Now showing items 1306-1325 of 2098

    • Observation of intrinsically bright terrestrial gamma ray flashes from the Mediterranean basin 

      Gjesteland, Thomas; Østgaard, Nikolai; Laviola, Sante; Miglietta, M M; Arnone, Enrico; Marisaldi, Martino; Fuschino, F; Collier, Andrew B.; Fabro, F; Montanya, J (Peer reviewed; Journal article, 2015-12-14)
      We present three terrestrial gamma ray flashes (TGFs) observed over the Mediterranean basin by the Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI) satellite. Since the occurrence of these events in the ...
    • Observation of light-by-light scattering in ultraperipheral Pb+Pb collisions with the ATLAS detector 

      Aad, Georges; Abbott, Brad; Abbott, Dale C.; Abdinov, Ovsat Bahram oglu; Abed Abud, Adam; Abeling, Kira; Abhayasinghe, Deshan Kavishka; Abidi, Syed Haider; AbouZeid, Hass; Abraham, Nadine L.; Bjørke, Kristian; Bugge, Magnar Kopangen; Cameron, David Gordon; Catmore, James Richard; Feigl, Simon; Garonne, Vincent; Gramstad, Eirik; Hellesund, Simen; Morisbak, Vanja; Oppen, Henrik; Ould-Saada, Farid; Pedersen, Maiken; Read, Alexander Lincoln; Rye, Eli Bæverfjord; Røhne, Ole Myren; Sandaker, Heidi; Vadla, Knut Oddvar Høie; Buanes, Trygve; Djuvsland, Julia Isabell; Eigen, Gerald; Fomin, Nikolai; Lee, Graham Richard; Lipniacka, Anna; Martin dit Latour, Bertrand; Mæland, Steffen; Stugu, Bjarne; Træet, Are Sivertsen; Abramowicz, Halina; Abreu, Henso; Abulaiti, Yiming; Acharya, Bobby S.; Achkar, Baida; Adachi, Shunsuke; Adam, Lennart; Adam-Bourdarios, Claire; Adamczyk, Leszek; Adamek, Lukas; Adelman, Jareed; Adersberger, Michael; Adigüzel, Aytül; ATLAS, Collaboration (Peer reviewed; Journal article, 2019)
    • Observation of photon-induced W+W− production in pp collisions at √s = 13 TeV using the ATLAS detector 

      Aad, Georges; Abbott, Brad; Abbott, Dale C.; Abed Abud, Adam; Abeling, Kira; Abhayasinghe, Deshan Kavishka; Abidi, Syed Haider; AbouZeid, Ossama Sherif Alexander; Abraham, Nadine L.; Abramowicz, Halina; Bjørke, Kristian; Bugge, Magnar Kopangen; Cameron, David Gordon; Catmore, James Richard; Garonne, Vincent; Gramstad, Eirik; Heggelund, Andreas; Hellesund, Simen; Håland, Even Simonsen; Morisbak, Vanja; Oppen, Henrik; Ould-Saada, Farid; Pedersen, Maiken; Read, Alexander Lincoln; Rye, Eli Bæverfjord; Røhne, Ole Myren; Sandaker, Heidi; Vadla, Knut Oddvar Høie; Buanes, Trygve; Djuvsland, Julia Isabell; Eigen, Gerald; Fomin, Nikolai; Latour, Bertrand Pascal Christian; Lee, Graham Richard; Lipniacka, Anna; Stugu, Bjarne Sandvik; Træet, Are Sivertsen; Abreu, Henso; Abulaiti, Yiming; Acharya, Bobby S.; Achkar, Baida; Adam, Lennart; Adam-Bourdarios, Claire; Adamczyk, Leszek; Adamek, Lukas; Adelman, Jareed; Adigüzel, Aytül; Adorni, Sofia; Adye, Tim; Affolder, Anthony Allen; ATLAS, Collaboration (Journal article; Peer reviewed, 2021)
      This letter reports the observation of photon-induced production of W -boson pairs, γγ → W W . The analysis uses 139 fb−1 of LHC proton–proton collision data taken at √s = 13 TeV recorded by the ATLAS experiment during the ...
    • Observation of Terrestrial Gamma-Ray Flashes at Mid Latitude 

