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Author name code: nobrega
ADS astronomy entries on 2022-09-14
author:"Nobrega-Siverio, Daniel"
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Title: A 2D Model for Coronal Bright Points: Association with
Spicules, UV Bursts, Surges, and EUV Coronal Jets
Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.
2022ApJ...935L..21N Altcode: 2022arXiv220804308N
Coronal bright points (CBPs) are ubiquitous structures in the
solar atmosphere composed of hot small-scale loops observed in
extreme-ultraviolet (EUV) or X-rays in the quiet Sun and coronal
holes. They are key elements to understanding the heating of
the corona; nonetheless, basic questions regarding their heating
mechanisms, the chromosphere underneath, or the effects of flux
emergence in these structures remain open. We have used the
Bifrost code to carry out a 2D experiment in which a coronal-hole
magnetic null-point configuration evolves perturbed by realistic
granulation. To compare with observations, synthetic SDO/AIA, Solar
Orbiter EUI-HRI, and IRIS images have been computed. The experiment
shows the self-consistent creation of a CBP through the action of
stochastic granular motions alone, mediated by magnetic reconnection
in the corona. The reconnection is intermittent and oscillatory,
and it leads to coronal and transition-region temperature loops that
are identifiable in our EUV/UV observables. During the CBP lifetime,
convergence and cancellation at the surface of its underlying opposite
polarities takes place. The chromosphere below the CBP shows a number
of peculiar features concerning its density and the spicules in
it. The final stage of the CBP is eruptive: Magnetic flux emergence
at the granular scale disrupts the CBP topology, leading to different
ejections, such as UV bursts, surges, and EUV coronal jets. Apart
from explaining observed CBP features, our results pave the way for
further studies combining simulations and coordinated observations in
different atmospheric layers.
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Title: Solar surges related to UV bursts: Characterization through
k-means, inversions, and density diagnostics
Authors: Nóbrega Siverio, Daniel; Guglielmino, Salvatore Luigi;
Sainz Dalda, A.
2022cosp...44.2530N Altcode:
Surges are dynamic, cool and dense ejections typically observed in
chromospheric lines and closely related to other solar phenomena
like UV bursts or coronal jets. Even though surges have been observed
for decades now, fundamental questions regarding the temperature and
density distribution, as well as their connection and impact on upper
layers of the solar atmosphere remain open. Our aim is to characterize
the chromospheric and transition region properties of these phenomena
taking advantage of high-resolution observations combined with advanced
techniques. We have analyzed four surges that appear related to UV
bursts observed with the Interface Region Imaging Spectrograph (IRIS)
on 2016 April. We have studied the mid- and low-chromosphere of the
surges by getting their representative Mg II h$&$k line profiles
through the k-means algorithm and performing inversions on them using
the STIC code. We have also studied the far-UV spectra, focusing
on the O IV 1399.8 and 1401.2 Å lines, and carrying out density
diagnostics. We obtain that the mid- and low-chromosphere of the surges
are characterized by temperatures between 5.5 and 6.9 kK, electronic
number densities from ∼1.5$\times$10$^{11}$ to 2.5$\times$10$ ^{12}$
cm$^{‑3}$, and line-of-sight velocities of a few km/s at optical
depths ranging from log$ _{10}$($\tau$)=‑6.0 to ‑3.2. We find,
for the first time, observational evidence of O IV emission within
the surges, indicating that these phenomena have a transition region
counterpart even in the weakest lines. The O IV emitting layers of the
surges have an electron number density between 2.5$\times$10$ ^{10}$
and 10$ ^{12}$ cm$ ^{‑3}$.
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Title: A textbook example of magnetic flux emergence leading to EBs,
UV bursts, surges and EUV signatures
Authors: Cabello, Iballa; Moreno-Insertis, . Fernando, , Prof; Rouppe
van der Voort, Luc; Bose, Souvik; Nóbrega Siverio, Daniel
2022cosp...44.2531C Altcode:
Small-scale eruptive phenomena (like Ellerman bombs (EBs), UV bursts,
surges) constitute both a true challenge and an opportunity for
progress in understanding the solar atmosphere since they involve very
different layers from the photosphere to the low corona. In our work,
we are aiming to characterize small-scale eruptive phenomena related to
emerging flux regions. In particular, we use coordinated observations
from the Swedish $1-$m Solar Telescope (SST), the Interface Region
Imaging Spectrograph (IRIS) and the Solar Dynamics Observatory (SDO,
both HMI and AIA) to analyze an episode of magnetic flux emergence
in an enhanced network that leads to an EB, a UV burst, a cool surge,
and coronal signatures in the EUV. Through Milne-Eddington inversions
of the {\ion{Fe}{I}} 6302 \AA\ line observed with SST/CRISP we
obtain high-resolution (0.057"/pixel) magnetograms that allow us to
reliably measure the magnetic field at the photosphere. A comparison
with the corresponding SDO/HMI magnetograms reveals that this type
of small-scale events are barely discernible in low-resolution (1")
observations. During the emergence, a roundish dark bubble is visible
in {\ion{Ca}{II} K} 3933 \AA\ at the location where the two opposite
polarities of the emerging dipole are splitting apart. Several minutes
later, indirect evidence of reconnection is found above the positive
polarity of the dipole through the appearance of an EB in the wings
of the {H$\alpha$} 6563 \AA\ and {\ion{Ca}{II} K} 3933 \AA\ lines
from SST, and also in the SDO/AIA 1600 and 1700 \AA~data. Later,
a surge shows up as an elongated structure visible in absorption in
{H$\alpha$} and {\ion{Ca}{II} K}, extending over 12 Mm projected size
on the disk. The shape of the surge is also apparent as an absorption
feature in the SDO/AIA channels. Simultaneously with the surge (and
at the location where the EB had appeared earlier on) a UV burst
is clearly discernible as a strong and bright emission feature both
in IRIS/SJI 1400 and 2796 \AA. Interestingly, this UV burst also has
counterpart in SDO/AIA 94, 171, 193, 211, 304, and 335 \AA, meaning that
we can find multi-thermal plasma up to a few MK in the reconnection
site. This observation clearly shows the impact of the emergence of
new magnetic field from the photosphere through the chromosphere and
transition region and up into the corona. In addition, it provides an
illustrative case to test new realistic simulations.
