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تسجيل مستخدم جديدNuSTAR observations of a repeatedly microflaring active region
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We investigate the spatial, temporal, and spectral properties of 10 microflares from AR12721 on 2018 September 9 and 10 observed in X-rays using the Nuclear Spectroscopic Telescope ARray (NuSTAR) and the Solar Dynamic Observatorys Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager (SDO/AIA and HMI). We find GOES sub-A class equivalent microflare energies of 10$^{26}$-10$^{28}$ erg reaching temperatures up to 10 MK with consistent quiescent or hot active region core plasma temperatures of 3-4 MK. One microflare (SOL2018-09-09T10:33), with an equivalent GOES class of A0.1, has non-thermal HXR emission during its impulsive phase (of non-thermal power $sim$7$times$10$^{24}$ erg s$^{-1}$) making it one of the faintest X-ray microflares to have direct evidence for accelerated electrons. In 4 of the 10 microflares, we find that the X-ray time profile matches fainter and more transient sources in the EUV, highlighting the need for observations sensitive to only the hottest material that reaches temperatures higher than those of the active region core ($>$5 MK). Evidence for corresponding photospheric magnetic flux cancellation/emergence present at the footpoints of 8 microflares is also observed.
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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 S
olar 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.
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.
The current $super-active$ state of the recurrent nova T CrB has been observed with unprecedented detail. Previously published observations provide strong evidence that this state is due to an enhancement of the flow of material through the accretion
disk, which increased the optical depth of its most internal region, the boundary layer. $NuSTAR$ and $Swift$ observed T CrB in 2015 September, roughly halfway through the rise to optical maximum. In our analysis of these data, we have found that: $i$) the UV emission, as observed with $Swift$/UVOT in 2015, was already as bright as it became in 2017, after the optical peak; $ii$) the soft X-ray emission (E $lesssim$ 0.6 keV) observed in 2017 after the optical peak, on the other hand, had not yet developed during the rising phase in 2015; $iii$) the hard X-ray emitting plasma (E $gtrsim$ 2 keV) had the same temperature and about half the flux of that observed during quiescence in 2006. This phenomenology is akin to that observed during dwarf novae in outburst, but with the changes in the spectral energy distribution happening on a far longer time scale.
We aim to investigate the temperature enhancements and formation heights of impulsive heating phenomena in solar active-regions such as Ellerman bombs (EBs), ultraviolet bursts (UVBs), and flaring active-region fibrils (FAFs) using interferometric ob
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marking data-driven modelling of the upflows (Galsgaard et al., 2015). The studied AR upflow displays blueshifted emission of 5-20 km/s in Fe XII and Fe XIII and its average electron density is 1.8x10^9 cm^3 at 1 MK. The time variation of the density shows no significant change (in a 3sigma error). The plasma density along a single loop drops by 50% over a distance of 20000 km. We find a second velocity component in the blue wing of the Fe XII and Fe XIII lines at 105 km/s. This component is persistent during the whole observing period of 3.5 hours with variations of only 15 km/s. We also study the evolution of the photospheric magnetic field and find that magnetic flux diffusion is responsible for the formation of the upflow region. High cadence Halpha observations of the chromosphere at the footpoints of the upflow region show no significant jet-like (spicule/rapid blue excursion) activity to account for several hours/days of plasma upflow. Using an image enhancement technique, we show that the coronal structures seen in the AIA 193A channel is comparable to the EIS Fe XII images, while images in the AIA 171A channel reveals additional loops that are a result of contribution from cooler emission to this channel. Our results suggest that at chromospheric heights there are no signatures that support the possible contribution of spicules to AR upflows. We suggest that magnetic flux diffusion is responsible for the formation of the coronal upflows. The existence of two velocity components possibly indicate the presence of two different flows which are produced by two different physical mechanisms, e.g. magnetic reconnection and pressure-driven.
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