Extreme ultraviolet structure of solar active regions

The extreme ultraviolet emissions from solar active regions have been analyzed to determine their temperatures, densities, pressures, emission measures, flow velocities and structures. The accuracy and limitations of the diagnostic techniques are discussed. It is found that the active region structures are well correlated with the inferred and observed magnetic fields and that the electron pressure of the plasma is essentially constant over a wide range of temperatures. Cool (105K less than or equal to T less than or equal to 106K) loop structures are shown to be composed of unresolved filaments. Static and steady flow one dimensional hydrodynamical models of active region loops have been constructed and used to interpret the observations. Both models require a loop cross section that diverges with height. This is found by comparing the static model with the observed emission measures and the steady flow model with the observed emission measures and the steady flow model with the observed flow velocities. No local heating is found to be needed in the transition zone. Transition zone downflows in active regions indicate a mass loss from the overlying coronal structures for which there is no known supply