  (Abridged). We present new high spatial resolution narrowband imaging
observations of extraplanar diffuse ionized gas (eDIG) in four late-type,
actively star forming edge-on spirals, obtained with ACS on-board HST. Our
F658N (H-alpha) observations reveal a multitude of structures on both small and
large scales. Whereas all four galaxies have been studied with ground-based
telescopes before, here the small scale structure of the extended emission line
gas is presented for the first time at a spatial resolution of 0.05",
corresponding to 5.0 pc at the mean distance to the target galaxies. The eDIG
morphology is very different for all four targets, probably as a result of
their different levels of star formation activity. We find that the morphology
of the eDIG, in particular the break-up of diffuse emission into filaments in
galaxy halos, shows a strong dependence on the level of star formation activity
per unit area, and eDIG can be arranged into a morphological sequence. NGC4634
and NGC5775 have the highest SF rate per unit area in our sample and the
observed morphology suggests that the break-up of the smooth eDIG layer into
individual resolved filaments occurs only above a certain threshold of SF
activity per unit area. Combined with ground-based data for samples that span a
larger range of galaxy mass our results indicate that the gravitational
potential also plays an important role in the eDIG morphology. In low-mass
galaxies the gas can be expelled due to shallow gravitational potentials more
easily and couple with strong star formation driven outflows on a local scale.
This is in contrast to the more massive galaxies, which show smooth eDIG
layers, unless they are powered by a superwind, as in the case of nucleated
starburst galaxies.
