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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 D<SUB>3</SUB> 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
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 D<SUB>3</SUB> and He I 10 830 Å
lines, suggesting that their temperatures are higher than previously
expected. <BR /> Aims: We aim to explain the line formation of He I
D<SUB>3</SUB> and He I 10 830 Å in small-scale reconnection events. <BR
/> Methods: We used a simulated EB in a Bifrost-generated radiative
magnetohydrodynamics snapshot. The resulting He I D<SUB>3</SUB> 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 D<SUB>3</SUB> obtained at the Swedish Solar Telescope
with the TRI-Port Polarimetric Echelle-Littrow Spectrograph. <BR />
Results: Emission in He I D<SUB>3</SUB> 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
D<SUB>3</SUB> 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 × 10<SUP>3</SUP> K
to 10<SUP>6</SUP> 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 D<SUB>3</SUB>
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 × 10<SUP>4</SUP> − 2 × 10<SUP>6</SUP>
K and electron densities between 10<SUP>11</SUP> and 10<SUP>13</SUP>
cm<SUP>−3</SUP>. The synthetic emission signals are a result of
coupling to local conditions in a thin shell around the EB, with
temperatures between 7 × 10<SUP>3</SUP> and 10<SUP>4</SUP> K and
electron densities ranging from ∼10<SUP>12</SUP> to 10<SUP>13</SUP>
cm<SUP>−3</SUP>. This shows that both strong non-LTE and thermal
processes play a role in the formation of He I D<SUB>3</SUB> and
He I 10 830 Å in the synthetic EB/UV burst that we studied. <BR />
Conclusions: In conclusion, the synthetic He I D<SUB>3</SUB> and He I 10
830 Å emission signatures are an indicator of temperatures of at least
2 × 10<SUP>4</SUP> K; in this case, as high as ∼10<SUP>6</SUP> K.
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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
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<SUP>-1</SUP> appearing near the foot points
of loops connecting two pores of opposite polarity, whereas strong
upflows of 22 km s<SUP>-1</SUP> appear near the apex of the loops. At
the location of supersonic downflows in the chromosphere, we observed
downflows of 3 km s<SUP>-1</SUP> 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.
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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.
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<SUP>-1</SUP> 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.
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Title: Chromospheric condensations and magnetic field in a C3.6-class
flare studied via He I D<SUB>3</SUB> spectro-polarimetry
Authors: Libbrecht, Tine; de la Cruz Rodríguez, Jaime; Danilovic,
Sanja; Leenaarts, Jorrit; Pazira, Hiva
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. <BR /> Aims:
We aim to study the chromosphere during a C3.6 class flare via
spectro-polarimetric observations of the He I D<SUB>3</SUB> line. <BR
/> Methods: We present the first SST/CRISP spectro-polarimetric
observations of He I D<SUB>3</SUB>. We analyzed the data using the
inversion code HAZEL, and estimate the line-of-sight velocity and
the magnetic field vector. <BR /> Results: Strong He I D<SUB>3</SUB>
emission at the flare footpoints, as well as strong He I D<SUB>3</SUB>
absorption profiles tracing the flaring loops are observed during the
flare. The He I D<SUB>3</SUB> traveling emission kernels at the flare
footpoints exhibit strong chromospheric condensations of up to ∼60
km s<SUP>-1</SUP> at their leading edge. Our observations suggest that
such condensations result in shocking the deep chromosphere, causing
broad and modestly blueshifted He I D<SUB>3</SUB> 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 D<SUB>3</SUB> 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 D<SUB>3</SUB> inversions. We propose a
fan-spine configuration as the flare magnetic field topology. <BR />
Conclusions: The He I D<SUB>3</SUB> 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.
