Dynamo Model for Magnetic Activity Saturation in Rapidly Rotating Solar-Type Stars

Stellar magnetic activity: dependence on rotation rate (observations)

Activity saturation for rapid rotation

A possible reason for the saturation

L. L. Kitchatinov & S. V. Olemskoy

Stellar dynamo model showing the saturation

Special dynamo regime in rapid rotators

Seoul – 2015

Institute for Solar-Terrestrial Physics, Irkutsk; kit@iszf.irk.ru

Magnetic Activity - RotationCoronal activity (Wright et al. 2011)

Chromospheric activity(Noyes et al. 1984)

Unsigned magnetic flux (Vidotto et al. 2014)

Ro = Prot/ is the Rossby number

is the convective turnover time

Prot is the rotation period

Dynamo = differential rotation + alpha-effect

Two versions of the alpha-effect: 1) Canonical alpha-effect of turbulent convection by Parker (1955). 2) The Babcock-Leighton mechanism (Babcock 1961; Leighton 1969)

Latitude

Joy’s law (Hale et al. 1919)

Saturation of the Babcock-Leighton mechanism for rapid rotation:

Rotation

Alpha-effect ~ sin(tilt angle)

Dynamo modelEquations:

Design:Differential rotation according to the helioseismology

Single-cell meridional flow

Non-local alpha-effect

Dependence of the alpha-effect on (co)latitude and rotation rate

The dependence of the mean tilt-angle on latitude was inferred from the extensive literature on the Joy’s law (Hale et al. 1919; Dasi-Espuig et al. 2010; Ivanov 2011; Stenflo & Kosovichev 2012; Wang et al. 2015; …):

The alpha-effect dependence on latitude and rotation rate was prescribed as follows:

Vidotto et al. (2014)

Toroidal magnetic flux as the function of rotation period:

Prot = 1day

Magnetic field oscillations about a (latitude-dependent) nonzero mean imply a double-periodic oscillations in magnetic energy.

Brandenburg et al. (1998)

Conclusions

Saturation of magnetic activity in rapidly rotating solar-type stars can be attributed to the saturation of the BL-type dynamos for the tilt angles approaching 90 with increasing rotation rate.

The dynamo model for the saturation of magnetic activity predicts a special dynamo regime with double-periodic oscillations in magnetic energy for rapid rotators.