Abstract.A sequence of 3 patches of high-density (10 12 m −3 ) cold plasma on a horizontal scale-size of 300-700 km was observed near magnetic noon by the EISCAT VHF radar above Svalbard on 17 December 2001. The patches followed a trajectory towards the cusp inflow region. The combination of radar and all-sky observations demonstrates that the patches must have been segmented equatorward of the cusp/cleft auroral display, and hence their properties had not yet been influenced by cusp particle showers and electrodynamics on open flux tubes. The last patch in the sequence was intersected by radio tomography observations, and was found to be located adjacent to a broader region of the same high electron density further south. The patches occurred under moderately active conditions (Kp=3) and the total electron content (TEC) of the high-density plasma was 45 TEC units. The train of patches appeared as a segmentation of the tongue of ionization. The sequence of patches occurred in association with a sequence of flow bursts in the dusk cell return flow. It is proposed that reconnection driven pulsed convection is able to create sub-auroral patches in the region where high density mid-latitude plasma is diverted poleward toward the cusp. It is the downward Birkeland current sheet located at the equatorward boundary of the flow disturbance that represents the actual cutting mechanism.

Abstract. This paper presents the first direct empirical evidence that mesoscale variations in ion velocities must be taken into consideration when calculating Joule heating and relating it to changes in ion temperatures and momentum transfer to the neutral gas. The data come from the first tristatic Fabry-Perot Interferometer (FPI) measurements of the neutral atmosphere co-located with tristatic measurements of the ionosphere made by the European Incoherent Scatter (EISCAT) radar which were carried out during the nights of 27-28 February 2003 and 28 February until 1 March 2003. Tristatic measurements mean that there are no assumptions of uniform wind fields and ion drifts, nor zero vertical winds. The independent, tristatic, thermospheric measurements presented here should provide unambiguous vector wind information, and hence reduce the need to supplement observations with information obtained from models of the neutral atmosphere, or with estimates of neutral parameters derived from ionospheric measurements. These new data can also test the assumptions used in models and in ion-neutral interactions. The FPIs are located close to the 3 radars of the EISCAT configuration in northern Scandinavia, which is a region well covered by a network of complementary instruments. These provide a larger scale context within which to interpret our observations of mesoscale variations on the scales of tens of kilometres spatially and minutes temporally. Initial studies indicate that the thermosphere is more dynamic and responsive to ionospheric forcing than expected. Calculations using the tristatic volume measurements show that the magnitude of the neutral wind dynamoCorrespondence to: A. L. Aruliah (a.aruliah@ucl.ac.uk) contribution was on average 29% of Joule heating during the first night of observation. At times it either enhanced or reduced the effective electric field by up to several tens of percent. The tristatic experiment also presents the first validation of absolute temperature measurements from a common volume observed by independently calibrated FPIs. Comparison of EISCAT ion temperatures at an altitude of 240 km with FPI neutral temperatures show that T i was around 200 K below T n for nearly 3 h on the first night during a period of strong geomagnetic activity. This is inconsistent with energy transfer. Comparison with FPI temperatures from surrounding regions indicate that it could not be accounted for by height variations. Indeed, these first results seem to indicate that the 630-nm emission did not stray too far from 240 km. There were also apparent drops in T e at the same time as the anomalous T i values which are energetically implausible. Incorrect assumptions of composition or nonMaxwellian spectra are likely to be the problem.