      Maiorana, Carolina; Marisaldi, M.; Füllekrug, Martin; Soula, Serge; Lapierre, J.; Mezentsev, Andrey; Skeie, Chris Alexander Kallevik; Heumesser, M.; Chanrion, Olivier; Østgaard, N.; Neubert, Torsten; Reglero, Victor (Journal article; Peer reviewed, 2021)
      We present a sample of Terrestrial Gamma-ray Flashes (TGFs) observed at mid latitudes by the Atmosphere Space Interaction Monitor (ASIM). The events were detected between June 2018 and August 2020 in the latitude bands ...
    • Observation of the associated production of a top quark and a Z boson in pp collisions at √s = 13 TeV with the ATLAS detector 

      Aad, Georges; Abbott, Brad; Abbott, Dale C.; Abed Abud, Adam; Abeling, Kira; Abhayasinghe, Deshan Kavishka; Abidi, Syed Haider; AbouZeid, Hass; Abraham, Nadine L.; Abramowicz, Halina; Bjørke, Kristian; Bugge, Magnar Kopangen; Cameron, David Gordon; Catmore, James Richard; Garonne, Vincent; Gramstad, Eirik; Hellesund, Simen; Morisbak, Vanja; Oppen, Henrik; Ould-Saada, Farid; Pedersen, Maiken; Read, Alexander Lincoln; Rye, Eli Bæverfjord; Røhne, Ole Myren; Sandaker, Heidi; Vadla, Knut Oddvar Høie; Buanes, Trygve; Djuvsland, Julia Isabell; Eigen, Gerald; Fomin, Nikolai; Lee, Graham Richard; Lipniacka, Anna; Martin dit Latour, Bertrand; Stugu, Bjarne; Træet, Are Sivertsen; Abreu, Henso; Abulaiti, Yiming; Acharya, Bobby S.; Achkar, Baida; Adachi, Shunsuke; Adam, Lennart; Adam-Bourdarios, Claire; Adamczyk, Leszek; Adamek, Lukas; Adelman, Jareed; Adersberger, Michael; Adigüzel, Aytül; Adorni, Sofia; Adye, Tim; Affolder, Anthony Allen; ATLAS, Collaboration (Journal article; Peer reviewed, 2020-07-20)
      Single top-quark production in association with a Z boson, where the Z boson decays to a pair of charged leptons, is measured in the trilepton channel. The proton-proton collision data collected by the ATLAS experiment ...
    • Observation of the boson peak in a two-dimensional material 

      Tømterud, Martin; Eder, Sabrina Daniela; Büchner, Christin; Wondraczek, Lothar; Simonsen, Ingve; Schirmacher, Walter; Manson, Joseph R.; Holst, Bodil (Journal article; Peer reviewed, 2023)
      The boson peak is an excess in the phonon vibrational density of states relative to the Debye model. It has been observed in a wide range of amorphous materials, from inorganic glasses to polymers. Two-dimensional random ...
    • Observation of the decay D0→K−π+e+e− 

      Lees, Jean-Pierre; Poireau, Vincent; Tisserand, Vincent; Grauges, Eugeni; Palano, Antimo; Eigen, Gerald; Brown, David Nathan; Kolomensky, Yury G.; Fritsch, Miriam; Koch, Helmut; Schroeder, Torsten; Hearty, Christopher; Mattison, Tom S.; McKenna, Janis A.; So, Rocky Y.; Blinov, Vladimir E.; Buzykaev, Alexey; Druzhinin, Vladimir P.; Golubev, Vladimir B.; Kozyrev, Evgeny A.; Kravchenko, Evgeniy A.; Onuchin, Alexei P.; Serednyakov, Sergey I.; Skovpen, Yuri I.; Solodov, Evgeni Petrovich; Todyshev, Korneliy Yu.; Lankford, Andrew J.; Gary, J. William; Long, Owen; Eisner, Alan M.; Lockman, William S.; Panduro-Vazquez, William; Chao, Daniel S.; Cheng, Chih-Hsiang; Echenard, Bertrand; Flood, Kevin T.; Hitlin, David G.; Kim, Jae; Lee, Myeongjae; Miyashita, Tomonari S.; Ongmongkolkul, Piti; Porter, Frank C.; Röhrken, Markus; Huard, Zachary; Meadows, Brian T.; Pushpawela, Buddhi G.; Sokoloff, Michael D.; Sun, Liang; Smith, Jim G.; Wagner, Stephen R.; BABAR, Collaboration (Peer reviewed; Journal article, 2019)
    • Observation of the Gamma-Ray Binary HESS J0632+057 with the H.E.S.S., MAGIC, and VERITAS Telescopes 

