• The 3rd AGILE Terrestrial Gamma Ray Flash Catalog. Part I: Association to Lightning Sferics 

      Lindanger, Anders; Marisaldi, Martino; Maiorana, Carolina; Sarria, David; Albrechtsen, Kjetil; Østgaard, Nikolai; Galli, M.; Ursi, Alessandro; Labanti, C.; Tavani, Marco; Pittori, C.; Verrecchia, F. (Journal article; Peer reviewed, 2020)
      We present a complete and systematic search for terrestrial gamma-ray flashes (TGFs), detected by AGILE, that are associated with radio sferics detected by the World Wide Lightning Location Network (WWLLN) in the period ...
    • The 3rd AGILE Terrestrial Gamma-ray Flashes Catalog. Part II: Optimized Selection Criteria and Characteristics of the New Sample 

      Maiorana, Carolina; Marisaldi, Martino; Lindanger, Anders; Østgaard, Nikolai; Ursi, Alessandro; Sarria, David; Galli, M.; Labanti, C.; Tavani, Marco; Pittori, C.; Verrecchia, F. (Journal article; Peer reviewed, 2020)
      We present in this work the third catalog of terrestrial gamma-ray flashes (TGFs) by the AGILE mission and the new search algorithm that was developed to produce it. We firstly introduce the new selection criteria, designed ...
    • An altitude and distance correction to the initial fluence distribution of TGFs 

      Nisi, Ragnhild Schrøder; Østgaard, Nikolai; Gjesteland, Thomas; Collier, Andrew B. (Peer reviewed; Journal article, 2014-10)
      The source fluence distribution of terrestrial gamma ray flashes (TGFs) has been extensively discussed in recent years, but few have considered how the TGF fluence distribution at the source, as estimated from satellite ...
    • The ASIM mission on the international space station 

      Neubert, Torsten; Østgaard, Nikolai; Reglero, Victor; Blanc, Elisabeth; Chanrion, Olivier; Oxborrow, Carol Anne; Orr, Astrid; Tacconi, Matteo; Hartnack, Ole; Bhanderi, Dan D.V. (Peer reviewed; Journal article, 2019-03-12)
      The Atmosphere-Space Interactions Monitor (ASIM) is an instrument suite on the International Space Station (ISS) for measurements of lightning, Transient Luminous Events (TLEs) and Terrestrial Gamma-ray Flashes (TGFs). ...
    • The asymmetric geospace as displayed during the geomagnetic storm on 17 August 2001 

      Østgaard, Nikolai; Reistad, Jone Peter; Tenfjord, Paul; Laundal, Karl Magnus; Rexer, Theresa; Haaland, Stein; Snekvik, Kristian; Hesse, Michael; Milan, Stephen Eric; Ohma, Anders (Peer reviewed; Journal article, 2018-11-29)
      Previous studies have shown that conjugate auroral features are displaced in the two hemispheres when the interplanetary magnetic field (IMF) has a transverse (Y) component. It has also been shown that a BY component is ...
    • Birkeland current effects on high-latitude ground magnetic field perturbations 

      Laundal, Karl Magnus; Haaland, Stein; Lehtinen, Nikolai; Gjerløv, Jesper; Østgaard, Nikolai; Tenfjord, Paul; Reistad, Jone Peter; Snekvik, Kristian; Milan, Steve; Ohtani, Shin-ichi; Anderson, Brian J. (Peer reviewed; Journal article, 2015-09-18)
      Magnetic perturbations on ground at high latitudes are directly associated only with the divergence-free component of the height-integrated horizontal ionospheric current, J⊥,df. Here we show how J⊥,df can be expressed as ...
    • Can magnetotail reconnection produce the auroral intensities observed in the conjugate ionosphere? 

