Rebecca Centeno

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.

Limitations and biases in the retrieval of the polar magnetic field I: the role of the magnetic filling factor in Milne-Eddington inversions of simulated Hinode/SP data

whawkins — Thu, 20 Apr 2023 19:00:57 +0000

Limitations and biases in the retrieval of the polar magnetic field I: the role of the magnetic filling factor in Milne-Eddington inversions of simulated Hinode/SP data whawkins Thu, 04/20/2023 - 13:00

Author:

whawkins

Apr 20, 2023

We study the extent to which Milne-Eddington inversions are able to retrieve and characterize the magnetic landscape of the solar poles from observations by the spectropolarimeter onboard Hinode. In particular, we evaluate whether a variable magnetic filling factor is an adequate modeling technique for retrieving the intrinsic magnetic properties from every pixel in the polar field of view. We first generate synthetic spectra emerging from a numerical simulation of a "plage" region at an inclined line of sight of 65 degrees, and degrade the data to emulate real observations. Then, we invert the synthetic spectra with two Milne-Eddington inversion codes that feature different treatments of the magnetic filling factor, and relate the retrieved magnetic quantities back to their original values in the simulation cube. We find that while the apparent retrieved magnetic properties map well the spatially-degraded simulation, the intrinsic magnetic quantities bear little relation to the magnetic field at the native resolution of the simulation. We discuss the systematic biases caused by line-of-sight foreshortening, spatial degradation, photon noise and modeling assumptions embedded in the inversion algorithm.

Continuum intensity derived from the synthetic spectra at three different stages in the process. A: synthetic continuum image from the spectra emerging from a 65 degree LOS at the native resolution of the simulation (i.e. 16 km sampling). B: continuum intensity after foreshortening the pixels along the LOS direction. C: continuum image after spatially convolving the synthetic data with an Airy disk of 240~km radius.

Effects of spectral resolution on simple magnetic field diagnostics of the Mg II h & k lines

whawkins — Fri, 23 Sep 2022 21:17:25 +0000

Effects of spectral resolution on simple magnetic field diagnostics of the Mg II h & k lines whawkins Fri, 09/23/2022 - 15:17

Author:

whawkins

Sep 23, 2022

The Astrophysical Journal: Rebecca Centeno, Matthias Rempel, Roberto Casini, and Tanausu del Pino Aleman study the effects of finite spectral resolution on the magnetic field values retrieved through the weak field approximation (WFA) from the cores of the Mg II h&k lines. The retrieval of the line-of-sight (LOS) component of the magnetic field, Blos, from synthetic spectra generated in a uniformly magnetized FAL-C atmosphere are accurate when restricted to the inner lobes of Stokes V. As we degrade the spectral resolution, partial redistribution (PRD) effects that more prominently affect the outer lobes of Stokes V, are brought into the line core through spectral smearing, degrading the accuracy of the WFA and resulting in an inference bias, which is more pronounced the poorer the resolution. When applied to a diverse set of spectra emerging from a sunspot simulation, we find a good accuracy in the retrieved Blos when comparing it to the model value at the height where the optical depth in the line core is unity. The accuracy is preserved up to field strengths of B~1700 G. Limited spectral resolution results in a bias toward weaker retrieved fields. The WFA for the transverse component of the magnetic field is also evaluated. We find the best estimates when the WFA is evaluated in the core of the line. Reduced spectral resolution degrades the accuracy of the inferences because spectral mixing results in the line effectively probing deeper layers of the atmosphere.

Scatter density plots of the retrieved value of Blos retrieved from Mg II h using the weak field approximation, against their MURaM model counterparts for the case of infinite spectral resolution (left), R = 45,000h (middle) and R = 30,000 (right). The model values are taken at the height where the core of the Mg II h line reaches optical depth unity. The darker the grey-level, the higher the number of samples in the bin. The bin size is 40 G. The red line represents the ideal solution and the blue line shows a linear fit through the data.

2nd Successful Spectropolarimetry School

whawkins — Wed, 07 Sep 2022 15:36:21 +0000

2nd Successful Spectropolarimetry School whawkins Wed, 09/07/2022 - 09:36

2022 Spectropolarimetry School

Author:

whawkins

Sep 7, 2022

The High Altitude Observatory and the National Solar Observatory organized their second successful Spectropolarimetry School in Boulder, Colorado. This two-week school held at the end of August presented an overview of the field of solar spectropolarimetry and the diagnostic techniques used to analyze and interpret spectropolarimetric observations of the Sun.

