hansteen

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Author name code: hansteen
ADS astronomy entries on 2022-09-14
author:"Hansteen, Viggo H."

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Title: A novel inversion method to determine the coronal magnetic
field including the impact of bound-free absorption
Authors: Martinez-Sykora, Juan; Hansteen, Viggo H.; De Pontieu, Bart;
Landi, Enrico
2022arXiv220813984M Altcode:
The magnetic field governs the corona; hence it is a crucial parameter
to measure. Unfortunately, existing techniques for estimating its
strength are limited by strong assumptions and limitations. These
techniques include photospheric or chromospheric field extrapolation
using potential or non-linear-force-free methods, estimates based on
coronal seismology, or by direct observations via, e.g., the Cryo-NIRSP
instrument on DKIST which will measure the coronal magnetic field,
but only off the limb. Alternately, in this work we investigate a
recently developed approach based on the magnetic-field-induced (MIT)
transition of the \fex~257.261~Å. In order to examine this approach,
we have synthesized several \fex\ lines from two 3D magnetohydrodynamic
simulations, one modeling an emerging flux region and the second an
established mature active region. In addition, we take bound-free
absorption from neutral hydrogen and helium and singly ionised
helium into account. The absorption from cool plasma that occurs at
coronal heights has a significant impact on determining the magnetic
field. We investigate in detail the challenges of using these \fex\
lines to measure the field, considering their density and temperature
dependence. We present a novel approach to deriving the magnetic field
from the MIT using inversions of the differential emission measure as a
function of the temperature, density, and magnetic field. This approach
successfully estimates the magnetic field strength (up to \%18 relative
error) in regions that do not suffer from significant absorption and
that have relatively strong coronal magnetic fields ($&gt;250$~G). This
method allows the masking of regions where absorption is significant.

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Title: Fine-scale, Dot-like, Brightenings in an Emerging Flux Region:
SolO/EUI Observations, and Bifrost MHD Simulations
Authors: Tiwari, Sanjiv Kumar; Berghmans, David; De Pontieu, Bart;
Hansteen, Viggo; Panesar, Navdeep Kaur
2022cosp...44.2529T Altcode:
Numerous tiny bright dots are observed in SolO's EUI/\hri\ data
of an emerging flux region (a coronal bright point) in 174 \AA,
emitted by the coronal plasma at $\sim$1 MK. These dots are roundish,
with a diameter of 675$\pm$300 km, a lifetime of 50$\pm$35 seconds,
and an intensity enhancement of 30% $\pm$10% from their immediate
surroundings. About half of the dots remain isolated during their
evolution and move randomly and slowly ($&lt;$10 \kms). The other half
show extensions, appearing as a small loop or surge/jet, with intensity
propagations below 30\,\kms. Some dots form at the end of a fine-scale
explosion. Many of the bigger and brighter EUI/HRI dots are discernible
in SDO/AIA 171 \AA\ channel, have significant EM in the temperature
range of 1--2 MK, and are often located at polarity inversion lines
observed in HMI LOS magnetograms. Bifrost MHD simulations of an emerging
flux region do show dots in synthetic Fe IX/X images, although dots
in simulations are not as pervasive as in observations. The dots
in simulations show distinct Doppler signatures -- blueshifts and
redshifts coexist, or a redshift of the order of 10 \kms\ is followed
by a blueshift of similar or higher magnitude. The synthetic images of
O V/VI and Si IV lines, which form in the transition region, also show
the dots that are observed in Fe IX/X images, often expanded in size,
or extended as a loop, and always with stronger Doppler velocities (up
to 100 \kms) than that in Fe IX/X lines. Our results, together with the
field geometry of dots in the simulations, suggest that most dots in
emerging flux regions form in the lower solar atmosphere (at $\approx$1
Mm) by magnetic reconnection between emerging and pre-existing/emerged
magnetic field. The dots are smaller in Fe IX/X images (than in O V/VI
&amp; Si IV lines) most likely because only the hottest counterpart of
the magnetic reconnection events is visible in the hotter emission. Some
dots might be manifestations of magneto-acoustic shocks (from the
lower atmosphere) through the line formation region of Fe IX/X. A
small number of dots could also be a response of supersonic downflows
impacting transition-region/chromospheric density.

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Title: Stirring the Base of the Solar Wind: On Heat Transfer and
Vortex Formation
Authors: Finley, Adam J.; Brun, Sacha A.; Carlsson, Mats; Szydlarski,
Mikolaj; Hansteen, Viggo; Shoda, Munehito
2022arXiv220702878F Altcode:
Current models of the solar wind must approximate (or ignore) the
small-scale dynamics within the solar atmosphere, however these are
likely important in shaping the emerging wave-turbulence spectrum and
ultimately heating/accelerating the coronal plasma. The Bifrost code
produces realistic simulations of the solar atmosphere that facilitate
the analysis of spatial and temporal scales which are currently at,
or beyond, the limit of modern solar telescopes. For this study, the
Bifrost simulation is configured to represent the solar atmosphere in
a coronal hole region, from which the fast solar wind emerges. The
simulation extends from the upper-convection zone (2.5 Mm below the
photosphere) to the low-corona (14.5 Mm above the photosphere), with
a horizontal extent of 24 Mm x 24 Mm. The twisting of the coronal
magnetic field by photospheric flows, efficiently injects energy
into the low-corona. Poynting fluxes of up to $2-4$ kWm$^{-2}$ are
commonly observed inside twisted magnetic structures with diameters
in the low-corona of 1 - 5 Mm. Torsional Alfvén waves are favourably
transmitted along these structures, and will subsequently escape into
the solar wind. However, reflections of these waves from the upper
boundary condition make it difficult to unambiguously quantify the
emerging Alfvén wave-energy flux. This study represents a first step
in quantifying the conditions at the base of the solar wind using
Bifrost simulations. It is shown that the coronal magnetic field is
readily braided and twisted by photospheric flows. Temperature and
density contrasts form between regions with active stirring motions
and those without. Stronger whirlpool-like flows in the convection,
concurrent with magnetic concentrations, launch torsional Alfvén waves
up through the magnetic funnel network, which are expected to enhance
the turbulent generation of magnetic switchbacks in the solar wind.

