arXiv subjects

Alexander G. M. Pietrow

Publications and source records attributed to Alexander G. M. Pietrow.

13 recordsLinked to original sources

arXiv2026

Multi-Height Spectropolarimetric Signatures of Magnetoacoustic Waves in Solar Plage

We analyze high-cadence spectropolarimetric observations of solar plage obtained with the Visible Spectro-Polarimeter (ViSP) on the Daniel K. Inouye Solar Telescope (DKIST) to investigate low-chromospheric waves sampled by the Na I D1 5896 Angstrom and Ca II 8542 Angstrom lines. Observations were taken at an oblique viewing angle, providing sensitivity to transverse motions. Magnetic oscillations within plage elements exhibit significant line-of-sight (LOS) RMS amplitudes of approximately 9 G on average in the Na I D1 line, whereas the corresponding Ca II oscillations are only marginally above the noise level. Adaptive profile tracking provided line-core intensities and Doppler velocities, the weak-field approximation provided LOS magnetic fields, and non-LTE inversions provided mass densities and formation heights. Cross-spectral analysis shows velocity-intensity phase distributions coupled at -90 degrees, consistent with compressive oscillations. The velocity phases in Na I-Ca II indicate upward propagation and the phase speeds are predominantly sub-Alfvenic, consistent with slow magnetoacoustic waves. Although the slit geometry prevents a definitive identification of the tube eigenmode, the differing magnetic phase relationships suggest a height-dependent sausage-like behavior in the coupling between the magnetic and compressive perturbations. Additionally, our inferred wave fluxes are insufficient to heat active-region plage.

astro-ph.SR↗

arXiv2026

Multi-wavelength synthesis of a flux rope-trapped mini-prominence eruption and post-flare coronal rain

Small-scale eruptive phenomena in the solar corona including miniature flux ropes and associated cool plasma condensations are not fully understood despite increasing high-resolution observations. We perform forward modeling based on a 2.5D MHD simulation capturing homologous flux rope eruptions, in-situ condensation leading to a mini-prominence, and subsequent post-flare coronal rain. Synthetic diagnostics are obtained using optically-thin EUV and UV emissions, and non-LTE radiative transfer treatment for the H$α$ line. The synthetic EUV emission reveals the flux ropes as bright rim-like structures. The corresponding UV diagnostic shows bright region, which is co-spatial with the dark core due to embedded cool plasma ($\sim$ tens of kK) inside the flux rope, identifying an erupting mini-prominence. Spectral synthesis of Si IV 1402.77 A indicates an upward motion of the mini-filament, and reveals the presence of two predominant velocity components during eruption. At a later stage, thermal instability in post-flare arcades produces coronal rain with temperatures of $\approx 10^4$ K. The EUV diagnostics reveal brightening at the downstream of the rain blob, indicating localized heating associated with compressional effects. The H$α$ spectral synthesis shows enhanced absorption signatures and red-shifted profiles corresponding to downflows of the coronal rain blobs up to $\approx 23$ km s$^{-1}$, whereas the Si IV 1402.77 A spectral profile shows the maximum downflow velocity of $\approx50$ km s$^{-1}$, highlighting the evidence of thermodynamic and kinematic structuring within the falling rain blobs. The synthetic diagnostics provide clear, multi-wavelength signatures that can guide future high-resolution observations, and highlight the importance of small-scale reconnection-driven processes in shaping the multi-thermal structure (between MK to kK) of the solar corona.

astro-ph.SR↗

arXiv2026

The Fe I 4377 Å Line as a Solar Faculae Indicator: Insights from Spectral Ratio Analysis

