Ingemar Haggstrom

Plasma convection and auroral precipitation processes associated with the main ionospheric trough at high latitudes

Abstract Intervals of F -region electron density depletions associated with the main (mid-latitude) ionospheric trough have been studied using latitude scanning experiments with the EISCAT UHF radar. From 450 h of measurements over a one year period at solar minimum (April 1986–April 1987) the local time of appearance of the trough at a given latitude is observed to vary by up to about 8 h. No seasonal dependence of location is apparent, but troughs are absent in the data from summertime experiments. A weak dependence of trough location on K p is found, and an empirical model predicting the latitude of the trough is proposed. The model is shown to be more appropriate than other available quantitative models for the latitudes covered by EISCAT. Detailed studies of four individual days show no relationship between local magnetic activity and time of observation of the trough. On all four of these days, however, the edge of the auroral oval, evidenced by enhanced electron densities in the E -region, is found to be approximately co-located with, or up to 1° poleward of, the F -region density minimum. Simultaneous ion drift velocity measurements show that the main trough is a region of strong (> several hundred metres per second) westward flow, with its boundary located approximately 1°–2° equatorward of the density minimum. Within the accuracy of the observations this relationship between the convection boundary, the trough minimum and the precipitation boundary is independent of local time and latitude. The relevance of these results is discussed in relation to theoretical models of the F -reregion at high latitudes.

Ion composition changes during F-region density depletions in the presence of electric fields at auroral latitudes

F-region density depletions in the afternoon/evening sector of the auroral zone are studied with the EISCAT UHF radar. Four case studies are presented, in which data from three experiment modes are used. In each case the density depletion can be identified with the main ionospheric trough. For the two cases occurring in sunlit conditions the electron densities recovered significantly after the trough minimum. Tristatic ion velocity measurements show the development of poleward electric fields of typically 50–100 m Vm−1, which maximize exactly in the trough minimum. A special analysis technique for incoherent scatter measurements is introduced, based on the ion energy equation. By assuming that the ion temperature should obey this equation it is possible to fix this parameter in a second analysis and to allow the ion composition to be a free parameter. The results from two experiments with accurate velocity measurements indicate that the proportion of O+ near the F-region peak decreased from 100% in the undisturbed ionosphere to only 10% and 30%, respectively, in the density minimum of the trough. The loss of O+ is explained by the temperature dependence of recombination with nitrogen molecules. Temperatures derived from radar measurements are very sensitive to the assumed ion composition. For the above case of 10% O+ the deduced electron temperature in the trough was transformed from a local minimum of < 2000 K to a local maximum of 4000 K.

On the development of folds in auroral arcs

Abstract The development of an auroral arc in the midnight sector, from diffuse to discrete with subsequent large scale folding, is studied with the aid of several ground-based observations, including incoherent scatter radar, and data from a HILAT satellite pass. Ion drift velocities in the F -region, as measured by EISCAT, were consistently eastward throughout and after the whole period of development, whilst the ion temperature showed two large enhancements just prior to the appearance of the main auroral fold. The fold moved eastwards and crossed the EISCAT antenna beam, appearing as a short-lived spike in electron density at altitudes between about 100 km and 400 km. The spike in electron density came progressively later at higher altitudes. The observations are interpreted as the result of enhanced convection in the ionosphere and in the magnetosphere. The auroral arc folding is suggested to be caused by the Kelvin-Helmholtz instability in a velocity shear zone in the magnetosphere.

On the relation of Langmuir turbulence radar signatures to auroral conditions

We present a statistical study of anomalous radar echoes observed in the auroral ionosphere thought to be signatures of Langmuir turbulence (LT). Data obtained with the European Incoherent Scatter Svalbard radar during the international polar year (IPY) were searched for these anomalous echoes in the auroral Fu2009region. In incoherent scatter radar experiments LT may in certain circumstances be observed as enhanced backscattered radar power at the ion line frequencies, plasma line frequencies, and at zero Doppler shift. The power enhancement at zero Doppler shift could arise due to Bragg scattering from nonpropagating density fluctuations caused by strong LT. In the IPY data set, around 0.02% of the data comply with our search criteria for altitudes above 190 km based on the ion line spectrum including enhancement at zero Doppler shift. The occurrence frequency of the identified events peaks in the premidnight sector and increases with local geomagnetic disturbance. Enhanced backscattered power is observed with limited altitude extent (below 20 km in 70% of the events), and the altitude distribution of identified radar signatures in the ion line channel has a peak at about 220 km. Enhancement of the plasma line is observed with the ion line enhancements in more than 60% of the events. Two classes of enhanced plasma lines occur. In the first class, plasma lines are limited in frequency and altitude and occur at altitudes of ion line enhancements. In the second class, the plasma lines are spread in frequency and range and are observed at lower altitudes than the first class (at about 170 km) with frequencies close to 3 MHz. Available optical data available indicate that the identified events to occur during auroral breakup with high-energy electron precipitation.

High-latitude ionospheric response to a geomagnetic sudden commencement

Abstract A geomagnetic sudden commencement on 25 March 1987 was simultaneous with the onset of a large-scale depletion of F -region electron density observed by the EISCAT incoherent scatter radar in the dusk sector of the auroral zone (69.6° N, 19.2° E). Ionospheric signatures of the impulse were a transient increase in the northward component of the electric field and in the ion temperature. The maximum density depletion occurred about 1 h after the magnetic impulse and corresponded to maxima in the electric field and ion temperature. Ionosonde measurements from stations south of the radar showed that the density depletion progressed steadily equatorwards by more than 10 of latitude during the 2 h following the impulse. A brief recovery of F -region densities after the passage of the depletion was observed by the ionosondes, but the ionosphere above the radar remained depleted. It is concluded that the main cause of the observed density depletion was sunward transport of lower density nightside plasma following an equatorward expansion of the high-latitude plasma convection pattern associated with the magnetic impulse.

A Critical Ionization Velocity Experiment on the ARGOS Satellite

Abstract : We report on a xenon gas release experiment conducted on the Advanced Research and Global Observations (ARGOS) Satellite in the F-region ionosphere above the European Incoherent Scatter (EISCAT) radar at Tromso, Norway, Oct 20, 2000. In this experiment, xenon gas was released in the ram direction of the satellite. This was intended to induce ionization through the critical ionization velocity (CIV) process proposed by Alfven in his theory of the formation of the planets in the solar system. If the CIV process had been operational and efficient, ionization of the xenon cloud might have been observed. Radar observations by EISCAT showed no detectable enhancement of the ambient plasma in the velocity of the satellite. We present a simple model calculation which predicts that the overall yield of xenon ions in the release would be low, owing merely to the initially high density of the rapidly expanding xenon cloud.