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Combining magneto-hydrostatic constraints with Stokes profiles inversions: III. Uncertainty in the inference of electric currents
Electric currents play an important role in the energy balance of the plasma in the solar atmosphere. They are also indicative of non-potential magnetic fields and magnetic reconnection. Unfortunately, the direct measuring of electric currents has traditionally been riddled with inaccuracies. Aims. We study how accurately we can infer electric currents under different scenarios. Methods. We carry out increasingly complex inversions of the radiative transfer equation for polarized light applied to Stokes profiles synthesized from radiative three-dimensional magnetohydrodynamic (MHD) simulations. The inversion yields the magnetic field vector. B. from which the electric current density, ./, is derived by applying Ampere's law. Results. We find that the retrieval of the electric current density is only slightly affected by photon noise or spectral resolution. However, the retrieval steadily improves as the Stokes inversion becomes increasingly elaborated. In the least complex case (a Milne- Eddington-like inversion applied to a single spectral region), it is possible to determine the individual components of the electric current density (jx, jy, jz) with an accuracy of cr = 0.90 - l.OOdex, whereas the modulus (|[/) can only be determined with cr - 0.75 dex. In the most complicated case (with multiple spectral regions, a large number of nodes, Tikhonov vertical regularization, and magnetohydrostatic equilibrium), these numbers improve to cr - 0.70-0.75 dex for the individual components and cr = 0.5 dex for the modulus. Moreover, in regions where the magnetic field is above 300 gauss, \\j\\ can be inferred with an accuracy of cr - 0.3 dex. In general, the x and y components of the electric current density are retrieved slightly better than the z component. In addition, the modulus of the electric current density is the best retrieved parameter of all, and thus it can potentially be used to detect regions of enhanced Joule heating. Conclusions. The fact that the accuracy does not worsen with decreasing spectral resolution or increasing photon noise, and instead increases as the Stokes inversion complexity grows, suggests that the main source of errors in the determination of electric currents is the lack of realism in the inversion model employed to determine variations in the magnetic field along the line of sight at scales smaller than the photon mean-free path, along with the intrinsic limitations of the model due to radiative transfer effects
Real-time multiframe blind deconvolution of solar images
The quality of images of the Sun obtained from the ground are severely
limited by the perturbing effect of the turbulent Earth's atmosphere. The
post-facto correction of the images to compensate for the presence of the
atmosphere require the combination of high-order adaptive optics techniques,
fast measurements to freeze the turbulent atmosphere and very time consuming
blind deconvolution algorithms. Under mild seeing conditions, blind
deconvolution algorithms can produce images of astonishing quality. They can be
very competitive with those obtained from space, with the huge advantage of the
flexibility of the instrumentation thanks to the direct access to the
telescope. In this contribution we leverage deep learning techniques to
significantly accelerate the blind deconvolution process and produce corrected
images at a peak rate of ~100 images per second. We present two different
architectures that produce excellent image corrections with noise suppression
while maintaining the photometric properties of the images. As a consequence,
polarimetric signals can be obtained with standard polarimetric modulation
without any significant artifact. With the expected improvements in computer
hardware and algorithms, we anticipate that on-site real-time correction of
solar images will be possible in the near future.Comment: 16 pages, 12 figures, accepted for publication in A&
Magnetic topology of the north solar pole
We study the polar magnetism near an activity maximum when these regions
change their polarity, from which it is expected that its magnetism should be
less affected by the global field. To fully characterise the magnetic field
vector, we use deep full Stokes polarimetric observations of the 15648.5 {\AA}
and 15652.8 {\AA} FeI lines. We observe the north pole as well as a quiet
region at disc centre to compare their field distributions. In order to
calibrate the projection effects, we observe an additional quiet region at the
east limb. We find that the two limb datasets share similar magnetic field
vector distributions. This means that close to a maximum, the poles look like
typical limb, quiet-Sun regions. However, the magnetic field distributions at
the limbs are different from the distribution inferred at disc centre. At the
limbs, we infer a new population of magnetic fields with relatively strong
intensities (600-800 G), inclined by 30 deg with respect to the
line of sight, and with an azimuth aligned with the solar disc radial
direction. We propose that this new population at the limbs is due to the
observation of unresolved magnetic loops as seen close to the limb. These loops
