Volume 580, August 2015
|Number of page(s)||6|
|Published online||03 August 2015|
LOFAR tied-array imaging and spectroscopy of solar S bursts⋆
1 School of Physics, Trinity College Dublin, Dublin 2, Ireland
2 ASTRON, Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
3 School of Physics and Astronomy, SUPA, University of Glasgow, Glasgow G12 8QQ, UK
4 Solar-Terrestrial Center of Excellence, SIDC, Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium
5 Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
6 RAL Space, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Oxford, Oxfordshire, OX11 OQX, UK
7 LESIA, UMR CNRS 8109, Observatoire de Paris, 92195 Meudon, France
8 Institute of Radio Astronomy, 4, Chervonopraporna Str., 61002 Kharkiv, Ukraine
9 Commission for Astronomy, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
10 Swedish Institute of Space Physics, Box 537, 75121 Uppsala, Sweden
11 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
12 Helmholtz-Zentrum Potsdam, DeutschesGeoForschungsZentrum GFZ, Department 1: Geodesy and Remote Sensing, Telegrafenberg, A17, 14473 Potsdam, Germany
13 Shell Technology Center, Bangalore, 560099 Karnataka, India
14 SRON Netherlands Insitute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
15 Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
16 University of Twente, 7522 NB Enxhede, The Netherlands
17 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
18 University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
19 Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
20 School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
21 Research School of Astronomy and Astrophysics, Australian National University, Mt Stromlo Obs., via Cotter Road, Weston, A.C.T. 2611, Australia
22 Max Planck Institute for Astrophysics, Karl Schwarzschild Str. 1, 85741 Garching, Germany
23 SmarterVision BV, Oostersingel 5, 9401 JX Assen, The Netherlands
24 Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
25 Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
26 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
27 LPC2E – Université d’Orléans/CNRS, 3A, Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France
28 Station de Radioastronomie de Nançay, Observatoire de Paris – CNRS/INSU, USR 704 – Univ. Orléans, OSUC, route de Souesmes, 18330 Nançay, France
29 Anton Pannekoek Institute, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
30 Astro Space Center of the Lebedev Physical Institute, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
31 Sodankylä Geophysical Observatory, University of Oulu, Tähteläntie 62, 99600 Sodankylä, Finland
32 STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK
33 Center for Information Technology (CIT), University of Groningen, PO Box 11044, 9700 CA Groningen, The Netherlands
Received: 10 March 2015
Accepted: 19 June 2015
Context. The Sun is an active source of radio emission that is often associated with energetic phenomena ranging from nanoflares to coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), numerous millisecond duration radio bursts have been reported, such as radio spikes or solar S bursts (where S stands for short). To date, these have neither been studied extensively nor imaged because of the instrumental limitations of previous radio telescopes.
Aims. Here, LOw Frequency ARray (LOFAR) observations were used to study the spectral and spatial characteristics of a multitude of S bursts, as well as their origin and possible emission mechanisms.
Methods. We used 170 simultaneous tied-array beams for spectroscopy and imaging of S bursts. Since S bursts have short timescales and fine frequency structures, high cadence (~50 ms) tied-array images were used instead of standard interferometric imaging, that is currently limited to one image per second.
Results. On 9 July 2013, over 3000 S bursts were observed over a time period of ~8 h. S bursts were found to appear as groups of short-lived (<1 s) and narrow-bandwidth (~2.5 MHz) features, the majority drifting at ~3.5 MHz s-1 and a wide range of circular polarisation degrees (2−8 times more polarised than the accompanying Type III bursts). Extrapolation of the photospheric magnetic field using the potential field source surface (PFSS) model suggests that S bursts are associated with a trans-equatorial loop system that connects an active region in the southern hemisphere to a bipolar region of plage in the northern hemisphere.
Conclusions. We have identified polarised, short-lived solar radio bursts that have never been imaged before. They are observed at a height and frequency range where plasma emission is the dominant emission mechanism, however, they possess some of the characteristics of electron-cyclotron maser emission.
Key words: Sun: corona / Sun: radio radiation / Sun: particle emission / Sun: magnetic fields
© ESO, 2015
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