Observing and Quantifying Solar Magnetism

Polarization fringes in optical systems: a compendium

whawkins — Tue, 23 Dec 2025 21:58:07 +0000

Polarization fringes in optical systems: a compendium whawkins Tue, 12/23/2025 - 14:58

Author:

whawkins

Dec 23, 2025

Applied Optics: Spectral and spatial fringes in polarized light are produced by the interference of transmitted and reflected waves at the interface between materials with different indexes of refraction. These instrumental artifacts can affect the accuracy of optical designs conceived for high-sensitivity spectroscopy and polarimetry. We review the fundamental mechanisms that are responsible for these artifacts and the possible design pathways that allow us to mitigate them. In order to do so, we also present an approximate treatment of the problem of the transmission and reflection of light through (possibly absorptive) birefringent layers, relying on a few fundamental results that can be found in the already extensive literature on the subject. Unfortunately, many of these results remain the domain of a niche of investigators working in the field of thin films and optical coatings, and are often overlooked even by experienced designers of spectro-polarimetric instrumentation. The treatment presented in this work is limited to isotropic materials and uniaxial crystals, which are the most common types of optics employed in polarimetric instrumentation, and it fundamentally relies on the approximation of small birefringence for its implementation. An extensive set of modeling examples is provided to highlight the salient characteristics of polarization fringes, as well as to assess how approximate treatments such as this compare to exact but more computational expensive formulations of the problem such as Berreman's calculus.

Wavelength dependence between 200 and 1000 nm of the (intensity normalized) Mueller matrix of a PCM optimized for full-Stokes polarimetry between 400 and 1000 nm. The polarization fringes are calculated with a spectral resolution of 20000. The PCM design uses three compound retarders in the configuration MgF2-SiO2-MgF2, where the two MgF2 elements are identical. The gray curves plotted over the fringes represent the ideal Mueller matrix from the PCM design.

Spectropolarimetric Inversion in Four Dimensions with Deep Learning (SPIn4D): II. A Physics-Informed Machine Learning Method for 3D Solar Photosphere Reconstruction

whawkins — Tue, 23 Dec 2025 16:21:35 +0000

Spectropolarimetric Inversion in Four Dimensions with Deep Learning (SPIn4D): II. A Physics-Informed Machine Learning Method for 3D Solar Photosphere Reconstruction whawkins Tue, 12/23/2025 - 09:21

Author:

whawkins

Dec 23, 2025

Astrophysical Journal: Inferring the three-dimensional (3D) solar atmospheric structures from observations is a critical task for advancing our understanding of the magnetic fields and electric currents that drive solar activity. In this work, we introduce a novel, Physics-Informed Machine Learning method to reconstruct the 3D structure of the lower solar atmosphere based on the output of optical depth sampled spectropolarimetric inversions, wherein both the fully disambiguated vector magnetic fields and the geometric height associated with each optical depth are returned simultaneously. Traditional techniques typically resolve the 180-degree azimuthal ambiguity assuming a single layer, either ignoring the intrinsic non-planar physical geometry of constant optical-depth surfaces (e.g., the Wilson depression in sunspots), or correcting the effect as a post-processing step. In contrast, our approach simultaneously maps the optical depths to physical heights, and enforces the divergence-free condition for magnetic fields fully in 3D. Tests on magnetohydrodynamic simulations of quiet Sun, plage, and a sunspot demonstrate that our method reliably recovers the horizontal magnetic field orientation in locations with appreciable magnetic field strength. By coupling the resolutions of the azimuthal ambiguity and the geometric heights problems, we achieve a self-consistent reconstruction of the 3D vector magnetic fields and, by extension, the electric current density and Lorentz force. This physics-constrained, label-free training paradigm is a generalizable, physics-anchored framework that extends across solar magnetic environments while improving the understanding of various solar puzzles.

Panel (a) shows the flowchart for training the model. The inputs are the 3D magnetic vector fields with azimuthal ambiguity, and an initial guess of the geometric heights associated with each optical-depth layer. The UNet3DB network predicts 3D vector magnetic field, and the UNet3DZ network provides the geometric heights. They are combined to compute a custom loss function that also considers physical constraints (divergence free magnetic field). Panel (b) shows the post-processing step that assembles the outputs from the neural networks (prediction) and the original input magnetic fields into the final output. Panel (c) shows an example of the 3D data, split with overlaps between nearby subfields. These individual subfields are used as the input for training. Panel (d) presents the UNet3D structure, reproduced from Figure 2 in Cicek et al. (2016).