have typical granular sizes as measured in the disc centre. At the limbs, where
the spatial resolution decreases, we observe them spatially unresolved, which
explains the new population of magnetic fields that is inferred. This is the
first (indirect) evidence of small-scale magnetic loops outside the disc centre
and would imply that these small-scale structures are ubiquitous on the entire
solar surface. This result has profound implications for the energetics not
only of the photosphere, but also of the outer layers since these loops have
been reported to reach the chromosphere and the low corona
Discovery of long-period magnetic field oscillations and motions in isolated sunspots
We analyse the temporal evolution of the inclination component of the
magnetic field vector for the penumbral area of 25 isolated sunspots. Compared
to previous works, the use of data from the HMI instrument aboard the SDO
observatory facilitates the study of very long time series (1 week),
compared to previous works, with a good spatial and temporal resolution. We
used the wavelet technique and we found some filamentary-shaped events with
large wavelet power. Their distribution of periods is broad, ranging from the
lower limit for this study of 48 minutes up to 63 hours. An interesting
property of these events is that they do not appear homogeneously all around
the penumbra but they seem to concentrate at particular locations. The
cross-comparison of these wavelet maps with AIA data shows that the regions
where these events appear are visually related to the coronal loops that
connect the outer penumbra to one or more neighbouring opposite polarity flux
patches
A reconnection driven magnetic flux cancellation and a quiet Sun Ellerman bomb
The focus of this investigation is to quantify the conversion of magnetic to
thermal energy initiated by a quiet Sun cancellation event and to explore the
resulting dynamics from the interaction of the opposite polarity magnetic
features. We used imaging spectroscopy in the H line, along with
spectropolarimetry in the \ion{Fe}{I} 6173~{\AA} and \ion{Ca}{II} 8542~{\AA}
lines from the Swedish Solar Telescope (SST) to study a reconnection-related
cancellation and the appearance of a quiet Sun Ellerman bomb (QSEB). We
observed, for the first time, QSEB signature in both the wings and core of the
\ion{Fe}{I} 6173~{\AA} line. We also found that, at times, the \ion{Fe}{I}
line-core intensity reaches higher values than the quiet Sun continuum
intensity. From FIRTEZ-dz inversions of the Stokes profiles in \ion{Fe}{I} and
\ion{Ca}{II} lines, we found enhanced temperature, with respect to the quiet
Sun values, at the photospheric ( = -1.5; 1000 K) and lower
chromospheric heights ( = -4.5; 360 K). From the calculation
of total magnetic energy and thermal energy within these two layers it was
confirmed that the magnetic energy released during the flux cancellation can
support heating in the aforesaid height range. Further, the temperature
stratification maps enabled us to identify cumulative effects of successive
reconnection on temperature pattern, including recurring temperature
enhancements. Similarly, Doppler velocity stratification maps revealed impacts
on plasma flow pattern, such as a sudden change in the flow direction.Comment: 16 pages, 12 figures, accepted for publication in MNRA
Magnetic fields of opposite polarity in sunspot penumbrae
Context. A significant part of the penumbral magnetic field returns below the
surface in the very deep photosphere. For lines in the visible, a large portion
of this return field can only be detected indirectly by studying its imprints
on strongly asymmetric and three-lobed Stokes V profiles. Infrared lines probe
a narrow layer in the very deep photosphere, providing the possibility of
directly measuring the orientation of magnetic fields close to the solar
surface.
Aims. We study the topology of the penumbral magnetic field in the lower
photosphere, focusing on regions where it returns below the surface.
Methods. We analyzed 71 spectropolarimetric datasets from Hinode and from the
GREGOR infrared spectrograph. We inferred the quality and polarimetric accuracy
of the infrared data after applying several reduction steps. Techniques of
spectral inversion and forward synthesis were used to test the detection
algorithm. We compared the morphology and the fractional penumbral area covered
by reversed-polarity and three-lobed Stokes V profiles for sunspots at disk
center. We determined the amount of reversed-polarity and three-lobed Stokes V
profiles in visible and infrared data of sunspots at various heliocentric
angles. From the results, we computed center-to-limb variation curves, which
were interpreted in the context of existing penumbral models.
Results. Observations in visible and near-infrared spectral lines yield a
significant difference in the penumbral area covered by magnetic fields of
opposite polarity. In the infrared, the number of reversed-polarity Stokes V
profiles is smaller by a factor of two than in the visible. For three-lobed
Stokes V profiles the numbers differ by up to an order of magnitude.Comment: 11 pages 10 figures plus appendix (2 pages 3 figures). Accepted as
part of the A&A special issue on the GREGOR solar telescop
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