Now showing items 118-137 of 393

    • F-region electron heating by X-mode radiowaves in underdense conditions 

      Gustavsson, Björn Johan; Löfås, H.; Ivchenko, N.; Rietveld, Michael T; Leyser, T. B. (Journal article; Tidsskriftartikkel; Peer reviewed, 2009)
    • Faraday rotation fluctuations of MESSENGER radio signals through the equatorial lower corona near solar minimum 

      Wexler, D.B.; Jensen, E.A.; Hollweg, J.V.; Heiles, C.; Efimov, A.I.; Vierinen, J.; Coster, A.J. (Journal article; Peer reviewed; Tidsskriftsartikkel, 2016-12-26)
      Faraday rotation (FR) of transcoronal radio transmissions from spacecraft near superior conjunction enables study of the temporal variations in coronal plasma density, velocity, and magnetic field. The MESSENGER spacecraft 8.4 GHz radio, transmitting through the corona with closest line-of-sight approach 1.63–1.89 solar radii and near-equatorial heliolatitudes, was recorded soon after the deep ...
    • First estimates of volume distribution of HF-pump enhanced emissions at 6300 and 5577 Å : a comparison between observations and theory 

      Gustavsson, Björn Johan; Kosch, Mike; Wong, Alfred; Pedersen, Todd; Heinselman, Craig; Mutiso, Charles; Bristow, Bill; Hughes, John; Wang, Weiyuan (Journal article; Tidsskriftartikkel; Peer reviewed, 2008)
      We present bi-static observations of radio-wave induced optical emissions at 6300 and 5577 Å from a night-time radio-induced optical emission ionospheric pumping experiment at the HIPAS (Fairbanks) facility in Alaska. The optical observations were made at HIPAS and from HAARP located 285 km south-east. From these observations the altitude distribution of the emissions is estimated with tomography-like ...
    • The First Magnetotelluric Image of the Lithospheric-Scale Geological Architecture in Central Svalbard, Arctic Norway 

      Beka, Thomas Ibsa; Smirnov, Maxim; Bergh, Steffen G; Birkelund, Yngve (Journal article; Tidsskriftartikkel; Peer reviewed, 2015-12-17)
      Magnetotelluric data, collected from 30 stations on Spitsbergen as part of a reconnaissance geothermal resource assessment along a profile with 0.53-km spacing in 0.0031000-s period range, were used to develop a lithospheric-scale two-dimensional (2D) resistivity model, heretofore unavailable for the region. Inverting the determinant of the impedance tensor in 2D, we found the smoothest model ...
    • First Observations of Recurring HF-Enhanced Topside Ion Line Spectra Near the Fourth Gyroharmonic 

      Rexer, Theresa; Gustavsson, Björn Johan; Leyser, Thomas; Rietveld, Michael T; Yeoman, Tim; Grydeland, Tom (Journal article; Tidsskriftartikkel; Peer reviewed, 2018-09-28)
      We present first incoherent scatter radar observations of systematically recurring, high‐frequency (HF)‐enhanced ion line spectra at the topside F‐region ionosphere, during magnetic field aligned HF pumping in an O‐mode polarization. The European Incoherent Scatter UHF radar was directed in magnetic zenith on 9–11 March 2016 while stepping the pump frequency across the double resonance of the fourth ...
    • First results from the Njord device for space related plasma experiments 

      Fredriksen, Åshild; Tribulato, Giulio; Watts, Christopher (Conference object; Konferansebidrag, 2007-08-05)
      The Njord device at Aurolab, University of Tromsø, has been constructed to carry out experiments on instabilities and heating in plasma flows and beams, with relevance to near-Earth space plasmas. For plasma production, Njord combines a 13.56 MHz RF source with a Double Plasma chamber equipped with filaments along the walls. The source is equipped dwith two coils operated at 0-6A, providing magnetic ...
    • First simultaneous measurements of waves generated at the bow shock in the solar wind, the magnetosphere and on the ground 