The ASPIICS solar coronagraph aboard the Proba-3 formation flying mission. Scientific objectives and instrument design

whawkins — Tue, 11 Nov 2025 21:28:07 +0000

The ASPIICS solar coronagraph aboard the Proba-3 formation flying mission. Scientific objectives and instrument design whawkins Tue, 11/11/2025 - 14:28

Author:

whawkins

Nov 11, 2025

Astronomy and Astrophysics: We describe the scientific objectives and instrument design of the ASPIICS coronagraph launched aboard the Proba-3 mission of the European Space Agency (ESA) on 5 December 2024. Proba-3 consists of two spacecraft in a highly elliptical orbit around the Earth. One spacecraft carries the telescope, and the external occulter is mounted on the second spacecraft. The two spacecraft fly in a precise formation during 6 hours out of 19.63 hour orbit, together forming a giant solar coronagraph called ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun). Very long distance between the external occulter and the telescope (around 144 m) represents an increase of two orders of magnitude compared to classical externally occulted solar coronagraphs. This allows us to observe the inner corona in eclipse-like conditions, i.e. close to the solar limb (down to 1.099 Rs) and with very low straylight. ASPIICS will provide a new perspective on the inner solar corona that will help solve several outstanding problems in solar physics, such as the origin of the slow solar wind and physical mechanism of coronal mass ejections.

Two spacecraft of the Proba-3 mission. Left panel: the Coronagraph Spacecraft (CSC). Right panel: the Occulter Spacecraft (OSC). The annotations highlight key subsystems of the mission: the entrance door of the ASPIICS coronagraph (1), GNSS antennas (2), antennas of the Inter-Satellite Link (ISL, 3), some mires of the Visual-Based System (4), Corner Cube Retro-Reflector (5), which is a part of the Fine Lateral and Longitudinal Sensor (FLLS), the edge of the external occulter (6), three LEDs of the Occulter Position Sensor Emitter (OPSE, 7), wide-angle and narrow-angle cameras of the VBS (8), laser of the FLLS (9). The axes of the coordinate systems are shown in each panel, with the x-axis pointing away from the Sun, z-axis pointing towards the ecliptic north, and y-axis complementing the right-handed system. The theoretical formation corresponds to the perfect alignment of the respectively x, y, and z axes attached to the two spacecraft.

Using the Hanle Effect in Mg II k to Quantify the Open Flux above the Solar Poles

whawkins — Tue, 16 Sep 2025 20:00:52 +0000

Using the Hanle Effect in Mg II k to Quantify the Open Flux above the Solar Poles whawkins Tue, 09/16/2025 - 14:00

Author:

whawkins

Sep 16, 2025

The Astrophysical Journal: We test the use of the Mg II resonant lines for measurement of the magnetic field at the top of the chromosphere of polar coronal holes (CHs). The Hanle effect in the core of Mg II k enables access to a regime of field strengths where the Zeeman effect has little diagnostic value (especially at the solar poles, where most of the field is transverse to the line of sight). Synthetic Stokes spectra computed from a radiation magnetohydrodynamic simulation of a CH emulating a high viewing angle are inverted with the HanleRT Tenerife Inversion Code, which accounts for the physical processes that lead to scattering-induced polarization and its modification due to the magnetic field and other symmetry-breaking mechanisms. We find that, while degeneracies in the atmospheric model lead to poor inferences of the thermodynamical properties, the magnetic inferences are highly consistent with the model values. The mean magnetic field strength in the simulation cube is typically retrieved with a relative error of δB ∼ 20% and an absolute error of ΔB ∼ 2 G at the top of the chromosphere. This opens up an avenue for promising chromospheric constraints for magnetic extrapolation models that ingest photospheric magnetograms, whose biases and uncertainties are troublesome to the reconstruction of the heliospheric magnetic field.

Inversion of an average synthetic Mg II k spectrum averaged over a 5"x5" area of a quiet Sun coronal hole simulation. The inversion strategy optimizes the fit of the spectral line core to extract the magnetic information at the top of the chromosphere. The synthetic observations are represented by black dots and the best-fit spectrum achieved by the HanleRT-TIC inversion is shown with red lines in the top four panels; additionally, the top-left panel shows the contribution function of the spectral line in blue. The bottom row shows the retrieved magnetic field stratification (red line) and the formal error bars at the inversion nodes derived by HanleRT-TIC. The inversion results are compared to the ensemble of values from the MURaM model atmospheres (gray histogram) used to create the synthetic observation. The height range of formation of the line core is indicated by the vertical blue stripes. The inversion inferences of the magnetic field are compatible with the average values of the simulation within the blue stripes.

