Modelling of coronal active regions in terms of loop structures began around 1974 (see Jordan 1975) and was stimulated by images of the corona in X-rays and the uv, obtained from rocket flights. Vaiana was a pioneer in this field (see Vaiana et al. 1973). The Skylab missions provided a fuller range of imaging and spectroscopic observations, and much of the available information is still based on these data (see Orrall 1981). Since 1975 a very large number of papers have been published on the various aspects of loop structure, heating and stability (see review by Mewe 1991). Here I can mention only a few points concerning the relation between observable parameters and the energy balance and heating requirements of active region loops.
Comparison of active region upflow and core properties using simultaneous spectroscopic observations from IRIS and Hinode
