Investigating small solar flares with NuSTAR

Kristopher Cooper

Hosted by University of Glasgow, UK on May 20, 2021


Small, highly frequent flares are thought to contribute to heating the Sun’s atmosphere, particularly in active regions. This impulsive energy release would heat plasma >5 MK and accelerate electrons, producing weak thermal and non-thermal signatures that could be observed by a very sensitive X-ray telescope. No such solar telescope exists currently so we use the Nuclear Spectroscopic Telescope Array (NuSTAR), an astrophysical X-ray telescope, with focusing optics imaging spectroscopy providing a unique sensitivity for observing the Sun above 2.5 keV. In this seminar, I will present an overview of the discoveries from NuSTAR solar observations where decreasing solar activity between cycle 24 and 25 has allowed GOES sub-A class microflares to be observed regularly within, and small brightenings outside, active regions. In particular, I will describe several X-ray microflares from a recently emerged active region, AR12721, that were observed on 2018 September 9-10 with NuSTAR. In combination with SDO/AIA, I will discuss the temporal, spatial, and spectral evolution of these sub-A class microflares and show that temperatures up to 10 MK are reached. Using SDO/HMI, I also present evidence of photospheric magnetic flux cancellation/emergence at the footpoints in 8 NuSTAR microflares.

Recorded video