Speaker
Description
The dynamo problem, in its simplest form, consists of identifying and quantifying material flows which lead to amplification of magnetic fields when inserted into the MHD induction equation [1]. In this connection, we focus on solar meridional flows and flow speeds, which may dictate the timing, amplitude, and shape of magnetic cycles in flux transport dynamo models [2]. While organized large-scale motions of solar materials have been under investigation for more than 40 years, their prognostic utility in dynamo simulations has been problematic, as the observed flow speeds and morphologies vary unpredictably from cycle to cycle. The source of this variability remains an open question. In this presentation, we introduce and briefly discuss a deterministic mechanism for exciting time-varying solar meridional flows that has not yet been incorporated in dynamo models. The orbit-spin coupling hypothesis [3] identifies a reversing torque whose axis lies in the solar equatorial plane. Pulsed intensification and relaxation of meridional flow speeds is predicted under this hypothesis [3, 4]. Global circulation model investigations for the Mars atmosphere, incorporating orbit-spin coupling [4, 5], provide proof of concept for this physics, which has enabled successful years-in-advance forecasting of global-scale weather and climate anomalies [6, 7]. The applicability of the orbit-spin coupling mechanism to the problem of solar dynamo excitation has accordingly become a question of interest. Current best estimates for the amplitude of the depth-dependent tangential orbit-spin coupling accelerations of solar materials exceed by nearly two orders of magnitude the peak accelerations of planetary tides [3, 8, 9]. We compare sunspot numbers with calculated orbit-spin coupling torques in the years 1712-2023. The comparisons yield a surprisingly simple explanation for the variability of Hale cycle period lengths and Schwabe cycle period lengths during this interval [9], if we additionally postulate a magnetic cycle time-delay mechanism associated with meridional flows, as first suggested in [10].
[1] Charbonneau, P., Ann. Rev. Astron. Astrophys. 214.52:251-290.
[2] Dikpati, M., Gilman, P. A., de Toma, G., & Ulrich, R. K. (2010), Geophys. Res. Lett. 37, L14107.
[3] Shirley, J. H. (2017), Plan. Space Sci. 141, 1-16, 10.1016/j.pss.2017.04.006
[4] Mischna, M. A. & Shirley, J. H. (2017), Plan. Space Sci. 141, 45-72, 10.1016/j.pss.2017.04.003
[5] Newman, C. E., et al. (2019), icarus 317, 649-668, https://doi.org/10.1016/j.icarus.2018.07.023
[6] Shirley, J. H., et al. (2020), J. Geophys. Res.-Planets, 125, https://doi.org/10.1029/2019JE006007
[7] Shirley, J. H. (2024), https://www.hou.usra.edu/meetings/tenthmars2024/eposter/3180.pdf
[8] Shirley, J. H. (2017b), arXiv:1706.01854 [Astro-ph.SR], https://doi.org/10.48550/arXiv.1706.01854
[9] Shirley, J. H. (2023), https://arxiv.org/abs/2309.13076
[10] Wilmot-Smith, A.L., et al. (2006), Ap. J. 652, 696-708.