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Title: Detailed Description of the Collision Frequency in the Solar
Atmosphere
Authors: Wargnier, Q. M.; Martínez-Sykora, J.; Hansteen, V. H.;
De Pontieu, B.
2022ApJ...933..205W Altcode:
This work aims to provide an accurate description and calculations of
collision frequencies in conditions relevant to the solar atmosphere. To
do so, we focus on the detailed description of the collision
frequency in the solar atmosphere based on a classical formalism with
Chapman-Cowling collision integrals, as described by Zhdanov. These
collision integrals allow linking the macroscopic transport fluxes
of multifluid models to the kinetic scales involved in the Boltzmann
equations. In this context, the collision frequencies are computed
accurately while being consistent at the kinetic level. We calculate
the collision frequencies based on this formalism and compare them with
approaches commonly used in the literature for conditions typical of the
solar atmosphere. To calculate the collision frequencies, we focus on
the collision integral data provided by Bruno et al., which is based on
a multicomponent hydrogen-helium mixture used for conditions typical for
the atmosphere of Jupiter. We perform a comparison with the classical
formalism of Vranjes &amp; Krstic and Leake &amp; Linton. We highlight
the differences obtained in the distribution of the cross sections as
functions of the temperature. Then, we quantify the disparities obtained
in numerical simulations of a 2.5D solar atmosphere by calculating
collision frequencies and ambipolar diffusion. This strategy allows
us to validate and assess the accuracy of these collision frequencies
for conditions typical of the solar atmosphere.

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Title: SolO/EUI Observations of Ubiquitous Fine-scale Bright Dots
in an Emerging Flux Region: Comparison with a Bifrost MHD Simulation
Authors: Tiwari, Sanjiv K.; Hansteen, Viggo H.; De Pontieu, Bart;
Panesar, Navdeep K.; Berghmans, David
2022ApJ...929..103T Altcode: 2022arXiv220306161T
We report on the presence of numerous tiny bright dots in and around
an emerging flux region (an X-ray/coronal bright point) observed with
SolO's EUI/HRIEUV in 174 Å. These dots are roundish and have
a diameter of 675 ± 300 km, a lifetime of 50 ± 35 s, and an intensity
enhancement of 30% ± 10% above their immediate surroundings. About
half of the dots remain isolated during their evolution and move
randomly and slowly (&lt;10 km s-1). The other half show
extensions, appearing as a small loop or surge/jet, with intensity
propagations below 30 km s-1. Many of the bigger and brighter
HRIEUV dots are discernible in the SDO/AIA 171 Å channel,
have significant emissivity in the temperature range of 1-2 MK, and
are often located at polarity inversion lines observed in SDO/HMI LOS
magnetograms. Although not as pervasive as in observations, a Bifrost
MHD simulation of an emerging flux region does show dots in synthetic
Fe IX/X images. These dots in the simulation show distinct Doppler
signatures-blueshifts and redshifts coexist, or a redshift of the
order of 10 km s-1 is followed by a blueshift of similar
or higher magnitude. The synthetic images of O V/VI and Si IV lines,
which represent transition region radiation, also show the dots that
are observed in Fe IX/X images, often expanded in size, or extended
as a loop, and always with stronger Doppler velocities (up to 100
km s-1) than that in Fe IX/X lines. Our observation and
simulation results, together with the field geometry of dots in the
simulation, suggest that most dots in emerging flux regions form in the
lower solar atmosphere (at ≍ 1 Mm) by magnetic reconnection between
emerging and preexisting/emerged magnetic field. Some dots might be
manifestations of magnetoacoustic shocks through the line formation
region of Fe IX/X emission.

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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 (&lt;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: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE): II. Flares and Eruptions
Authors: Cheung, Chun Ming Mark; Martinez-Sykora, Juan; Testa, Paola;
De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
Vanessa; Kerr, Graham; Reeves, Katharine; Fletcher, Lyndsay; Jin,
Meng; Nobrega, Daniel; Danilovic, Sanja; Antolin, Patrick; Allred,
Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward; Longcope,
Dana; Takasao, Shinsuke; DeRosa, Marc; Boerner, Paul; Jaeggli, Sarah;
Nitta, Nariaki; Daw, Adrian; Carlsson, Mats; Golub, Leon
2021AGUFMSH51A..08C Altcode:
Current state-of-the-art spectrographs cannot resolve the fundamental
spatial (sub-arcseconds) and temporal scales (less than a few tens
of seconds) 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 IRIS 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
(&lt;20 s), sub-arcsecond 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 EUV Solar Telescope (EUVST) 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. (2021, also submitted to SH-17),
which focuses on investigating coronal heating with MUSE.

