Density Fluctuations in the Solar Wind Based on Type III Radio Bursts Observed by Parker Solar Probe

Abstract

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 $\epsilon =⟨\delta n⟩/⟨n⟩$ 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 ${\tau }_{\mathrm{d}}$ 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 $\epsilon$ at the effective turbulence scale length at radial distances between 2.5$R_{\odot }$ and 14$R_{\odot }$ to range from $0.22$ and $0.09$. Finally, we calculated relative density fluctuations $\epsilon$ measured in situ by PSP at a radial distance from the Sun of $35.7$~$R_{\odot }$ 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 ($\epsilon =0.06-0.07$) obtained by remote measurements of radio sources generated at this radial distance.

12 pages, 10 figures, accepted for publication in ApJS

References in corpus (3)

• Statistical Survey of Type III Radio Bursts at Long Wavelengths Observed by the Solar TErrestrial RElations Observatory (STEREO)/Waves Instruments: Radio Flux Density Variations with Frequency
• Statistical Survey of Type III Radio Bursts at Long Wavelengths Observed by the Solar TErrestrial RElations Observatory (STEREO)/Waves Instruments: Goniopolarimetric Properties and Radio Source Locations
• Determination of Electromagnetic Source Direction as an Eigenvalue Problem

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• Harmonic radio emission in randomly inhomogeneous plasma
• Parametric simulation studies on the wave propagation of solar radio emission: the source size, duration, and position
• Three-dimensional simulations of the inhomogeneous Low Solar Wind
• LOFAR observations of radio burst source sizes and scattering in the solar corona
• Properties of Type III and Type IIIb Bursts in the Frequency Band of 8-80 MHz during PSP Perihelion at the Beginning of April 2019
• Electromagnetic radiation by turbulent, magnetized and randomly inhomogeneous solar radio sources generated by electron beams
• Coronal electron density: Insights from radio and in situ observations, and EUHFORIA modeling
• Observations of shock propagation through turbulent plasma in the solar corona