Frank Stefani

Biography

Dr Frank Stefani studied Physics at Dresden University of Technology (Dipl.-Phys. 1988). In 1995, he obtained a doctorate in theoretical solid-state physics under Wolfgang Weller at University of Leipzig. After two years as a Postdoc at the Institute of Applied Geodesy, Leipzig, where he worked on geoid determination and remote sensing, he moved to Forschungszentrum Rossendorf, now Helmholtz-Zentrum Dresden-Rossendorf (HZDR). He has been strongly involved in the Riga dynamo experiment and led the PROMISE experiments on the helical and azimuthal magnetorotational instability. As group leader for geo- and astrophysics, he is now responsible for the large-scale precession-driven dynamo experiment in the frame of the DRESDYN project at HZDR. He has published more than 200 papers in basic and applied magnetohydrodynamics, geophysics, astrophysics, inverse problems, and measurement techniques. In 2018, he was was awarded an ERC Advanced Grant.

Precession, tides, et cetera: Astronomically forced and synchronized dynamos

In the first part of the talk, we discuss a model of the solar dynamo that tries to explain its various periodicities on widely different
time scales in a self-consistent manner. Starting with Rieger-type periods, we show that the two-planet spring-tides of Venus, Earth and Jupiter can excite magneto-Rossby waves in the solar tachocline with periods between 100 and 300 days and amplitudes of m/s or even more. We further show that the dynamo-relevant quadratic action of these waves contains a beat period of 11.07 years, and that its axisymmetric part might indeed be strong enough to synchronize the entire solar dynamo via parametric resonance. Then we argue that a second beat between the arising Hale cycle and the 19.86-year periodic motion of the Sun around the barycenter of the solar system may explain the longer-term Gleissberg and Suess-de Vries cycles. The spectrum emerging from this double-synchronized dynamo model shows amazing correspondence with climate-related data.

In the second part of the talk, we examine the present status of the DRESDYN precession-driven dynamo experiment. Its motivation stems, first, from the possible influence of the various Milankovic cycles on the geodynamo and, second, from the not yet fully understood spin-orbit coupling effects in our solar dynamo model. We discuss in some detail the combined numerical
and experimental efforts to identify dynamo-optimizing precession ratios and nutation angles, and illustrate the recent steps in finalizing the set-up of the machine.

Ramada Sukarmadji

Biography

I am a Solar Physics PhD student at Northumbria University studying the coronal nanojet phenomenon. My thesis focuses on the observational and numerical MHD modelling aspects of nanojets, to understand their generation mechanism and role in heating coronal structures. I mainly use the Interface Region Imaging Spectrograph (IRIS) satellite for observations and the PLUTO code for my simulations. Before my PhD, I received my integrated master's degree from the University of St Andrews, where I studied theoretical physics and applied mathematics.

Numerical experiments on the role of MHD waves in triggering nanojets

The coronal heating problem is the question of why the temperature of the Sun’s corona is on the order of a million degrees higher than the temperature of the solar surface. This open question has been investigated for several decades with two leading theories: the dissipation of magnetohydrodynamic (MHD) waves and magnetic reconnection. For reconnection-based heating in particular, no observations have indicated a direct link towards heating by magnetic reconnection until the discovery of nanojets by Antolin et al. (2021). Nanojets are small and rapid energy bursts driven by small-scale reconnections that have been observed in many coronal loop-like structures (Sukarmadji et al. 2022; Sukarmadji et al. 2024). However, we have a limited number of nanojet observations due to their small scales (<1500 km in length) and short timescales (<25 s in duration), presenting a challenge to understand how nanojets are generated. We present numerical simulation results of nanojets based on the model in Antolin et al. (2021), which is of two straight and adjacent flux tubes driven to form a small misalignment. MHD waves are introduced through footpoint driving, and we conducted a parameter investigation of the effects of footpoint driving on the reconnection by varying the driving amplitudes. Our results show that driving the footpoints with amplitudes of 10km/s and larger produces a singular nanojet-like formation event characterised by the fast bi-directional flows, with an energy release of 10^24 erg similar to Antolin et al. (2021). Smaller driving amplitudes produce smaller scale and bursty reconnection events, or continuous energy release on the order of 10^23 erg, without clear nanojet-like features. In all cases, the simulations suggest that magnetic reconnection can be triggered by propagating MHD waves in a braided field, which locally increases the current.

