T. Hagfors

Radar Soundings of the Subsurface of Mars

The martian subsurface has been probed to kilometer depths by the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express orbiter. Signals penetrate the polar layered deposits, probably imaging the base of the deposits. Data from the northern lowlands of Chryse Planitia have revealed a shallowly buried quasi-circular structure about 250 kilometers in diameter that is interpreted to be an impact basin. In addition, a planar reflector associated with the basin structure may indicate the presence of a low-loss deposit that is more than 1 kilometer thick.

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.

High-latitude pump-induced optical emissions for frequencies close to the third electron gyro-harmonic

It has been long established that high-power O-mode HF pumping of the ionosphere can produce artificial optical emissions. 630 nm O(1D) photons are produced by pump-accelerated electrons colliding with the F-layer neutral oxygen. However, the mechanism for artificial electron acceleration remains unclear. Competing theories include Langmuir and upper-hybrid turbulence. Pump-induced HF coherent radar backscatter power is closely linked with upper-hybrid turbulence, both of which are known to reduce when pumping on an electron gyro-harmonic frequency. On 3 November 2000, the EISCAT HF facility was systematically stepped in frequency through the 3rd gyro-harmonic. A significant reduction in the artificial optical intensity coincides with that of CUTLASS radar backscatter power. This is conclusive proof that upper-hybrid turbulence is intimately linked to the mechanism for high-latitude pump-induced aurora, at least for 630 nm photons and the steady state.

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

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 arc 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.

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.

High-latitude space plasma physics

This book constitutes the proceedings of the Nobel Symposium No. 54 on High Latitude Magnetospheric/Ionospheric Plasma Physics. The main purpose of the symposium was to prepare for the European research effort in space plasma physics in the mid-1980s, in which two major constituents are the European Incoherent Scatter Association (EISCAT) facilities and the Swedish satellite Viking. The physics of the high-latitude ionosphere and how this part of near space is affected by the properties of the solar wind and the interplanetary magnetic field are explored. A detailed discussion is provided on high-latitude magnetospheric physics at altitudes of 1-2 earth radii, the main focus of the Viking project. Specific topics considered include the role of the auroral ionosphere in magnetospheric substorms, the low altitude cleft, ionospheric modification and stimulated emissions, plasma physics on auroral field lines, solar wind-magnetosphere energy coupling, cold plasma distribution above a few thousand kilometers at high latitudes, hot electrons in and above the auroral ionosphere, the correlation of auroral kilometric radiation with visual auroras and with Birkeland currents, electrostatic waves in the topside ionosphere, solitary waves and double layers, and an Alfven wave model of auroral arcs.

On the electron distribution function in the F region and airglow enhancements during HF modification experiments

The variation with height and plasma frequency of the distribution function of ambient electrons (EDF) in the F region ionosphere during high-power, HF radio wave modification experiments is discussed. It is shown that the deviation of the EDF from a Maxwellian distribution (MD) in the high energy, >2 eV, range may be quite significant (decreasing by a factor of 2 to 5) depending on the background parameters. As a result, the cooling and excitation rates are reduced with respect to a MD calculation. This, for example, can improve agreement of the model based on excitation by heated thermal electrons with the observations of 630.0 nm airglow during HF modification experiments.

A new digital all-sky imager experiment for optical auroral studies in conjunction with the Scandinavian twin auroral radar experiment

Studies of the relationship between the optical aurorae and the ionospheric electric fields, as observed by the bi-static Scandinavian twin auroral coherent backscatter radar experiment (STARE) and the tri-static European incoherent backscatter radar facility (EISCAT), are to be undertaken in Scandinavia. For this purpose, an unmanned and fully automatic low-light-level television camera system, coupled to an all-sky lens, has been constructed. A personal computer controls all aspects of the instrument, operating it for all dark and moon-free periods. Monochrome optical data, usually at 557.7 nm, are pre-processed in real time at the recording site. The transformed images are stored digitally to magneto-optical disk with a temporal and spatial resolution directly compatible with the STARE radar data, thus making comparisons easy. Simultaneous TV recordings to tape may be made on a campaign basis. The camera has been calibrated for all gain settings, thereby permitting auroral images to be recalled in any ...

A characterization of a comet nucleus interior: inversion of simulated radio frequency data

Abstract Radio transmission through the nucleus of the comet Wirtanen, as developed in the CONSERT experiment on the ROSETTA mission, is designed to provide information on the electrical properties of the cometary interior. In order to determine what information can most readily be obtained, we first introduce an analytical model of the nucleus permittivity. We then use the WKB approximation to describe the propagation of waves through the nucleus model assuming a radio frequency of 90 MHz, the frequency chosen for the experiment. With one end of the propagation path on the surface of the comet, and the other in a vehicle orbiting the nucleus, it is shown that the signal at the orbiting end of the path must be back-propagated to the surface of the nucleus of the comet in order to obtain a correct interpretation of the data. We show how the mean permittivity along each propagation path can be obtained and how this can be used to obtain information about the large scale structure of the nucleus. Finally we discuss how the permittivity distribution found can be used to constrain the nature of the material in the interior of the nucleus.

High‐latitude ground‐based observations of the thermospheric ion‐drag time constant

From previous studies, it has been conclusively demonstrated that F-region thermospheric winds follow, but generally lag behind, the ion drift pattern of magnetospheric convection. Analysis of the ion-neutral momentum exchange equation shows that ion-drag and thermal pressure are the major contributors to neutral momentum forcing at F-region heights with relatively minor effects from coriolis, advection and viscous forces. An ion-neutral coupling time constant (e-folding time) has been defined which describes the time taken for the neutral gas velocity to approach the ion velocity after a step change in convection. In this study, F-region ion drift and neutral winds have been observed by the EISCAT tristatic incoherent scatter facility and a ground-based Fabry-Perot interferometer, respectively, from northern Scandinavia. The e-folding time varies in the range 0.5 - 6.5 hours, with an average of 1.8 and 3.3 hours for a geomagnetically active and quiet period, respectively, which compares well with previous satellite measurements of 0.5 - 3.5 hours.