Magnetic Field Diagnostics Research Articles

Two-dimensional magnetic field diagnostics of plasma based on nano-thin-film probe

In this paper, we propose a method to determine the diagnostics of magnetic fields based on arrays of nano-thin-film probes that are manufactured by using magnetron sputtering-based fabrication and arranged in a two-dimensional (2D) topological design. Measurements of the magnetic field and its wave number based on the proposed method were validated in a linear cylindrical plasma device, the linear experimental advanced device. We manufactured nano-thin-film probes, each with a thickness of 500 nm, a diameter of 9 mm, and a shape similar to the Greek letter “omega” by depositing silver nanoparticles generated through magnetron sputtering to improve the accuracy of the fabrication as well as their time response. The 2D topological arrangement of the array of probes enabled the determination of the diagnostics of the magnetic field and its wave number at a high spatial resolution. Measurements of the amplitude of the magnetic field and its wavenumber obtained using the proposed method were in good agreement with the results of theoretical simulations in COMSOL, which verifies its high reliability and accuracy in obtaining low-pressure plasma diagnostics. In future work, we plan to apply our diagnostic method based on the array of thin-film probes to scenarios that require a high spatial resolution.

Magnetic field diagnostics of prominences with the Mg II k line 3D Stokes inversions versus traditional methods

The Mg II k resonance line is commonly used for diagnosing the solar chromosphere. We theoretically investigated its intensity and polarization in solar prominences, taking 3D radiative transfer and Hanle and Zeeman effects into account. We used an optically thick 3D model representative of a solar prominence and applied several inversion methods to the synthetic Stokes profiles, clarifying their pros and cons for inferring prominence magnetic fields. We conclude that the self-consistent 3D inversion with radiative transfer is necessary to determine the magnetic field vector, although its geometry cannot be inferred with full fidelity. We also demonstrate that more traditional methods, such as those based on the weak field approximation or the constant-property slab assumption, can offer useful information under certain conditions.

Diagnostic potential of wavelength-integrated scattering polarisation in the solar He II Ly-αline

Aims.The main goal of this work is to study the potential of the He IILy-αwavelength-integrated scattering polarisation for probing the magnetism of the solar upper chromosphere. Meanwhile, the suitability of different modelling approximations is investigated.Methods.Radiative transfer calculations are performed in semi-empirical 1D solar atmospheres, out of local thermodynamic equilibrium, considering a two-term atomic model and accounting for the Hanle, Zeeman, and magneto-optical effects. The problem is suitably linearised and discretised, and the resulting numerical system is solved with a matrix-free iterative method. The results obtained by modelling scattering processes with three different descriptions, namely in the limit of complete frequency redistribution (CRD), and accounting for partial frequency redistribution (PRD) effects under the angle-averaged (AA) approximation and in the general angle-dependent (AD) formulation, are compared.Results.The synthetic Stokes profiles resulting from CRD, PRD–AA, and PRD–AD calculations show a very good agreement in the line core, while some differences are observed inQ/Ioutside this spectral region. Moreover, the precise structure of the atmospheric model does not noticeably affect the line-core profiles, but it strongly impacts theQ/Isignals outside the line core. As most of the He IILy-αphotons originate in the core region, it turns out that wavelength-integrated linear polarisation signals are almost insensitive to both the scattering description and the atmospheric model. Appreciable wavelength-integratedŪ/Īsignals, showing observable sensitivity to horizontal magnetic fields in the 0–1000 G range, are also found, particularly near the limb. While the integration time required to detect magnetic fields in the quiet chromosphere with this line is too long for sounding-rocket missions, magnetic fields corresponding to typical plage areas would produce detectable signals, especially near the limb.Conclusions.These results, to be confirmed by 3D calculations including the impact of horizontal inhomogeneities and bulk velocity gradients, show that filter polarimetry in the He IILy-αline has a promising potential for chromospheric magnetic-field diagnostics. In near-limb plage regions, this could already be assessed through sounding-rocket experiments.

