Technical Summary: The differential emission measure distribution, DEM(T), of the solar transition region and corona--i.e., the relative amounts of hot and cool plasma--has been a long-standing puzzle. This distribution has a minimum near 10^5 K, increasing for both hotter and cooler temperatures. The hot part of the distribution is well explained by standard coronal loop models, which consist of a roughly isothermal coronal section near 10^6 K and an ultra-narrow transition region at the base. These models predict far less emission below 10^5 K than is observed, however (e.g., Klimchuk 1992).
A second class of solution to the equilibrium hydrodynamic equations gives rise to the so-called "cool" loop models, which have a very gradual temperature gradient throughout and a peak temperatue of < 10^5 K. It has been suggested by Antiochos and Noci (1986) that a mixture of hot and cool loops is responsible for the observed DEM distribution. It is possible to test this hypothesis. Because of the nature of the cool loop solution, these loops have a maximum height of < 5,000 km. Thus, we might expect them to be more abundant in regions of mixed magnetic polarity, where many short field lines are present, than in unipolar magnetic regions, where a majority of field lines are long and must reach heights substantially above 5,000 km.
The purpose of our investigation is compare ratios of hot (> 10^5 K) and cool (< 10^5 K) emissions in different magnetic regions. If the solar DEM distribution is indeed explained by a mixture of hot and cool loops, then the hot/cool ratio should be greater in unipolar regions than in mixed polarity regions.