Small-Scale Dynamos: From Idealized Models to Solar and Stellar Applications

Abstract

In this article we review small-scale dynamo processes that are responsible for magnetic field generation on scales comparable to and smaller than the energy carrying scales of turbulence. We provide a review of critical observation of quiet Sun magnetism, which have provided strong support for the operation of a small-scale dynamo in the solar photosphere and convection zone. After a review of basic concepts we focus on numerical studies of kinematic growth and non-linear saturation in idealized setups, with special emphasis on the role of the magnetic Prandtl number for dynamo onset and saturation. Moving towards astrophysical applications we review convective dynamo setups that focus on the deep convection zone and the photospheres of solar-like stars. We review the critical ingredients for stellar convection setups and discuss their application to the Sun and solar-like stars including comparison against available observations.

Dynamics of Photospheric Magnetic Flux Distribution and Variations in Solar RVs: A Study Using HARPS-N Solar and SDO Observations

“…We stress that the relative constancy of IN on longer timescales, thus, is important to correctly determine the longterm baseline in the RV fluctuations due to magnetism: in the absence of full coverage of a solar minimum period, a meansubtracted RV would be biased by the strongly varying contributions from active region fields and hence would cause an offset as can be seen in Figure 7. We further point out that weak background fluctuations from the IN fields on the long cycle timescale are of significant consequence for the following reasons: (i) we still do not understand the origin of the weak fields (holding a large fraction of the total flux) on the Sun (Lites et al 2008;Bellot Rubio & Orozco Suárez 2019), although simulation studies show the possible operation of the so-called local (small-scale) dynamos with wider implications for stellar magnetism (Rempel et al 2023;Warnecke et al 2023), and (ii) such fields may be of greater relevance in other stars that possibly maintain them much more efficiently and hence extreme precision measurements of RVs of these stars should provide pathways to explore the existence of these fields and the underlying dynamo mechanisms. We note that at 1″ resolution SDO/HMI captures only about 1/3 of flux in IN (Khomenko & Collados 2006;Zhou et al 2013), and hence our inferences on the contributions of IN to RV variations are likely to be much lower than the actual ones.…”

A Floor in the Sun's Photospheric Magnetic Field: Implications for an Independent Small-scale Dynamo

“…In this study, we have directly addressed the question raised in the above reviews. The different ways in which the Sunʼs total magnetic flux and SW magnetic field strength approach their respective floors at 11 yr minima-as shown by direct observations over the last 50 yr (Figures 1 and 2) and proxy data for the ∼1840-1965 (Figure 4) interval-argue for the existence of two separate dynamos at the Sun: a time-varying cyclic dynamo (Charbonneau 2020) that generates the strong fields underlying large-scale magnetic features such as active regions and coronal holes that dominate the photospheric and SW magnetic flux at solar maximum and an independent constant small-scale turbulent dynamo (Rempel et al 2023) that maintains a floor at all times in the Sunʼs surface magnetism and arguably in the SW as well (Section 3.2).…”

“…Neither of these conjectures is new. The possibility of an independent small-scale dynamo was proposed theoretically several decades ago (see Rempel et al 2023 for a review), with the hypothesized linkage of such a dynamo to the slow solar wind more recent (Cliver & Ling 2011;Cliver & von Steiger 2015). Both suggestions warrant revisiting because of the increasing evidence for the reality of solar and solar wind magnetic floors, which includes the support for a firm floor in F 10.7 (Clette 2021), the correlation of the Sunʼs total magnetic flux with F 10.7 (Figure 1), the contrasting behavior of observed/inferred solar surface and SW B above their respective floors over the last ∼180 yr (Figures 2 and 4), and the quasi-constancy of the SSW contribution to SW B during the last ∼60 yr (Figure 6), in contrast to the cyclic contributions of CMEs and HSSs.…”

“…In Section 2 we present evidence that F 10.7 is a proxy for the Sunʼs surface magnetism and compare how the magnetic field strength at the Sunʼs surface and in the SW approach their respective floor values at 11 yr cycle minima. In Section 3, we make conjectures based on these floors regarding (1) the existence of a turbulent small-scale dynamo (Durney et al 1993;Petrovay & Szakaly 1993;Cattaneo 1999;Rempel et al 2023) that produces a constant unsigned photospheric flux independently of the large-scale dynamo (Charbonneau 2020;Charbonneau & Sokoloff 2023) that produces the 11 yr cycle; and (2) the role of this small-scale dynamo in the generation of the floor-like slow SW (SSW). Our results are summarized and discussed in Section 4.…”

The Cycles of Alpha Centauri: Double Dipping of AB

“…The residual high-energy FUV and X-ray emissions would then arise from processes that are present at all phases of the cycle but disconnected from the dynamo oscillation itself. Examples are chromospheric shock waves associated with photospheric p-modes (e.g., Rutten & Uitenbroek 1991;Carlsson & Stein 1992) and the diminutive magnetic elements of the supergranulation pattern, which are thought to arise from a purely convective "local" or "small-scale" dynamo (e.g., Rempel et al 2023, and references to previous work therein). However, if the low-state high-energy emissions are not invariable, but rather differ from cycle to cycle, then there must be more to the "basal" emission-level story.…”