arXiv subjects

Laurent Gizon

Publications and source records attributed to Laurent Gizon.

At least 19 recordsLinked to original sources

arXiv2026

Cause of chromospheric opposite polarity intrusions discovered in Sunrise III/SCIP data: MURaM-ChE simulations point to twisted flux ropes

The Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) instrument onboard the balloon-borne Sunrise III observatory provided new high-resolution observations of the solar chromosphere in the Ca II 854.2 nm line. The Stokes-V signal in magnetic network regions was found to show fine-structured details, which suggests the magnetic field above the network elements does not simply expand as a unipolar feature but displays opposite-polarity-intrusions (OPIs). These features appear as elongated structures in Stokes-V observations. In this work, we demonstrate that such features appear ubiquitously in a numerical simulation of the solar chromosphere. We use a simulation that is computed with the recently developed chromospheric extension of MURaM (MURaM-ChE) and resembles an enhanced network region. We find that OPIs appear ubiquitously in the vertical component of the magnetic field at around 1 Mm above the surface and are visible in the synthetic Stokes-V signal of the Ca II 854.2 nm line. The structures have lengths of 2 Mm to 7 Mm and widths of approximately 1 Mm. The magnetic field configurations associated with the OPI features appear to belong to twisted flux ropes (TFRs) and are visible for most of the time in the presented 21 min time series. Our results show that the magnetic structure of the chromosphere is more complex than previously thought, with even seemingly simple flux tubes showing embedded twisted fields pointing in the opposite direction. This may help in explaining new high-resolution observations from the Sunrise III mission.

astro-ph.SR↗

arXiv2026

Determining low-$\ell$ p-mode frequency shifts in Sun-like stars: Enhancing the cross-correlation technique with filters

Acoustic mode frequencies in the Sun and Sun-like stars change due to magnetic activity, on time-scales much larger than the star's rotation and much smaller than its evolution. Given the poor S/N of the observed stellar p-modes, it is challenging to measure the changes of individual mode frequencies. Typically, power spectra of different time series segments are cross-correlated to estimate a mean p-mode frequency change, which ends up averaging over the individual mode contributions. We seek to enhance the cross-correlation method, by introducing a novel and computationally cheap method, thus enabling us to disentangle p-mode frequency changes for different spherical harmonic degree $\ell$. Assuming that the inclination angle and rotation rate are already measured, filters are designed, which enable the isolation of $δω_\ell$, frequency changes of modes with a given $\ell$, while preventing bias creeping in from neighbouring modes. Monte-Carlo simulations are performed to quantify uncertainty in the estimation of $δω_\ell$. We validate our method against well-studied solar data (SOHO/VIRGO and BiSON) and demonstrate its applicability to the solar-like Kepler star KIC 8006161.

astro-ph.SR↗

arXiv2026

Chromospheric Dynamics of an Umbral Flare Kernel - Based on Coordinated SUNRISE III SCIP and Domeless Solar Telescope Observations

We report imaging spectroscopic observations of an M1.4 solar flare obtained during a coordinated observation between the infrared spectropolarimeter SCIP onboard the SUNRISE-III balloon mission and Domeless Solar Telescope (DST) at Hida Observatory, Kyoto University. The flare that occurred on 2024 July 13 in NOAA Active Region 13738 exhibited a compact flare kernel located within a sunspot umbra. SCIP performed rapid slit-scan observations over a field of view of 58" x 58" around the umbra with a cadence of 40 s, covering infrared chromospheric and upper-photospheric lines including Ca II 8498/8542 A and K I D1. At the same time, DST observed a wider surrounding region with a cadence of 25 s in H-alpha, Ca II 8542 A, and Na I D1/D2. Clear flare-related brightenings are detected in all chromospheric lines observed by SCIP and DST, while no significant enhancement is found in photospheric lines. The high spatial resolution of SCIP reveals fine substructures within the kernel on spatial scales of order 1000 km, which appear smeared in ground-based observations. The spectral profiles exhibit temporally and spatially varying Doppler shifts and line broadenings, indicating complex, fine-scale plasma motions in the chromosphere. These results suggest that the observed red asymmetry arises from the temporal succession of multiple fine-scale kernels, as revealed by SCIP, rather than from a single continuous process.