      Adams, C.B.; Benbow, W.; Brill, A.; Buckley, J.H.; Capasso, M.; Chromey, A.J.; Errando, M.; Falcone, A.; A Farrell, Farrell; Feng, Q.; Finley, J.P.; M Foote, Foote; Fortson, L.; Furniss, A.; Gent, A.; Gillanders, G.H.; Giuri, C.; Gueta, O.; Hanna, D.; Hassan, T.; Hervet, O.; Holder, J.; Hona, B.; Humensky, T.B.; Jin, W.; Kaaret, P.; Kertzman, M.; Kieda, D.; K Kleiner, Kleiner; Krennrich, F.; Kumar, S.; Lang, M.J.; Lundy, M.; Maier, G.; E McGrath, McGrath; Moriarty, P.; Mukherjee, R.; Nieto, D.; Nievas-Rosillo, M.; O'Brien, S.; Ong, R.A.; Otte, A.N.; Park, N.; Patel, S.; Pfrang, K.; Pichel, A.; Pohl, M.; Prado, R.R.; Quinn, J.; Ragan, K.; Reynolds, P.T.; Ribeiro, D.; Roache, E.; Rovero, A.C.; Ryan, J.L.; Santander, M.; Schlenstedt, S.; Sembroski, G.H.; Shang, R.; Tak, D.; Vassiliev, V.V.; Weinstein, A.; Williams, D.A.; J Williamson, Williamson; Acciari, V.A.; Ansoldi, S.; Antonelli, L.A.; Arbet Engels, Engels; Artero, M.; Asano, K.; Baack, D.; Babić, A.; Baquero, A.; Barres De Almeida, De; Barrio, J.A.; Batković, I.; Becerra González, Gonzalez; Bednarek, W.; Bellizzi, L.; Bernardini, E.; Bernardos, M.; Berti, A.; Besenrieder, J.; Bhattacharyya, W.; Bigongiari, C.; Biland, A.; Blanch, O.; Bökenkamp, H.; Bonnoli, G.; Bošnjak, Z.; Busetto, G.; Carosi, R.; Ceribella, G.; Cerruti, M.; Chai, Y.; Chilingarian, A.; Cikota, S.; Colak, S.M.; Colombo, E; Contreras, J.L.; Cortina, J.; Covino, S; D'Amico, Giacomo; D'Elia, V; Da Vela, Vela; Dazzi, F.; De Angelis, Angelis; De Lotto, Lotto; Delfino, M.; Delgado, J.; Delgado Mendez, Mendez; Depaoli, D.; Di Pierro, Pierro; Di Venere, Venere; Do Souto Espieira, Souto; Dominis Prester, Prester; Donini, A.; Dorner, D.; Doro, M.; Elsaesser, D.; Fallah Ramazani, Ramazani; Fattorini, A.; Fonseca, M.V.; Font, L.; Fruck, C; Fukami, S.; Fukazawa, Y.; García López, Lopez; Garczarczyk, M.; Gasparyan, S.; Gaug, M.; Giglietto, N.; Giordano, F.; Gliwny, P.; Godinović, N.; Green, J.G.; Green, D.; Hadasch, D.; Hahn, A.; Heckmann, L.; Herrera, J.; Hoang, J.; Hrupec, D.; Hütten, M.; Inada, T.; Ishio, K.; Iwamura, Y.; Jiménez Martínez, Martinez; Jormanainen, J.; Jouvin, L.; Karjalainen, M.; Kerszberg, D.; Kobayashi, Y.; Kubo, H.; Kushida, J.; Lamastra, A.; Lelas, D.; Leone, F.; Lindfors, E.; Linhoff, L.; Lombardi, S.; Longo, F.; López-Coto, R.; López-Moya, M.; López-Oramas, A.; Loporchio, S.; Machado De Oliveira Fraga, De; Maggio, C.; Majumdar, P.; Makariev, M.; Mallamaci, M.; Maneva, G.; Manganaro, M.; Mannheim, K; Maraschi, L; Mariotti, M.; Martínez, M.; Mazin, D.; Menchiari, S.; Mender, S.; Mićanović, S.; Miceli, D.; Miener, T.; Miranda, J.M.; Mirzoyan, R.; Molina, E.; Moralejo, A.; Morcuende, D.; Moreno, V.; Moretti, E.; Nakamori, T.; Nava, L.; Neustroev, V.; Nigro, C.; Nilsson, K.; Nishijima, K.; Noda, K.; Nozaki, S.; Ohtani, Y.; Oka, T.; Otero-Santos, J.; Paiano, S.; Palatiello, M.; Paneque, D.; Paoletti, R.; Paredes, J.M.; Pavletić, L.; Peil, P.; Persic, M.; Pihet, M.; Prada