Now showing items 28-47 of 393

    • Can we extend local sea-ice measurements to satellite scale? An example from the N-ICE2015 expedition 

      Rösel, Anja; King, Jennifer; Doulgeris, Anthony P.; Wagner, Penelope M.; Johansson, Malin; Gerland, Sebastian (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-12-17)
      Knowledge of Arctic sea-ice conditions is of great interest for Arctic residents, as well as for commercial usage, and to study the effects of climate change. Information gained from analysis of satellite data contributes to this understanding. In the course of using in situ data in combination with remotely sensed data, the question of how representative local scale measurements are of a wider ...
    • Capillary suction model as pipes of different sizes: flow conditions and comparison with experiments 

      Nguyen, Hung Thanh; Jacobsen, Stefan; Melandsø, Frank (Conference object; Konferansebidrag, 2012)
      ABSTRACT Absorption of water in concrete is often described by the simple linear water uptake vs. square-root-of-time law. However, a deviation from this behaviour is frequently seen depending on factors such as initial water content, water/binder ratio and specimen thickness. The deviation increases with thickness (typically from 25 to 100 mm) and is seen even for very dry specimens with ...
    • Carbon Nanotube Spectrally Selective Solar Absorbers 

      Chen, Zhonghua; Nguyen, Quang Hong; Boström, Tobias (Journal article; Tidsskriftartikkel; Peer reviewed, 2015)
      The use of multi-walled carbon nanotubes (MWCNTs) as spectrally selective solar thermal absorbers (SSA) in solar thermal collectors is presented in this paper. CNT layers were coated by electrophoretic deposition (EPD) on aluminum substrates. Stable aqueous CNT suspensions consisted of multi-walled CNTs, DI water and surfactant was used for EPD. The deposited samples were heat treated in a tube ...
    • A case study on generation mechanisms of a sporadic sodium layer above Tromsø (69.6° N) during a night of high auroral activity 

      Takahashi, Toru; Nozawa, Satonaori; Tsuda, Takuo T.; Ogawa, Yasunobu; Saito, Norihito; Hidemori, Takehiro; Kawahara, Takuya D.; Hall, Chris; Fujiwara, Hitoshi; Matuura, Nobuo; Brekke, Asgeir; Tsutsumi, Masaki; Wada, Satoshi; Kawabata, Tetsuya; Oyama, Shinichiro; Fujii, Ryoichi (Journal article; Tidsskriftartikkel; Peer reviewed, 2015-08-05)
      We have quantitatively evaluated generation mechanisms of a sporadic sodium layer (SSL) based on observational data obtained by multiple instruments at a high-latitude station: Ramfjordmoen, Tromsø, Norway (69.6° N, 19.2° E). The sodium lidar observed an SSL at 21:18 UT on 22 January 2012. The SSL was observed for 18 min, with a maximum sodium density of about 1.9 × 1010 m−3 at 93 km with a 1.1 km ...
    • CFD-DEM and Experimental Study of Bubbling in a Fluidized Bed 

      Khawaja, Hassan Abbas (Journal article; Tidsskriftartikkel; Peer reviewed, 2015-12)
      In this study, phenomenon of bubbling is investigated using CFD-DEM and experiments. A CFD-DEM simulation is setup to model the fluidized beds of different sizes. Geldart D particles of 1.2 mm diameter and 1000 Kg/m3 density are modelled. Study revealed different types of fluidization regimes as stated in the literature. An experimental setup is built to obtain the results for the comparison. ...
    • Challenges and Strategic Research Plans for Earth and Heliosphere: Research Infrastructures, Projects and Initiatives 

      Mann, Ingrid; Kauristie, Kirsti; Bamford, Ruth; McCrea, Ian; Moen, Jøran Idar; Oksavik, Kjellmar; Yamauchi, Masanori; Johnsen, Magnar Gullikstad; Turunen, Esa (Journal article; Tidsskriftartikkel; Peer reviewed, 2018-07-24)
      We describe existing research infrastructures relevant for space weather and open issues of space weather research including the need for sustainable observation networks and for high-quality data products as basis for model development. The local relevance in Europe for studies of the ionosphere at high latitude is described. We propose as possible a way forward to sustain space weather research ...
    • A change detector for polarimetric SAR data based on the relaxed Wishart distribution 