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Title: Description of collisional frequencies for multifluid MHD
models with Chapman-Cowling collision integrals
Authors: Wargnier, Q.; Martinez-Sykora, Juan; Hansteen, Viggo;
Magin, Thierry
2021AGUFMSH45B2362W Altcode:
We focus on the detailed description of the collisional frequency in the
solar atmosphere based on a classical formalism with Chapman-Cowling
collision integrals, as described by Zhdanov (2002) in the context of
the 13N-moment model derived with a method of Grad (Grad 1949). These
collision integrals allow linking the macroscopic transport fluxes
of multifluid models to the kinetic scales involved in the Boltzmann
equations. In this context, the collisional frequencies are computed
accurately while being consistent at the kinetic level. We calculate
the collisional frequencies based on this formalism and compare them
with approaches commonly used in the literature in solar atmosphere
conditions. To calculate the collisional frequencies, we focus on
the collision integrals data provided by Bruno et al. (2010), which
is based on a multicomponent hydrogen-helium mixture used in Jupiter
atmosphere conditions. We propose a comparison with the classical
formalism of Vranjes &amp; Krstic (2013) and Leake &amp; Linton
(2013). We compare it with the formalism used in the three approaches
and highlight the differences obtained in the distribution of the
cross sections as functions of the temperature. Then, we quantify
the disparities obtained in postprocessed simulations of a 2.5D solar
atmosphere with the Bifrost code (see Gudiksen et al. 2011). Finally,
we assess the impact of the collisional frequency in a simulated
2.5D solar atmosphere with a single-fluid radiative MHD model with
ambipolar diffusion to consider ion-neutral interactions. Significant
disparities in the cross sections have been obtained between these
three formalisms. or instance, we note that Vranjes &amp; Krstic 2013
did no integrate the transport cross sections. We will describe the
impact of these discrepancies from previous results and the importance
of doing these calculations properly.

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Title: Linking the Sun to the Heliosphere Using Composition Data
and Modelling
Authors: Parenti, Susanna; Chifu, Iulia; Del Zanna, Giulio; Edmondson,
Justin; Giunta, Alessandra; Hansteen, Viggo H.; Higginson, Aleida;
Laming, J. Martin; Lepri, Susan T.; Lynch, Benjamin J.; Rivera, Yeimy
J.; von Steiger, Rudolf; Wiegelmann, Thomas; Wimmer-Schweingruber,
Robert F.; Zambrana Prado, Natalia; Pelouze, Gabriel
2021SSRv..217...78P Altcode: 2021arXiv211006111P
Our understanding of the formation and evolution of the corona and
the heliosphere is linked to our capability of properly interpret the
data from remote sensing and in-situ observations. In this respect,
being able to correctly connect in-situ observations with their source
regions on the Sun is the key for solving this problem. In this work
we aim at testing a diagnostics method for this connectivity.

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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
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.

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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: ALMA and IRIS Observations of the Solar
Chromosphere. II. Structure and Dynamics of Chromospheric Plages
Authors: Chintzoglou, Georgios; De Pontieu, Bart; Martínez-Sykora,
Juan; Hansteen, Viggo; de la Cruz Rodríguez, Jaime; Szydlarski,
Mikolaj; Jafarzadeh, Shahin; Wedemeyer, Sven; Bastian, Timothy S.;
Sainz Dalda, Alberto
2021ApJ...906...83C Altcode: 2020arXiv201205970C
We propose and employ a novel empirical method for determining
chromospheric plage regions, which seems to better isolate a plage from
its surrounding regions than other methods commonly used. We caution
that isolating a plage from its immediate surroundings must be done
with care in order to successfully mitigate statistical biases that,
for instance, can impact quantitative comparisons between different
chromospheric observables. Using this methodology, our analysis suggests
that λ = 1.25 mm free-free emission in plage regions observed with
the Atacama Large Millimeter/submillimeter Array (ALMA)/Band6 may
not form in the low chromosphere as previously thought, but rather
in the upper chromospheric parts of dynamic plage features (such as
spicules and other bright structures), i.e., near geometric heights
of transition-region temperatures. We investigate the high degree of
similarity between chromospheric plage features observed in ALMA/Band6
(at 1.25 mm wavelengths) and the Interface Region Imaging Spectrograph
(IRIS)/Si IV at 1393 Å. We also show that IRIS/Mg II h and k are
not as well correlated with ALMA/Band6 as was previously thought,
and we discuss discrepancies with previous works. Lastly, we report
indications of chromospheric heating due to propagating shocks supported
by the ALMA/Band6 observations.

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Title: ALMA and IRIS Observations of the Solar Chromosphere. I. An
On-disk Type II Spicule
Authors: Chintzoglou, Georgios; De Pontieu, Bart; Martínez-Sykora,
Juan; Hansteen, Viggo; de la Cruz Rodríguez, Jaime; Szydlarski,
Mikolaj; Jafarzadeh, Shahin; Wedemeyer, Sven; Bastian, Timothy S.;
Sainz Dalda, Alberto
2021ApJ...906...82C Altcode: 2020arXiv200512717C
We present observations of the solar chromosphere obtained
simultaneously with the Atacama Large Millimeter/submillimeter Array
(ALMA) and the Interface Region Imaging Spectrograph. The observatories
targeted a chromospheric plage region of which the spatial distribution
(split between strongly and weakly magnetized regions) allowed the
study of linear-like structures in isolation, free of contamination
from background emission. Using these observations in conjunction with
a radiative magnetohydrodynamic 2.5D model covering the upper convection
zone all the way to the corona that considers nonequilibrium ionization
effects, we report the detection of an on-disk chromospheric spicule
with ALMA and confirm its multithermal nature.

