H. Kopka

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 modification experiments in northern Scandinavia

Abstract The heating facility at Ramfjordmoen near Tromso, Norway, is briefly described, and a survey is given of the experiments performed with this facility until now. These experiments comprise D -region modification, polar electrojet modulation at VLF, ELF and ULF, HF absorption and backscatter due to short-scale field-aligned irregularities, stimulated radio wave emission of the modified ionospheric plasma, short-time scale HF absorption due to the parametric decay instability, airglow modification, excitation of large-scale irregularities, and F -region cross modulation.

ELF and VLF wave generation by modulated HF heating of the current carrying lower ionosphere

Abstract This paper presents further experimental results on ionospheric current modulation, using powerful amplitude modulated HF waves produced by the new heating facility at Ramfjordmoen near Tromso, Norway. As a result of the current modulation, waves in the ULF, ELF and VLF range can be efficiently generated. The experiments discussed here cover the range from low ELF up to 7 kHz. The observed signal strengths are of the order 1 pT. Decomposition of the received ELF/VLF waves into R- and L-mode shows that both modes are usually of comparable strength. The signal strength as a function of modulation frequency shows pronounced maxima at multiples of approximately 2 kHz. The paper also presents a brief theoretical discussion of the processes involved in the generation of ELF/VLF waves by HF induced current modulation.

Ionospheric modification experiments with the Tromsø heating facility

Abstract The experiments performed up to mid 1984 with the heating facility at Ramfjordmoen near Tromso, Norway, are summarized. These experiments comprise D -region modification, polar electrojet modulation at VLF, ELF and ULF frequencies, excitation of E -region small-scale irregularities and of F -region small- and large-scale irregularities, anomalous absorption of HF wave on long and short time scales, excitation of incoherent backscatter plasma and ion lines, stimulated radio wave emission and F -region in situ measurements.

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.

EISCAT observations of the heated ionosphere

Abstract EISCAT observations of ionospheric heating by means of high power HF radio waves are reported. These results indicate that the ionospheric response to heating involves a number of plasma instabilities with different characteristic time scales, ranging from a few milliseconds to several tens of seconds.

Frequency dependence of anomalous absorption caused by high power radio waves

Abstract High power radio waves can modify the ionospheric electron density distribution to produce field aligned plasma irregularities which give rise to the anomalous absorption of HF radio waves. The coefficient of anomalous absorption of a vertically propagated radio wave due to scattering from field aligned irregularities has been calculated, taking into account the effects of the geomagnetic field on electron motions. These results are compared with those of other theoretical models. Furthermore, the scale lengths of field aligned irregularities produced by a high power radio wave during recent high latitude modification experiments have been determined from measurements of the anomalous absorption by means of this theory.

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.

Long-range detection of VLF radiation produced by heating the auroral electrojet

This paper presents the first evidence of long-range detection (>1000 km) of calibrated VLF signals resulting from the HF heating of the auroral electrojet, that is, signal detection at a point of direct “line of sight” of the heated patch of ionosphere. Other workers have presented calibrated data from shorter ranges (190–550 km) or claimed the detection of uncalibrated signals at ranges greater than 6000 km, but we believe that no such calibrated and clear signals, like those presented here from a range of greater than 2000 km, have previously been reported. Also, in contrast to earlier long-range detection experiments we record the “radial” as well as the “azimuthal” magnetic component of the signals and from their ratio obtain the waveguide mode polarization. Observed absolute magnetic field strengths and waveguide polarizations are found to be in line with the predictions of simple waveguide models.

Observations of ionospheric modification by the Tromsø heating facility with the mobile diagnostic equipment of IZMIRAN

Abstract Results are presented of ionospheric modification experiments carried out at Tromso, Norway, in October and November 1988. These experiments were jointly conducted by IZMIRAN and MPAe. In these experiments, small scale electron density perturbations (having a vertical size ~ 1 km and a density variation ~0.1%) were found to be excited near the reflection and upper hybrid resonance (UHR) points of an O -mode pump wave. When the pump frequency was close to the triple electron gyrofrequency, there were no noticeable perturbations near the UHR point. This peculiarity correlated well with a decrease of the intensity of stimulated electromagnetic emission (SEE). When SEE is comparatively weak, the SEE intensity can vary quasi-periodically, with a quasi-period of approximately 1 s. When this happens, anticorrelations between different parts of the SEE spectrum become possible.