Faculae are a dominant source of stellar activity noise in radial velocity measurements, yet their low contrast and broad surface distribution make them difficult to track in disc-integrated observations. We apply Spectral Ratio Analysis (SRA) to HARPS-N Sun-as-a-star observations to isolate and characterize the spectral imprint of facular regions over rotational timescales. The resulting SRA spectra show coherent, line-dependent variability sensitive to surface magnetic activity, with the Fe I 4377 Angstrom line exhibiting a particularly strong diagnostic response to facular coverage. We interpret the observed signatures using two complementary synthetic frameworks: composite PHOENIX spectra, from which we derive best-fit facular temperature contrasts in the range 200-400 K, and MPS-ATLAS spectra synthesized using MURaM simulations of the quiet Sun including a small-scale dynamo and magnetically-enhanced facular analogues with initial mean vertical magnetic fields of 100G, 200G, and 300G. Both approaches are benchmarked against facular filling factors measured from Solar Dynamics Observatory (SDO) disc-resolved images. We find good agreement between SDO-measured and SRA-inferred filling factors using the Fe I 4377 Angstrom line, with Pearson R coefficients of 0.587-0.927 across models and timescales. The estimated filling factors track the solar activity cycle, rising from ~1.5% at lower activity to ~5.5% at higher activity, consistent with SDO-measured filling factors. These results demonstrate that SRA offers a means to reliably track surface magnetic activity in disc-resolved spectra, which is necessary for mitigating the effects of activity on RV characterization of exoplanet masses and atmospheres at modern precision.

astro-ph.SR↗

arXiv2026

Multi-wavelength observations of substructures in solar flare ribbons

Solar flare ribbons are extensive brightenings in the chromosphere during flares, often showing fine scale structuring that reflects the underlying energy release. Using high cadence imaging from the Swedish 1-m Solar Telescope/CRISP during an X1.5-class limb flare on 10 June 2014, we identify and track 232 coherent, thread-like substructures, which we term for the first time ``riblets''. From a statistical analysis, riblets have well defined lifetimes and plane-of-sky speeds (typically 5-15 s and 50-150 km/s respectively), establishing them as distinct ribbon substructures. From analysis of their temporal distributions, their distance-time (X-T) evolution uniquely reveal approximately linear and non-linear (accelerating/decelerating) classes, a discrepancy that may be influenced by projection geometry. From analysis of their spatial distributions, we find no clear correspondence between the properties of adjacent riblets, suggesting that local atmospheric conditions (fine-scale thermodynamic and/or magnetic structuring) govern their kinematics more than spatial variations in electron-beam energy flux. From analysis of their spectral distributions, clusters of riblets do show temporal and spatial coincidence with hard X-ray emission signatures, consistent with episodic electron-beam injection into the chromosphere. Using Fermi/GBM spectroscopy, we derive thick-target parameters suitable for flare simulations, with representative values $δ_{\rm low}\approx 5.93$, $E_{\rm c}\approx 24.7$ keV, and an implied beam energy flux $\mathcal{F}_{\rm beam}\approx 1.5\times 10^{10}$ erg cm$^{-2}$ s$^{-1}$ (based on RHESSI footpoint area). Together, these results identify riblets as the fundamental building block of flare ribbons and provide quantitative constraints for forward tests of riblet formation mechanisms.

astro-ph.SR↗

arXiv2026

Fine-scale downflows above flare ribbons captured by Solar Orbiter/EUI

In solar flares, flare ribbons map chromospheric footpoints where flare energy deposition occurs. These locations are associated with field aligned energy transport from the corona that results from energy liberated during magnetic reconnection. Recent chromospheric observations in the H$α$ and H$β$ bands have revealed fine-scale downflow structures above flare ribbons, referred to as riblets. In this study, we identify similar downflow structures in the extreme-ultraviolet (EUV) wavelength using high-resolution observations from Solar Orbiter/EUI. These fine-scale downflows appear as downward-propagating, bright, and thread-like structures. They exhibit typical velocities of $\sim100~\mathrm{km\ s^{-1}}$, lifetimes of $\sim15$~s, and lengths of $\sim1.6$~Mm. Based on their morphological and dynamical properties, we interpret these observed downflows as the EUV counterparts of the riblets that have previously been reported from chromospheric observations. This study presents EUV imaging of $\sim 10^6$~K downflows above flare ribbons. We interpret these downflows as a result of (1) the energisation and subsequent compression of pre-existing chromospheric fibrils due to particle beams or (2) adiabatic or shock-driven compression induced by the downward-propagating plasma from the corona. These fine-scale EUV riblets provide a new diagnostic tool for probing the dynamics of magnetic reconnection as well as energy transport and deposition during solar flares.

