Long-term solar flux observations with Irkutsk incoherent scatter radar (IISR) in 2011-2019

Irkutsk incoherent scatter radar (IISR) is an oblongish horn antenna that operates in a meter waveband (154-162 MHz), has a 0.5°×20° beam, and a frequency steering allowing us to tilt the beam by 30° to the south. Besides active measurements of ionospheric conditions and monitoring of space objects, the radar is regularly used for passive radio astronomical observations. From May to August, the Sun crosses the radar field of view and can be in the maximum of the radiation pattern for about two hours. The known shape of the radiation pattern and the high sensitivity of the receiver allow us to conduct calibrated measurements of the solar flux in solar flux units during this period. We have developed a new approach to the calibration, which can be applied to all IISR archival passive data. In the paper, we present long-term observations (2011-2019) of the solar flux in May and summer. We describe the measurement method, present daily average values of the solar flux for this period of passive measurements, and compare it with the solar activity F 10.7 index and solar flux measurements made at the Australian observatory Learmonth at 245 MHz. We show that the daily average flux for the period of observations at a frequency of ~161 MHz generally has values from 5 to 10 sfu.