arXiv subjects

F. Stefani

Publications and source records attributed to F. Stefani.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic Dynamo Driven by Inertial Waves

We demonstrate, by studying precession-driven flows, that inertial wave hydrodynamic turbulence can drive a robust magnetic dynamo action. Motivated by the stronger damping of large-scale geostrophic vortices in rapidly rotating planetary and stellar interiors, we introduce a controlled damping of the vortices, which usually accompany inertial wave turbulence and feed on wave energy. It is shown that even a small vortex damping results in a significant increase of the growth rate of the dynamo due to inertial waves in the kinematic regime, allowing it to persist for magnetic Prandtl numbers as low as $Pm \sim 10^{-3}$ and Poincar\'e numbers $Po\sim 0.025$. These critical values of $Po$ and $Pm$ for the dynamo onset decrease with increasing Reynolds number. The onset and growth of the dynamo appear to correlate with the coherent fluctuations of kinetic helicity. Spectral analysis shows that magnetic energy growth is primarily due to inertial-wave-induced induction over a broad range of scales. These results establish inertial waves as an efficient mechanism for magnetic field amplification in rapidly rotating low-$Pm$ flows relevant to planetary and stellar interiors.

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arXiv2026

An optimized tidal-trigger model of the QBO, and some implications for the Carrington event

Magneto-Rossby waves in the solar tachocline are currently being discussed as a potential cause of the quasi-biennial oscillation (QBO). By analyzing sequences of ground-level enhancement (GLE) events and S-flares, the dominant period of the QBO was recently shown to be close to 1.723 years, which is the dominant beat between the periods of the two-planet spring tides of Venus, Earth and Jupiter. We improve upon this model by taking into account the dependence of the three tidally-triggered magneto-Rossby waves on the actual strength of the toroidal field at the tachocline, which we infer from the averaged monthly sunspot number. When optimizing the parameters of this magnetic-field dependence, the correlation of the tidal-forcing function with the 109 extreme solar events reaches values of up to 0.8. This is much higher than the corresponding value for the field-independent tidal forcing function (appr. 0.4), and also higher than the correlation with the sunspot number (appr. 0.56). Based on this improved model, we discuss some interesting parallels between the Carrington event of 1859 and the clustering of strong solar events in summer and autumn 1989. We also make some cautious forecasts for the remainder of cycle 25.

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arXiv2026

Tidal triggers and the predictability of solar activity

Magneto-Rossby waves in the solar tachocline are currently considered to be one of the main determinants of solar activity. In particular, they can give rise to the quasi-biennial oscillation (QBO). The latter was recently shown to be dominated by a phase-stable period of around 1.7 years. By analyzing 72 ground-level enhancement (GLE) events and 37 S-flares, we determine that this period is close to 1.723 years. This, in turn, is the dominant beat between the periods of the spring tides of the tidally dominant planets Venus, Earth, and Jupiter, which are suspected to synchronize not only the QBO, but also the 11.07-year Schwabe cycle. We demonstrate that recent events, such as the solar storm of 2024 May 10 and the strong X-flare of 2026 February 1, align well with maxima of the combined tidal forcing.

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arXiv2025

Reappraising the Elatina series: What if the Solar Interpretation Were Correct?

We revisit the sedimentary laminae from the Neoproterozoic Elatina Formation of South Australia. These were first interpreted as varves driven by solar activity and later as tidalites formed by tidal processes, although the debate has remained open. This series exhibits remarkably consistent periodicities, including a primary and a secondary period of approximately 12 and 314 laminae, respectively. Our present analysis reveals a statistically significant negative correlation between the length of a cycle and the amplitude of the subsequent cycle. By analyzing the residuals of the series' minima with respect to a linear trend and calculating Dicke's ratio, we show that this series exhibits a high degree of phase stability, except for a single break-point which possibly indicates a 90{\deg} phase shift. As these findings are difficult to reconcile with the tidal model, we reconsider the original interpretation as solar-driven sedimentation and tentatively discuss it in terms of a recently developed synchronization model of the solar dynamo. This model is used to calculate the orbital periods of Venus, Earth, Jupiter and Saturn that would be required to explain the 12-year cycle, when interpreted as a modified solar Schwabe cycle, and the 314-year cycle, when interpreted as a prolonged Suess-de Vries cycle. Assuming that the sums of the angular momenta of Jupiter and Saturn and Venus and Earth are pairwise conserved, we find that the planetary orbits change surprisingly little. The plausibility of such changes over a period of seven hundred million years is discussed in light of solar system dynamics.