The Polarization of the Solar Ba ii D1 Line with Partial Frequency Redistribution and Its Magnetic Sensitivity

We investigate the main physical mechanisms that shape the intensity and polarization of the Ba ii D1 line at 4934 Å via radiative transfer numerical experiments. We focus especially on the scattering linear polarization arising from the spectral structure of the anisotropic radiation in the wavelength interval spanned by the line’s hyperfine structure (HFS) components in the odd isotopes of barium. After verifying that the presence of the low-energy metastable levels only impacts the amplitude, but not the shape, of the D1 linear polarization, we relied on a two-term atomic model that neglects such metastable levels but includes HFS. The D1 fractional linear polarization shows a very small variation with the choice of atmospheric model, enhancing its suitability for solar magnetic field diagnostics. Tangled magnetic fields with strengths of tens of gauss reduce the linear polarization, and saturation is reached at roughly 300 G. Deterministic inclined magnetic fields produce a U/I profile and, if they have a significant longitudinal component, a V/I profile, whose modeling requires accounting for HFS and the Paschen–Back effect. Because of the overlap between HFS components, the magnetograph formula cannot be applied to infer the longitudinal magnetic field. Accurately modeling the D1 intensity and polarization requires an atomic system that includes the metastable levels and the HFS, the detailed spectral structure of the radiation field, the incomplete Paschen–Back regime for magnetic fields, and an accurate treatment of collisions.

An Optically Thin View of the Solar Chromosphere from Observations of the O i 1355 Å Spectral Line

The O i 1355 Å spectral line is one of the only optically thin lines that are both routinely observed and thought to be formed in the chromosphere. We present an analysis of a variety of observations of this line with the Interface Region Imaging Spectrograph (IRIS), and compare it with other IRIS diagnostics as well as diagnostics of the photospheric magnetic field. We utilize special deep exposure modes on IRIS and provide an overview of the statistical properties of this spectral line for several different regions on the Sun. We analyze the spatiotemporal variations of the line intensity and find that it is often significantly enhanced when and where magnetic flux of opposite polarities cancel. Significant emission occurs in association with chromospheric spicules. Because of the optically thin nature of the O i line, the nonthermal broadening can provide insight into unresolved small-scale motions. We find that the nonthermal broadening is modest, with typical values of 5–10 km s−1, and shows some center-to-limb variation, with a modest increase toward the limb. The dependence with the height of the intensity and line broadening off-limb is compatible with the line broadening being dominated by the superposition of Alfvén waves on different structures. The nonthermal broadening shows a modest but significant enhancement above locations that are in between photospheric magnetic flux concentrations in plage, i.e., where the magnetic field is likely to be more inclined with respect to the line of sight. Our measurements provide strict constraints on future theoretical models of the chromosphere.

Magnetic Field and Plasma Diagnostics for Solar Coronal Mass Ejections: A Case Study Using the Forward Modeling Approach

Magnetic Field and Plasma Diagnostics for Solar Coronal Mass Ejections: A Case Study Using the Forward Modeling Approach

30-min decayless kink oscillations in a very long bundle of solar coronal plasma loops

The energy balance in the corona of the Sun is the key to the long-standing coronal heating dilemma, which could be potentially revealed by observational studies of decayless kink oscillations of coronal plasma loops. A bundle of very long off-limb coronal loops with the length of 736pm 80 Mm and a lifetime of about 2 days are found to exhibit decayless kink oscillations. The oscillations are observed for several hours. The oscillation amplitude is measured at 0.3–0.5 Mm, and the period at 28–33 min. The existence of 30-min periodicity of decayless kink oscillations indicates that the mechanism compensating the wave damping is still valid in such a massive plasma structure. It provides important evidence for the non-resonant origin of decayless kink oscillations with 2–6 min periods, i.e., the lack of their link with the leakage of photospheric and chromospheric oscillations into the corona and the likely role of the broadband energy sources. Magnetohydrodynamic seismology based on the reported detection of the kink oscillation, with the assistance of the differential emission measure analysis and a background coronal model provides us with a comprehensive set of plasma and magnetic field diagnostics, which is of interest as input parameters of space weather models.