astro-ph.SR↗

arXiv2025

The calm before the storm: High spatial resolution mosaic of active region NOAA 14274 at the onset of an X1.2 flare

Active region NOAA 14274 produced some of the strongest flares of Solar Cycle 25, including the X1.2 and X5.1 flares on 10 and 11 November 2025, respectively. We present the first large mosaic of speckle-restored images obtained with the improved High-resolution Fast Imager (HiFI+) at the 1.5-meter GREGOR solar telescope at the Observatorio del Teide in Izaña, Tenerife, Spain. The observations were obtained approximately 30 minutes before the onset of the X1.2 flare. The active region exhibited strongly curved penumbral filaments, sunspot rotation, and shear motions along the polarity inversion line (PIL), which led to a highly stressed magnetic field configuration that stored sufficient energy to release multiple M- and X-class flares. The first flare signatures appeared as small-scale brightenings, each with a width of a few tenths of an arcsecond, that trace penumbral filaments in the trailing sunspot.

astro-ph.SR↗

arXiv2025

Differential rotation of solar α sunspots and implications for stellar light curves

Differential rotation is a key driver of magnetic activity and dynamo processes in the Sun and other stars, especially as the rate differs across the solar layers, but also in active regions. We aim to accurately quantify the velocity at which round α-spots traverse the solar disk as a function of their latitude, and compare these rates to those of the quiet-Sun and other sunspot types. We then extend this work to other stars and investigate how differential rotation affects the modulation of stellar light curves by introducing a generalized stellar differential rotation law. We manually identify and track 105 α-sunspots in the 6173 Å continuum using the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO). We measure the angular velocities of each spot through center-of-mass and geometric ellipse-fitting methods to derive a differential rotation law for round α-sunspots. Results. Using over a decade of HMI data we derive a differential rotation law for α-sunspots. When compared to previous measurements we find that α-sunspots rotate 1.56% faster than the surrounding quiet-Sun, but 1.35% slower than the average sunspot population. This supports the hypothesis that the depth at which flux tubes are anchored influences sunspot motions across the solar disk. We extend this analysis to other stars by introducing a scaling law based on the rotation rates of these stars. This scaling law is implemented into the Stellar Activity Grid for Exoplanets (SAGE) code to illustrate how differential rotation alters the photometric modulation of active stars. Our findings emphasize the necessity of considering differential rotation effects when modeling stellar activity and exoplanet transit signatures

astro-ph.SR↗

arXiv2025

High-spectral Resolution, Multi-wavelength Center-to-limb Observations of the Sun

The center-to-limb variations (CLVs) of photospheric and chromospheric spectral lines were obtained in 2025 July and August using drift scans from the echelle spectrograph of the 0.7 m Vacuum Tower Telescope at the Observatorio del Teide (ODT) in Tenerife, Spain. This instrument can observe four spectral regions simultaneously, enabling multi-line spectroscopy with high spectral resolution of various activity features and the quiet Sun in the lower solar atmosphere. The initial results of Halpha observations demonstrate the diagnostic potential of drift scans obtained with a ground-based, high-resolution telescope. Data products include spectroheliograms and maps of physical parameters such as line-of-sight velocity, line width, and line-core intensity. The combination of the CLV from photospheric and chromospheric lines, as well as the wide range of formation heights of the selected lines, renders this dataset ideal for characterizing stellar and exoplanet atmospheres.

astro-ph.SR↗

arXiv2023

Center-to-limb variation of spectral lines and their effect on full-disk observations

An accurate description of the center-to-limb variation (CLV) of stellar spectra is becoming an increasingly critical factor in both stellar and exoplanet characterization. In particular, the CLV of spectral lines is extremely challenging as its characterization requires highly detailed knowledge of the stellar physical conditions. To this end, we present the Numerical Empirical Sun-as-a-Star Integrator (NESSI) as a tool for translating high-resolution solar observations of a partial field of view into disk-integrated spectra that can be used to test common assumptions in stellar physics.