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Title: Probing chromospheric heating with millimeter interferometry
Authors: da Silva Santos, J. M.; de la Cruz Rodriguez, J.; White,
S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H.; Danilovic, S.
2020AGUFMSH0010001D Altcode:
Observations at visible and ultraviolet wavelengths have shown that
solar active regions host different kinds of small-scale, transient,
bright structures that are believed to be heating events resulting
from the release of magnetic energy in the low atmosphere of the Sun,
especially at the early stages of flux emergence. It is of great
scientific interest to be able to accurately infer temperatures and
formation heights of the most localized events, which are still
matter of debate, in the hope of learning about the evolution of
active regions where occasionally more energetic phenomena lead to
much larger outbursts that propagate across the Solar System. The
millimeter (mm) continuum is a new complementary diagnostic for
chromospheric heating that is now available thanks to the Atacama
Large Millimeter/submillimeter Array (ALMA). We report on the
first ALMA 3 mm observations of small-scale heating events in a
solar active region. In contrast with the low-amplitude brightness
temperature variations in the quiet-Sun, the interferometric maps show
that the active region consists of long, warm, fibril-like structures
that connect magnetic concentrations of opposite polarity and often
flare up along with compact, flickering mm-bursts -- reminiscent of
ultraviolet bursts -- with brightness temperatures of up to 14000 K at
1.2" scales. These events also show simultaneous EUV emission observed
by the Solar Dynamics Observatory (SDO). We find a weak correlation
between the photospheric bright patches and the 3 mm continuum
brightness and, in particular, we do not detect any mm counterpart of
Ellerman bombs which confirms that they are photospheric phenomena. Our observations and modelling highlight the diagnostic capabilities
of ALMA for local heating in solar active regions and emphasize the
need for coordinated observations with IRIS and DKIST in the future.

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Title: ALMA and IRIS Observations Highlighting the Dynamics and
Structure of Chromospheric Plage
Authors: Chintzoglou, G.; De Pontieu, B.; Martinez-Sykora, J.;
Hansteen, V. H.; de la Cruz Rodriguez, J.; Szydlarski, M.; Jafarzadeh,
S.; Wedemeyer, S.; Bastian, T.; Sainz Dalda, A.
2020AGUFMSH0010009C Altcode:
We present observations of the solar chromosphere obtained
simultaneously with the Atacama Large Millimeter/submillimeter Array
(ALMA) and the Interface Region Imaging Spectrograph (IRIS). The
observatories targeted a chromospheric plage region of which the spatial
distribution (split between strongly and weakly magnetized regions)
allowed the study of linear-like structures in isolation, free of
contamination from background emission. Using these observations
in conjunction with a radiative magnetohydrodynamic 2.5D model
covering the upper convection zone all the way to the corona
that considers non-equilibrium ionization effects, we report the
detection of an on-disk chromospheric spicule with ALMA and confirm
its multithermal nature. In addition, we discuss the strikingly high
degree of similarity between chromospheric plage features observed
in ALMA/Band6 and IRIS/\ion{Si}{4} (also reproduced in our model)
suggesting that ALMA/Band6 does not observe in the low chromosphere as
previously thought but rather observes the upper chromospheric parts
of structures such as spicules and other bright structures above plage
at geometric heights near transition region temperatures. We also show
that IRIS/\ion{Mg}{2} is not as well correlated with ALMA/Band6 as was
previously thought. For these comparisons, we propose and employ a novel
empirical method for the determination of plage regions, which seems
to better isolate plage from its surrounding regions as compared to
other methods commonly used. We caution that isolating plage from its
immediate surroundings must be done with care to mitigate statistical
bias in quantitative comparisons between different chromospheric
observables. Lastly, we report indications for chromospheric heating
due to traveling shocks supported by the ALMA/Band6 observations.

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Title: On the velocity drift between ions in the solar atmosphere
Authors: Martínez-Sykora, J.; Szydlarski, M.; Hansteen, V. H.;
De Pontieu, B.
2020AGUFMSH0010017M Altcode:
Very recent results that compare ALMA and IRIS observations with 2D
radiative MHD including non-equilibrium ionization and ambipolar
diffusion models of the type II spicules reveal that these models
may underestimate the energy dissipated in the chromosphere. The
solar atmosphere is composed of many species that are populated at
different ionization and excitation levels. The upper chromosphere,
transition region, and corona are nearly collisionless. Consequently,
slippage between, for instance, ions and neutral particles, or
interactions between separate species, may play an important role
in the local momentum and energy balance. The interaction between
species is missing in the 2D radiative MHD model. We have developed a
3D multi-fluid and multi-species numerical code (Ebysus) to investigate
such effects. Ebysus is capable of treating species (e.g., hydrogen,
helium, etc) and fluids (neutrals, excited and ionized elements)
separately. Treating different species as different fluids leads
to drifts between different ions and an electric field that couple
these motions. Different ionized species and momentum exchange can
dissipate this velocity drift, i.e., convert wave kinetic energy into
thermal energy. High-frequency Alfven waves, driven for instance by
reconnection, thought to occur in the solar atmosphere, can drive such
multi-ion velocity drifts.

