The Cosmic Hot Interstellar Plasma Spectrometer ( CHIPS ) was designed to study diffuse emission from hot gas in the local interstellar cavity in the wavelength range 90 - 265 Å .
Between launch in January 2003 and early 2004 , the instrument was operated in narrow-slit mode , achieving a peak spectral resolution of about 1.4 Å FWHM .
Observations were carried out preferentially at high galactic latitudes ; weighted by observing time , the mean absolute value of the galactic latitude for all narrow-slit observations combined is about 45 degrees .
The total integration time is about 13.2 Msec ( 74 % day , 26 % night ) .
In the context of a standard collisional ionization equilibrium plasma model , the CHIPS data set tight constraints on the emission measure at temperatures between 10 ^ { 5.55 } K and 10 ^ { 6.4 } K. At 10 ^ { 6.0 } K , the 95 % upper limit on the emission measure is about 0.0004 cm ^ { -6 } pc for solar abundance plasma with foreground neutral hydrogen column of 2 x 10 ^ { 18 } cm ^ { -2 } .
This constraint , derived primarily from limits on the extreme ultraviolet emission lines of highly ionized iron , is well below the range for the local hot bubble estimated previously from soft X-ray studies .
If the pattern of elemental depletion in the hot gas follows that observed in much denser interstellar clouds , the gas phase abundance of iron , relative to other heavy elements that contribute more to the soft X-ray emission , might be much lower than solar .
However , to support the emission measures inferred previously from X-ray data would require depletions much higher than the moderate values reported previously for hot gas .
Excluding the He II Lyman lines , which are known to be primarily terrestrial in origin , the brightest feature we find in the integrated spectrum is an Fe IX line at 171.1 Å .
The sky-averaged flux of the feature is about 6 photons cm ^ { -2 } s ^ { -1 } ster ^ { -1 } , a flux that exceeds the 1-sigma shot noise significantly but is comparable to the systematic uncertainty .
We find ” bright ” 171.1 Å emission ( flux greater than 10 photons cm ^ { -2 } s ^ { -1 } ster ^ { -1 } and S/N > 2 ) in about 10 % of the observing time .
However , these ” bright ” observations overwhelmingly select for day time ( 96 % of 1.3 Msec ) .
Thus , a local rather than interstellar origin for much of the 171.1 Å emission seems likely .