Magnetic Connectivity in the Time-Dependent Corona and Heliosphere

Abstract

Magnetic flux fills the heliosphere, expands outward from the solar corona, and is fundamentally related to the structure and dynamics of the solar corona and solar wind. Open magnetic flux and the fast wind are thought to originate from open magnetic field lines in coronal holes. Less understood processes in the streamer belt and the boundaries of coronal holes, associated with the more variable slow wind, may be formed by interchange reconnection between open and closed magnetic flux. Interchange reconnection is thought to give rise to field lines that are "folded," i.e. that turn back on themselves. The properties of strahl electrons measured in the solar wind give clues to the heliospheric magnetic connectivity. Unidirectionally outward strahl indicates open field lines, while bidirectional strahl is associated with closed magnetic flux and CMEs. Inward directed, unidirectional strahl is believed to indicate folded flux. We use two time-dependent, flux-evolutionary MHD models of the combined corona and heliosphere, one for a solar-minimum configuration, one for the 2024 total solar eclipse, to investigate the magnetic connectivity of the corona/heliosphere system. We examine how magnetic connectivity varies with distance from the Sun in the two configurations. We evaluate the evolutionary effects by contrasting time-dependent results with the corresponding steady-state calculations, and compare the model connectivities with statistical studies of strahl. The connectivities in the time-evolving simulations are roughly consistent with observed strahl occurrence rates, while those from the steady-state models are not. Our results suggest that complex magnetic connectivities are ubiquitous in the heliosphere.

Figures (4)

• Figure 1: Area fractions of the spherical surface at different radii occupied by magnetic field lines of different connectivity: (a) SM runs; (b) E24 runs. The dotted vertical line is 1 AU.
• Figure 2: Connectivity maps for the runs of Table \ref{['tab:runs']}. Color codes: navy, open negative flux; maroon, open positive flux; green, closed flux; sky, disconnected flux; white dashed line, $B_r=0$. (a) Mercator projection at 1 AU for SMTD (b) Cut at $0^\circ$ longitude for SMTD. (c) Same as (a) for SMSS. (d) Same as (b) for SMSS. (e) and (f): respectively the same as (a), (b) for E24TD. (g) and (h): respectively the same as (c) and (d) for E24SS.
• Figure 3: Connectivity fractions as a function of time in the $\pm 6^\circ$ latitude zone: (a) SM runs; (b) E24 runs. We have removed the first 200 h hours of the simulations, during which heliospheric relaxation phase occurs. Closed flux is green, disconnected is sky, open folded flux rose, open not-folded gold, total open flux orange. Average values with standard deviation errors from 200 h to the end of the TD simulations are shown on the right within each plot. The values for the SS simulations are shown as circles at 526 h for SMSS and 320 h for E24SS.
• Figure 4: Field line connectivity along the trajectory of Earth in the E24TD calculation, from March 24, 2024 at 19:00 UT (200 h in the simulation) to the end (April 16, 2024, 17:00 UT). Top: 3D rendering. An animation showing all angles is available in the HTML version. Bottom: the trajectory in the longitude-latitude plane. As in Fig. \ref{['fig-temporal']}, closed field lines are green, open not folded rose, open and folded gold, and disconnected sky. The double blue arrow shows the beginning of the trajectory.