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Title: On the relationship between spicules and coronal bright points
Authors: Bose, Souvik; De Pontieu, Bart; Rouppe van der Voort, Luc;
Nóbrega Siverio, Daniel
2022cosp...44.2522B Altcode:
Coronal bright points (CBPs) are a set of small-scale, lower coronal
loop systems connecting opposite magnetic polarities and are primarily
characterized by enhanced emission in the extreme ultraviolet (EUV)
wavelengths and X-rays. Being ubiquitous they are thought to play a
definite role in heating the solar corona. This study aims to explore
the chromospheric components associated with a CBP by focusing on
spicules and small-scaled flux emergence. We used high-resolution
observations in H$\beta$ and Fe I 617.3 nm spectral lines obtained
from the Swedish 1-m Solar Telescope (SST) in coordination with the
images acquired from the Atmospheric Imaging Assembly (AIA) instrument
on-board the Solar Dynamics Observatory (SDO). On-disk spicules were
automatically detected by employing advanced image processing techniques
on the Dopplergrams derived from H$\beta$, and Mile-Eddington inversions
of the Fe I 617.3 nm line provided the photospheric vector magnetic
field. The AIA co-observations were co-aligned to SST with the latter
serving as a reference. We find abundant occurrences of chromospheric
spicules close to the "footpoints" of the CBP. The orientation of the
spicules is predominantly aligned along with CBP loops which further
indicates that they form a fundamental part of the same magnetic
structure. Several examples of the spatio-temporal evolution indicate
that much of the chromospheric plasma is heated to coronal temperatures
implying that spicules potentially supply mass and energy to the CBP
loops. Furthermore, we study chromospheric and corresponding coronal
responses to two magnetic flux emergence events and their impact on the
dynamics of the CBP. This study presents unique and unambiguous evidence
that connects chromospheric spicular dynamics and flux emergence with
a CBP for the very first time using high-resolution observations.
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Title: Ambipolar diffusion: Self-similar solutions and MHD code
testing. Cylindrical symmetry
Authors: Moreno-Insertis, F.; Nóbrega-Siverio, D.; Priest, E. R.;
Hood, A. W.
2022A&A...662A..42M Altcode: 2022arXiv220306272M
Context. Ambipolar diffusion is a process occurring in partially
ionised astrophysical systems that imparts a complicated mathematical
and physical nature to Ohm's law. The numerical codes that solve the
magnetohydrodynamic (MHD) equations have to be able to deal with the
singularities that are naturally created in the system by the ambipolar
diffusion term. <BR /> Aims: The global aim is to calculate a set of
theoretical self-similar solutions to the nonlinear diffusion equation
with cylindrical symmetry that can be used as tests for MHD codes which
include the ambipolar diffusion term. <BR /> Methods: First, following
the general methods developed in the applied mathematics literature,
we obtained the theoretical solutions as eigenfunctions of a nonlinear
ordinary differential equation. Phase-plane techniques were used to
integrate through the singularities at the locations of the nulls,
which correspond to infinitely sharp current sheets. In the second
half of the paper, we consider the use of these solutions as tests
for MHD codes. To that end, we used the Bifrost code, thereby testing
the capabilities of these solutions as tests as well as (inversely) the
accuracy of Bifrost's recently developed ambipolar diffusion module. <BR
/> Results: The obtained solutions are shown to constitute a demanding,
but nonetheless viable, test for MHD codes that incorporate ambipolar
diffusion. Detailed tabulated runs of the solutions have been made
available at a public repository. The Bifrost code is able to reproduce
the theoretical solutions with sufficient accuracy up to very advanced
diffusive times. Using the code, we also explored the asymptotic
properties of our theoretical solutions in time when initially perturbed
with either small or finite perturbations. <BR /> Conclusions: The
functions obtained in this paper are relevant as physical solutions
and also as tests for general MHD codes. They provide a more stringent
and general test than the simple Zeldovich-Kompaneets-Barenblatt-Pattle
solution. <P />Movies associated to Figs. 4 and 7 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202141449/olm">https://www.aanda.org</A>
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Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE). II. Flares and Eruptions
Authors: Cheung, Mark C. M.; Martínez-Sykora, Juan; Testa, Paola;
De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
Vanessa; Kerr, Graham S.; Reeves, Katharine K.; Fletcher, Lyndsay; Jin,
Meng; Nóbrega-Siverio, Daniel; Danilovic, Sanja; Antolin, Patrick;
Allred, Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward;
Longcope, Dana; Takasao, Shinsuke; DeRosa, Marc L.; Boerner, Paul;
Jaeggli, Sarah; Nitta, Nariaki V.; Daw, Adrian; Carlsson, Mats; Golub,
Leon; The
2022ApJ...926...53C Altcode: 2021arXiv210615591C
Current state-of-the-art spectrographs cannot resolve the fundamental
spatial (subarcseconds) and temporal (less than a few tens of
seconds) scales of the coronal dynamics of solar flares and eruptive
phenomena. The highest-resolution coronal data to date are based on
imaging, which is blind to many of the processes that drive coronal
energetics and dynamics. As shown by the Interface Region Imaging
Spectrograph for the low solar atmosphere, we need high-resolution
spectroscopic measurements with simultaneous imaging to understand the
dominant processes. In this paper: (1) we introduce the Multi-slit Solar
Explorer (MUSE), a spaceborne observatory to fill this observational
gap by providing high-cadence (<20 s), subarcsecond-resolution
spectroscopic rasters over an active region size of the solar transition
region and corona; (2) using advanced numerical models, we demonstrate
the unique diagnostic capabilities of MUSE for exploring solar coronal
dynamics and for constraining and discriminating models of solar flares
and eruptions; (3) we discuss the key contributions MUSE would make
in addressing the science objectives of the Next Generation Solar
Physics Mission (NGSPM), and how MUSE, the high-throughput Extreme
Ultraviolet Solar Telescope, and the Daniel K Inouye Solar Telescope
(and other ground-based observatories) can operate as a distributed
implementation of the NGSPM. This is a companion paper to De Pontieu
et al., which focuses on investigating coronal heating with MUSE.
