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norskBlar i utgivelsesdato Fakultet for naturvitenskap og teknologi
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Theory of linear G-difference equations
(Journal article; Tidsskriftartikkel; Peer reviewed, 1997-12-17)We introduce the notion of difference equation defined on a structured set. The symmetry group of the structure determines the set of difference operators. All main notions in the theory of difference equations are introduced as invariants of the action of the symmetry group. Linear equations are modules over the skew group algebra, solutions are morphisms relating a given equation to other ... -
Superposed epoch analysis applied to large-amplitude travelling convection vortices.
(Journal article; Tidsskriftartikkel, 1998)For the six months from 1 October 1993 to 1 April 1994 the recordings of the IMAGE magnetometer network have been surveyed in a search for large- amplitude travelling convection vortices (TCVs). The restriction to large amplitudes (>100 nT) was chosen to ensure a proper detection of evens also during times of high activity. Readings of all stations of the northern half of the IMAGE network ... -
Observations of thermospheric neutral winds within the polar cusp and the auroral oval using a Doppler imaging system (DIS).
(Journal article; Tidsskriftartikkel, 1998)Two Doppler imaging systems (DIS) or wide-field imaging Fabry-Perot interferometers (FPI), have recently been commissioned, one at the Auroral Station, Adventdalen, Longyearbyen, Svalbard, and the second at the IRF, Kiruna, Sweden. These instruments can provide wide-field (600 * 800 km) images of neutral wind flows in the upper thermosphere, by measuring the Doppler shift of the atomic oxygen forbidden ... -
An approach towards an agent computing environment
(Research report; Forskningsrapport, 1998)We devise a mobile agent middleware architecture for supporting distributed applications in a wide-area network. The architecture provides a structural framework for functional components that are needed to support mobile agents in asymmetric networking environments. -
Spectral characteristics of spring arctic mesosphere dynamics.
(Journal article; Tidsskriftartikkel, 1998)The spring of 1997 has represented a stable period of operation for the joint University of Tromsù / University of Saskatchewan MF radar, being between refurbishment and upgrades. We examine the horizontal winds from the February to June inclusive and also include estimates of energy dissipation rates derived from signal fading times and presented as upper limits on the turbulent energy ... -
Observations of auroral plasma lines
(Master thesis; Mastergradsoppgave, 1998-03-01)The thesis describes observations of strongly enhanced incoherent scatter plasma lines in the auroral ionosphere which were made with the EISCAT UHF radar in 1985. The linear theory of plasma instabilities is reviewed, and several possible explanations are discussed. The observations were found to be of insufficient resolution to unambiguously resolve which one would be the correct explanation. ... -
Bidirectional and Spectral Reflectance Properties of the Cladina stellaris and Flavocetraria nivalis lichens and the Racomitrium lanuginosum moss
(Doctoral thesis; Doktorgradsavhandling, 1998-06-04)In this thesis we present a scientific investigation of the spectral and bidirectional reflectance properties of two lichen species, <i>Flavocetraria nivalis</i> and <i>Cladina stellaris</i>, and one moss, <i>Racomitrium lanuginosum</i>. The spectral and bidirectional characterizations were based on high-quality laboratory measurements made in the European Goniometer facility, at the Joint Research ... -
Three photometric methods tested on ground-based data of Q 2237+0305
(Journal article; Tidsskriftartikkel; Peer reviewed, 1998-09-09)The Einstein Cross, Q 2237+0305, has been photometrically observed in four bands on two successive nights at NOT (La Palma, Spain) in October 1995. Three independent algorithms have been used to analyse the data: an automatic image decomposition technique, a CLEAN algorithm and the new MCS deconvolution code. The photometric and astrometric results obtained with the three methods are presented. ... -
Electron velocity distribution function in a plasma with temperature gradient and in the presence of suprathermal electrons: application to incoherent-scatter plasma lines.
(Journal article; Tidsskriftartikkel, 1998-10-31)The plasma dispersion function and the reduced velocity distribution function are calculated numerically for any arbitrary velocity distribution function with cylindrical symmetry along the magnetic field. The electron velocity distribution is separated into two distributions representing the distribution of the ambient electrons and the suprathermal electrons. The velocity distribution function of ... -
Studies of ionospheric parameters by means of electron plasma lines observed by EISCAT
(Doctoral thesis; Doktorgradsavhandling, 1998-12-18)This work presents a study of the electron plasma lines observed by the incoherent scatter radar EISCAT. The work is focusing on two parts. On one hand, the design of a plasma line experiment for the EISCAT system with an improved spatial resolution. On the other hand, the comparison of the plasma line data collected with the EISCAT radar with an improved model for the intensity and the Doppler ... -
ELF wave generation in the ionosphere using pulse modulated HF heating. Initial tests of a technique for increasing ELF wave generation efficiency.
