Stellar Dynamos in the Transition Regime: Multiple Dynamo Modes and Antisolar Differential Rotation

Abstract

Global and semi-global convective dynamo simulations of solar-like stars are known to show a transition from an anti-solar (fast poles, slow equator) to solar-like (fast equator, slow poles) differential rotation (DR) for increasing rotation rate. The dynamo solutions in the latter regime can exhibit regular cyclic modes, whereas in the former one, only stationary or temporally irregular solutions have been obtained so far. In this paper we present a semi-global dynamo simulation in the transition region, exhibiting two coexisting dynamo modes, a cyclic and a stationary one, both being dynamically significant. We seek to understand how such a dynamo is driven by analyzing the large-scale flow properties (DR and meridional circulation) together with the turbulent transport coefficients obtained with the test-field method. Neither an αΩ dynamo wave nor an advection-dominated dynamo are able to explain the cycle period and the propagation direction of the mean magnetic field. Furthermore, we find that the α effect is comparable or even larger than the Ω effect in generating the toroidal magnetic field, and therefore, the dynamo seems to be α 2 Ω or α 2 type. We further find that the effective large-scale flows are significantly altered by turbulent pumping.

Rotational dependence of turbulent transport coefficients in global convective dynamo simulations of solar-like stars

“…For Co > 1.5, it becomes positive (negative), and finally approaches zero, being weaker than α rr for the highest rotation rates. This change was also reported by Viviani et al (2019) by comparing their run with Co = 2.8 to the run of Warnecke et al (2018) with Co = 8.3. At high latitudes, the distribution is similar as for α K .…”

“…To determine the turbulent transport coefficients from our simulations, we use the test-field method (Schrinner et al 2005(Schrinner et al , 2007Brandenburg et al 2008;Warnecke et al 2018) with the new convention introduced in Viviani et al (2019). In the testfield method, nine independent test fields are used to calculate how the flow acts on these fields to generate a small-scale magnetic field and therefore an electromotive force E. It is important to note that these test fields do not have a feedback effect on the simulated hydromagnetic quantities, and are therefore only diagnostics of the system.…”

“…None of these dynamo solutions can be explained by a pure αΩ dynamo as in the moderate rotation case. The study of Viviani et al (2019) revealed that the α effect generating the toroidal magnetic field is comparable to or even larger than the Ω effect of differential rotation. In the Coriolis number range between the regime described above, we often find a mixture of both dynamo types (e.g., Viviani et al 2018;Warnecke 2018).…”

“…In this regime, most of the simulations produce irregular in-time dynamo solutions (Karak et al 2015;Warnecke 2018). However, Viviani et al (2018Viviani et al ( , 2019 discovered a cyclic solution in this regime. None of these dynamo solutions can be explained by a pure αΩ dynamo as in the moderate rotation case.…”

“…We use the test-field method (Schrinner et al 2005(Schrinner et al , 2007 to determine the turbulent transport coefficients. This method has been shown to give a good description of the dynamo processes in global dynamo simulations at moderate Reynolds numbers (Schrinner 2011;Schrinner et al 2011Schrinner et al , 2012Warnecke et al 2018;Warnecke 2018;Viviani et al 2019). As the current testfield method only works for cases where the large-scale magnetic field is axisymmetric, we restrict our setup to azimuthal wedges of one-quarter of a sphere.…”