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LOFAR tied-array imaging and spectroscopy of solar S bursts*

D. E. Morosan 1 P. T. Gallagher 1 P. Zucca 1 A. O’flannagain R. Fallows 2 H. Reid 3 J. Magdalenić G. Mann 4 M. M. Bisi 5 A. Kerdraon 6 A. A. Konovalenko 7 A. L. Mackinnon 8 H. O. Rucker 9 B. Thidé 10 C. Vocks 4 A. Alexov 2 J. Anderson 11 A. Asgekar 2 I. M. Avruch 12 M. J. Bentum 2 G. Bernardi 13 A. Bonafede 14 F. Breitling 4 J. W. Broderick 15 W. N. Brouw 16 H. R. Butcher 17 B. Ciardi 18 E. de Geus 2 J. Eislöffel 19 H. Falcke 20 W. Frieswijk 2 M. A. Garrett 21 Jean-Mathias Griessmeier 22 A. W. Gunst 2 J. W. T. Hessels 23 M. Hoeft 19 A. Karastergiou 24 V. I. Kondratiev 2 G. Kuper 2 J. van Leeuwen 23 D. Mckay-Bukowski 25 J. P. Mckean 2 H. Munk 2 E. Orru 2 H. Paas 26 R. Pizzo 2 A. G. Polatidis 2 A. M. M. Scaife 27 J. Sluman 2 C. Tasse 28 M. C. Toribio 2 R. Vermeulen 2 P. Zarka 6 
Abstract : 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 hours. 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 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.
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D. E. Morosan, P. T. Gallagher, P. Zucca, A. O’flannagain, R. Fallows, et al.. LOFAR tied-array imaging and spectroscopy of solar S bursts*. Astronomy and Astrophysics - A&A, EDP Sciences, 2015, 580, pp.A65. ⟨10.1051/0004-6361/201526064⟩. ⟨insu-01323436⟩



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