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arXiv2025

Adding further pieces to the synchronization puzzle: QBO, bimodality, and phase jumps

This work builds on a recently developed self-consistent synchronization model of the solar dynamo which attempts to explain Rieger-type periods, the Schwabe/Hale cycle and the Suess-de Vries and Gleissberg cycles in terms of resonances of various wave phenomena with gravitational forces exerted by the orbiting planets. We start again from the basic concept that the spring tides of the three pairs of the tidally dominant planets Venus, Earth and Jupiter excite magneto-Rossby waves at the solar tachocline. While the quadratic action of the sum of these three waves comprises the secondary beat period of 11.07 years, the main focus is now on the action of the even more pronounced period of 1.723 years. Our dynamo model provides oscillations with exactly that period, which is also typical for the quasi-biennial oscillation (QBO). Most remarkable is its agreement with Ground Level Enhancement (GLE) events which preferentially occur in the positive phase of an oscillation with a period of 1.724 years. While bimodality of the sunspot distribution is shown to be a general feature of synchronization, it becomes most strongly expressed under the influence of the QBO. This may explain the observation that the solar activity is relatively subdued when compared to that of other sun-like stars. We also discuss anomalies of the solar cycle, and subsequent phase jumps by 180 degrees. In this connection it is noted that the very 11.07-year beat period is rather sensitive to the time-averaging of the quadratic functional of the waves and prone to phase jumps of 90 degrees. On this basis, we propose an alternative explanation of the observed 5.5-year phase jumps in algae-related data from the North Atlantic and Lake Holzmaar that were hitherto attributed to optimal growth conditions.

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arXiv2025

The effect of split endcaps on the flow dynamics in a tall Taylor-Couette setup

The effects of axial boundaries, or endcaps are of fundamental interest in many Taylor-Couette (TC) flow experiments. A main challenge in those experiments has been to minimize these effects, which can substantially alter the flow structure compared to the axially unbounded idealized case. Therefore, understanding and disentangling the influence of endcaps on the TC flow dynamics is essential for the unambiguous interpretation of experimental results, particularly when other dynamical processes (instabilities) in TC flows are involved. In this paper, we study the hydrodynamic evolution of a quasi-Keplerian TC flow in the presence of split endcaps for high Reynolds numbers, $Re$, up to $2\times 10^5$, which are larger than those considered in related previous studies. At these $Re$, the flow deviates from the ideal TC flow profile without endcaps, resulting in about $15\%$ deviation in angular velocity at the mid-height of the cylinders. Aside from turbulent fluctuations caused by shearing instability near the endcaps, the bulk flow remains nearly axially independent and exhibits overall Rayleigh-stability. We characterize the scalings of the Ekman and Stewartson layer sizes with $Re$ as well as examine the effect of the ratio of the outer to inner cylinders' angular velocities on the flow. The implications of these findings for ongoing magnetorotational instability (MRI) experiments based on the similar axially bounded TC setup are also discussed. Specifically, it is shown that when imposing a constant axial magnetic field in all the considered configurations, the flow profile modified by the endcaps lowers the critical threshold for the onset of MRI that in turn can facilitate its emergence and detection in those experiments.

physics.flu-dyn↗

arXiv2025

Strong and weak dynamo regimes in Taylor-Couette flows

We reveal a nonlinear magnetic dynamo in a Taylor-Couette flow at small magnetic Prandtl numbers $Pm\leq 1$, which has been previously believed to exist only at higher $Pm\gtrsim 10$ in this flow. The amplitude of initial perturbations, $Pm$ and domain aspect ratio play a key role in the onset and evolution of the dynamo. It exists in two main states -- a weak state dominated by large-scale modes and a strong, turbulent state with higher amplitude dominated by small-scale modes. These findings can be important for dynamo processes in various astrophysical objects with small $Pm$.

