arXiv subjects

R. Casini

Publications and source records attributed to R. Casini.

At least 19 recordsLinked to original sources

arXiv2025

Detection of "diffuse" coronal He I 1083 during the April 8 2024 Solar Eclipse: evidence for terrestrial atmospheric scattering origin

Strong He I 1083 nm atomic line signals have been previously measured during total solar eclipses at coronal heights above the lunar limb. This rather unexpected measurement has kindled a discussion about the hypothesized presence of significant amounts of neutral helium at coronal conditions. We performed spectroscopic observations of the He I 1083 nm spectroscopic region with the newly built CHEESE instrument during the April 8th 2024 total solar eclipse to test the presence of He I 1083 in the solar corona. We detected the He I 1083, the forbidden coronal line Fe XIII 1074.7 nm, as well as the chromospheric H I 1093.8 nm Paschen-{\gamma} line in our eclipse observations. The chromospheric He I 1083 and H I 1093.8 nm Paschen-{\gamma} lines are detected in the corona as well as on the lunar disc. Our findings point toward a non-solar origin of the He I 1083 signal during the April 8th 2024 eclipse that challenge the notion of abundant neutral helium in the solar corona inferred from eclipse observations.

astro-ph.SR↗

arXiv2024

A unifying polarization formalism for electric- and magnetic-multipole interactions

We extend the spherical tensorial formalism for polarization to the treatment of electric- and magnetic-multipole transitions of any order. We rely on the spherical-wave expansion to derive the tensor form of the operator describing the interaction of the atomic system with a polarized radiation field, which naturally leads to the introduction of spherical tensors describing the polarization properties of the interacting field. As a direct application, the formalism is used to model the radiation anisotropy affecting the scattering of radiation in an electric-quadrupole transition, and the associated Hanle effect in the presence of a magnetic field.

physics.atom-ph↗

arXiv2022

Optimal spectral lines for measuring chromospheric magnetic fields

This paper identifies spectral lines from X-ray to infrared wavelengths which are optimally suited to measuring vector magnetic fields as high as possible in the solar atmosphere. Instrumental and Earth's atmospheric properties, as well as solar abundances, atmospheric properties and elementary atomic physics are considered without bias towards particular wavelengths or diagnostic techniques. While narrowly-focused investigations of individual lines have been reported in detail, no assessment of the comparative merits of all lines has ever been published. Although in the UV, on balance the Mg+ h and k lines near 2800 Angstroms are optimally suited to polarimetry of plasma near the base of the solar corona. This result was unanticipated, given that longer-wavelength lines offer greater sensitivity to the Zeeman effect. While these lines sample optical depths photosphere to the coronal base, we argue that cores of multiple spectral lines provide a far more discriminating probe of magnetic structure as a function of optical depth than the core and inner wings of a strong line. Thus, together with many chromospheric lines of Fe+ between 2585 and the h line at 2803 Angstrom, this UV region promises new discoveries concerning how the magnetic fields emerge, heat, and accelerate plasma as they battle to dominate the force and energy balance within the poorly-understood chromosphere.

astro-ph.SR↗

arXiv2022

Ultraviolet Spectropolarimetry: Investigating stellar magnetic field diagnostics

Magnetic fields are important for stellar photospheres and magnetospheres, influencing photospheric physics and sculpting stellar winds. Observations of stellar magnetic fields are typically made in the visible, although infrared observations are becoming common. Here we consider the possibility of directly detecting magnetic fields at ultraviolet (UV) wavelengths using high resolution spectropolarimetry, specifically considering the capabilities of the proposed Polstar mission. UV observations are particularly advantageous for studying wind resonance lines not available in the visible, but they can also provide many photospheric lines in hot stars. Detecting photospheric magnetic fields using the Zeeman effect and Least Squares Deconvolution is potentially more effective in the UV due to the much higher density of strong lines. We investigate detecting magnetic fields in the magnetosphere of a star using the Zeeman effect in wind lines, and find that this could be detectable at high S/N in an O or B star with a strong magnetic field. We consider detecting magnetic fields using the Hanle effect in linear polarization, which is complementary to the Zeeman effect, and could be more sensitive in photospheric lines of rapid rotators. The Hanle effect can also be used to infer circumstellar magnetism in winds. Detecting the Hanle effect requires UV observations, and a multi-line approach is key for inferring magnetic field properties. This demonstrates that high resolution spectropolarimetry in the UV, and the proposed Polstar mission, has the potential to greatly expand our ability to detect and characterize magnetic fields in and around hot stars.

astro-ph.SR↗

arXiv2022

Ultraviolet Spectropolarimetry With Polstar: Using Polstar to test Magnetospheric Mass-loss Quenching

