Exploring the Cycle Period and Parity of Stellar Magnetic Activity with Dynamo Modeling

Abstract

Observations of chromospheric and coronal emissions from various solar-type stars show that the stellar magnetic activity varies with the rotation rates of the stars. The faster the star rotates, its magnetic activity gets stronger but activity cycle period does not show a straightforward variation with the rotation rate. For slowly rotating stars, the cycle period decreases with the increase of rotation rate, while for the fast rotators dependency of cycle period on rotation is presently quite complicated. We aim to provide an explanation of these observational trends of stellar magnetic activity using a dynamo model. We construct a theoretical dynamo model for stars of mass 1 M based on the kinematic flux transport dynamo model including radial pumping near the surface of the stars. The inclusion of this near surface downward radial pumping is found to be necessary to match the observed surface magnetic field in case of the Sun. The main ingredients of our dynamo model, meridional circulation and differential rotation for stars are obtained from a mean-field hydrodynamic model. Our model shows a decrease of cycle period with increasing rotation rate in the slowly rotating regime and a slight increase of cycle period with rotation rate for the rapid rotators. The strength of the magnetic field is found to be increasing as the rotation rate of the star increases. We also find that the parity of the stellar magnetic field changes with rotation. According to our model, the parity flips to quadrupolar from dipolar if the rotation period of the star is less than 17 days.

Stellar dynamos with solar and antisolar differential rotations: Implications to magnetic cycles of slowly rotating stars

“…In the solar-like DR branch, we find the magnetic field strength increases with the decrease of rotation period. This result is, in general, agreement with the observational findings (Noyes et al 1984a;Petit et al 2008;Wright et al 2011;Wright & Drake 2016), the global MHD convection simulations (Viviani et al 2018;Warnecke 2018), and mean-field dynamo modellings (Jouve et al 2010;Karak et al 2014a;Kitchatinov & Olemskoy 2015;Hazra et al 2019). However, our model does not produce the observed saturation of magnetic field in the very rapidly rotating stars, which in the kinematic models, requires some additional dynamo saturation (Karak et al 2014a;Kitchatinov & Olemskoy 2015).…”

“…As the meridional flow transports the magnetic fields from source regions, the cycle duration tends to be longer with the decrease of meridional flow (Dikpati & Charbonneau 1999;Karak 2010). This was usually seen in previous flux transport dynamo models of stellar cycles (Nandy 2004;Jouve et al 2010;Karak et al 2014a), but not in the turbulent pumping-dominated regime (Karak & Cameron 2016;Hazra et al 2019). Therefore, in Set C in which meridional circulation is decreased with the rotation rate following Equation ( 12), we expected an increase of cycle period.…”

The Meridional Circulation of the Sun: Observations, Theory and Connections with the Solar Dynamo

“…As we pointed out in §3.4, theoretical considerations suggest that faster rotating stars have weaker meridional circulation, which would lead to longer cycle periods [112,62]. Hazra et al [113] have suggested that the inclusion of the downward turbulent pumping may help in closing the gap between observations and theory.…”

Influence of the Sun-like magnetic cycle on exoplanetary atmospheric escape

“…Magnetic field is a ubiquitous property of stars and many of them show a cyclic magnetic activity. The underlying dynamo mechanism i.e., the non-linear interaction between velocity field and magnetic field in the convection zone, drives the cyclic magnetic activity in stars (Parker 1955;Choudhuri et al 1995;Charbonneau 2014;Hazra et al 2014Hazra et al , 2017Hazra et al , 2019. The CaII H&K project of the Mount Wilson Observatory (Wilson 1978;Noyes et al 1984;Baliunas et al 1995) has monitored chromospheric activity of 111 stars of spectral types F2-M2 on or near the main sequence, and found that Sun-like cyclic activity is common and exhibited by many cool dwarfs.…”

“…Temporal variation was also observed in the Hα transit of HD189733b (Barnes et al 2016), confirming once more that evaporating atmospheres undergo temporal evolution. One likely explanation is that these temporal changes are caused by stellar variability, i.e., time-variations in the properties of host stars, which can take place in the form of, e.g., cyclic magnetic activity, strong flares or variation in stellar winds or total stellar output radiation (e.g., Baliunas et al 1995;Tokumaru et al 2010;Boro Saikia et al 2018;Hazra et al 2019).…”