Abstract
Small and elongated, cool and dense blob-like structures are being reported with high resolution telescopes in physically different regions throughout the solar atmosphere. Their detection and the understanding of their formation, morphology, and thermodynamical characteristics can provide important information on their hosting environment, especially concerning the magnetic field, whose understanding constitutes a major problem in solar physics. An example of such blobs is coronal rain, a phenomenon of thermal non-equilibrium observed in active region loops, which consists of cool and dense chromospheric blobs falling along loop-like paths from coronal heights. So far, only off-limb coronal rain has been observed, and few reports on the phenomenon exist. In the present work, several data sets of on-disk Hα observations with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m Solar Telescope (SST) are analyzed. A special family of on-disk blobs is selected for each data set, and a statistical analysis is carried out on their dynamics, morphology, and temperature. All characteristics present distributions which are very similar to reported coronal rain statistics. We discuss possible interpretations considering other similar blob-like structures reported so far and show that a coronal rain interpretation is the most likely one. The chromospheric nature of the blobs and the projection effects (which eliminate all direct possibilities of height estimation) on one side, and their small sizes, fast dynamics, and especially their faint character (offering low contrast with the background intensity) on the other side, are found as the main causes for the absence until now of the detection of this on-disk coronal rain counterpart.
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Notes
Most of these observations are done with imaging instruments; thus the speeds correspond to projected values in the plane of the sky. However, since all reports correspond to off-limb observations, the values are close to the total velocities, as confirmed by the spectropolarimetric observations in Antolin and Rouppe van der Voort (2012).
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Acknowledgements
The authors would like to thank the organizers of the 13th European Solar Physics Meeting for a very good conference (abundant with the cheerful Greek spirit) and the opportunity to present this recent work. The Swedish 1-m Solar Telescope is operated on the island of La Palma by the Institute for Solar Physics of the Royal Swedish Academy of Sciences in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. We would further like to acknowledge Mats Carlsson, Viggo Hansteen, Jorrit Leenaarts, Ada Ortiz, and Sven Wedemeyer for co-observation of the 2008 and 2010 data sets.
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Advances in European Solar Physics
Guest Editors: Valery M. Nakariakov, Manolis K. Georgoulis, and Stefaan Poedts
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Antolin, P., Vissers, G. & Rouppe van der Voort, L. On-Disk Coronal Rain. Sol Phys 280, 457–474 (2012). https://doi.org/10.1007/s11207-012-9979-7
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DOI: https://doi.org/10.1007/s11207-012-9979-7