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 = 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.

Using radio triangulation to understand the origin of two subsequent type II radio bursts

“…Therefore, we do not discuss the geometric radio source sizes, but only the radio source regions as defined above. A Similar procedure was used in previous studies (Reiner et al 1998;Martínez Oliveros et al 2012b;Martínez-Oliveros et al 2015;Mäkelä et al 2016Mäkelä et al , 2018Krupar et al 2020) • The estimated distance between the two wave vectors is generally smaller at high frequencies. Hence, In this analysis we used the highest available frequencies of direction finding observations, and we did not use frequencies below 500 kHz.…”

Subsecond Time Evolution of Type III Solar Radio Burst Sources at Fundamental and Harmonic Frequencies

“…In-situ (Celnikier et al 1983;Krupar et al 2020) and remote-sensing radio observations (Wohlmuth et al 2001) suggest that the density fluctuations in the interplanetary space have an inverse power-law spectrum S(q) ∝ q −(p+2) , with the spectral index p close to 5/3. The power-law is normally observed over a broad inertial range, from the outer scales l 0 = 2π/q 0 to the inner scales l i = 2π/q i , so we consider…”

“…The grey-shaded area represents the decay time of the burst observed by LOFAR. Krupar et al (2020)). The one standard deviation uncertainty calculated during fitting is used as the error in the measurements of decay time.…”

Coronal Electron Density Fluctuations Inferred from Akatsuki Spacecraft Radio Observations

“…Hollweg, Cranmer, and Chandran (2010), evaluating a wave-heating model based on Helios data, showed ∼ 0.023 -0.031 at SO 2R , and rising an order of magnitude by SO 20R . Krupar et al (2018) evaluated density fluctuations from type III radio bursts measured by STEREO spacecraft, and more recently Krupar et al (2020) gave results from the Parker Solar Probe first and second perihelia. Both studies revealed fractional density fluctuation of 0.06 -0.07 in the extended corona.…”