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Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE). I. Coronal Heating
Authors: De Pontieu, Bart; Testa, Paola; Martínez-Sykora, Juan;
Antolin, Patrick; Karampelas, Konstantinos; Hansteen, Viggo; Rempel,
Matthias; Cheung, Mark C. M.; Reale, Fabio; Danilovic, Sanja; Pagano,
Paolo; Polito, Vanessa; De Moortel, Ineke; Nóbrega-Siverio, Daniel;
Van Doorsselaere, Tom; Petralia, Antonino; Asgari-Targhi, Mahboubeh;
Boerner, Paul; Carlsson, Mats; Chintzoglou, Georgios; Daw, Adrian;
DeLuca, Edward; Golub, Leon; Matsumoto, Takuma; Ugarte-Urra, Ignacio;
McIntosh, Scott W.; the MUSE Team
2022ApJ...926...52D Altcode: 2021arXiv210615584D
The Multi-slit Solar Explorer (MUSE) is a proposed mission composed of
a multislit extreme ultraviolet (EUV) spectrograph (in three spectral
bands around 171 Å, 284 Å, and 108 Å) and an EUV context imager (in
two passbands around 195 Å and 304 Å). MUSE will provide unprecedented
spectral and imaging diagnostics of the solar corona at high spatial
(≤0.″5) and temporal resolution (down to ~0.5 s for sit-and-stare
observations), thanks to its innovative multislit design. By obtaining
spectra in four bright EUV lines (Fe IX 171 Å, Fe XV 284 Å, Fe XIX-Fe
XXI 108 Å) covering a wide range of transition regions and coronal
temperatures along 37 slits simultaneously, MUSE will, for the first
time, "freeze" (at a cadence as short as 10 s) with a spectroscopic
raster the evolution of the dynamic coronal plasma over a wide range of
scales: from the spatial scales on which energy is released (≤0.″5)
to the large-scale (~170″ × 170″) atmospheric response. We use
numerical modeling to showcase how MUSE will constrain the properties of
the solar atmosphere on spatiotemporal scales (≤0.″5, ≤20 s) and
the large field of view on which state-of-the-art models of the physical
processes that drive coronal heating, flares, and coronal mass ejections
(CMEs) make distinguishing and testable predictions. We describe the
synergy between MUSE, the single-slit, high-resolution Solar-C EUVST
spectrograph, and ground-based observatories (DKIST and others), and
the critical role MUSE plays because of the multiscale nature of the
physical processes involved. In this first paper, we focus on coronal
heating mechanisms. An accompanying paper focuses on flares and CMEs.
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Title: Solar surges related to UV bursts. Characterization through
k-means, inversions, and density diagnostics
Authors: Nóbrega-Siverio, D.; Guglielmino, S. L.; Sainz Dalda, A.
2021A&A...655A..28N Altcode: 2021arXiv210813960N
Context. Surges are cool and dense ejections typically observed in
chromospheric lines and closely related to other solar phenomena
such as UV bursts or coronal jets. Even though surges have been
observed for decades now, questions regarding their fundamental
physical properties such as temperature and density, as well as their
impact on upper layers of the solar atmosphere remain open. <BR />
Aims: Our aim is to address the current lack of inverted models and
diagnostics of surges, as well as to characterize the chromospheric
and transition region plasma of these phenomena. <BR /> Methods: We
have analyzed an episode of recurrent surges related to UV bursts
observed with the Interface Region Imaging Spectrograph (IRIS)
in April 2016. The mid- and low-chromosphere of the surges were
unprecedentedly examined by getting their representative Mg IIh&k
line profiles through the k-means algorithm and performing inversions
on them using the state-of-the-art STiC code. We have studied the
far-UV spectra focusing on the O IV 1399.8 Å and 1401.2 Å lines,
which were previously unexplored for surges, carrying out density
diagnostics to determine the transition region properties of these
ejections. We have also used numerical experiments performed with
the Bifrost code for comparisons. <BR /> Results: Thanks to the
k-means clustering, we reduced the number of Mg IIh&k profiles
to invert by a factor 43.2. The inversions of the representative
profiles show that the mid- and low-chromosphere of the surges are
characterized, with a high degree of reliability, by temperatures
mainly around T = 6 kK at −6.0 ≤ log<SUB>10</SUB>(τ)≤
− 3.2. For the electronic number density, n<SUB>e</SUB>, and
line-of-sight velocity, V<SUB>LOS</SUB>, the most reliable results
from the inversions are within −6.0 ≤ log<SUB>10</SUB>(τ)≤
− 4.8, with n<SUB>e</SUB> ranging from ∼1.6 × 10<SUP>11</SUP>
cm<SUP>−3</SUP> up to 10<SUP>12</SUP> cm<SUP>−3</SUP>, and
V<SUB>LOS</SUB> of a few km s<SUP>−1</SUP>. We find, for the first
time, observational evidence of enhanced O IV emission within the
surges, indicating that these phenomena have a considerable impact
on the transition region even in the weakest far-UV lines. The O IV
emitting layers of the surges have an electron number density ranging
from 2.5 × 10<SUP>10</SUP> cm<SUP>−3</SUP> to 10<SUP>12</SUP>
cm<SUP>−3</SUP>. The numerical simulations provide theoretical
support in terms of the topology and location of the O IV emission
within the surges. <P />Movie associated with Fig. 2 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202141472/olm">https://www.aanda.org</A>
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Title: Evidence of the multi-thermal nature of spicular
downflows. Impact on solar atmospheric heating
Authors: Bose, Souvik; Rouppe van der Voort, Luc; Joshi, Jayant;
Henriques, Vasco M. J.; Nóbrega-Siverio, Daniel; Martínez-Sykora,
Juan; De Pontieu, Bart
2021A&A...654A..51B Altcode: 2021arXiv210802153B
Context. Spectroscopic observations of the emission lines formed in the
solar transition region commonly show persistent downflows on the order
of 10−15 km s<SUP>−1</SUP>. The cause of such downflows, however, is
still not fully clear and has remained a matter of debate. <BR /> Aims:
We aim to understand the cause of such downflows by studying the coronal
and transition region responses to the recently reported chromospheric
downflowing rapid redshifted excursions (RREs) and their impact on the
heating of the solar atmosphere. <BR /> Methods: We have used two sets
of coordinated data from the Swedish 1 m Solar Telescope, the Interface
Region Imaging Spectrograph, and the Solar Dynamics Observatory for
analyzing the response of the downflowing RREs in the transition
region and corona. To provide theoretical support, we use an already
existing 2.5D magnetohydrodynamic simulation of spicules performed
with the Bifrost code. <BR /> Results: We find ample occurrences of
downflowing RREs and show several examples of their spatio-temporal
evolution, sampling multiple wavelength channels ranging from the cooler
chromospheric to the hotter coronal channels. These downflowing features
are thought to be likely associated with the returning components of
the previously heated spicular plasma. Furthermore, the transition
region Doppler shifts associated with them are close to the average
redshifts observed in this region, which further implies that these
