We report experimental observations and computer modeling results of large-scale density and temperature modifications (several km extent, STe/Teo--3, \dne\/neo--25%) created in a low-density, midlatitude, night-time ionosphere by nonlinear refraction of an hf beam launched from a ground-based antenna. The process consists of the reorientation of the reflection surface parallel to the geomagnetic field lines and results in intense heating. PACS numbers: 52.35.Mw, 52.40.Db, 94.20.Bb, 94.20.Vv Large-scale density modifications can be produced in the ionosphere by localized temperature perturbations generated by a powerful hf wave (total power -400 kW, effective radiated power -100 MW) near its reflection layer. A schematic of the relevant experimental geometry is shown in Fig. 1. Large perturbations (5Te/Teo--3, \Sne\/neo-'25%) caused by hf heating were first observed by Duncan, Sheerin, and Behnke' at the Arecibo Observatory in 1985 during solar minimum. Subsequent experiments (1986)(1987) examined several features of these modifications including cavity dynamics in a high-neutral-wind environment,^ but produced no quantitative conclusions regarding the mechanism responsible for their generation. In this Letter we report experimental observations of steady-state large perturbations obtained during 3-6 May 1988 and make quantitative comparisons with a two-dimensional transport model. It is found that reorientation of the wave reflection surface parallel to the geomagnetic field and subsequent heating confined to a narrow flux tube (i.e., nonlinear refraction) is the principal mechanism responsible for the generation of the large perturbations.Experimental results from the heating campaign of 3-6 May 1988 at the Arecibo Observatory clearly indicate that large modifications evolve in time from a broad xZmax = 650km North Modelled density z^~270km ZQ=150km HF transmitter i i Diagnostic radar FIG. 1. Schematic of midlatitude ionospheric-modification experiment and night-time density profile.and symmetric heating profile (characteristic of the linear hf beam envelope) to narrow hot flux tubes shifted northward of the original heated region. The nonlinear evolution is observed to attain a highly reproducible universal asymptotic state. The principal diagnostic used is the received backscattered power of the 430-MHz radar at Arecibo as a function of altitude. The received backscattered power can be approximated^ bywith a a constant dependent on radar parameters, Te/Ti the electron-ion temperature ratio, and fte the electron density. Both density depletions and temperature increases simultaneously cause a decrease in the received signal strength. Figure 2 shows the backscattered signal of a typical asymptotic state achieved after 10-15 min of heating; for reference, the averaged unperturbed profile is also shown. The sharp decrease seen in Fig. 2 corresponds to simultaneous heating and density depletion along a narrow flux tube intersected at an angle of about 40° by the diagnostic radar, as illustrated in Fig. 1.Altitu...

A particle-in-cell code is used to investigate the evolution of a density plume moving through a background plasma with supersonic speed directed along the confinement magnetic field. For scale lengths representative of laboratory and auroral phenomena, the major nonlinear effects identified by the present simulations are the formation of a bipolar current system from the ballistic electrons, the appearance of transient potential layers, and the carving of deep density cavities. A 3D magnetic topology is generated by the self-consistent ballistic and diamagnetic currents that accompany highly localized potential layers.

A generalized resonance-tunneling equation containing a first-derivative term having the coefficient a ^ 0 is found to exhibit stimulated emission of waves by the resonance. The expected Budden absorption occurs for a = 27V, TV = 0,1,2, ... . However, in bands centered around a = IN +1, the modulus of the reflection coefficient can be larger than unity.PACS numbers: 03.40. Kf, 02.90.+p, The propagation of waves through nonuniform media is a subject of broad interest to various areas of basic and applied physics. The two principal effects encountered in related problems are the appearance of cutoffs, i.e., points where the local wave number vanishes [Hz) = 0], and resonances, points where short wavelengths develop [/Kz)--oo]. It is intuitively expected that cutoffs result in wave reflection while resonances lead to wave absorption. Because of the two seemingly opposing roles played by cutoffs and resonances, it is of considerable importance to investigate model systems in which both effects are present and in a sense compete with each other; this is the essence of resonance-tunneling problems. The prototype equation used to approximate physical systems exhibiting such features is Budden's equation. 1 This secondorder differential equation describes systems in which propagation regions exist on both sides of a resonance, but in which one of the propagation regions also contains a cutoff. An intrinsic property of Budden's equation is that for waves approaching the resonance point (z = 0) from the cutoff side (z > 0), the modulus of the reflection coefficient is less than unity. Such a result is consistent with the intuitive expectation that a fraction of the incident wave is absorbed at the resonance, while yet another part can tunnel to the propagation region on the other side of the resonance (z < 0). In the present Letter, we describe the unusual reflection properties exhibited by an equation that can be viewed as a generalization of Budden's equation. Although our initial motivation to study the generalized equation arose from a specific problem 2 (electrostatic whistler waves in magnetized plasmas with longitudinal density gradients), we discuss here only its general mathematical properties. We hope that by exposure of this topic to a broad physics audience, some novel applications may be found in areas outside our research experience.The generalized resonance-tunneling second-order differential equation for the dependent variable (z), where z is the independent variable ranging over -oo < z < oo, is (z + iv)"(z) + <*' (z) -(£i-$z) 0 serves to ensure that (f> is finite as |z| -• 0; physically it is typically associated with a dissipative mechanism. Howeve...