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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 & k lines. The retrieval of the line-of-sight (LOS) component of the magnetic field, $B_{\rm LOS}$, 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 $B_{\rm LOS}$ 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., Accepted for publication in ApJ

In ultraviolet (UV) spectropolarimetric observations of the solar corona, the existence of a magnetic field, solar wind velocity, and temperature anisotropies modify the linear polarization associated with resonant scattering. Unlike previous empirical models or global models, which present blended results of the above physical effects, in this work, we forward-model expected signals in the H i Lyα line (121.6 nm) by adopting an analytic model that can be adjusted to test the roles of different effects separately. We find that the impact of all three effects is most evident in the rotation of the linear polarization direction. In particular, (1) for magnetic fields between ∼10 and ∼100 G, the Hanle effect modifies the linear polarization at low coronal heights, rotating the linear polarization direction clockwise (counterclockwise) when the angle between the magnetic field and the local vertical is greater (less) than the van Vleck angle, which is consistent with the result of Zhao et al.; (2) solar wind velocity, which increases with height, has a significant effect through the Doppler dimming effect at higher coronal heights, rotating the linear polarization direction in an opposite fashion to the Hanle effect; and (3) kinetic temperature anisotropies are most significant at lower heights in open nonradial magnetic field regions, producing tilt opposite to isotropic Doppler dimming. The fact that the three effects operate differently in distinct spatial regimes opens up the possibility for using linear polarization measurements in UV lines to diagnose these important physical characteristics of the solar corona.

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.

著者人数: 29名, Accepted: 2017-04-04, 資料番号: SA1170029000

Spectro-polarimetric observations with the Solar Optical Telescope onboard Hinode reveal the emission spectrum of the Fe I 630 nm lines at the solar limb. The emission shell extends for less than 1'' thereby making it extremely difficult to detect from ground-based observatories viewing the limb through the Earth's atmosphere. The linear polarization signal is clearly due to scattering and it is predominantly oriented in the radial direction. Using a comprehensive atomic model of iron, we are able to interpret qualitatively the observed signals, including the radial orientation of the linear polarization. The Hanle effect causes the linear polarization of the Fe I 630 nm lines to be sensitive to magnetic fields between ~0.1 G and ~40 G, and also to be sensitive to the field's topology for stronger fields. The overall degree of observed polarization can be reproduced by randomly oriented horizontal magnetic fields of strength ≈2 G. The discovery of their scattering polarization signals thus opens a new diagnostic opportunity for these lines.

For about a decade, spectro-polarimetry of HeI 10830 has been applied to the magnetic diagnostics of the solar chromosphere. This resonance line is very versatile, as it is visible both on disk and in off-limb structures, and it has a good sensitivity to both the weak-field Hanle effect and the strong-field Zeeman effect. Recent observations of an active-region filament showed that the linear polarization was dominated by the transverse Zeeman effect, with very little or no hint of scattering polarization. This is surprising, since the HeI levels should be significantly polarized in a conventional scattering scenario. To explain the observed level of atomic depolarization by collisional or radiative processes, one must invoke plasma densities larger by several orders of magnitude than currently known values for prominences. We show that such depolarization can be explained quite naturally by the presence of an unresolved, highly entangled magnetic field, which averages to give the ordered field inferred from spectro-polarimetric data, over the typical temporal and spatial scales of the observations. We present a modeling of the polarized HeI 10830 in this scenario, and discuss its implications for the magnetic diagnostics of prominences and spicules, and for the general study of unresolved magnetic field distributions in the solar atmosphere.

We report on a dynamical event observed in a quiescent prominence on 2007 November 8: a well-formed dark "bubble" with a bright core rose vertically through the prominence without causing it to erupt. This event was observed in Hα and He I 1083 nm with the instruments of the Mauna Loa Solar Observatory. The dark bubble had a size of over 40'' and rose from the prominence base, at an average speed of ~12 km s−1, forming a bright compression front as it traversed the prominence. It finally assumed a "keyhole" shape before fading. The bright core embedded in the dark bubble was observed to rise from the solar limb, accelerating from ~12 to ~20 km s−1, leaving a thin trail of material behind. Subsequent observations indicate that this was not an exceptional event, but rather that similar disturbances do occur occasionally in prominences without disrupting them. In this Letter we present the November 8 observations, and propose a possible interpretation of the physical mechanism behind these dynamic events.

