Blar i forfatter "Gulbrandsen, Njål"

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    • Diagnostics of Ion Beam and Current Free Double Layer in Helicon Plasma Devices with Expanding Magnetic Field 

      Gulbrandsen, Njål (Doctoral thesis; Doktorgradsavhandling, 2017-09-18)
      The main focus of this thesis is the diagnostics of ion beams in helicon plasma devices with expanding magnetic field, and running in an inductive mode. These ion beams are closely connected to the concept of current-free double layers (CFDL). We started by investigating how to best interpret the measurements from a Retarding Field Energy Analyzer (RFEA) through 3-dimensional particle-in-cell ...

    • Identifying gravity waves launched by the Hunga Tonga-Hunga Ha′apai volcanic eruption in mesosphere/lower-Thermosphere winds derived from CONDOR and the Nordic Meteor Radar Cluster 

      Stober, Gunter; Liu, Alan; Kozlovsky, Alexander; Qiao, Zishun; Krochin, Witali; Shi, Guochun; Kero, Johan; Tsutsumi, Masaki; Gulbrandsen, Njål; Nozawa, Satonori; Lester, Mark; Baumgarten, Kathrin; Belova, Evgenia; Mitchell, Nicholas (Journal article; Tidsskriftartikkel; Peer reviewed, 2023-04-18)
      The Hunga Tonga–Hunga Ha'apai volcano eruption was a unique event that caused many atmospheric phenomena around the globe. In this study, we investigate the atmospheric gravity waves in the mesosphere/lowerthermosphere (MLT) launched by the volcanic explosion in the Pacific, leveraging multistatic meteor radar observations from the Chilean Observation Network De Meteor Radars (CONDOR) and the ...

    • Impact of sudden stratospheric warmings on the neutral density, temperature and wind in the MLT region 

      Zhou, Baozhu; Yi, Wen; Xue, Xianghui; Ye, Hailun; Zeng, Jie; Li, Guozhu; Tsutsumi, Masaki; Gulbrandsen, Njål; Chen, Tingdi; Dou, Xiankang (Journal article; Tidsskriftartikkel; Peer reviewed, 2023-05-04)
      In this study, the neutral density and horizontal wind observed by the four meteor radars, as well as the temperature measured by the Microwave Limb Sounder (MLS) onboard the Aura satellite are used to examine the response of neutral density, wind, and temperature in the MLT region to the stratospheric sudden warmings (SSWs) during 2005 to 2021 in the Northern Hemisphere. The four meteor radars ...

    • Inferring neutral winds in the ionospheric transition region from atmospheric-gravity-wave traveling-ionospheric-disturbance (AGW-TID) observations with the EISCAT VHF radar and the Nordic Meteor Radar Cluster 

      Günzkofer, Florian; Pokhotelov, Dimitry; Stober, Gunter; Mann, Ingrid Brigitte; Vadas, Sharon L.; Becker, Erich; Tjulin, Anders; Kozlovsky, Alexander; Tsutsumi, Masaki; Gulbrandsen, Njål; Nozawa, Satonori; Lester, Mark; Belova, Evgenia; Kero, Johan; Mitchell, Nicholas J.; Borries, Claudia (Journal article; Tidsskriftartikkel; Peer reviewed, 2023-10-18)
      Atmospheric gravity waves and traveling ionospheric disturbances can be observed in the neutral atmosphere and the ionosphere at a wide range of spatial and temporal scales. Especially at medium scales, these oscillations are often not resolved in general circulation models and are parameterized. We show that ionospheric disturbances forced by upward-propagating atmospheric gravity waves can be ...

    • Long-Term studies of the summer wind in the mesosphere and lower thermosphere at middle and high latitudes over Europe 

      Jaen, Juliana; Renkwitz, Toralf; Liu, Huixin; Jacobi, Christoph; Wing, Robin; KuchaÅ™, Aleš; Tsutsumi, Masaki; Gulbrandsen, Njål; Chau, Jorge L. (Journal article; Tidsskriftartikkel; Peer reviewed, 2023-12-01)
      Continuous wind measurements using partial-reflection radars and specular meteor radars have been carried out for nearly 2 decades (2004–2022) at middle and high latitudes over Germany (∼ 54<sup>∘</sup> N) and northern Norway (∼ 69<sup>∘</sup> N), respectively. They provide crucial data for understanding the long-term behavior of winds in the mesosphere and lower thermosphere. Our investigation ...

