Modeling the ionospheric response to artificially produced density enhancements
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The motion of plasma density enhancements (barium clouds) artificially introduced into the postsunset equatorial F region is investigated with a two‐dimensional model incorporating flux tube integrated quantities. The temporal development of the ionosphere, in which the density perturbations are imbedded, is derived from a one‐dimensional set of relations modeling plasma transport and the vertical electric field from initial conditions and a prescribed variation of the horizontal electric field as a function of time. The calculations show that the strong horizontal shear flows existing at the nighttime F region ledge (where the perturbations were placed) reduce the growth of polarization fields associated with the enhancements and adjacent relative depletions of plasma for weak perturbations. The reason is that the perturbations develop a strong tilt with respect to the horizontal. More massive density perturbations lead to stronger drop velocities with respect to the rising ambient plasma. At a later time they develop secondary horizontal density perturbations on the side unstable to E × B drift instability due to the motion of neutral constituents. When rising “bubbles” of low density are produced, they (1) form on the steepened eastward side of the enhancement perturbation, (2) have a width comparable to the scale size of the enhancement perturbation, and (3) are most easily produced when the enhancement perturbation size is comparable to the scale height of the integrated density. These simulations show why experimental efforts of initiating rising bubbles and equatorial spread F have not been successful. The experiments require larger‐scale and stronger density perturbations than what can be achieved with conventional sounding rocket releases of barium vapors.This paper deals with the analysis of the perturbation growth at the interface between the colliding mediums, one of them has an initial perturbation field of density. The main difference of this hydrodynamic instability from the classical Richtmyer–Meshkov instability is the nature of the initial disturbance. The analyzed hydrodynamic instability is caused by the initial perturbation of the medium density.
Richtmyer–Meshkov instability
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Middle latitudes
Critical frequency
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Terminator (solar)
Middle latitudes
Scale height
Longitude
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Middle latitudes
Sporadic E propagation
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We make a numerical study of the effect that spatial perturbations have in normal Saffman-Taylor fingers driven at constant pressure gradients. We use a phase field model that allows for spatial variations in the Hele-Shaw cell. We find that, regardless of the specific way in which spatial perturbations are introduced, a lateral instability develops on the sides of the propagating Saffman-Taylor finger. Moreover, the instability exists regardless of the intensity of spatial perturbations in the cell as long as the perturbations are felt by the finger tip. If, as the finger propagates, the spatial perturbations felt by the tip change, the instability is nonperiodic. If, as the finger propagates, the spatial perturbations felt by the tip are persistent, the instability developed is periodic. In the later case, the instability is symmetrical or asymmetrical depending on the intensity of the perturbation.
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A dynamic mitigation mechanism for instability growth was proposed and discussed in the paper [S. Kawata, Phys. Plasmas 19, 024503 (2012)]. In the present paper, the robustness of the dynamic instability mitigation mechanism is discussed further. The results presented here show that the mechanism of the dynamic instability mitigation is rather robust against changes in the phase, the amplitude, and the wavelength of the wobbling perturbation applied. Generally, instability would emerge from the perturbation of the physical quantity. Normally, the perturbation phase is unknown so that the instability growth rate is discussed. However, if the perturbation phase is known, the instability growth can be controlled by a superposition of perturbations imposed actively: If the perturbation is induced by, for example, a driving beam axis oscillation or wobbling, the perturbation phase could be controlled, and the instability growth is mitigated by the superposition of the growing perturbations.
Oscillation (cell signaling)
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Anomaly (physics)
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By using the high order compact finite difference scheme,and through the numerical simulations of two-dimensional hydrodynamic perturbation equations,the evolution of spatio-temporal patterns of the traveling wave convection in fluid mixtures in an intermediated-aspect-ratio rectangular cell with small temperature perturbation under the weakly Soret effect is performed.The result shows that counterpropagating waves are generated and modulated until the system evolves to spatially modulated counterpropagating and finally to a stable stationary overturning convection.Based on this,the Rayleigh effect on the course of spatio-temporal evolution from modulated counterpropagating wave to stationary overturning convection is discussed.
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Ionospheric electron density data from the Sondrestrom incoherent scatter radar (ISR) have been used to characterize the structure of the F region ionosphere during ground‐based LF/MF/HF receiver observations of natural ionospheric radio emissions known as auroral roar. In five out of six cases, the F region ionosphere has significant horizontal N e gradient scale lengths ( , measured with 23–137 km spatial resolution). In three of these cases, localized F region auroral ionospheric cavities, with horizontal scales ∼50 km, are observed. In one of six cases, the ionosphere lacks either of these features, and a laminar, mostly unstructured, F region is observed. The data suggest that auroral roar events may occur for a range of large‐scale (>30 km) ionospheric conditions. Some theories for the generation of auroral roar require that the relationship between the electron plasma frequency (ƒ pe ) and the electron gyrofrequency (ƒ ce ) in the source region is , where n is the harmonic number of the observed emission. Comparisons between observed auroral roar emission frequencies, ISR observations of electron density, and the IGRF model for the magnetic field show that this frequency‐matching condition holds somewhere in the ionosphere in 16 out of 18 cases studied and in all 3 cases of ISR elevation scans capable of measuring a source located directly overhead.
Ionogram
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We study a low-amplitude, long-wavelength lateral instability of the Saffman-Taylor finger by means of a phase-field model. We observe such an instability in two situations in which small dynamic perturbations are overimposed to a constant pressure drop. We first study the case in which the perturbation consists of a single oscillatory mode and then a case in which the perturbation consists of temporal noise. In both cases the instability undergoes a process of selection.
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