Quentin Noraz

Hunting for anti-solar differentially rotating stars using the Rossby number. An application to the Kepler field, Noraz Q., Breton S. N., Brun A. S., García R. A., Strugarek A., Santos A. R. G., Mathur S., Amard L., 2022, A&A 667, A50

Impact of anti-solar differential rotation in mean-field solar-type dynamos. Exploring possible magnetic cycles in slowly rotating stars, Noraz Q., Brun A. S., Strugarek A., Depambour G., 2022, A&A, 658, A144

Powering Stellar Magnetism: Energy Transfers in Cyclic Dynamos of Sun-like Stars, Brun A. S., Strugarek A., Noraz Q., Perri B., Varela J., Augustson K., Charbonneau P., Toomre J., 2022, ApJ, 926, 21

Se mer på: orcid: 0000-0002-7422-1127

• Sand, Mats Ola; Noraz, Quentin; Aulanier, Guillaume; Martinez-Sykora, Juan; Carlsson, Mats & Voort, Luc Rouppe van der (2026). Shock-induced magnetic reconnection driving Ellerman bomb emission and a spicule. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202557602. Fulltekst i vitenarkiv Vis sammendrag

  Context. The formation mechanism for the dynamic type II spicules has remained elusive for many years. Their dynamical behaviour has long been linked to magnetic reconnection, yet no conclusive evidence has been provided. However, one recent observational study found signs of magnetic reconnection, as traced by Ellerman bombs (EBs), at the footpoints of many spicules. The triggering of EBs is generally linked to magnetic reconnection due to flux emergence and convective motions in the photosphere. Aims. We aim to explore whether we can connect EBs to type II spicules, and determine to what extent we can use EBs as an observational proxy to probe magnetic reconnection in this dynamic. We also aim to provide further insight into the mechanisms that trigger EBs. Methods. We used a simulation run with the radiative magnetohydrodynamics code Bifrost to track spicules and study the physical processes underlying their formation. To detect EBs and classify the spicules, we synthesised the chromospheric H α spectral line using the multilevel radiative transfer code RH 1.5D. We also traced shocks and current sheets to decipher the origin of EBs and spicules. We selected one type II spicule with a strong EB near its footpoint and studied their formation in detail. Results. A magnetoacoustic shock advects the magnetic field lines towards an oppositely directed ambient field, creating a current sheet. The current sheet accelerates dense plasma via a whiplash effect generated by magnetic reconnection into the inclined ambient field, launching the spicule. Several EB profiles trace shock- and magnetic-reconnection-induced dynamics during this process at the spicule footpoint. Conclusions. We present a new EB triggering mechanism in which a shock-induced current sheet reconnects, triggering an EB in the lower solar atmosphere. The shock-induced current sheet generates the upwards propagation of a type II spicule via reconnection outflows. These results provide a plausible physical origin for the recently observed connection between EBs and spicules.

• Cherry, George; Gudiksen, Boris Vilhelm; Finley, Adam J. & Noraz, Quentin (2026). Decomposing wave activity in the solar atmosphere: Shocks, jets, and swirls in the quiet Sun. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202557226. Fulltekst i vitenarkiv Vis sammendrag

  Context. There remains much mystery about how wave energy in the photosphere can be transferred sufficiently upward through the solar atmosphere to contribute to coronal heating. In light of a plethora of theoretical and idealised studies, we must complement our understanding with realistic and self-driven simulations in order to confidently quantify such contributions. Aims. In this study we aim to connect wave drivers in the photosphere with their impact on the low corona, transitions, and dissipation mechanisms. We analyse the effects of the presence of twisted magnetic features and vortical flows on the transport of such wave modes as the structures evolve. Methods. We adopted the most significant frequency (MSF) decomposition method to trace wave activity through a 3D realistic quiet Sun simulation. We focused on vertical and temporal evolution, identifying wave sources and shifts in the dominant modes. Results. We identify two frequencies, at 3.5 and 5 mHz, that connect oscillations in the upper convection zone to the dynamics in the solar atmosphere. We see distinct differences in the absence and presence of swirling structures on the upward propagation of these oscillations. Furthermore, we validate the use of the highest MSF bin as a proxy for the location of shocks in the chromosphere and use the results to understand the connection between shocks and the propagation of oscillations in the upper atmosphere. We discuss the relation of energy transfer via shocks, mode conversion, and jets. Finally, we find the contribution of 3.5 and 5 mHz signals to the overall wave power in the domain to be significant, up to 50%.