      Clausen, L. B. N.; Behlke, Rico; Yeoman, T. K.; Fear, R. C.; Lucek, E. A.; Engebretson, M. J. (Journal article; Tidsskriftartikkel; Peer reviewed, 2009)
    • First Simultaneous Rocket and Radar Detections of Rare Low Summer Mesospheric Clouds 

      Havnes, O.; Latteck, R.; Hartquist, T.W.; Antonsen, T. (Journal article; Peer reviewed, 2018-06-10)
      On 30 June 2016 a layer of dust, possibly meteoric smoke particles (MSPs), was observed with a rocket borne probe at 69.29°N, 16.02°E and altitudes of ~74 km where patchy thin cloud layers, detected with the Middle Atmosphere Alomar Radar System, were present. The rocket traversed a layer with a net positive dust charge density of ~107 unit charges per cubic meters and a number density of neutral ...
    • First wind shear observation in PMSE with the tristatic EISCAT VHF radar 

      Mann, Ingrid; Häggström, I.; Tjulin, A; Rostami, S; Anyairo, CC; Dalin, P (Journal article; Tidsskriftartikkel; Peer reviewed, 2016-11-05)
      The Polar Summer Mesosphere has the lowest temperatures that occur in the entire Earth system. Water ice particles below the optically observable size range participate there in the formation of strong radar echoes (Polar Mesospheric Summer Echoes, PMSE). To study PMSE we carried out observations with the European Incoherent Scatter (EISCAT) VHF and EISCAT UHF radar simultaneously from a site near ...
    • Fluctuation statistics in the scrape-off layer of Alcator C-Mod 

      Kube, Ralph; Theodorsen, Audun; Garcia, Odd Erik; LaBombard, Brian; Terry, James L. (Peer reviewed, 2016-03-30)
      We study long time series of the ion saturation current and floating potential, sampled by Langmuir probes dwelled in the outboard mid-plane scrape-off layer and embedded in the lower divertor baffle of Alcator C-Mod. A series of ohmically heated L-mode plasma discharges is investigated with line-averaged plasma density ranging from ${{\bar{n}}_{\text{e}}}/{{n}_{\text{G}}}=0.15$ to 0.42, where n G ...
    • Fluctuations in electron cyclotron resonance plasma in a divergent magnetic field 

      Bhattacharjee, Sudeep; Fredriksen, Åshild; Chandra, Sayan (Journal article; Tidsskriftartikkel; Peer reviewed, 2016-02)
      The dependence of fluctuations on electron-neutral collision frequency (V<sub>en</sub>) and the radial location is investigated in an electron cyclotron resonance plasma in a divergent magnetic field region for a set of magnetic fields. Results indicate that the fluctuations depend strongly on the collision frequency. At lower magnetic fields and V<sub>en</sub>, the fluctuation levels are small and ...
    • A Framework for Mellin Kind Series Expansion Methods 

      Brenn, Torgeir; Anfinsen, Stian Normann (Preprint; Manuskript, 2017-07-31)
      Mellin kind statistics (MKS) is the framework which arises if the Fourier transform is replaced with the Mellin transform when computing the characteristic function from the probability density function. We may then proceed to retrieve logarithmic moments and cumulants, that have important applications in the analysis of heavy-tailed distribution models for nonnegative random variables. In this paper ...
    • Frequencies of wave packets of whistler-mode chorus inside its source region : a case study 

      Santolik, O.; Kozelov, Boris V; Macusova, E.; Titova, E. E.; Gurnett, D.; Pickett, J. S.; Trakhtengerts, V. Y.; Demekhov, A. G. (Journal article; Tidsskriftartikkel; Peer reviewed, 2008)
    • Fysikk i den polare atmosfæren og nordlyset 

      Brekke, Asgeir (Research report; Forskningsrapport, 2008)
      Den polare atmosfæren har fått mer oppmerksomhet i den senere tid enn den vanligvis har vært forunt. Dette henger sammen med tanken om at den er så ren og upåvirket av eventuelle forurensninger som vi pumper ut i atmosfæren i de mer urbane strøk. Dersom atmosfæren tar skade av denne aktiviteten, forventer en å se skadevirkningene først og klarest i den polare atmosfæren. Svekkelsen av ozonlaget ...
    • Gaussian Markov random field priors in ionospheric 3D multi-instrument tomography 