      Akbari, Vahid; Anfinsen, Stian Normann; Doulgeris, Anthony Paul; Eltoft, Torbjørn (Journal article; Tidsskriftartikkel; Peer reviewed, 2015-11-12)
      In this paper, we present an unsupervised change detection method for polarimetric synthetic aperture radar (PolSAR) images based on the relaxed Wishart distribution. Most polarimetric change detectors assume the Gaussian-based complex Wishart model for multilook covariance matrices, which is only satisfied for homogeneous areas with fully developed speckle and no texture. Liu et al. recently proposed ...
    • Characterization and discrimination of evolving mineral and plant oil slicks based on L-band synthetic aperture radar (SAR) 

      Jones, Cathleen; Espeseth, Martine Mostervik; Holt, Benjamin; Brekke, Camilla; Skrunes, Stine (Peer reviewed, 2016-10-18)
      Evolution of the damping ratio for Bragg wavenumbers in the range 32-43 rad/m is evaluated for oil slicks of different composition released in the open ocean and allowed to develop naturally. The study uses quad-polarimetric L-band airborne synthetic aperture radar data acquired over three mineral oil emulsion releases of different, known oil-to-water ratio, and a near-coincident release of 2-ethylhexyl ...
    • Characterization of a waveguide Mach-Zehnder interferometer using PDMS as a cover layer 

      Lindecrantz, Susan Mariah; Tinguely, Jean-Claude; Ahluwalia, Balpreet Singh; Hellesø, Olav Gaute (Journal article; Tidsskriftartikkel; Peer reviewed, 2015)
      A Mach-Zehnder interferometer made with shallow rib waveguides is studied experimentally and using simulations. The rib-height giving single-mode guidance is found as function of core thickness and polarization. Devices have been made using shallow rib waveguides (5 nm rib height) in silicon nitride. The sensitivity and the limit of detection (LOD) is studied experimentally regarding the length of ...
    • Characterization of color cross-talk of CCD detectors and its influence in multispectral quantitative phase imaging 

      Ahmad, Azeem; Kumar, Anand; Dubey, Vishesh Kumar; Butola, Ankit; Ahluwalia, Balpreet Singh; Mehta, Dalip Singh (Journal article; Tidsskriftartikkel; Peer reviewed, 2019-02-08)
      Multi-spectral quantitative phase imaging (QPI) is an emerging imaging modality for wavelength dependent studies of several biological and industrial specimens. Simultaneous multi-spectral QPI is generally performed with color CCD cameras. Here, we present a new approach for accurately measuring the color crosstalk of 2D area detectors, without needing prior information about camera specifications. ...
    • Chip Based Nanoscopy: Towards Integration and High-throughput Imaging 

      Ahluwalia, Balpreet Singh; Coucheron, David Andre; Helle, Øystein Ivar; Øie, Cristina Ionica; Dullo, Firehun Tsige (Journal article; Tidsskriftartikkel; Peer reviewed, 2017)
      Super-resolution optical microscopy, commonly referred to as optical nanoscopy, has enabled imaging of biological samples with a resolution that was only achievable previously using electron microscopy. Optical nanoscopy is a rapidly growing field, with several different techniques and implementations that overcome the diffraction limit of light. However, the common nanoscope continues to be a rather ...
    • Chip-based optical microscopy for imaging membrane sieve plates of liver scavenger cells 

      Helle, Øystein Ivar; Øie, Cristina Ionica; McCourt, Peter Anthony; Ahluwalia, Balpreet Singh (Journal article; Tidsskriftartikkel; Peer reviewed, 2015-08-26)
      The evanescent field on top of optical waveguides is used to image membrane network and sieve-plates of liver endothelial cells. In waveguide excitation, the evanescent field is dominant only near the surface (~100-150 nm) providing a default optical sectioning by illuminating fluorophores in close proximity to the surface and thus benefiting higher signal-to-noise ratio. The sieve plates of liver ...
    • Chip-based wide field-of-view nanoscopy 