Moroni, Moroni; Prandini, E.; Priyadarshi, C.; Puljak, I.; Rhode, W.; Ribó, M.; Rico, J.; Righi, C.; Rugliancich, A.; Saha, L.; Sahakyan, N.; Saito, T.; Sakurai, S.; Satalecka, K.; Saturni, F.G.; Schleicher, B.; Schmidt, K. B.; Schweizer, T; Sitarek, J; Snidaric, I; Sobczynska, D.; Spolon, A.; Stamerra, A.; Strišković, J.; Strom, D.; Strzys, M.; Suda, Y.; Surić, T.; Takahashi, M.; Takeishi, R.; Tavecchio, F.; Temnikov, P.; Terzić, T.; Teshima, M.; Tosti, L.; Truzzi, S.; Tutone, A.; Ubach, S.; Van Scherpenberg, Scherpenberg; Vanzo, G.; Vazquez Acosta, Acosta; Ventura, S.; Verguilov, V.; Vigorito, C.F.; Vitale, V.; Vovk, I.; Will, M.; Wunderlich, C.; Yamamoto, T.; Zarić, D.; Abdalla, H.; Aharonian, F.; Ait Benkhali, Benkhali; Angüner, E.O.; Arcaro, C.; Ashkar, H.; Backes, M.; Barbosa Martins, Martins; Barnard, M.; Batzofin, R.; Becherini, Y.; Berge, D.; Bernlöhr, K.; Bi, B.; Böttcher, M.; Boisson, C.; Bolmont, J.; De Bony De Lavergne, Bony; Breuhaus, M.; Brose, R.; Brun, F.; Bulik, T.; Caroff, S.; Casanova, S.; Chand, T.; Chen, A.; Cotter, G.; Damascenev Mbarubucyeye, Mbarubucyeye; Devin, J.; Djannati-Ataï, A.; Egberts, K.; Ernenwein, J.-P.; Fegan, S.; Fiasson, A.; Fichet De Clairfontaine, De; Fontaine, G.; Füßling, M.; Funk, S.; Gabici, S.; Giavitto, G.; Glawion, D.; Glicenstein, J.F.; Grondin, M.-H.; Hinton, J.A.; Hofmann, W.; Holch, T.L.; Holler, M.; Horns, D.; Huang, Zhiqiu; Jamrozy, M.; Jankowsky, F.; Joshi, V.; Jung-Richardt, I.; Kasai, E.; Katarzyński, K.; Khélifi, B.; Komin, Nu.; Kosack, K.; Kostunin, D.; Le Stum, Stum; Lemiere, A.; Lenain, J.-P.; Leuschner, F.; Levy, C.; Lohse, T.; Luashvili, A.; Lypova, I.; Mackey, J.; Majumdar, J.; Malyshev, D.; Marandon, V.; Marchegiani, P.; Marcowith, A.; Martí-Devesa, G.; Marx, R.; Maurin, G.; Meintjes, P.J.; Mitchell, A.; Moderski, R.; Mohrmann, L.; Montanari, A.; Moulin, E.; Muller, J.; Murach, T.; De Naurois, Naurois; Nayerhoda, A.; Niemiec, J.; Priyana Noel, Noel; O'Brien, P.; Ohm, S.; Olivera-Nieto, L.; De Ona Wilhelmi, Ona; Ostrowski, M.; Panny, S.; Panter, M.; Parsons, RD; Peron, G.; Poireau, V; Prokhorov, D.A.; Prokoph, H.; Pühlhofer, G.; Punch, M.; Quirrenbach, A.; Reichherzer, P.; Reimer, A.; Reimer, O.; Renaud, M.; Rieger, F.; Romoli, C.; Rowell, G.; Rudak, B.; Rueda Ricarte, Ricarte; Ruiz-Velasco, E.; Sahakian, V.; Sailer, S.; Salzmann, H.; Sanchez, D.A.; Santangelo, A.; Sasaki, M.; Schutte, H.M.; Schwanke, U.; Schüssler, F.; Senniappan, M.; Shapopi, J.N.S.; Simoni, R.; Sol, H.; Specovius, A.; Spencer, S.; Steenkamp, R.; Steinmassl, S.; Sun, L.; Takahashi, T.; Tanaka, T.; Terrier, R.; Tsuji, N.; Uchiyama, Y.; Van Eldik, Eldik; Van Soelen, Soelen; Veh, J.; Venter, C.; Vink, J.; Wagner, S.J.; White, R.; Wierzcholska, A.; Wun Wong, Yu; Zacharias, M.; Zargaryan, D.; Zdziarski, A.A.; Zech, A.; Zhu, Sylvia J.; Zouari, S.; Żywucka, N.; Moritani, Y.; Torres, D.F. (Journal article; Peer reviewed, 2021)
      The results of gamma-ray observations of the binary system HESS J0632 + 057 collected during 450 hr over 15 yr, between 2004 and 2019, are presented. Data taken with the atmospheric Cherenkov telescopes H.E.S.S., MAGIC, ...
    • Observation of WWW production in pp collisions at √s=13 TeV with the ATLAS detector 

