FINE-STRUCTURE SUPERIMPOSED ON MILLIMETER-WAVELENGTH (23-18 GHZ) SPECTRA OF SOLAR ACTIVE REGIONS

A millimeter-wavelength (23-18 GHz) variable frequency radiometer with frequency resolution of 1 GHz and time resolution of 0.1-16 s has bee n developed in conjunction with a 13.7 m-diameter antenna. In this pap er we describe briefly this new instrument, and its use to observe act ive regions. Spectra of four active regions show that: (i) spectra of the quiet-Sun region and those of the active region were of the same n ature and the spectral index of both varied between -0.4 and +0.3; (ii ) for two cases the spectra of the active region remained almost flat during the observing period of about one hour; (iii) spectra of the tw o other active regions exhibited frequency fine structures similar to the trough and crest type with a width of the order of 2 GHz in freque ncy and lasting about 40 to 90 min. The spectra of the quiet-Sun regio n and those of the active regions are attributed to bremsstrahlung emi ssion. Fine structures in frequency type crest and trough are attribut ed to the radio signatures of the temperature plateau in the chromosph ere.