SPEAR far UV spectral imaging of highly ionized emission from the North Galactic Pole region

Aims. We present far ultraviolet (FUV: 912-1750 angstrom) spectral imaging observations recorded with the SPEAR satellite of the interstellar OVI (1032 angstrom), CIV (1550 angstrom), SiIV (1394 angstrom), SiII* (1533 angstrom) and AlII (1671 angstrom) emission lines originating in a 60 degrees x 30 degrees rectangular region lying close to the North Galactic Pole. These data represent the first large area, moderate spatial resolution maps of the distribution of UV spectral-line emission originating the both the highly ionized medium (HIM) and the warm ionized medium (WIM) recorded at high galactic latitudes. Methods. By assessing and removing a local continuum level that underlies these emission line spectra, we have obtained interstellar emission intensity maps for the aforementioned lines constructed in 8 degrees x 8 degrees spatial bins on the sky. Results. Our maps of OVI, CIV, SiIV and SiII* line emission show the highest intensity levels being spatially coincident with similarly high levels of soft X-ray emission originating in the edge of the Northern Polar Spur feature. However, the distribution of the low ionization AlII emission does not show this spatial correlation, and suggests that warm-neutral and/or partially ionized gas with a temperature < 20 000 K may be quite pervasive at high galactic latitudes. The variation of the emission line intensity ratios as a function of sky position is contrasted with theoretical predictions concerning the physical state of interstellar gas in the galactic halo. The observed line ratios alone unfortunately do not provide us with a clear diagnostic tool to distinguish between the various physical production mechanisms responsible for both high and low ion states. However, our results do favor the hybrid model of Shull & Slavin (1994, ApJ, 427, 784) which incorporates the combined effects of turbulent mixing layers and isobarically cooling supernova remnant gas. For this highly ionized gas, our results are best explained assuming that the observed OVI halo emission is somewhat clumpy in nature, consistent with its production at interfaces between warm (T = 10(3)-10(4) K) and hotter (T = 10(6) K) soft X-ray emitting gas. CIV emission at these interfaces occurs in the intermediate temperature (T = 105 K) gas, which seems always present whenever OVI is strongly detected. Alternately, the data are also consistent with CIV emission being ubiquitous throughout the halo with a fairly constant level of emission line intensity (of similar to 4000 LU), and our observations mostly reflect the superposition of spatially separate OVI emission originating at the cloud interfaces of random clumps of high latitude gas.