Cold Solar Flares

Gregory Fleishman,1,2,3 Alexandra Lysenko,1 Valentin Pal'shin,1 Natalia Meshalkina,4 Dmitriy Zhdanov,4 Larissa Kashapova,4 Alexander Altyntsev4

1Ioffe Institute, Polytekhnicheskaya 26, 194021, St. Petersburg, Russia
2Center For Solar-Terrestrial Research, New Jersey Institute of Technology, Newark, NJ 07102, USA
3Central Astronomical Observatory at Pulkovo of RAS, Saint-Petersburg 196140, Russia
4Institute of Solar-Terrestrial Physics SB RAS, Lermontov St. 126A, Irkutsk 664033, Russia

Recently, a number of "cold" flares, which demonstrate significant non-thermal particle signatures without any noticeable thermal emission, have been reported. The interest to these events is motivated by the implied close association of the observed emission with the primary energy release/electron acceleration region. In this talk I briefly review a few published cases presenting a few cold flares and outline the most recent activity in search and statistical analysis of the cold flares using Konus-Wind data and microwave data from a number of radio instruments. The main part of my talk is dedicated to a flare that appears a "cold" one at the impulsive phase, while displaying clear evidence of a delayed heating later on. Using HXR data obtained by Konus-Wind, microwave observations by SSRT, RSTN, NoRH and NoRP, relevant context observations, and 3D modeling, we study the process of the energy release, particle acceleration and transport, and the relationships between the nonthermal and thermal signatures in this flare. We found ample evidence that the flaring process involved interaction between a small and a big loops and the accelerated particles divided in roughly equal numbers between these two loops. Precipitation of the electrons from the small loop produced only weak thermal response because the loop volume was small, while the electrons trapped in the big loop lost most of their energy in the coronal part of the loop, which resulted in the coronal plasma heating but no or only weak chromospheric evaporation, resulting in unusually weak GOES emission. Energy losses of fast electrons in the big tenuous loop were relatively slow resulting in the observed delay of the plasma heating. We determined that the impulsively accelerated electron population had a beamed angular distribution in the direction of electric force along the magnetic field of the small loop. Also, we found that the accelerated particle transport in big loop is primarily mediated by turbulent waves like in the other reported cold flares.

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