Statistics of Low Frequency Cutoffs for Type III Radio Bursts Observed by Parker Solar Probe during Its Encounters 1–5

The low frequency cutoffs flo and the observed plasma frequency fp of 176 type III radio bursts are investigated in this paper. These events are observed by the Parker Solar Probe when it is in the encounter phase from the first to the fifth orbit. The result shows that the distribution of cutoffs flo is widely spread between 200 kHz and 1.6 MHz. While the plasma frequency fp at the spacecraft is between 50 and 250 kHz, which is almost all smaller than flo. The result also shows that the maximum probability distribution of flo (∼680 kHz) is remarkably higher than that observed by Ulysses and Wind (∼100 kHz) in previous research. Three possible reasons, i.e., solar activity intensity, event electing criteria, and radiation attenuation effect, are also preliminarily discussed.