astro-ph.SR↗

arXiv2026

Numerical simulations of oscillations for axisymmetric solar backgrounds with differential rotation and gravity

Local helioseismology comprises of imaging and inversion techniques employed to reconstruct the dynamic and interior of the Sun from correlations of oscillations observed on the surface, all of which require modeling solar oscillations and computing Green's kernels. In this context, we implement and investigate the robustness of the Hybridizable Discontinuous Galerkin (HDG) method in solving the equation modeling stellar oscillations for realistic solar backgrounds containing acoustic attenuation, gravity, and differential rotation. While a common choice for modeling stellar oscillations is the Galbrun's equation, our working equations are derived from an equivalent variant, involving less regularity in its coefficients, working with Lagrangian displacement and pressure perturbation as unknowns. Under differential rotation and axisymmetric assumption, the system is solved in azimuthal decomposition with the HDG method. Compared to no-gravity approximations, the mathematical nature of the wave operator is now linked to the profile of the solar buoyancy frequency $N$ which encodes gravity, and leads to distinction into regions of elliptic or hyperbolic behavior of the wave operator at zero attenuation. While small attenuation is systematically included to guarantee theoretical well-posedness, the above phenomenon affects the numerical solutions in terms of amplitude and oscillation pattern, and requires a judicious choice of stabilization. We investigate the stabilization of the HDG discretization scheme, and demonstrate its importance to ensure the accuracy of numerical results, which is shown to depend on frequencies relative to $N$, and on the position of the Dirac source. As validations, the numerical power spectra reproduce accurately the observed effects of the solar rotation on acoustic waves.

astro-ph.SR↗

arXiv2026

Temporal variations of solar inertial mode parameters from GONG (2002-2024) and HMI (2010-2024): Rossby modes ($3 \leq m \leq 16 $) and $m=1$ high-latitude mode

We studied the temporal evolution of solar inertial modes over the solar cycle using observations from GONG and SDO/HMI. We focused on the high-latitude mode with azimuthal wavenumber $m=1$ and the equatorial Rossby modes with $3 \le m \le 16$. We used horizontal flow maps near the solar surface from GONG and HMI ring-diagram pipelines at a cadence of approximately one day, covering the period 2002-2024. The data were divided into overlapping 4-year windows, with central times separated by six months. Within each time window and for each inertial mode, we measured the frequency and power of the mode from GONG and HMI data. We find good agreement between GONG and HMI measurements throughout their overlapping period from 2010 to 2024. In general, the magnitude of the frequency variations increases with increasing $m$, and relative changes in mode power typically exceed 100%. For the $m=1$ high-latitude mode, the measured power is anticorrelated with the sunspot number (SSN), while its frequency shows no significant temporal variation. For the equatorial Rossby modes, the frequencies are generally anticorrelated with the SSN, whereas the mode powers tend to correlate positively with the SSN. An exception is the $m=3$ equatorial Rossby mode, whose mode power is strongly anticorrelated with the SSN, in contrast to the other equatorial Rossby modes, highlighting its distinct behavior. We find that the frequencies and power of the Sun's inertial modes have varied significantly on solar-cycle timescales over the past 23 years. The mode parameters are however not uniformly synchronized with the SSN; variations are observed both between modes and from one solar cycle to the next, with some of the differences likely reflecting observational uncertainties. The sensitivity of inertial modes to solar-cycle changes indicates their potential as a diagnostic of solar interior dynamics and magnetism.

astro-ph.SR↗

arXiv2026

Sunrise III: Instrument, mission, data, and first results

Sunrise III is a stratospheric balloon-borne solar observatory with a 1-m diameter telescope and three post-focus instruments, along with an image stabilisation system, all within a protective gondola. It samples the lower solar atmosphere, from the solar surface to the middle chromosphere, at a resolution approaching 50~km on the Sun. Sunrise III flew successfully for 6.5 days suspended from a zero-pressure stratospheric balloon from northern Sweden to north-western Canada in July 2024, gathering around 200 TB of data. The present issue of ApJL focuses on the first scientific results from the data collected during that flight. This paper introduces this Focus Issue, providing a very brief overview of the capabilities of the instrumentation, the flight and of the gathered data. Challenges for the measurements, data reduction and interpretation are also briefly touched upon. The paper ends with an overview of the first set of science results obtained from these data, as presented in the current Focus Issue.

