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 10 Be 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.

Solar Magnetism in the Polar Regions

“…Following the long-term flux-transport simulations of Schrijver et al (2002) whose polar fields did not reverse during every cycle, Wang et al (2002) demonstrated that polar field reversals could be maintained if the surface meridional flow speeds were systematically higher during large-amplitude cycles than during small-amplitude cycles. Schrijver and Liu (2008) studied the evolution of the axial dipole moment of the global photospheric field from 1997 to 2008.…”

“…Schrijver and Liu (2008) studied the evolution of the axial dipole moment of the global photospheric field from 1997 to 2008. They used three distinct methods to reconstruct the axial dipole: they recorded the axial dipole moments of the standard MDI synoptic maps, they built maps based on the flux transport model described in Section 3.5 and recorded the axial dipole moments of the resulting model fields, and they calculated a pure flux-transport simulation, based on the model of Schrijver et al (2002), with parameters chosen such that the model photospheric field matched the observations. They adopted a standard meridional flow profile from Schrijver (2001) and varied the gradient of the flow near the equator, effectively controlling the communication between the two hemispheres.…”

Solar-cycle precursors and predictions

“…We need longer time series data. Surface flux transport (SFT) model is an effective way to reconstruct solar polar magnetic field with the input of sunspot group data (Wang et al 1989;Schrijver et al 2002;Mackay et al 2002;Baumann et al 2004). derived from geomagnetic indices (Lockwood 2003) and the reversal times of polar fields (Makarov et al 2003).…”

Modeling the Sun’s Small-Scale Global Photospheric Magnetic Field

“…Several authors have constructed models for the evolution of small-scale magnetic features, building in a variety of the observed flux evolution processes including emergence, cancellation, coalescence, and fragmentation (e.g., Schrijver et al 1997;Crouch et al 2007), as well as the influence of underlying convective flows (e.g., Parnell 2001;Meyer et al 2011). Schrijver (2001) and Schrijver et al (2002) modeled the global evolution of the Sun's photospheric magnetic field, incorporating magnetic features that spanned from active regions (few times 10 22 Mx) down to ephermeral regions (few times 10 19 Mx, e.g., Harvey 1993). They included differential rotation and meridional flow, but did not explicitly simulate supergranulation.…”

“…The four flux evolution processes of emergence, cancellation, coalescence, and fragmentation were built into the model, although the emergence phase itself was not simulated -new magnetic features were inserted into the simulation under the assumption that the polarities had already separated. From the simulated photospheric magnetic fields, they considered the radiative losses of the Sun and other cool stars (Schrijver 2001), and the long-term evolution (several hundred years) of the photosphere (Schrijver et al 2002). Thibault et al (2012Thibault et al ( , 2014 extended the magnetic carpet model of Crouch et al (2007) to cover the full solar surface, and included active regions with properties determined from the cycle 21 sunspot database of Wang and Sheeley, as well as differential rotation and meridional flow.…”