Type III bursts are generated by fast electron beams originated from magnetic reconnection sites of solar flares. As propagation of radio waves in the interplanetary medium is strongly affected by random electron density fluctuations, type III bursts provide us with a unique diagnostic tool for solar wind remote plasma measurements. Here, we performed a statistical survey of 152 simple and isolated type III bursts observed by the twin-spacecraft Solar TErrestrial RElations Observatory mission. We investigated their time–frequency profiles in order to retrieve decay times as a function of frequency. Next, we performed Monte Carlo simulations to study the role of scattering due to random electron density fluctuations on time–frequency profiles of radio emissions generated in the interplanetary medium. For simplification, we assumed the presence of isotropic electron density fluctuations described by a power law with the Kolmogorov spectral index. Decay times obtained from observations and simulations were compared. We found that the characteristic exponential decay profile of type III bursts can be explained by the scattering of the fundamental component between the source and the observer despite restrictive assumptions included in the Monte Carlo simulation algorithm. Our results suggest that relative electron density fluctuations in the solar wind are 0.06–0.07 over wide range of heliospheric distances.

Radio waves are strongly scattered in the solar wind, so that their apparent sources seem to be considerably larger and shifted than the actual ones. Since the scattering depends on the spectrum of density turbulence, better understanding of the radio wave propagation provides indirect information on the relative density fluctuations = δn / n at the effective turbulence scale length. Here, we have analyzed 30 type III bursts detected by Parker Solar Probe (PSP). For the first time, we have retrieved type III burst decay times τ d between 1 MHz and 10 MHz thanks to an unparalleled temporal resolution of PSP. We observed a significant deviation in a power-law slope for frequencies above 1 MHz when compared to previous measurements below 1 MHz by the twin-spacecraft Solar TErrestrial RElations Observatory (STEREO) mission. We note that altitudes of radio bursts generated at 1 MHz roughly coincide with an expected location of the Alfvén point, where the solar wind becomes super-Alfvénic. By comparing PSP observations and Monte Carlo simulations we predict relative density fluctuations at the effective turbulence scale length at radial distances between 2.5 R and 14 R to range from 0.22 and 0.09. Finally, we calculated relative density fluctuations measured in situ by PSP at a radial distance from the Sun of 35.7 R during the perihelion #1, and the perihelion #2 to be 0.07 and 0.06, respectively. It is in a very good agreement with previous STEREO predictions ( = 0.06 − 0.07) obtained by remote measurements of radio sources generated at this radial distance.

The Radio and Plasma Waves (RPW) instrument on the ESA Solar Orbiter mission is described in this paper. This instrument is designed to measure in-situ magnetic and electric fields and waves from the continuous to a few hundreds of kHz. RPW will also observe solar radio emissions up to 16 MHz. The RPW instrument is of primary importance to the Solar Orbiter mission and science requirements since it is essential to answer three of the four mission overarching science objectives. In addition RPW will exchange on-board data with the other in-situ instruments in order to process algorithms for interplanetary shocks and type III langmuir waves detections.

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