Browsing by Author "Oppenheim, M. M."
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Item Restricted Ion thermal effects on E-region instabilities: 2D kinetic simulations(Elsevier, 2004-11) Oppenheim, M. M.; Dimant, Y. S.Radars frequently receive strong returns from naturally occurring plasma density irregularities centered around 100 km altitude in the atmosphere. These E-region ionospheric irregularities occur when electrojet polarization electric fields drive plasma instabilities, most commonly the Farley–Buneman (FB) and gradient drift instabilities. This paper presents results from two fully kinetic, 2D simulations of field driven instabilities in the E-region. These simulations show the important impact of ion thermal effects on wave growth and turbulence. The first simulation applies parameters meant to replicate a strongly driven high-latitude FB instability where the polarization electric field exceeds the FB threshold by a factor of 4. In this case, waves grow and saturate with characteristics dominated by ion thermal perturbations resulting from a balance between frictional ion heating and cooling. These characteristics differ substantially from those expected for FB waves. In the second simulation, the driving field exceeds the threshold by a factor of 2, causing FB waves to grow with distinct modifications by ion thermal effects. The thermal effects reproduced by these simulations may account for some of the previously unexplained observational characteristics of E-region irregularities, particularly at high latitude and in the upper E-region. They also explain the results seen in the simulations of Janhunen (J. Geophys. Res. 99 (1994b) 11461).Item Restricted Ion thermal effects on E-region instabilities: linear theory(Elsevier, 2004-11) Dimant, Y. S.; Oppenheim, M. M.Linear instabilities and turbulent processes in ionospheric electrojets lead to the formation of plasma density irregularities which create strong radar reflections. Studies of these irregularities have focused on the role of the Farley–Buneman (FB) and gradient drift instabilities with the assumption that the plasma behaves isothermally or adiabatically. In the last decade, this restriction has been relaxed, resulting in the prediction of D and E-region thermally driven instabilities, which now have some supporting observational evidence. Even more recently, fully kinetic particle-in-cell (PIC) simulations show that ion thermal effects strongly modify the nonlinear behavior of the FB instability, especially at the top of the electrojet. This paper describes the linear theory of the combined FB and thermal instabilities. The theory predicts that instabilities will develop over a wider range of altitudes than predicted for the adiabatic or isothermal FB instability. It also predicts that preferred directions of thermally modified FB waves may differ significantly from that of standard Farley–Buneman waves. These theoretical developments have important observational consequences.Item Restricted Kinetic simulations of 3‐D Farley‐Buneman turbulence and anomalous electron heating(American Geophysical Union, 2013-02-27) Oppenheim, M. M.; Dimant, Y. S.Electric fields map from the magnetosphere to the E region ionosphere where they drive the intense currents of the auroral electrojet. Particularly during geomagnetic storms and substorms, these currents become sufficiently intense to develop Farley‐Buneman (FB) streaming instabilities and become turbulent. This leads to anomalous electron heating which can raise the electron temperature from 300 K to as much as 4000 K and, also, modifies auroral conductivities. This paper describes the first fully kinetic 3‐D simulations of electric field‐driven turbulence in the electrojet and compares the results with 2‐D simulations and observations. These simulations show that 3‐D turbulence can dramatically elevate electron temperatures, enough to explain the observed heating. They also show the saturated amplitude of the waves; coupling between linearly growing modes and damped modes; the propagation of the dominant modes at phase velocities near the acoustic velocity, slower than in 2‐D simulations; and anomalous cross‐field electron transport, leading to a greatly increased E region Pedersen conductivity. These simulations provide information useful in accurately modeling FB turbulence and represent significant progress in understanding the electrojet.Item Restricted Meteor trail diffusion and fields: 1. Simulations(American Geophysical Union, 2006-12-20) Dimant, Y. S.; Oppenheim, M. M.A meteoroid penetrating the Earth's atmosphere leaves behind a trail of dense plasma embedded in the lower E/upper D region ionosphere. While radar measurements of meteor trail evolution have been collected and used to infer meteor and atmospheric properties since the 1950s, no accurate quantitative model of trail fields and diffusion exists. This paper describes finite element simulations of trail plasma physics applicable to the majority of small meteors. Unlike earlier research, our simulations resolve both the trail and a vast current closure area in the background ionosphere. This paper also summarizes a newly developed analytical theory of meteor electrodynamics and shows that our simulations and theory predict nearly identical fields and diffusion rates. This study should enable meteor and atmospheric researchers to more accurately interpret radar observations of specular and nonspecular meteor echoes.Item Restricted Meteor trail diffusion and fields: 2. Analytical theory(American Geophysical Union, 2006-12-20) Dimant, Y. S.; Oppenheim, M. M.A meteoroid penetrating the Earth's atmosphere leaves behind a trail of dense plasma embedded in the lower E/upper D region ionosphere. While radar measurements of meteor trail evolution have been collected and used to infer meteor and atmospheric properties since the 1950s, no accurate quantitative model of trail fields and diffusion exists. This paper describes a two‐dimensional (2‐D) analytical theory of meteor trail plasma physics that fills the gap between the studies of dense trail physics without background plasma, 1‐D models, and those for small disturbances. Major new results include an estimate of the spatial distribution of a trail's ambipolar electric potential and a quantitative model of diffusion rates, both parallel and perpendicular to the geomagnetic field, with a prediction that in the course of their diffusion, dense trails transform from anisotropic to more isotropic. These results are important for interpreting specular and nonspecular radar measurements of meteor trails and for accurate modeling of plasma instabilities. A companion paper shows that the results from this analytical theory agree well with simulations results.Item Open Access Nonspecular meteor trail altitude distributions and durations observed by a 50 MHz high‐power radar(American Geophysical Union, 2010-12-30) Sugar, G.; Oppenheim, M. M.; Bass, E.; Chau Chong Shing, Jorge LuisHigh-power large-aperture (HPLA) radars frequently observe nonspecular meteor trail echoes that result from plasma turbulence driven by the intense pressure gradients on the trail edges. This paper analyzes the altitude range, duration, and dependence on head echo strength of nonspecular trails using two large data sets from the 50 MHz HPLA radar at the Jicamarca Radio Observatory. Over 2100 trails were used to build altitude profiles that extend from 86 km to 120 km altitude, with 97% of events occurring between 90 km and 110 km. Longer-duration trails tend to form at lower altitudes than shorter-duration echoes. The peak of the altitude distribution of trails lasting at least 5 s can be up to 12 km lower than the peak of the distribution of trails shorter than 5 s. Further, the data show a clear power law relationship for the frequency of both head echo and nonspecular trail power. An improved knowledge of nonspecular trails will allow researchers to better understand meteor trail evolution and their use in monitoring lower thermospheric winds.