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

Abstract

We present a new model for the Sun's global photospheric magnetic field during a deep minimum of activity, in which no active regions emerge. The emergence and subsequent evolution of small-scale magnetic features across the full solar surface is simulated, subject to the influence of a global supergranular flow pattern. Visually, the resulting simulated magnetograms reproduce the typical structure and scale observed in quiet Sun magnetograms. Quantitatively, the simulation quickly reaches a steady state, resulting in a mean field and flux distribution that are in good agreement with those determined from observations. A potential coronal magnetic field is extrapolated from the simulated full Sun magnetograms to consider the implications of such a quiet photospheric magnetic field on the corona and inner heliosphere. The bulk of the coronal magnetic field closes very low down, in short connections between small-scale features in the simulated magnetic network. Just 0.1% of the photospheric magnetic flux is found to be open at 2.5 R e , around 10-100 times less than that determined for typical Helioseismic and Magnetic Imager synoptic map observations. If such conditions were to exist on the Sun, this would lead to a significantly weaker interplanetary magnetic field than is currently observed, and hence a much higher cosmic ray flux at Earth.

Revisiting the connection between magnetic activity, rotation period, and convective turnover time for main-sequence stars

“…Once the Rossby number reaches the value of unity and hence any chromospheric emission is reduced to the basal flux level, there is no magnetic dynamo action with emerging active regions any more, rather only small-scale activity is expected to be present on the star. Following models of the global magnetic field of the Sun under such conditions (Meyer & Mackay 2016), we speculate that magnetic braking becomes less efficient because of the decrease of the magnetic field strength, which provides another reason why the rotation periods of basal flux stars form an envelope to the observed rotation periods.…”

Magnetic Helicity Condensation and the Solar Cycle

“…Because the flux transport model does not have a mechanism to regenerate the network-scale features, they are absent from the simulation once those in the initial condition have been removed. Some global magnetic field models have incorporated both the maintenance and evolution of the Sun's magnetic network (Schrijver 2001;Meyer & Mackay 2016). However, for the present study, which aims to consider large-scale longlived features, this is not necessary and so is not included.…”

Investigation of the Middle Corona with SWAP and a Data-Driven Non-Potential Coronal Magnetic Field Model

“…In a follow-up study, we will examine the effect on the coronal evolution of an active region that has decayed as the result of such smaller-scale processes, compared to the standard flux transport model decay of an active region through diffusion. Active region decay via smaller-scale processes will be simulated by coupling the current flux transport model to the Magnetic Carpet model of Meyer et al (2011) and Meyer and Mackay (2016), which successfully reproduced many observed properties of the small-scale photospheric magnetic field.…”

“…Active region decay via smaller-scale processes will be simulated by coupling the current flux transport model to the Magnetic Carpet model of Meyer et al. ( 2011 ) and Meyer and Mackay ( 2016 ), which successfully reproduced many observed properties of the small-scale photospheric magnetic field.…”