Abstract
Defining the solar brightness temperature accurately at millimeter wavelengths has always been challenging. One of the main reasons has been the lack of a proper calibration source. New Moon was used earlier as a calibration source. We carried out a new extensive set of observations at 8 mm using the New Moon for calibration. The solar and Moon observations were made using the 14-meter radiotelescope operated by the Aalto University Metsähovi Radio Observatory in Finland. In this article, we present our method for defining the brightness temperature of the quiet-Sun level (QSL). Based on these observations, we found 8100~mbox{K} pm 300~mbox{K} to be the mean value for the QSL temperature. This value is between the values that were reported in earlier studies.
Highlights
The Sun is a powerful emitting source at radio wavelengths, and partly because of this, defining its brightness temperature is challenging
The mean brightness temperature of the Moon (Tmoon) at millimeter wavelengths consists of two different components, and it depends on the observing frequency (ν) (Hafez et al, 2014; Krotikov and Pelyushenko, 1987)
From our measurements we conclude that the solar brightness temperature at 8 mm is 8100 K ± 300 K
Summary
The Sun is a powerful emitting source at radio wavelengths, and partly because of this, defining its brightness temperature is challenging. The Moon has been used as a calibrator source (Wrixon, 1974; Zirin, Baumert, and Hurford, 1991; Iwai et al, 2017). In this study we use the New Moon as a calibrator source at 8 mm, and we produce several datasets from which we obtain a new reference value for the solar brightness temperature. All the other solar features (e.g. radio brightenings) can be scaled to the quiet-Sun level (QSL) temperature. The more precise QSL temperature value could explain which atmospheric layer the 8 mm emission is coming from. In this article we present our instrumentation, observations, analysis method, and results for calibrating the QSL temperature
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