Supernova explosions are a basic process that define the structure of
the ISM. The interaction between supernova remnants (SNR) and the
interstellar medium (ISM) is complex and symbiotic. Unlike other
well-known SNR's, e.g., Cas A, Tycho, or Kepler, the structure of the
Cygnus Loop is dominated by its interaction with the ISM. The Cygnus
Loop is the prime candidate for a case study of a SNR interacting with
a multiphase medium; it is close and has a low foreground column
ensuring unrivaled signal-to-noise and spatial resolution. From an
observational perspective the Cygnus Loop is unique in terms of X-ray
brightness, shock front resolution ( 
), and
interaction with a range of ISM environments. The large diameter of the
Cygnus Loop ( 
) assures that it is
interacting with different components of the ISM: from molecular gas on
the NW edge (Scoville et al. 1977), to diffuse atomic gas along the NE
and eastern limbs (DeNoyer 1975; Hester et al. 1994), and low density,
hot, ionized gas to the south (Ku et al. 1984).
High resolution X-ray observations are crucial for studying SNR gas
dynamics because: 1) shock codes predict fully developed turbulence
when the blastwave collides with a cloud (Klein 1996); 2) the length
scale, 
, for cooling and recombination is small: 
, where 
is the preshock
number density, and 
is the shock velocity in units of
(McKee 1987); 3) clouds sizes and substructure range
from scales of tens of pc and downward. Consequently, the improved
ROSAT-HRI sensitivity and spatial resolution compared to Einstein
make it invaluable for exploring cloud-blastwave interactions. The
ROSAT-HRI combines high 0.1-2 keV sensitivity ( 
better than the Einstein HRI [Zombeck et al. 1990]) and high
resolution ( 
on axis [ROSAT Status Report #85]).