arXiv subjects

Quentin Noraz

Publications and source records attributed to Quentin Noraz.

14 recordsLinked to original sources

arXiv2026

COCONUT: Toward practical time-evolving Sun-to-Earth magnetohydrodynamic modeling

Due to computational efficiency and numerical stability limitations, coronal simulations constrained by static magnetograms are typically performed first and then used to drive inner-heliosphere (IH) models. In this paper, we calculate the Sun-to-Earth coronal and wind evolutions using a single time-evolving MHD model, showing that implicit MHD models have the potential to meaningfully simplify and improve the overall Sun-to-Earth modelling pipeline. We extend the implicit time-evolving coronal MHD model COCONUT out to 1 AU, and utilise it to investigate solar coronal and wind evolutions around a solar maximum Carrington rotation (CR). We compare quasi-steady-state and time-evolving Sun-to-Earth simulations to evaluate the impact of the inner-boundary magnetic field evolution, which is neglected in steady-state simulations, on background plasma parameters. Comparisons with commonly used coupled Sun-to-Earth simulations are also conducted to further validate and assess the Sun-to-Earth model COCONUT. The results show that the time-evolving implicit MHD modelling approach yields noticeable differences compared to oversimplified steady-state simulations, and is efficient enough for practical applications. Modelling the solar corona and wind using a single MHD model simplifies the modelling pipeline and avoids uncertainties associated with coupling different coronal and IH models. The noticeable differences in the temporal evolution of plasma parameters at the L1 and L5 points highlight the need to use continuously evolving, synchronised magnetic field observations to improve global coronal and solar wind simulations, whereas the overall consistent evolutionary trend reveals the reliability of using L5 observations to forecast solar wind conditions near Earth about four days in advance.

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arXiv2026

Chromosphere of the quiet sun: II. Atmospheric response to small-scale magnetic flux emergence

Coupling between the photosphere, chromosphere and corona in the quiet Sun (QS) is governed by a complex interplay between magnetic structuring, heating, mass loading, and radiative cooling. Constraining how this balance responds to variations in small-scale magnetic flux remains limited. We investigate how chromospheric heating and its thermodynamic coupling to higher atmospheric layers vary as a function of small-scale magnetic flux emergence. We performed a parametric set of 3D radiative-MHD simulations with the Bifrost code, starting from a weakly magnetised QS reference model and injecting horizontal magnetic flux of increasing amplitude into the sub-surface convection zone. The resulting chromospheric dynamics, heating, mass loading, and coronal response were analysed. Chromospheric temperatures and mechanical heating rise monotonically with increasing magnetic-field strength. Although the fractional contribution of shocks decreases, reconnecting current sheets keeps maintaining about 50%. In contrast, the temperature at the base of the corona exhibits a non-monotonic response, reaching a maximum at intermediate magnetic amplitudes and decreasing for the strongest-field case. We show that stronger magnetic-field strength increases chromospheric heating, which increases the coronal-base density through efficient mass loading, and amplifies radiative losses. These density-driven radiative losses dominate the coronal energy balance and thus lead to reduced coronal-base temperatures despite increased heating. Our results demonstrate the sensitivity of chromospheric structure and dynamics to small-scale flux emergence, and its key role in regulating coronal thermodynamics. This result illustrates the chromosphere-s role as a thermodynamic gatekeeper, and further warrants future investigations of atmospheric models relevant to global solar-wind models and space-weather forecasts.

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arXiv2026

MHD modeling of magnetic flux evolution around solar maximum by the coronal model COCONUT

In this paper, we simulate the magnetic flux evolution at different heliocentric distances during two solar-maximum Carrington rotations (CRs) using the time-evolving coronal magnetohydrodynamic (MHD) model COCONUT to investigate the ``open flux problem". The simulated open magnetic flux (OMF) near the solar surface is comparable to that derived from \textit{in situ} observations by PSP and WIND satellites, and is about 5 times larger than that derived from SDO coronal hole (CH) observations, and the variation in the simulated radial solar wind speed is consistent with the evolution of the OMF evaluated around the corresponding solar disk center. We find that the OMF is reduced by up to $45\%$ from 1.01~$R_s$ to 0.1~AU and increases with a higher-resolution mesh. The OMF decreases mainly within 3~$R_s$, where the closed magnetic flux drops more rapidly, from about $60\%$ of the total magnetic flux at 1.01~$R_s$ to about $4\%$ at 3~$R_s$. Moderate adjustment of the heating source term can effectively regulate the simulated OMF. Preprocessing the photospheric magnetograms with a potential field solver that removes many high-order spherical harmonic components reduces the OMF in the low corona, while having little impact beyond 3~$R_s$. Additionally, the ratio of the maximum to the minimum OMF can reach 1.4 during a single solar maximum CR. These findings highlight the necessity of considering higher grid resolution, more realistic heating mechanisms, and the time-evolving regime of coronal MHD modeling when further addressing the ``open flux problem".