    • Meteor radar vertical wind observation biases and mathematical debiasing strategies including the 3DVAR+DIV algorithm 

      Stober, Gunter; Liu, Alan; Kozlovsky, Alexander; Qiao, Zishun; Kuchar, Ales; Jacobi, Christoph; Meek, Chris; Janches, Diego; Liu, Guiping; Tsutsumi, Masaki; Gulbrandsen, Njål; Nozawa, Satonori; Lester, Mark; Belova, Evgenia; Kero, Johan; Mitchell, Nicholas (Journal article; Tidsskriftartikkel; Peer reviewed, 2022-10-13)
      Meteor radars have become widely used instruments to study atmospheric dynamics, particularly in the 70 to 110 km altitude region. These systems have been proven to provide reliable and continuous measurements of horizontal winds in the mesosphere and lower thermosphere. Recently, there have been many attempts to utilize specular and/or transverse scatter meteor measurements to estimate vertical ...

    • On the measurement of subsonic flow in a capacitively coupled helicon plasma source 

      Fredriksen, Åshild; Mishra, Lekha Nath; Gulbrandsen, Njål; Miloch, Wojciech Jacek (Journal article; Tidsskriftartikkel; Peer reviewed, 2010)
      Plasma parameters and the subsonic flow from a capacitively coupled, cylindrical plasma source of the Njord helicon device are investigated by means of a Mach probe and a retarding field energy analyzer (RFEA). 13.56 MHz and 600 W RF power is inserted into the argon working gas under low-pressure conditions and moderate magnetic field. By means of a downstream field coil, the magnetic field is shaped ...

    • On the symmetry of ionospheric polar cap patch exits around magnetic midnight 

      Moen, Jøran Idar; Gulbrandsen, Njål; Clausen, Lasse Boy Novock (Journal article; Tidsskriftartikkel; Peer reviewed, 2015-07-14)
      In this paper we examine how polar cap patches, which have been frozen into the antisolar flow over the polar cap, are transported into the nighttime auroral oval. First we present a detailed case study from 12 January 2002, with continuous observations of polar cap patches exiting into the nighttime auroral oval in the Scandinavian sector. Satellite images of the auroral oval and all-sky camera ...

    • Results from the intercalibration of optical low-light calibration sources 2011 

      Brändström, B. Urban E.; Enell, Carl-Fredrik; Widell, Ola; Hansson, Thord; Whiter, Daniel; Mäkinen, Sanna; Mikhaylova, Daria; Axelsson, Katarina; Sigernes, Fred; Gulbrandsen, Njål; Schlatter, Nicola M.; Gjendem, Amund Gjerde; Cai, L.; Reistad, Jone Peter; Daae, Marianne; Demissie, Teferi Dejene; Andalsvik, Yngvild Linnea; Roberts, Owen; Poluyanov, Stepan Viktorovich; Chernouss, Sergey (Journal article; Tidsskriftartikkel; Peer reviewed, 2012-05-03)
      Following the 38th Annual European Meeting on Atmospheric Studies by Optical Methods in Siuntio in Finland, an intercalibration workshop for optical low light calibration sources was held in Sodankylä, Finland. The main purpose of this workshop was to provide a comparable scale for absolute measurements of aurora and airglow. All sources brought to the intercalibration workshop were compared to the ...

    • RFEA Measurements of High-Energy Electrons in a Helicon Plasma Device with Expanding Magnetic Field 

      Gulbrandsen, Njål; Fredriksen, Åshild (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-01-26)
      In the inductively coupled plasma of the Njord helicon device we have, for the same parameters as for which an ion beam exists, measured a downstream population of high-energy electrons emerging from the source. Separated measurements of energetic tail electrons was carried out by Retarding Field Energy Analyzer (RFEA) with a grounded entrance grid, operated in an electron collection mode. In a ...

    • Solar Cycle and Long-Term Trends in the Observed Peak of the Meteor Altitude Distributions by Meteor Radars 

      Dawkins, E.C.M.; Stober, G.; Janches, D.; Carrillo-Sánchez, J.D.; Lieberman, R.S.; Jacobi, C.; Moffat-Griffin, T.; Mitchell, N.J.; Cobbett, N.; Batista, P.P.; Andrioli, V.F.; Buriti, R.A.; Murphy, D.J.; Kero, J.; Gulbrandsen, Njål; Tsutsumi, M.; Kozlovsky, A.; Kim, J.H.; Lee, C.; Lester, M. (Journal article; Tidsskriftartikkel; Peer reviewed, 2023-01-25)
      The mesosphere/lower thermosphere (MLT, 80–100 km) region is an important boundary between Earth's atmosphere below and space above and may act as a sensitive indicator for anthropogenic climate change. Existing observational and modeling studies have shown the middle atmosphere and the MLT is cooling and contracting because of increasing greenhouse gas emissions. However, trend analyses are ...