The magnetic field vector of the Sun-as-a-star – II. Evolution of the large-scale vector field through activity cycle 24

Abstract

In the present work, we investigate how the large-scale magnetic field of the Sun, in its three vector components, has evolved during most of cycle 24, from 2010 Jan to 2018 Apr. To filter out the small-scale field of the Sun, present in high-resolution synoptic maps, we use a spherical harmonic decomposition method, which decomposes the solar field in multipoles with different degrees. By summing together the low-multipoles, we reconstruct the large-scale field at a resolution similar to observed stellar magnetic fields, which allows the direct comparison between solar and stellar magnetic maps. During cycle 24, the 'Sun-as-a-star' magnetic field shows a polarity reversal in the radial and meridional components, but not in the azimuthal component. The large-scale solar field remains mainly poloidal with 70% of its energy contained in the poloidal component. During its evolution, the large-scale field is more axisymmetric and more poloidal when near minima in sunspot numbers, and with a larger intensity near maximum. There is a correlation between toroidal energy and sunspot number, which indicates that spot fields are major contributors to the toroidal large-scale energy of the Sun. The solar large-scale magnetic properties fit smoothly with observational trends of stellar magnetism reported in See et al. The toroidal ( B 2 tor ) and poloidal ( B 2 pol ) energies are related as B 2 tor ∝ B 2 pol 1.38±0.04 . Similar to the stellar sample, the large-scale field of the Sun shows a lack of toroidal non-axisymmetric field.

Direct evidence of a full dipole flip during the magnetic cycle of a sun-like star

“…This transition of the large-scale dipole field is also seen in the Sun (DeRosa et al 2012;Finley et al 2018). Recent analyses by Vidotto et al (2018) indicate that the solar large-scale field, filtered for the large-scale components, shows a similar trend to that presented in Figs. A.1 and A.2. During epochs 2017.89 and 2018.52 the activity decreases even further, as shown in Fig.…”

Evolution of Magnetic Helicity in Solar Cycle 24

“…Figure 4 shows the time-latitude diagrams for the axisymmetric components of magnetic field and vector potential. The time-latitude diagrams of the magnetic field evolution are in agreement with Vidotto et al (2018). The reconstructed potentials disagree with results of Pipin & Pevtsov (2014), who used profiles ofB r from different longitudinal distances from the central meridian to reconstructB φ andĀ r .…”

The evolution of the solar wind

“…10). When the Sun is at minimum activity, its large-scale magnetic field is dominated by a dipolar field, whose axis is roughly aligned with the rotation axis (Sanderson et al 2003;DeRosa et al 2012;Vidotto et al 2018a). In this case, the solar wind velocity distribution is organised into a faster stream ( $ 700-800 km/s) emerging from high latitude coronal holes and a slower stream ( $ 400 km/s) around the equatorial plane (McComas et al 1998).…”

“…In this case, the solar wind velocity distribution is organised into a faster stream ( $ 700-800 km/s) emerging from high latitude coronal holes and a slower stream ( $ 400 km/s) around the equatorial plane (McComas et al 1998). Conversely, at solar maximum, the large-scale magnetic field geometry is more complex, with a dipolar component vanishing and giving rise to higher-order fields, in particular, even-mode components become more important, such as the quadrupole component (DeRosa et al 2012;Vidotto et al 2018a). The solar wind speed in this case has a more complex distribution, as shown in the right panel of Fig.…”