Polstar is a proposed NASA MIDEX space telescope that will provide high-resolution, simultaneous full-Stokes spectropolarimetry in the far ultraviolet, together with low-resolution linear polarimetry in the near ultraviolet. This observatory offers unprecedented capabilities to obtain unique information on the magnetic and plasma properties of the magnetospheres of hot stars. We describe an observing program making use of the known population of magnetic hot stars to test the fundamental hypothesis that magnetospheres should act to rapidly drain angular momentum, thereby spinning the star down, whilst simultaneously reducing the net mass-loss rate. Both effects are expected to lead to dramatic differences in the evolution of magnetic vs. non-magnetic stars.

astro-ph.SR↗

arXiv2022

TIC: A Stokes inversion code for scattering polarization with partial frequency redistribution and arbitrary magnetic fields

We present the Tenerife Inversion Code (TIC), which has been developed to infer the magnetic and plasma properties of the solar chromosphere and transition region via full-Stokes inversion of polarized spectral lines. The code is based on the HanleRT forward engine, which takes into account many of the physical mechanisms that are critical for a proper modeling of the Stokes profiles of spectral lines originating in the tenuous and highly dynamic plasmas of the chromosphere and transition region: quantum level population imbalance and interference (atomic polarization), frequency coherence effects in polarized resonance scattering (partial frequency redistribution), and the impact of arbitrary magnetic fields on the atomic polarization and the radiation field. We present first results of atmospheric and magnetic inversions, and discuss future developments for the project.

astro-ph.SR↗

arXiv2022

The Visible Spectro-Polarimeter of the Daniel K. Inouye Solar Telescope

The Daniel K. Inouye Solar Telescope (DKIST) Visible Spectro-Polarimeter (ViSP) is a traditional slit-scanning spectrograph, with the ability to observe solar regions up to a $120\times78~\mathrm{arcsec}^2$ area. The design implements dual-beam polarimetry, a polychromatic polarization modulator, a high-dispersion echelle grating, and three spectral channels that can be automatically positioned. A defining feature of the instrument is its capability to tune anywhere within the 380-900~nm range of the solar spectrum, allowing for a virtually infinite number of combinations of three wavelengths to be observed simultaneously. This enables the ViSP user to pursue well-established spectro-polarimetric studies of the magnetic structure and plasma dynamics of the solar atmosphere, as well as completely novel investigations of the solar spectrum. Within the suite of first-generation instruments at the DKIST, ViSP is the only wavelength-versatile spectro-polarimeter available to the scientific community. It was specifically designed to be a discovery instrument, for the exploration of new spectroscopic and polarimetric diagnostics, and to test improved models of polarized line formation, through high spatial-, spectral-, and temporal-resolution observations of the Sun's polarized spectrum. In this instrument article, we describe the science requirements and design drivers of ViSP, and we present preliminary science data collected during the commissioning of the instrument.

astro-ph.IM↗

arXiv2021

Ultraviolet Spectropolarimetry With Polstar: Hot Star Magnetospheres

Polstar is a proposed NASA MIDEX space telescope that will provide high-resolution, simultaneous full-Stokes spectropolarimetry in the far ultraviolet, together with low-resolution linear polarimetry in the near ultraviolet. In this white paper, we describe the unprecedented capabilities this observatory would offer in order to obtain unique information on the magnetic and plasma properties of the magnetospheres of hot stars. This would enable a test of the fundamental hypothesis that magnetospheres should act to rapidly drain angular momentum, thereby spinning the star down, whilst simultaneously reducing the net mass-loss rate. Both effects are expected to lead to dramatic differences in the evolution of magnetic vs. non-magnetic stars.

astro-ph.IM↗

arXiv2020

The magnetic sensitivity of the resonance and subordinate lines of Mg II in the solar chromosphere

We carry out a theoretical study of the polarization of the solar Mg II h-k doublet (including its extended wings) and the subordinate UV triplet around 280 nm. These lines are of great diagnostic interest, as they encode information on the physical properties of the solar atmosphere from the upper photosphere to the chromosphere-corona transition region. We base our study on radiative transfer calculations of spectral line polarization in one-dimensional models of quiet and plage regions of the solar atmosphere. Our calculations take into account the combined action of atomic polarization, quantum level interference, frequency redistribution, and magnetic fields of arbitrary strength. In particular, we study the sensitivity of the emergent Stokes profiles to changes in the magnetic field through the Zeeman and Hanle effects. We also study the impact of the chromospheric plasma dynamics on the emergent Stokes profiles, taking into account the angle-dependent frequency redistribution in the h-k resonance transitions. The results presented here are of interest for the interpretation of spectropolarimetric observations in this important region of the solar ultraviolet spectrum.