      Aad, Georges; Abbott, Brad; Abbott, Dale C.; Abed Abud, Adam; Abeling, Kira; Abhayasinghe, Deshan Kavishka; Abidi, Syed Haider; Aboulhorma, Asmaa; Abramowicz, Halina; Abreu, Henso; Ananiev, Viktor; Bjørke, Kristian; Bugge, Magnar Kopangen; Cameron, David Gordon; Catmore, James Richard; Gramstad, Eirik; Heggelund, Andreas; Håland, Even Simonsen; Morisbak, Vanja; Ould-Saada, Farid; Pedersen, Maiken; Read, Alexander Lincoln; Rye, Eli Bæverfjord; Røhne, Ole Myren; Sandaker, Heidi; Vadla, Knut Oddvar Høie; Buanes, Trygve; Djuvsland, Julia Isabell; Eigen, Gerald; Fomin, Nikolai; Huiberts, Simon Kristian; Latour, Bertrand Pascal Christian; Lee, Graham Richard; Lipniacka, Anna; Stugu, Bjarne Sandvik; Træet, Are Sivertsen; Abulaiti, Yiming; Abusleme Hoffman, Angel C.; Acharya, Bobby S.; Achkar, Baida; Adam, Lennart; Adam-Bourdarios, Claire; Adamczyk, Leszek; Adamek, Lukas; Addepalli, Sagar Vidya; Adelman, Jareed; Adigüzel, Aytül; Adorni, Sofia; Adye, Tim; Affolder, Anthony Allen; ATLAS, Collaboration (Journal article; Peer reviewed, 2022)
      This Letter reports the observation of WWW production and a measurement of its cross section using 139  fb−1 of proton-proton collision data recorded at a center-of-mass energy of 13 TeV by the ATLAS detector at the Large ...
    • Observational evidence for throat aurora being associated with magnetopause reconnection 

