12 research outputs found
Formation of the Mg II h and k polarization profiles in a solar plage model and their suitability to infer magnetic fields
The Mg II h and k lines are among the strongest in the near-ultraviolet solar
spectrum and their line core originates in the upper chromosphere, just below
the transition region. Consequently, they have become one of the main targets
for investigating the magnetism of the upper solar atmosphere. The recent
CLASP2 mission obtained unprecedented spectropolarimetric data of these lines
in an active region plage, which have already been used to infer the
longitudinal component of the magnetic field by applying the weak field
approximation. In this paper, we aim at improving our understanding of the
diagnostic capabilities of these lines by studying the emergent Stokes profiles
resulting from radiative transfer calculations in a radiative
magneto-hydrodynamic (rMHD) time-dependent model representative of a solar
plage. To this end, we create a synthetic observation with temporal and spatial
resolutions similar to those of CLASP2. We find strong asymmetries in the
circular polarization synthetic profiles which considerably complicate the
application of the weak field approximation. We demonstrate that the selective
application of the weak field approximation to fit different spectral regions
in the profile allows to retrieve information about the longitudinal component
of the magnetic field at different regions of the model atmosphere, even when
the circular polarization profiles are not anti-symmetric and are formed in the
presence of strong velocity and magnetic field gradients.Comment: Accepted for publication in Ap
Magnetic field information in the near-ultraviolet Fe II lines of the CLASP2 space experiment
We investigate theoretically the circular polarization signals induced by the
Zeeman effect in the Fe II lines of the 279.3-280.7 nm spectral range of the
CLASP2 space experiment and their suitability to infer solar magnetic fields.
To this end, we use a comprehensive Fe II atomic model to solve the problem of
the generation and transfer of polarized radiation in semi-empirical models of
the solar atmosphere, comparing the region of formation of the Fe II spectral
lines with those of the Mg II h and k and the Mn I resonance lines. These are
present in the same near ultraviolet (near-UV) spectral region and allowed the
mapping of the longitudinal component of the magnetic field ()
through several layers of the solar chromosphere in an active region plage. We
compare our synthetic intensity profiles with observations from the IRIS and
CLASP2 missions, proving the suitability of our model atom to characterize
these Fe II spectral lines. The CLASP2 observations show two Fe II spectral
lines at 279.79 and 280.66 nm with significant circular polarization signals.
We demonstrate the suitability of the Weak-Field Approximation (WFA) applied to
the Stokes and profiles of these Fe II lines to infer in
the plage atmosphere. We conclude that the near-UV spectral region of CLASP2
allows to determine from the upper photosphere to the top of the
chromosphere of active region plages.Comment: Accepted for publication in The Astrophysical Journa
The impact of angle-dependent partial frequency redistribution on the scattering polarization of the solar Na i D lines
The long-standing paradox of the linear polarization signal of the Na i D1
line was recently resolved by accounting for the atom's hyperfine structure and
the detailed spectral structure of the incident radiation field. That modeling
relied on the simplifying angle-averaged (AA) approximation for partial
frequency redistribution (PRD) in scattering, which potentially neglects
important angle-frequency couplings. This work aims at evaluating the
suitability of a PRD-AA modeling for the D1 and D2 lines through comparisons
with general angle-dependent (AD) PRD calculations, both in the absence and
presence of magnetic fields. We solved the radiative transfer problem for
polarized radiation in a one-dimensional semi-empirical atmospheric model with
microturbulent and isotropic magnetic fields, accounting for PRD effects,
comparing PRD-AA and PRD-AD modelings. The D1 and D2 lines are modeled
separately as two-level atomic system with hyperfine structure. The numerical
results confirm that a spectrally structured radiation field induces linear
polarization in the D1 line. However, the PRD-AA approximation greatly impacts
the Q/I shape, producing an antisymmetric pattern instead of the more symmetric
PRD-AD one, while presenting a similar sensitivity to magnetic fields between
10 and 200 G. Under the PRD-AA approximation, the Q/I profile of the D2 line
presents an artificial dip in its core, which is not found for the PRD-AD case.
