The Spectral Features of Single-band Type III-like Radio Bursts Observed by PSP during Its First Nine Orbital Encounters

Abstract The spectral features of 1122 single-band type III-like radio bursts were analyzed statistically using data from the Parker Solar Probe (PSP) during the first nine orbital encounters. After excluding overlapping events, 923 bursts were classified into 706 complete-spectrum (CS) bursts and 217 incomplete-spectrum (IS) bursts depending on radiation completeness. The distribution of start frequency f st for IS events (20–54 MHz) was predicted by a lognormal fit. The higher cutoff frequency ( f lo ∼ 1.2–1.6 MHz) than previous results may be attributed to the limitation of exciting conditions or the radial expansion of guiding magnetic flux tubes. Short durations (1–4 minutes) and low peak fluxes (∼10 3 sfu) indicate that most events are short-time weak bursts. The spectral index of duration versus frequency is near −1, which reflects the influence of velocity dispersion and radio-wave scattering. For the denoised spectra of CS events, the peak frequency of maximal flux (∼4.5 MHz) is obviously higher than those from STEREO (∼1 MHz) and WIND (∼ 2 MHz). This implies that the radiation may be the second or third harmonic wave rather than the fundamental wave. Statistical frequency drift rates follow the classical power-law relation ( D abs ∝ f 1.84 ) and are consistent with simulation results of typical type III radio bursts. CS and IS bursts show similarities in f lo , duration, and drift features, but obviously differ in the peak frequency of maximal flux. These results can provide a useful reference for further exploring the emission mechanisms of single-band type III-like radio bursts and the associated energetic electron acceleration processes.