Directional features of the downshifted peak observed in HF-induced stimulated electromagnetic emission spectra obtained using an interferometer

A high frequency (HF) ionospheric modification experiment was carried out between 25 September and 8 October 2004, using the EISCAT HF transmitter located near Tromsø, Norway. During this experiment the spectra of the stimulated HF sideband waves (stimulated electromagnetic emission or SEE) induced by the HF pump were observed using an interferometer consisting of three spaced receiving antennas with baselines both along and perpendicular to the meridian, and a multi-channel coherent receiver, installed in the vicinity of the HF facility. The transmitter operated at 4040kHz and its antenna beam was scanned to angles of 0°, 7°, 14°, and 21° south from vertical, pausing 4min at each position. This paper focuses on features of the downshifted peak (DP) emission, which has not been as thoroughly studied as many of the other SEE spectral features observable within the EISCAT pump frequency range. It was found that the signal-weighted direction of the DP source region remained within 5° of magnetic zenith as the HF beam was tilted between 0 and 21° south of vertical

An interferometer experiment to explore the aspect angle dependence of stimulated electromagnetic emission spectra

International audienceWhen the Earth's ionosphere is irradiated by a radiofrequency (RF) electromagnetic wave of sufficiently high power density and tuned to match a natural E- or F-region plasma frequency, ionospheric magnetoionic wave modes may be excited and may generate RF electromagnetic sideband waves via nonlinear interactions. These secondary emissions, which may then escape from the ionosphere, have been termed stimulated electromagnetic emission or SEE. The frequency spectra of this radiation has been studied extensively, and a number of characteristic spectral features have been identified and in some cases related to particular plasma processes. The separation in frequency between the RF pump and the harmonics of the local electron gyrofrequency is critical in determining the amount of anomalous absorption suffered by the pump wave and the spectral properties of the stimulated sidebands. The pump can excite electrostatic waves which do not propagate away but can in some cases be observed via radio-wave scattering from the electron density fluctuations associated with them. These enhanced density fluctuations are created by processes commonly referred to as upper-hybrid and Langmuir turbulence. Langmuir turbulence has been the subject of 930-MHz scattering observations with antenna scanning through several pre-selected angles between the geographic and geomagnetic zenith directions, and a preference for pointing angles between the Spitze angle and geomagnetic field-aligned was identified. Other phenomena, such as the generation of enhanced electron temperatures and artificial aurora, have more recently been shown to have special behavior at similar angles, near but apparently not quite at field-aligned. In view of this evidence for angular structure in several pump-induced effects, in light of the rich variety of SEE phenomena strongly dependent on the geomagnetic field via the frequency interval between the pump and the gyrofrequency harmonics, and in view of the not yet understood but complex relationship between electrostatic fluctuations and SEE, it is of interest to investigate experimentally whether a similar angular structure is present in the various spectral features of the SEE signals and to compare the results with radar and other observations of RF-pump-induced effects. To this end we describe a simple two-element radio interferometer designed to search for aspect angle dependence of SEE features. We present an example of the initial data produced by this system, and draw preliminary conclusions based on the example data

Interpretation of ionospheric F-region structures in the vicinity of ionisation troughs observed by satellite radio tomography

Tomographic images of the spatial distribution of electron density in the ionospheric F-region are presented from the Russian-American Tomography Experiment (RATE) in November 1993 as well as from campaigns carried out in northern Scandinavia in November 1995 and in Russia in April 1990. The reconstructions selected display the ionisation troughs above the tomographic chains of receivers during geomagnetically quiet and disturbed periods. Two mathematical models of the high-latitude ionosphere developed in the Polar Geophysical Institute have been applied for interpretation of the observed tomographic images.Key words. Ionosphere (electric fields and currents; ion chemistry and composition; plasma convection

Features of stable diffuse arcs observed by means of auroral tomography

In this paper we study the spatial distribution of optical volume emission rates and peculiarities of the luminosity intensity within weak diffuse auroral arcs recovered by means of auroral tomography. The tomographic images are obtained from sets of scanning multi-channel photometer data obtained in February 1999 on the Kola Peninsula in Russia at three sites of a chain extending 226 km along the geomagnetic meridian. The 427.8- and 557.7-nm emissions of a 15-s time resolution observed within one hour during low geomagnetic activity are analyzed. We found that the intensity profile of an individual arc along the geomagnetic meridian has an inverted-V-shape. The luminosity maximum altitude decrease by 4–14 km at about 140 km distance in the south-north direction can be observed during two or more diffuse arcs. The parameters of the precipitating electron flux are obtained from an integral equation, which determines the best relationship between the 427.8-nm intensity height profile and an arbitrary particle energy spectrum. A dimensionless function of the energy dissipation is used as the core in the integral equation. The estimated average energy of electron flux, which generated the isolated diffuse arc, is 1–2 keV higher in the central part of the arc in comparison to values at its borders