Weak Solar Radio Bursts from the solar wind acceleration region observed by the Parker Solar Probe and its probable emission mechanism

We report a type of weak solar radio burst (SRB) that was detected by PSP when it passed a low-density magnetic channel during its second encounter phase.

These weak SRBs have a low starting frequency of similar to 20 MHz and a narrow frequency range from a few tens of MHz to a few hundred kHz. Their dynamic spectra display a strongly evolving feature of the intermediate relative drift rate decreasing rapidly from above 0.01 s(-1) to below 0.01 s(-1). Analyses based on common empirical models of solar coronal plasmas indicate that these weak SRBs originate from a heliocentric distance of similar to 1.1-6.1 R-S (the solar radius), a typical solar wind acceleration region with a low-beta plasma, and that their sources have a typical motion velocity of similar to v(A) (Alfven velocity) obviously lower than that of the fast electrons required to effectively excite SRBs.