Solitary Wave Radiation and the Related Type IV-like Burst from Solar Wind Acceleration Region Observed by PSP

As the latest near-Sun spacecraft, Parker Solar Probe (PSP) supports a unique viewpoint to explore solar radio bursts generated from the solar wind acceleration region. We will report two significant types of solar radio bursts detected by PSP. One is the weak radio burst observed when PSP passed through a low-density magnetic channel. It has a starting frequency of about 20 MHz and a narrow frequency range from tens of MHz to hundreds of kHz. The relative frequency drift rate of this burst rapidly decreases from above 0.01 s^-1 to below 0.01 s^-1. The other is a type IV-like radio burst. It lasts about 20 hours and consists of a series of short-time (ST) bursts with the central frequency slowly drifting from approximately 5 MHz to 1 MHz. By analyzing the empirical models of the solar atmosphere and the in-situ measurement data of PSP, it is found that the source regions of both types of radio bursts have similar characteristic plasma parameters. The electron cyclotron frequency in these regions is higher than the plasma frequency, which means that the source region is a low-β plasma environment and these radio bursts are likely to be generated by the electron cyclotron maser (ECM) emission mechanism.  We proposed that both types of bursts may be generated by solitary kinetic Alfvén waves (SKAWs). In a low-β plasma, SKAWs can accelerate electrons to excite the electron cyclotron maser (ECM) instability and cause radiation. The frequency drift is related to the propagation of SKAWs and the movement of magnetic loops. However, due to the uncertainty of empirical models, further verification is needed with the help of PSP's future observations closer to the Sun and the possible local measurements in the source regions.