Jesper Schou

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.

We report on the measurement of the height-dependent time shifts of wave pulses in the lower solar atmosphere from high-resolution spectropolarimetric observations obtained with the SUSI instrument on board the S unrise iii balloon-borne solar observatory during its successful science flight in 2024 July. The line-of-sight velocities derived from the line-core positions of 19 spectral lines in a 2 nm-wide window around the Ca ii H line were used to determine the time shifts of propagating pulses at their respective formation heights. Our analysis reveals that these shifts are roughly ordered according to the computed formation heights of the respective spectral lines. A statistical analysis of the time shifts using sit-and-stare observations with a total duration of 1 hr reveals that wave pulses propagating upwards from near the solar surface to heights of approximately 500–700 km are most common, with average time lags of 20–30 s between these heights. Also present are pulses with close-to-zero phase shifts, predominantly above intergranular lanes and areas of enhanced magnetic activity. Additionally, downward propagating wave pulses with negative time lags of 10–15 s are seen, mostly above areas of enhanced magnetic activity. A common feature of all the observed pulses is that in the lower 250 km they show small time lags of zero to a few seconds, and only at higher layers do the propagating pulses become more dominant. This study demonstrates the potential of the many-line approach for investigating the height dependence of the physical conditions in the solar atmosphere.

The Polarimetric and Helioseismic Imager (SO/PHI) on board Solar Orbiter is a spectropolarimeter scanning the Fe I line at 617.3 nm, providing data of the solar photosphere. The same line is sampled by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) and many other ground-based instruments. In this paper our aim is to assess the consistency between the line-of-sight (LoS) velocity measurements from the two instruments. Reliable measurements of upflows and downflows from SO/PHI are crucial and unique when Solar Orbiter is facing the far side of the Sun. Also, a combination of measurements from two vantage points to study horizontal flows must rely on consistent observations. For this purpose, we compare the LoS velocity measured by SO/PHI’s High Resolution Telescope (SO/PHI-HRT) and SDO/HMI on 29 March 2023, when Solar Orbiter was crossing the Sun–Earth line at a distance of 0.39 au from the Sun. Because such co-alignments are rare, this configuration offered an almost unique opportunity to directly compare data products from the two telescopes. The data are aligned and remapped to allow a pixel-by-pixel comparison of the whole time series of 4 hours. Temporal and spatial variations are considered for a direct combination of the measurements. The LoS velocity distributions are evaluated and a clear linear relation is found between the two instruments with a slope of 0.96 and a correlation of 92%. We find that the signals form at similar heights, with a separation of 9 ± 12 km, which is larger than previous estimates. A close-up look at the penumbra of a sunspot and its Evershed flow is presented. We conclude that the signals inferred by SO/PHI-HRT and SDO/HMI show very good agreement and high correlation when instrumental effects and large-scale velocities on the Sun are properly accounted for.

We report a point-spread function (PSF) and deconvolution procedure to remove stray light from the Helioseismic and Magnetic Imager (HMI) data. Prelaunch calibration observations, postlaunch Venus transit, and lunar transit data were used to develop the PSF and evaluate how well it reproduced the observed scattering. The PSF reported differs from previous stray light removal efforts since we do not use Gaussians as the central mathematical component. Instead, we use a Lorenztian convolved with an Airy function. In 2018, the HMI team began providing full-disk, stray-light-corrected data daily. Intensity, Doppler, magnetogram, and vector magnetic field data are provided. The deconvolution uses a Richardson–Lucy algorithm and takes less than 1 s per full-disk image. The results, on average, show decreases in umbral continuum intensity, a doubling of the granulation intensity contrast, increases in the total field strength, most notably in plage by ∼1.4–2.5 times the original value, and a partial correction for the convective blueshift. Local helioseismology analyses using corrected data yield results that are consistent with those from uncorrected data, with only negligible differences, except in sunspot regions. The new data are found in JSOC with names similar to the original but with the qualifying term “_ dcon ” or “_ dconS ” appended, denoting whether the deconvolution was applied to the filtergrams or Stokes images. The HMI team recommends using the corrected data for improved visual clarity, more accurate irradiance reconstruction, better coalignment with high-resolution data, reduced errors in tracking algorithms, and improved magnetic field strengths.

Direct remote-sensing observations of the solar poles have been hindered by the restricted view obtained from the ecliptic plane. For the first time ever, Solar Orbiter with its remote-sensing instruments observed the poles of the Sun from out of the ecliptic in the spring of 2025. Here, we report the first measurements of the magnetic field of the solar poles taken when Solar Orbiter was at heliographic latitudes ranging between 14 . ° 9 and 16 . ° 7. The data sets were collected by the High Resolution Telescope of the Polarimetric and Helioseismic Imager on board Solar Orbiter (SO/PHI-HRT). Two sets of observations, approximately one month apart, for the south and north pole are considered in this work. The magnetic flux and flux density measured during these campaigns are reported as a function of the heliographic latitude observed by SO/PHI-HRT. The net fluxes show a different latitudinal distribution for the two polar caps. We also discuss the observed dependence of the measured fluxes on the viewing angle. These first results highlight the importance of high-resolution direct measurements of the polar field, paving the way for the high-latitude observations planned for SO/PHI-HRT in the coming years.

