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...

No abstract

No abstract