temperature is 50 kK, and can be thought of as a map of the height of the onset of the transition region. It shows elongated structures that reach up to 8 Mm and connect the sunspot pair.

In the model atmosphere, there is a long arm of material at chromospheric temperatures that has erupted up to a height of 18 Mm into the corona, reminiscent of a coronal mass ejection. For brevity we refer to this arm as CME.

Since the CME has chromospheric temperatures, it should ideally be included in our radiative transfer calculations. We choose not to do so because this would increase the computation time by a factor of 3, which we could not afford. Fortunately, the CME covers only 2.3% of the surface area of the simulation domain. The surface area of the removed CME arm is shown as red patches in Fig. \[fig:atmos\_photosphere\]c.

We removed the upper part of the CME, so that the model atmosphere size is reduced to $1024 \times 512 \times 290$ grid points, corresponding to heights from $-$1.3 Mm beneath the photosphere up to 8 Mm above it. The zero point of our height scale is defined as the height where 