Shock location and CME 3D reconstruction of a solar type II radio burst with LOFAR

¹ ASTRON, Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
e-mail: zucca@astron.nl
² Astrophysics Research Group, School of Physics, Trinity College Dublin, Dublin 2, Ireland
³ Institut de Recherche en Astrophysique et Planétologie, 9 Av. du Colonel Roche, 31028 Toulouse Cedex 4, France
⁴ Solar-Terrestrial Center of Excellence, Royal Observatory of Belgium, Av. Circulaire 3, 1180 Brussels, Belgium
⁵ LESIA, UMR CNRS 8109, Observatoire de Paris, 92195 Meudon, France
⁶ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁷ RAL Space, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Oxfordshire, UK
⁸ SUPA School of Physics and Astronomy, University of Glasgow, G12 8QQ, UK
⁹ Space Research Centre of the Polish Academy of Science, 18A Bartycka 00-716 Warsaw, Poland
¹⁰ Space Radio-Diagnostics Research Centre, University of Warmia and Mazury in Olsztyn, Poland
¹¹ Helmholtz-Zentrum Potsdam, DeutschesGeoForschungsZentrum GFZ, Geodesy and Remote Sensing, Telegrafenberg, A17, 14473 Potsdam, Germany
¹² Shell Technology Center, Bangalore, India
¹³ University of Technology Sydney, 15 Broadway, Ultimo NSW 2007, Australia
¹⁴ Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands
¹⁵ Institute for Astronomy, University of Edinburgh, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁶ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁷ University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁸ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611 Australia
¹⁹ Max Planck Institute for Astrophysics, Karl Schwarzschild Str. 1, 85741 Garching, Germany
²⁰ SmarterVision BV, Oostersingel 5, 9401 JX Assen, The Netherlands
²¹ Centre for Astrophysics & Supercomputing, Swinburne University of Technology John St, Hawthorn VIC 3122, Australia
²² Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
²³ Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
²⁴ Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
²⁵ LPC2E - Univ. d’Orléans/CNRS, 3A Av. de la Recherche Scientifique, 45071 Orléans Cedex 2, France
²⁶ Station de Radioastronomie de Nancay, Observatoire de Paris CNRS/INSU, USR 704 - Univ., OSUC route de Souesmes, 18330 Nancay, France
²⁷ CSIRO Astronomy and Space Science, 26 Dick Perry Av., Kensington, WA 6151, Australia
²⁸ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²⁹ Astronomisches Institut der Ruhr-Universität Bochum, Universitaetsstrasse 150, 44780 Bochum, Germany
³⁰ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
³¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³² Department of Physics and Technology, University of Troms, Norway
³³ STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK
³⁴ Radboud University Radio Lab, Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
³⁵ Department of Physics and Astronomy, University of California Irvine, Irvine, CA 92697, USA
³⁶ Center for Information Technology (CIT), University of Groningen, Groningen, The Netherlands
³⁷ Poznan Supercomputing and Networking Center (PCSS) Poznan, Poland
³⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³⁹ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501, Bielefeld, Germany
⁴⁰ Department of Physics and Elelctronics, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
⁴¹ SKA South Africa, 3rd Floor, The Park, Park Road, Pinelands, 7405, South Africa
⁴² International Centre for Radio Astronomy Research - Curtin University, GPO Box U1987, Perth, WA 6845, Australia
⁴³ Jagiellonian University, Astronomical Observatory, Orla 171, 30-244 Krakow, Poland
⁴⁴ Department of Physics and Electrical Engineering, Linnaeus University 35195, Vaexjoe, Sweden

Received: 16 November 2017
Accepted: 24 March 2018

Abstract

Context. Type II radio bursts are evidence of shocks in the solar atmosphere and inner heliosphere that emit radio waves ranging from sub-meter to kilometer lengths. These shocks may be associated with coronal mass ejections (CMEs) and reach speeds higher than the local magnetosonic speed. Radio imaging of decameter wavelengths (20–90 MHz) is now possible with the Low Frequency Array (LOFAR), opening a new radio window in which to study coronal shocks that leave the inner solar corona and enter the interplanetary medium and to understand their association with CMEs.

Aims. To this end, we study a coronal shock associated with a CME and type II radio burst to determine the locations at which the radio emission is generated, and we investigate the origin of the band-splitting phenomenon.

Methods. Thetype II shock source-positions and spectra were obtained using 91 simultaneous tied-array beams of LOFAR, and the CME was observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) and by the COR2A coronagraph of the SECCHI instruments on board the Solar Terrestrial Relation Observatory(STEREO). The 3D structure was inferred using triangulation of the coronographic observations. Coronal magnetic fields were obtained from a 3D magnetohydrodynamics (MHD) polytropic model using the photospheric fields measured by the Heliospheric Imager (HMI) on board the Solar Dynamic Observatory (SDO) as lower boundary.

Results. The type II radio source of the coronal shock observed between 50 and 70 MHz was found to be located at the expanding flank of the CME, where the shock geometry is quasi-perpendicular with θ_Bn ~ 70°. The type II radio burst showed first and second harmonic emission; the second harmonic source was cospatial with the first harmonic source to within the observational uncertainty. This suggests that radio wave propagation does not alter the apparent location of the harmonic source. The sources of the two split bands were also found to be cospatial within the observational uncertainty, in agreement with the interpretation that split bands are simultaneous radio emission from upstream and downstream of the shock front. The fast magnetosonic Mach number derived from this interpretation was found to lie in the range 1.3–1.5. The fast magnetosonic Mach numbers derived from modelling the CME and the coronal magnetic field around the type II source were found to lie in the range 1.4–1.6.