Measurements of HF‐enhanced plasma and ion lines at EISCAT with high‐altitude resolution

Abstract

Abstract. Measurements of plasma and ion lines induced during HF ionospheric interaction experiments have been made with the European Incoherent Scatter (EISCAT) facility at TromsO with sufficiently high-altitude resolution to compare with theories of Langmuir turbulence. Recent Langmuir turbulence models predict a change from broad structureless spectra to line or cascade spectra within a few hundred meters for VHF (224 MHz) observations assuming typical ionospheric density gradients. In a campaign in May 1994 we found VHF spectra that were grouped into two regions separated in altitude by -2 km, with broad, unstructured plasma line spectra in the upper region and cascade type spectra in the lower region. The ion line channels showed detectable spectra mainly in the upper altitude region, which corresponds to that which had the broad plasma lines. The background ionospheric density profile showed an unusually low plasma density gradient near the HF reflection heights, thus allowing the two regions, which are normally so close together that one only sees a transition from one type of spectra to the other, to be clearly separated in height. Thus, in the high-latitude ionosphere there can, at times, be a simultaneous existence in spatially separate regions of cavitation (often referred to as strong turbulence) and cascading (normally associated with saturated parametric decay) as predicted by some simulations. Another new feature is a height variation in the plasma line cascades with the highest-order cascades strongest at the lowest heights, in accordance with expectations based on the parametric decay instability. Using the pulse-to-pulse technique, a continuous transition 7429

Excitation thresholds of field-aligned irregularities and associated ionospheric hysteresis at very high latitudes observed using SPEAR-induced HF radar backscatter

“…Figure 7 is reproduced from Wright et al (2006) and shows a typical example of hysteresis identified in ionospheric electron temperatures within the heated volume observed over Tromsø using the EISCAT UHF radar, over a height range which encompasses that where the artificial FAI are generated (the upper-hybrid height, as identified using the ion-line overshoot effect; e.g. Rietveld et al, 2000). Similar hysteresis was observed in changes in electron density induced by the heating effect.…”

EISCAT Svalbard radar observations of SPEAR-induced E- and F-region spectral enhancements in the polar cap ionosphere

“…These ionacoustic and Langmuir waves can be detected in the interCorrespondence to: R. S. Dhillon (rsd6@ion.le.ac.uk) action region by radars pointing in a wide range of directions, including those close to the geomagnetic field direction (e.g. Stubbe et al, 1992;Kohl et al, 1993;Djuth et al, 1994;Isham et al, 1999;Honary et al, 1999;Rietveld et al, 2000;Dhillon and Robinson, 2005). These wave modes give rise to enhancements in both the E-and F-region ion and plasma line incoherent scatter spectra.…”

A statistical study of the spatial distribution of Co‐operative UK Twin Located Auroral Sounding System (CUTLASS) backscatter power during EISCAT heater beam‐sweeping experiments

“…For the following investigation, electron density profiles provided by the EISCAT UHF radar were used to estimate a value of the upper hybrid height, h U . In these EISCAT UHF data a heater-induced ionline overshoot occurs at the altitude where the plasma frequency is equal to the heater frequency, causing an apparent enhancement in the electron density [Jones et al, 1986;Rietveld et al, 2000]. The height difference, Dz, between this altitude level and the upper hybrid level, U, can be determined, and Rietveld et al [1993] state a typical value of 6 km.…”