M. T. Rietveld

Introduction to ionospheric heating at Tromsø - I. Experimental overview

Abstract The HF ionospheric modification (heating) facility at Ramfjordmoen will become a part of the EISCAT association from January 1993. This paper, which is intended for the new user, describes the technical capabilities of the facility and the broad range of geophysical and plasma physical experiments which are possible. An overview is presented of the physical effects that a powerful HF electromagnetic wave incident on the ionosphere can produce on timescales ranging from tens of microseconds to minutes in height regions ranging from 50 to hundreds of km. Emphasis is placed on the practical implementation of ionospheric heating experiments using the EISCAT incoherent scatter radars as the main diagnostic, but other diagnostic techniques using ground-based radars, radio links, radio receivers, photometers, rocket and satellite instrumentation are also described. A companion paper presents in greater depth some of the current scientific issues being addressed in ionospheric modification research.

Ionospheric electron heating, optical emissions, and striations induced by powerful HF radio waves at high latitudes: Aspect angle dependence

radio-induced aurora showed that the enhancement caused by the HF radio waves also remained localized near the field-aligned position. Coherent HF radar backscatter also appeared strongest when the pump beam was pointed field-aligned. These results are similar to some Langmuir turbulence phenomena which also show a strong preference for excitation by HF rays launched in the field-aligned direction. The correlation of the position of largest temperature enhancement with the position of the radio-induced aurora suggests that a common mechanism, upper-hybrid wave turbulence, is responsible for both effects. Why the strongest heating effects occur for HF rays directed along the magnetic field is still unclear, but self-focusing on field-aligned striations is a candidate mechanism, and possibly ionospheric tilts may be important. INDEX TERMS: 2403 Ionosphere: Active experiments; 6929 Radio Science: Ionospheric physics (2409); 7839 Space Plasma Physics: Nonlinear phenomena; KEYWORDS: HF-heating, ionospheric modification, electron heating, EISCAT, striations, aspect angle

Langmuir turbulence and ionospheric modification

Experimental results, obtained with the Heating facility and the EISCAT UHF and VHF radars at Tromso, Norway, have been used to test existing Langmuir turbulence theories and to provide further observational facts to be taken into account in a new theoretical formulation of Langmuir turbulence. It is found that none of the available theories is able to describe the diversity of observational facts. The dominance of propagating Langmuir and ion acoustic waves in the observed spectra, and the fact that these waves closely follow their linear dispersion relations, lend support to a weak turbulence description. On the other hand, the observation that the number of cascade steps is limited to two, independent of input power, strongly contradicts such an approach. Unmistakable indications for the occurrence of caviton collapse are not seen in the experimental results at Tromso. The low-frequency turbulence is found to consist of subsonic and sonic contributions, showing widely independent properties. The sonic turbulence is seen to evolve from the natural, thermally excited, ion acoustic background.

Eiscat radar observations of enhanced incoherent scatter spectra; Their relation to red aurora and field‐aligned currents

Enhancements of one, or both, of the ion-acoustic peaks of incoherent scatter spectra in the auroral ionosphere have been observed with the EISCAT UHF and VHF radars. All occurrences for which optical data are available show these events to coincide with active, unusually intense, red auroral forms in the vicinity of the radar beam at high altitudes. Both the optical and the radar signatures are expected to be caused by large fluxes of low energy electrons. Analyses of the measured spectra, in which the electron drift speed is estimated, imply field-aligned current densities up to several mA m{sup {minus}2}. The vertically-directed VHF observations from {approximately}1,000 km altitude reveal that the spectral enhancements, which are transient features in field-aligned measurements, can exist for up to several minutes.

First tomographic estimate of volume distribution of HF‐pump enhanced airglow emission

This report presents the first estimates of the three-dimensional volume emission rate of enhanced O(1D) 6300 A airglow caused by HF radio wave pumping in the ionosphere. Images of the excitation show how the initially speckled spatial structure of excitation changes to a simpler shape with a smaller region that contains most of the excitation. A region of enhanced airglow was imaged by three stations in the Auroral Large Imaging System (ALIS) in northern Scandinavia. These images allowed for a tomography-like inversion of the volume emission of the airglow. The altitude of maximum emission was found to be around 235 ± 5 km with typical horizontal and vertical scale sizes of 20 km. The shape of the O(1D) excitation rate varied from flatish to elongated along the magnetic field. The altitude of maximum emission is found to be approximately 10 km below the altitude of the enhanced ion line and 15 km above the altitude of maximum electron temperature. Comparisons of the measured altitude and temporal variations of the 6300 A emission with modelled emission caused by O(1D) excitation from the high energy tail of a Maxwellian electron distribution show significant deviations. The 6300 A emission from excitation of the high energy tail is about a factor of 4 too large compared with what is observed. This shows that the source of O(1D) excitation is electrons from a “sub-thermal” distribution function, i.e. the electron distribution is Maxwellian at low energies and at energies above 1.96 eV there is a depletion.

