between the formation height of [H$\alpha$]{} and the formation heights of the rest of the lines. On average, the core of H$\alpha$, k$_3$, and K$_3$ features are formed at similar heights and [roughly 150 km]{} above the core of 8542 Å throughout the field-of-view.

We note that the distributions have long tails towards positive values meaning that [H$\alpha$]{} [can be formed]{} much higher than the other lines. These points, where the height difference exceeds 1000 km, constitute for the field-of-view ${\sim}$2% for both [k]{} and [K]{}, and $\sim$5% for 8542 Å. Figure \[fig:formation\_height2\] illustrates this case. At 3.22 Mm where the [H$\alpha$]{} core reaches optical depth unity, is only 97.5% ionized and at the same location the metals ( and ) are 99.9% ionized. [H$\alpha$]{} is formed highest in the atmosphere. [It retains opacity where the lines from the metals have none because  does not ionize as quickly as  and  when the temperature increases.]{}

Synthetic images  {#sec:synthetic_images}
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In Fig. \[fig:fig3\_height\] we show the vertically emergent intensity in the line core of  8542 Å, [K]{}, [k]{}, and [H$\alpha$]{}. We first describe what we see at nominal line center (Fig. \[fig:fig3\_height\], top panels). In all four spectral lines we see large 