and in the westward motion of plasma bubbles observed by satellite in situ measurements.more » Numerical simulations, incorporating an eastward neutral wind in the equatorial F region and E region Pedersen conductivity effects, are presented to further support the model and analysis. The simulations also show the result that it may be the eastward as well as the westward wall of a bubble which is subject to secondary instabilities in the presence of an eastward neutral wind. In addition, even without the neutral wind the numerical simulations show that E region Pedersen conductivity effects can result in a slowing down of equatorial spread F and attendant bubble evolution.« less

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Nonlinear equatorial spread F: The effect of neutral winds and background Pedersen conductivity

A numerical simulation of the non-linear evolution of the collisional Rayleigh-Taylor instability using a set of equations appropriate for the equatorial F region ionosphere has been performed. Our results show that the irregularities produced by the instability grow on the bottomside of the F region peak, as predicted by linear theory, and then the irregularities nonlinearly bubble through to the topside, where linear theory predicts no irregularities. Fourier analysis of the irregularities show one dimensional power law power spectrum for both the vertical and horizontal directions.

Nonlinear equatorial spread F

Analytical electrodynamic morphological studies of rising equatorial spread F bubbles, within the context of the collisional Rayleigh-Taylor regime, are presented. The analogy between bubbles (plasma density depletions) and barium clouds (plasma density enhancements) is noted. Bubbles tend to steepen on their tops and elongate in the vertical direction, and they narrow horizontally as they rise through the equatorial F region ionosphere. Linear and nonlinear models of constant shape bubbles are presented. It is shown for these models that the vertical bubble rise velocity VB = (g/νin)f(δn/n0), where νin is the ion-neutral collision frequency and f is an increasing function of δn/n0 (fractional density depletion), depending on the shape of the bubble. Consequently, high altitudes and/or large percentage plasma depletions yield large vertical bubble rise velocities. The models are extended to include arbitrary electric fields in addition to the gravitational force.

Morphological studies of rising equatorial spread F bubbles

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 ∼ 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 × B forces, of 2.5, 12, and 160 m/s and percentage depletions of 16, 40, and 85, respectively. In one case, ESF 0, the bubble did not rise to the topside, but in ESF 1 and 3, topside irregularities were produced by the bubbles (where linear theory predicts no irregularities). In these three cases, spread F could be described from weak to strong. In the fourth case (ESF 2) the altitude of the F peak was 350 km, but the minimum L on the bottomside was changed to 5 km. This resulted in a bubble rise velocity of ∼23 m/s and a 60% depletion with strong bottomside and moderate topside spread F and a time scale for development between ESF 1 and 3. Two other cases, ESF 0′ and 0″ with peaks at 330 and 300 km, respectively, and bottomside L ∼ 10 km, were investigated via linear theory. These cases resulted in extremely weak bottomside spread F and no spread F (entire bottomside linearly stable), respectively. These simulations show that under appropriate conditions, the collisional Rayleigh-Taylor instability causes linear growth on the bottomside of the F region. This causes the formation of plasma density depletions (bubbles) which rise to the topside (under appropriate conditions) F region by polarization E × B motion. 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 Defense Nuclear Agency at Kwajalein.

Sidney L. Ossakow

Papers

Spread-F theories—a review

Nonlinear equatorial spread F: The effect of neutral winds and background Pedersen conductivity

Nonlinear equatorial spread F

Morphological studies of rising equatorial spread F bubbles

Nonlinear equatorial spread F: Dependence on altitude of the F peak and bottomside background electron density gradient scale length