We present the first theoretical, forward calculations of the Stokes profiles of several magnetic dipole (‘‘M1’’) coronal emission lines produced in current-carrying magnetic structures. An idealized coronal model of Low, Fong, and Fan is used, which describes a spherically symmetric, hydrostatic background atmosphere, isothermal at a coronal temperature of 1:6 ;10 6 K. Embedded is a global, axisymmetric magnetic field that is everywhere potential except at a quiescent prominence, consisting of an infinitesimally thin, equatorial current sheet whose weight is supported by the outward discrete Lorentz force in the sheet. This model contains a physically nontrivial, localized magnetic structure, although the atmospheric plasma is thermally of the simplest stratification possible. The calculated M1 coronal lines show clear and distinct signatures of the presence and magnitude of this localized magnetic structure, in both linear and circular polarizations, eventhough the thermal structure isalmost homogeneous. Themorphologyof mapsoflinearpolarizationisparticularlysensitive to the existenceand strengthofthe current sheets, asfield lines wrap around them according to the Biot-Savart law, and the linear polarization responds to different projections offield line directions onto local radius vectors. Of the M1 lines studied, those of Fe xiii (1074.7 nm) and Si x (1430.1 nm) are especiallypromisingbecauseoftheirrelativelystronglinearpolarization.Theseforwardcalculationsprovideabasisfor optimismthatemission-linemeasurementsmayrevealthepresenceandnatureofcurrentsystemsinthecorona,and provide motivation for developing instruments capable ofroutinely measuring polarized light in forbidden coronal lines.

Within the framework of the quantum theory of polarized line formation, in the limit of complete frequency redistribution and of the collisionless regime, we derive explicit formulae describing the statistical equilibrium and the radiative emission of a multiterm atom with hyperfine structure, in the presence of an external magnetic field. The formulae we obtained for the radiative rates of the statistical equilibrium equations and for the radiative coefficients of the transfer equation for polarized radiation can be applied to investigate the formation of spectral lines for which both fine-structure and hyperfine-structure effects are important (e.g., the D1 and D2 lines of Na I in the solar atmosphere).

In this paper we discuss some general aspects of Stokes profile inversion in prominences using principal component analysis (PCA). First, we address the problem of departure of the formation of He I D3 from the optically thin approximation, dealing in a heuristic way with the inversion of the two visible components of the line, which are differently affected by radiative transfer. This modified inversion does not alter recently published results on the magnetic structure of quiescent prominences, in particular confirming the presence of magnetic fields significantly larger than average in limited regions. Second, we investigate the effect of line-of-sight integration of the polarization signals from different magnetic configurations, to determine whether PCA inversion could interpret an observed signal in terms of a single magnetic model with unrealistically large field strength. Our results show that this possibility is not statistically significant, and in addition the few models that are wrongly inverted show very peculiar geometries that are not found in our inversion of real data. This result also supports the reality of stronger than average fields in prominences. Finally, we further investigated the importance of full Stokes inversion for a correct diagnostic of magnetic fields in prominences. We show how the exclusion of Stokes V induces the appearance of a 90° ambiguity in the position angle of the magnetic field in the plane of the sky (in addition to the well-known 180° ambiguity). Inclusion of Stokes V proves to be important at all field strengths, even though Stokes Q and U should in theory be sufficient to retrieve the complete vector field information for field strengths in the Hanle depolarization regime. We also show how the inversion noise determined by the discreteness of the PCA database may be an additional source of the 90° ambiguity, even in the presence of Stokes V, therefore imposing a lower bound on the model density of the database.

International audience; We report on spectropolarimetric observations of Halpha in prominences made with the Télescope Héliographique pour l'Etude du Magnétisme et des Instabilités Solaires and the High Altitude Observatory/Advanced Stokes Polarimeter. Stokes Q and U show the expected profile shape from resonance scattering polarization and the Hanle effect. In contrast, most of the time, Stokes V does not show the antisymmetric profile shape typical of the Zeeman effect but a profile that indicates the presence of strong atomic orientation in the hydrogen levels, to an extent that cannot be explained by invoking the alignment-to-orientation transfer mechanism induced by the prominence magnetic field. We found that the largest signal amplitudes of Stokes V (comparable to that of Stokes Q and U) could be produced by a process of selective absorption of circularly polarized radiation from the photosphere, which requires that the prominence be in the vicinity of an active region. Although recent observations of active region filaments indicate such a selective absorption mechanism as a plausible explanation of the anomalous signals observed, the particular set of conditions that must be met suggest that a different explanation may be required to explain the almost ubiquitous symmetric V signal observed in Halpha prominences. Therefore, we speculate that an alternative mechanism inducing strong atomic orientation at the observed level could be due to the presence of electric fields inducing an electric Hanle effect on Halpha. Although we are still working toward a careful modeling of this effect, including both electric and magnetic fields, we present some preliminary considerations that seem to support this possibility.

We present new results on the inversion of spectropolarimetric data of the He I D3 line in a prominence. These data were taken with the High Altitude Observatory Stokes II polarimeter on 1980 August 5. Full Stokes profiles in I, Q, U, and V were recorded at 13 locations of a prominence at the northeast limb. This prominence erupted 3 hr later, giving rise to the day of year 218 Solar Maximum Mission coronal transient. The data were originally analyzed in a paper by Querfeld et al. Here we analyze the data using the new inversion algorithm proposed by Lopez Ariste & Casini based on pattern recognition techniques. This method employs full Stokes profiles, and in tests on synthetic data, it proved to be much more precise than previous diagnostic procedures. When applied to the Stokes data of the prominence under study, it gives magnetic field strengths at least 2 times larger than previous estimates.