D-region characteristics deduced from pulsed ionospheric heating under auroral electrojet conditions

Abstract Characteristic times for conductivity changes in the auroral D -region caused by HF heating are deduced from the VLF/ELF signals recorded on the ground underneath the heated region. The VLF/ELF signals are caused by a heating-induced redistribution of the horizontal ionospheric currents when d.c. electric fields exist in the ionosphere. Measurements were made in the late afternoon with a dark, moderately disturbed ionosphere. The time constant for the Hall conductivity to change due to heating with 240 MW of effective radiated power is approximately 70 μs, while the time constant for cooling is approximately 120 μs. These results agree with model calculations. Delay times of the VLF/ELF pulses and their subsequent echoes result in a virtual source height of approximately 88 km and a VLF reflection height of approximately 75 km.

High‐latitude HF‐induced airglow displaced equatorwards of the pump beam

HF-induced airglow at 630 nm was observed by the Digital All-sky Imager, located near Skibotn in Norway, at F-region altitudes above the EISCAT HF facility near Tromso on 21 February 1999. The transmitter was operated in a 4-min on, 4-min off sequence at 4.04 MHz O-mode with the beam pointing vertically. The airglow reached a peak intensity of about 100 R above background and appeared equatorward of the HF beam’s projection on the reflection altitude, which was obtained from ionograms. Generally, the region of maximum airglow was displaced towards the magnetic field line (zenith angle = 12.8° S) passing through the HF facility. This is a unique feature of these observations. From mid-latitude studies, such airglow is thought to be excited either by electrons energised to several eV by plasma turbulence, or by thermal electron temperature enhancement. Such localisation towards the magnetic field is unexpected for both mechanisms of airglow generation and suggests this feature may be important at high latitudes.

Aspect angle dependence of HF enhanced incoherent backscatter

Abstract An HF ionospheric interaction experiment was performed in November and December of 1997 using the EISCAT HF transmitter and 931 and 224 MHz incoherent scatter radars, all co-located near Tromso, Norway. During this experiment the pointing of the UHF radar was varied in a predetermined and repeating cycle between elevation angles of 90 and 77.2 degrees south, that is, between vertical and geomagnetic field aligned. The HF transmitter duty cycle was intentionally kept to the relatively low value of 2% (200 ms every 10 s) in order to minimize the effects of ionospheric irregularities. Here we report on variations in the intensity of the enhanced incoherent scatter ion and plasma lines observed during the experiment. Bottomside and topside F region enhanced lines were seen with both radars, and while intensity enhancements observed with the UHF radar were clearly correlated with pointing angles between the Spitze angle and field aligned, no correlation between the intensity of the lines observed with the scanning UHF radar and the vertically pointing VHF radar was observed. Consistent with HF propagation theory, the field aligned backscatter observed by the UHF radar originated several kilometers below the HF reflection height.

Introduction to ionospheric heating experiments at Tromsø. II: Scientific problems

Abstract In the first section the theory of parametric decay instabilities (PDI) is briefly described. Then results of joint heating/incoherent scatter observations at Tromso are presented. It turns out that most of the observed features are in good agreement with that theory, while some others remain still unexplained. Among the latter features the most striking is the existence of a ‘space-time blob structure’, which means that the time variation of scattered power from adjacent altitudes seems to be correlated. Experiments at Arecibo often lead to results different from ours. Some scientists in the field explain these observations in terms of a ‘strong turbulence’ in which also caviton formation is involved. We think that most of the Tromso results can be adequately explained by the parametric decay process.

The phase speed of artificial field-aligned irregularities observed by CUTLASS during HF modification of the auroral ionosphere

The RF heater facility at Ramfjordmoen, Tromso can generate field-aligned plasma irregularities in the field of view of the Cooperative UK Twin-Located Auroral Sounding System (CUTLASS) coherent backscatter radar at Hankasalmi, Finland. In a recent set of experiments conducted in April 1996, the phase speed of the generated plasma irregularities has been compared with independent measurements of the plasma drift velocity observed by the European Incoherent Scatter radar facility. The phase speed of the plasma irregularities is found to be equal to the component of the plasma drift velocity in the direction of propagation of the plasma wave. This result is a further verification of the high performance of the CUTLASS radar and also demonstrates how artificially generated plasma irregularities can be employed to detect small plasma drift velocities and contribute to geophysical research. The characteristics of the Doppler spectrum of the artificial plasma waves are also described and discussed.