Explosive heating of low-density coronal plasma

Abstract

Context. This paper addresses the impulsive heating of very diffuse coronal loops, such as can occur in a nanoflare-heated corona with low filling factor. Aims. We study the physics associated with nanoflare heating in this scenario and aim to determine whether there exist any observable signatures. Methods. We derive an analytical model in order to gain some simple physical insights into the system and use a one dimensional hydrodynamic model that treats the electrons and ions as a coupled fluid to simulate nanoflare heating with time-scales of 30 s. Our analytical model also provide a means of verifying our numerical results. Results. We find that diffuse loops containing plasma at T > 20 MK can be rapidly created and subsequently filled by the violent evaporation of chromospheric plasma driven by near-saturated thermal conduction. Most importantly, we find order-of-magnitude departures from equilibrium of the ionisation balance for iron and use this result to identify a potential signature of this heating mechanism. Conclusions. We conclude that nanoflare heating can account for the presence of extremely high temperature plasma in a corona with low filling factor. We find that near-saturated thermal conduction may play a key role at the onset of chromospheric evaporation and a non-equilibrium ionisation balance is absolutely inevitable. The high temperatures could never be directly measured in the corona due to the small emission measure and the most promising signature of such heating is blue-shifted plasma from the loop footpoints. We find reason for cautious optimism that this signature can be detected by future space-based spectroscopic instrumentation (e.g. SolarB-EIS).

Measurements of Non-Thermal Line Widths in Solar Active Regions

“…Non-equilibrium ionization could contribute to surpressing the emission too (Bradshaw & Cargill 2006). In this picture, the non-thermal broadening of the spectral lines would appear as it does in real flares, with the hot AR core loops corresponding to post-flare loops observed in their cooling phase.…”

“…For example, measurements of non-thermal velocities of 40 km s −1 or so would tend to support a chromospheric evaporation or Alfvén wave turbulence explanation, though clearly they could not discriminate between them. Conversely, the nanoflare model may predict high non-thermal velocities, but these may only be apparent immediately after the energy release, and could be difficult to detect due to the low emission measure (EM) of any hot plasma, a problem that may be further exacerbated by non-equilibrium conditions (Bradshaw & Cargill 2006).…”

Forward Modelling of a Brightening Observed by AIA

“…Müller, Hansteen, and Peter (2003) investigated the effects of non-equilibrium ionisation on condensation in transition region spectral lines. Bradshaw and Cargill (2006) found significant departure from equilibrium in models of coronal loops heated by nanoflares. In this work we factor in the effects of non-equilibrium ionisation by using the established HYDRAD code (Bradshaw and Cargill, 2013) for our simulations and modelling.…”

A numerical tool for the calculation of non-equilibrium ionisation states in the solar corona and other astrophysical plasma environments

“…Recent interest in the detectability of dynamic heating events localised in the solar corona (Bradshaw & Cargill 2006;Reale & Orlando 2008), believed to be responsible for its high temperature, has brought the importance of non-equilibrium ionisation (NEI) states to the forefront of research concerning the nature of coronal heating. One example of such heating is the reconnection of adjacent magnetic field lines that have become twisted and braided in the corona due to the random motions of their photospheric footpoints.…”

“…One example of such heating is the reconnection of adjacent magnetic field lines that have become twisted and braided in the corona due to the random motions of their photospheric footpoints. Often called nanoflares (Parker 1988;Cargill 1993Cargill , 1994Cargill 1997Klimchuk 2006), with reference to the fraction of energy released in comparison to a large scale flare, models have shown that they may heat the corona to temperatures ≥10 7 K (Cargill & Klimchuk 2004;Patsourakos & Klimchuk 2005, 2006Bradshaw & Cargill 2006;Klimchuk 2006).…”