flows could (partly) be responsible for the persistent downflows
observed in the transition region. We also propose two mechanisms -
(i) a typical upflow followed by a downflow and (ii) downflows along a
loop -from the perspective of a numerical simulation that could explain
the ubiquitous occurrence of such downflows. A detailed comparison
between the synthetic and observed spectral characteristics reveals a
distinctive match and further suggests an impact on the heating of the
solar atmosphere. <BR /> Conclusions: We present evidence that suggests
that at least some of the downflowing RREs are the chromospheric
counterparts of the transition region and lower coronal downflows. <P
/>Movies associated to Figs. 1-3, 8, and 10 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202141404/olm">https://www.aanda.org</A>
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Title: A New View of the Solar Interface Region from the Interface
Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, Bart; Polito, Vanessa; Hansteen, Viggo; Testa,
Paola; Reeves, Katharine K.; Antolin, Patrick; Nóbrega-Siverio,
Daniel Elias; Kowalski, Adam F.; Martinez-Sykora, Juan; Carlsson,
Mats; McIntosh, Scott W.; Liu, Wei; Daw, Adrian; Kankelborg, Charles C.
2021SoPh..296...84D Altcode: 2021arXiv210316109D
The Interface Region Imaging Spectrograph (IRIS) has been obtaining
near- and far-ultraviolet images and spectra of the solar atmosphere
since July 2013. IRIS is the highest resolution observatory to provide
seamless coverage of spectra and images from the photosphere into the
low corona. The unique combination of near- and far-ultraviolet spectra
and images at sub-arcsecond resolution and high cadence allows the
tracing of mass and energy through the critical interface between the
surface and the corona or solar wind. IRIS has enabled research into the
fundamental physical processes thought to play a role in the low solar
atmosphere such as ion-neutral interactions, magnetic reconnection, the
generation, propagation, and dissipation of waves, the acceleration of
non-thermal particles, and various small-scale instabilities. IRIS has
provided insights into a wide range of phenomena including the discovery
of non-thermal particles in coronal nano-flares, the formation and
impact of spicules and other jets, resonant absorption and dissipation
of Alfvénic waves, energy release and jet-like dynamics associated
with braiding of magnetic-field lines, the role of turbulence and the
tearing-mode instability in reconnection, the contribution of waves,
turbulence, and non-thermal particles in the energy deposition during
flares and smaller-scale events such as UV bursts, and the role of flux
ropes and various other mechanisms in triggering and driving CMEs. IRIS
observations have also been used to elucidate the physical mechanisms
driving the solar irradiance that impacts Earth's upper atmosphere,
and the connections between solar and stellar physics. Advances in
numerical modeling, inversion codes, and machine-learning techniques
have played a key role. With the advent of exciting new instrumentation
both on the ground, e.g. the Daniel K. Inouye Solar Telescope (DKIST)
and the Atacama Large Millimeter/submillimeter Array (ALMA), and
space-based, e.g. the Parker Solar Probe and the Solar Orbiter, we aim
to review new insights based on IRIS observations or related modeling,
and highlight some of the outstanding challenges.
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Title: The chromospheric component of coronal bright points. Coronal
and chromospheric responses to magnetic-flux emergence
Authors: Madjarska, Maria S.; Chae, Jongchul; Moreno-Insertis,
Fernando; Hou, Zhenyong; Nóbrega-Siverio, Daniel; Kwak, Hannah;
Galsgaard, Klaus; Cho, Kyuhyoun
2021A&A...646A.107M Altcode: 2020arXiv201209426M
Context. We investigate the chromospheric counterpart of small-scale
coronal loops constituting a coronal bright point (CBP) and its
response to a photospheric magnetic-flux increase accompanied by
co-temporal CBP heating. <BR /> Aims: The aim of this study is
to simultaneously investigate the chromospheric and coronal layers
associated with a CBP, and in so doing, provide further understanding on
the heating of plasmas confined in small-scale loops. <BR /> Methods:
We used co-observations from the Atmospheric Imaging Assembly and
Helioseismic Magnetic Imager on board the Solar Dynamics Observatory,
together with data from the Fast Imaging Solar Spectrograph taken
in the Hα and Ca II 8542.1 Å lines. We also employed both linear
force-free and potential field extrapolation models to investigate
the magnetic topology of the CBP loops and the overlying corona,
respectively. We used a new multi-layer spectral inversion technique
to derive the temporal variations of the temperature of the Hα loops
(HLs). <BR /> Results: We find that the counterpart of the CBP, as
seen at chromospheric temperatures, is composed of a bundle of dark
elongated features named in this work Hα loops, which constitute an
integral part of the CBP loop magnetic structure. An increase in the
photospheric magnetic flux due to flux emergence is accompanied by
a rise of the coronal emission of the CBP loops, that is a heating
episode. We also observe enhanced chromospheric activity associated
with the occurrence of new HLs and mottles. While the coronal emission
and magnetic flux increases appear to be co-temporal, the response of
the Hα counterpart of the CBP occurs with a small delay of less than
3 min. A sharp temperature increase is found in one of the HLs and
in one of the CBP footpoints estimated at 46% and 55% with respect
to the pre-event values, also starting with a delay of less than 3
min following the coronal heating episode. The low-lying CBP loop
structure remains non-potential for the entire observing period. The
magnetic topological analysis of the overlying corona reveals the
presence of a coronal null point at the beginning and towards the end
of the heating episode. <BR /> Conclusions: The delay in the response
of the chromospheric counterpart of the CBP suggests that the heating
may have occurred at coronal heights. <P />Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202039329/olm">https://www.aanda.org</A>
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Title: High-resolution observations of the solar photosphere,
chromosphere, and transition region. A database of coordinated IRIS
and SST observations
Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Carlsson,
M.; de la Cruz Rodríguez, J.; Bose, S.; Chintzoglou, G.; Drews, A.;
Froment, C.; Gošić, M.; Graham, D. R.; Hansteen, V. H.; Henriques,
V. M. J.; Jafarzadeh, S.; Joshi, J.; Kleint, L.; Kohutova, P.;
Leifsen, T.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Ortiz, A.;
Pereira, T. M. D.; Popovas, A.; Quintero Noda, C.; Sainz Dalda, A.;
Scharmer, G. B.; Schmit, D.; Scullion, E.; Skogsrud, H.; Szydlarski,
M.; Timmons, R.; Vissers, G. J. M.; Woods, M. M.; Zacharias, P.