Magnetoconvection at the solar surface governs the dynamics in the upper solar atmosphere and sustains the heliosphere. Properties of this fundamental process are poorly described near the solar poles. Here we report the first out-of-ecliptic remote-sensing observations of the south pole of the Sun from a high-latitude campaign of the Solar Orbiter spacecraft, which reveal spatial and temporal evolution of supergranular convective cells. The supergranular cells have spatial scales of 20–40 Mm. From 8 days of observations starting on 2025 March 16, our analysis shows that the magnetic network migrates poleward, on average, at high latitudes (above 60°), with speeds in the range of 10–20 m s ⁻¹ , depending on the structures being tracked. These results shed light on the buildup of the polar magnetic field that is central to our understanding of the solar cycle and the heliospheric magnetic field.

Context. Helioseismology aims to infer the properties of the solar interior by analyzing observations of acoustic oscillations. Interpreting helioseismic data, however, is complicated by the non-trivial relationship between helioseismic observables and the physical perturbations associated with acoustic modes as well as by various instrumental effects. Aims. We aim to improve our understanding of the signature of acoustic modes measured in the Helioseismic and Magnetic Imager (HMI) continuum intensity and Doppler velocity observables by accounting for radiative transfer, solar background rotation, and spacecraft velocity. Methods. We started with a background model atmosphere that accurately reproduces solar limb darkening and the Fe I 6173Å spectral line profile. We employed first-order perturbation theory to model the effect of acoustic oscillations on inferred intensity and velocity. By solving the radiative transfer equation in the atmosphere, we synthesized the spectral line, convolved it with the six HMI spectral windows, and deduced the continuum intensity (hmi.Ic_45s) and Doppler velocity (hmi.V_45s) according to the HMI algorithm. Results. We analytically derived the relationship between mode displacement in the atmosphere and the HMI observables and show that both the intensity and velocity deviate significantly from simple approximations. Specifically, the continuum intensity does not simply reflect the true continuum value, while the line-of-sight velocity does not correspond to a straightforward projection of the velocity at a fixed height in the atmosphere. Our results indicate that these deviations are substantial, with amplitudes of approximately 10% and phase shifts of around 10° across the detector for both observables. Moreover, these effects are highly dependent on the acoustic mode under consideration and the position on the solar disk. To achieve accurate modeling of the observables, it is important to account for the impact of radiative transfer on oscillation velocities and perturbations in atmospheric thermodynamic quantities, which influence the line profile. Conclusions. The combination of these effects leads to non-trivial systematic errors (in amplitude and phase) across the detector that must be taken into account to understand the observables. This framework can be used to study mode visibility across the solar disk and for asteroseismology applications.

Solar pores are intense concentrations of magnetic flux that emerge through the solar photosphere. When compared to sunspots, they are much smaller in diameter and can therefore be affected and buffeted by neighbouring granular activity to generate significant magnetohydrodynamic (MHD) wave energy flux within their confines. However, observations of solar pores from ground-based telescope facilities may struggle to capture subtle motions that are synonymous with higher-order MHD wave signatures because of the seeing effects that are produced in the Earth’s atmosphere. Hence, we exploited timely seeing-free and high-quality observations of four small magnetic pores from the High Resolution Telescope (HRT) of the Polarimetric and Helioseismic Imager (PHI) on board the Solar Orbiter spacecraft during its first close perihelion passage in March 2022 (at a distance of 0.5 au from the Sun). Through acquisition of data under stable observing conditions, we were able to measure the area fluctuations and horizontal displacements of the solar pores. Cross correlations between perturbations in intensity, area, line-of-sight velocity, and magnetic fields, coupled with the first-time application of novel proper orthogonal decomposition techniques on the boundary oscillations, provided a comprehensive diagnosis of the embedded MHD waves as sausage and kink modes. Additionally, the previously elusive m = 2 fluting mode is identified in the most magnetically isolated of the four pores. An important consideration lies in how the identified wave modes contribute to the transfer of energy into the upper solar atmosphere. Approximately 56%, 72%, 52%, and 34% of the total wave energy of the four pores we examined is associated with the identified sausage modes and about 23%, 17%, 39%, and 49% with their kink modes, while the first pore also receives a contribution of about 11% linked to the fluting mode. This study reports the first-time identification of concurrent sausage, kink, and fluting MHD wave modes in solar magnetic pores.