behaviour is seen in the movies of line-depth $L_{\rm d}$, only as darkenings, essentially meaning that the flare ribbons are connected with the shallower spectral line at the same place.

A negative \[one has to keep in mind that the “orientation” of $I_{\rm c}$ is opposite to $L_{\rm d}$, e.g., the shallower (lesser) $L_{\rm d}$ means a larger $I_{\rm c}$\] correlation between $I_{\rm c}$ and $L_{\rm d}$ is apparently visible from (\[eq:ic\]). By plotting cuts for various position in the field-of-view as a function of time, the deviations of both the $I_{\rm c}$ and $L_{\rm d}$ transients with respect to the secular trends may be estimated. We found that in most cases, the decrease of line-depth almost agrees (to within say 10%) with the increase in continuum intensity. This issue deserves a future investigation by looking directly to the shape of the spectral line sampled by six band-pass filters of SDO/HMI, and we tested this idea using a simple model.

We followed closely a recipe given by [@2012SoPh..278..217C], where the means of derivation of HMI data products are given explicitly. We simulated two situations: first the quiet Sun situation and the situation, when the spectral line has a line-core emission. 