Parity fluctuations in stellar dynamos

Abstract

Observations of the solar butterfly diagram from sunspot records suggest persistent fluctuation in parity, away from the overall, approximately dipolar structure. We use a simple mean-field dynamo model with a solar-like rotation law, and perturb the $\alpha$-effect. We find that the parity of the magnetic field with respect to the rotational equator can demonstrate what we describe as resonant behaviour, while the magnetic energy behaves in a more or less expected way. We discuss possible applications of the phenomena in the context of various deviations of the solar magnetic field from dipolar symmetry, as reported from analysis of archival sunspot data. We deduce that our model produces fluctuations in field parity, and hence in the butterfly diagram, that are consistent with observed fluctaions in solar behaviour

A Model of a Tidally Synchronized Solar Dynamo

“…This difference in behaviour at the first and second minimum of α p 0 indicates a high sensitivity of these transitions. Note that in particular the transition between quadrupole and dipole looks similar to that after the Maunder minimum (Arlt (2009); Moss and Sokoloff (2017)).…”

“…In section 5, we show how long-term changes of various dynamo parameters (e.g., the portion of the periodic α part or the term which governs field losses by magnetic buoyancy) are capable of producing transitions between dipole and quadrupole fields, a behaviour for which some observational evidence exists from the Maunder minimum (Sokoloff and Nesme-Ribes, 1994;Arlt, 2009;Moss and Sokoloff, 2017;Weiss and Tobias, 2016). A robust feature of our synchronization model is the phase coherence which is maintained throughout such transitions.…”

Shaken and Stirred: When Bond Meets Suess–de Vries and Gnevyshev–Ohl

“…Without a detailed model for spin-orbit coupling at hand, we hypothesized that this coupling would lead to a periodic variation of the field loss parameter κ in the tachocline region. Numerically, the combination of such a κ-variation with the synchronized component of the α-effect led to a modulation of the dynamo wave with a beat period of 193 years, which manifests itself in a modulation of the North-South asymmetry and, closely related to that, in a change of the dipole-quadrupole relation (Knobloch, Tobias and Weiss, 1998;Moss and Sokoloff, 2017) and the Gnevyshev-Ohl rule. We would like to point out that the emergence of this beat period depends critically on the phase stability of the two underlying 11.07-year and 19.86-year processes, or, to put it otherwise: the existence of the long-term Suess-de Vries cycle gives a "backward argument" for the synchronized character of the short-term Hale cycle.…”

“…comprises the same smoothing term (although more conveniently written here) as in Stefani, Giesecke and Weier (2019), which avoids a steep jump of α at the equator. The term κ(t)B 3 (θ, t) in Equation 1, as originally introduced by Jones (1983) as well as Jennings and Weiss (1991), has been included to account for field losses owing to magnetic buoyancy, on the assumption that the escape velocity is proportional to B 2 (a modified and physically better substantiated version was discussed by Moss, Tuominen and Brandenburg, 1990). While we openly admit that the spin-orbit coupling of the angular momentum of the Sun around the barycenter into some dynamo relevant parameters remains an open question (for ideas, see Zaqarashvili, 1997;Juckett, 2000;Palus et al, 2000;Javaraiah, 2003;Shirley, 2006;Wilson, 2008;and Sharp, 2013) we employ in the following a time variation of the parameter κ(t) proportional to the time series of the angular momentum.…”

Shaken and stirred: When Bond meets Suess-de Vries and Gnevyshev-Ohl

“…Numerically, the combination of such a κ-variation with the synchronized component of the α-effect led to a modulation of the dynamo wave Figure 12. Comparison of the wavelet power density for the "chaotic" intervals (separated by the green dashed lines in Figure 11(d)) with that for the remaining "regular" intervals with a beat period of 193 years, which manifests itself in a modulation of the North-South asymmetry and, closely related to that, in a change of the dipolequadrupole relation (Knobloch, Tobias and Weiss, 1998;Moss and Sokoloff, 2017) and the Gnevyshev-Ohl rule. We would like to point out that the emergence of this beat period depends critically on the phase stability of the two underlying 11.07-years and 19.86-years processes, or, to put it otherwise: the existence of the long-term Suess-de Vries cycle gives a "backward argument" for the synchronized character of the short-term Hale cycle.…”