Ubiquitous chromospheric structures observed in the quiet Sun at millimeter and centimeter wavelengths

Abstract

Unique long-term visibility variations are detected when the quiet Sun is observed with interferometers operating at 8 mm and 11 cm wavelength with angular resolutions of ∽0.5′. Quasiperiodic fluctuations in fringe amplitude are observed with periods between 20 and 30 min, and with amplitude nulls which are correlated with 180° phase changes. These variations are interpreted in terms of a changing projected baseline while viewing a few sources with angular sizes of ∽0.5′ which are distributed within the main interferometer beam with typical angular separations of 7′. The observed variations cannot be due to expanding or contracting sources of the type envisaged by Bocchia and Poumeyrol (1976) when explaining similar variations observed at 8 mm wavelength. A comparison of the flux observed at 8 mm and 11 cm indicates that the individual sources are optically thick thermal radiators with a flux which decreases with increasing wavelength, but with a temperature which increases with increasing wavelength where higher, hotter levels of the chromosphere are observed. For a source whose angular size is 0.5′, the observed flux values correspond to respective temperatures of 5000 K and 19 000 K at 8 mm and 11cm - suggesting that elements of the chromospheric network are being observed. A thermal origin for the individual sources is consistent with the lack of any detectable circular or linear polarization (< 10%) in the interferometer signal.