• Lamarre, Henri; Charbonneau, Paul; Noraz, Quentin; Strugarek, Antoine; Blaise, Alexis & Brun, Allan Sacha [Vis alle 8 forfattere av denne artikkelen] (2025). Avalanches in magnetohydrodynamical simulations. Physical Review E (PRE). ISSN 2470-0045. 112(6). doi: 10.1103/28ws-d57z. Fulltekst i vitenarkiv Vis sammendrag

  Scale invariance is a hallmark of many natural systems, including solar flares, where energy release spans a vast range of scales. Recent computational advances, at the level of both algorithmics and hardware, have enabled high-resolution magnetohydrodynamical (MHD) simulations to span multiple scales, offering new insights into magnetic energy dissipation processes. Here, we study the scale invariance of magnetic energy dissipation in two distinct MHD simulations. Current sheets are identified and analyzed over time. Results demonstrate that dissipative events exhibit scale invariance, with power-law distributions characterizing their energy dissipation and lifetimes. Remarkably, these distributions are consistent across the two simulations, despite differing numerical and physical setups, suggesting universality in the process of magnetic energy dissipation. Comparisons between the evolution of dissipation regions reveal distinct growth behaviors in high plasma-𝛽 regions (convective zone) and low plasma-𝛽 regions (atmosphere). The latter display spatiotemporal dynamics similar to those of avalanche models, suggesting self-organized criticality and a common universality class.

• Noraz, Quentin; Brun, Allan Sacha & Strugarek, Antoine (2025). Global Turbulent Solar Convection: A Numerical Path Investigating Key Force Balances in the Context of the Convective Conundrum. The Astrophysical Journal (ApJ). ISSN 0004-637X. 981(2). doi: 10.3847/1538-4357/adad59. Fulltekst i vitenarkiv Vis sammendrag

  Understanding solar turbulent convection and its influence on differential rotation has been a challenge over the past two decades. Current models often overestimate giant convection cells' amplitude, leading to an effective Rossby number (Ro) too large and a shift toward an antisolar rotation regime. This convective conundrum underscores the need for improved comprehension of solar convective dynamics. We propose a numerical experiment in the parameter space that controls Ro while increasing the Reynolds number (Re) and maintaining solar parameters. By controlling the Nusselt number (Nu), we limit the energy transport by convection while reducing viscous dissipation. This approach enabled us to construct a Sun-like rotating model (SBR97n035) with strong turbulence (Re ∼ 800) that exhibits prograde equatorial rotation and aligns with observational data from helioseismology. We compare this model with an antisolar rotating counterpart and provide an in-depth spectral analysis to investigate the changes in convective dynamics. We also find the appearance of vorticity rings near the poles, whose existence on the Sun could be probed in the future. The Sun-like model shows reduced buoyancy over the spectrum, as well as an extended quasi-geostrophic equilibrium toward smaller scales. This promotes a Coriolis–inertia (CI) balance rather than a Coriolis–inertia–Archimedes (CIA) balance, in order to favor the establishment of a prograde equator. The presence of convective columns in the bulk of the convection zone, with limited surface manifestations, also hints at such structures potentially occurring in the Sun.

• Mathur, Savita; Santos, Ângela R. G.; Claytor, Zachary R.; García, Rafael A.; Strugarek, Antoine & Finley, Adam J. [Vis alle 19 forfattere av denne artikkelen] (2025). Magnetic Activity Evolution of Solar-like Stars. II. S_ph-Ro Evolution of Kepler Main-sequence Targets. The Astrophysical Journal (ApJ). ISSN 0004-637X. 982(2). doi: 10.3847/1538-4357/adb8cc. Fulltekst i vitenarkiv Vis sammendrag

  There is now a large sample of stars observed by the Kepler satellite with measured rotation periods and photometric activity index Sph. We use this data, in conjunction with stellar interiors models, to explore the interplay of magnetism, rotation, and convection. Stellar activity proxies other than Sph are correlated with the Rossby number, Ro, or ratio of rotation period to convective overturn timescale. We compute the latter using the Yale Rotating Evolution Code stellar models. We observe different Sph–Ro relationships for different stellar spectral types. Though the overall trend of decreasing magnetic activity versus Rossby number is recovered, we find a localized dip in Sph around Ro/Ro⊙ ∼ 0.3 for the G and K dwarfs. F dwarfs show little to no dependence of Sph on Rossby number due to their shallow convective zone, further accentuated as Teff increases. The dip in activity for the G and K dwarfs corresponds to the intermediate rotation period gap, suggesting that the dip in Sph could be associated with the redistribution of angular momentum between the core and convective envelope inside stars. For G-type stars, we observe enhanced magnetic activity above the solar Rossby number. Compared to other Sun-like stars with similar effective temperature and metallicity, we find that the Sun’s current level of magnetic activity is comparable to its peers and lies near the transition to increasing magnetic activity at high Rossby number. We confirm that metal-rich stars have a systematically larger Sph level than metal-poor stars, which is likely a consequence of their deeper convective zones.