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arXiv2025

Shock-induced magnetic reconnection driving Ellerman bomb emission and a spicule

The mechanism that forms 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 reconnection, as traced by Ellerman bombs (EBs), at the footpoints of many spicules. The triggering of EBs is generally linked to reconnection due to flux emergence and convective motions in the photosphere. We aim to explore whether we can connect EBs to type II spicules, and to what extent we can use EBs as an observational proxy to probe reconnection in this dynamic. We also aim to provide further insight into the mechanisms that trigger EBs. We used a simulation run with the radiative magnetohydrodynamics code Bifrost to track spicules and study the physical processes behind their formation. To detect EBs and classify the spicules, we synthesised the H-alpha line using the multilevel radiative transfer code RH1.5D. 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. 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 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. 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 launch of a type II spicule via reconnection outflows. These results provide a physical origin for the observed connection between EBs and spicules.

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arXiv2025

Chromosphere of the quiet sun -- I. Shock and current-sheet dynamics and heating

The solar chromosphere is a crucial interface between the solar interior and its interplanetary environment, regulating how energy is locally deposited into heat and transported into the upper atmospheric layers. Despite significant progress, the dominant processes responsible for chromospheric heating remain debated, particularly under quiet-Sun (QS) conditions. We aim to disentangle and quantify the respective roles of shocks and current sheets (CS) in QS chromospheric modeling. We use a simulation performed with the radiation-magnetohydrodynamics code Bifrost. In order to identify shocks and CS events across space and time, we develop and apply physics-based criteria, allowing us to describe their dynamics and evaluate their contributions to both dissipative (viscous and ohmic) and mechanical (including compressive work) heating. Shocks are found to dominate the energy deposition in the lower chromosphere (up to $59\%$ of the mechanical heating), while CS become the primary contributor in the upper chromosphere, as both plasma $\beta$ and Mach number $Ma$ drop. Overall, $66\%$ of the mechanical chromospheric heating is powered by the combined action of shocks and CS. These results support a multi-process view of the chromospheric heating in the QS, dominated by shocks, CS, and non-steep gradient dynamics. In addition to viscous and ohmic dissipation, compressive heating can play a major role locally in the model, particularly in chromospheric shock structures, where it offsets non-reversibly cooling from expansion and radiation, and therefore constitutes a key heating contribution to consider in the energy budget. This study further highlights the need for next-generation observations to resolve the intermittent and small-scale nature of chromospheric dynamics, in order to bring new constraints on the coupling between the different layers of the solar atmosphere.

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arXiv2025

Avalanches in Magnetohydrodynamical simulations

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 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 reveals distinct growth behaviors in high plasma-beta regions (convective zone) and low plasma-beta regions (atmosphere). The latter display spatiotemporal dynamics similar to those of avalanche models, suggesting self-organized criticality and a common universality class.

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arXiv2025

Decomposing wave activity in the solar atmosphere: Shocks, jets and swirls in the quiet Sun

Context. There remains much mystery about how or if wave-energy in the photosphere can be transferred sufficiently upwards 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 the various environments from the photosphere to low corona and identify wave drivers, transitions and dissipation mechanisms. We will 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 adopt the most significant frequency (MSF) decomposition method to trace wave activity through a 3D realistic quiet Sun simulation. We focus 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 upwards propagation of these oscillations. Furthermore, we validate the use of the highest frequency MSFs 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%.

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arXiv2025

Magnetic activity evolution of solar-like stars: II. $S_{\rm ph}$-Ro evolution of Kepler main-sequence targets

There is now a large sample of stars observed by the Kepler satellite with measured rotation periods and photometric activity index $S_{\rm ph}$. We use this data, in conjunction with stellar interiors models, to explore the interplay of magnetism, rotation, and convection. Stellar activity proxies other than $S_{\rm ph}$ 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 $S_{\rm ph}$-$Ro$ relationships for different stellar spectral types. Though the overall trend of decreasing magnetic activity versus $Ro$ is recovered, we find a localized dip in $S_{\rm ph}$ around $Ro/Ro_{\odot} \sim$\,0.3 for the G and K dwarfs. F dwarfs show little to no dependence of $S_{\rm ph}$ on $Ro$ due to their shallow convective zones; further accentuated as $T_{\rm eff}$ increases. The dip in activity for the G and K dwarfs corresponds to the intermediate rotation period gap, suggesting that the dip in $S_{\rm ph}$ 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 solar $Ro$. 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 $Ro$. We confirm that metal-rich stars have a systematically larger $S_{\rm ph}$ level than metal-poor stars, which is likely a consequence of their deeper convective zones.

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arXiv2025

Global Turbulent Solar Convection: a Numerical Path Investigating Key Force Balances in the context of the Convective Conundrum

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 too large and a shift towards an anti-solar 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 ($Re \sim 800$) that exhibits prograde equatorial rotation and aligns with observational data from helioseismology. We compare this model with an anti-solar 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, which 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 towards 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.

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arXiv2024

Magnetochronology of solar-type star dynamos

Aims. In this study, we analyse the magnetic field properties of a set of 15 global magnetohydrodynamics (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.

