  We present Infrared Space Observatory (ISO) observations of fine structure
lines toward the Radio Arc region, close to Sgr A, in the Galactic center
region (GCR). The ionization at large scale is dominated by the clusters of
massive stars in this region: the Quintuplet and the Arches cluster. Any other
possible mechanism as interaction with the magnetic fields revealed by the
Non-Thermal Filaments (NTFs) must play a minor role. The radiation in this area
is relatively hard ($T_\mathrm{eff}\sim 33-35$ kK) but diluted. It reaches
large distances due to the inhomogeneity of the medium.
