Author name code: libbrecht-tine
ADS astronomy entries on 2022-09-14
author:"Libbrecht, Tine"
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Title: Line formation of He I D3 and He I 10 830 Å in
a small-scale reconnection event
Authors: Libbrecht, Tine; Bjørgen, Johan P.; Leenaarts, Jorrit;
de la Cruz Rodríguez, Jaime; Hansteen, Viggo; Joshi, Jayant
Bibcode: 2021A&A...652A.146L
Altcode: 2020arXiv201015946L
Context. Ellerman bombs (EBs) and UV bursts are small-scale reconnection
events that occur in the region of the upper photosphere to the
chromosphere. It has recently been discovered that these events can
have emission signatures in the He I D3 and He I 10 830 Å
lines, suggesting that their temperatures are higher than previously
expected.
Aims: We aim to explain the line formation of He I
D3 and He I 10 830 Å in small-scale reconnection events.
Methods: We used a simulated EB in a Bifrost-generated radiative
magnetohydrodynamics snapshot. The resulting He I D3 and He
I 10 830 Å line intensities were synthesized in 3D using the non-local
thermal equilibrium (non-LTE) Multi3D code. The presence of coronal
extreme UV (EUV) radiation was included self-consistently. We compared
the synthetic helium spectra with observed raster scans of EBs in He I
10 830 Å and He I D3 obtained at the Swedish Solar Telescope
with the TRI-Port Polarimetric Echelle-Littrow Spectrograph.
Results: Emission in He I D3 and He I 10 830 Å is formed
in a thin shell around the EB at a height of ∼0.8 Mm, while the He I
D3 absorption is formed above the EB at ∼4 Mm. The height
at which the emission is formed corresponds to the lower boundary of the
EB, where the temperature increases rapidly from 6 × 103 K
to 106 K. The synthetic line profiles at a heliocentric angle
of μ = 0.27 are qualitatively similar to the observed profiles at the
same μ-angle in dynamics, broadening, and line shape: emission in the
wing and absorption in the line core. The opacity in He I D3
and He I 10 830 Å is generated through photoionization-recombination
driven by EUV radiation that is locally generated in the EB at
temperatures in the range of 2 × 104 − 2 × 106
K and electron densities between 1011 and 1013
cm−3. The synthetic emission signals are a result of
coupling to local conditions in a thin shell around the EB, with
temperatures between 7 × 103 and 104 K and
electron densities ranging from ∼1012 to 1013
cm−3. This shows that both strong non-LTE and thermal
processes play a role in the formation of He I D3 and
He I 10 830 Å in the synthetic EB/UV burst that we studied.
Conclusions: In conclusion, the synthetic He I D3 and He I 10
830 Å emission signatures are an indicator of temperatures of at least
2 × 104 K; in this case, as high as ∼106 K.
Title: Three-dimensional magnetic field structure of a flux-emerging
region in the solar atmosphere
Authors: Yadav, Rahul; de la Cruz Rodríguez, Jaime; Díaz Baso,
Carlos José; Prasad, Avijeet; Libbrecht, Tine; Robustini, Carolina;
Asensio Ramos, Andrés
Bibcode: 2019A&A...632A.112Y
Altcode: 2019arXiv191013279Y
We analyze high-resolution spectropolarimetric observations of a
flux-emerging region (FER) in order to understand its magnetic and
kinematic structure. Our spectropolarimetric observations in the He
I 10830 Å spectral region of a FER were recorded with GRIS at the
1.5 m aperture GREGOR telescope. A Milne-Eddington-based inversion
code was employed to extract the photospheric information of the Si I
spectral line, whereas the He I triplet line was analyzed with the Hazel
inversion code, which takes into account the joint action of the Hanle
and the Zeeman effects. The spectropolarimetric analysis of the Si I
line reveals a complex magnetic structure near the vicinity of the FER,
where a weak (350-600 G) and horizontal magnetic field was observed. In
contrast to the photosphere, the analysis of the He I triplet presents
a smooth variation of the magnetic field vector (ranging from 100 to
400 G) and velocities across the FER. Moreover, we find supersonic
downflows of ∼40 km s-1 appearing near the foot points
of loops connecting two pores of opposite polarity, whereas strong
upflows of 22 km s-1 appear near the apex of the loops. At
the location of supersonic downflows in the chromosphere, we observed
downflows of 3 km s-1 in the photosphere. Furthermore,
nonforce-free field extrapolations were performed separately at
two layers in order to understand the magnetic field topology of
the FER. We determine, using extrapolations from the photosphere and
the observed chromospheric magnetic field, that the average formation
height of the He I triplet line is ∼2 Mm from the solar surface. The
reconstructed loops using photospheric extrapolations along an arch
filament system have a maximum height of ∼10.5 Mm from the solar
surface with a foot-point separation of ∼19 Mm, whereas the loops
reconstructed using chromospheric extrapolations reach around ∼8.4
Mm above the solar surface with a foot-point separation of ∼16 Mm at
the chromospheric height. The magnetic topology in the FER suggests
the presence of small-scale loops beneath the large loops. Under
suitable conditions, due to magnetic reconnection, these loops can
trigger various heating events in the vicinity of the FER.
Title: Dissecting bombs and bursts: non-LTE inversions of
low-atmosphere reconnection in SST and IRIS observations
Authors: Vissers, G. J. M.; de la Cruz Rodríguez, J.; Libbrecht,
T.; Rouppe van der Voort, L. H. M.; Scharmer, G. B.; Carlsson, M.
