Nested active regions anchor the heliospheric current sheet and stall the reversal of the coronal magnetic field

TL;DR

This study investigates how nested flux emergence influences the solar magnetic field reversal and the structure of the heliospheric current sheet, revealing that nested active regions can stall the reversal process and affect solar wind sources.

Contribution

The paper demonstrates that nested flux emergence significantly impacts the coronal magnetic field topology and the heliospheric current sheet, providing new insights into solar magnetic field evolution.

Findings

Nested flux emergence produces a coherent contribution to the photospheric magnetic field.

Nested active regions can anchor the heliospheric current sheet above them.

Flux emergence and cancellation influence the stability and position of the HCS.

Abstract

During the solar cycle, the Sun's magnetic field polarity reverses due to the emergence, cancellation, and advection of magnetic flux towards the rotational poles. Flux emergence events occasionally cluster together, although it is unclear if this is due to the underlying solar dynamo or simply by chance. Regardless of the cause, we aim to characterise how the reversal of the Sun's magnetic field and the structure of the solar corona are influenced by nested flux emergence. From the spherical harmonic decomposition of the Sun's photospheric magnetic field, we identify times when the reversal of the dipole component stalls for several solar rotations. Using observations from sunspot cycle 23 to present, we locate the nested active regions responsible for each stalling and explore their impact on the coronal magnetic field using potential field source surface extrapolations. Nested flux emergence has a more significant impact on the topology of the coronal magnetic field than isolated emergences as it produces a coherent (low spherical harmonic order) contribution to the photospheric magnetic field. The heliospheric current sheet (HCS), that separates oppositely directed coronal magnetic field, can become anchored above these regions due to the formation of strong opposing magnetic fluxes. Further flux emergence, cancellation, differential rotation, and diffusion, then effectively advects the HCS and shifts the dipole axis. Nested flux emergence can restrict the evolution of the HCS and impede the reversal of the coronal magnetic field. The sources of the solar wind can be more consistently identified around nested active regions as the magnetic field topology remains self-similar for multiple solar rotations. This highlights the importance of identifying nested active regions to guide the remote-sensing observations of modern heliophysics missions.