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Title: The diagnostic potential of the He I D3 spectral line in the
solar atmosphere
Authors: Libbrecht, Tine
2019PhDT.......127L Altcode:
No abstract at ADS
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Title: Observations of Ellerman bomb emission features in He I
D<SUB>3</SUB> and He I 10 830 Å
Authors: Libbrecht, Tine; Joshi, Jayant; de la Cruz Rodríguez, Jaime;
Leenaarts, Jorrit; Ramos, Andrés Asensio
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 D<SUB>3</SUB> have
never been reported. <BR /> Aims: We aim to study the signature of
EBs in neutral helium triplet lines. <BR /> Methods: The observations
consisted of ten consecutive SST/TRIPPEL raster scans close to the
limb, featuring the Hβ, He I D<SUB>3</SUB> 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. <BR /> Results: Three EBs in our data show distinct
emission signatures in neutral helium triplet lines, most prominently
visible in the He I D<SUB>3</SUB> 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 × 10<SUP>4</SUP>-10<SUP>5</SUP>
K. <P />Movies associated to Figs. 3-5 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201629266/olm">http://www.aanda.org</A>
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Title: Ellerman bomb emission features in He I D3 and He I 10830:
observations and modelling
Authors: Libbrecht, Tine
2017psio.confE..51L Altcode:
No abstract at ADS
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Title: The cause of spatial structure in solar He I 1083 nm multiplet
images
Authors: Leenaarts, Jorrit; Golding, Thomas; Carlsson, Mats; Libbrecht,
Tine; Joshi, Jayant
2016A&A...594A.104L Altcode: 2016arXiv160800838L
Context. The He I 1083 nm is a powerful diagnostic for inferring
properties of the upper solar chromosphere, in particular for the
magnetic field. The basic formation of the line in one-dimensional
models is well understood, but the influence of the complex
three-dimensional structure of the chromosphere and corona has however
never been investigated. This structure must play an essential role
because images taken in He I 1083 nm show structures with widths
down to 100 km. <BR /> Aims: We aim to understand the effect of
the three-dimensional temperature and density structure in the
solar atmosphere on the formation of the He I 1083 nm line. <BR />
Methods: We solved the non-LTE radiative transfer problem assuming
statistical equilibrium for a simple nine-level helium atom that
nevertheless captures all essential physics. As a model atmosphere we
used a snapshot from a 3D radiation-MHD simulation computed with the
Bifrost code. Ionising radiation from the corona was self-consistently
taken into account. <BR /> Results: The emergent intensity in the He
I 1083 nm is set by the source function and the opacity in the upper
chromosphere. The former is dominated by scattering of photospheric
radiation and does not vary much with spatial location. The latter
is determined by the photonionisation rate in the He I ground state
continuum, as well as the electron density in the chromosphere. The
spatial variation of the flux of ionising radiation is caused
by the spatially-structured emissivity of the ionising photons
from material at T ≈ 100 kK in the transition region. The hotter
coronal material produces more ionising photons, but the resulting
radiation field is smooth and does not lead to small-scale variation
of the UV flux. The corrugation of the transition region further
increases the spatial variation of the amount of UV radiation in the
chromosphere. Finally we find that variations in the chromospheric
electron density also cause strong variation in He I 1083 nm
opacity. We compare our findings to observations using SST, IRIS and
SDO/AIA data. <P />A movie associated to Fig. 4 is available at <A
href="http://www.aanda.org/10.1051/0004-6361/201628490/olm">http://www.aanda.org</A>
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Title: The diagnostic potential of the He I D3 spectral line in the
solar atmosphere
Authors: Libbrecht, Tine
2016PhDT.......231L Altcode:
The research question of my PhD is in a way a simple one: what
can observations of the He I D3 line teach us about the solar
chromosphere? This optical spectral line at 5876 Å is generally
formed in the upper chromosphere, and is sensitive to the local
magnetic field. The He I D3 line is also indirectly sensitive to
heating of the transition region and corona, since it is resulting
from a transition that occurs between levels in the triplet system
of neutral helium. These levels are generally populated via an
ionization-recombination mechanism under the influence of EUV radiation
originating in the transition region and corona. <P />The He I D3 line
was used as a flare diagnostic in the seventies and in the subsequent
decades also to measure magnetic fields in prominences. However, due
to the poor spatial resolution and low signal-to-noise of that data,
almost exclusively off-limb targets have been studied. The on-disk
absorption of He I D3 is very weak and localized. Recent instrumental
developments allow for the acquisition of high spatial resolution
on-disk spectroscopic and spectro-polarimetric data of He I D3 with
different instruments at the SST, opening the possibility of studying
all types of targets in the chromosphere in a new light. During
my PhD, I have focused on the study of reconnection targets via
high-resolution observations of He I D3 with TRIPPEL and CRISP at the
SST, in co-observation with space-borne instruments. Subsequently, a
theoretical study has aimed at in-depth understanding of He I D3 line
formation in small-scale reconnection events. <P />The data which I
have obtained and analyzed during my PhD has provided new insights in
Ellerman bombs and flares. Our He I D3 observations have suggested that
the temperature of Ellerman Bombs is higher than 2×1^04 K based on the
discovery of helium emission signatures in these events. This result
is unexpected, since previous modeling in the literature estimates
the temperatures of Ellerman Bombs below 10^4 K. Subsequently, 3D
non-LTE radiative transfer calculations have revealed the detailed
physical mechanisms to generate He I D3 emission in these events. The
calculations also confirmed that temperatures between 2×10^4 - 10^6
K are required to populate the helium triplet levels. <P />In the
context of flares, we measured strong downflows in the chromosphere
via He I D3, revealing detailed dynamics in the deep atmosphere during
a flare. Spectro-polarimetry was used to measure the magnetic field
during a flare and to propose its magnetic topology. In conclusion,
the He I D3 line is an excellent probe for reconnection targets in
the solar atmosphere. Detailed dynamics as well as the magnetic field
configuration can be derived using the line. Our findings encourage the
use of the He I D3 spectral line as a diagnostic for the chromosphere
and open up a range of applications that is yet to be exploited.
