LOFAR tied-array imaging of Type III solar radio bursts^⋆

¹ School of Physics, Trinity College Dublin, 2, Dublin, Ireland
e-mail: morosand@tcd.ie
² Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, The Netherlands
³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁴ RAL Space, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Oxford, Oxfordshire, OX11 OQX, UK
⁵ LESIA, UMR CNRS 8109, Observatoire de Paris, 92195 Meudon, France
⁶ Institute of Radio Astronomy, 4, Chervonopraporna Str., 61002 Kharkiv, Ukraine
⁷ School of Physics and Astronomy, SUPA, University of Glasgow, Glasgow G12 8QQ, UK
⁸ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
⁹ Swedish Institute of Space Physics, Box 537, 75121 Uppsala, Sweden
¹⁰ Solar-Terrestrial Center of Excellence, SIDC, Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium
¹¹ Shell Technology Center, 560099 Karnata ha, Bangalore, India
¹² SRON Netherlands Insitute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
¹³ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁵ Institute for Astronomy, University of Edinburgh, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁶ University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁷ School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
¹⁸ Research School of Astronomy and Astrophysics, Australian National University, Mt Stromlo Obs., via Cotter Road, Weston, A.C.T. 2611, Australia
¹⁹ Max Planck Institute for Astrophysics, Karl Schwarzschild Str. 1, 85741 Garching, Germany
²⁰ Onsala Space Observatory, Dept. of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
²¹ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
²² Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²³ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²⁴ Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
²⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²⁶ LPC2E – Université d’Orléans/CNRS 45071 Orléans, Cedex 2, France
²⁷ Station de Radioastronomie de Nançay, Observatoire de Paris – CNRS/INSU, USR 704 - Univ. Orléans, OSUC, route de Souesmes, 18330 Nançay, France
²⁸ Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
²⁹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
³⁰ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³¹ Astronomisches Institut der Ruhr-Universität Bochum, Universitaetsstrasse 150, 44780 Bochum, Germany
³² Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
³³ Astro Space Center of the Lebedev Physical Institute, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
³⁴ Center for Information Technology (CIT), University of Groningen, The Netherlands
³⁵ Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 av. Charles André, 69561 Saint-Genis-Laval Cedex, France
³⁶ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
³⁷ Department of Physics and Elelctronics, Rhodes University, PO Box 94, 6140 Grahamstown, South Africa
³⁸ SKA South Africa, 3rd Floor, The Park, Park Road, 7405 Pinelands, South Africa
³⁹ Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 3 April 2014
Accepted: 11 July 2014

Abstract

Context. The Sun is an active source of radio emission which is often associated with energetic phenomena such as solar flares and coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), the Sun has not been imaged extensively because of the instrumental limitations of previous radio telescopes.

Aims. Here, the combined high spatial, spectral, and temporal resolution of the LOw Frequency ARray (LOFAR) was used to study solar Type III radio bursts at 30–90 MHz and their association with CMEs.

Methods. The Sun was imaged with 126 simultaneous tied-array beams within ≤5 R_⊙ of the solar centre. This method offers benefits over standard interferometric imaging since each beam produces high temporal (~83 ms) and spectral resolution (12.5 kHz) dynamic spectra at an array of spatial locations centred on the Sun. LOFAR’s standard interferometric output is currently limited to one image per second.

Results. Over a period of 30 min, multiple Type III radio bursts were observed, a number of which were found to be located at high altitudes (~4 R_⊙ from the solar center at 30 MHz) and to have non-radial trajectories. These bursts occurred at altitudes in excess of values predicted by 1D radial electron density models. The non-radial high altitude Type III bursts were found to be associated with the expanding flank of a CME.

Conclusions. The CME may have compressed neighbouring streamer plasma producing larger electron densities at high altitudes, while the non-radial burst trajectories can be explained by the deflection of radial magnetic fields as the CME expanded in the low corona.