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Register a new userTurbulent cross-field transport of non-thermal electrons in coronal loops: theory and observations
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A fundamental problem in astrophysics is the interaction between magnetic turbulence and charged particles. It is now possible to use emph{Ramaty High Energy Solar Spectroscopic Imager (RHESSI)} observations of hard X-rays (HXR) emitted by electrons to identify the presence of turbulence and to estimate the magnitude of the magnetic field line diffusion coefficient at least in dense coronal flaring loops.} {We discuss the various possible regimes of cross-field transport of non-thermal electrons resulting from broadband magnetic turbulence in coronal loops. The importance of the Kubo number $K$ as a governing parameter is emphasized and results applicable in both the large and small Kubo number limits are collected.} {Generic models, based on concepts and insights developed in the statistical theory of transport, are applied to the coronal loops and to the interpretation of hard X-ray imaging data in solar flares. The role of trapping effects, which become important in the non-linear regime of transport, is taken into account in the interpretation of the data.} For this flaring solar loop, we constrain the ranges of parallel and perpendicular correlation lengths of turbulent magnetic fields and possible Kubo numbers. We show that a substantial amount of magnetic fluctuations with energy $sim 1%$ (or more) of the background field can be inferred from the measurements of the magnetic diffusion coefficient inside thick-target coronal loops.
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Thermal non-equilibrium (TNE) is a phenomenon that can occur in solar coronal loops when the heating is quasi-constant and highly-stratified. Under such heating conditions, coronal loops undergo cycles of evaporation and condensation. The recent observations of ubiquitous long-period intensity pulsations in coronal loops and their relationship with coronal rain have demonstrated that understanding the characteristics of TNE cycles is an essential step in constraining the circulation of mass and energy in the corona. We report unique observations with the Solar Dynamics Observatory (SDO) and the Swedish 1-m Solar Telescope (SST) that link the captured thermal properties across the extreme spatiotemporal scales covered by TNE processes. Within the same coronal loop bundle, we captured 6 hr period coronal intensity pulsations in SDO/AIA and coronal rain observed off-limb in the chromospheric Halpha and Ca II K spectral lines with SST/CRISP and SST/CHROMIS. We combined a multi-thermal analysis of the cycles with AIA and an extensive spectral characterisation of the rain clumps with the SST. We find clear evidence of evaporation-condensation cycles in the corona which are linked with periodic coronal rain showers. The high-resolution spectroscopic instruments at the SST reveal the fine-structured rain strands and allow us to probe the cooling phase of one of the cycles down to chromospheric temperatures. These observations reinforce the link between long-period intensity pulsations and coronal rain. They also demonstrate the capability of TNE to shape the dynamics of active regions on the large scales as well as on the smallest scales currently resolvable.
Recent observations from {em RHESSI} have revealed that the number of non-thermal electrons in the coronal part of a flaring loop can exceed the number of electrons required to explain the hard X-ray-emitting footpoints of the same flaring loop. Such sources cannot, therefore, be interpreted on the basis of the standard collisional transport model, in which electrons stream along the loop while losing their energy through collisions with the ambient plasma; additional physical processes, to either trap or scatter the energetic electrons, are required. Motivated by this and other observations that suggest that high energy electrons are confined to the coronal region of the source, we consider turbulent pitch angle scattering of fast electrons off low frequency magnetic fluctuations as a confinement mechanism, modeled as a spatial diffusion parallel to the mean magnetic field. In general, turbulent scattering leads to a reduction of the collisional stopping distance of non-thermal electrons along the loop and hence to an enhancement of the coronal HXR source relative to the footpoints. The variation of source size $L$ with electron energy $E$ becomes weaker than the quadratic behavior pertinent to collisional transport, with the slope of $L(E)$ depending directly on the mean free path $lambda$ again pitch angle scattering. Comparing the predictions of the model with observations, we find that $lambda sim$$(10^8-10^9)$ cm for $sim30$ keV, less than the length of a typical flaring loop and smaller than, or comparable to, the size of the electron acceleration region.
Long-period EUV pulsations, recently discovered to be common in active regions, are understood to be the coronal manifestation of thermal non-equilibrium (TNE). The active regions previously studied with EIT/SOHO and AIA/SDO indicated that long-period intensity pulsations are localized in only one or two loop bundles. The basic idea of this study is to understand why. For this purpose, we tested the response of different loop systems, using different magnetic configurations, to different stratifications and strengths of the heating. We present an extensive parameter-space study using 1D hydrodynamic simulations (1,020 in total) and conclude that the occurrence of TNE requires specific combinations of parameters. Our study shows that the TNE cycles are confined to specific ranges in parameter space. This naturally explains why only some loops undergo constant periodic pulsations over several days: since the loop geometry and the heating properties generally vary from one loop to another in an active region, only the ones in which these parameters are compatible exhibits TNE cycles. Furthermore, these parameters (heating and geometry) are likely to vary significantly over the duration of a cycle, which potentially limits the possibilities of periodic behavior. This study also confirms that long-period intensity pulsations and coronal rain are two aspects of the same phenomenon: both phenomena can occur for similar heating conditions and can appear simultaneously in the simulations.
The physical processes causing energy exchange between the Suns hot corona and its cool lower atmosphere remain poorly understood. The chromosphere and transition region (TR) form an interface region between the surface and the corona that is highly sensitive to the coronal heating mechanism. High resolution observations with the Interface Region Imaging Spectrograph (IRIS) reveal rapid variability (about 20 to 60 seconds) of intensity and velocity on small spatial scales at the footpoints of hot dynamic coronal loops. The observations are consistent with numerical simulations of heating by beams of non-thermal electrons, which are generated in small impulsive heating events called coronal nanoflares. The accelerated electrons deposit a sizable fraction of their energy in the chromosphere and TR. Our analysis provides tight constraints on the properties of such electron beams and new diagnostics for their presence in the nonflaring corona.
Active region EUV loops are believed to trace a subset of magnetic field lines through the corona. Malanushenko et al. (2009) proposed a method, using loop images and line-of-sight photospheric magnetograms, to infer the three-dimensional shape and field strength along each loop. McCarthy et al. (2019) used this novel method to compute the total magnetic flux interconnecting a pair of active regions observed by SDO/AIA. They adopted the common assumption that each loop had a circular cross section. The accuracy of inferred shape and circularity of cross sections can both be tested using observations of the same loops from additional vantage points as provided by STEREO/EUVI. Here, we use multiple viewing angles to confirm the three-dimensional structure of loops. Of 151 viable cases, 105 (69.5%) matched some form of visible coronal structure when viewed approximately in quadrature. A loop with a circular cross-section should appear of a similar width in different perspectives. In contradiction to this, we find a puzzling lack of correlation between loop diameters seen from different perspectives, even an anti-correlation in some cases. Features identified as monolithic loops in AIA may, in fact, be more complex density enhancements. The 30.5% of reconstructions from AIA which did not match any feature in EUVI might be such enhancements. Others may be genuine loop structures, but with elliptical cross sections. We observe an anti-correlation between diameter and brightness, lending support to the latter hypothesis. Of 13 suitable for width analysis, four loops are consistent with non-circular cross sections, where we find anti-correlation in both comparisons.
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