Properties of EUV and X-ray emission in solar active regions

Using observations from the Coronal Diagnostic Spectrometer (CDS) on SoHO a nd the Soft. X-ray Telescope (SXT) on Yohkoh we investigate how the spatial properties of active region emission observed in the EUV and X-ray range v aries with temperature. We examine the contrast per unit area of the EUV em ission from a number of active regions, and employ correlation techniques a nd Fourier methods with which we obtain the two dimensional power spectrum of the intensity distribution for a number of images in emission lines form ed at different temperatures. Integrating this over polar angle we find iso tropic power-law behaviour at all temperatures irt a number of topologicall y different active regions, with a tendency for flatter spectra at lower te mperatures. The existence of power-law spectra indicates that there is no p referred length scale within the regions, at least not a resolvable one, wh ile flatter spectra at lower temperatures indicate that the structures are relatively smaller in this temperature range, possibly providing support fo r the idea of a multi-component transition region (TR). Implications for va rious heating models are discussed.