astro-ph.SR↗

arXiv2019

Spectropolarimetry of the Solar Mg II h and k Lines

We report on spectropolarimetric observations across the Mg II h and k-lines at 2800 angstrom made by the Ultraviolet Spectrometer and Polarimeter onboard the Solar Maximum Mission satellite. Our analysis confirms the strong linear polarization in the wings of both lines observed near the limb, as previously reported, but also demonstrates the presence of a negatively (i.e., radially oriented) polarized signal between the two lines. We find evidence for fluctuations of the polarization pattern over a broad spectral range, resulting in some depolarization with respect to the pure scattering case when observed at very low spatial and temporal resolutions. This is consistent with recent theoretical modeling that predicts this to be the result of redistribution effects, quantum interference between the atomic levels of the upper term, and magneto-optical effects. A first attempt at a quantitative exploitation of these signals for the diagnosis of magnetic fields in the chromosphere is attempted. In active regions, we present observations of circular polarization dominated by the Zeeman effect. We are able to constrain the magnetic field strength in the upper active chromosphere using an analysis based on the magnetograph formula, as justified by theoretical modeling. We inferred a significantly strong magnetic field (~500 G) at the 2.5 sigma level on an exceptionally active, flaring region.

astro-ph.SR↗

arXiv2018

Experimental Testing of Scattering Polarization Models

We realized a laboratory experiment to study the scattering polarization of the Na I D-doublet at 589.0 and 589.6 nm in the presence of a magnetic field. This work was stimulated by solar observations of that doublet, which have proven particularly challenging to explain through available models of polarized line formation, even to the point of casting doubts on our very understanding of the underlying physics. The purpose of the experiment was to test a quantum theory for the polarized scattering of spectrally flat incident radiation, on which much of the current magnetic diagnostics of stellar atmospheres is based. The experiment has confirmed the predictions of that theory, and its adequacy for the modeling of scattering polarization under flat-spectrum illumination.

astro-ph.SR↗

arXiv2018

CLASP Constraints on the Magnetization and Geometrical Complexity of the Chromosphere-Corona Transition Region

The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a suborbital rocket experiment that on 3rd September 2015 measured the linear polarization produced by scattering processes in the hydrogen Ly-$α$ line of the solar disk radiation, whose line-center photons stem from the chromosphere-corona transition region (TR). These unprecedented spectropolarimetric observations revealed an interesting surprise, namely that there is practically no center-to-limb variation (CLV) in the $Q/I$ line-center signals. Using an analytical model, we first show that the geometrical complexity of the corrugated surface that delineates the TR has a crucial impact on the CLV of the $Q/I$ and $U/I$ line-center signals. Secondly, we introduce a statistical description of the solar atmosphere based on a three-dimensional (3D) model derived from a state-of-the-art radiation magneto-hydrodynamic simulation. Each realization of the statistical ensemble is a 3D model characterized by a given degree of magnetization and corrugation of the TR, and for each such realization we solve the full 3D radiative transfer problem taking into account the impact of the CLASP instrument degradation on the calculated polarization signals. Finally, we apply the statistical inference method presented in a previous paper to show that the TR of the 3D model that produces the best agreement with the CLASP observations has a relatively weak magnetic field and a relatively high degree of corrugation. We emphasize that a suitable way to validate or refute numerical models of the upper solar chromosphere is by confronting calculations and observations of the scattering polarization in ultraviolet lines sensitive to the Hanle effect.

astro-ph.SR↗

arXiv2018

A Statistical Inference Method for Interpreting the CLASP Observations

On 3rd September 2015, the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) successfully measured the linear polarization produced by scattering processes in the hydrogen Lyman-$α$ line of the solar disk radiation, revealing conspicuous spatial variations in the $Q/I$ and $U/I$ signals. Via the Hanle effect the line-center $Q/I$ and $U/I$ amplitudes encode information on the magnetic field of the chromosphere-corona transition region (TR), but they are also sensitive to the three-dimensional structure of this corrugated interface region. With the help of a simple line formation model, here we propose a statistical inference method for interpreting the Lyman-$α$ line-center polarization observed by CLASP.

astro-ph.SR↗

arXiv2017

Rayleigh Scattering in Spectral Series with L-Term Interference

We derive a formalism to describe the scattering of polarized radiation over the full spectral range encompassed by atomic transitions belonging to the same spectral series (e.g., the H I Lyman and Balmer series, the UV multiplets of Fe I and Fe II). This allows us to study the role of radiation-induced coherence among the upper terms of the spectral series, and its contribution to Rayleigh scattering and the polarization of the solar continuum. We rely on previous theoretical results for the emissivity of a three-term atom of the $Λ$-type taking into account partially coherent scattering, and generalize its expression in order to describe a "multiple $Λ$" atomic system underlying the formation of a spectral series. Our study shows that important polarization effects must be expected because of the combined action of partial frequency redistribution and radiation-induced coherence among the terms of the series. In particular, our model predicts the correct asymptotic limit of 100% polarization in the far wings of a \emph{complete} (i.e., $ΔL=0,\pm 1$) group of transitions, which must be expected on the basis of the principle of spectroscopic stability.