The Magnetic Sensitivity of the (250–278 nm) Fe ii Polarization Spectrum

This paper presents a theoretical investigation of the polarization and magnetic sensitivity of the near-ultraviolet (near-UV) solar spectral lines of Fe ii between 250 and 278 nm. In recent years, UV spectropolarimetry has become key to uncovering the magnetism of the upper layers of the solar chromosphere. The unprecedented data obtained by the CLASP2 suborbital space experiment across the Mg ii h and k lines around 280 nm are a clear example of the capabilities of near-UV spectropolarimetry for magnetic field diagnostics throughout the whole solar chromosphere. Recent works have pointed out the possible complementary diagnostic potential of the many Fe ii lines in the unexplored spectral region between 250 and 278 nm, but no quantitative analysis of the polarization and magnetic sensitivity of those spectral lines has been carried out yet. To study the polarization signals in these spectral lines, we create a comprehensive atomic model including all the atomic transitions resulting in strong spectral lines. We then study the magnetic sensitivity of the linear and circular polarization profiles in a semiempirical model representative of the quiet Sun. We present a selection of Fe ii spectral lines with significant linear and circular polarization signals and evaluate their diagnostic capabilities by studying their formation heights and magnetic sensitivity through the action of the Hanle and Zeeman effects. We conclude that when combined with the CLASP2 spectral region these Fe ii lines are of interest for the inference of magnetic fields throughout the solar chromosphere, up to near the base of the corona.

Effects of Spectral Resolution on Simple Magnetic Field Diagnostics of the Mg ii H and K Lines

We 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 and k lines. The retrieval of the line-of-sight component of the magnetic field, B LOS, from synthetic spectra generated in a uniformly magnetized FAL-C atmosphere is accurate when restricted to the inner lobes of Stokes V. As we degrade the spectral resolution, partial redistribution effects, which 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 as the resolution becomes poorer. When applied to a diverse set of spectra emerging from a sunspot simulation, we find good accuracy in the retrieved B LOS 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 ∼ 1500 G. Limited spectral resolution results in a small bias toward weaker retrieved fields. The WFA for the transverse component of the magnetic field is also evaluated. Reduced spectral resolution degrades the accuracy of the inferences, because spectral mixing results in the line effectively probing deeper layers of the atmosphere.

Magnetic Field Diagnostics in the Solar Upper Atmosphere

The magnetic field is the main driver of the activity in the solar upper atmosphere, but its measurement is notoriously difficult. In order to determine the magnetic field in the chromosphere, transition region, and corona, we need to measure and interpret the polarization signals that the scattering of anisotropic radiation and the Hanle and Zeeman effects introduce in the emitted spectral line radiation. A number of recent advances have activated the development of this research field. ▪ The quantum theory of the generation and transfer of polarized radiation allows us to explain the polarization signals observed in chromospheric and coronal lines and to make successful predictions in unexplored spectral regions. ▪ The development of diagnostic techniques for the solar upper atmosphere has served to improve our empirical knowledge of the magnetic field in a variety of plasma structures, as well as to pave the way for their application to the unprecedented data that the new generation of solar telescopes are expected to provide. However, further improvements are required. ▪ The CLASP suborbital experiments have opened a new diagnostic window, namely ultraviolet (UV) spectropolarimetry as a tool for probing the magnetism and geometry of the upper chromosphere and transition region. A space telescope equipped with a UV spectropolarimeter would lead to major advances in our empirical understanding of solar magnetism.