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Title: The correlation between chromospheric and coronal heating in
active region plage
Authors: De Pontieu, B.; Hansteen, V. H.; Winebarger, A. R.
2020AGUFMSH0010010D Altcode:
The upper transition region at the footpoints of the hottest loops in
active regions is known as moss, highly structured and dynamic 1 MK
plasma that is formed at the same heights as dynamic chromospheric jets
emanating from the underlying plage regions. Moss provides an excellent
laboratory to disentangle the complex interface between chromosphere and
corona and to study how chromospheric and coronal heating mechanisms
are spatio-temporally correlated (if at all). This is because moss is
very sensitive to changes in the local heating rate and, since it is
formed in a thin, corrugated layer, avoids the confusion introduce
by line-of-sight superposition taht affects optically thin coronal
diagnostics. Previous results based on lower-resolution instruments
(e.g., TRACE, SDO/AIA) suggested a puzzling mismatch between low
chromospheric and upper TR emission. We will present results based on
analysis of a unique coordinated dataset from IRIS and the sounding
rocket HiC. The HiC 2.1 flight took place in 2018 and obtained several
minutes of sub-arcsecond resolution images of the upper TR in Fe IX
171A, while IRIS obtained high-resolution rasters in the Mg II h &amp;
k lines at high cadence. Our analysis will focus on spatio-temporal
correlations between the properties of the optically thick Mg II h
&amp; k lines, and the intensities of the HiC 2.1 images. We will
also exploit the recently developed IRIS2 database to invert the
Mg II h &amp; k profiles and study correlations between the derived
chromospheric temperature, density, and micro-turbulence (as a function
of height in the chromosphere) and the overlying upper TR and coronal
emission. Our analysis provides insight and constraints on the nature
and (dis)similarities of the heating mechanisms in both the chromosphere
and corona.

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Title: A Multi-Fluid Multi-Species (MFMS) numerical code for
simulating the solar atmosphere
Authors: Wargnier, Q.; Martínez-Sykora, J.; Hansteen, V. H.;
Szydlarski, M.; Evans, S.
2020AGUFMSH0370007W Altcode:
The solar atmosphere is characterized by multiple periods and sizes
involving a large spectrum of temporal and spatial scales. It is
regulated through complex interactions between different species and
chemical reactions, amongst other physical processes . Because of
this complexity, an accurate description of all of these multi-scale
phenomena in the solar atmosphere is out of the reach of standard
single-fluid MHD models (Hartlep et al. 2012). Furthermore, the
enrichment of low first ionization potential elements in the outer
layer of the solar atmosphere (the FIP effect) is not fully described
by the current theoretical models (e.g. Laming et al. 2017), since
they employ semi-empirical static atmospheres.

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Title: Chromospheric and TR diagnostics in a large scale numerical
simulation of flux emergence: Synthetic vs Real observables
Authors: Hansteen, V. H.; De Pontieu, B.; Testa, P.; Gosic, M.;
Martinez-Sykora, J.
2020AGUFMSH0010021H Altcode:
Field stored just below or rising to the photosphere will break through
the surface and enter the upper atmosphere once the gradient of the
subphotospheric field strength becomes sufficiently large. Opposite
polarity flux bundles will reconnect in the photosphere and above,
to form steadily longer loops that expand into the outer solar
atmosphere, forming the corona. Some of the emerging flux is likely
due to a local dynamo, but also the direct emergence of large scale
magnetic structures from below is important, even in the quiet Sun. A
significant proportion of this field likely reaches the chromosphere
and may leave imprint on chromospheric dynamics and energetics. Using
large scale numerical models (72x72x60) Mm and the high resolution
spectra and slit jaw images from IRIS, as well as photospheric data
from Hinode/SOT, and SDO/HMI we study the interactions between the
magnetic flux caught in the granular flow field and the chromosphere
and chromospheric field above. We will compare synthetic observables
of the photospheric Fe I 617.3 nm line, the chromospheric Mg II h&amp;k
lines, and the transition region Si IV lines, with their observational
counterparts. We will also generate synthetic ALMA band 3 images. The
comparison of synthetic observational data will let us draw conclusions
as to the validity of the numerical modeling and the importance of flux
emergence for the dynamics and energetics of the outer solar atmosphere.

