Fig. 1

Overview of a type III burst observed by multiple spacecraft on 2020 July 21 near 06:30 UT. All observed flux densities are scaled to 1 au. (a) Dynamic spectra observed by each spacecraft with the background noise subtracted. (b) Time profiles from PSP (blue), SolO (red), ST-A (black), and WIND (green), at select frequencies. The vertical dashed lines denote the observed peak times, and the dotted lines denote the median background level. (c) Type III burst peak intensities (open symbols) observed by each spacecraft at different frequencies. The solid curves show the fits using Eq. (1); the vertical dashed lines mark the longitude \(\theta _0\) of the fitted maximum intensity. (d) Longitudes of maximum intensity \(\theta _0\) (dashed coloured lines) with colour mapped to the frequencies shown in panel (c), and the angular uncertainty shown at 1 au. The blue arc highlights the longitude of maximum intensity \(\theta _0\) at 0.9 MHz. The relative spacecraft positions are shown by the open symbols. The curved grey lines show the possible Parker spiral structure based on the radial solar wind speeds measured at each spacecraft (scaled to 1 au) of 310 km \(\hbox {s}^{-1}\) (SolO), 340 km \(\hbox {s}^{-1}\) (ST-A), and 330 km \(\hbox {s}^{-1}\) (WIND).