Analysis and modeling of laser-driven ion-beam trace probe diagnostics of poloidal magnetic fields in field-reversed configurations

The field-reversed configuration (FRC) is a promising magnetic confinement fusion concept [M. Tuszewski, Nucl. Fusion 28, 2033 (1988)] and is often chosen as the target plasma for magneto inertial fusion [S. A. Slutz and M. R. Gomez, Phys. Plasmas 28, 042707 (2021)]. In FRCs, the toroidal magnetic field is essentially zero, and the poloidal magnetic field (Bp) pressure is comparable with the plasma pressure. Applying the traditional Bp diagnostics to FRCs is a major challenge because Bp is small, and reversal occurs across the core region of FRCs. The laser-driven ion-beam trace probe (LITP) is a newly developing diagnostic method to measure Bp and the radial electric field (Er) in tokamak. Here, the principles of using LITP to diagnose Bp in FRCs are proposed, verified, and numerically implemented using an iterative method to reconstruct the Bp profile. Least square tomography employing a dissipative term is used to solve the nonlinear tomography problem, which arises when applying LITP to the unique FRC magnetic topology. Numerical modeling results show that the relative errors of the reconstruction are mostly below 10%, verifying the feasibility of LITP diagnostics for FRC internal magnetic field measurements. Ion beam orbits and detector arrangements are optimized to meet the experimental requirements of FRCs. LITP can still be applied to diagnose Bp in FRCs when there is 5% measurement errors.

The Coronal Magnetograph – CorMag: mission for the Stratospheric Hemera Mission

The Coronal Magnetograph - CorMag - experiment aims at studying the magnetic field topology of the solar corona. The direction of the coronal magnetic field vector would be derived from narrow-wavelength bandpass observations of the linearly polarized FeXIV line-emission (530.3 nm), interpreted through the saturated Hanle effect. CorMag is an internally-occulted coronagraph whose design was derived from the externally- occulted, formation-flying ASPIICS coronagraph of the PROBA-3 ESA solar mission. Prelim- inary tests of CorMag were carried out during two campaigns in Antarctica at the Concordia Italian-French Station in 2018/2019 and 2019/2020 (Fig. 1). This presentation reports the results of those tests. The European Union-funded HEMERA Program has selected CorMag for a stratospheric balloon flight is 2021. The configuration for this mission will be illustrated, together with the expected polarization sensitivity. This mission is a demonstrator for future space-based coronagraphs with the capability of visible-light and infrared polarization measurements for coronal magnetic field diagnostics.

Infrared diagnostics of the solar magnetic field with Mg I 12 μm lines: forward-model results

Context. The Mg I 12.32 and 12.22 μm lines are a pair of emission lines that present a great advantage for accurate solar magnetic field measurement. They potentially contribute to the diagnosis of solar atmospheric parameters through their high magnetic sensitivity. Aims. The goal of this study is to understand the radiation transfer process of these lines in detail and explore the ability of magnetic field diagnosis in the infrared. Methods. We calculated the Stokes profiles and response functions of the two Mg I 12 μm lines based on one-dimensional solar atmospheric models using the Rybicki-Hummer (RH) radiative transfer code. The integration of these profiles with respect to the wavelength was used to generate calibration curves related to the longitudinal and transverse fields. The traditional single-wavelength calibration curve based on the weak-field approximation was also tested to determine if it is suitable for the infrared. Results. The 12.32 μm line is more suitable for a magnetic field diagnosis because its relative emission intensity and polarization signal are stronger than that of the 12.22 μm line. The result from the response functions illustrates that the derived magnetic field and velocity with 12.32 μm line mainly originate from the height of 450 km, while that for the temperature is about 490 km. The calibration curves obtained by the wavelength-integrated method show a nonlinear distribution. For the Mg I 12.32 μm line, the longitudinal (transverse) field can be effectively inferred from Stokes V/I (Q/I and U/I) in the linear range below ∼600 G (∼3000 G) in quiet regions and below ∼400 G (∼1200 G) in penumbrae. Within the given linear range, the method is a supplement to the magnetic field calibration when the Zeeman components are incompletely split.

