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.

The minimum of the previous solar cycle, Solar Cycle 24, occurred in December 2019, which also marked the start of the new Solar Cycle 25. The first radio bursts of the new solar cycle were observed in the spring season 2020. In this work we will present three type III solar bursts which were observed in May and June 2020 at radio frequencies between 18 – 90 MHz. There are two radio observatories in Finland that are capable of doing low-frequency solar radio observations: Aalto University Metsähovi Radio Observatory (MRO) and Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) of the Sodankylä Geophysical Observatory, University of Oulu. The instruments of the two institutes have different design and characteristics, and they operate in rather different radio interference environments. We will compare simultaneous observations from these two instruments and we will also discuss the properties of these type III solar bursts.

The increasing terrestrial and space-borne communications are causing major problems to the radio astronomy observations. Only a minor part of the frequencies is allocated to the passive services. such as Radio Astronomy Services (RAS). There are only a few, relatively narrow frequency bands below 20 GHz, which are still suitable for the radio astronomical observations. In addition. Out-of-Band (OoB) emissions will be a real threat to the observations on these bands. On behalf of all European radio astronomers. the Committee on Radio Astronomy Frequencies (CRAF) of the European Science Foundation (ESF) coordinates activities to keep the frequency bands used by radio astronomy and space sciences free of interference. Along with interference caused by active radio communication services. the local electronic device selection should be considered in the observatories. For instance, more common LED based lamps could cause harmful interference for the observations. Thus. it is very important to perform continuous radio frequency interference (RFI) monitoring locally. in each radio observatory.