We conclude that accounting for PRD-AD effects is essential to suitably model
the scattering polarization of the Na i D lines. These results bring us closer
to exploiting the full diagnostic potential of these lines for the elusive
chromospheric magnetic fields
Evidence for the Operation of the Hanle and Magneto-Optical Effects in the Scattering Polarization Signals Observed by CLASP2 Across the Mg II h and k Lines
Radiative transfer investigations of the solar Mg II h and k resonance lines
around 280~nm showed that, while their circular polarization (Stokes V) signals
arise from the Zeeman effect, the linear polarization profiles (Stokes Q and U)
are dominated by the scattering of anisotropic radiation and the Hanle and
magneto-optical (MO) effects. Using the unprecedented observations of the Mg II
and Mn I resonance lines obtained by the Chromospheric LAyer
Spectro-Polarimeter (CLASP2), here we investigate how the linear polarization
signals at different wavelengths (i.e., at the center, and at the near and far
wings of the k line) vary with the longitudinal component of the magnetic field
() at their approximate height of formation. The is estimated
from the V signals in the aforementioned spectral lines. Particular attention
is given to the following quantities that are expected to be influenced by the
presence of magnetic fields through the Hanle and MO effects: the sign of the U
signals, the total linear polarization amplitude () and its direction
() with respect to a reference direction. We find that at the center and
near wings of the line, the behavior of these quantities is significantly
different in the observed quiet and plage regions, and that both and
seem to depend on . These observational results are indicative of
the operation of the Hanle effectComment: 26 pages, 18 figures, accepted for publication in the Astrophysical
Journa
Novel framework for the three-dimensional NLTE inverse problem
The inversion of spectropolarimetric observations of the solar upper atmosphere is one of the most challenging goals in solar physics. If we account for all relevant ingredients of the spectral line formation process, such as the three-dimensional (3D) radiative transfer out of local thermodynamic equilibrium (NLTE), the task becomes extremely computationally expensive. Instead of generalizing 1D methods to 3D, we have developed a new approach to the inverse problem. In our meshfree method, we do not consider the requirement of 3D NLTE consistency as an obstacle, but as a natural regularization with respect to the traditional pixel-by-pixel methods. This leads to more robust and less ambiguous solutions. We solve the 3D NLTE inverse problem as an unconstrained global minimization problem that avoids repetitive evaluations of the Λ operator. Apart from the 3D NLTE consistency, the method allows us to easily include additional conditions of physical consistency such as the zero divergence of the magnetic field. Stochastic ingredients make the method less prone to ending up within the local minima of the loss function. Our method is capable of solving the inverse problem faster by several orders of magnitude than by using grid-based methods. The method can provide accurate and physically consistent results if sufficient computing time is available, along with approximate solutions in the case of very complex plasma structures or limited computing time
Modeling the scattering polarization of the solar Ca 
Context. The correct modeling of the scattering polarization signals observed in several strong resonance lines requires taking partial frequency redistribution (PRD) phenomena into account. Modeling scattering polarization with PRD effects is very computationally demanding and the simplifying angle-averaged (AA) approximation is therefore commonly applied.
Aims. This work aims to assess the impact and the range of validity of the AA approximation with respect to the general angle-dependent (AD) treatment of PRD effects in the modeling of scattering polarization in strong resonance lines, with a focus on the solar Ca 
Evidence of a flare ignited above a low-latitude spotted active region in the ultrafast rotator HK Aqr
We study the magnetic activity in the ultra fast rotator dMe HK Aqr using tomography techniques with high resolution spectroscopy. We aim to characterise how this magnetic activity appears in a regime of very fast rotation without external forces, given that HK Aqr is, very likely, a single star. We find dark spots located at low latitudes. We also detect prominences below the co-rotation radius and at low latitudes, coinciding with the spot latitudes. This apparent low-latitude activity contrasts with what is typically observed in fast rotators, which tend to form large polar spots. Moreover, we detect a stellar flare which produces an enhancement of the continuum and additional emission in the core of most photospheric and chromospheric lines. We find evidence that the flare is ignited above an active region, as seen in solar flares. This means that, with high probability, the flare is initiated by magnetic reconnection in complex active regions. We also present evidence of bulk red-shifted velocities of about 15 km s^(−1) during the rise of the flare, and velocities of 5-10 km s^(−1) during the decay phase. An estimation of the heating during the flare results in about 200 kK close to the peak and in 100 kK at the end of the observations