The low frequency cutoffs f _lo and the observed plasma frequency f _p 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 f _lo is widely spread between 200 kHz and 1.6 MHz. While the plasma frequency f _p at the spacecraft is between 50 and 250 kHz, which is almost all smaller than f _lo . The result also shows that the maximum probability distribution of f _lo (∼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.

本文研究了176次III型射电暴的低频截止频率f _lo和观测到的等离子体频率f _p。当帕克太阳探测器处于从第一到第五轨道的相遇阶段时，会观察到这些事件。结果表明，截止频率f _lo的分布在 200 kHz 和 1.6 MHz 之间广泛分布。而航天器处的等离子体频率f _p在50到250 kHz之间，几乎都小于f _lo。结果还表明，f _lo的最大概率分布 (∼680 kHz) 明显高于 Ulysses 和 Wind (∼100 kHz) 在先前研究中观察到的值。还初步讨论了太阳活动强度、事件选择标准和辐射衰减效应三个可能的原因。