Influence of Solar Polar Magnetic Fields on the Propagation of Coronal Mass Ejection

TL;DR

This study uses numerical simulations to show how variations in solar polar magnetic fields influence CME propagation, significantly affecting their speed, expansion, and arrival times at planets, with implications for space weather forecasting.

Contribution

It provides a systematic analysis of how polar magnetic field strength variations alter CME dynamics and propagation in the heliosphere, a previously poorly quantified aspect.

Findings

Stronger polar fields slow CME propagation and reduce expansion.

Enhanced polar fields increase plasma density and magnetic field strength in the solar wind.

Background magnetic pressure can surpass aerodynamic drag at large distances.

Abstract

Understanding the propagation of coronal mass ejections (CMEs) through interplanetary space is essential for space weather forecasting. Due to observational limitations, measurements of the photospheric polar magnetic fields remain highly uncertain, and their influence on CME propagation in the heliosphere is still poorly quantified. In this study, we systematically investigate how variations in the photospheric polar magnetic fields affect the Sun-Mars propagation of the 4 December 2021 CME using numerical simulations. The results show that stronger polar fields modify the background solar wind, producing higher plasma density, enhanced magnetic field strength, a flattened heliospheric current sheet, and weakened high-speed streams in the ecliptic plane. These changes markedly slow the CME's radial propagation and inhibit its lateral and radial expansion, leading to notably delayed arrivals at BepiColombo and MAVEN/Tianwen-1. Quantitatively, an enhancement of the polar magnetic fields with a peak value of 6 G at the pole decreases the mean propagation and expansion speeds by roughly 200 km s$^{-1}$ and halves the CME volume. Force analysis reveals that strengthening the polar fields produces only minor changes in the internal force balance of the CME, where the thermal pressure gradient force dominates over the Lorentz force, while it strongly affects the forces acting on the CME surface. At large heliocentric distances, the magnetic pressure of the background solar wind becomes comparable to or even exceeds the aerodynamic drag force, producing a strong confining effect that hinders the CME's motion.