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Title: ALMA observations of transient heating in a solar active region
Authors: da Silva Santos, J. M.; de la Cruz Rodríguez, J.; White,
S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H.
2020A&A...643A..41D Altcode: 2020arXiv200614564D
 Aims: We aim to investigate the temperature enhancements and
formation heights of solar active-region brightenings such as Ellerman
bombs (EBs), ultraviolet bursts (UVBs), and flaring active-region
fibrils (FAFs) using interferometric observations in the millimeter
(mm) continuum provided by the Atacama Large Millimeter/submillimeter
Array (ALMA). Methods: We examined 3 mm signatures of heating
events identified in Solar Dynamics Observatory observations of an
active region and compared the results with synthetic spectra from a 3D
radiative magnetohydrodynamic simulation. We estimated the contribution
from the corona to the mm brightness using differential emission measure
analysis. Results: We report the null detection of EBs in the 3
mm continuum at ∼1.2″ spatial resolution, which is evidence that
they are sub-canopy events that do not significantly contribute to
heating the upper chromosphere. In contrast, we find the active region
to be populated with multiple compact, bright, flickering mm-bursts -
reminiscent of UVBs. The high brightness temperatures of up to ∼14 200
K in some events have a contribution (up to ∼7%) from the corona. We
also detect FAF-like events in the 3 mm continuum. These events show
rapid motions of &gt; 10 kK plasma launched with high plane-of-sky
velocities (37 - 340 km s-1) from bright kernels. The mm FAFs
are the brightest class of warm canopy fibrils that connect magnetic
regions of opposite polarities. The simulation confirms that ALMA
should be able to detect the mm counterparts of UVBs and small flares
and thus provide a complementary diagnostic for localized heating in the
solar chromosphere. Movie associated to Fig. 5 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202038755/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: IRIS Observations of the Low-atmosphere Counterparts of Active
Region Outflows
Authors: Polito, Vanessa; De Pontieu, Bart; Testa, Paola; Brooks,
David H.; Hansteen, Viggo
2020ApJ...903...68P Altcode: 2020arXiv201015945P
Active region (AR) outflows have been studied in detail since
the launch of Hinode/EIS and are believed to provide a possible
source of mass and energy to the slow solar wind. In this work, we
investigate the lower atmospheric counterpart of AR outflows using
observations from the Interface Region Imaging Spectrograph (IRIS). We
find that the IRIS Si IV, C II&gt; and Mg II transition region (TR)
and chromospheric lines exhibit different spectral features in the
outflows as compared to neighboring regions at the footpoints ("moss")
of hot AR loops. The average redshift of Si IV in the outflow region
(≍5.5 km s-1) is smaller than typical moss (≍12-13
km s-1) and quiet Sun (≍7.5 km s-1) values,
while the C II line is blueshifted (≍-1.1-1.5 km s-1),
in contrast to the moss where it is observed to be redshifted by
about ≍2.5 km s-1. Further, we observe that the low
atmosphere underneath the coronal outflows is highly structured, with
the presence of blueshifts in Si IV and positive Mg II k2 asymmetries
(which can be interpreted as signatures of chromospheric upflows)
which are mostly not observed in the moss. These observations show a
clear correlation between the coronal outflows and the chromosphere
and TR underneath, which has not been shown before. Our work strongly
suggests that these regions are not separate environments and should
be treated together, and that current leading theories of AR outflows,
such as the interchange reconnection model, need to take into account
the dynamics of the low atmosphere.

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Title: The Solar Orbiter SPICE instrument. An extreme UV imaging
spectrometer
Authors: SPICE Consortium; Anderson, M.; Appourchaux, T.; Auchère, F.;
Aznar Cuadrado, R.; Barbay, J.; Baudin, F.; Beardsley, S.; Bocchialini,
K.; Borgo, B.; Bruzzi, D.; Buchlin, E.; Burton, G.; Büchel, V.;
Caldwell, M.; Caminade, S.; Carlsson, M.; Curdt, W.; Davenne, J.;
Davila, J.; Deforest, C. E.; Del Zanna, G.; Drummond, D.; Dubau,
J.; Dumesnil, C.; Dunn, G.; Eccleston, P.; Fludra, A.; Fredvik, T.;
Gabriel, A.; Giunta, A.; Gottwald, A.; Griffin, D.; Grundy, T.; Guest,
S.; Gyo, M.; Haberreiter, M.; Hansteen, V.; Harrison, R.; Hassler,
D. M.; Haugan, S. V. H.; Howe, C.; Janvier, M.; Klein, R.; Koller,
S.; Kucera, T. A.; Kouliche, D.; Marsch, E.; Marshall, A.; Marshall,
G.; Matthews, S. A.; McQuirk, C.; Meining, S.; Mercier, C.; Morris,
N.; Morse, T.; Munro, G.; Parenti, S.; Pastor-Santos, C.; Peter, H.;
Pfiffner, D.; Phelan, P.; Philippon, A.; Richards, A.; Rogers, K.;
Sawyer, C.; Schlatter, P.; Schmutz, W.; Schühle, U.; Shaughnessy,
B.; Sidher, S.; Solanki, S. K.; Speight, R.; Spescha, M.; Szwec, N.;
Tamiatto, C.; Teriaca, L.; Thompson, W.; Tosh, I.; Tustain, S.; Vial,
J. -C.; Walls, B.; Waltham, N.; Wimmer-Schweingruber, R.; Woodward,
S.; Young, P.; de Groof, A.; Pacros, A.; Williams, D.; Müller, D.
2020A&A...642A..14S Altcode: 2019arXiv190901183A; 2019arXiv190901183S
 Aims: The Spectral Imaging of the Coronal Environment (SPICE)
instrument is a high-resolution imaging spectrometer operating at
extreme ultraviolet wavelengths. In this paper, we present the concept,
design, and pre-launch performance of this facility instrument on the
ESA/NASA Solar Orbiter mission. Methods: The goal of this paper
is to give prospective users a better understanding of the possible
types of observations, the data acquisition, and the sources that
contribute to the instrument's signal. Results: The paper
discusses the science objectives, with a focus on the SPICE-specific
aspects, before presenting the instrument's design, including optical,
mechanical, thermal, and electronics aspects. This is followed by a
characterisation and calibration of the instrument's performance. The
paper concludes with descriptions of the operations concept and data
processing. Conclusions: The performance measurements of the
various instrument parameters meet the requirements derived from the
mission's science objectives. The SPICE instrument is ready to perform
measurements that will provide vital contributions to the scientific
success of the Solar Orbiter mission.

