Now showing items 263-282 of 393

    • Particle precipitation during NEIAL events : simultaneous ground based nighttime observations at Svalbard 

      Lunde, June; Løvhaug, Unni Pia; Gustavsson, Björn Johan (Journal article; Tidsskriftartikkel; Peer reviewed, 2009)
    • Particle precipitations during NEIAL events : simultaneous ground based observations at Svalbard 

      Lorentzen, Dag Arne; Lunde, June; Gustavsson, B; Ogawa, Y; Løvhaug, U.P.; Ogawa, Yasunobu (Journal article; Tidsskriftartikkel; Peer reviewed, 2007)
      In this paper we present Naturally Enhanced Ion Acoustic Lines (NEIALs) observed with the EISCAT Svalbard Radar (ESR). For the first time, long sequences of NEIALs are recorded, with more than 50 events within an hour, ranging from 6.4 to 140 s in duration. The events took place from ~08:45 to 10:00 UT, 22 January 2004. We combine ESR data with observations of optical aurora by a meridian scanning ...
    • Phase calibration of the EISCAT Svalbard Radar interferometer using optical satellite signatures. 

      Grydeland, Tom; Sullivan, J.M.; Blixt, Erik Mårten; Ivchenko, N.; Lockwood, M.; Lanchester, B.S. (Journal article; Peer reviewed; Tidsskriftartikkel, 2006-09-20)
      The link between natural ion-line enhancements in radar spectra and auroral activity has been the subject of recent studies but conclusions have been limited by the spatial and temporal resolution previously available. The next challenge is to use shorter sub-second integration times in combination with interferometric programmes to resolve spatial structure within the main radar beam, and so ...
    • Phenomena induced by powerful HF pumping towards magnetic zenith with a frequency near the F-region critical frequency and the third electron gyro harmonic frequency 

      Brekke, Asgeir; Blagoveshchenskaya, N. F.; Carlson, H. C.; Kornienko, V. A.; Borisova, T. D.; Rietveld, M. T.; Yeoman, T. K. (Journal article; Tidsskriftartikkel; Peer reviewed, 2009-01-08)
      Multi-instrument observational data from an experiment on 13 October 2006 at the EISCAT/HEATING facility at Tromsø, Norway are analysed. The experiment was carried out in the evening hours when the electron density in the F-region dropped, and the HF pump frequency fH was near and then above the critical frequency of the F2 layer. The distinctive feature of this experiment is that the pump frequency ...
    • Plasma density over Svalbard during the ISBJØRN campaign. 

      Hall, Chris; Van Eyken, A.P.; Svenes, K.R. (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 ...
    • Polarimetric SAR Change Detection with the Complex Hotelling-Lawley Trace Statistic 

      Akbari, Vahid; Anfinsen, Stian Normann; Doulgeris, Anthony Paul; Eltoft, Torbjørn; Moser, Gabriele; Serpico, Sebastian Bruno (Journal article; Tidsskriftartikkel; Peer reviewed, 2016-03-15)
      In this paper, we propose a new test statistic for unsupervised change detection in polarimetric radar images. We work with multilook complex covariance matrix data, whose underlying model is assumed to be the scaled complex Wishart distribution. We use the complex-kind Hotelling-Lawley trace statistic for measuring the similarity of two covariance matrices. The distribution of the Hotelling-Lawley ...
    • Poleward energy transport: is the standard definition physically relevant at all time scales? 

      Liang, Minyi; Czaja, Arnaud; Graversen, Rune; Tailleux, Remi (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-05-17)
      Poleward energy transport in the atmosphere and oceans constitutes an important branch of the global energy budget, and its role in the climate system has been the subject of many studies. In the atmosphere, the transport is affected by “eddies” and large scale meridional cells, both with zero net mass transport across latitude circles, but also partly by processes associated with a net transport ...
    • Power law spectra and intermittent fluctuations due to uncorrelated Lorentzian pulses 

      Garcia, Odd Erik; Theodorsen, Audun (Peer reviewed, 2017-02)
      A stochastic model for intermittent fluctuations due to a super-position of uncorrelated Lorentzian pulses is presented. For a constant pulse duration, this is shown to result in an exponential power spectral density for the stationary process. A random distribution of pulse durations modifies the frequency spectrum, and several examples are shown to result in power law spectra. The distribution of ...
    • Preliminary Assessment of Remote Wind Sites 

