Coronal electron density: Insights from radio and in situ observations, and EUHFORIA modeling

TL;DR

This work addresses the challenge of validating coronal electron-density estimates by integrating three complementary approaches: radio triangulation of 11 type III bursts observed during Parker Solar Probe's second perihelion, in situ PSP density measurements, and EUHFORIA MHD modeling. By locating 3D radio sources from direction-finding data, mapping radio-derived densities along burst paths, and comparing them with PSP densities and EUHFORIA outputs, the study reveals large density spreads driven by propagation paths and model limitations, while demonstrating that EUHFORIA can identify large-scale high-density regions along the bursts. The results also indicate relatively small small-scale density fluctuations in PSP data, suggesting limited scattering in the studied distances, and show that type III bursts often trace magnetic field lines rather than the Parker spiral. The work highlights the value of multi-instrument cross-validation for coronal-density estimation and points to improvements such as 360° solar-magnetic-field coverage, time-dependent coronal modeling, and more accurate coronal models to enhance predictive capabilities for space weather.

Abstract

The distribution of the coronal electron density at different distances from the Sun strongly influences the physical processes in the solar corona and is therefore a very important topic in solar physics. Most methods, including radio observations, used for estimating coronal electron density were not fully validated due to the absence of in situ observations closer to the Sun. Consequently, space weather forecasting models that simulate coronal density lacked proper validation. Newly available PSP in situ observations at distances close to the Sun provide an opportunity to study plasma properties near the Sun and to compare observational and modeling results. This work studies type III bursts, estimates their propagation path, and validates coronal electron density obtained from radio, in situ observations, and modeling with EUHFORIA. Type III bursts observed during the second PSP perihelion are analyzed using radio triangulation and modeling. We determine 3D positions of radio sources and use EUHFORIA to estimate electron densities at various locations. The electron densities derived from radio observations and EUHFORIA modeling are inter-validated with in situ PSP measurements. We studied 11 type III bursts during the second PSP perihelion, with radio triangulation showing propagation paths southward from the solar ecliptic plane. Radio source sizes ranged from 0.5 to 40 deg (0.5 to 25 Rs) with no clear frequency dependence, indicating that scattering of radio waves was not very significant. Comparison of electron densities from radio triangulation, PSP data, and EUHFORIA modeling showed a large range of values, influenced by different propagation paths and model limitations. Despite these variations, EUHFORIA identified high-density regions along type III burst paths.

Figures (16)

• Figure 1: Schematic illustration of the radio source size, calculated in degrees from the shortest distance between the wave vectors $W_{wind}$ and $W_{STA}$. The distance d is derived from the Wind and STEREO-A DF observations. The sizes of the Sun, radio source and the distance between them are not in the real proportions.
• Figure 2: Panel (a) shows spacecraft locations in equatorial plane during the type III burst on 05 April 2019 ($\sim$ 17:00 UT). Panel (b) sketches the part of EUHFORIA's modeling domain up to 0.5 au. It also shows the arbitrary virtual spacecraft positions at which we consider modeled time series of the solar wind parameters.
• Figure 3: Dynamic radio spectra of the events 8 and 9 observed on 05 April 2019 at $\sim$ 17:00 UT. Events 8 and 9 (see also \ref{['table1']}) were observed by Wind, STEREO-A, and Parker Solar Probe, from left to right panel. The dashed lines in red, green, and blue indicate the direction-finding frequencies for the Wind and STEREO-A spacecraft. For comparison similar frequencies are shown for PSP observations.
• Figure 4: Panel (a) shows time profiles at 708 kHz (highest), 548 kHz (middle) and 428 kHz (lowest) direction-finding frequencies for all the studies type III radio bursts observed by Wind spacecraft. Panel (b) presents time profiles at 725 kHz (highest), 525 kHz (middle) and 425 kHz (lowest) direction-finding frequencies for all the studied type III radio bursts observed by STEREO-A spacecraft.
• Figure 5: EUV image shows associated jet structure, while white-light observations capture one of the CMEs observed during the second PSP perihelion. Panel (a) displays a jet associated with Event 8 originating from the NOAA AR 12738, observed in the 304 Å wavelength channel of the EUVI instrument onboard STEREO-A. Panel (b) presents a combined image from 03 April 2019, featuring the EUVI 304 Å observation alongside white-light observations from the SECCHI COR2 instrument, highlighting a CME. Image created using Jhelioviewer Muller17