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arXiv2024

Helicity oscillations in Rayleigh-B\'enard convection of liquid metal in a cell with aspect ratio 0.5

In this paper, we present numerical and experimental results on helicity oscillations in a liquid-metal Rayleigh-B\'enard (RB) convection cell, with an aspect ratio of 0.5. We find that helicity oscillations occur during transitions of flow states that are characterised by significant changes in the Reynolds number. Moreover, we also observe helicity oscillations at flow conditions where the temporal gradient of the change in the Reynolds number is significantly smaller than that of the helicity. Notably, the helicity oscillations observed during the transient double-roll state exhibit characteristics remarkably similar to those associated with the Tayler Instability.

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arXiv2024

One-winged butterflies: mode selection for azimuthal magnetorotational instability by thermal convection

The effects of thermal convection on turbulence in accretion discs, and particularly its interplay with the magnetorotational instability (MRI), are of significant astrophysical interest. Despite extensive theoretical and numerical studies, such an interplay has not been explored experimentally. We conduct linear analysis of the azimuthal version of MRI (AMRI) in the presence of thermal convection and compare the results with our experimental data published before. We show that the critical Hartmann number ($Ha$) for the onset of AMRI is reduced by convection. Importantly, convection breaks symmetry between $m = \pm 1$ instability modes ($m$ is the azimuthal wavenumber). This preference for one mode over the other makes the AMRI-wave appear as a ``one-winged butterfly''.

astro-ph.SR↗

arXiv2023

Dynamo action driven by precessional turbulence

We reveal and analyze an efficient magnetic dynamo action due to precession-driven hydrodynamic turbulence in the local model of a precessional flow, focusing on the kinematic stage of this dynamo. The growth rate of magnetic field monotonically increases with Poincar\'{e} number, $\rm Po$, characterizing precession strength, and magnetic Prandtl number, $\rm Pm$, equal to the ratio of viscosity to resistivity, for the considered ranges of these parameters. The critical ${\rm Po}_c$ for the dynamo onset decreases with increasing $\rm Pm$. To understand the scale-by-scale evolution (growth) of the precession dynamo and its driving processes, we perform spectral analysis by calculating the spectra of magnetic energy and of different terms in the induction equation in Fourier space. To this end, we decompose the velocity field of precession-driven turbulence into 2D vortical and 3D inertial wave modes. It is shown that the dynamo operates across a broad range of scales and exhibits a remarkable transition from a primarily vortex-driven regime at lower $\rm Po$ to a more complex regime at higher $\rm Po$ where it is driven jointly by vortices, inertial waves and the shear of the background precessional flow. The vortices and shear drive the dynamo mostly at large scales, comparable to the flow system size, and at intermediate scales, while at smaller scales it is mainly driven by inertial waves. This study can be important not only for understanding the magnetic dynamo action in precession-driven flows, but also in a general context of flows where vortices emerge and govern the flow dynamics and evolution.

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arXiv2023

Rieger, Schwabe, Suess-de Vries: The Sunny Beats of Resonance

We propose a self-consistent explanation of Rieger-type periodicities, the Schwabe cycle, and the Suess-de Vries cycle of the solar dynamo in terms of resonances of various wave phenomena with gravitational forces exerted by the orbiting planets. Starting on the high-frequency side, we show that the two-planet spring tides of Venus, Earth and Jupiter are able to excite magneto-Rossby waves which can be linked with typical Rieger-type periods. We argue then that the 11.07-year beat period of those magneto-Rossby waves synchronizes an underlying conventional $\alpha-\Omega$-dynamo, by periodically changing either the field storage capacity in the tachocline or some portion of the $\alpha$-effect therein. We also strengthen the argument that the Suess-de Vries cycle appears as an 193-year beat period between the 22.14-year Hale cycle and a spin-orbit coupling effect related with the 19.86-year rosette-like motion of the Sun around the barycenter.

astro-ph.SR↗

arXiv2023

Non-axisymmetric modes of magnetorotational and possible hydrodynamical instabilities in the upcoming DRESDYN-MRI experiments -- linear and nonlinear dynamics