Imaging spectropolarimetry for magnetic field diagnostics in solar prominences

Context. Narrowband imaging spectropolarimetry is one of the most powerful tools available to infer information about the intensity and topology of the magnetic fields present in extended plasma structures in the solar atmosphere. Aims. We describe the instrumental set-up and the observing procedure that we have developed and optimized at the Istituto Ricerche Solari Locarno in order to perform imaging spectropolarimetry. A measurement that highlights the potential of the ensuing observations for magnetic field diagnostics in solar prominences is presented. Methods. Monochromatic images of solar prominences were obtained by combining a tunable narrowband filter, based on two Fabry-Perot etalons, with a Czerny-Turner spectrograph. Linear and circular polarization were measured at every pixel of the monochromatic image with the Zurich Imaging Polarimeter, ZIMPOL. A wavelength scan was performed across the profile of the considered spectral line. The HAZEL inversion code was applied to the observed Stokes profiles to infer a series of physical properties of the observed structure. Results. We carried out a spectropolarimetric observation of a prominence, consisting of a set of quasi-monochromatic images across the He I D3 line at 5876 Å in the four Stokes parameters. The map of observed Stokes profiles was inverted with HAZEL, finding magnetic fields with intensities between 15 and 30 G and directed along the spine of the prominence, which is in agreement with the results of previous works.

SUMER Measurement of the Fe x 3p 43d 4D5/2,7/2 Energy Difference

Abstract Recent studies have shown that magnetic fields in the solar corona are strong enough to significantly mix the two 3p 43d 4D5/2,7/2 levels in Cl-like Fe x. This mixing gives rise to a magnetically induced transition (MIT) component in the bright Fe x 257.3 Å line, commonly observed by current instrumentation, that can be used for coronal magnetic field diagnostics. This line, commonly observed by the still operational EIS spectrometer on board the Hinode satellite since 2007, opens a new window into the coronal magnetic field. However, the strength of this MIT transition depends on the square of the energy difference ΔE of the two levels, so that an accurate determination of ΔE is of critical importance to accurately measure coronal magnetic field strengths. In the present work we present a new measurement of ΔE obtained determining the separation of the two component of the Fe x doublet close to 1603.3 Å from deep-exposure spectra of a quiescent streamer at the solar limb taken with the SUMER instrument on board SoHO. Our measurement of ΔE = 2.29 ± 0.50 cm−1 agrees with, and improves upon, an earlier measurements by Judge et al. by decreasing its uncertainty from 80% to approximately 20%, improving the attainable accuracy of magnetic field strength measurements obtainable with the Fe x 257.26 Å line.

Mapping the magnetic field in the solar corona through magnetoseismology

Magnetoseismology, a technique of magnetic field diagnostics based on observations of magnetohydrodynamic (MHD) waves, has been widely used to estimate the field strengths of oscillating structures in the solar corona. However, previously magnetoseismology was mostly applied to occasionally occurring oscillation events, providing an estimate of only the average field strength or one-dimensional distribution of field strength along an oscillating structure. This restriction could be eliminated if we apply magnetoseismology to the pervasive propagating transverse MHD waves discovered with the Coronal Multi-channel Polarimeter (CoMP). Using several CoMP observations of the Fe xiii 1074.7 nm and 1079.8 nm spectral lines, we obtained maps of the plasma density and wave phase speed in the corona, which allow us to map both the strength and direction of the coronal magnetic field in the plane of sky. We also examined distributions of the electron density and magnetic field strength, and compared their variations with height in the quiet Sun and active regions. Such measurements could provide critical information to advance our understanding of the Sun's magnetism and the magnetic coupling of the whole solar atmosphere.

H$$\alpha $$ full line spectropolarimetry as diagnostics of chromospheric magnetic field

Analysis of spectropolarimetric observations of two circular sunspots located close to disk centre in H $${\alpha }$$ (6562.8 A) and Fe i (6569.22 A) is presented in this paper. The corresponding active region numbers are NOAA 10940 and NOAA 10941 referred to as AR1 and AR2, respectively. The vector magnetic field at the photosphere is derived through inversion of Stokes profiles of Fe i under Milne–Eddington atmospheric model. The chromospheric vector magnetic field is derived from H $${\alpha }$$ Stokes profiles under weak-field approximation. Azimuthally averaged magnetic field as a function of radial distance from the centre of sunspot at the photosphere and chromosphere are studied. At the photosphere, the radial variation shows a well known behaviour that the total field and the line-of-sight (LOS) component monotonically decrease from centre to the edge of the sunspot and the transverse component initially increases, reaches a maximum close to half the sunspot radius and then decreases. LOS and the transverse components become equal close to half the sunspot radius consistent with the earlier findings. At the chromosphere, all the components of the magnetic field decrease with the sunspot radius. However, the LOS component decreases monotonically whereas the transverse component decreases monotonically up to about 0.6 times the sunspot radius after which it reaches a constant value. Azimuthally averaged magnetic field gradient from photosphere to chromosphere is also presented here.