2020A&A...641A.146R Altcode: 2020arXiv200514175R
NASA's Interface Region Imaging Spectrograph (IRIS) provides
high-resolution observations of the solar atmosphere through ultraviolet
spectroscopy and imaging. Since the launch of IRIS in June 2013, we
have conducted systematic observation campaigns in coordination with
the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides
complementary high-resolution observations of the photosphere and
chromosphere. The SST observations include spectropolarimetric imaging
in photospheric Fe I lines and spectrally resolved imaging in the
chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present
a database of co-aligned IRIS and SST datasets that is open for
analysis to the scientific community. The database covers a variety
of targets including active regions, sunspots, plages, the quiet Sun,
and coronal holes.
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Title: Case study of multi-temperature coronal jets for emerging
flux MHD models
Authors: Joshi, Reetika; Chandra, Ramesh; Schmieder, Brigitte;
Moreno-Insertis, Fernando; Aulanier, Guillaume; Nóbrega-Siverio,
Daniel; Devi, Pooja
2020A&A...639A..22J Altcode: 2020arXiv200506064J
Context. Hot coronal jets are a basic observed feature of the solar
atmosphere whose physical origin is still actively debated. <BR />
Aims: We study six recurrent jets that occurred in active region NOAA
12644 on April 4, 2017. They are observed in all the hot filters
of AIA as well as cool surges in IRIS slit-jaw high spatial and
temporal resolution images. <BR /> Methods: The AIA filters allow us
to study the temperature and the emission measure of the jets using
the filter ratio method. We studied the pre-jet phases by analysing
the intensity oscillations at the base of the jets with the wavelet
technique. <BR /> Results: A fine co-alignment of the AIA and IRIS
data shows that the jets are initiated at the top of a canopy-like
double-chambered structure with cool emission on one and hot emission
on the other side. The hot jets are collimated in the hot temperature
filters, have high velocities (around 250 km s<SUP>-1</SUP>) and
are accompanied by cool surges and ejected kernels that both move
at about 45 km s<SUP>-1</SUP>. In the pre-phase of the jets, we find
quasi-periodic intensity oscillations at their base that are in phase
with small ejections; they have a period of between 2 and 6 min,
and are reminiscent of acoustic or magnetohydrodynamic waves. <BR />
Conclusions: This series of jets and surges provides a good case study
for testing the 2D and 3D magnetohydrodynamic emerging flux models. The
double-chambered structure that is found in the observations corresponds
to the regions with cold and hot loops that are in the models below
the current sheet that contains the reconnection site. The cool surge
with kernels is comparable with the cool ejection and plasmoids that
naturally appears in the models. <P />Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202037806/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: Ambipolar diffusion in the Bifrost code
Authors: Nóbrega-Siverio, D.; Martínez-Sykora, J.; Moreno-Insertis,
F.; Carlsson, M.
2020A&A...638A..79N Altcode: 2020arXiv200411927N
Context. Ambipolar diffusion is a physical mechanism related to the
drift between charged and neutral particles in a partially ionized
plasma that is key to many different astrophysical systems. However,
understanding its effects is challenging due to basic uncertainties
concerning relevant microphysical aspects and the strong constraints it
imposes on the numerical modeling. <BR /> Aims: Our aim is to introduce
a numerical tool that allows us to address complex problems involving
ambipolar diffusion in which, additionally, departures from ionization
equilibrium are important or high resolution is needed. The primary
application of this tool is for solar atmosphere calculations, but the
methods and results presented here may also have a potential impact
on other astrophysical systems. <BR /> Methods: We have developed a
new module for the stellar atmosphere Bifrost code that improves its
computational capabilities of the ambipolar diffusion term in the
generalized Ohm's law. This module includes, among other things,
collision terms adequate to processes in the coolest regions in
the solar chromosphere. As the main feature of the module, we have
implemented the super time stepping (STS) technique, which allows an
important acceleration of the calculations. We have also introduced
hyperdiffusion terms to guarantee the stability of the code. <BR />
Results: We show that to have an accurate value for the ambipolar
diffusion coefficient in the solar atmosphere it is necessary to
include as atomic elements in the equation of state not only hydrogen
and helium, but also the main electron donors like sodium, silicon,
and potassium. In addition, we establish a range of criteria to set
up an automatic selection of the free parameters of the STS method
that guarantees the best performance, optimizing the stability and
speed for the ambipolar diffusion calculations. We validate the STS
implementation by comparison with a self-similar analytical solution.
