Laurent Gizon

Context . On the Sun, the inertial mode with the largest observed amplitude (rms velocity exceeding 10 m/s) is the high-latitude mode with a longitudinal wavenumber of m = 1. In two dimensions, linear theory predicts this mode to be unstable due to a shear instability associated with latitudinal differential rotation (i.e., fast equator, slower polar regions). Aims . In this work, we investigated the evolution of this instability numerically and theoretically. Methods . The nonlinear vorticity equation was solved using direct numerical simulations in the time domain. The only control parameter was set as the Ekman number, E . For 10 ⁻³ ≲ E < E _c ≈ 1.5×10 ⁻³ , only the high-latitude m = 1 mode was unstable. We extracted its saturation amplitude as a function of E and compared the results with predictions from two perturbative approaches in nonlinear stability theory. Results . The simulations reveal a supercritical Hopf bifurcation. Near the onset, the mode amplitude is aptly described by the Stuart-Landau equation d | A |/ dt = σ _I | A |+ β _I | A | ³ , derived from perturbation theories, with a positive linear growth rate, σ _I , and a negative nonlinear coefficient, β _I . This description remains valid throughout the range of 10 ⁻³ ≲ E < E _c . The coefficient β _I depends weakly on E , implying that the saturated amplitude scales approximately as | A |∝ σ _I ^1/2 . The equilibrium mode shape consists of the m = 1 fundamental mode and higher harmonics, m = 2 and m = 3, whose amplitudes scale approximately as σ _I ^{m /2} . The saturation mechanism arises from the Reynolds stress that feeds back on and smooths the latitudinal differential rotation. At E = E _⊙ = 4×10 ⁻⁴ , consistent with solar-like values inferred from supergranular turbulent viscosity, the saturated velocity reaches 28 m/s, comparable to the observed value on the Sun. Conclusions . In two dimensions, the onset and saturation of the solar m = 1 inertial mode are governed by a supercritical Hopf bifurcation associated with a shear instability that is aptly described by weakly nonlinear theory. Future works will benefit from extending the model to include solar-cycle variations in the differential rotation and moving toward a 3D setup that incorporates baroclinic effects into the dynamics.

Context. Acoustic mode frequencies in the Sun and Sun-like stars change due to magnetic activity on timescales that are much longer than that of the star’s rotation and much shorter than that of 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 being an average of the individual mode contributions. Aims. We seek to enhance the cross-correlation method by introducing a novel and computationally cheap method that enables us to disentangle p-mode frequency changes for different spherical harmonic degrees, ℓ . Methods. We designed filters that, if the inclination angle and rotation rate are already measured, enable the isolation of δω _ℓ and frequency changes of modes with a given ℓ while preventing bias creeping in from neighbouring modes. We performed Monte Carlo simulations to quantify the uncertainty in the estimation of δω _ℓ . Results. We validated our method against well-studied solar data (from SOHO/VIRGO and BiSON) and demonstrate its applicability to the solar-like Kepler star KIC 8006161.

The Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) instrument on board the balloon-borne S unrise iii observatory provided new high-resolution observations of the solar chromosphere in the Ca ii λ 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 λ 854.2 nm line. The structures have lengths of ≈2–7 Mm and widths of approximately 1 Mm. The magnetic field configurations associated with the OPI features appear to belong to twisted flux ropes and are visible for most of the time in the presented 21 minutes 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 S unrise iii mission.

Aims . We studied the temporal evolution of solar inertial modes over the solar cycle using observations from GONG and SDO/HMI. We focused our attention on the high-latitude mode with azimuthal wavenumber m = 1 and the equatorial Rossby modes with 3 ≤ m ≤ 16. Methods . We used maps of horizontal flows near the solar surface from the GONG and HMI (p-mode) 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 the power of the mode from the GONG and HMI data. Results . We find good agreement between the 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, while its frequency shows no significant temporal variation. For the equatorial Rossby modes, the frequencies are generally anticorrelated with the sunspot number, whereas the mode powers tend to correlate positively with the sunspot number. An exception is the m = 3 equatorial Rossby mode, whose mode power is strongly anticorrelated with the sunspot number, in contrast to the other equatorial Rossby modes, highlighting its distinct behavior. Conclusions . 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 sunspot number; 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.

We report imaging spectroscopic observations of an M1.4 solar flare obtained during a coordinated observation between the infrared spectropolarimeter SCIP onboard the S unrise 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″ × 58″ around the umbra with a cadence of 40 s, covering infrared chromospheric and upper-photospheric lines including Ca ii 8498/8542 Å and K i D ₁ . At the same time, DST observed a wider surrounding region with a cadence of 25 s in H α , Ca ii 8542 Å, and Na i D ₁ /D ₂ . 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.