astro-ph.SR↗

arXiv2026

Quantifying the effect of passband on observations in the Ca II K line

Full-disk observations of the Sun in the Ca II K line have been carried out since the late 19th century at various observatories worldwide. These long-term records of solar activity are crucial for reducing discrepancies among solar irradiance reconstructions and for advancing our understanding of the solar dynamo. To construct a consistent composite record, data from different observatories must be cross-calibrated to account for variations in spectral passband and spatial resolution, which are the primary sources of discrepancies between archives. In this study, we use high spectral and spatial resolution observations in the Ca II K line from the state-of-the-art Sunrise III mission to emulate different passbands and derive empirical contrast-contrast relationships between them. We find that these relationships are well described by a power law and provide coefficients for different combinations of passband widths in the range 0.1--9 Angstroms and spatial resolutions between 1 arcsec and 6 arcsec. Applying such a relationship to observations from two major Ca II K archives demonstrates its potential to improve their cross-calibration. The results provide a foundation for the construction of a consistent, century-long time series of solar activity from historical and modern Ca II K observations.

astro-ph.SR↗

arXiv2026

A single power law for the TRAPPIST-1 flare distribution across four orders of magnitude in energy

TRAPPIST-1 is an ultra-cool dwarf that flares frequently. These flares shape the surrounding planets' high-energy irradiation environments, with consequences for atmospheric chemistry and escape, and they can contaminate transmission spectroscopy of those planets. A quantitative flare-frequency distribution (FFD) spanning the full energy range is therefore essential for both interpreting JWST spectra and modeling the planets' irradiation histories. Here we present a unified FFD over four orders of magnitude in energy by jointly analyzing $\approx$87\,hr of JWST/NIRISS and JWST/NIRSpec time-series spectroscopy together with $\approx$74\,days of \textit{Kepler}/K2 photometry. To enable a consistent comparison across these heterogeneous datasets, we convert all events to energies in the TESS bandpass. For the Kepler-to-TESS conversion we adopt a cooler flare continuum appropriate for ultra-cool dwarfs ($T_{\rm flare}=3500$\,K). After correcting for flare-detection sensitivities, the combined JWST+K2 cumulative FFD is consistent with a single power law, $N(\ge E_\mathrm{TESS})\propto E_\mathrm{TESS}^{-β}$, with $β=0.753$ over $E_{\rm TESS}\simeq10^{29}$-$10^{33}$\,erg. The slope of the distribution indicates that the time-averaged flare energy budget is dominated by rare, high-energy events rather than by the more numerous low-energy flares. Moreover, we found that strong flares with energies $E_\mathrm{TESS} > 10^{32}$~erg occur once every 25 days, about an order of magnitude more frequently than inferred from previous TRAPPIST-1/analog FFD estimates. This elevated rate of energetic flares has important implications for atmospheric escape, photochemistry, and habitability assessments of the TRAPPIST-1 planets.

astro-ph.SR↗

arXiv2026

Towards inertial-mode helioseismology: Direct sensing of solar rotation at 75 deg latitude and 0.8 Rsun

