Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA

Abstract

NuSTAR is a highly sensitive focusing hard X-ray (HXR) telescope and has observed several small microflares in its initial solar pointings. In this paper, we present the first joint observation of a microflare with NuSTAR and Hinode/ XRT on 2015 April 29 at ∼11:29 UT. This microflare shows the heating of material to several million Kelvin, observed in soft X-rays with Hinode/XRT, and was faintly visible in the extreme ultraviolet with SDO/AIA. For three of the four NuSTAR observations of this region (pre-flare, decay, and post-flare phases), the spectrum is well fitted by a single thermal model of 3.2-3.5 MK, but the spectrum during the impulsive phase shows additional emission up to 10 MK, emission equivalent to the A0.1 GOES class. We recover the differential emission measure (DEM) using SDO/AIA, Hinode/XRT, and NuSTAR, giving unprecedented coverage in temperature. We find that the pre-flare DEM peaks at ∼3 MK and falls off sharply by 5 MK; but during the microflare's impulsive phase, the emission above 3 MK is brighter and extends to 10 MK, giving a heating rate of about2.5 10 25 erg s −1. As the NuSTAR spectrum is purely thermal, we determined upper limits on the possible non-thermal bremsstrahlung emission. We find that for the accelerated electrons to be the source of heating, a power-law spectrum of  d 7 with a low-energy cutoff  E 7 c keV is required. In summary, this first NuSTAR microflare strongly resembles much more powerful flares.

Joint X-Ray, EUV, and UV Observations of a Small Microflare

“…In this paper, we presented the smallest microflare seen yet with NuSTAR , about an order of magnitude weaker than those previously observed with NuSTAR (Wright et al 2017;Glesener et al 2017) and well beyond the microflares observed with RHESSI (Hannah et al 2008a). This event is similar in thermal energy to quiet Sun flares seen with NuSTAR (Kuhar et al 2018), however the microflare presented in this paper demonstrates higher temperature emission and is from an active region.…”

“…It has targeted the Sun several times since the first pointing in late 2014 , and has observed active region microflares as well as quiet Sun brightenings. These microflares are about an order of magnitude weaker than RHESSI could observe, down to an estimated GOES level of ∼A0.1, and showed heating up to about 10MK, with thermal energies of 10 27 erg (Wright et al 2017;Glesener et al 2017). These events were also well observed at longer wavelengths, in softer X-rays with Hinode/XRT and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA ).…”

NuSTAR Hard X-Ray Observation of a Sub-A Class Solar Flare

“…Other NuSTAR microflares are the subjects of recent or upcoming papers, including one by Wright et al (2017) that shows a detailed differential emission measure obtained via Hinode XRT coordination. We expect future NuSTAR events with higher livetime and correspondingly better statistics.…”

Solar microflares: a case study on temperatures and the Fe XVIII emission

“…Most of the events that have been studied with RHESSI are much larger than those considered here, though. Recently, NuSTAR, the very sensitive astrophysics mission, has also been used to observe quiescent active regions, and on one occasion a microflare was observed, as described in Wright et al (2017). Although the NuSTAR resolution does not allow to resolve spectral lines, the (thermal) continuum measurements indicated very low temperatures (of the order of 3-5 MK) for the event of GOES class A0.1.…”