(Journal article; Peer reviewed; Tidsskriftartikkel, 1999)This paper describes the results of a preliminary study to determine the effective heating and cooling time constants of ionospheric currents in a simulated modulated HF heating, `beam painting' configuration. It has been found that even and odd harmonics of the fundamental ELF wave used to amplitude modulate the HF heater are sourced from different regions of the ionosphere which support significantly ... -
Observations of plasma density structures in association with the passage of traveling convection vortices and the occurrence of large plasma jets.
(Journal article; Peer reviewed; Tidsskriftartikkel, 1999)We report important results of the first campaign specially designed to observe the formation and the initial convection of polar cap patches. The principal instrumentation used in the experiments comprised the EISCAT, the Sondrestrom, and the Super DARN network of radars. The experiment was conducted on February 18, 1996 and was complemented with additional sensors such as the Greenland chain of ... -
Effect of suprathermal electrons on the intensity and Doppler frequency of electron plasma lines.
(Journal article; Tidsskriftartikkel, 1999)In an incoherent scattering radar experiment, the spectral measurement of the so-called up- and downshifted electron plasma lines provides information about their intensity and their Doppler frequency. These two spectral lines correspond, in the backscatter geom- etry, to two Langmuir waves travelling towards and away from the radar. In the daytime ionosphere, the presence of a small percentage ... -
Authorization and access control in a distributed file repository
(Research report; Forskningsrapport, 1999-01-18)A distributed file repository is described. It supports interaction between different machines used by a single user, as well as between users that share data. Files can be replicated and consistency will be maintained, or files can be shipped (copied) to a remote site. As with more traditional systems, the servers are trusted not to leak information. However, the rôle servers play is not as much ... -
On the possibility of ion-drag to induce dynamic instability in the lower thermosphere neutral gas
(Journal article; Tidsskriftartikkel; Peer reviewed, 2000)Strong wind shears may result in dynamic instability, often characterised by the Richardson number lying between zero and 0.25. The extent to which electric-field driven ion flow may induce such neutral wind shears is examined. Further, it is proposed that, in the ionosphere, it is possible for electric fields to drive ion winds such that the collisionally induced neutral air response may be comparable ... -
Adding mobility to non-mobile web robots
(Research report; Forskningsrapport, 2000)In this paper we will show that it is possible to combine mobile agent technology with existing non-mobile data mining applications. The motivation for this is the advantage mobile agents offer in moving the computation closer to the data in a distributed system. This can save bandwidth and increase performance when the data is condensed as a result of data mining. -
MIDAS-W: a workstation-based incoherent scatter radar data aquisition system.
(Journal article; Tidsskriftartikkel, 2000)The Millstone Hill Incoherent Scatter Data Acquisition System (MIDAS) is based on an abstract model of an incoherent scatter radar. This model is implemented in a hierarchical software system, which serves to isolate hardware and low-level software implementation details from higher levels of the system. Inherent in this is the idea that implementation details can easily be changed in response to ... -
Irregular structures of the F layer at high latitudes during ionospheric heating.
(Journal article; Peer reviewed; Tidsskriftartikkel, 2000)Results on heating the ionospheric F region above Tromsø, Norway are presented. The ionosphere was monitored by satellite tomography and amplitude scintillation methods as well as the EISCAT incoherent scatter radar. No effect of heating was observed in the daytime. In the evening and in the pre-midnight sector, noticeable tilts of the F region were observed during heating periods. The tilts overlapped ... -
Comparison of the orientation of small-scale electron density irregularities and F region plasma flow direction.
(Journal article; Tidsskriftartikkel; Peer reviewed, 2000)Results are shown from an experimental campaign where satellite scintillation was observed at three sites at high latitudes and, simultaneously, the F region plasma flow was measured by the nearby EISCAT incoherent scatter radar. The anisotropy parameters of field-aligned irregularities are determined from amplitude scintillation using a method based on the variance of the relative logarithmic ... -
Plasma density over Svalbard during the ISBJØRN campaign.
(Journal article; Peer reviewed; Tidsskriftartikkel, 2000)In 1997, reliable operation of the EISCAT Svalbard Radar (ESR) was achieved and a rocket launching facility at Ny Ålesund on Svalbard (79°N, 12°E) (SVALRAK) was established. On 20 November, 1977, the first instrumented payload was launched from SVALRAK. Although the payload configuration had been flown many times previously from Andøya Rocket Range on the Norwegian mainland, this presented an ...