Solar internal rotation at high latitudes is poorly constrained by acoustic-mode helioseismology. Global inertial modes observed on the Sun are highly sensitive to solar differential rotation and may provide new diagnostics of rotation in these regions. We aim to constrain solar rotation with the measured frequency of the $m=1$ high-latitude inertial mode, starting from the HMI/SDO reference rotation profile given by p-mode helioseismology for 2010-2024. Using a validated and accurate eigenvalue solver, we compute the perturbation to the mode frequency resulting from localised changes in the differential rotation rate throughout the solar interior. We find that the linear sensitivity kernel of the $m=1$ high-latitude mode peaks at latitude 75 deg and radius $0.8 R_\odot$, with full widths of 7 deg and $0.13 R_\odot$. From the observed mode frequency in the Carrington frame, $-87.9 \pm 1.9$ nHz (retrograde, averaged over 2010-2024), we infer that the solar rotation rate near this location is $365.3\pm 2.0$ nHz, which exceeds the reference p-mode estimate by $8.1$ nHz. Additionally, we propose a latitudinally smooth, radially independent modification to the rotation rate at high latitudes beyond the linear (small-perturbation) regime. This work demonstrates that individual inertial modes can provide direct constraints on rotation in the bulk of the solar convection zone, well below the surface, representing the first example of spatially resolved inertial-mode helioseismology.

astro-ph.SR↗

arXiv2026

Effects of the radiative interior on solar inertial modes

Solar inertial modes are believed to play important diagnostic and dynamical roles in the Sun's differentially rotating convection zone. However, the coupling of these modes to the radiative interior has not yet been discussed. We aim to understand the dependence of the modes on the uniformly rotating sub-adiabatic region below the convection zone and determine whether this leads to measurable changes at the surface. We used the Dedalus code to compute linear eigenmodes in the inertial frequency range in a setup that includes both the convection zone and the radiative interior down to $0.5R_\odot$. We imposed free-surface boundary conditions at both radial boundaries. For comparison, we also computed the eigenmodes in a setup restricted to the convection zone. We find that including the radiative zone only slightly modifies the frequencies and surface eigenfunctions, except for some modes with significant radial motion (high-frequency retrograde and prograde columnar modes). On the other hand, most modes penetrate significantly into the overshooting layer below the convection zone. This reduces their growth rates and distorts their eigenfunctions near the base of the convection zone. Furthermore, the uniformly rotating sub-adiabatic radiative zone supports oscillations due to Rossby modes of all possible spherical harmonics and radial nodes. In particular, when the nearest inertial mode in frequency space lies within around 10 nHz and shares the same north-south symmetry, these Rossby modes evolve into mixed modes characterized by significant motions within both the radiative and convection zones. However, such mixed modes have a high mode mass in the radiative interior and thus will be difficult to excite stochastically via convection.

astro-ph.SR↗

arXiv2026

Linear toroidal-inertial waves on a differentially rotating sphere with application to helioseismology: Modeling, forward and inverse problems

This paper develops a mathematical framework for interpreting observations of solar inertial waves in an idealized setting. Under the assumption of purely toroidal linear waves on the sphere, the stream function of the flow satisfies a fourth-order scalar equation. We prove well-posedness of wave solutions under explicit conditions on differential rotation. Moreover, we study the inverse problem of simultaneously reconstructing viscosity and differential rotation parameters from either complete or partial surface data. We establish convergence guarantee of iterative regularization methods by verifying the tangential cone condition, and prove local unique identifiability of the unknown parameters. Numerical experiments with Nesterov-Landweber iteration confirm reconstruction robustness across different observation strategies and noise levels.

math.AP↗

arXiv2026

Assessment of PLATO Science Performance

The PLATO mission is scheduled for launch early 2027. In this paper we present an overview of the performance drivers for the mission at the time where all flight models of the cameras have been tested and integrated on the optical bench. The PLATO consortium needs an estimate of the planet detection yield to dimension the ground-based radial velocity follow-up resources. We provide updated estimates on the yield of planet detections that can be expected from the mission under certain assumptions. As of today, large uncertainties remain on the planet occurrence rates, especially for small planets in long-period orbits, and on our ability to detect these planets in the presence of stellar variability and instrumental noise. To partially overcome these limitations, we compare results using different planet occurrence rates, detectability rates, and we include an estimate on the expected contribution of stellar variability to the noise budget. The final detection yield of PLATO will provide constraints to planet occurrence rates which in turn will help constraining planet formation models.

astro-ph.EP↗

arXiv2026

The PLATO Science Calibration and Validation Plan: Targets for the First Long-pointing Field