      Norberg, J.; Vierinen, Juha; Roininen, L; Orispää, M.; Kauristie, K; Rideout, W.; Coster, A. J.; Lehtinen, M (Journal article; Tidsskriftartikkel; Peer reviewed, 2018-08-22)
      In ionospheric tomography, the atmospheric electron density is reconstructed from different electron density related measurements, most often from ground-based measurements of satellite signals. Typically, ionospheric tomography suffers from two major complications. First, the information provided by measurements is insufficient and additional information is required to obtain a unique solution. ...
    • Gaussian Process Sensitivity Analysis for Oceanic Chlorophyll Estimation 

      Blix, Katalin; Camps-Valls, Gustau; Jenssen, Robert (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-01-04)
      Gaussian process regression (GPR) has experienced tremendous success in biophysical parameter retrieval in the past years. The GPR provides a full posterior predictive distribution so one can derive mean and variance predictive estimates, i.e., point-wise predictions and associated confidence intervals. GPR typically uses translation invariant covariances that make the prediction function very ...
    • Generation of the lower-thermospheric vertical wind estimated with the EISCAT KST radar at high latitudes during periods of moderate geomagnetic disturbance 

      Brekke, Asgeir; Oyama, S; Watkins, B. J.; Maeda, S.; Shinagawa, H.; Nozawa, S.; Ogawa, Y.; Lathuillere, C.; Kofman, W. (Journal article; Tidsskriftartikkel; Peer reviewed, 2008-06-11)
      Lower-thermospheric winds at high latitudes during moderately-disturbed geomagnetic conditions were studied using data obtained with the European Incoherent Scatter (EISCAT) Kiruna-Sodankylä-Tromsø (KST) ultrahigh frequency (UHF) radar system on 9–10 September 2004. The antenna-beam configuration was newly designed to minimize the estimated measurement error of the vertical neutralwind speed in the ...
    • Geomagnetic control of polar mesosphere summer echoes. 

      Bremer, J.; Hansen, T.L.; Hoffmann, P. (Journal article; Tidsskriftartikkel, 2000)
      Using observations with the ALOMAR SOUSY radar near Andenes (69.3°N, 16.0°E) from 1994 until 1997 polar mesosphere summer echoes (PMSE) have been investigated in dependence on geomagnetic K indices derived at the Auroral Observa- tory Tromsù (69.66°N, 18.94°E). During night-time and morning hours a significant correlation between the signal-to-noise ratio (SNR) of the radar results and ...
    • GNSS Observations of Ionospheric Variations During the 21 August 2017 Solar Eclipse 

      Coster, A. J.; Goncharenko, Larisa; Zhang, Shun-Rong; Erickson, P. J.; Rideout, William; Vierinen, Juha (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-11-20)
      On 21 August 2017, during daytime hours, a total solar eclipse with a narrow ∼160 km wide umbral shadow occurred across the continental United States. Totality was observed from the Oregon coast at ∼9:15 local standard time (LST) (17:20 UT) to the South Carolina coast at ∼13:27 LST (18:47 UT). A dense network of Global Navigation Satellite Systems (GNSS) receivers was utilized to produce total ...
    • High frequency copolymer ultrasonic transducer array of size-effective elements 

      Decharat, Adit; Wagle, Sanat; Habib, Anowarul; Jacobsen, Svein Ketil; Melandsø, Frank (Journal article; Tidsskriftartikkel; Peer reviewed, 2018-01-05)
      A layer-by-layer deposition method for producing dual-layer ultrasonic transducers from piezoelectric copolymers has been developed. The method uses a combination of customized and standard processing to obtain 2D array transducers with electrical connection of the individual elements routed directly to the rear of the substrate. A numerical model was implemented to study basic parameters effecting ...