Emission Mechanism of radio bursts from the Solar Wind Acceleration Region observed by Parker Solar Probe in near-Sun plasmas

    Solar and interplanetary radio bursts are important phenomena to reflect the electron acceleration and kinetic process in the corona and space plasmas. The near-Sun radio observation by Parker Solar Probe (PSP) provides an good chance to make the approach and in situ measurements for the emission source of radio bursts. According to the radio dynamic spectrum, we found that a large number of weak radio bursts with higher cutoff frequency f_lo can only be detected by PSP. These bursts have several obvious characteristics: (1) short duration (1-5 min); (2) narrow frequency band (0.5-15 MHz); (3) weak peak intensity (~ 10^-15 V²/Hz). Their relative frequency drift rate decreases from > 0.01 s^-1 to < 0.01 s^-1 implies that they are not the typical type III and II radio bursts. Based on the plasma empirical models and the data from in situ detection by PSP, the fitted models indicate that the radiation of these bursts might be generated by the electron cyclotron maser emission in the acceleration region of solar wind (1.1-10 R_S). The spectral characteristics of these bursts manifest that these bursts come from small-scale emission source, which have experienced strong kinetic evolution. We propose that these weak bursts are possibly the solitary wave radiation (SWR) generated from electron cyclotron maser instability with the energetic electrons trapped and accelerated by solitary kinetic Alfvén wave (SKAW) in the close magnetic structure. The decay of SKAW can well explain the deceleration of the emitting sources and the short duration.