In order to meet the science goals of the PLATO space mission, an extensive science calibration and validation plan has been designed. This paper describes this plan, as well as the methodology adopted to select the science calibration and validation stars that have entered its input catalogue. This is the so-called {\tt scvPIC}, which is part of the general PLATO Input Catalogue (PIC) for the first selected long pointing field in the Southern Hemisphere known as LOPS2. While many of PLATO's science requirements needed dedicated stars as calibrators as discussed here, its most stringent requirement is the delivery of the age of the host stars of exoplanetary systems with an accuracy better than 10\% for a G0V star of {\it V} = 10 mag, i.e. a nearby Sun-like star. This is presently not within reach for large populations of dwarfs and subgiants in the Milky Way as it requires the models of their stellar interiors to be improved. We discuss how this ambitious age requirement led to the selection of tens of thousands of red giants, and of thousands of main-sequence early F-type gravity-mode pulsators in order to deduce their internal rotation profile across stellar evolution. This asteroseismic observable will then be imported as key information into improved models of dwarfs and subgiants in the Milky Way as optimal modelling tools for ever better age-dating of the exoplanet hosts as the PLATO mission moves along. Additional calibrators and validators included in the {\tt scvPIC} are a few thousands of binaries, a few hundreds of legacy and benchmark stars, a few hundred photometrically stable stars, and six transiting brown dwarfs.

astro-ph.SR↗

arXiv2026

PLATO input catalogs for technical calibration and fine guidance

A few weeks after launch, the PLATO spacecraft is expected to start its payload commissioning, which will be completed within the first three months of the mission. This phase includes the in-orbit verification, calibration, and configuration of the instrument prior to nominal science operations. During this mission-critical period, and again later during regular spacecraft rotations and re-pointings, a set of reference stars is required to complete various calibration steps. This set, referred to as the calibration PLATO Input Catalog (cPIC), is part of the PIC. The cPIC comprises various stellar samples, each serving a dedicated technical calibration purpose, and it contains 71671 unique stellar targets across PLATO's entire field of view (FoV). Once the spacecraft commences science observations, the on-board Fine Guidance System (FGS) will rely on a small set of guide stars. These stars must be particularly bright and will be observed with the two fast cameras, which cover only a smaller central region of PLATO's FoV. This target list, referred to as the fine-guidance PLATO Input Catalog (fgPIC), contains 2640 unique targets, of which about 30 are used by the FGS at any given time. In this paper, we present the selection criteria for both the cPIC and the fgPIC, and asses their impact on the construction of these calibration catalogs for PLATO.

astro-ph.IM↗

arXiv2026

Nonlinear evolution of unstable solar inertial modes: The case of viscous modes on a differentially rotating sphere

On the Sun, the inertial mode with the largest observed amplitude (rms velocity exceeding $10$ m/s) is the high-latitude mode with longitudinal wavenumber $m=1$. In two dimensions, on the sphere, linear theory predicts that this mode is unstable due to a shear instability associated with latitudinal differential rotation (fast equator, slower polar regions). We investigate the evolution of this instability numerically and theoretically. The nonlinear vorticity equation is solved using direct numerical simulations in the time domain. The only control parameter is the Ekman number $E$. For $10^{-3}\lesssim E< E_c \approx 1.5\times10^{-3}$, only the high-latitude $m=1$ mode is unstable. We extract its saturation amplitude as a function of $E$ and compare the results with predictions from two perturbative approaches in nonlinear stability theory. The simulations reveal a supercritical Hopf bifurcation. Near onset, the mode amplitude is well described by the Landau equation $d|A|/dt=σ_I |A|+β_I |A|^3$, with a positive linear growth rate $σ_I$ and a negative nonlinear coefficient $β_I$. The coefficient $β_I$ depends weakly on $E$, implying that the saturated amplitude scales approximately as $|A|\proptoσ_I^{1/2}$. The equilibrium mode contains the $m=1$ fundamental and harmonics $m=2$ and $m=3$, whose amplitudes scale as $σ_I^{m/2}$. Saturation results from Reynolds stresses that smooth the latitudinal differential rotation. For $E=4\times10^{-4}$, consistent with solar-like turbulent viscosity, the saturated velocity reaches $28$ m/s, comparable to solar observations. These results should be interpreted cautiously, since in three dimensions the instability is baroclinic and involves different physics.

