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tokens: beappropriate for an optically thin spectral line with a given formation temperature). At a temperature of 6.3 kK we already see low lying loop structures connecting magnetic field of opposite polarities. There is a multitude of these low lying, short loops but much of the plasma at that temperature is also distributed in structures that are less loop-like. At 10 kK most of the lowest lying loops have disappeared and we have fewer, more pronounced loops that reach higher. At higher temperatures we basically see the same loops, all the way up to 316 kK () when a new set of hotter, higher lying loops start to appear. These loops dominate up to about 1 MK. The maximum temperature in this simulation is 2.2 MK and this hottest plasma is located in loops that do not reach as high as the loops with temperatures up to 1 MK. The lower lying loops with temperatures below 300 kK evolve on shorter timescales than the hotter loops and give rise to the “Unresolved Fine Structure” (UFS) loops discussed in Hansteen et al. (2014) . Fig. 13 Probability density function (PDF) of the electron density as function of height at s ( upper panel ) and at s ( lower panel ). Note the logarithmic electron density scale. 3.5. Ionization balance The simulation includes the effects of non-equilibrium ionization of hydrogen, see Leenaarts et al. (2007) . Figure 13 shows the electron density as a function of height for two times, just 10 s after the non-equilibrium ionization of hydrogen was switched on at s and for the first published snapshot, at s. The non-equilibrium hydrogen ionization leads to much higher electron density in the cool pockets at 0.5–2 Mm and also higher electron densities up to 3.5 Mm height. There is not much change with time of this probability density function after s. 3.6. Velocity field Spectral lines are normally observed to be broader than what the thermal broadening of the opacity profile would give. An extra free parameter, called microturbulence, is often introduced in 1D semi-empirical models to account for this broadening. The “micro” in the name comes from the fact that this parameter is introduced as an extra broadening of the opacity profile, acting in exactly the same way as thermal broadening. This would be a physically correct description in the limit of zero length-scale for the velocity field. It is also often necessary to introduce a second free parameter to account for the observed lineshape. This is called macroturbulence and is equivalent to a Gaussian convolution of the emergent intensity profile (rather than a convolution of the opacity profile as is the case for microturbulence). Realistic 3D radiation hydrodynamic simulations of the solar photosphere give line profiles that are close to the observed profiles without the addition of extra free parameters – the non-thermal broadening comes from Doppler shifts arising from the convective flows and oscillations (e.g., Asplund et al. 2000 ). Also spectral lines formed in the