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Context. Solar magnetic activity exhibits variations with periods between 1.5 and 4 years, the so-called quasi-biennial oscillations (QBOs), in addition to the well-known 11-year Schwabe cycles. Solar dynamo is thought to be the mechanism responsible for the generation of QBOs. Aims. In this work, we analyse sunspot areas to investigate the spatial and temporal behaviour of the QBO signal and study the physical mechanisms responsible using simulations from fully non-linear mean-field flux-transport dynamos. Methods. We investigated the behaviour of the QBOs in the sunspot area data for the full disk, and the northern and southern hemispheres, using wavelet and Fourier analyses. We also ran solar dynamos with two different approaches to generating a poloidal field from an existing toroidal field, namely Babcock–Leighton and turbulent α mechanisms. We then studied the simulated magnetic field strengths as well as meridional circulation and differential rotation rates using the same methods. Results. The results from the sunspot areas show that the QBOs are present in the full disk and hemispheric sunspot areas. These QBOs show slightly different spatial and temporal behaviours, indicating slightly decoupled solar hemispheres. The QBO signal is generally intermittent and in-phase with the sunspot area data, surfacing when the solar activity is at its maximum. The results from the BL-dynamos show that they are neither capable of generating the slightly decoupled behaviour of solar hemispheres nor can they generate QBO-like signals. The turbulent α-dynamos on the other hand generated decoupled hemispheres and some QBO-like shorter cycles. Conclusions. In conclusion, our simulations show that the turbulent α-dynamos with the Lorentz force seem more efficient in generating the observed temporal and spatial behaviour of the QBO signal compared with the BL-dynamos. [ABSTRACT FROM AUTHOR]

Context. The (Zeeman-)Doppler imaging studies of solar-type stars very often reveal large high-latitude spots. This also includes F stars that possess relatively shallow convection zones, indicating that the dynamo operating in these stars differs from the solar dynamo. Aims. We aim to determine whether mean-field dynamo models of late-F type dwarf stars can reproduce the surface features recovered in Doppler maps. In particular, we wish to test whether the models can reproduce the high-latitude spots observed on some F dwarfs. Methods. The photometric inversions and the surface temperature maps of AF Lep were obtained using the Occamian-approach inversion technique. Low signal-to-noise spectroscopic data were improved by applying the least-squares deconvolution method. The locations of strong magnetic flux in the stellar tachocline as well as the surface fields obtained from mean-field dynamo solutions were compared with the observed surface temperature maps. Results. The photometric record of AF Lep reveals both long- and short-term variability. However, the current data set is too short for cycle-length estimates. From the photometry, we have determined the rotation period of the star to be 0.9660 ± 0.0023 days. The surface temperature maps show a dominant, but evolving, high-latitude (around +65°) spot. Detailed study of the photometry reveals that sometimes the spot coverage varies only marginally over a long time, and at other times it varies rapidly. Of a suite of dynamo models, the model with a radiative interior rotating as fast as the convection zone at the equator delivered the highest compatibility with the obtained Doppler images [ABSTRACT FROM AUTHOR]

The question of the origin of magnetic fields of Ap/Bp stars is still regarded as an interesting puzzle of stellar astrophysics. We investigate the possibility that the randomness and relative complexity of these fields are remnants of a magnetic instability. In the studied scenario it is assumed a priori that the surface of an Ap star is slowed down in its early evolutionary stage more than its analogous A star. This leads to a significant differential rotation in its interior, making it possible to generate a strong toroidal magnetic field in the radiative zone. Under such circumstances the kink-type Tayler instability is likely to set in. The presented numerical simulations in a compressible, spherical domain show that the instability can produce large surface magnetic fields, even of the order of 0.01-1 of the internal toroidal component (depending on the setup). The resulting magnetic fields can then serve as "initial conditions" evolving into a stable magnetic configuration (however, the matter of long-term stability is not addressed here). This theory naturally supports the fact that Ap stars rotate typically slower than normal A stars (the Tayler instability is suppressed when rotation is too fast), it also qualitatively explains the dependence of the apparent obliquity of the main magnetic axis on the rotation period, as well as the existence of the minimum field threshold (no Ap stars have been observed with fields weaker than ≈102 G). Given that the generation of the initial differential rotation and initial poloidal fields are not discussed here, the results have a speculative nature and can be viewed as a possible step toward a full understanding of Ap star magnetism. [ABSTRACT FROM AUTHOR]

The sunspot drawings of Johann Staudacher of 1749-1799 were used to determine the solar differential rotation in that period. These drawings of the full disk lack any indication of their orientation. We used a Bayesian estimator to obtain the position angles of the drawings, the corresponding heliographic spot positions, a time offset between the drawings and the differential rotation parameter δΩ, assuming the equatorial rotation period is the same as today. The drawings are grouped in pairs, and the resulting marginal distributions for δΩ were multiplied. We obtain δΩ = -0.048 ± 0.025 d-1 (-2.75 ° /d) for the entire period. There is no significant difference to the value of the present Sun. We find an (insignificant) indication for a change of δΩ throughout the observing period from strong differential rotation, δΩ ≈ -0.07 d-1, to weaker differential rotation, δΩ ≈ -0.04 d-1. [ABSTRACT FROM AUTHOR]