We propose the use of the Zeeman effect of the hyperfine structure (HFS) as a diagnostic of solar photospheric magnetic fields. The HFS induces unique signatures in the polarized spectra of particular atoms, which can be used to directly get information on the magnetic field strength. To explore the effects of HFS, we adopt a numerical model of line formation and radiative transfer of polarized light in a magnetized Milne-Eddington atmosphere. We concentrate our studies on several lines of Mn I, which show particularly striking HFS signatures in the solar atlas. For these lines, anomalous Stokes profiles are produced for fields in the range 0-1000 G. The presence of these unusual profiles for weak magnetic fields constitutes a novel and potentially powerful diagnostic of the strength of solar magnetic fields in the quiet photosphere and plages.

We propose the use of principal component analysis (PCA) to invert spectropolarimetric data from prominences. Observation of the full Stokes profiles in prominences is very important for a deeper understanding of magnetic-field topology in these solar structures, and for the testing of theoretical models. The line formation problem, however, is complicated by the special conditions of prominences: anisotropy of light, low magnetic intensities, temperature and density ranges, etc. We created a code to solve this problem in the limit of optically thin plasma and of a collisionless regime, and use it in combination with PCA techniques to invert synthetic data. The results show that inversion is feasible.

We propose a simple classical model for the damped, magnetic dipole oscillator based on a circuit analogy. The solution for the dynamical equation of the associated magnetic moment is found to be similar in form to the well-known solution for the damped, electric dipole oscillator, but with the magnetic vector of the incident electromagnetic wave as the forcing field, instead of the electric vector. This model has been successfully applied to a classical derivation of the polarization properties of the forbidden (M1) coronal emission lines.

We apply the weak-field approximation of the line profiles, ωB Δω (ωB being the Larmor frequency and Δω the width of the line profile), adopting a second-order Taylor expansion in the Larmor frequency, to the polarized emission coefficients of the resonance transition in a two-level atom with values of the total angular momentum Ju and Jl, for the upper and lower levels, respectively. Using methods of Racah's algebra, we then derive rather compact algebraic expressions for the Stokes parameters of the radiation scattered locally in this transition. The advantage of using a second-order Taylor expansion of the emission coefficients is that the contributions of the Hanle effect and the Zeeman effect to the Stokes vector of the scattered radiation are easily identified and their respective roles in determining the polarization properties of the line better understood. Under the further assumption of unpolarized lower level, these expressions can be applied to derive explicit diagnostic formulae for polarized, resonance-scattering radiation, in terms of the relative geometry of the observer with respect to the solar magnetic field at the scattering center. The typical case of a spinless, two-level atom with Ju = 1 and Jl = 0 is explicitly worked out, and the corresponding diagnostic formulae are used to clarify some interesting properties of the Hanle effect. Finally, we compare these results, derived from the quantum-mechanical theory of line formation, with a recently proposed, classical formulation of the Hanle effect.

We demonstrate that the atomic alignment of the hyperfine-structure components of the ground level S$_{1/2}$ of Na {\sc i} and of the upper level P$_{1/2}$ of the D$_1$ line are practically negligible for magnetic strengths $B>10 \rm G$, and virtually zero for $B\ga 100 \rm G$. This occurs independently of the magnetic-field inclination on the stellar surface (also, in particular, for vertical fields). Consequently, the characteristic antisymmetric linear-polarization signature of the scattered light in the D$_1$ line is practically suppressed in the presence of magnetic fields larger than 10 G, regardless of their inclination. Remarkably, we find that the scattering polarization amplitude of the D$_2$ line increases steadily with the magnetic strength, for vertical fields above 10 G, while the contribution of alignment to the polarization of the D$_1$ line rapidly decreases. Therefore, we suggest that spectropolarimetric observations of the ``quiet'' solar chromosphere showing significant linear polarization peaks in both D$_1$ and D$_2$ cannot be interpreted in terms of one-component magnetic field models, implying that the magnetic structuring of the solar chromosphere could be substantially more complex than previously thought.

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 $\Lambda$-type taking into account partially coherent scattering, and generalize its expression in order to describe a "multiple $\Lambda$" 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., $\Delta L=0,\pm 1$) group of transitions, which must be expected on the basis of the principle of spectroscopic stability.

We investigate atomic transitions that have previously been identified as having zero polarization from the Zeeman effect. Our goal is to identify spectral lines that can be used for the calibration of instrumental polarization of large astronomical and solar telescopes, such as the Daniel K. Inouye Solar Telescope, which is currently under construction on Haleakala. We use a numerical model that takes into account the generation of scattering polarization and its modification by the presence of a magnetic field of arbitrary strength. We adopt values for the Lande factors from spectroscopic measurements or semi-empirical results, thus relaxing the common assumption of LS-coupling previously used in the literature. The mechanisms dominating the polarization of particular transitions are identified, and we summarize groups of various spectral lines useful for the calibration of spectropolarimetric instruments, classified according to their polarization properties.