The quest for an unambiguous detection of magnetorotational instability (MRI) in experiments is still ongoing despite recent promising results. To conclusively identify MRI in the laboratory, a large cylindrical Taylor-Couette experiment with liquid sodium is under construction within the DRESDYN project. In this paper, we investigate the linear and nonlinear dynamics of non-axisymmetric MRI in the magnetized Taylor-Couette flow of liquid sodium, which is a model flow in this experiment. We show that the achievable highest Lundquist $Lu = 10$ and magnetic Reynolds $Rm = 40$ numbers in this experiment are large enough for the linear instability of non-axisymmetric modes with azimuthal wavenumber $|m|=1$, although the corresponding critical values of these numbers are usually higher than those for the axisymmetric mode. The structure of the ensuing nonlinear saturated state and its scaling properties with respect to Reynolds number $Re$ are analyzed, which are important for the DRESDYN-MRI experiment having very high $Re \gtrsim 10^6$. It is shown that for $Re \lesssim 4\times 10^4$, the non-axisymmetric MRI modes eventually decay, since the modified shear profile of the mean azimuthal velocity due to the nonlinear axisymmetric MRI appears to be stable against non-axisymmetric instabilities. By contrast, for larger $Re \gtrsim 4\times 10^4$, a rapid growth and saturation of the non-axisymmetric modes of nonmagnetic origin occurs, which are radially localized near the inner cylinder wall, forming a turbulent boundary layer. However, for all the parameters considered, the saturation amplitude of these non-axisymmetric modes is always a few orders smaller than that of the axisymmetric MRI mode. Therefore, the results of our previous axisymmetric study on the scaling properties of nonlinear MRI states also hold when non-axisymmetric modes are included.

physics.flu-dyn↗

arXiv2023

No evidence for absence of solar dynamo synchronization

Context: The old question of whether the solar dynamo is synchronized by the tidal forces of the orbiting planets has recently received renewed interest, both from the viewpoint of historical data analysis and in terms of theoretical and numerical modelling. Aims: We aim to contribute to the solution of this longstanding puzzle by analyzing cosmogenic radionuclide data from the last millennium. Methods: We reconsider a recent time-series of $^{14}$C-inferred sunspot data and compare the resulting cycle minima and maxima with the corresponding conventional series down to 1610 A.D., enhanced by Schove's data before that time. Results: We find that, despite recent claims to the contrary, the $^{14}$C-inferred sunspot data are well compatible with a synchronized solar dynamo, exhibiting a relatively phase-stable period of 11.07 years, which points to a synchronizing role of the spring tides of the Venus-Earth-Jupiter system.

astro-ph.SR↗

arXiv2023

Alfv\'en wave experiments with liquid rubidium in a pulsed magnetic field

Magnetic fields are key ingredients for heating the solar corona to temperatures of several million Kelvin. A particularly important region with respect to this is the so-called magnetic canopy below the corona, where sound and Alfv\'en waves have roughly the same speed and can, therefore, easily transform into each other. We present the results of an Alfv\'en-wave experiment with liquid rubidium carried out in a pulsed field of up to 63 T. At the critical point of 54 T, where the speeds of Alfv\'en waves and sound coincide, a new 4 kHz signal appears in addition to the externally excited 8 kHz torsional wave. This emergence of an Alfv\'en wave with a doubled period is in agreement with the theoretical predictions of a parametric resonance between the two wave types. We also present preliminary results from numerical simulations of Alfv\'en and magneto-sonic waves using a compressible MHD code.

astro-ph.SR↗

arXiv2023

Numerical and theoretical framework for the DRESDYN precession dynamo experiment

The upcoming DRESDYN (DREsden Sodium facility for DYNnamo and thermohydraulic studies) precession experiment will test the possibility to achieve magnetohydrodynamic dynamo action solely driven by precession. Here, after the description of the experimental facility, we present the results from direct numerical simulations with the aim to understand the flow behavior and its dynamo capability. The main conclusion is that in the nonlinear regime the nutation angle is an essential governing parameter which determines the flow structures and the possibility of dynamo action. We obtain clear indications about the optimum configuration for the future experimental runs.