• Breton, S.N.; Lanza, A.F.; Messina, S.; Pagano, I.; Bugnet, L. & Corsaro, E. [Vis alle 23 forfattere av denne artikkelen] (2024). Measuring stellar surface rotation and activity with the PLATO mission: I. Strategy and application to simulated light curves. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 689. doi: 10.1051/0004-6361/202449893. Fulltekst i vitenarkiv Vis sammendrag

  The Planetary Transits and Oscillations of stars mission (PLATO) will allow us to measure surface rotation and monitor photometric activity of tens of thousands of main sequence solar-type and subgiant stars. This paper is the first of a series dedicated to the preparation of the analysis of stellar surface rotation and photospheric activity with the near-future PLATO data. We describe in this work the strategy that will be implemented in the PLATO pipeline to measure stellar surface rotation, photometric activity, and long-term modulations. The algorithms are applied on both noise-free and noisy simulations of solar-type stars, which include activity cycles, latitudinal differential rotation, and spot evolution. PLATO simulated systematics are included in the noisy light curves. We show that surface rotation periods can be recovered with confidence for most of the stars with only six months of observations and that the recovery rate of the analysis significantly improves as additional observations are collected. This means that the first PLATO data release will already provide a substantial set of measurements for this quantity, with a significant refinement on their quality as the instrument obtains longer light curves. Measuring the Schwabe-like magnetic activity cycle during the mission will require that the same field be observed over a significant timescale (more than four years). Nevertheless, PLATO will provide a vast and robust sample of solar-type stars with constraints on the activity-cycle length. Such a sample is lacking from previous missions dedicated to space photometry.

• Noraz, Quentin Jean Francois Alain; Brun, A.S. & Strugarek, A. (2024). Magnetochronology of solar-type star dynamos. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 684. doi: 10.1051/0004-6361/202347939. Fulltekst i vitenarkiv Vis sammendrag

  Aims. In this study, we analyse the magnetic field properties of a set of 15 global magnetohydrodynamic (MHD) simulations of solar-type star dynamos conducted using the ASH code. Our objective is to enhance our understanding of these properties by comparing theoretical results to current observations, and to finally provide fresh insights into the field. Methods. We analysed the rotational and magnetic properties as a function of various stellar parameters (mass, age, and rotation rate) in a ‘Sun in time’ approach in our extended set of 3D MHD simulations. To facilitate direct comparisons with stellar magnetism observations using various Zeeman-effect techniques, we decomposed the numerical data into vectorial spherical harmonics. Results.A comparison of the trends we find in our simulations set reveals a promising overall agreement with the observational context of stellar magnetism, enabling us to suggest a plausible scenario for the magneto-rotational evolution of solar-type stars. In particular, we find that the magnetic field may reach a minimum amplitude at a transition value of the Rossby number near unity. This may have important consequences on the long-term evolution of solar-type stars, by impacting the relation between stellar age, rotation, and magnetism. This supports the need for future observational campaigns, especially for stars in the high Rossby number regime.

• Hotta, Hideyuki; Bekki, Yuto; Gizon, Laurent; Noraz, Quentin Jean Francois Alain & Rast, Mark (2023). Dynamics of Large-Scale Solar Flows. Space Science Reviews. ISSN 0038-6308. 219(8). doi: 10.1007/s11214-023-01021-6. Fulltekst i vitenarkiv Vis sammendrag

  Abstract The Sun’s axisymmetric large-scale flows, differential rotation and meridional circulation, are thought to be maintained by the influence of rotation on the thermal-convective motions in the solar convection zone. These large-scale flows are crucial for maintaining the Sun’s global magnetic field. Over the last several decades, our understanding of large-scale motions in the Sun has significantly improved, both through observational and theoretical efforts. Helioseismology has constrained the flow topology in the solar interior, and the growth of supercomputers has enabled simulations that can self-consistently generate large-scale flows in rotating spherical convective shells. In this article, we review our current understanding of solar convection and the large-scale flows present in the Sun, including those associated with the recently discovered inertial modes of oscillation. We discuss some issues still outstanding, and provide an outline of future efforts needed to address these.