astro-ph.SR↗

arXiv2025

3D Modeling of Solar Oscillations with Hybridizable Discontinuous Galerkin Method

With increasing quantity and quality of solar observations, it becomes essential to account for three-dimensional heterogeneities in wave modeling for seismic data interpretation. In this context, we present a 3D solver of the time-harmonic adiabatic stellar oscillation equations without background flows on a domain consisting of the Sun and its photosphere. The background medium consists of 3D heterogeneities on top of a radial strongly-stratified standard solar model. The oscillation equations are solved with the Hybridizable Discontinuous Galerkin (HDG) method, considering a first-order formulation in terms of the vector displacement and the pressure perturbation. This method combines the high-order accuracy and the parallelism of DG methods while yielding smaller linear systems. These are solved with a direct solver, with block low-rank compression and mixed-precision arithmetic to reduce memory footprint. The trade-off between compression and solution accuracy is investigated, and our 3D solver is validated by comparing with resolution under axial symmetry for solar backgrounds. The capacity of the solver is illustrated with wave speed heterogeneities characteristic of two physical phenomena: active regions and convection. We show the importance of global 3D gravito-acoustic wave simulations, in particular when the amplitudes of the perturbations are strong and their effect on the wavefield cannot be estimated by linear approximations.

math.AP↗

arXiv2025

Evaluating Solar Wind Forecast Using Magnetic Maps That Include Helioseismic Far-Side Information

To model the structure and dynamics of the heliosphere well enough for high-quality forecasting, it is essential to accurately estimate the global solar magnetic field used as inner boundary condition in solar wind models. However, our understanding of the photospheric magnetic field topology is inherently constrained by the limitation of systematically observing the Sun from only one vantage point, Earth. To address this challenge, we introduce global magnetic field maps that assimilate far-side active regions derived from helioseismology into solar wind modeling. Through a comparative analysis between the combined surface flux transport and helioseismic Far-side Active Region Model (FARM) magnetic maps and the base surface flux transport model without far-side active regions (SFTM), we assess the feasibility and efficacy of incorporating helioseismic far-side information in space weather forecasting. We are employing the Wang-Sheeley-Arge Solar Wind (WSA) model for statistical evaluation and leveraging the EUropean Heliospheric FOrecasting Information Asset (EUHFORIA), a three-dimensional heliospheric MHD model, to analyze a case study. Using the WSA model, we show that including far-side magnetic data improves solar wind forecasts for 2013-2014 by up to 50% in correlation and 3% in root mean square error and mean absolute error, especially near Earth and Solar TErrestrial RElations Observatory - Ahead (STEREO-A). Additionally, our 3D modeling shows significant localized differences in heliospheric structure that can be attributed to the presence or absence of active regions in the magnetic maps used as input boundaries. This highlights the importance of including far-side information to more accurately model and predict space weather effects caused by solar wind, solar transients, and geomagnetic disturbances.

astro-ph.SR↗

arXiv2025

Oscillations of the solar photospheric magnetic field caused by the m = 1 high-latitude inertial mode

Periodic oscillations at 338 nHz in the Earth frame are observed at high latitudes in direct Doppler velocity measurements. These oscillations correspond to the $m=1$ high-latitude global mode of inertial oscillation. In this study, we investigate the signature of this mode in the photospheric magnetic field using long-term series of line-of-sight magnetograms from the Helioseismic and Magnetic Imager (HMI) and the Global Oscillation Network Group (GONG). Through direct observations and spectral analysis, we detect periodic magnetic field oscillations at high latitudes ($65^\circ$--$70^\circ$) with a frequency of 338 nHz in the Earth frame, matching the known frequency of the $m = 1$ high-latitude inertial mode. The observed line-of-sight magnetic field oscillations are predominantly symmetric across the equator. We find a peak magnetic oscillation amplitude of up to $0.2$~gauss and a distinct spatial pattern, both consistent with simplified model calculations in which the radial component of the magnetic field is advected by the mode's horizontal flow field.

astro-ph.SR↗