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arXiv2023

A synchronized two-dimensional $\alpha-\Omega$ model of the solar dynamo

We consider a conventional $\alpha-\Omega$-dynamo model with meridional circulation that exhibits typical features of the solar dynamo, including a Hale cycle period of around 20 years and a reasonable shape of the butterfly diagram. With regard to recent ideas of a tidal synchronization of the solar cycle, we complement this model by an additional time-periodic $\alpha$-term that is localized in the tachocline region. It is shown that amplitudes of some dm/s are sufficient for this $\alpha$-term to become capable of entraining the underlying dynamo. We argue that such amplitudes of $\alpha$ may indeed be realistic, since velocities in the range of m/s are reachable, e.g., for tidally excited magneto-Rossby waves.

astro-ph.SR↗

arXiv2022

Nonlinear evolution of magnetorotational instability in a magnetized Taylor-Couette flow: scaling properties and relation to upcoming DRESDYN-MRI experiment

Magnetorotational instability (MRI) is the most likely mechanism driving angular momentum transport in astrophysical disks. However, despite many efforts, a conclusive experimental evidence of MRI is still missing. Recently, performing 1D linear analysis of the standard MRI (SMRI) in a cylindrical Taylor-Couette (TC) flow with an axial magnetic field, we showed that SMRI can be detected in the upcoming DRESDYN-MRI experiment based on a magnetized TC flow of liquid sodium. In this study, also related to DRESDYN-MRI experiments, we focused on the nonlinear evolution and saturation properties of SMRI and analyzed its scaling behavior with respect to the main parameters of the TC flow. We did a detailed analysis over the extensive ranges of magnetic Reynolds number $Rm\in [8.5, 37.1]$, Lundquist number $Lu\in[1.5, 15.5]$ and Reynolds number, $Re\in[10^3, 10^5]$. We considered small magnetic Prandtl numbers, $Pm \ll 1$, down to $Pm\sim 10^{-4}$, aiming at values typical of liquid sodium in the experiments. In the saturated state, the magnetic energy of SMRI and torque due to perturbations on the cylinders, which characterizes angular momentum transport, both increase with $Rm$ for fixed $(Lu, Re)$, while for fixed $(Lu, Rm)$, the magnetic energy decreases and torque increases with increasing $Re$. We studied the scaling of the magnetic energy and torque in the saturated state as a function of $Re$ and find a power law dependence $Re^{-0.6...-0.5}$ for the magnetic energy and $Re^{0.4...0.5}$ for the torque at all $(Lu, Rm)$ and high $Re\geq 4000$. We also explored the dependence on Lundquist number and angular velocity of the cylinders. These scaling laws will be instrumental in the subsequent analysis of more realistic finite-length TC flows and comparison of numerical results with those obtained from the DRESDYN-MRI experiments to unambiguously identify SMRI in laboratory.

physics.flu-dyn↗

arXiv2022

Interplay between geostrophic vortices and inertial waves in precession-driven turbulence

The properties of rotating turbulence driven by precession are studied using direct numerical simulations and analysis of the underlying dynamical processes in Fourier space. The study is carried out in the local rotating coordinate frame, where precession gives rise to a background shear flow, which becomes linearly unstable and breaks down into turbulence. We observe that this precession-driven turbulence is in general characterized by coexisting two dimensional (2D) columnar vortices and three dimensional (3D) inertial waves, whose relative energies depend on the precession parameter $Po$. The vortices resemble the typical condensates of geostrophic turbulence, are aligned along the rotation axis (with zero wavenumber in this direction, $k_z=0$) and are fed by the 3D waves through nonlinear transfer of energy, while the waves (with $k_z\neq0$) in turn are directly fed by the precessional instability of the background flow. The vortices themselves undergo inverse cascade of energy and exhibit anisotropy in Fourier space. For small $Po<0.1$ and sufficiently high Reynolds numbers, the typical regime for most geo- and astrophysical applications, the flow exhibits strongly oscillatory (bursty) evolution due to the alternation of vortices and small-scale waves. On the other hand, at larger $Po>0.1$ turbulence is quasi-steady with only mild fluctuations, the coexisting columnar vortices and waves in this state give rise to a split (simultaneous inverse and forward) cascade. Increasing the precession magnitude causes a reinforcement of waves relative to vortices with the energy spectrum approaching Kolmogorov scaling and, therefore, the precession mechanism counteracts the effects of the rotation.

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