What Is Missing from Our Understanding of Long‐Term Solar and Heliospheric Activity?

Abstract

The heliospheric magnetic field is associated with changes in space weather, cosmic-ray flux, and likely climate. This field is determined by the largest scale patterns of magnetism at the solar surface, dominated by the lower latitude active regions during cycle maximum and by the circumpolar fields during cycle minimum. Whereas the magnetic field in the activity belt is readily studied, the high-latitude field is much less accessible, and its study requires a combination of modeling and observation. Current models hold that the high-latitude magnetic field on the Sun is determined solely by the accumulation of field transported poleward from lower latitude active regions. We test this hypothesis by simulating the evolution of the magnetic field at the solar surface and in the heliosphere during the last 340 yr using a state-of-the-art model that incorporates all processes that are known to contribute significantly to the evolution of the large-scale patterns in the solar field. We find that if only the emergence frequency of magnetic bipoles is varied in accordance with observed sunspot records, the polar- cap field reservoir does not match measurements during past years. Based on comparisons of our simulations with observed polar fluxes over the last few decades and with the proxy for the heliospheric flux formed by 340 yr of Be-10 ice-core data, we suggest that the high-latitude field may be subject to decay on a timescale of 5-10 yr. We discuss the consequences of this finding for our understanding of the Sun-Earth connection and explore inferences for the coupling of the Sun's internal magnetic field to the heliospheric field.