      Bilal, Muhammad; Araya, Guillermo; Birkelund, Yngve (Journal article; Tidsskriftartikkel; Peer reviewed, 2015-08)
      Wind energy is becoming a reliable and affordable source of clean energy and is rapidly expanding to remote places around the world. A crucial input for wind farming prospect is the assessment of potential wind sites. Sites, especially remotely located, often do not have a wind resource map and thus lack credible historical records of wind resources. Measurement campaigns to map these sites are ...
    • Probability distribution functions for intermittent scrape-off layer plasma fluctuations 

      Theodorsen, Audun; Garcia, Odd Erik (Journal article; Peer reviewed; Tidsskriftartikkel, 2018-01-24)
      A stochastic model for intermittent fluctuations in the scrape-off layer of magnetically confined plasmas has been constructed based on a super-position of uncorrelated pulses arriving according to a Poisson process. In the most common applications of the model, the pulse amplitudes are assumed exponentially distributed, supported by conditional averaging of large-amplitude fluctuations in experimental ...
    • A procedure to correct the effects of a relative delay between the quadrature components of radar signals at base band. 

      Westman, A.; Grydeland, Tom; Belyey, Vasyl; La Hoz, Cesar (Journal article; Peer reviewed; Tidsskriftartikkel, 2005-01-31)
      The real and imaginary parts of baseband signals are obtained from a real narrow–band signal by quadrature mixing, i.e. by mixing with cosine and sine signals at the narrow band’s selected center frequency. We address the consequences of a delay between the outputs of the quadrature mixer, which arise when digital samples of the quadrature baseband signals are not synchronised, i.e. when ...
    • Pulsating aurora and cosmic noise absorption associated with growth-phase arcs 

      McKay, Derek; Partamies, Noora; Vierinen, Juha (Journal article; Tidsskriftartikkel; Peer reviewed, 2018-01-18)
      The initial stage of a magnetospheric substorm is the growth phase, which typically lasts 1–2 h. During the growth phase, an equatorward moving, east–west extended, optical auroral arc is observed. This is called a growth-phase arc. This work aims to characterize the optical emission and riometer absorption signatures associated with growth-phase arcs of isolated substorms. This is done using ...
    • The pulsation modes of the pre-white dwarf PG 1159-035 

      Costa, J. E. S.; Solheim, Jan Erik; Kepler, S. O.; Winget, D. E.; O’Brien, M. S.; Kawaler, S. D.; Costa, A. F. M.; Giovannini, O.; Kanaan, A.; Mukadam, A. S.; Mullally, F.; Nitta, A.; Provenc, J. L.; Shipman, H.; Wood, M. A.; Ahrens, T. J.; Grauer, A.; Kilic, M.; Bradley, P. A.; Sekiguchi, K.; Crowe, R.; Jiang, X. J.; Sullivan, D.; Sullivan, T.; Rosen, R.; Clemens, J. C.; Janulis, R.; O’Donoghue, D.; Ogloza, W.; Baran, A.; Silvotti, R.; Marinoni, S.; Vauclair, G.; Dolez, N.; Chevreton, M.; Dreizler, S.; Schuh, S.; Deetjen, J.; Nagel, T.; Gonzalez Perez, J. M.; Ulla, A.; Barstow, Martin; Burleigh, M.; Good, S.; Metcalfe, T.S.; Kim, S.-L.; Lee, H.; Sergeev, A.; Akan, M.C.; Çakırlı, Ö.; Paparo, M.; Viraghalmy, G.; Ashoka, B. N.; Handler, G.; Hürkal, Özlem; Johannessen, F.; Kleinman, S. J.; Kalytis, R.; Krzesinski, J.; Klumpe, E.; Larrison, J.; Lawrence, T.; Meištas, E.; Martinez, P.; Nather, R. E.; Fu, J.-N.; Pakštienė, E.; Rosen, R.; Romero-Colmenero, E.; Riddle, R.; Seetha, S.; Silvestri, N. M.; Vučković, M.; Warner, B.; Zola, S.; Althaus, L. G.; Córsico, A. H.; Montgomery, M. H. (Journal article; Tidsskriftartikkel; Peer reviewed, 2008)
      Context. PG 1159-035, a pre-white dwarf with Teff ≃ 140 000 K, is the prototype of both two classes: the PG 1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035 photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. Analyzing the periods of pulsation, it is possible to ...
    • Quad-Polarimetric SAR for Detection and Characterization of Icebergs 