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Abstract. Troughs in the latitudinal distribution of electron density are a well-known feature of the ionosphere from subauroral to polar latitudes. The location and depth of the trough minimum, the width of the feature, and the horizontal gradients in electron density associated with the trough walls are all quantities of interest or concern to practical applications of radio systems involving the ionosphere. In practice, the precise characteristics of trough-like structures have been difficult to monitor using ground-based methods. Ionospheric tomography represents a new development that is maturing into a technique ideally suited to the study of electron density troughs. Results are presented from a variety of observations made during tomographic campaigns in northern Europe. A long-term investigation has been made of the main trough from a network of stations in the United Kingdom. The position of the trough minimum and the wall gradients have been studied on a diurnal basis using tomographic images reconstructed from measurements for a succession of passes of Navy Navigation Satellite System satellites. With stations deployed for more than 6 months, the average behavior has also been studied. Examples are shown of extreme behavior of the trough under very disturbed geomagnetic conditions, during which tomography continues to yield images while the limitations of ionosondes are exposed. Studies of narrow troughs with very steep gradients seen at auroral latitudes have been used to investigate some of the successes and limitations of the tomographic method. Measurements made in the polar cap show the depleted densities of the polar hole in the center of the dawn convection cell and illustrate the power of the tomographic method at high latitudes. Finally, the dayside trough at the high-latitude boundary between corotating and counterstreaming flux tubes in the afternoon sector has been revealed in a tomographic image extending over some 30 ø latitude, made using a chain of six stations in Scandinavia.

Pryse, Sian; Kersley, L.; Malan, D.; Bishop, G. J., 'Parameterization of the main ionospheric trough in the European sector, Radio Science (2006) 41 pp.RS5S14 RAE2008Measurements of total electron content, obtained by monitoring satellite transmissions at stations in the United Kingdom during a period of 12 months, have been used to characterize the structure and dynamics of the main ionospheric trough in terms of a set of defined parameters. The primary aim of the investigation was to represent the position and shape of the trough in such a way that the experimental observations can be used for direct comparison and validation of ionospheric models.Peer reviewe

Tomographic imaging of the ionosphere is a recently developed technique that uses integrated measurements and computer reconstructions to determine electron densities. The integral of electron density along vertical or oblique paths is obtained with radio transmissions from low-earth-orbiting (LEO) satellite transmitters to a chain of receivers on the earth’s surface. Similar measurements along horizontal paths can be made using transmissions from Global Position System (GPS) navigation satellites to GPS receivers on LEO spacecraft. Also, the intensities of extreme ultraviolet (EUV) emissions can be measured with orbiting spectrometers. These intensities are directly related to the integral of the oxygen ion and electron densities along the instrument line of sight. Two-dimensional maps of the ionospheric plasma are produced by analyzing the combined radio and EUV data using computerized ionospheric tomography (CIT). Difficulties associated with CIT arise from the nonuniqueness of the reconstructions, owing to limited angle measurements or nonoptimal receiver location. Improvements in both reconstruction algorithms and CIT measurement systems are being implemented to overcome these difficulties. New imaging systems being developed employ CIT for large area mapping of the plasma densities in the ionosphere.

A campaign was conducted in late September 1991 to obtain experimental measurements for use in ionospheric tomography. Four stations receiving signals from the Navy Navigation Satellite System satellites were set up in a meridional chain in Scandinavia covering some 10° of latitude. Measurements of total electron content were made for six satellite passes over a 2‐day period coinciding with an extended run of the CP3f latitudinal scan common program of the European incoherent scatter radar. A new reconstruction technique involving the use of two‐dimensional basis vectors has been used to convert the total electron content measurements to images of electron density in a height versus latitude plane. Comparisons of the tomographic images and the independent measurements of electron density from the radar show good general agreement. Broadly similar troughs and enhancements are observed by the two techniques, and a latitudinal gradient in the height of the peak density is reproduced in both data sets.

Abstract. This paper compares the performance of three different reconstruction algorithms used for tomographic imaging of the ionosphere. The test cases chosen make use of experimental electron content observations obtained from a meridional chain of receivers in Scandinavia at times when the European incoherent scatter radar facility provided independent measurements of electron density for verification of the reconstructions. The examples include ionospheres with layers peaking at both anomalously high and very low altitudes, in addition to a smooth horizontally stratified layer. The results expose the limitations of an algorithm based on use of a standard ionospheric model but demonstrate that with appropriate choice of background a priori profiles, reliable images are obtained from both discrete inverse theory and quadratic programing. For these two latter algorithms the resultant reconstructions are essentially independent of the algorithm used. The study highlights the critical role of the range of the background input to the algorithms. A density value at a single ghd cell provides sufficient additional a priori information to constrain the imaging of a horizontally stratified ionosphere.