LOFAR tied-array imaging and spectroscopy of solar S bursts
2015 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 580, A65Article in journal (Refereed) Published
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 (similar to 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 similar to 8 h. S bursts were found to appear as groups of short-lived (<1 s) and narrow-bandwidth (similar to 2.5 MHz) features, the majority drifting at similar to 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.
Place, publisher, year, edition, pages
2015. Vol. 580, A65
Sun: corona, Sun: radio radiation, Sun: particle emission, Sun: magnetic fields
Astronomy, Astrophysics and Cosmology Fusion, Plasma and Space Physics
IdentifiersURN: urn:nbn:se:uu:diva-263462DOI: 10.1051/0004-6361/201526064ISI: 000360020200065OAI: oai:DiVA.org:uu-263462DiVA: diva2:858092