Mother's Day Superstorms: Pre- and Post-storm Evolutionary Patterns of AR 13664/8

whawkins — Tue, 24 Jun 2025 18:13:37 +0000

Mother's Day Superstorms: Pre- and Post-storm Evolutionary Patterns of AR 13664/8 whawkins Tue, 06/24/2025 - 12:13

Author:

whawkins

Jun 24, 2025

Top: Global-scale toroid patterns indicate ARs 13664 and 13668 were located in the south-toroid in such a way as to be sufficiently away from ARs in the north-toroid, indicating probability of their eruption; close proximity of ARs 13664 and 13668 indicates the possibility of complex interactions between them. Bottom: Small-scale evolution of AR13664/8 is shown with three snapshots from HMI continuum (left) and radial magnetic field(right). The active region at the right side of the first frame was denoted as AR13664 first, and AR 13668 emerged to the east of it. Multiple pairs of bipoles emerged with the positive and negative polarities seen in white and black, respectively.

The Astrophysical Journal: In the week including Mother's day 2024, active region (AR) 13664 became superactive when AR\,13668 emerged nearby, causing multiple X-class flares and CMEs, and activity level increased similar to that inferred from geomagnetic storms associated with the historic 1859 events. By analyzing bot global warped toroids on which the active regions are strung, and active-region-scale magnetic flux and helicity, we find: (i) North and South toroids have nearly identical warped patterns, with mostly longitudinal wave numbers $m=1-3$; (ii) in three longitude intervals North and South toroids were tipped away from each other in latitude, with a longitude phase-shift between them, creating locations most prone for AR eruptions; (iii) on active-region-scale, vector magnetic fields deviate far from potential fields, and therefore contain large amounts of magnetic 'free energy' available for conversion into kinetic energy and high temperature radiation; (iv) the positive and negative polarities converge toward each other, facilitating reconnection and magnetic energy release; (v) rapid changes in magnetic helicity caused by helicity injection from below that creates helicity imbalances. We conclude that the Mother's day superstorms were caused by enhanced magnetic complexity occurring due to intricate interactions among multiple active regions emerging at nearly the same locations. This suggests, predicting locations of magnetically complex ARs, and studying and tracking their eruptive states using different proxy parameters, can greatly improve our ability to forecast intense storms, not only hours but potentially weeks in advance.

A deep learning framework for instrument-to-instrument translation of solar observation data

whawkins — Tue, 06 May 2025 19:39:28 +0000

A deep learning framework for instrument-to-instrument translation of solar observation data whawkins Tue, 05/06/2025 - 13:39

Author:

whawkins

May 6, 2025

Nature Communications: The constant improvement of astronomical instrumentation provides the foundation for scientific discoveries. In general, these improvements have only implications forward in time, while previous observations do not benefit from this trend, and the joint use of data sets from different instruments is typically limited by differences in calibration and quality.
Researches from the High Altitude Observatory (NSF NCAR; USA), in collaboration with researchers from the University of Graz (Austria), and Skolkovo Institute of Science and Technology (Skoltech; Russia), recently developed a new deep learning framework for Instrument-To-Instrument translation of solar observation data, enabling homogenized data series across multi-instrument datasets. The approach uses unpaired domain translations with Generative Adversarial Networks, which eliminate the need for spatial or temporal overlap to relate instruments. The study demonstrates that the available data sets can directly profit from instrumental improvements, by applying the method to four different applications of ground- and space-based solar observations. The authors obtain a homogenized data series of 24 years of space-based observations of the solar EUV corona and line-of-sight magnetic field, solar full-disk observations with increased spatial resolution, real-time mitigation of atmospheric degradations in ground-based observations, and unsigned magnetic field estimates from the solar far-side based on EUV imagery. The direct comparison to simultaneous high-quality observations shows that the method produces images that are perceptually similar, and enables more homogeneous multi-instrument data sets without the requirement of spatial or temporal alignment.

Occasional overlapping observations between different instruments allow for direct comparison between AI-enhanced images and high-quality reference data. This figure presents side-by-side views of original space-based observations from SOHO/EIT, their AI-enhanced counterparts, and reference observations from SDO/AIA. The three rows highlight different solar features: the Extreme Ultraviolet observations of the solar limb (top), an active region (middle), and the magnetic field of a sunspot (bottom).

