INTERFEROMETRIC OBSERVATIONS OF THE QUIET SUN AT DECAMETER WAVELENGTHS UNDER STRONG RADIO FREQUENCY INTERFERENCE

Abstract

Studies of the quiet Sun radio emission were carried out in a wide range of wavelengths from extremely short up to decameter ones. At the longest wavelengths, the measurements of angular sizes of the solar corona were previously carried out using the UTR-2 radio telescope in the scanning mode. We have developed a simple interferometric technique for measuring the angular diameter of an extended radio source. It uses a set of interferometers formed from the antenna sections of the north-south and east-west arms of the UTR-2 radio telescope to measure the size of the quiet Sun in the equatorial and polar directions. The first interferometric observations with this approach were carried out using the receivers and software of the URAN interferometers back in 2014. That study allowed us to determine equatorial and polar solar sizes at the fixed frequencies of 20 and 25 MHz. To expand the frequency range of the studies in the following observations, we used broadband digital DSPZ receivers in the correlation mode. However, in the daytime, broad-band observations are complicated by radio frequency interference of various types, which often significantly exceed the level of wanted signals. To limit the effect of RFI, software has been developed that automatically detects and mitigates narrowband and impulse interference in a recorded signal. The paper describes the methods of RFI mitigation and criteria for the degree of signal clearing, which are used in this software. We also present the measurement results of the angular parameters of the quiet Sun radio emission, which were obtained by the interferometric method in the frequency range of 10 – 30 MHz. The observations were carried out during the minimum of solar activity in 2018 – 2020.