MICROWAVE AND SOFT-X-RAY STUDY OF SOLAR ACTIVE-REGION EVOLUTION

We have studied the properties and evolution of several active regions observed at multiple wavelengths over a period of about 10 days. We h ave used simultaneous microwave (1.5 and 17 GHz) and soft X-ray measur ements made with the Very Large Array (VLA), the Nobeyama Radio Heliog raph (NRH) and the Soft X-ray Telescope (SXT) on board the Yohkoh spac ecraft, as well as photospheric magnetograms from WNO. This is the fir st detailed comparison between observations at radio wavelengths diffe ring by one order of magnitude. We have performed morphological and qu antitative studies of active region properties by making inter-compari son between observations at different wavelengths and tracking the day -to-day variations. We have found good general agreement between the 1 .5 and 17 GHz radio maps and the soft X-rays images. The 17 GHz emissi on is consistent with thermal bremsstrahlung (free-free) emission from electrons at coronal temperatures plus a small component coming from plasma at lower temperatures. We did not find any systematic limb dark ening of the microwave emission from active regions. We discuss the di fference between the observed microwave brightness temperature and the one expected from X-ray data and in terms of emission of a low temper ature plasma at the transition region level. We found a coronal optica l thickness of similar to 10(-3) and similar to 1 for radiation at 17 and 1.5 GHz, respectively. We have also estimated the typical coronal values of emission measure (similar to 5 x 10(28) cm(-5)), electron te mperature (similar to 4.5 x 10(6) K) and density (similar to 1.2 x 10( 9) cm(-3)). Assuming that the emission mechanism at 17 GHz is due to t hermal free-free emission, we calculated the magnetic field in the sou rce region using the observed degree of polarization. From the degree of polarization, we infer that the 17 GHz radiation is confined to the low-lying inner loop system of the active region. We also extrapolate d the photospheric magnetic field distribution to the coronal level an d found it to be in good agreement with the coronal magnetic field dis tribution obtained from microwave observations.