  We present the second extensive study of the coronal line variability in an
active galaxy. Our data set for the well-studied Seyfert galaxy NGC 5548
consists of five epochs of quasi-simultaneous optical and near-infrared
spectroscopy spanning a period of about five years and three epochs of X-ray
spectroscopy overlapping in time with it. Whereas the broad emission lines and
hot dust emission varied only moderately, the coronal lines varied strongly.
However, the observed high variability is mainly due to a flux decrease. Using
the optical [FeVII] and X-ray OVII emission lines we estimate that the coronal
line gas has a relatively low density of n~10^3/cm^3 and a relatively high
ionisation parameter of log U~1. The resultant distance of the coronal line gas
from the ionising source of about eight light years places this region well
beyond the hot inner face of the dusty torus. These results imply that the
coronal line region is an independent entity. We find again support for the
X-ray heated wind scenario of Pier & Voit; the increased ionising radiation
that heats the dusty torus also increases the cooling efficiency of the coronal
line gas, most likely due to a stronger adiabatic expansion. The much stronger
coronal line variability of NGC 5548 relative to that of NGC 4151 can also be
explained within this picture. NGC 5548 has much stronger coronal lines
relative to the low ionisation lines than NGC 4151 indicating a stronger wind,
in which case a stronger adiabatic expansion of the gas and so fading of the
line emission is expected.
