Abstract
Cool main-sequence stars, such as the Sun, have magnetic fields which are generated by an internal dynamo mechanism. In the Sun, the dynamo mechanism produces a balance between the amounts of magnetic flux generated and lost over the Sun’s 11-year activity cycle and it is visible in the Sun’s different atmospheric layers using multi-wavelength observations. We used the same observational diagnostics, spanning several decades, to probe the emergence of magnetic flux on the two close by, active- and low-mass K dwarfs: 61 Cygni A and ϵ Eridani. Our results show that 61 Cygni A follows the Solar dynamo with a regular cycle at all wavelengths, while ϵ Eridani represents a more extreme level of the Solar dynamo, while also showing strong Solar-like characteristics. For the first time we show magnetic butterfly diagrams for stars other than the Sun. For the two K stars and the Sun, the rate at which the toroidal field is generated from surface poloidal field is similar to the rate at which toroidal flux is lost through flux emergence. This suggests that the surface field plays a crucial role in the dynamos of all three stars. Finally, for ϵ Eridani, we show that the two chromospheric cycle periods, of ∼3 and ∼13 years, correspond to two superimposed magnetic cycles.
Highlights
Magnetic activity is ubiquitous on the Sun and other solar-type stars
Assuming that the surface toroidal field on all three stars corresponds to flux emergence, in Section 4 we show that if the differential rotation of Eridani and 61 Cygni A are comparable to that of the Sun, the amount of net axisymmetric toroidal flux generated in the convection zone associated with the surface poloidal field is similar to the amount of toroidal flux that is lost through the surface
The Sun’s line-of-sight magnetic field has been observed daily with the Wilcox Solar Observatory (WSO) since the mid-1970s covering more than four solar activity cycles
Summary
The Sun’s varying magnetic activity is well established to be generated by an internal dynamo with a cyclic periodicity of approximately 11 years as revealed by multi-wavelength synoptic observations that probe the different layers in the Sun’s atmosphere. The magnetic nature of the Sun’s activity patterns has been confirmed using polarimetric observations (Hale 1908), and regular synoptic observations during the last 40 years have revealed the spatio-temporal structure of the Sun’s largescale magnetic field (Cameron et al 2018) The combination of these multi-wavelength diagnostics shows that the normal cyclic mode of the solar dynamo, as monitored with observations of sunspots over the last 400 years approximately, can be explained by the Babcock-Leighton dynamo model where magnetic flux is generated and lost over the Sun’s 11-year cycle (Babcock 1961; Leighton 1969). Our aim is to understand if the solar-dynamo model can generate more extreme levels of magnetic activity and if we can fill the
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