A MODEL FOR ACTIVE-REGION EMISSION AT CENTIMETER WAVELENGTHS

We present multi-frequency observations and model computations of the microwave emission of a solar active region. The radio observations we re obtained with the RATAN-600 at several wavelengths between 0.8 and 31.6 cm and with the VLA at 6 and 20 cm. The active region was also ob served in the EUV O IV lines by the HRTS instrument aboard the Space S huttle Spacelab-2 mission. These lines are formed in the chromosphere- corona transition region and their intensity ratio is sensitive to pre ssure. Photospheric magnetograms provided both the longitudinal and th e transverse component of the magnetic field. The microwave observatio ns were checked against model computations taking into account both th e free-free and the gyro-resonance emission mechanisms and using the p ressure data from the O IV lines. The magnetic field was computed thro ugh constant-a! force-free extrapolations of the longitudinal photosph eric field. We computed both the flux from 2 to 20 cm and the spatial structure of the microwave emission at 6 and 20 cm. The comparison of the computed and observed flux spectra allowed us to estimate the magn etic field strength at the base of the transition region and in the lo w corona, as well as the values of the conductive flux and the height of the base of the transition region. The model maps at 6 cm and 20 cm showed that ct was not constant above the active region; the same con clusion was reached on the basis of the photospheric observations. The use of pressure measurements allowed us to identify microwave structu res which were determined by pressure enhancements. At 6 cm the comput ations confirmed the fact that the magnetic field is the principal fac tor that determines the structure of sunspot-associated sources and sh owed that the effect of pressure variations was small. Pressure variat ions were more important at 20 cm, where the peak of the emission was associated with the sunspot and a diffuse component was associated wit h the plage which had an average pressure higher by a factor of 1.54 t han the sunspot.