---------------------------------------------------------
Title: Quasi Periodic Oscillations in the Pre Phases of Recurrent
Jets Highlighting Plasmoids in Current Sheet
Authors: Joshi, Reetika; Chandra, Ramesh; Schmieder, Brigitte;
Aulanier, Guillaume; Devi, Pooja; Moreno-Insertis, Fernando;
Nóbrega-Siverio, Daniel
2020EGUGA..2222351J Altcode:
Solar jets observed at the limb are important to determine the location
of reconnection sites in the corona. In this study, we investigate
six recurrent hot and cool jets occurring in the active region NOAA
12644 as it is crossing the west limb on April 04, 2017. These jets
are observed in all the UV/EUV filters of SDO/AIA and in cooler
temperature formation lines in IRIS slit jaw images. The jets are
initiated at the top of a double chamber vault with cool loops on one
side and hot loops on the other side. The existence of such double
chamber vaults suggests the presence of emerging flux with cool
loops, the hot loops being the reconnected loops similarly as in the
models of Moreno-Insertiset al. 2008, 2013 and Nóbrega-Siverio et
al. 2016. In the preliminary phase of the main jets, quasi periodic
intensity oscillations accompanied by smaller jets are detected in the
bright current sheet between the vault and the preexisting magnetic
field. Individual kernels and plasmoids are ejected in open field lines
along the jets. Plasmoids may launch torsional Alfven waves and the
kernels would be the result of the untwist of the plasmoids in open
magnetic field as proposed in the model of Wyper et al. 2016.
---------------------------------------------------------
Title: Ion-neutral Interactions and Nonequilibrium Ionization in
the Solar Chromosphere
Authors: Martínez-Sykora, Juan; Leenaarts, Jorrit; De Pontieu,
Bart; Nóbrega-Siverio, Daniel; Hansteen, Viggo H.; Carlsson, Mats;
Szydlarski, Mikolaj
2020ApJ...889...95M Altcode: 2019arXiv191206682M
The thermal structure of the chromosphere is regulated through a
complex interaction of various heating processes, radiative cooling,
and the ionization degree of the plasma. Here, we study the impact on
the thermal properties of the chromosphere when including the combined
action of nonequilibrium ionization (NEI) of hydrogen and helium and
ion-neutral interaction effects. We have performed a 2.5D radiative
magnetohydrodynamic simulation using the Bifrost code. This model
includes ion-neutral interaction effects by solving the generalized
Ohm' s law (GOL) as well as NEI for hydrogen and helium. The GOL
equation includes ambipolar diffusion and the Hall term. We compare
this simulation with another simulation that computes the ionization in
local thermodynamic equilibrium (LTE) including ion-neutral interaction
effects. Our numerical models reveal substantial thermal differences
in magneto-acoustic shocks, the wake behind the shocks, spicules,
low-lying magnetic loops, and the transition region. In particular,
we find that heating through ambipolar diffusion in shock wakes is
substantially less efficient, while in the shock fronts themselves it
is more efficient, under NEI conditions than when assuming LTE.
---------------------------------------------------------
Title: Ellerman bombs and UV bursts: reconnection at different
atmospheric layers
Authors: Ortiz, Ada; Hansteen, Viggo H.; Nóbrega-Siverio, Daniel;
Rouppe van der Voort, Luc
2020A&A...633A..58O Altcode: 2019arXiv191010736O
The emergence of magnetic flux through the photosphere and into
the outer solar atmosphere produces, amongst other dynamical
phenomena, Ellerman bombs (EBs), which are observed in the wings of
Hα and are due to magnetic reconnection in the photosphere below
the chromospheric canopy. Signs of magnetic reconnection are also
observed in other spectral lines, typical of the chromosphere or the
transition region. An example are the ultraviolet (UV) bursts observed
in the transition region lines of Si IV and the upper chromospheric
lines of Mg II. In this work we analyze high-cadence, high-resolution
coordinated observations between the Swedish 1m Solar Telescope (SST)
and the Interface Region Imaging Spectrograph (IRIS) spacecraft. Hα
images from the SST provide us with the positions, timings, and
trajectories of EBs in an emerging flux region. Simultaneous, co-aligned
IRIS slit-jaw images at 133 (C II, transition region), 140 (Si IV,
transition region), and 279.6 (Mg II k, core, upper chromosphere)
nm as well as spectroscopy in the far- and near-ultraviolet from the
fast spectrograph raster allow us to study the possible chromospheric
and transition region counterparts of those EBs. Our main goal is
to study the possible temporal and spatial relationship between
several reconnection events at different layers in the atmosphere
(namely EBs and UV bursts), the timing history between them, and the
connection of these dynamical phenomena to the ejection of surges in
the chromosphere. We also investigate the properties of an extended
UV burst and their variations across the burst domain. Our results
suggest a scenario where simultaneous and co-spatial EBs and UV bursts
are part of the same reconnection system occurring sequentially along
a vertical or nearly vertical current sheet. Heating and bidirectional
jets trace the location where reconnection takes place. These results
support and expand those obtained from recent numerical simulations
of magnetic flux emergence. <P />The movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201936574/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: Nonequilibrium ionization and ambipolar diffusion in solar
magnetic flux emergence processes
Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martínez-Sykora,
J.; Carlsson, M.; Szydlarski, M.