Robert Kingham

Biography

Dr Robert Kingham is a Reader in the Plasma Physics Group, Imperial College London, working on theoretical and computational research projects spanning fusion physics and laser-plasma interactions.   Growing up in a sleepy Cotswold town, he sought out adventure in a big city and ended up studying Physics at Imperial College London, followed by a PhD in laser-plasma acceleration there.  After a 2 year stint in a high-intensity laser laboratory in the Institute of Optics and Quantum Electronics in Jena (Germany), where he was eventually deemed competent enough to turn off the laser, he returned to Imperial College London for a further postdoc on modelling non-local transport in inertial confinement fusion, securing a faculty position in 2004 and remaining there to this day.  The main thread of his research has been kinetic theory and modelling of electron transport including coupling to magnetic-field dynamics.  He and his team have developed several kinetic and extended-MHD simulation codes including IMPACT and CTC to study this.  Other research interests include non-linear optics of laser propagation in plasma, instabilities and twisted light.  As he matured, the countryside started to lure him back and he now lives near Oxford.  Since 2016, he has been increasingly working in the field of magnetic confinement fusion, particularly exhaust physics in the open magnetic-field lines outside the fusion core in tokamaks.  He is currently a Visiting Fellow at UKAEA and collaborates extensively with the Culham Centre for Fusion Energy.

Extended-MHD phenomena and modelling in high energy density plasmas

Interest in magnetic fields in high energy density (HED) plasmas, particularly those generated by high-power nanosecond-duration lasers, has been increasing over the last decade or so.  A prime application is inertial confinement fusion (ICF).  These magnetic fields are either externally applied or grow spontaneously via the Biermann battery.  Conventional MHD models (ideal, resistive) need to be extended to include the full Braginskii transport prescription in this interesting regime, which the HED/ICF community call extended-MHD [1].  This talk will survey the rich set of phenomena that occur in such "magnetized" HED plasmas due to a close coupling between complex magnetic induction (due to the extended Ohm's law involved) and the magnetization of the electron heat flow.  These phenomena include Nernst advection of magnetic field and thermo-magnetic instabilities [2].  The talk will also briefly touch on efforts to incorporate these effects into simulations, the thorny question of kinetic corrections [3], recent experimental work to verify the models [4] and applications of extended-MHD phenomena in ICF [3,5].

[1]  C.A. Walsh, et al., “Extended-magnetohydrodynamics in under-dense plasmas”, Phys. Plasmas 27, 022103 (2020) [ DOI: 10.1063/1.5124144 ]

[2]  J. Bissell, et al., “Field Compressing Magnetothermal Instability in Laser Plasmas”, Phys. Rev. Lett. 105, 175001 (2010)  [ DOI: 10.1103/PhysRevLett.105.175001 ] 

[3]  D.W. Hill & R.J. Kingham, “Enhancement of pressure perturbations in ablation due to kinetic magnetized transport effects under direct-drive inertial confinement fusion relevant conditions”, Phys. Rev. E 98, 021201(R) (2018)  [ DOI:  10.1103/PhysRevE.98.021201 ]

[4]  C. Arran, et al., “Measurement of magnetic cavitation driven by heat flow in a plasma”, Phys. Rev. Lett. 131, 015101 (2023)  [ DOI:  10.1103/PhysRevLett.131.015101  ]

[5]  G. Perez-Callejo, et al., “Cylindrical implosion platform for the study of highly magnetized plasmas at Laser MegaJoule”, Phys. Rev. E 106, 035206 (2022)  [ DOI:  10.1103/PhysRevE.106.035206 ]

Joanne Mason

Biography

Dr Joanne Mason is a senior lecturer in applied mathematics at the University of Exeter, where she has been since 2012. Prior to this, she held postdoctoral appointments in the Department of Astronomy and Astrophysics at the University of Chicago and the High Altitude Observatory in Boulder, Colorado. She obtained her PhD from the University of Leeds. Her main research interests are in dynamo theory and the fundamental properties of MHD turbulence. More generally, she is interested in fluid instabilities, nonlinear phenomena in the physical sciences, and using the techniques of high-performance computing to solve problems that cannot be addressed otherwise.

Statistical zonostrophic instability and the effects of magnetic fields

Zonal flows are long-lived, mean flows, oriented in the east-west direction with respect to rotation and alternating in latitude. The intriguing banded structure of Jupiter’s atmosphere is perhaps the most widely known example, though others exist in geophysics, astrophysics and laboratory plasmas. A variety of mechanisms are proposed for their existence and much remains to be understood regarding their generation, structure and evolution.

In `zonostrophic instability’, within a sea of small-scale turbulence a weak large-scale zonal flow grows exponentially to become prominent. We will discuss the statistical behaviour of the zonostrophic instability and the effects of magnetic fields. Using a simplified model in which a stochastic forcing drives random waves, the dispersion relation for the growth rate of the expectation of the mean flow can be derived. In the limits of weak and strong magnetic diffusivity, the dispersion relation reduces to manageable expressions that provide insights into the effect of the magnetic field and scaling laws for the threshold of instability.

Direct numerical simulations of the stochastic flow will be compared with the theory. The results confirm that the magnetic field mainly plays a stabilising role and thus impedes the formation of the zonal flow. The results also indicate that the field can significantly increase the randomness of the flow: it is found that the zonal flow of an individual realisation may behave very differently from the expectation. For weak magnetic diffusivity and moderate magnetic field strengths, this leads to considerable variation of the outcome, that is whether zonostrophic instability takes place or not in individual realisations.

This is joint work with Chen Wang and Andrew Gilbert.