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arXiv2023

Dynamics of solar large-scale flows

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 chapter, 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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arXiv2022

Hunting for anti-solar differentially rotating stars using the Rossby number -- An application to the Kepler field

Anti-solar differential rotation profiles have been found for decades in numerical simulations of convective envelopes of solar-type stars. These profiles are characterized by a slow equator and fast poles (i.e., reversed with respect to the Sun) and have been found in simulations for high Rossby numbers (slow rotators). Rotation profiles like this have been reported observationally in evolved stars, but have never been unambiguously observed for cool solar-type stars on the main sequence. In this context, detecting this regime in main-sequence solar-type stars would improve our understanding of their magnetorotational evolution. The goal of this study is to identify the most promising cool main-sequence stellar candidates for anti-solar differential rotation in the \textit{Kepler} sample. First, we introduce a new theoretical formula to estimate fluid Rossby numbers, $Ro_{\rm f}$, of main-sequence solar-type stars, from observational quantities, and taking the influences of the internal structure and metallicity into account. We obtain a list of the most promising stars that are likely to show anti-solar differential rotation. We identify two samples: one at solar metallicity, including 14 targets, and another for other metallicities, including 8 targets. We find that the targets with the highest $Ro_{\rm f}$ are likely to be early-G or late-F stars at about log$_{10}g=4.37$~dex. We conclude that cool main-sequence stellar candidates for anti-solar differential rotation exist in the \textit{Kepler} sample. The most promising candidate is KIC~10907436, and two other particularly interesting candidates are the solar analog KIC~7189915 and the seismic target KIC~12117868. Future characterization of these 22 stars is expected to help us understand how dynamics can impact magnetic and rotational evolution of old solar-type stars at high Rossby number.

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arXiv2022

Powering Stellar Magnetism: Energy Transfers in Cyclic Dynamos of Sun-like Stars

We use the ASH code to model the convective dynamo of solar-type stars. Based on a series of 15 3-D MHD simulations spanning 4 bins in rotation and mass, we show what mechanisms are at work in these stellar dynamos with and without magnetic cycles and how global stellar parameters affect the outcome. We also derive scaling laws for the differential rotation and magnetic field based on these simulations. We find a weaker trend between differential rotation and stellar rotation rate, ($\Delta \Omega \sim (|\Omega|/\Omega_{\odot})^{0.46}$) in the MHD solutions than in their HD counterpart $(|\Omega|/\Omega_{\odot})^{0.66})$, yielding a better agreement with the observational trends based on power laws. We find that for a fluid Rossby number between $0.15 \lesssim Ro_f \lesssim 0.65$ the solutions possess long magnetic cycle, if $Ro_f \lesssim 0.42$ a short cycle and if $Ro_f \gtrsim 1$ (anti-solar-like differential rotation) a statistically steady state. We show that short-cycle dynamos follow the classical Parker-Yoshimura rule whereas the long-cycle period ones do not. We further demonstrate that the Rossby number dependency of the large-scale surface magnetic field in the simulation ($B_{L,surf} \sim Ro_{f}^{-1.26}$) agrees better with observations ($B_{V} \sim Ro_{s}^{-1.4 \pm 0.1}$) and differs from dynamo scaling based on the global magnetic energy ($B_{bulk} \sim Ro_{f}^{-0.5}$). We also show that up to few percents of the stellar luminosity can be channelled into the star's magnetism, hence providing a large energy reservoir for possible surface eruptive events.

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arXiv2021

Impact of anti-solar differential rotation in mean-field solar-type dynamos -- Exploring possible magnetic cycles in slowly rotating stars

Over the course of their lifetimes, the rotation of solar-type stars goes through different phases. Once they reach the zero-age main sequence, their global rotation rate decreases during the main sequence until at least the solar age, approximately following the empirical Skumanich's law and enabling gyrochronology. Older solar-type stars might then reach a point of transition when they stop braking, according to recent results of asteroseismology. Additionally, recent 3D numerical simulations of solar-type stars show that different regimes of differential rotation can be characterized with the Rossby number. In particular, anti-solar differential rotation (fast poles, slow equator) may exist for high Rossby number (slow rotators). If this regime occurs during the main sequence and, in general, for slow rotators, we may consider how magnetic generation through the dynamo process might be impacted. In particular, we consider whether slowly rotating stars are indeed subject to magnetic cycles. We find that kinematic $\alpha$ $\Omega$ dynamos allow for the presence of magnetic cycles and global polarity reversals for both rotation regimes, but only if the $\alpha$-effect is saddled on the tachocline. If it is distributed in the convection zone, solar-type cases still possess a cycle and anti-solar cases do not. Conversely, we have not found any possibility for sustaining a magnetic cycle with the traditional Babcock-Leighton flux-transport dynamos in the anti-solar differential rotation regime due to flux addition. Graphic interpretations are proposed in order to illustrate these cases. However, we find that hybrid models containing both prescriptions can still sustain local polarity reversals at some latitudes. We conclude that stars in the anti-solar differential rotation regime can sustain magnetic cycles only for very specific dynamo processes.

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