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Register a new userHard X-Ray Constraints on Small-Scale Coronal Heating Events
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Much evidence suggests that the solar corona is heated impulsively, meaning that nanoflares may be ubiquitous in quiet and active regions (ARs). Hard X-ray (HXR) observations with unprecedented sensitivity $>$3~keV are now enabled by focusing instruments. We analyzed data from the textit{Focusing Optics X-ray Solar Imager (FOXSI)} rocket and the textit{Nuclear Spectroscopic Telescope Array (NuSTAR)} spacecraft to constrain properties of AR nanoflares simulated by the EBTEL field-line-averaged hydrodynamics code. We generated model X-ray spectra by computing differential emission measures for homogeneous nanoflare sequences with heating amplitudes $H_0$, durations $tau$, delay times between events $t_N$, and filling factors $f$. The single quiescent AR observed by textit{FOXSI-2} on 2014 December 11 is well fit by nanoflare sequences with heating amplitudes 0.02 erg cm$^{-3}$ s$^{-1}$ $<$ $H_0$ $<$ 13 erg cm$^{-3}$ s$^{-1}$ and a wide range of delay times and durations. We exclude delays between events shorter than $sim$900 s at the 90% confidence level for this region. Three of five regions observed by { ustar} on 2014 November 1 are well fit by homogeneous nanoflare models, while two regions with higher fluxes are not. Generally, the { ustar} count spectra are well fit by nanoflare sequences with smaller heating amplitudes, shorter delays, and shorter durations than the allowed textit{FOXSI-2} models. These apparent discrepancies are likely due to differences in spectral coverage between the two instruments and intrinsic differences among the regions. Steady heating ($t_N$ = $tau$) was ruled out with $>$99% confidence for all regions observed by either instrument.
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This paper reports on the re-analysis of solar flares in which the hard X-rays (HXRs) come predominantly from the corona rather than from the more usual chromospheric footpoints. All of the 26 previously analyzed event time intervals, over 13 flares, are re-examined for consistency with a flare model in which electrons are accelerated near the top of a magnetic loop that has a sufficiently high density to stop most of the electrons by Coulomb collisions before they can reach the footpoints. Of particular importance in the previous analysis was the finding that the length of the coronal HXR source increased with energy in the 20 - 30 keV range. However, after allowing for the possibility that footpoint emission at the higher energies affects the inferred length of the coronal HXR source, and using analysis techniques that suppress the possible influence of such footpoint emission, we conclude that there is no longer evidence that the length of the HXR coronal sources increase with increasing energy. In fact, for the 6 flares and 12 time intervals that satisfied our selection criteria, the loop lengths decreased on average by 1.0 +/- 0.2 arcsec between 20 and 30 keV, with a standard deviation of 3.5 arcsec. We find strong evidence that the peak of the coronal HXR source increases in altitude with increasing energy. For the thermal component of the emission, this is consistent with the standard CHSKP flare model in which magnetic reconnection in a coronal current sheet results in new hot loops being formed at progressively higher altitudes. The explanation for the nonthermal emission is not so clear.
The hot solar corona exists because of the balance between radiative and conductive cooling and some counteracting heating mechanism which remains one of the major puzzles in solar physics. The coronal thermal equilibrium is perturbed by magnetoacoustic waves which are abundantly present in the corona, causing a misbalance between the heating and cooling rates. Due to this misbalance, the wave experiences a back-reaction, either losing or gaining energy from the energy supply that heats the plasma, at the time scales comparable to the wave period. In particular, the plasma can be subject to wave-induced instability or over-stability, depending on the specific choice of the coronal heating function. In the unstable case, the coronal thermal equilibrium would be violently destroyed, which does not allow for the existence of long-lived plasma structures typical for the corona. Based on this, we constrained the coronal heating function using observations of slow magnetoacoustic waves in various coronal plasma structures.
Even in the absence of resolved flares, the corona is heated to several million degrees. However, despite its importance for the structure, dynamics, and evolution of the solar atmosphere, the origin of this heating remains poorly understood. Several observational and theoretical considerations suggest that the heating is driven by small, impulsive energy bursts which could be Parker-style nanoflares (Parker 1988) that arise via reconnection within the tangled and twisted coronal magnetic field. The classical smoking gun (Klimchuk 2009; Cargill et al. 2013) for impulsive heating is the direct detection of widespread hot plasma (T > 6 MK) with a low emission measure. In recent years there has been great progress in the development of Transition Edge Sensor (TES) X-ray microcalorimeters that make them more ideal for studying the Sun. When combined with grazing-incidence focusing optics, they provide direct spectroscopic imaging over a broad energy band (0.5 to 10 keV) combined with extremely impressive energy resolution in small pixels, as low as 0.7 eV (FWHM) at 1.5 keV (Lee 2015), and 1.56 eV (FWHM) at 6 keV (Smith 2012), two orders of magnitude better than the current best traditional solid state photon-counting spectrometers. Decisive observations of the hot plasma associated with nanoflare models of coronal heating can be provided by new solar microcalorimeters. These measurements will cover the most important part of the coronal spectrum for searching for the nanoflare-related hot plasma and will characterize how much nanoflares can heat the corona both in active regions and the quiet Sun. Finally, microcalorimeters will enable to study all of this as a function of time and space in each pixel simultaneously a capability never before available.
We present the first results of a search for transient hard X-ray (HXR) emission in the quiet solar corona with the textit{Nuclear Spectroscopic Telescope Array} (textit{NuSTAR}) satellite. While textit{NuSTAR} was designed as an astrophysics mission, it can observe the Sun above 2~keV with unprecedented sensitivity due to its pioneering use of focusing optics. textit{NuSTAR} first observed quiet Sun regions on 2014 November 1, although out-of-view active regions contributed a notable amount of background in the form of single-bounce (unfocused) X-rays. We conducted a search for quiet Sun transient brightenings on time scales of 100 s and set upper limits on emission in two energy bands. We set 2.5--4~keV limits on brightenings with time scales of 100 s, expressed as the temperature T and emission measure EM of a thermal plasma. We also set 10--20~keV limits on brightenings with time scales of 30, 60, and 100 s, expressed as model-independent photon fluxes. The limits in both bands are well below previous HXR microflare detections, though not low enough to detect events of equivalent T and EM as quiet Sun brightenings seen in soft X-ray observations. We expect future observations during solar minimum to increase the textit{NuSTAR} sensitivity by over two orders of magnitude due to higher instrument livetime and reduced solar background.
Rapid Blue- and Red-shifted Excursions (RBEs and RREs) are likely to be the on-disk counterparts of Type II spicules. Recently, heating signatures from RBEs/RREs have been detected in IRIS slit-jaw images dominated by transition-region lines around network patches. Additionally, signatures of Type II spicules have been observed in AIA diagnostics. The full-disk, ever-present nature of the AIA diagnostics should provide us with sufficient statistics to directly determine how important RBEs and RREs are to the heating of the transition region and corona. We find, with high statistical significance, that at least 11% of the low-coronal brightenings detected in a quiet-Sun region in 304, can be attributed to either RBEs or RREs as observed in Halpha, and a 6% match of 171 detected events to RBEs or RREs with very similar statistics for both types of Halpha features. We took a statistical approach that allows for noisy detections in the coronal channels and provides us with a lower, but statistical significant, bound. Further, we consider matches based on overlapping features in both time and space, and find strong visual indications of further correspondence between coronal events and co-evolving but non-overlapping, RBEs and RREs.
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