      Akbari, Vahid; Brekke, Camilla; Doulgeris, Anthony Paul; Storvold, Rune; Sivertsen, Agnar H. (Conference object; Konferansebidrag, 2016)
      This paper evaluates the performance of fully polarimetric SAR data in iceberg detection and characterization. The study aims to explore the potential of RADARSAT-2 SAR data to detect icebergs and growlers in Svalbard that have broken off from the glaciers nearby. To be able to detect iceberg/growlers in a SAR image, a significant contrast between iceberg and background clutter is required. The ...
    • Quantitative phase microscopy of red blood cells during planar trapping and propulsion 

      Ahmad, Azeem; Dubey, Vishesh; Singh, Vijay; Tinguely, Jean-Claude; Øie, Cristina Ionica; Wolfson, Deanna; Mehta, Dalip Singh; So, Peter T. C.; Ahluwalia, Balpreet Singh (Journal article; Tidsskriftartikkel; Peer reviewed, 2018-08-22)
      Red blood cells (RBCs) have the ability to undergo morphological deformations during microcirculation, such as changes in surface area, volume and sphericity. Optical waveguide trapping is suitable for trapping, propelling and deforming large cell populations along the length of the waveguide. Bright field microscopy employed with waveguide trapping does not provide quantitative information about ...
    • Radar observations of thermal plasma oscillations in the ionosphere 

      Vierinen, Juha; Gustavsson, Björn Johan; Hysell, David; Sulzer, M.; Perillat, Phil; Kudeki, Erhan (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-06-02)
      Incoherent scatter radar observations of ionospheric plasmas rely on echoes from electron density fluctuations with properties governed by the dispersion relations for ion acoustic and Langmuir waves. Radar observations of echoes associated with Langmuir waves (plasma lines) from thermal plasma are weak, and only a few near‐thermal level measurements have been reported. Plasma line echoes are typically ...
    • Radar studies of ionospheric dusty plasma phenomena 

      Mann, Ingrid; Gunnarsdottir, Tinna; Häggström, Ingemar; Eren, Saliha; Tjulin, Anders; Myrvang, Margaretha; Rietveld, Mike T; Dalin, Peter; Jozwicki, Dorota; Trollvik, Henriette Marie Tveitnes (Journal article; Tidsskriftartikkel; Peer reviewed, 2019-06-17)
      We discuss the influence of charged dust on radar observations in the Earth ionosphere. This region in the upper Earth atmosphere can be described as a partially ionized, low‐temperature plasma. Plasma parameters vary by orders of magnitude spatially and in time. Dust particles influence the charge balance, in some cases dusty plasma condition is met. The polar mesospheric echoes are an example of ...
    • Radiometric temperature reading of a hot ellipsoidal object inside the oral cavity by a shielded microwave antenna put flush to the cheek 

      Klemetsen, Øystein; Jacobsen, Svein Ketil; Birkelund, Yngve (Journal article; Tidsskriftartikkel; Peer reviewed, 2012)
      A new scheme for detection of vesicoureteral reflux (VUR) in children has recently been proposed in the literature. The idea is to warm bladder urine via microwave exposure to at least fever temperatures and observe potential urine reflux from the bladder back to the kidney(s) by medical radiometry. As a preliminary step toward realization of this detection device, we present non-invasive temperature ...
    • Range-Doppler Mapping of Space-Based Targets Using the JRO 50 MHz Radar 

      Kesaraju, Saiveena; Mathews, J. D.; Milla, M.; Vierinen, Juha (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-12-02)
      The Jicamarca Radio Observatory (JRO) two-dimensional square array radar system operating at ~ 6-m wavelength was used to study the Moon and low Earth orbit satellites using the Range-Doppler inverse synthetic aperture radar technique also known as Delay-Doppler imaging. The radar data was collected on Oct 21, 2015. A circularly polarized coded pulse was transmitted from a quarter-array antenna ...
    • Ranking Using Transition Probabilities Learned from Multi-Attribute Data 

      Løkse, Sigurd; Jenssen, Robert (Journal article; Tidsskriftartikkel; Peer reviewed, 2018-09-13)
      In this paper, as a novel approach, we learn Markov chain transition probabilities for ranking of multi-attribute data from the inherent structures in the data itself. The procedure is inspired by consensus clustering and exploits a suitable form of the PageRank algorithm. This is very much in the spirit of the original PageRank utilizing the hyperlink structure to learn such probabilities. ...