Using the Stokes V widths of Fe I lines for diagnostics of the intrinsic solar photospheric magnetic field

Aims. The goal of this study is to explore a novel method for the solar photospheric magnetic field diagnostics using Stokes V widths of different magnetosensitive Fe I spectral lines. Methods. We calculate Stokes I and V profiles of several Fe I lines based on a one-dimensional photospheric model VAL C using the NICOLE radiative transfer code. These profiles are used to produce calibration curves linking the intrinsic magnetic field values with the widths of blue peaks of Stokes V profiles. The obtained calibration curves are then tested using the Stokes profiles calculated for more realistic photospheric models based on magnetohydrodynamic of magneto-convection. Results. It is shown that the developed Stokes V widths method can be used with various optical and near-infrared lines. Out of six lines considered in this study, Fe I 6301 line appears to be the most effective: it is sensitive to fields over ∼200 G and does not show any saturation up to ∼2 kG. Other lines considered can also be used for the photospheric field diagnostics with this method, however, only in narrower field value ranges, typically from about 100 G to 700–1000 G. Conclusions. The developed method can be a useful alternative to the classical magnetic line ratio method, particularly when the choice of lines is limited.

Polarized radiative transfer, rotation measure fluctuations, and large-scale magnetic fields

ABSTRACT Faraday rotation measure (RM) at radio wavelengths is commonly used to diagnose large-scale magnetic fields. It is argued that the length-scales on which magnetic fields vary in large-scale diffuse astrophysical media can be inferred from correlations in the observed RM. RM is a variable which can be derived from the polarized radiative transfer equations in restrictive conditions. This paper assesses the usage of rotation measure fluctuation (RMF) analyses for magnetic field diagnostics in the framework of polarized radiative transfer. We use models of various magnetic field configurations and electron density distributions to show how density fluctuations could affect the correlation length of the magnetic fields inferred from the conventional RMF analyses. We caution against interpretations of RMF analyses when a characteristic density is ill defined, e.g. in cases of lognormal-distributed and fractal-like density structures. As the spatial correlations are generally not the same in the line-of-sight longitudinal direction and the sky plane direction, one also needs to clarify the context of RMF when inferring from observational data. In complex situations, a covariant polarized radiative transfer calculation is essential to capture all aspects of radiative and transport processes, which would otherwise ambiguate the interpretations of magnetism in galaxy clusters and larger scale cosmological structures.

Three-dimensional reconstruction of CME-driven shock–streamer interaction from radio and EUV observations: a different take on the diagnostics of coronal magnetic fields

On 2014 October 30, a band-splitted type II radio burst associated with a coronal mass ejection (CME) observed by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) occurred over the southeast limb of the Sun. The fast expansion in all directions of the plasma front acted as a piston and drove a spherical fast shock ahead of it, whose outward progression was traced by simultaneous images obtained with the Nançay Radioheliograph (NRH). The geometry of the CME/shock event was recovered through 3D modeling, given the absence of concomitant stereoscopic observations, and assuming that the band-splitted type II burst was emitted at the intersection of the shock surface with two adjacent low-Alfvén speed coronal streamers. From the derived spatiotemporal evolution of the standoff distance between shock and CME leading edge, we were finally able to infer the magnetic field strength B in the inner corona. A simple radial profile of the form B(r) = (12.6 ± 2.5)r−4 nicely fits our results, together with previous estimates, in the range r = 1.1−2.0 R⊙.