Bibcode: 2019A&A...627A.101V
Altcode: 2019arXiv190502035V
Ellerman bombs and UV bursts are transient brightenings that are
ubiquitously observed in the lower atmospheres of active and emerging
flux regions. As they are believed to pinpoint sites of magnetic
reconnection in reconfiguring fields, understanding their occurrence
and detailed evolution may provide useful insight into the overall
evolution of active regions. Here we present results from inversions
of SST/CRISP and CHROMIS, as well as IRIS data of such transient
events. Combining information from the Mg II h & k, Si IV, and Ca
II 8542 Å and Ca II H & K lines, we aim to characterise their
temperature and velocity stratification, as well as their magnetic
field configuration. We find average temperature enhancements of
a few thousand kelvin, close to the classical temperature minimum
and similar to previous studies, but localised peak temperatures
of up to 10 000-15 000 K from Ca II inversions. Including Mg II
appears to generally dampen these temperature enhancements to below
8000 K, while Si IV requires temperatures in excess of 10 000 K at
low heights, but may also be reproduced with secondary temperature
enhancements of 35 000-60 000 K higher up. However, reproducing Si
IV comes at the expense of overestimating the Mg II emission. The
line-of-sight velocity maps show clear bi-directional jet signatures
for some events and strong correlation with substructure in the
intensity images in general. Absolute line-of-sight velocities range
between 5 and 20 km s-1 on average, with slightly larger
velocities towards, rather than away from, the observer. The inverted
magnetic field parameters show an enhancement of the horizontal
field co-located with the brightenings at heights similar to that of
the temperature increase. We are thus able to largely reproduce the
observational properties of Ellerman bombs with the UV burst signature
(e.g. intensities, profile asymmetries, morphology, and bi-directional
jet signatures), with temperature stratifications peaking close
to the classical temperature minimum. Correctly modelling the Si
IV emission in agreement with all other diagnostics is however an
outstanding issue and remains paramount in explaining its apparent
coincidence with Hα emission. Fine-tuning the approach (accounting for
resolution differences, fitting localised temperature enhancements,
and/or performing spatially coupled inversions) is likely necessary
in order to obtain better agreement between all considered diagnostics.
Title: Chromospheric condensations and magnetic field in a C3.6-class
flare studied via He I D3 spectro-polarimetry
Authors: Libbrecht, Tine; de la Cruz Rodríguez, Jaime; Danilovic,
Sanja; Leenaarts, Jorrit; Pazira, Hiva
Bibcode: 2019A&A...621A..35L
Altcode: 2018arXiv180606880L
Context. Magnetic reconnection during flares takes place in the
corona, but a substantial part of flare energy is deposited in
the chromosphere. However, high-resolution spectro-polarimetric
chromospheric observations of flares are very rare. The most used
observables are Ca II 8542 Å and He I 10830 Å.
Aims:
We aim to study the chromosphere during a C3.6 class flare via
spectro-polarimetric observations of the He I D3 line.
Methods: We present the first SST/CRISP spectro-polarimetric
observations of He I D3. We analyzed the data using the
inversion code HAZEL, and estimate the line-of-sight velocity and
the magnetic field vector.
Results: Strong He I D3
emission at the flare footpoints, as well as strong He I D3
absorption profiles tracing the flaring loops are observed during the
flare. The He I D3 traveling emission kernels at the flare
footpoints exhibit strong chromospheric condensations of up to ∼60
km s-1 at their leading edge. Our observations suggest that
such condensations result in shocking the deep chromosphere, causing
broad and modestly blueshifted He I D3 profiles indicating
subsequent upflows. A strong and rather vertical magnetic field of up
to ∼2500 G is measured in the flare footpoints, confirming that the He
I D3 line is likely formed in the deep chromosphere at those
locations. We provide chromospheric line-of-sight velocity and magnetic
field maps obtained via He I D3 inversions. We propose a
fan-spine configuration as the flare magnetic field topology.
Conclusions: The He I D3 line is an excellent diagnostic to
study the chromosphere during flares. The impact of strong condensations
on the deep chromosphere has been observed. Detailed maps of the flare
dynamics and the magnetic field are obtained.
Title: The diagnostic potential of the He I D3 spectral line in the
solar atmosphere
Authors: Libbrecht, Tine
Bibcode: 2019PhDT.......127L
Altcode:
No abstract at ADS
Title: Observations of Ellerman bomb emission features in He I
D3 and He I 10 830 Å
Authors: Libbrecht, Tine; Joshi, Jayant; de la Cruz Rodríguez, Jaime;
Leenaarts, Jorrit; Ramos, Andrés Asensio
Bibcode: 2017A&A...598A..33L
Altcode: 2016arXiv161001321L
Context. Ellerman bombs (EBs) are short-lived emission features,
characterised by extended wing emission in hydrogen Balmer lines. Until
now, no distinct signature of EBs has been found in the He I 10 830 Å
line, and conclusive observations of EBs in He I D3 have
never been reported.
Aims: We aim to study the signature of
EBs in neutral helium triplet lines.
Methods: The observations
consisted of ten consecutive SST/TRIPPEL raster scans close to the
limb, featuring the Hβ, He I D3 and He I 10 830 Å spectral
regions. We also obtained raster scans with IRIS and made use of the
SDO/AIA 1700 Å channel. We used Hazel to invert the neutral helium
triplet lines.
Results: Three EBs in our data show distinct
emission signatures in neutral helium triplet lines, most prominently
visible in the He I D3 line. The helium lines have two
components: a broad and blueshifted emission component associated with
the EB, and a narrower absorption component formed in the overlying
chromosphere. One of the EBs in our data shows evidence of strong
velocity gradients in its emission component. The emission component of
the other two EBs could be fitted using a constant slab. Our analysis
hints towards thermal Doppler motions having a large contribution to
the broadening for helium and IRIS lines. We conclude that the EBs
must have high temperatures to exhibit emission signals in neutral
helium triplet lines. An order of magnitude estimate places our
observed EBs in the range of T 2 × 104-105
K.