      Østgaard, Nikolai; Snekvik, Kristian; Borg, Anette Lauen; Åsnes, Arne; Pedersen, Arne; Øieroset, Marit; Phan, Tai D.; Haaland, Stein (Peer reviewed; Journal article, 2009-06)
      [1] In a recent case study, Borg et al. (2007) reported that an inverted V structure, caused by a field-aligned potential drop of 30 kV producing very strong X-ray aurora, was found in connection with tail reconnection. ...
    • Cluster observations of a field aligned current at the dawn flank of a bursty bulk flow 

      Snekvik, Kristian; Haaland, Stein; Østgaard, Nikolai; Hasegawa, H.; Nakamura, R.; Takada, T.; Juusola, L.; Amm, O.; Pitout, F.; Rème, H.; Klecker, B.; Lucek, E. A. (Peer reviewed; Journal article, 2007-06-29)
      This article describes observations of a bursty bulk flow (BBF) in the outer central plasma sheet. The observations are made with the Cluster satellites, located approximately 19RE downtail, close to the midnight sector ...
    • Confining the angular distribution of terrestrial gamma ray flash emission 

      Gjesteland, Thomas; Østgaard, Nikolai; Collier, A. B.; Carlson, B. E.; Cohen, M. B.; Lehtinen, N. G. (Peer reviewed; Journal article, 2011-11-11)
    • Constraints on Recoil Leader Properties Estimated from X-ray Emissions in Aircraft-Triggered Discharges 

      Skeie, Chris Alexander; Østgaard, Nikolai; Lehtinen, Nikolai; Sarria, David; Kochkin, Pavlo; de Boer, Boer; Bardet, M.; Allasia, C.; Flourens, F. (Journal article; Peer reviewed, 2020)
      During Airbus aircraft campaigns flying into thunderstorms in 2014–2016, X-rays were observed during two stages of aircraft-triggered lightning: nanosecond pulses of X-rays associated with negative leader steps and bursts ...
    • Constraints to do realistic modeling of the electric field ahead of the tip of a lightning leader 

      Skeltved, Alexander Broberg; Østgaard, Nikolai; Mezentsev, Andrey; Lehtinen, Nikolai; Carlson, Brant Edward (Peer reviewed; Journal article, 2017-08)
      Several computer models exist to explain the observation of terrestrial gamma‐ray flashes (TGFs). Some of these models estimate the electric field ahead of lightning leaders and its effects on electron acceleration and ...
    • Dayside and nightside magnetic field responses at 780 km altitude to dayside reconnection 

      Snekvik, Kristian; Østgaard, Nikolai; Tenfjord, Paul; Reistad, Jone Peter; Laundal, Karl Magnus; Milan, Stephen Eric; Haaland, Stein (Peer reviewed; Journal article, 2017-02)
      During southward interplanetary magnetic field, dayside reconnection will drive the Dungey cycle in the magnetosphere, which is manifested as a two‐cell convection pattern in the ionosphere. We address the response of the ...
    • Dynamic effects of restoring footpoint symmetry on closed magnetic field lines 

      Reistad, Jone Peter; Østgaard, Nikolai; Tenfjord, Paul; Laundal, Karl Magnus; Snekvik, Kristian; Haaland, Stein; Milan, Steve; Oksavik, Kjellmar; Frey, H. U.; Grokott, A. (Peer reviewed; Journal article, 2016-05-05)
      Here we present an event where simultaneous global imaging of the aurora from both hemispheres reveals a large longitudinal shift of the nightside aurora of about 3 h, being the largest relative shift reported on from ...
    • Effects of dead time losses on terrestrial gamma ray flash measurements with the Burst and Transient Source Experiment 

      Gjesteland, Thomas; Østgaard, Nikolai; Connell, P. H.; Stadsnes, J.; Fishman, G. J. (Peer reviewed; Journal article, 2010-05-05)
    • Enhanced detection of terrestrial gamma-ray flashes by AGILE 

