Observational Signatures of Coronal Loop Heating and Cooling Driven by Footpoint Shuffling

Abstract

The evolution of a coronal loop is studied by means of numerical simulations of the fully compressible threedimensional magnetohydrodynamic equations using the HYPERION code. The footpoints of the loop magnetic field are advected by random motions. As a consequence the magnetic field in the loop is energized and develops turbulent nonlinear dynamics characterized by the continuous formation and dissipation of fieldaligned current sheets: energy is deposited at small scales where heating occurs. Dissipation is non-uniformly distributed so that only a fraction of the coronal mass and volume gets heated at any time. Temperature and density are highly structured at scales which, in the solar corona, remain observationally unresolved: the plasma of our simulated loop is multi-thermal, where highly dynamical hotter and cooler plasma strands are scattered throughout the loop at sub-observational scales. Numerical simulations of coronal loops of 50000 km length and axial magnetic field intensities ranging from 0.01 to 0.04 Tesla are presented. To connect these simulations to observations we use the computed number densities and temperatures to synthesize the intensities expected in emission lines typically observed with the Extreme ultraviolet Imaging Spectrometer (EIS) on Hinode. These intensities are used to compute differential emission measure distributions using the Monte Carlo Markov Chain code, which are very similar to those derived from observations of solar active regions. We conclude that coronal heating is found to be strongly intermittent in space and time, with only small portions of the coronal loop being heated: in fact, at any given time, most of the corona is cooling down.

The Heating of Solar Coronal Loops by Alfvén Wave Turbulence

“…The authors simulate the response of the coronal plasma to such heating, and find that the heating produces large variations in coronal temperature across the loop, but only small density variations. Dahlburg et al (2016) compare the simulated DEM distributions with observations, and find that a good fit to the observations is obtained for a coronal field strength of 400 G. However, the Dahlburg et al model has several limitations that may cause the coronal heating rates and temperatures to be overestimated. First, the model does not include the chromosphere, which may cause the energy available to the corona to be overestimated.…”

“…For example, in our previous work on observed coronal loops in the cores of active regions we obtained minimum field strengths in the range 13 -30 G (paper II), 30 -120 G (paper III), and 20 -45 G (paper IV). Finally, in the model by Dahlburg et al (2016) the temperature and density are held fixed at the TR boundaries, so there is no chromospheric "evaporation" in response to coronal heating events. Therefore, the model may overestimate the temperature increase resulting from a given heating event.…”

“…The velocities v x (x, y, t) and v y (x, y, t) at the merging height are assumed to be continuous functions of position, so there are no topologically distinct flux tubes in the corona. This assumption of spatial continuity of the footpoint motions is often used in numerical simulations of the build-up and evolution of braided magnetic fields (e.g., van Ballegooijen 1988;Mikić et al 1989;Galsgaard & Nordlund 1996;Rappazzo et al 2007Rappazzo et al , 2008Rappazzo et al , 2013Dahlburg et al 2016). Other authors focus on the turbulent relaxation of braided fields, and assume that the braided field is present in the initial state of the simulation (e.g., Wilmot-Smith 2015; Pontin & Hornig 2015;Pontin et al 2017), so in these models the build-up of the braided field is not simulated.…”

3d MHD Modeling of Twisted Coronal Loops

“…This approach is able to describe the formation and powering of coronal loops in a qualitative or semi-quantitative way (Gudiksen & Nordlund 2005a;Bingert & Peter 2011), including the emergence of flux tubes by magnetic twisting (Martínez-Sykora et al 2008, 2009, so as to reproduce several observed features, such as a constant cross-section (Peter & Bingert 2012), and to help interpret and use data analysis tools . Recent work has supported episodic and structured heating due to the fragmentation of current sheets and/or turbulent cascades (Hansteen et al 2015;Dahlburg et al 2016).…”

Multifractal Solar Euv Intensity Fluctuations and Their Implications for Coronal Heating Models

“…In recent work Dahlburg et al (2016) performed three-dimensional fully compressible MHD simulations of a coronal loop in Cartesian geometry with setups very similar to those of Rappazzo et al (2007Rappazzo et al ( , 2008. The dynamics are indeed very similar for the two cases, but the use of the energy equation (with field-aligned thermal conduction and radiative losses) in the fully compressible case allows one to compute the emitted radiation.…”