Properties of Type III and Type IIIb Bursts in the Frequency Band of 8 – 80 MHz During PSP Perihelion at the Beginning of April 2019

Properties of type III and type IIIb bursts in the frequency band of 8 – 80 MHz observed by the radio telescopes Ukrainian Radio Interferometer of NASU-2 (URAN-2) (Poltava) and Giant Ukrainian Radio Telescope (GURT) (Kharkiv) during the Parker Solar Probe (PSP) perihelion in April 2019 are discussed. These correspond to those that were observed by PSP at frequencies <19 MHz. We analyze dependencies of drift rates and durations on frequency for these bursts. We show that drift rate dependencies on frequency agree well with those derived from the Newkirk corona if source velocities are between 0.17 and 0.2 $c$ for both type IIIb bursts and type III bursts under the assumption that the first ones are fundamentals and the second ones are their harmonics. However, all observational dependencies are flatter in comparison with the dependencies for a Newkirk corona. We assume that this can be related with coronal temperature decreasing at heliocentric distances from 1.35 to 6.5 solar radii. Duration dependencies of type IIIb and type III bursts on frequency in the range of 10 – 70 MHz are also obtained. We note that the durations and drift rates of type III bursts as well as their dependencies on frequency differ essentially from those for type IIIb bursts.