Abstract
A gradual microwave darkening during several days of the preflare phase in the active region (AR) NOAA 9077, before the powerful proton event on 14 July 2000 was detected. The associated change of the circular polarization structure in the form of narrow-band double inversion of the sign was marked. The effects were revealed in a wide range of radio observations at the RATAN-600 radiotelescope and were confirmed by data from the Nobeyama Radio Heliograph (NRH) at 17 GHz and the Siberian Solar Radio Telescope (SSRT, Irkutsk) at 5.8 GHz. We propose an interpretation of the observed effects as an indication of gradual formation of a filament in the vertical structure of the AR. Destabilization of this filament apparently caused the major flare with a coronal mass ejection (CME).
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