      Han, De-Sheng; Xu, Tong; Jin, Yaqi; Oksavik, Kjellmar; Chen, Xiangcai; Liu, Jian-Jun; Zhang, Qinghe; Baddeley, Lisa; Herlingshaw, Katie (Peer reviewed; Journal article, 2019)
      Throat auroras have been suggested to be related to indentations on the subsolar magnetopause. However, the indentation generation process and the resulting ionospheric responses have remained unknown. An EISCAT Svalbard ...
    • Observational Evidence of Magnetic Reconnection in the Terrestrial Bow Shock Transition Region 

      Wang, Shan; Chen, Li-Jen; Bessho, Naoki; Hesse, Michael; Wilson, Lynn B.; Giles, Barbara; Moore, Thomas E.; Russell, Christopher T.; Torbert, Roy B.; Burch, James L. (Journal article; Peer reviewed, 2019)
      We report evidence of magnetic reconnection in the transition region of the Earth's bow shock when the angle between the shock normal and the immediate upstream magnetic field is 65°. An ion‐skin‐depth‐scale current sheet ...
    • Observations of Asymmetric Lobe Convection for Weak and Strong Tail Activity 

      Ohma, Anders; Østgaard, Nikolai; Reistad, Jone Peter; Tenfjord, Paul; Laundal, Karl Magnus; Moretto, Therese; Haaland, Stein; Krcelic, Patrik; Milan, Stephen Eric (Journal article; Peer reviewed, 2019)
      In this study we use high‐quality convection data from the Electron Drift Instrument on board Cluster to investigate how near‐Earth tail activity affects the average convection pattern in the magnetotail lobes when the ...
    • Observations of asymmetries in ionospheric return flow during different levels of geomagnetic activity 

      Reistad, Jone Peter; Østgaard, Nikolai; Laundal, Karl Magnus; Ohma, Anders; Snekvik, Kristian; Tenfjord, Paul; Grocott, Adrian; Oksavik, Kjellmar; Milan, Steve; Haaland, Stein (Peer reviewed; Journal article, 2018-06)
      It is known that the magnetic field of the Earth's closed magnetosphere can be highly displaced from the quiet‐day configuration when interacting with the interplanetary magnetic field (IMF), an asymmetry largely controlled ...
    • Observations of Blue Corona Discharges in Thunderclouds 

      Husbjerg, Lasse Skaaning; Neubert, Torsten; Chanrion, Olivier; Dimitriadou, Krystallia; Li, Dongshuai; Stendel, Martin; Kaas, Eigil; Østgaard, Nikolai; Reglero, Victor (Journal article; Peer reviewed, 2022)
      Blue electric streamer discharges in the upper reaches of thunderclouds are observed as flashes of 337.0 nm (blue) with faint or no emissions of 777.4 nm (red). Analyzing 3 years of measurements by the Atmosphere-Space ...
    • Observations of Electron Precipitation During Pulsating Aurora and Its Chemical Impact 

      Kebede, Fasil Tesema; Partamies, Noora; Tyssøy, Hilde Nesse; Kero, Antti; Smith-Johnsen, Christine (Journal article; Peer reviewed, 2020)
      Pulsating auroras (PsAs) are low‐intensity diffuse aurora, which switch on and off with a quasiperiodic oscillation period from a few seconds to ∼10 s. They are predominantly observed after magnetic midnight, during the ...
    • Observations of planetary waves in the mesosphere-lower thermosphere during stratospheric warming events 

      Stray, Nora Henriette; Orsolini, Yvan; Espy, Patrick Joseph; Limpasuvan, Varavut; Hibbins, Robert (Peer reviewed; Journal article, 2015-05-04)
      This study investigates the effect of stratospheric sudden warmings (SSWs) on planetary wave (PW) activity in the mesosphere–lower thermosphere (MLT). PW activity near 95 km is derived from meteor wind data using a chain ...
    • Observations of precipitation energies during different types of pulsating aurora 

      Kebede, Fasil Tesema; Partamies, Noora; Tyssøy, Hilde Nesse; McKay, Derek (Journal article; Peer reviewed, 2020)
      Pulsating aurora (PsA) is a diffuse type of aurora with different structures switching on and off with a period of a few seconds. It is often associated with energetic electron precipitation (>10 keV) resulting in the ...
    • Observations of sunlit N2+ aurora at high altitudes during the RENU2 flight 

      Ellingsen, Pål Gunnar; Lorentzen, Dag Arne; Kenward, David; Hecht, James H.; Evans, J. Scott; Sigernes, Fred; Lessard, Marc (Journal article; Peer reviewed, 2021)
      We present measurements of sunlit aurora during the launch of the Rocket Experiment for Neutral Upwelling 2 (RENU2) on the 13 December 2015, 07:34 UTC. The in situ auroral conditions coincide with those of sunlit aurora ...
    • Occurrence Statistics of Horse Collar Aurora 

      Bower, G.E.; Milan, Stephen Eric; Paxton, L.J.; Anderson, B.J. (Journal article; Peer reviewed, 2022)
      Horse collar aurora (HCA) are an auroral feature where the dawn and dusk sector auroral oval moves polewards and the polar cap becomes teardrop shaped. They form during prolonged periods of northward interplanetary magnetic ...