      Marisaldi, Martino; Argan, A; Ursi, Alessandro; Gjesteland, Thomas; Fuschino, F; Labanti, C; Galli, M; Tavani, M; Pittori, C; Verrecchia, F; D´Amico, F.; Østgaard, Nikolai; Mereghetti, S; Campana, R; Cattaneo, PW; Bulgarelli, S; Colafrancesco, S; Dietrich, S; Longo, F; Gianotti, F.; Giommi, P; Rappoldi, A; Trifoglio, M; Trois, A (Peer reviewed; Journal article, 2015-11-06)
      At the end of March 2015 the onboard software configuration of the Astrorivelatore Gamma a Immagini Leggero (AGILE) satellite was modified in order to disable the veto signal of the anticoincidence shield for the minicalorimeter ...
    • First 10 Months of TGF Observations by ASIM 

      Østgaard, Nikolai; Neubert, Torsten; Reglero, Victor; Ullaland, Kjetil; Yang, Shiming; Genov, Georgi; Marisaldi, Martino; Mezentsev, Andrey; Kochkin, Pavlo; Lehtinen, Nikolai; Sarria, David; Qureshi, Bilal Hasan; Solberg, Arne Olav; Maiorana, Carolina; Albrechtsen, Kjetil; Budtz-Jørgensen, Carl; Kuvvetli, Irfan; Christiansen, Freddy; Chanrion, Olivier; Heumesser, Matthias; Navarro-Gonzalez, Javier; Connell, Paul H.; Eyles, Christopher J; Christian, Hugh J.; Al-Nussirat, Samer (Peer reviewed; Journal article, 2019)
      The Atmosphere‐Space Interactions Monitor (ASIM) was launched to the International Space Station on 2 April 2018. The ASIM payload consists of two main instruments, the Modular X‐ray and Gamma‐ray Sensor (MXGS) for imaging ...
    • Gamma Ray Glow Observations at 20-km Altitude 

      Østgaard, Nikolai; Christian, Hugh J.; Grove, J. Eric; Sarria, David; Mezentsev, Andrey; Kochkin, Pavlo; Lehtinen, Nikolai; Quick, Mason; Al-Nussirat, Samer; Wulf, Eric; Genov, Georgi; Ullaland, Kjetil; Marisaldi, Martino; Yang, Shiming; Blakeslee, Richard J. (Peer reviewed; Journal article, 2019)
      In the spring of 2017 an ER‐2 aircraft campaign was undertaken over continental United States to observe energetic radiation from thunderstorms and lightning. The payload consisted of a suite of instruments designed to ...
    • The Hall current system revealed as a statistical significant pattern during fast flows 

      Snekvik, Kristian; Nakamura, R.; Østgaard, Nikolai; Haaland, Stein; Retinò, A. (Peer reviewed; Journal article, 2008-11-05)
      We have examined the dawn-dusk component of the magnetic field, BY , in the night side current sheet during fast flows in the neutral sheet. 237 h of Cluster data from the plasma sheet between 2 August 2002 and 2 October ...
    • How the IMF By induces a By component in the closed magnetosphere and how it leads to asymmetric currents and convection patterns in the two hemispheres 

      Tenfjord, Paul; Østgaard, Nikolai; Snekvik, Kristian; Laundal, Karl Magnus; Reistad, Jone Peter; Haaland, Stein; Milan, Steve (Peer reviewed; Journal article, 2015-11-12)
      We used the Lyon-Fedder-Mobarry global magnetohydrodynamics model to study the effects of the interplanetary magnetic field (IMF) By component on the coupling between the solar wind and magnetosphere-ionosphere system. ...
    • The impact of sunlight on high-latitude equivalent currents 

      Laundal, Karl Magnus; Gjerløv, Jesper; Østgaard, Nikolai; Reistad, Jone Peter; Haaland, Stein; Snekvik, Kristian; Tenfjord, Paul; Ohtani, Shin-ichi; Milan, Steve (Peer reviewed; Journal article, 2016-03)
      Ground magnetic field measurements can be mathematically related to an overhead ionospheric equivalent current. In this study we look in detail at how the global equivalent current, calculated using more than 30 years of ...