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

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.

Figures (11)

• Figure 1: Background solar wind structures under the three magnetic configurations: Case 1 ($B_\textrm{polar}$) with the original polar field, Case 2 ($B_\textrm{polar}+3G$) with the enhanced radial polar field of $3 \sin^7\theta$ (G), and Case 3 ($B_\textrm{polar}+6G$) with the enhanced radial polar field of $6 \sin^7\theta$ (G). The top row shows meridional-plane (XZ) slices of the coronal quasi-steady solar wind solution, displaying the radial speed $v_{r}$ and magnetic field lines from 1 to 18 $R_s$. The middle row presents ecliptic-plane (XY) slices of the interplanetary solar wind, displaying the radial speed $v_{r}$ and magnetic field lines from 20 to 560 $R_s$. The bottom row provides synoptic maps of the radial magnetic field $B_r$ at 1 AU, with the black solid line indicating the heliospheric current sheet.
• Figure 2: Radial profiles of solar wind parameters under the three magnetic configurations: $B_\textrm{polar}$ (red), $B_\textrm{polar}+3G$ (green), and $B_\textrm{polar}+6G$ (blue). High-speed streams are shown by solid lines, and low-speed streams are indicated by dashed lines. The left panels show the radial speed $V_r$, number density $N$, total magnetic field strength $B_\textrm{tot}$, and thermal pressure $P_\textrm{th}$ in the coronal region (from 1 to 20 $R_s$), and the right panels display the corresponding variations in interplanetary space (from 20 to 500 $R_s$). In the second row, the insets in the density $N$ panels present zoomed-in views of the density profiles, highlighting the differences among the three cases.
• Figure 3: Time evolution of CME propagation under the three magnetic configurations: Case 1 ($B_\textrm{polar}$), Case 2 ($B_\textrm{polar}+3G$), and Case 3 ($B_\textrm{polar}+6G$). The left and right parts show the CME structure at $t=50\ \textrm{hr}$ and $t=90 \ \textrm{hr}$, respectively. In each part, the first column illustrates the CME evolution in the ecliptic plane (XY) (20-500 $R_s$), and the second column presents the corresponding meridional-plane slices (20-480 $R_s$). The CME body is outlined by shaded isosurfaces (3D views) and solid-line contours (2D slices), superimposed on the radial speed $V_r$. The dash–dotted lines denote the trajectories of BepiColombo and MAVEN/Tianwen-1, with their positions shown by the red circles.
• Figure 4: Magnetic structure of the simulated CME under the three magnetic configurations: Case 1 ($B_\textrm{polar}$), Case 2 ($B_\textrm{polar}+3G$), and Case 3 ($B_\textrm{polar}+6G$). The top and bottom panels show the CME structure at $t=50\ \textrm{hr}$ and $t=90\ \textrm{hr}$, respectively. A semi-transparent slice on the equatorial plane from 20 to 500 $R_s$ displays the background magnetic field distribution, with the colors indicating the normalized magnetic field strength $B_\textrm{tot}(r/r_0)^2$. In the top panels, the magnetic field lines are colored by $\rho_c$ (yellow for the CME and purple for the background solar wind), while in the bottom panels, they are colored according to the radial velocity $V_r$. The dashed lines denote the trajectories of BepiColombo and MAVEN/Tianwen-1, with their positions shown by the black circles.
• Figure 5: Comparison of the simulated and in situ CME signatures at BepiColombo (left panels) and at MAVEN/Tianwen-1 (right panels) under the three magnetic configurations: Case 1 ($B_\textrm{polar}$), Case 2 ($B_\textrm{polar}+3G$), and Case 3 ($B_\textrm{polar}+6G$). For each spacecraft, the panels display time series of the magnetic field magnitude $B_\textrm{tot}$, the magnetic field vector $(B_r, B_t, B_n)$ in RTN coordinates, the number density $N$, and the radial speed $V_r$. The simulated profiles (colored lines: red for Case 1, green for Case 2, and blue for Case 3) are overlaid with the corresponding in situ measurements (gray for BepiColombo; gray and purple for MAVEN and Tianwen-1, respectively). The vertical dashed lines mark the observed CME arrival times at each spacecraft.