The  linear filaments of the Galactic center radio arch are believed to
be  highly  ordered  and  strong  magnetic  field  lines  (~milliGauss)
illuminated  by  the  collision  with  a  molecular cloud.  A prominent
ionized  region  (G0.18-0.4)  delineates  the cloud/filament interface,
perhaps  evidence  an  extremely strong shock.  We propose to observe a
number  of  shock/photoionization  diagnostics  (including  Fe_II,  H2,
Si_II) toward G0.18-0.04 in order to:

(1)  determine  whether  the  ionized interface is indeed the result of
extreme    shock  activity  or  is  instead  enhanced  by  some  nearby
photoionizing (e.g. stellar) source or cluster,
(2)  better  understand  the  interaction that is occurring between the
cloud and the filament,
(3)  better  determine  the  role that the collision process has in the
illuminating  the  magnetic field lines (both precisely how and to what
degree) and,
(4)  better  determine  the nature of the remarkable magnetic filaments
themselves --- an aspect yet poorly understood.

Nearly all of the proposed transitions can only be observed effectively
from above the atmosphere.