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

¹ Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Uccle, Belgium
² Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, B-3001 Leuven, Belgium
³ Department of Physics and Astronomy, University of Turku, 20500 Turku, Finland
⁴ Goddard Planetary Heliophysics Institute, University of Maryland, Baltimore County, Baltimore, MD 21250, USA
⁵ Heliospheric Physics Laboratory, Heliophysics Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

^⋆ Corresponding author: ketaki.deshpande@oma.be

Received: 12 April 2025
Accepted: 19 October 2025

Abstract

Context. The distribution of the coronal electron density at different distances from the Sun strongly influences the physical processes in the solar corona, and it is therefore a very important topic in solar physics. The majority of the methods used to estimate coronal electron density, including radio observations, were up to now 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 Parker Solar Probe (PSP) in situ observations at distances close to the Sun provide an opportunity to study the properties of plasma near the Sun and to compare observational and modeling results.

Aims. The focus of this work is to study type III radio bursts, estimate their propagation path, and validate the coronal electron density obtained from in situ radio observations and modeling with the EUropean Heliospheric FORecasting Information Asset (EUHFORIA).

Methods. In this study of type III radio bursts observed during the second PSP perihelion, we employ radio triangulation and modeling to analyze coronal electron density. Using the radio triangulation method, we determined the 3D positions of the radio sources. Additionally, we utilized the state-of-the-art EUHFORIA model to estimate electron densities at various locations. The electron densities derived from radio observations and EUHFORIA modeling were then inter-validated with in situ measurements from PSP.

Results. We studied 11 type III radio bursts during the second PSP perihelion, with radio triangulation showing their propagation path in the southward direction from the solar ecliptic plane. The obtained radio source sizes ranged from 0.5 to 40 deg (0.5–25 R_⊙), showing no clear frequency dependence. This indicates that scattering of radio waves was not very significant for the studied events and in this frequency range. A comparison of electron densities derived from radio triangulation, in situ PSP data, and EUHFORIA modeling showed a large range of obtained values. This result is influenced by the different propagation paths across different coronal structures and model limitations. Despite these variations, EUHFORIA successfully identified high-density regions along type III burst paths, demonstrating its capability to capture large-scale density structures.

Conclusions. Our study emphasizes that type III bursts do not always follow the Parker spiral but instead trace distinct magnetic field lines that can be very differently oriented. The study shows constant radio source sizes and confirms that small-scale density fluctuations in PSP data remain relatively low. These two characteristics indicate that scattering effects do not significantly change observed radio source positions within the studied distances.