---------------------------------------------------------
Title: On the Velocity Drift between Ions in the Solar Atmosphere
Authors: Martínez-Sykora, Juan; Szydlarski, Mikolaj; Hansteen,
Viggo H.; De Pontieu, Bart
2020ApJ...900..101M Altcode: 2020arXiv200800069M
The solar atmosphere is composed of many species, which are populated
at different ionization and excitation levels. The upper chromosphere,
transition region, and corona are nearly collisionless. Consequently,
slippage between, for instance, ions and neutral particles, or
interactions between separate species, may play important roles. We
have developed a 3D MFMS numerical code (Ebysus) to investigate such
effects. Ebysus is capable of treating species (e.g., hydrogen,
helium, etc.) and fluids (neutrals, excited and ionized elements)
separately, including nonequilibrium ionization, momentum exchange,
radiation, thermal conduction, and other complex processes in the solar
atmosphere. Treating different species as different fluids leads to
drifts between different ions and an electric field that couples these
motions. The coupling for two ionized fluids can lead to an anti-phase
rotational motion between them. Different ionized species and momentum
exchange can dissipate this velocity drift, i.e., convert wave kinetic
energy into thermal energy. High-frequency Alfvén waves driven by,
e.g., reconnection thought to occur in the solar atmosphere, can drive
such multi-ion velocity drifts.

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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: Self-consistent 3D radiative magnetohydrodynamic simulations
of coronal rain formation and evolution
Authors: Kohutova, P.; Antolin, P.; Popovas, A.; Szydlarski, M.;
Hansteen, V. H.
2020A&A...639A..20K Altcode: 2020arXiv200503317K
Context. Coronal rain consists of cool and dense plasma condensations
formed in coronal loops as a result of thermal instability. 
Aims: Previous numerical simulations of thermal instability and
coronal rain formation have relied on the practice of artificially
adding a coronal heating term to the energy equation. To reproduce
large-scale characteristics of the corona, the use of more realistic
coronal heating prescription is necessary. Methods: We analysed
coronal rain formation and evolution in a three-dimensional radiative
magnetohydrodynamic simulation spanning from convection zone to
corona which is self-consistently heated by magnetic field braiding
as a result of convective motions. Results: We investigate
the spatial and temporal evolution of energy dissipation along
coronal loops which become thermally unstable. Ohmic dissipation in
the model leads to the heating events capable of inducing sufficient
chromospheric evaporation into the loop to trigger thermal instability
and condensation formation. The cooling of the thermally unstable
plasma occurs on timescales that are comparable to the duration
of the individual impulsive heating events. The impulsive heating
has sufficient duration to trigger thermal instability in the
loop but does not last long enough to lead to coronal rain limit
cycles. We show that condensations can either survive and fall into
the chromosphere or be destroyed by strong bursts of Joule heating
associated with a magnetic reconnection events. In addition, we find
that condensations can also form along open magnetic field lines. 
Conclusions: We modelled, for the first time, coronal rain formation in
a self-consistent 3D radiative magnetohydrodynamic simulation, in which
the heating occurs mainly through the braiding and subsequent Ohmic
dissipation of the magnetic field. The heating is stratified enough
and lasts for long enough along specific field lines to produce the
necessary chromospheric evaporation that triggers thermal instability
in the corona. Movie associated to Fig. 1 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202037899/olm">https://www.aanda.org</A>

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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. The movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201936574/olm">https://www.aanda.org</A>

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Title: The Multi-slit Approach to Coronal Spectroscopy with the
Multi-slit Solar Explorer (MUSE)
Authors: De Pontieu, Bart; Martínez-Sykora, Juan; Testa, Paola;
Winebarger, Amy R.; Daw, Adrian; Hansteen, Viggo; Cheung, Mark C. M.;
Antolin, Patrick
2020ApJ...888....3D Altcode: 2019arXiv190908818D
The Multi-slit Solar Explorer (MUSE) is a proposed mission aimed
at understanding the physical mechanisms driving the heating of the
solar corona and the eruptions that are at the foundation of space
weather. MUSE contains two instruments, a multi-slit extreme ultraviolet
(EUV) spectrograph and a context imager. It will simultaneously
obtain EUV spectra (along 37 slits) and context images with the
highest resolution in space (0.″33-0.″4) and time (1-4 s) ever
achieved for the transition region (TR) and corona. The MUSE science
investigation will exploit major advances in numerical modeling, and
observe at the spatial and temporal scales on which competing models
make testable and distinguishable predictions, thereby leading to a
breakthrough in our understanding of coronal heating and the drivers
of space weather. By obtaining spectra in four bright EUV lines (Fe
IX 171 Å, Fe XV 284 Å, Fe XIX 108Å, Fe XXI 108 Å) covering a wide
range of TR and coronal temperatures along 37 slits simultaneously,
MUSE will be able to “freeze” the evolution of the dynamic
coronal plasma. We describe MUSE’s multi-slit approach and show
that the optimization of the design minimizes the impact of spectral
lines from neighboring slits, generally allowing line parameters to
be accurately determined. We also describe a Spectral Disambiguation
Code to resolve multi-slit ambiguity in locations where secondary lines
are bright. We use simulations of the corona and eruptions to perform
validation tests and show that the multi-slit disambiguation approach
allows accurate determination of MUSE observables in locations where
significant multi-slit contamination occurs.

