Browsing by Author "Scales, Wayne"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Nonlinear Three-Dimensional Simulations of the Gradient Drift and Secondary Kelvin-Helmholtz Instabilities in Ionospheric Plasma Clouds
    Almarhabi, Lujain; Skolar, Chirag; Scales, Wayne; Srinivasan, Bhuvana (MDPI, 2023-04-03)
    A newly developed three-dimensional electrostatic fluid model solving continuity and current closure equations aims to study phenomena that generate ionospheric turbulence. The model is spatially discretized using a pseudo-spectral method with full Fourier basis functions and evolved in time using a four-stage, fourth-order Runge Kutta method. The 3D numerical model is used here to investigate the behavior and evolution of ionospheric plasma clouds. This problem has historically been used to study the processes governing the evolution of the irregularities in the F region of the ionosphere. It has been shown that these artificial clouds can become unstable and structure rapidly (i.e., cascade to smaller scales transverse to the ambient magnetic field). The primary mechanism which causes this structuring of ionospheric clouds is the E×B, or the gradient drift instability (GDI). The persistence and scale sizes of the resulting structures cannot be fully explained by a two-dimensional model. Therefore, we suggest here that the inclusion of three-dimensional effects is key to a successful interpretation of mid-latitude irregularities, as well as a prerequisite for a credible simulation of these processes. We investigate the results of 2D and 3D nonlinear simulations of the GDI and secondary Kelvin–Helmholtz instability (KHI) in plasma clouds for three different regimes: highly collisional (≈200 km), collisional (≈300 km), and inertial (≈450 km). The inclusion of inertial effects permits the growth of the secondary KHI. For the three different regimes, the overall evolution of structuring of plasma cloud occurs on longer timescales in 3D simulations. The inclusion of three-dimensional effects, in particular, the ambipolar potential in the current closure equation, introduces an azimuthal “twist“ about the axis of the cloud (i.e., the magnetic field B). This azimuthal “twist” is observed in the purely collisional regime, and it causes the perturbations to have a non-flute-like character (k‖≠0). However, for the 3D inertial simulations, the cloud rapidly diffuses to a state in which the sheared azimuthal flow is substantially reduced; subsequently, the cloud becomes unstable and structures, by retaining the flute-like character of the perturbations (k‖=0).
  • Thumbnail Image
    Statistical Analysis of Refractive and Diffractive Scintillation at High Latitudes
    Conroy, James P.; Deshpande, Kshitija; Scales, Wayne; Zaghloul, Amir (American Geophysical Union, 2022-02)
    A comprehensive statistical analysis was performed on Global Positioning System scintillation data acquired at high latitudes from 2014 to 2017 after separating phase scintillation events originating from refraction and/or diffraction. Events exceeding a prescribed threshold were identified and analyzed statistically as a function of time, latitude, and propagation angle. The statistical analysis indicates that at high latitudes phase scintillation, which occurs more frequently than amplitude scintillation, is generated through refractive processes which can typically be treated as a stochastic Total Electron Content effect at high latitudes for Global Navigation Satellite System frequencies, and have the highest probability around magnetic noon in the Cusp. On average the phase scintillation index values decrease as a function of latitude, particularly during the first 6 hr of the evening. In addition, irregularities on the poleward side of the aurora are predominantly smaller than the Fresnel scale, when amplitude scintillation events are observed. By comparison, the mean of the phase scintillations on the equatorial side of the aurora, when amplitude scintillations are also present, indicates the existence of irregularities which are larger and smaller than the Fresnel scale. We also found that, during the day and at dusk, the spectral content of the irregularities apparently changes with decreasing off-B Angle. No such increase is readily apparent at night or at dawn.