      Ahluwalia, Balpreet Singh; Helle, Øystein Ivar; Diekmann, Robin; Øie, Cristina Ionica; McCourt, Peter A. G.; Schuttpelz, Mark (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-04-24)
      Present optical nanoscopy techniques use a complex microscope for imaging and a simple glass slide to hold the sample. Here, we demonstrate the inverse: the use of a complex, but mass-producible optical chip, which hosts the sample and provides a waveguide for the illumination source, and a standard low-cost microscope to acquire super-resolved images via two different approaches. Waveguides composed ...
    • Chirp coded ultrasonic pulses used for scanning acoustic microscopy 

      Habib, Anowarul; Melandsø, Frank (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-11-02)
      In the present study, chirp coded pulses are investigated with respect to high frequency imaging. These pulses were used drive broadband polymer transducers assembled from an adhesive-free layer-be-layer deposition method. The image quality obtained from using a long chirp coded pulse was compared with images generated from shorter pulses. This includes the Ricker wavelet and the square pulse most ...
    • Classification of low backscatter ocean regions using log-cumulants 

      Skrunes, Stine; Brekke, Camilla (Conference object; Konferansebidrag, 2015-01)
      In a synthetic aperture radar image, low backscatter regions of various origin can be observed in ocean areas. Operational oil spill detection services work to discriminate anthropogenic oil spills from natural phenomena such as seeps, low wind fields, thin ice and biogenic slicks. In this paper, we investigate the potential of using matrix log-cumulants for this purpose.
    • Classification of Micro-Damage in Piezoelectric Ceramics Using Machine Learning of Ultrasound Signals 

      Tripathi, Gaurav; Anowarul, Habib; Agarwal, Krishna; Prasad, Dilip K. (Journal article; Peer reviewed, 2019-09-28)
      Ultrasound based structural health monitoring of piezoelectric material is challenging if a damage changes at a microscale over time. Classifying geometrically similar damages with a difference in diameter as small as 100 m is difficult using conventional sensing and signal analysis approaches. Here, we use an unconventional ultrasound sensing approach that collects information of the entire ...
    • Climatic trends in E-region critical frequency and virtual height above Tromsø (70 degrees N, 10 degrees E) 

      Cannon, P. S.; Hall, Chris; Brekke, Asgeir (Journal article; Tidsskriftartikkel; Peer reviewed, 2007-11-29)
      We have examined the long time series of observations of E-region virtual height (1948–2006) and critical frequency (1935-2006) hitherto made by the Tromsø ionosonde at 70 degrees N, 19 degrees E. Combining a simplistic trend analysis with a rigorous treatment of errors we identify a negative trend in critical frequency. While a similar analysis of the virtual height h'E also suggests a negative ...
    • Combination of large and small basis sets in electronic structure calculations on large systems 

      Røeggen, Inge Arvid; Gao, Bin (Journal article; Tidsskriftartikkel; Peer reviewed, 2018-04)
      Two basis sets-a large and a small one-are associated with each nucleus of the system. Each atom has its own separate one-electron basis comprising the large basis set of the atom in question and the small basis sets for the partner atoms in the complex. The perturbed atoms in molecules and solids model is at core of the approach since it allows for the definition of perturbed atoms in a system. It ...
    • Comets as a possible source of nanodust in the Solar System cloud and in planetary debris discs 

      Mann, Ingrid (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-05-29)
      Comets, comet-like objects and their fragments are the most plausible source for the dust in both the inner heliosphere and planetary debris discs around other stars. The smallest size of dust particles in debris discs is not known and recent observational results suggest that the size distribution of the dust extends down to sizes of a few nanometres or a few tens of nanometres. In the Solar System, ...
    • A comparative study of sea clutter covariance matrix estimators 

      Ding, Tao; Anfinsen, Stian Normann; Brekke, Camilla (Journal article; Tidsskriftartikkel; Peer reviewed, 2013-10-23)
      Estimation of the polarimetric covariance matrix is an important task in statistical modeling of sea clutter for maritime applications of polarimetric synthetic aperture radar (PolSAR) data. This work provides a comprehensive study of four covariance matrix estimators: the maximum likelihood estimators under the Gaussian distribution (G-ML) and the K distribution (K-ML), an approximation of the ...