Sketch of the geometry proposed to explain the appearance of the ${\text{L}}_{{\text{DM}}^2/\text{EM}}$ sky in Fig. 7. In a low-electron-density hot ISM (light orange), small but dense electron clouds are embedded (deep purple ellipses). The observer’s position is marked by the blue star symbol. An LoS at high galactic latitudes (blue arrow from top right) usually misses the dense clouds, and therefore probes the geometric length of the dilute ISM (long red bar) resulting in an ${\text{L}}_{{\text{DM}}^2/\text{EM}}~O(\text{kpc})$(kpc). An LoS in the Galactic plane (horizontal blue arrow) usually passes through at least one such cloud, and therefore probes their much smaller size (short red bars), leading to ${\text{L}}_{{\text{DM}}^2/\text{EM}}~O(10\text{pc})$(10pc). Some high-latitude LoSs pass through such clouds as well (blue arrow from top left) and exhibit a very short ${\text{L}}_{{\text{DM}}^2/\text{EM}}$ as well (see short red bar next to that LoS). As high-latitude clouds need to be nearby to be within the thin Galactic disk, the set of LoSs passing through them span a larger area of the high-latitude sky. We note that the scales or shape of this illustration are not intended to realistically match the Milky Way.