The Dependence of Stellar Activity Cycles on Effective Temperature

This paper proposes the idea that the observed dependence of stellar activity cycles on rotation rate can be a manifestation of a stronger dependence on the effective temperature. Observational evidence is recalled and theoretical arguments are given for the presence of cyclic activity in the case of sufficiently slow rotation only. Slow rotation means proximity to the observed upper bound on the rotation period of solar-type stars. This maximum rotation period depends on temperature and shortens for hotter stars. The maximum rotation period is interpreted as the minimum rotation rate for operation of a large-scale dynamo. A combined model for differential rotation and the dynamo is applied to stars of different mass rotating with a rate slightly above the threshold rate for the dynamo.Computations show shorter dynamo cycles for hotter stars. As the hotter stars rotate faster, the computed cycles are also shorter for faster rotation. The observed smaller upper bound for rotation period of hotter stars can be explained by the larger threshold amplitude of the α-effect for onset of their dynamos: a larger α demands faster rotation. The amplitude of the(cycling) magnetic energy in the computations is proportional to the difference between the rotation period and its upper bound for the dynamo. Stars with moderately different rotation rates can differ significantly in super-criticality of their dynamos and therefore in their magnetic activity, as observed.

Subcritical dynamo and hysteresis in a Babcock-Leighton type kinematic dynamo model

In the Sun and Sun-like stars,it is believed that cycles of the large-scale magnetic field are produced due to the existence of differential rotation and helicity in the plasma flows in their convection zones(CZs).Hence,it is expected that for each star,there is a critical dynamo number for the operation of a large-scale dynamo.As a star slows down,it is expected that the large-scale dynamo ceases to operate above a critical rotation period.In our study,we explore the possibility of the operation of the dynamo in the subcritical region using the Babcock–Leighton type kinematic dynamo model.In some parameter regimes,we find that the dynamo shows hysteresis behavior,i.e.,two dynamo solutions are possible depending on the initial parameters—decaying solution if starting with weak field and strong oscillatory solution(subcritical dynamo)when starting with a strong field.However,under large fluctuations in the dynamo parameter,the subcritical dynamo mode is unstable in some parameter regimes.Therefore,our study supports the possible existence of subcritical dynamo in some stars which was previously demonstrated in a mean-field dynamo model with distributedαand MHD turbulent dynamo simulations.

Inter-correlation between Sunspot Oscillations and Their Internal Structures

Three-and five-minute oscillations are commonly observed in any sunspot.Because they are modulated by the internal thermal and magnetic structures of a sunspot,they could be used as an effective tool for researching sunspot seismology.In this paper,we investigate the properties of oscillations in sunspot groups with varying sizes and magnetic fields,and aim to establish the relationships between sunspot oscillations and its internal structure comparatively.We selected three groups of the unipolar sunspot with approximately axial-symmetric magnetic field and calculated their Fourier spectra based on the ultraviolet/extreme ultraviolet emission intensity variations recorded by the Solar Dynamics Observatory/Atmospheric Imaging Assembly.We found that the distribution of three-minute oscillation is defined by the joint effect of diverging magnetic field and the stratification of the sunspot atmosphere.Its distribution could be modified by any invading magnetic structures in the umbra.In contrast,the five-minute oscillations are more prominent in small spots,implying that five-minute oscillation is very closely connected with umbral dynamics.

Magneto-acoustic waves in magnetic twisted flux tubes

At present,many works about MHD wave diagnostics in magnetic flux tubes are based on some pioneering works not considering the contributions of magnetic twist.Other works considered the effect on MHD waves,but the dispersion relationship they presented only gave the wave modes of m=0,1,2...The kink mode of m=−1 was absent.Therefore,in this work we present a complete dispersion relationship that includes both magnetic twist and the wave mode of m=−1.Analogous to the m=+1 wave mode,the mode of m=−1 also exhibits the mode change at finite kr0,from body to surface mode.The phase speeds of this mode are usually less than those of m=+1 mode.The harmonic curves of m=±1 modes in dispersion relationship diagrams are approximately symmetric in respect to a characteristic velocity,e.g.the tube velocity in flux tubes.Based on the present dispersion relationship,we revisit the issue of spiral wave patterns in sunspots and find that the magnetic twist has no great influence on their morphology in the frame of linear perturbation analysis.