Dynamo Wave Patterns inside of the Sun Revealed by Torsional Oscillations

Abstract

Torsional oscillations represent bands of fast and slow zonal flows around the Sun, which extend deep into the convection zone and migrate during solar cycles towards the equator following the sunspot butterfly diagram. Analysis of helioseismology data obtained in 1996-2018 for almost two solar cycles reveals zones of deceleration of the torsional oscillations inside the Sun due to dynamo-generated magnetic field. The zonal deceleration originates near the bottom of the convection zone at high latitudes, and migrates to the surface revealing patterns of magnetic dynamo waves predicted by the Parker's dynamo theory. The analysis reveals that the primary seat of the solar dynamo is located in a high-latitude zone of the tachocline. It suggests a dynamo scenario that can explain 'extended solar cycles' previously observed in the evolving shape of the solar corona. The results show a substantial decrease of the zonal acceleration in the current solar cycle and indicate further decline of activity in the next solar cycle. Although the relationship between the magnitude of zonal deceleration and the amount of emerged toroidal field that leads to formation of sunspots is not yet established, the results open a new perspective for solar cycle modeling and prediction using helioseismology data.

On the Origin of Solar Torsional Oscillations and Extended Solar Cycle

“…At lower latitudes, they propagate upward from the bottom of the convection zone, and form almost stationary oscillatory patterns in the subsurface layer, the depth of which increases with the latitude decrease. These effects qualitatively correspond to the results of our recent helioseismology analysis (Kosovichev & Pipin 2019). Snapshots for a half of the model M1 dynamo cycle of: a) the toroidal magnetic field (background image) and streamlines of the poloidal field (contours); b) variations of the zonal acceleration (background image) and the azimuthal force caused by variations of the meridional circulation, δFU , (contour lines are plotted in the range ±50 m/s 2 ); c) variations of the azimuthal velosity and contours show streamlines of the meridional circulation, dashed lines are for the counter clockwise circulation; d) the azimuthal force caused by the dynamo-induced Λ-effect, δF H (0,ρ) (density stratification, background image), δF H (0,a) (effect of anisotropy, contours are in the same range of magnitudes), see the Eqs (10) in the upper part of the convection zone and downward in the tachocline.…”

“…The robustness of our conclusions depends on the radial distribution of the large-scale field, which determines the zonal flow acceleration in the convection zone. Comparing results of our model with those from the recent paper by Kosovichev & Pipin (2019), (e.g., with their Figure 2) we see that in our models the latitudinal width of the zonal flow acceleration pattern is somewhat smaller than in the observations. At the mid latitudes, the models show the axially aligned zonal flow acceleration patterns.…”

“…Therefore, we come to conclusion that a global connections between the dynamo-induced flows at the bottom and top of the convection zone is essential for existence of the extended 22-year mode of the torsional oscillations. This fact is suggested by the recent results of the helioseismology, as well (Kosovichev & Pipin 2019). Perturbations of the meridional circulation are concentrated close to the boundaries of the dynamo domain.…”

“…Doppler measurements of the solar rotation by Ulrich (2001) and results of helioseismic inversion by Howe et al (2018) and Kosovichev & Pipin (2019) have demonstrated properties of the "extended" 22-year mode of the zonal flow variations. In this mode, the zonal flow pattern drifts equatorward, starting at high latitudes nearly simultaneously with activity of solar ephemeral active regions of a new solar cycle.…”

Changes in Solar Rotation over Two Solar Cycles

“…Basu et al (2012) reported that low-degree helioseismic data indicated that Cycle 24 would be very different, and recently Howe et al (2017) have shown that the difference in characteristics has continued to date. An early examination of solar rotation during the minimum just before Cycle 24 (Antia & Basu 2010 has revealed that the solar rotation profile was different from that of the minimum before Cycle 23, and studies show that the differences have continued (see e.g., Howe et al 2013b;Komm et al 2014;Howe et al 2018;Kosovichev & Pipin 2019). We present the results of an independent helioseismic study sarbani.basu@yale.edu arXiv:1908.05282v1 [astro-ph.SR] 14 Aug 2019 of solar dynamics using both ground-based and space-based helioseismic data.…”