Plasma motions and non-thermal line broadening in flaring twisted coronal loops

Abstract

Context. Observation of coronal extreme ultra-violet (EUV) spectral lines sensitive to different temperatures offers an opportunity to evaluate the thermal structure and flows in flaring atmospheres. This, in turn, can be used to estimate the partitioning between the thermal and kinetic energies released in flares. Aims. Our aim is to forward-model large-scale (50-10 000 km) velocity distributions to interpret non-thermal broadening of different spectral EUV lines observed in flares. The developed models allow us to understand the origin of the observed spectral line shifts and broadening, and link these features to particular physical phenomena in flaring atmospheres. Methods. We use ideal magnetohydrodynamics (MHD) to derive unstable twisted magnetic fluxtube configurations in a gravitationally stratified atmosphere. The evolution of these twisted fluxtubes is followed using resistive MHD with anomalous resistivity depending on the local density and temperature. The model also takes thermal conduction and radiative losses in the continuum into account. The model allows us to evaluate average velocities and velocity dispersions, which would be interpreted as non-thermal velocities in observations, at different temperatures for different parts of the models. Results. Our models show qualitative and quantitative agreement with observations. Thus, the line-of-sight (LOS) velocity dispersions demonstrate substantial correlation with the temperature, increasing from about 20-30 km s −1 around 1 MK to about 200-400 km s −1 near 10-20 MK. The average LOS velocities also correlate with velocity dispersions, although they demonstrate a very strong scattering compared to the observations. We also note that near footpoints the velocity dispersions across the magnetic field are systematically lower than those along the field. We conclude that the correlation between the flow velocities, velocity dispersions, and temperatures are likely to indicate that the same heating mechanism is responsible for heating the plasma, its turbulisation, and expansion/evaporation.

Turbulent Kinetic Energy in the Energy Balance of a Solar Flare

“…While K attains its peak value around the same time (∼01∶25 UT) as the SXR flux, it notably has a value equal to some 20% of its peak value as early as 01∶40 UT, well before the peak in the HXR flux. Similar behavior is also seen in MHD simulations [38]. Figure 4 also shows the ratio of the turbulent kinetic energy K (ergs) to the power P (erg s −1 ) in energetic electrons; the ratio K=P (which is a measure of the time it takes a power P to energize or deplete a reservoir of energy K) has a relatively steady value of order 1-10 s.…”

“…In relation to particle acceleration, there are, broadly speaking, two representations of turbulence (stochasticity): "wave turbulence" [20,40,41] and a "stochastic ensemble of current sheets" [38,[42][43][44]. These two concepts are not necessarily unrelated, since there is a tendency for MHD turbulence to form current sheets [45].…”

Observational Signatures of a Kink-unstable Coronal Flux Rope Using Hinode/EIS

“…Williams et al 2005;Török et al 2014) or non-eruptive (e.g. Srivastava et al 2010;Browning et al 2008;Botha et al 2011;Pinto et al 2015;Gordovskyy et al 2013Gordovskyy et al , 2016. The observations of the kink instability, however, show no clear evidence of small-scale reconnection.…”

“…The ideal MHD kink instability triggers reconnection in numerical simulations of coronal loops (Browning et al 2008;Hood et al 2009;Botha et al 2011;Pinto et al 2015;Gordovskyy et al 2016). A cylindrically-twisted unstable magnetic field is specified as an initial condition, evolving into the kink instability.…”

Forward Modeling of Particle Acceleration and Transport in an Individual Solar Flare

“…For instance, Turkmani et al (2005) and Gordovskyy & Browning (2011, 2012 used 3D MHDTP simulations to show that diffuse, large-scale acceleration regions with fragmented, stochastic electric field configurations can be efficient particle accelerators, and an alternative to the standard solar flare scenario (Shibata et al 1995). Gordovskyy et al (2014Gordovskyy et al ( , 2016Gordovskyy et al ( , 2017 and Pinto et al (2016) successfully used MHDTP modeling to investigate solar flares involving unstable twisted coronal loops, and compare the evolution of thermal and nonthermal emissions associated with these events.…”