2020A&A...633A..66N Altcode: 2019arXiv191201015N
Context. Magnetic flux emergence from the solar interior has
been shown to be a key mechanism for unleashing a wide variety of
phenomena. However, there are still open questions concerning the
rise of the magnetized plasma through the atmosphere, mainly in
the chromosphere, where the plasma departs from local thermodynamic
equilibrium (LTE) and is partially ionized. <BR /> Aims: We aim to
investigate the impact of the nonequilibrium (NEQ) ionization and
recombination and molecule formation of hydrogen, as well as ambipolar
diffusion, on the dynamics and thermodynamics of the flux emergence
process. <BR /> Methods: Using the radiation-magnetohydrodynamic
Bifrost code, we performed 2.5D numerical experiments of magnetic flux
emergence from the convection zone up to the corona. The experiments
include the NEQ ionization and recombination of atomic hydrogen, the NEQ
formation and dissociation of H<SUB>2</SUB> molecules, and the ambipolar
diffusion term of the generalized Ohm's law. <BR /> Results: Our
experiments show that the LTE assumption substantially underestimates
the ionization fraction in most of the emerged region, leading to an
artificial increase in the ambipolar diffusion and, therefore, in the
heating and temperatures as compared to those found when taking the
NEQ effects on the hydrogen ion population into account. We see that
LTE also overestimates the number density of H<SUB>2</SUB> molecules
within the emerged region, thus mistakenly magnifying the exothermic
contribution of the H<SUB>2</SUB> molecule formation to the thermal
energy during the flux emergence process. We find that the ambipolar
diffusion does not significantly affect the amount of total unsigned
emerged magnetic flux, but it is important in the shocks that cross
the emerged region, heating the plasma on characteristic times ranging
from 0.1 to 100 s. We also briefly discuss the importance of including
elements heavier than hydrogen in the equation of state so as not to
overestimate the role of ambipolar diffusion in the atmosphere. <P
/>Movies associated to Figs. 2-5, 8, 9, and A.1 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201936944/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: Signatures of Magnetic Reconnection at the Footpoints of
Fan-shaped Jets on a Light Bridge Driven by Photospheric Convective
Motions
Authors: Bai, Xianyong; Socas-Navarro, Hector; Nóbrega-Siverio,
Daniel; Su, Jiangtao; Deng, Yuanyong; Li, Dong; Cao, Wenda; Ji, Kaifan
2019ApJ...870...90B Altcode: 2018arXiv181103723B
Dynamical jets are generally found on light bridges (LBs), which are
key to studying sunspot decay. So far, their formation mechanism
is not fully understood. In this paper, we used state-of-the-art
observations from the Goode Solar Telescope, the Interface Region
Imaging Spectrograph, the Spectro-polarimeter on board Hinode, and
the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory to analyze the fan-shaped jets on LBs in detail. A
continuous upward motion of the jets in the ascending phase is found
from the Hα velocity that lasts for 12 minutes and is associated
with the Hα line wing enhancements. Two mini jets appear on the
bright fronts of the fan-shaped jets visible in the AIA 171 and 193
Å channels, with a time interval as short as 1 minute. Two kinds of
small-scale convective motions are identified in the photospheric
images, along with the Hα line wing enhancements. One seems to be
associated with the formation of a new convection cell, and the other
manifests as the motion of a dark lane passing through the convection
cell. The finding of three-lobe Stokes V profiles and their inversion
with the NICOLE code indicate that there are magnetic field lines
with opposite polarities in LBs. From the Hα -0.8 Å images, we found
ribbon-like brightenings propagating along the LBs, possibly indicating
slipping reconnection. Our observation supports the idea that the
fan-shaped jets under study are caused by magnetic reconnection, and
photospheric convective motions play an important role in triggering
the magnetic reconnection.
---------------------------------------------------------
Title: On the Importance of the Nonequilibrium Ionization of Si IV
and O IV and the Line of Sight in Solar Surges
Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martínez-Sykora,
J.
2018ApJ...858....8N Altcode: 2018arXiv180310251N
Surges are ubiquitous cool ejections in the solar atmosphere that often
appear associated with transient phenomena like UV bursts or coronal
jets. Recent observations from the Interface Region Imaging Spectrograph
show that surges, although traditionally related to chromospheric
lines, can exhibit enhanced emission in Si IV with brighter spectral
profiles than for the average transition region (TR). In this paper,
we explain why surges are natural sites to show enhanced emissivity
in TR lines. We performed 2.5D radiative-MHD numerical experiments
using the Bifrost code including the nonequilibrium (NEQ) ionization of
silicon and oxygen. A surge is obtained as a by-product of magnetic flux
emergence; the TR enveloping the emerged domain is strongly affected
by NEQ effects: assuming statistical equilibrium would produce an
absence of Si IV and O IV ions in most of the region. Studying the
properties of the surge plasma emitting in the Si IV λ1402.77 and O IV
λ1401.16 lines, we find that (a) the timescales for the optically thin
losses and heat conduction are very short, leading to departures from
statistical equilibrium, and (b) the surge emits in Si IV more and has
an emissivity ratio of Si IV to O IV larger than a standard TR. Using
synthetic spectra, we conclude the importance of line-of-sight effects:
given the involved geometry of the surge, the line of sight can cut the
emitting layer at small angles and/or cross it multiple times, causing
prominent, spatially intermittent brightenings in both Si IV and O IV.
---------------------------------------------------------
Title: Surges and Si IV Bursts in the Solar Atmosphere: Understanding
IRIS and SST Observations through RMHD Experiments
Authors: Nóbrega-Siverio, D.; Martínez-Sykora, J.; Moreno-Insertis,
F.; Rouppe van der Voort, L.
2017ApJ...850..153N Altcode: 2017arXiv171008928N
Surges often appear as a result of the emergence of magnetized
plasma from the solar interior. Traditionally, they are observed
in chromospheric lines such as Hα 6563 \mathringA and Ca II 8542
\mathringA . However, whether there is a response to the surge
appearance and evolution in the Si IV lines or, in fact, in many
other transition region lines has not been studied. In this paper,
we analyze a simultaneous episode of an Hα surge and a Si IV burst
that occurred on 2016 September 03 in active region AR 12585. To that
end, we use coordinated observations from the Interface Region Imaging
Spectrograph and the Swedish 1-m Solar Telescope. For the first time,
we report emission of Si IV within the surge, finding profiles that
are brighter and broader than the average. Furthermore, the brightest
Si IV patches within the domain of the surge are located mainly near
its footpoints. To understand the relation between the surges and the
emission in transition region lines like Si IV, we have carried out 2.5D
radiative MHD (RMHD) experiments of magnetic flux emergence episodes
using the Bifrost code and including the nonequilibrium ionization of
silicon. Through spectral synthesis, we explain several features of
the observations. We show that the presence of Si IV emission patches
within the surge, their location near the surge footpoints and various
observed spectral features are a natural consequence of the emergence of
magnetized plasma from the interior to the atmosphere and the ensuing
reconnection processes.
---------------------------------------------------------
Title: Intermittent Reconnection and Plasmoids in UV Bursts in the
Low Solar Atmosphere
Authors: Rouppe van der Voort, L.; De Pontieu, B.; Scharmer, G. B.;
de la Cruz Rodríguez, J.; Martínez-Sykora, J.; Nóbrega-Siverio,
D.; Guo, L. J.; Jafarzadeh, S.; Pereira, T. M. D.; Hansteen, V. H.;
Carlsson, M.; Vissers, G.