Jarolim et al., 2025

The results were published in Nature Communications (https://doi.org/10.1038/s41467-025-58391-4).

Image instabilities and polarization cross-talk

whawkins — Tue, 28 Jan 2025 18:06:38 +0000

Image instabilities and polarization cross-talk whawkins Tue, 01/28/2025 - 11:06

Author:

whawkins

Jan 28, 2025

The Astrophysical Journal: We expand on our previous study of the impact of atmospheric seeing on polarization cross-talk, and show how the formalism that was developed in that work can be applied to treat the case of spatial modulators of polarization. Beside formally demonstrating how the problem of cross-talk is fully eliminated in such devices, we also gain insight on the meaning of polarimetric noise of temporal modulation schemes in the limit of very high modulation frequency. We also describe the problem of spectrograph instabilities, and how the spectral gradients that are naturally associated with a line spectrum feed into the problem of polarimetric errors induced by mechanical vibrations, thermal drifts, and pointing jitter. Finally, we show how this formalism can be used to estimate the contribution of polarization cross-talk to the errors on the elements of the 4$\times$4 Stokes response matrix, for the purpose of producing realistic error budgets for polarimetric instrumentation.

Figure 2. Top two rows: Example of spectrally resolved Stokes profiles of a narrow line (FWHM ∼6.3 pm), observed with a resolving power of 200 000 (2-px critical-sampling), and assuming maximum linear polarization of 10% and circular polarization of 30%. The spectral gradients reported inside the panels are obtained through Eq. (15), and must be used in Eq. (3) to estimate the corresponding polarimetric cross-talk errors. Bottom row: (left) modeled PSD of the image jitter at the detector in the spectral direction, delivering a target 1/8 px rms (de Wijn et al. 2022); (right) polarization cross-talk errors induced by the same PSD, for a spectrograph configuration representing the observation of the synthetic Stokes profiles shown at the top, assuming a total integration time of 10 s and a 10-state modulation cycle.)

Cutting-edge SPIn4D project combines AI and Astronomy

whawkins — Tue, 26 Nov 2024 23:52:25 +0000

Cutting-edge SPIn4D project combines AI and Astronomy whawkins Tue, 11/26/2024 - 16:52

Author:

whawkins

Nov 26, 2024

Astrophysical Journal: The National Science Foundation's Daniel K. Inouye Solar Telescope (DKIST) will provide high-resolution, multiline spectropolarimetric observations that are poised to revolutionize our understanding of the Sun. Given the massive data volume, novel inference techniques are required to unlock its full potential. Here, we provide an overview of our "SPIn4D" project, which aims to develop deep convolutional neural networks (CNNs) for estimating the physical properties of the solar photosphere from DKIST spectropolarimetric observations. We describe the magnetohydrodynamic (MHD) modeling and the Stokes profile synthesis pipeline that produce the simulated output and input data, respectively. These data will be used to train a set of CNNs that can rapidly infer the four-dimensional MHD state vectors by exploiting the spatiotemporally coherent patterns in the Stokes profile time series. Specifically, our radiative MHD model simulates the small-scale dynamo actions that are prevalent in quiet-Sun and plage regions. Six cases with different mean magnetic fields have been explored; each case covers six solar-hours, totaling 109 TB in data volume. The simulation domain covers at least 25 × 25 × 8 Mm, with 16 × 16 × 12 km spatial resolution, extending from the upper convection zone up to the temperature minimum region. The outputs are stored at a 40 s cadence. We forward model the Stokes profile of two sets of Fe i lines at 630 and 1565 nm, which will be simultaneously observed by DKIST and can better constrain the parameter variations along the line of sight. The MHD model output and the synthetic Stokes profiles are publicly available, with 13.7 TB in the initial release.

Schematic representation of the SPIn4D model workflow. The core of the model is the DL neural network, highlighted in blue in the middle of the diagram. The network training step is outlined by the broken green line and uses data derived from the MURaM simulations, both the MHD variables themselves and the Stokes profiles (I, Q, U, V) synthesized from the MHD data cubes (green lines). Once trained on the simulated data, observed Stokes data can be input into the network (red arrow) to produce the most likely 3D MHD state as output (labeled "Predicted MHD Variables"). The network may be trained to receive single-time input Stokes data to produce a reduced dimensional output MHD state ( B , vz , P, T) or to receive multitime input Stokes data to produce a full-dimensional MHD output, including additional derived outputs, such as vector velocities, Poynting flux, and so on. The network may also be trained to produce the derived outputs directly.

