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 A. A. Konovalenko 6 A. L. Mackinnon 7 H. O. Rucker 8 B. Thidé 9 C. Vocks 4 A. Alexov 2 J. Anderson 10 A. Asgekar 2 I. M. Avruch 11 M. J. Bentum 2 G. Bernardi 12 A. Bonafede 13 F. Breitling 4 J. W. Broderick 14 W. N. Brouw 15 H. R. Butcher 16 B. Ciardi 17 E. de Geus 2 J. Eislöffel 18 H. Falcke 19 W. Frieswijk 2 M. A. Garrett 20 Jean-Mathias Grießmeier 21 A. W. Gunst 2 J. W. T. Hessels 22 M. Hoeft 18 A. Karastergiou 23 V. I. Kondratiev 2 G. Kuper 2 J. van Leeuwen 22 D. Mckay-Bukowski 24 J. P. Mckean 2 H. Munk 2 E. Orru 2 H. Paas 25 R. Pizzo 2 A. G. Polatidis 2 A. M. M. Scaife 26 J. Sluman 2 C. Tasse 27 M. C. Toribio 2 R. Vermeulen 2 P. Zarka 27
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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