The discrete and localized nature of the variable emission from active regions

Using data from the Extreme Ultraviolet (EUV) Spectroheliometer onSkylab, we study the empirical characteristics of the variable emission in active regions. These simultaneous multi-wavelength observations clearly confirm that active regions consist of a complex of loops at different temperatures. The variable emission from this complex has very well-defined properties that can be quantitatively summarized as follows: (1) It is localized predominantly around the footpoints where it occurs at discrete locations. (2) The strongest variability does not necessarily coincide with the most intense emission. (3) The fraction of the area of the footpoints,δn/N, that exhibits variable emission, varies by ±15% as a function of time, at any of the wavelengths measured. It also varies very little from footpoint to footpoint. (4) This fractional variation is temperature dependent with a maximum around 10⁵ K. (5) The ratio of the intensity of the variable to the average background emission, δI/Ī, also changes with temperature. In addition, we find that these distinctive characteristics persist even when flares occur within the active region.