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 been developed in conjunction with a 13.7 m-diameter antenna. In this paper we describe briefly this new instrument, and its use to observe active 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 nature 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 two other active regions exhibited frequency fine structures similar to the trough and crest type with a width of the order of 2 GHz in frequency and lasting about 40 to 90 min.The spectra of the quiet-Sun region and those of the active regions are attributed to bremsstrahlung emission. Fine structures in frequency type crest and trough are attributed to the radio signatures of the temperature plateau in the chromosphere.