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Title: The high-redshift gamma-ray burst GRB 140515A. A comprehensive
X-ray and optical study
Authors: Melandri, A.; Bernardini, M. G.; D'Avanzo, P.;
Sánchez-Ramírez, R.; Nappo, F.; Nava, L.; Japelj, J.; de Ugarte
Postigo, A.; Oates, S.; Campana, S.; Covino, S.; D'Elia, V.; Ghirlanda,
G.; Gafton, E.; Ghisellini, G.; Gnedin, N.; Goldoni, P.; Gorosabel, J.;
Libbrecht, T.; Malesani, D.; Salvaterra, R.; Thöne, C. C.; Vergani,
S. D.; Xu, D.; Tagliaferri, G.
2015A&A...581A..86M Altcode: 2015arXiv150603079M
High-redshift gamma-ray bursts (GRBs) offer several advantages
when studying the distant Universe, providing unique information
about the structure and properties of the galaxies in which they
exploded. Spectroscopic identification with large ground-based
telescopes has improved our knowledge of this kind of distant events. We
present the multi-wavelength analysis of the high-zSwift GRB GRB 140515A
(z = 6.327). The best estimate of the neutral hydrogen fraction of the
intergalactic medium towards the burst is x<SUB>HI</SUB> ≤ 0.002. The
spectral absorption lines detected for this event are the weakest
lines ever observed in GRB afterglows, suggesting that GRB 140515A
exploded in a very low-density environment. Its circum-burst medium
is characterised by an average extinction (A<SUB>V</SUB> ~ 0.1) that
seems to be typical of z ≥ 6 events. The observed multi-band light
curves are explained either with a very hard injected spectrum (p =
1.7) or with a multi-component emission (p = 2.1). In the second case
a long-lasting central engine activity is needed in order to explain
the late time X-ray emission. The possible origin of GRB 140515A in a
Pop III (or in a Pop II star with a local environment enriched by Pop
III) massive star is unlikely. <P />Based on observations collected
at the European Southern Observatory, ESO, the VLT/Kueyen telescope,
Paranal, Chile (proposal code: 093.A-0069), on observations made
with the Nordic Optical Telescope, operated by the Nordic Optical
Telescope Scientific Association at the Observatorio del Roque de
los Muchachos, La Palma, Spain, of the Instituto de Astrofísica de
Canarias (programme 49-008), and on observations made with the Italian
3.6-m Telescopio Nazionale Galileo (TNG), operated by the Fundación
Galileo Galilei of the INAF (Instituto Nazionale di Astrofisica)
at the Spanish Observatorio del Roque de los Muchachos, La Palma,
Spain, of the Instituto de Astrofísica de Canarias (programme
A26TAC_63).Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201526660/olm">http://www.aanda.org</A>
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Title: GRB 140512A: Optical observations from the 2.5 m NOT.
Authors: de Ugarte Postigo, A.; Gorosabel, J.; Xu, D.; Kruehler, T.;
Djupvik, A. A.; Gafton, E.; Libbrecht, T.
2014GCN.16253....1D Altcode: 2014GCN..16253...1D
No abstract at ADS
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Title: GRB 140515A: Optical observations from the 2.5 m NOT.
Authors: de Ugarte Postigo, A.; Gorosabel, J.; Xu, D.; Fynbo, J. P. U.;
Jakobsson, P.; Malesani, D.; Tanvir, N. R.; Gafton, E.; Libbrecht, T.
2014GCN.16278....1D Altcode: 2014GCN..16278...1D
No abstract at ADS
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Title: GRB 150416A: NOT optical observations.
Authors: Gorosabel, J.; Xu, D.; de Ugarte Postigo, A.; Fynbo, J. P. U.;
Jakobsson, P.; Malesani, D.; Tanvir, N. R.; Watson, D.; Gafton, E.;
Libbrecht, T.
2014GCN.16290....1G Altcode: 2014GCN..16290...1G
No abstract at ADS
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Title: GRB 140512A: Redshift from NOT.
Authors: de Ugarte Postigo, A.; Gorosabel, J.; Xu, D.; Malesani, D.;
Leloudas, G.; Jakobsson, P.; Kruehler, T.; Djupvik, A. A.; Gafton,
E.; Libbrecht, T.
2014GCN.16310....1D Altcode: 2014GCN..16310...1D
No abstract at ADS
