Polar coronal holes are relatively stable structures persisting over many solar rotations. The appearance of coronal holes in remote observations, however, can change on a daily basis due to variations of the denser and hotter plasma surrounding them. We explore the effect of these denser and hotter surrounding regions on coronal hole observations, using daily intensity measurements at 1.15 RS of the green Fe XIV 5303 ¿ and red Fe X 6374 ¿ spectral lines. The observations, which were carried out at the National Solar Observatory a Sacramento Peak, New Mexico, cover at time period of about four solar rotations.

We show that the ''coronal hole'' temperatures derived using the line ratio technique, vary by more than 0.8¿106 K over the time interval considered here. We also provide a short discussion of the expected accuracy of the atomic data for these two iron spectral lines. Using intensity measurements as a function of distance from the Sun, we briefly discuss how the regions surrounding the coronal holes might influence the inference of the temperature gradient in the coronal holes. The line of sight effect on the temperature gradient should be explored in more detail in the future using daily observations of the line intensities as a function of distance. These observations could be provided by ground-based coronagraphs and by instruments on board SOHO. ¿American Geophysical Union 1995