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• Noraz, Quentin Jean Francois Alain; Brun, Allan Sacha & Strugarek, Antoine (2024). Dynamo of the solar interior: Powering the decadal cycle and Its comparison to stellar magnetic cycles. Fulltekst i vitenarkiv
• Noraz, Quentin Jean Francois Alain; Carlsson, Mats & Aulanier, Guillaume (2024). What is heating the chromosphere of Bifrost simulations? Fulltekst i vitenarkiv
• Noraz, Quentin Jean Francois Alain; Brun, Allan Sacha & Strugarek, Antoine (2024). Impact of the Nusselt number in global models of solar turbulent convection. Fulltekst i vitenarkiv
• Noraz, Quentin Jean Francois Alain (2024). Chromospheric heating and flux emergence: from Bifrost to observations. Fulltekst i vitenarkiv
• Noraz, Quentin Jean Francois Alain; Brun, Allan Sacha & Strugarek, Antoine (2024). Magnetochronology of solar-type stars dynamos. Fulltekst i vitenarkiv
• Noraz, Quentin Jean Francois Alain; Brun, Allan Sacha & Strugarek, Antoine (2024). The Sun in Time: Evolution of cool-stars rotation and magnetism. Fulltekst i vitenarkiv
• Popovas, Andrius; Nordlund, Åke; Szydlarski, Mikolaj; Kohutova, Petra; Hansteen, Viggo Haraldson & Noraz, Quentin Jean Francois Alain [Vis alle 7 forfattere av denne artikkelen] (2023). Update on global MHD simulations of the solar convective region. Fulltekst i vitenarkiv
• Popovas, Andrius; Nordlund, Åke; Szydlarski, Mikolaj; Kohutova, Petra; Hansteen, Viggo Haraldson & Noraz, Quentin Jean Francois Alain [Vis alle 7 forfattere av denne artikkelen] (2023). Global MHD simulation of the solar convective region. Fulltekst i vitenarkiv
• Popovas, Andrius; Nordlund, Åke; Szydlarski, Mikolaj; Kohutova, Petra & Noraz, Quentin Jean Francois Alain (2023). Global MHD simulations of the solar convective zone using a volleyball mesh decomposition: Pilot. Fulltekst i vitenarkiv
• Noraz, Quentin (2023). New insight about magneto~rotational evolution of solar-type stars. Fulltekst i vitenarkiv
• Noraz, Quentin (2023). Power supply and evolution of the solar dynamo. Fulltekst i vitenarkiv
• Noraz, Quentin (2023). Quantifying Rossby numbers in the Kepler field: Promising candidates for anti-solar differential rotation. Fulltekst i vitenarkiv
• Noraz, Quentin (2023). Impact of the Nusselt number on the energy distribution among solar convection scales. Fulltekst i vitenarkiv
• Noraz, Quentin (2023). Role of Poynting flux injection by magneto-convection on the chromospheric energy balance. Fulltekst i vitenarkiv
• Noraz, Quentin (2022). New insights about the past, present and future of solar magnetism. Fulltekst i vitenarkiv Vis sammendrag

  The magnetic field of solar-type stars is generated and sustained through an internal dynamo process. This process is mostly determined by the combined action of turbulent convective motions and differential rotation. It can sometimes lead to magnetic cyclic variabilities, like the 11-years solar cycle. Evidence of magnetic cycles have been detected for other solar-type stars as well, ranging from a few years to a few tens of years. How are these cycles controlled? Observations and stellar evolution models show that solar-like stars spin-down during their main-sequence. In parallel, numerical simulations of these stars show that different regimes of differential rotation can be reach and are characterized with the Rossby number. In particular, anti-solar differential rotation (fast poles, slow equator) may exist for high Rossby numbers (slow rotators), which grows when the rotation spins-down. If this regime appears during the main sequence, we may wonder how the dynamo process will be impacted, especially if our Sun is in such a transition. In particular, can slowly rotating stars have magnetic cycles? We performed a numerical multi-D parametric study with the STELEM and ASH codes to understand the magnetic field generation of solar-type stars under various differential rotation regimes. We particularly focused on the energy transfers powering these stellar dynamos, and on the existence of magnetic cycles for different stages of the main sequence. We find that short cycles are favoured for small Rossby numbers (fast rotators), and long cycles for intermediate (solar-like) Rossby numbers. We further assess that energy transfers can reach up to 3% of the stellar luminosity to sustains these dynamos, and ultimately powering surface eruptive events. Finally, we find that anti-solar rotating stars (high Rossby numbers) can only sustain magnetic cycles for specific dynamo processes. This led us to develop a theoretical criterion to select anti-solar candidates with the perspective to bring new constraints on our models, stellar evolution, and more particularly on the future of the Sun.

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