A Magnetohydrodynamic Mechanism for the Formation of Solar Polar Vortices

whawkins — Wed, 13 Nov 2024 16:03:34 +0000

A Magnetohydrodynamic Mechanism for the Formation of Solar Polar Vortices whawkins Wed, 11/13/2024 - 09:03

Author:

whawkins

Nov 13, 2024

PNAS (Proceedings of National Academy of Sciences): Polar vortices are ubiquitous features of planetary atmospheric flows, from the Earth-like rocky planets to Jupiter- and Saturn-like gas giant planets. Very little is known about their existence or dynamics on the Sun. What should be expected near the Sun’s pole for the upcoming solar multi-viewpoint and polar missions? Here we report the first magnetohydrodynamic nonlinear simulations for the formation and evolution of solar polar vortices using a near-surface magnetohydrodynamic shallow-water model. Our findings indicate that the rush-to-the-poles, the migration of magnetic fields towards the pole following the Sun’s magnetic cycle, can positively contribute to the formation of polar vortices. The mechanism proposed here for the formation of polar vortices is the first one to involve the role of magnetic fields and may be relevant to any star with a magnetic cycle. The Sun’s polar vortices resulting from this mechanism are predominantly magnetohydrodynamic, consisting of a tight pair of cyclonic and anticyclonic swirls. This mechanism is likely to operate during all solar cycle phases except the peak, when the polar field reverses. Polar vortices can impact dynamical evolution of global flows and polar fields, which seed the next activity cycle, hence better knowledge of physics of polar regions may lead to improved solar cycle and space weather forecasts. See: NSF-NCAR news website.

Simulations of magnetohydrodynamically governed polar vortices for a 30-degree inclined view (left) and for a polar view (right). Magnetohydrodynamic simulations show that the initial mid-latitude perturbations tend to drift and cluster around the forming multiple complex cyclonic and anticyclonic swirls. Following the polar rush, these swirls ultimately lead to an evolved configuration consisting of a pair of tight swirls with m=1 pattern.

Global and Local Dynamics of X-flare Producing Active Regions During Solar Cycle 25 Peak-Phase

whawkins — Fri, 06 Sep 2024 16:54:25 +0000

Global and Local Dynamics of X-flare Producing Active Regions During Solar Cycle 25 Peak-Phase whawkins Fri, 09/06/2024 - 10:54

Author:

whawkins

Sep 6, 2024

Astronomy & Astrophysics: As solar cycle (SC) 25 approaches its peak, a number of significant (X-class) flares have been produced. Here, we investigate the circumstances under which two of the most flare-prolific active regions of solar cycle 25, namely ARs 13590 and 13514, flared. Two aspects of the evolution of these active regions are investigated: the global-scale magnetic toroid configuration and small-scale magnetic field morphology and topology, prior, during and after the onset of major flares.

All active regions numbered by NOAA are spotted and marked on 21st February 2024 synoptic map constructed from SDO/HMI daily fits files. The double-arrowed top bar indicates which active regions are on the front side. Here, active regions resulting in X-class Flares are highlighted in red, while active regions resulting in M-class flares are highlighted in orange. Blue and red curves represent, respectively, the Northern and Southern Hemisphere magnetic toroid.

Observing and Quantifying Solar Magnetism

Polarization fringes in optical systems: a compendium

Recent News

Spectropolarimetric Inversion in Four Dimensions with Deep Learning (SPIn4D): II. A Physics-Informed Machine Learning Method for 3D Solar Photosphere Reconstruction

Recent News

The ASPIICS solar coronagraph aboard the Proba-3 formation flying mission. Scientific objectives and instrument design

Recent News

Using the Hanle Effect in Mg II k to Quantify the Open Flux above the Solar Poles

Recent News

Mother's Day Superstorms: Pre- and Post-storm Evolutionary Patterns of AR 13664/8

Recent News

A deep learning framework for instrument-to-instrument translation of solar observation data

Recent News

Image instabilities and polarization cross-talk

Recent News

Cutting-edge SPIn4D project combines AI and Astronomy

Recent News

A Magnetohydrodynamic Mechanism for the Formation of Solar Polar Vortices

Recent News

Global and Local Dynamics of X-flare Producing Active Regions During Solar Cycle 25 Peak-Phase

Recent News