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Register a new userAccelerated electrons observed down to <7 keV in a NuSTAR solar microflare
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We report the detection of emission from a non-thermal electron distribution in a small solar microflare (GOES class A5.7) observed by the Nuclear Spectroscopic Telescope Array (NuSTAR), with supporting observation by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The flaring plasma is well accounted for by a thick-target model of accelerated electrons collisionally thermalizing within the loop, akin to the coronal thick target behavior occasionally observed in larger flares. This is the first positive detection of non-thermal hard X-rays from the Sun using a direct imager (as opposed to indirectly imaging instruments). The accelerated electron distribution has a spectral index of 6.3 +/- 0.7, extends down to at least 6.5 keV, and deposits energy at a rate of ~2x1027 erg/s, heating the flare loop to at least 10 MK. The existence of dominant non-thermal emission in X-rays down to <5 keV means that RHESSI emission is almost entirely non-thermal, contrary to what is usually assumed in RHESSI spectroscopy. The ratio of non-thermal to thermal energies is similar to that of large flares, in contrast to what has been found in previous studies of small RHESSI flares. We suggest that a coronal thick target may be a common property of many small microflares based on the average electron energy and collisional mean free path. Future observations of this kind will enable understanding of how flare particle acceleration changes across energy scales, and will aid the push toward the observational regime of nanoflares, which are a possible source of significant coronal heating.
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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 heating of material to several million Kelvin, observed in Soft X-rays (SXRs) with Hinode/XRT, and was faintly visible in Extreme Ultraviolet (EUV) with SDO/AIA. For three of the four NuSTAR observations of this region (pre-, decay, and post 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 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 the pre-flare DEM peaks at ~3 MK and falls off sharply by 5 MK; but during the microflares impulsive phase the emission above 3 MK is brighter and extends to 10 MK, giving a heating rate of about $2.5 times 10^{25}$ 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 the heating requires a power-law spectrum of $delta ge 7$ with a low energy cut-off $E_{c} lesssim 7$ keV. In summary, this first NuSTAR microflare strongly resembles much more powerful flares.
We present X-ray imaging spectroscopy of one of the weakest active region (AR) microflares ever studied. The microflare occurred at $sim$11:04 UT on 2018 September 9 and we studied it using the Nuclear Spectroscopic Telescope ARray (NuSTAR) and the Solar Dynamic Observatorys Atmospheric Imaging Assembly (SDO/AIA). The microflare is observed clearly in 2.5-7 keV with NuSTAR and in Fe XVIII emission derived from the hotter component of the 94 $unicode{x212B}$ SDO/AIA channel. We estimate the event to be three orders of magnitude lower than a GOES A class microflare with an energy of 1.1$times$10$^{26}$ erg. It reaches temperatures of 6.7 MK with an emission measure of 8.0$times$10$^{43}$ cm$^{-3}$. Non-thermal emission is not detected but we instead determine upper limits to such emission. We present the lowest thermal energy estimate for an AR microflare in literature, which is at the lower limits of what is still considered an X-ray microflare.
We have found an interesting event registered by the solar neutron telescopes installed at high mountains in Bolivia (5250 m a.s.l.) and Mexico (4600 m a.s.l.). The event was observed November 7th of 2004 in association with a large solar flare of magnitude X2.0. Some features in our registers and in two satellites (GOES 11 and SOHO) reveal the presence of electrons and protons as possible products of neutron decay. Solar neutron decay protons (sndp) were recorded on board ISEE3 satellite in June 3rd, 1982 . On October 19th, 1989, the ground level detectors installed in Goose Bay and Deep River revealed the registration of solar neutron decay protons (sndp). Therefore this is the second example that such an evidence is registered on the Earths surface.
Solar flares are sudden energy release events in the solar corona, resulting from magnetic reconnection, that accelerates particles and heats the ambient plasma. During a flare, there are often multiple, temporally and spatially separated individual energy release episodes that can be difficult to resolve depending on the observing instrument. We present multi-wavelength imaging and spectroscopy observations of multiple electron acceleration episodes during a GOES B1.7-class two-ribbon flare on 2012 February 25, observed simultaneously with the Karl G. Jansky Very Large Array (VLA) at 1--2 GHz, the Reuven Ramatay High Energy Solar Spectroscopic Imager (RHESSI) in X-rays, and the Solar Dynamics Observatory in extreme ultraviolet (EUV). During the initial phase of the flare, five radio bursts were observed. A nonthermal X-ray source was seen co-temporal, but not co-spatial, with the first three radio bursts. Their radio spectra are interpreted as optically thick gyrosynchrotron emission. By fitting the radio spectra with a gyrosynchrotron model, we derive the magnetic field strength and nonthermal electron spectral parameters in each acceleration episode. Notably, the nonthermal parameters derived from X-rays differ considerably from the nonthermal parameters inferred from the radio. The observations are indicative of multiple, co-temporal acceleration episodes during the impulsive phase of a solar microflare. The X-ray and radio burst sources likely originate from separate electron distributions in different magnetic loops.
Investigate particle acceleration and heating in a solar microflare. In a microflare with non-thermal emission to remarkably high energies ($>50$ keV), we investigate the hard X-rays with RHESSI imaging and spectroscopy and the resulting thermal emission seen in soft X-rays with Hinode/XRT and in EUV with TRACE. The non-thermal footpoints observed with RHESSI spatially and temporally match bright footpoint emission in soft X-rays and EUV. There is the possibility that the non-thermal spectrum extends down to 4 keV. The hard X-ray burst clearly does not follow the expected Neupert effect, with the time integrated hard X-rays not matching the soft X-ray time profile. So although this is a simple microflare with good X-ray observation coverage it does not fit the standard flare model.
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