astro-ph.SR↗

arXiv2017

Magnetic Field Diagnostics with Strong Chromospheric Lines

The complex spectropolarimetric patterns around strong chromospheric lines, the result of subtle spectroscopic and transport mechanisms, are sensitive, sometimes in unexpected ways, to the presence of magnetic fields in the chromosphere, which may be exploited for diagnostics. We apply numerical polarization radiative transfer implementing partially coherent scattering by polarized multi-term atoms, in the presence of arbitrary magnetic fields, in planeparallel stellar atmospheres to study a few important spectroscopic features: Mg II h-k doublet; Ca II H-K doublet and IR triplet. We confirm the importance of partial redistribution effects in the formation of the Mg II h-k doublet in magnetized atmospheres, as previously pointed out for the non-magnetic case. Morevover, we show, numerically and analytically, that a magnetic field produces measurable modications of the broadband linear polarization even for relatively small field strengths, while circular polarization remains well represented by the magnetograph formula. We note that this phenomenon has already (unknowingly) been observed by UVSP/SMM, and the interest and possibility of its observation in stars other than the Sun. The interplay between partial redistribution in the H-K doublet of Ca II and metastable level polarization in its IR triplet allow diagnosing the chromospheric magnetic field at different layers and strengths. Our results suggest several new avenues to investigate empirically the magnetism of the solar and stellar chromospheres.

astro-ph.SR↗

arXiv2017

Explicit Form of the Radiative and Collisional Branching Ratios in Polarized Radiation Transport with Coherent Scattering

We consider the vector emissivity of the polarized radiation transfer in a $Λ$-type atomic transition, which we recently proposed in order to account for both CRD and PRD contributions to the scattered radiation. This expression can concisely be written as $$\varepsilon=\left(\varepsilon^{(1)} -\varepsilon^{(2)}_{\rm f.s.} \right) +\varepsilon^{(2)}\;$$ where $\varepsilon^{(1)}$ and $\varepsilon^{(2)}$ are the emissivity terms describing, respectively, one-photon and two-photon processes in a $Λ$-type atom, %the fully and partially redistributed radiations, and where "f.s." means that the corresponding term must be evaluated assuming an appropriate "flat spectrum" average of the incident radiation across the spectral line. In this follow up study, we explicitly consider the expressions of these various terms for the case of a polarized multi-term atom, in order to derive the algebraic forms of the branching ratios between the CRD and PRD contributions to the emissivity. In the limit of a two-term atom with non-coherent lower-term, our results are shown to be in full agreement with those recently derived by Bommier (2017)

astro-ph.SR↗

arXiv2017

Discovery of Scattering Polarization in the Hydrogen Lyman-$α$ Line of the Solar Disk Radiation

There is a thin transition region (TR) in the solar atmosphere where the temperature rises from 10,000 K in the chromosphere to millions of degrees in the corona. Little is known about the mechanisms that dominate this enigmatic region other than the magnetic field plays a key role. The magnetism of the TR can only be detected by polarimetric measurements of a few ultraviolet (UV) spectral lines, the Lyman-$α$ line of neutral hydrogen at 121.6 nm (the strongest line of the solar UV spectrum) being of particular interest given its sensitivity to the Hanle effect (the magnetic-field-induced modification of the scattering line polarization). We report the discovery of linear polarization produced by scattering processes in the Lyman-$α$ line, obtained with the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) rocket experiment. The Stokes profiles observed by CLASP in quiet regions of the solar disk show that the $Q/I$ and $U/I$ linear polarization signals are of the order of 0.1 % in the line core and up to a few percent in the nearby wings, and that both have conspicuous spatial variations with scales of $\sim 10$ arcsec. These observations help constrain theoretical models of the chromosphere-corona TR and extrapolations of the magnetic field from photospheric magnetograms. In fact, the observed spatial variation from disk to limb of polarization at the line core and wings already challenge the predictions from three-dimensional magnetohydrodynamical models of the upper solar chromosphere.

astro-ph.SR↗

arXiv2016

Laboratory Frequency Redistribution Function for the Polarized $Λ$-Type Three-Term Atom

We present the frequency redistribution function for the polarized three-term atom of the $Λ$-type in the collisionless regime, and we specialize it to the case where both the initial and final terms of the three-state transition are metastable (i.e., with infinitely sharp levels). This redistribution function represents a generalization of the well-known $R_{\rm II}$ function to the case where the lower terms of the transition can be polarized and carry atomic coherence, and it can be applied to the investigation of polarized line formation in tenuous plasmas, where collisional rates may be low enough that anisotropy induced atomic polarization survives even in the case of metastable levels.

astro-ph.GA↗