astro-ph.SR↗

arXiv2023

Spectral Background-Subtracted Activity Maps

High-resolution solar spectroscopy provides a wealth of information from photospheric and chromospheric spectral lines. However, the volume of data easily exceeds hundreds of millions of spectra on a single observation day. Therefore, methods are needed to identify spectral signatures of interest in multidimensional datasets. Background-subtracted activity maps (BaSAMs) have previously been used to locate features of solar activity in time series of images and filtergrams. This research note shows how this method can be extended and adapted to spectral data.

astro-ph.SR↗

arXiv2023

Why every observatory needs a disco ball

Commercial disco balls provide a safe, effective and instructive way of observing the Sun. We explore the optics of solar projections with disco balls, and find that while sunspot observations are challenging, the solar disk and its changes during eclipses are easy and fun to observe. We explore the disco ball's potential for observing the moon and other bright astronomical phenomena.

physics.ed-ph↗

arXiv2022

COCOPLOT: COlor COllapsed PLOTting software : Using color to view 3D data as a 2D image

Most modern solar observatories deliver data products formatted as 3D spatio-temporal data cubes, that contain additional, higher dimensions with spectral and/or polarimetric information. This multi-dimensional complexity presents a major challenge when browsing for features of interest in several dimensions simultaneously. We developed the COlor COllapsed PLOTting (COCOPLOT) software as a quick-look and context image software, to convey spectral profile or time evolution from all the spatial pixels ($x,y$) in a 3D [$n_x,n_y,n_λ$] or [$n_x,n_y,n_t$] data cube as a single image, using color. This can avoid the need to scan through many wavelengths, creating difference and composite images when searching for signals satisfying multiple criteria. Filters are generated for the red, green, and blue channels by selecting values of interest to highlight in each channel, and their weightings. These filters are combined with the data cube over the third dimension axis to produce an $n_x \times n_y \times 3$ cube displayed as one true color image. Some use cases are presented for data from the Swedish 1-m Solar Telescope (SST) and IRIS, including H$α$ solar flare data, a comparison with $k$-means clustering for identifying asymmetries in the Ca II K line and off-limb coronal rain in IRIS C II slit-jaw images. These illustrate identification by color alone using COCOPLOT of locations including line wing or central enhancement, broadening, wing absorption, and sites with intermittent flows or time-persistent features. COCOPLOT is publicly available in both IDL and Python.

astro-ph.IM↗

arXiv2016

The coronagraphic Modal Wavefront Sensor: a hybrid focal-plane sensor for the high-contrast imaging of circumstellar environments

The raw coronagraphic performance of current high-contrast imaging instruments is limited by the presence of a quasi-static speckle (QSS) background, resulting from instrumental non-common path errors (NCPEs). Rapid development of efficient speckle subtraction techniques in data reduction has enabled final contrasts of up to 10-6 to be obtained, however it remains preferable to eliminate the underlying NCPEs at the source. In this work we introduce the coronagraphic Modal Wavefront Sensor (cMWS), a new wavefront sensor suitable for real-time NCPE correction. This pupil-plane optic combines the apodizing phase plate coronagraph with a holographic modal wavefront sensor, to provide simultaneous coronagraphic imaging and focal-plane wavefront sensing using the science point spread function. We first characterise the baseline performance of the cMWS via idealised closed-loop simulations, showing that the sensor successfully recovers diffraction-limited coronagraph performance over an effective dynamic range of +/-2.5 radians root-mean-square (RMS) wavefront error within 2-10 iterations. We then present the results of initial on-sky testing at the William Herschel Telescope, and demonstrate that the sensor is able to retrieve injected wavefront aberrations to an accuracy of 10nm RMS under realistic seeing conditions. We also find that the cMWS is capable of real-time broadband measurement of atmospheric wavefront variance at a cadence of 50Hz across an uncorrected telescope sub-aperture. When combined with a suitable closed-loop adaptive optics system, the cMWS holds the potential to deliver an improvement in raw contrast of up to two orders of magnitude over the uncorrected QSS floor. Such a sensor would be eminently suitable for the direct imaging and spectroscopy of exoplanets with both existing and future instruments, including EPICS and METIS for the E-ELT.

astro-ph.IM↗