2017ApJ...851L...6R Altcode: 2017arXiv171104581R
Magnetic reconnection is thought to drive a wide variety of dynamic
phenomena in the solar atmosphere. Yet, the detailed physical mechanisms
driving reconnection are difficult to discern in the remote sensing
observations that are used to study the solar atmosphere. In this
Letter, we exploit the high-resolution instruments Interface Region
Imaging Spectrograph and the new CHROMIS Fabry-Pérot instrument at
the Swedish 1-m Solar Telescope (SST) to identify the intermittency
of magnetic reconnection and its association with the formation of
plasmoids in so-called UV bursts in the low solar atmosphere. The Si IV
1403 Å UV burst spectra from the transition region show evidence of
highly broadened line profiles with often non-Gaussian and triangular
shapes, in addition to signatures of bidirectional flows. Such profiles
had previously been linked, in idealized numerical simulations, to
magnetic reconnection driven by the plasmoid instability. Simultaneous
CHROMIS images in the chromospheric Ca II K 3934 Å line now provide
compelling evidence for the presence of plasmoids by revealing highly
dynamic and rapidly moving brightenings that are smaller than 0.″2 and
that evolve on timescales of the order of seconds. Our interpretation
of the observations is supported by detailed comparisons with synthetic
observables from advanced numerical simulations of magnetic reconnection
and associated plasmoids in the chromosphere. Our results highlight
how subarcsecond imaging spectroscopy sensitive to a wide range of
temperatures combined with advanced numerical simulations that are
realistic enough to compare with observations can directly reveal the
small-scale physical processes that drive the wide range of phenomena
in the solar atmosphere.
---------------------------------------------------------
Title: Two-dimensional Radiative Magnetohydrodynamic Simulations of
Partial Ionization in the Chromosphere. II. Dynamics and Energetics
of the Low Solar Atmosphere
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Carlsson, Mats;
Hansteen, Viggo H.; Nóbrega-Siverio, Daniel; Gudiksen, Boris V.
2017ApJ...847...36M Altcode: 2017arXiv170806781M
We investigate the effects of interactions between ions and
neutrals on the chromosphere and overlying corona using 2.5D
radiative MHD simulations with the Bifrost code. We have extended
the code capabilities implementing ion-neutral interaction effects
using the generalized Ohm’s law, I.e., we include the Hall term
and the ambipolar diffusion (Pedersen dissipation) in the induction
equation. Our models span from the upper convection zone to the corona,
with the photosphere, chromosphere, and transition region partially
ionized. Our simulations reveal that the interactions between ionized
particles and neutral particles have important consequences for the
magnetothermodynamics of these modeled layers: (1) ambipolar diffusion
increases the temperature in the chromosphere; (2) sporadically the
horizontal magnetic field in the photosphere is diffused into the
chromosphere, due to the large ambipolar diffusion; (3) ambipolar
diffusion concentrates electrical currents, leading to more violent
jets and reconnection processes, resulting in (3a) the formation of
longer and faster spicules, (3b) heating of plasma during the spicule
evolution, and (3c) decoupling of the plasma and magnetic field in
spicules. Our results indicate that ambipolar diffusion is a critical
ingredient for understanding the magnetothermodynamic properties in the
chromosphere and transition region. The numerical simulations have been
made publicly available, similar to previous Bifrost simulations. This
will allow the community to study realistic numerical simulations with
a wider range of magnetic field configurations and physics modules
than previously possible.
---------------------------------------------------------
Title: The Cool Surge Following Flux Emergence in a Radiation-MHD
Experiment
Authors: Nóbrega-Siverio; D.; Moreno-Insertis, F.; Martínez-Sykora,
J.
2016usc..confE..68N Altcode:
Cool and dense ejections, typically Hα surges, often appear alongside
EUV or X-ray coronal jets as a result of the emergence of magnetized
plasma from the solar interior. Idealized numerical experiments explain
those ejections as being indirectly associated with the magnetic
reconnection taking place between the emerging and preexisting
systems. However, those experiments miss basic elements that can
importantly affect the surge phenomenon. In this paper we study the
cool surges using a realistic treatment of the radiation transfer and
material plasma properties. To that end, the Bifrost code is used,
which has advanced modules for the equation of state of the plasma,
photospheric and chromospheric radiation transfer, heat conduction,
and optically thin radiative cooling. We carry out a 2.5D experiment of
the emergence of magnetized plasma through (meso) granular convection
cells and the low atmosphere to the corona. Through detailed Lagrange
tracing we study the formation and evolution of the cool ejection and,
in particular, the role of the entropy sources; this allows us to
discern families of evolutionary patterns for the plasma elements. In
the launch phase, many elements suffer accelerations well in excess
of gravity; when nearing the apex of their individual trajectories,
instead, the plasma elements follow quasi-parabolic trajectories with
accelerations close to the solar gravity . We show how the formation
of the cool ejection is mediated by a wedge-like structure composed
of two shocks, one of which leads to the detachment of the surge from
the original emerged plasma dome.
---------------------------------------------------------
Title: The Cool Surge Following Flux Emergence in a Radiation-MHD
Experiment
Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martínez-Sykora,
J.
2016ApJ...822...18N Altcode: 2016arXiv160104074N
Cool and dense ejections, typically Hα surges, often appear alongside
EUV or X-ray coronal jets as a result of the emergence of magnetized
plasma from the solar interior. Idealized numerical experiments explain
those ejections as being indirectly associated with the magnetic
reconnection taking place between the emerging and preexisting
systems. However, those experiments miss basic elements that can
importantly affect the surge phenomenon. In this paper we study the
cool surges using a realistic treatment of the radiation transfer and
material plasma properties. To that end, the Bifrost code is used,
which has advanced modules for the equation of state of the plasma,
photospheric and chromospheric radiation transfer, heat conduction,
and optically thin radiative cooling. We carry out a 2.5D experiment of
the emergence of magnetized plasma through (meso) granular convection
cells and the low atmosphere to the corona. Through detailed Lagrange
tracing we study the formation and evolution of the cool ejection and,
in particular, the role of the entropy sources; this allows us to
discern families of evolutionary patterns for the plasma elements. In
the launch phase, many elements suffer accelerations well in excess
of gravity; when nearing the apex of their individual trajectories,
instead, the plasma elements follow quasi-parabolic trajectories with
accelerations close to {g}<SUB>⊙ </SUB>. We show how the formation
of the cool ejection is mediated by a wedge-like structure composed
of two shocks, one of which leads to the detachment of the surge from
the original emerged plasma dome.
