Evidence of nonthermal particles in coronal loops heated impulsively by nanoflares

Abstract

Abstract:The physical processes resulting in energy exchange between the Sun's hot corona and its cool lower atmosphere are still 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 (~20-60s) of intensity and velocity on small spatial scales (≲500km) at the footpoints of hot and dynamic coronal loops. Comparison with numerical simulations reveal that the observations are consistent with heating by beams of non-thermal electrons and that these beams are generated even in small impulsive (≲30s) heating events called "coronal nanoflares". The accelerated electrons deposit a significant fraction of their energy (≲10 25 erg) 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 non-flaring corona. Main Text:Though it is established that the magnetic field plays a major role in the energetics of the bright corona, determining the details of the physical mechanisms that heat the solar corona remains one of the outstanding open issues in astrophysics. There are several candidate physical processes for heating the corona, including dissipation of magnetic stresses via reconnection, and dissipation of magnetohydrodynamic waves (1,2,3). In many heating models, the energy release is characterized by small spatial and temporal scales. For instance, in the "nanoflare" model, random photospheric motions lead to braiding or shearing of magnetic field lines and to reconnection which yields impulsive heating of the coronal plasma (4,5). Several statistical studies of large numbers of solar flares (6-8) have suggested that the mechanisms producing flares are likely similar within a large range from micro-to X-class flares. If nanoflares behave as a scaled down version of larger flares, particles accelerated in the corona by reconnection processes could play a significant role in the heating of plasma even in absence of large flares. Hard X-ray observations of microflares (E~10 27 erg) in active regions reveal the presence of non-thermal particles (8,9), but nanoflare size events (E~10 24 erg) are not currently accessible to hard X-ray studies due to limited sensitivity. As a result, the properties and generation of non-thermal particles in the solar atmosphere and their role in quiescent coronal heating remain poorly known.The observational tracers of the coronal heating are elusive because the corona is highly conductive, washing out the signatures of heating release. However, the emission of the TR, where the temperature steeply increases to MK values in a narrow layer (~1-3 ×10 8 cm), is instead highly responsive to heating since its density, temperature gradients and spatial dimensions change rapidly during heating events (10)(11)(12). This is the also the case for coronal heating events where ...

Transition Region and Chromospheric Signatures of Impulsive Heating Events. Ii. Modeling

“…At that cut-off, the threshold between gentle and explosive evaporation is ≈3´10 9 erg s −1 cm −2 (Reep et al 2015). For lower beam fluxes, the Si IV line is in fact blueshifted (compare Testa et al 2014), which was never observed during this event. We therefore can reasonably conclude that the maximum beam flux must be greater than this value.…”

“…In this work we focus on the Si IV 1402.770 Å and C II 1334.535 Å lines, which are useful diagnostics of heat transport to the lower atmosphere (e.g., Testa et al 2014), and strongly correlated with energy input (Cheng et al 1981). …”

Evidence for two-loop interaction from IRIS and SDO observations of penumbral brightenings

“…In any case, small scale events, being relatively simpler in their Article number, page 1 of 11 arXiv:1704.07344v1 [astro-ph.SR] 24 Apr 2017 spatial structure and time evolution than their large-scale counterparts, provide a good opportunity to check existing models of energy release and atmospheric structure and to develop new models (e.g. Klimchuk 2006;Brooks & Warren 2008;Raouafi et al 2010;Testa et al 2014;Raouafi et al 2016, among others).…”

Transition Region and Chromospheric Signatures of Impulsive Heating Events. I. Observations

“…Lower resolution observations of the moss, in contrast, had suggested that the heating was relatively steady (e.g., Antiochos et al 2003;Brooks & Warren 2009). Similarly, Testa et al (2014) identified several events in 9.5 s cadence IRIS sit-and-stare observations that showed strong blueshifts in Si IV. Numerical simulations indicate that these blueshifts are a signature of energy deposition at heights below the region where Si IV is typically formed, perhaps because the energy transport in these events is driven by electron beams rather than thermal conduction.…”