Solar magnetism is at the root of all solar activity and is the driving force of Space Weather. The characteristics of the magnetic field are encoded in the polarization of the light emitted by the Sun, and sensitive analysis techniques are necessary to reveal them. This school explored the observations, the theory and the tools that help us determine the state and evolution of the Sun's magnetic field through its polarized spectrum. The curriculum covered theoretical topics such as polarized radiative transfer, polarimetric instrumentation, scattering polarization and the Hanle effect, as well as dozens of hours of hands-on tutorials with different spectral line inversion codes. The school brought together 19 domestic and 7 international participants as well as multiple leading experts from around the world.

Group portrait of the 2022 Spectropolarimetry school students and lecturers taken on the outdoor patio at Center Green 1 building.

On the (Mis)interpretation of the Scattering Polarization Signatures In the Ca II 8542 A line Through Spectral Line Inversions

kolinski — Mon, 11 Jul 2022 20:45:28 +0000

On the (Mis)interpretation of the Scattering Polarization Signatures In the Ca II 8542 A line Through Spectral Line Inversions kolinski Mon, 07/11/2022 - 14:45

Publication Name: The Astrophysical Journal; Author's names: Centeno, R., de la Cruz Rodriguez, J., del Pino Aleman, T.

Author:

kolinski

Jul 11, 2022

Scattering polarization tends to dominate the linear polarization signals of the Ca II 8542 A line in weakly magnetized areas (B ~ 100 G), especially when the observing geometry is close to the limb. In this paper we evaluate the degree of applicability of existing non-LTE spectral line inversion codes (which assume that the spectral line polarization is due to the Zeeman effect only) at inferring the magnetic field vector and, particularly, its transverse component. To this end, we use the inversion code STiC to extract the strength and orientation of the magnetic field from synthetic spectropolarimetric data generated with the Hanle-RT code. The latter accounts for the generation of polarization through scattering processes as well as the joint actions of the Hanle and the Zeeman effects. We find that, when the transverse component of the field is stronger than ~80 G, the inversion code is able to retrieve accurate estimates of the transverse field strength as well as its azimuth in the plane of the sky. Below this threshold, the scattering polarization signatures become the major contributors to the linear polarization signals and often mislead the inversion code into severely over- or under-estimating the field strength. Since the line-of-sight component of the field is derived from the circular polarization signal, which is not affected by atomic alignment, the corresponding inferences are always good.

Inversion results for observing geometries away from the disk center. The panels show the relative (percent) differences between the magnetic field values retrieved by the STiC inversions and those in the model atmospheres used for the Hanle-RT syntheses. Left and right correspond, respectively, to the Zeeman-only and the full calculation with atomic polarization cases. The top two rows show the results for the strength and the azimuth of the transverse component of the magnetic field, while the bottom panels present the results for the LOS magnetic field.

AAS Nova Features a Recent Publication by Philip Judge

kolinski — Thu, 18 Nov 2021 19:36:28 +0000

AAS Nova Features a Recent Publication by Philip Judge kolinski Thu, 11/18/2021 - 12:36

Author:

kolinski

Nov 18, 2021

Understanding the magnitude and occurrence of solar storms is key to predicting events that can be harmful to thousands of Earth-orbiting satellites and the welfare of astronauts. A recent paper by Philip Judge et al was highlighted in AAS Nova in a their featured article entitled, "A Better Space Weather Forecasting Tool."

Images to highlight the corrugated surfaces at which the centers of UV lines are formed (close to τ = 1) in MURaM calculations of the upper solar chromosphere. The calculations are from a numerical experiment where a magnetic flux system is emerging from beneath the solar atmosphere. The emergent fields carry with them plasma, which is revealed by the extended heights of the τ = 1 surfaces, which can exceed 10 Mm. Typically the Doppler line shifts in the lower transition region are smaller than the line widths (Athay & Dere 1991). Therefore the τ = 1 surfaces were computed without taking into account Doppler shifts, at the centers of lines of Mg ii k, and several lines of Fe ii. The x–y images of height of formation are plotted in order of decreasing opacity from Mg ii to Fe ii 2769.75 Å. In the bottom right panel, slices of these τ = 1 surfaces are shown, taken along the black line in the five other panels. But the black lines in this final panel are magnetic lines of force in the y–z plane. Evidently, a combination of these and other lines of iron with a wide range of opacities spans the ranges of excursions of the surfaces away from a horizontal plane.

Link to paper: Measuring the Magnetic Origins of Solar Flares, Coronal Mass Ejections, and Space Weather

Rebecca Centeno

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

Recent News

Limitations and biases in the retrieval of the polar magnetic field I: the role of the magnetic filling factor in Milne-Eddington inversions of simulated Hinode/SP data

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Effects of spectral resolution on simple magnetic field diagnostics of the Mg II h & k lines

Recent News

2nd Successful Spectropolarimetry School

Recent News

On the (Mis)interpretation of the Scattering Polarization Signatures In the Ca II 8542 A line Through Spectral Line Inversions

Recent News

AAS Nova Features a Recent Publication by Philip Judge

Recent News