Deciphering the physical properties of flares is crucial for understanding the nature of one of the most energetic events on the Sun. We analyze high spatial and spectral resolution observations of a C-class flare, focusing on a complex light bridge in the active region. The observations show Stokes profiles with opposite Doppler shifts and asymmetries between photospheric and chromospheric spectral lines, and Ca ii infrared spectral lines in emission, tracing areas of extreme atmospheric heating. Inversion results are consistent with these spectral features, showing regions of opposite-sign plasma flows between the photosphere and chromosphere, adjacent to areas with extreme chromospheric temperatures. The latter areas show relatively weak line-of-sight velocities despite the intense heating. The magnetic field configuration shows abrupt changes with height in the regions with opposite-sign flows, while remaining nearly constant with height in the areas of extreme heating. These findings point to a scenario of small-scale magnetic reconnection in the photosphere, suggesting a flare triggering mechanism based on an avalanche of many small reconnection events.

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 S unrise 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 Å and spatial resolutions between 1″ and 6″. 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.

S unrise iii is a stratospheric balloon-borne solar observatory with a 1 m diameter telescope and three postfocus 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. S unrise iii flew successfully for 6.5 days suspended from a zero-pressure stratospheric balloon from northern Sweden to northwestern Canada in 2024 July, gathering around 200 TB of data. The present issue of the Astrophysical Journal Letters focuses on the first scientific results from the data collected during that flight. This Letter 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 Letter ends with an overview of the first set of science results obtained from these data, as presented in the current Focus Issue.

The PLanetary Transits and Oscillations of stars (PLATO) mission will begin its four-year nominal mission in early 2027 by monitoring its Long-duration Observation Phase field at South (LOPS2) for at least two years continuously. The primary aim of PLATO is a very ambitious and challenging one: the discovery of Earth-like planets in the habitable zone of nearby and bright solar analogs. To this purpose, the PLATO Mission Consortium, through its Ground-based Observing Program, will perform the follow-up needed to confirm part of the candidate planets photometrically detected by PLATO and measure their masses through radial velocity curves. For the LOPS2, the Ground-based Observing Program is committed (as part of the PLATO mission) to follow up the candidate exoplanets discovered orbiting the 15 000 high-quality target subset of the PLATO Input Catalog (PIC) known as the Prime Sample. The Prime Sample will be made public nine months before launch in the context of the first Guest Observer call for proposals to be issued by the European Space Agency. Here, we present the quantitative metrics and thresholds defined to select and prioritize the Prime Sample. Our method is perfectly general and suitable to rank any list of stars surveyed for transiting planets. We also describe the astrophysical properties of the LOPS2 Prime Sample, both in a statistical sense and for some specific targets of interest.

Stellar granulation is a source of radial-velocity (RV) jitter of 1 m s ⁻¹ on Sun-like stars, limiting the detection of Earth analogs. One way to move beyond this limit is to use line-weighting schemes to mitigate granulation-driven RV variability in a spectral-type-dependent manner. A physics-based granulation-sensitivity diagnostic is essential. Here, we use a line-by-line sensitivity measure from 3D magnetoconvection simulations that quantifies how strongly each spectral line’s Doppler shift and strength respond to convective velocity and thermodynamic fluctuations. Extending our previous solar analysis, which used the spatial variability of line profiles across a single granulation snapshot as a computationally efficient proxy for temporal variability, we test the transferability of this diagnostic to cooler stars and characterize how line sensitivity varies with spectral type. We synthesize high-resolution spectra with the Merged Parallelised Simplified ATLAS code from 3D time-dependent MURaM simulations representative of the Sun and late-G and K dwarfs, focusing on neutral and singly ionized iron lines spanning a wide range of excitation potentials and strengths. As T _eff decreases, convective velocities weaken, and the ionization balance shifts, producing a clearer separation between line families: Fe i lines show lower velocity sensitivity and smaller fractional strength variability, whereas Fe ii lines show the opposite. Using cumulative contribution functions, we further relate spectroscopic velocity jitter to characteristic line-formation temperature. The diagnostic continues to separate robust and sensitive lines in late-G and K dwarfs, enabling spectral-type-aware cross-correlation masks and line-by-line weights. Solar-optimized line selections are not generally portable to cooler stars, especially when based on equivalent-width stability rather than velocity sensitivity.