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C.W. Danforth (JHU), R.H. Cornett (Raytheon ITSS/GSFC), N.A. Levenson, W.P. Blair (JHU), T.P. Stecher (GSFC)
During the Astro-1 and Astro-2 Space Shuttle missions in 1990 and 1995, far ultraviolet (FUV) images of five 40' diameter fields around the rim of the Cygnus Loop SNR were observed with the Ultraviolet Imaging Telescope (UIT). These fields sample a broad range of SNR conditions including both radiative and non-radiative shocks in various geometries and scales. The UIT B5 band images discussed here sample predominantly \ion{C}{4} \lambda 1550 and the 2-photon continuum. Smaller contributions are made by emission lines of \ion{He}{2} \lambda 1640 and \ion{O}{3}] \lambda 1666. A unique aspect of the B5 band is its ability to sample the hydrogen 2-photon continuum from regions where the gas is recombining.
We present these new FUV images and compare them with optical H\alpha and [\ion{O}{3}], and ROSAT HRI X-ray images. In non-radiative shocks, existing 2-photon flux measurements from spectra and the H\alpha images suggest we are seeing approximately equal contributions from 2-photon and \ion{C}{4} emission. In radiative filaments, however, shock models and our images suggest \ion{C}{4} should dominate while spectra of specific locations seem to indicate that 2-photon emission dominates. We surmise that spectral observations on specific bright filaments have decreased locally-observed levels of \ion{C}{4} emission due to resonance scattering in that line.
Funding for the UIT project has been through the Spacelab Office at NASA headquarters under project number 440-551.
The author(s) of this abstract have provided an email address for comments about the abstract: danforth@pha.jhu.edu