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; [email protected]
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.