---------------------------------------------------------
Title: Ebysus: a multi-fluid and multi-species numerical code:
on coupling between ionized species
Authors: Martínez-Sykora, J.; Szydlarski, M.; Hansteen, V. H.
2019AGUFMSH33D3412M Altcode:
The solar atmosphere is composed of many species which are populated at
different ionized/excited levels. The upper chromosphere, transition
region and corona are nearly collisionless. Consequently, ion-neutral
interaction effects or interactions between species may play a role. We
have developed a 3D multi-fluid and multi-species numerical code
(Ebysus) to investigate this scenario. Ebysus is capable of treating
species (e.g., hydrogen, helium etc) and fluids (neutrals, excited and
ionized elements) separately including non-equilibrium ionization,
momentum exchange, radiation, thermal conduction, and other complex
processes in the solar atmosphere. The development of the Ebysus
code started from the already existing and advanced 3D radiative MHD
Bifrost code designed to model the upper convection zone and outer
solar atmosphere. Treating different species as different fluids leads
to the presence of an electric coupling due to drifts on different ion
velocities. This coupling leads to anti-correlated high frequency waves
between the different ionized species. Momentum exchange can dissipate
this kinetic energy. Alfven waves and reconnection can produce such
high frequency waves and thus result in reconnection instabilities,
and in addition reduce the reconnection rate.

---------------------------------------------------------
Title: Ellerman bombs and UV bursts: reconnection at different
atmospheric layers
Authors: Hansteen, V. H.; Ortiz-Carbonell, A. N.; Nobrega, D. E.;
Rouppe van der Voort, L.
2019AGUFMSH13B..06H Altcode:
The emergence of magnetic flux through the photosphere and into the
outer solar atmosphere produces, amongst many other dynamical phenomena,
the appearance of Ellerman bombs (EBs) in the photosphere. EBs are
observed in the wings of H(alpha) and are highly likely to be due to
reconnection in the photosphere, below the chromospheric canopy. Signs
of the reconnection process are also observed in several other spectral
lines, typical of the chromosphere or the transition region. An example
are the 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
1-m Solar Telescope (SST) and the Interface Region Ima ging Spectrograph
(IRIS) spacecraft in order to study the possible relationship between
reconnection events at different layers in the atmosphere and, in
particular, the timing history between them. H(alpha) 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
1330 (C II, transition region), 1400 (Si IV, transition region) and 2796
(Mg II k, core, upper chromosphere) Ã…, as well as spectroscopy in
the far and near ultraviolet from the fast spectrograph raster, allow
us to study the possible chromospheric/transition region counterparts
of those photospheric EBs. Our main goal is to study whether there is
a temporal and spatial relationship between the appearance of an EB
and the appearance of a UV burst and the connection of these dynamical
phenomena to the appearance of surges in the chromosphere. We also
investigate in detail the properties of an extended UV burst and their
v ariations 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. This scenario is in agreement
with the most recent 3D numerical experiments modeling flux emergence.

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Title: Unfolding Overlappogram Data: Preparing for the COOL-AID
instrument on Hi-C FLARE
Authors: Winebarger, A. R.; De Pontieu, B.; Cheung, C. M. M.;
Martinez-Sykora, J.; Hansteen, V. H.; Testa, P.; Golub, L.; Savage,
S. L.; Samra, J.; Reeves, K.
2019AGUFMSH33A..06W Altcode:
During a solar flare, energy released in the corona streams to the solar
chromosphere, where plasma is heated and then evaporated upward. The
magnitude of these velocities and their evolution as a function of time
can provide quantitative information on the magnitude of energy released
and the method by which it is transported in a solar flare. Measuring
these velocities, however, is quite challenging. Typically, they are
measured with single slit spectrometers, where light passing through
a long but narrow slit is dispersed and emission lines formed across
a range of temperatures are observed. The main issue with using
single slit spectrometers to make this measurement is that they are
rarely pointed at the right place at the right time. Additionally,
their fields of view are limited by narrow slit widths, and although
rastering can effectively expand the field of view, it does so at the
cost of time. This combination means that single slit spectrometers
cannot adequately capture the evolution of the flare velocities. On
the contrary, slitless spectrometers can make "overlappograms”,
which provide both imaging and spectral information over a large field
of view. However, spatial information from different spectral lines
can overlap in the dispersion direction, making the data difficult
to interpret. Furthermore, the spectral resolution of slitless
spectrometers are limited and typically worse than single-slit
spectrometers, since no line fitting (and hence sub-pixel sampling) is
possible. For the next generation of the High-resolution Coronal
Imager (Hi-C) Rocket Experiment, which we are proposing to launch during
a solar flare, we are including the COronal OverLapagram - Ancillary
Imaging Diagnostics (COOL-AID) instrument. COOL-AID is a slitless
spectrometer based on the COronal Spectrographic Imager in the EUV
(COSIE) design, but with a narrow passband coating around 12.9 nm (the
same passband as the primary Hi-C telescope), a spatial resolution of
~1"x2", and a velocity resolution of ~5 km/s. The goal of the COOL-AID
instrument is to determine the velocity associated with the Fe XXI
12.9 nm spectral line during a solar flare. In this talk, we will
demonstrate the unfolding method developed by Cheung et al (2019) to
determine the velocity information from a simulated COOL-AID data set.

---------------------------------------------------------
Title: A comprehensive three-dimensional radiative magnetohydrodynamic
simulation of a solar flare
Authors: Cheung, M. C. M.; Rempel, M.; Chintzoglou, G.; Chen, F.;
Testa, P.; Martínez-Sykora, J.; Sainz Dalda, A.; DeRosa, M. L.;
Malanushenko, A.; Hansteen, V.; De Pontieu, B.; Carlsson, M.; Gudiksen,
B.; McIntosh, S. W.
2019NatAs...3..160C Altcode: 2018NatAs...3..160C
Solar and stellar flares are the most intense emitters of X-rays and
extreme ultraviolet radiation in planetary systems1,2. On
the Sun, strong flares are usually found in newly emerging sunspot
regions3. The emergence of these magnetic sunspot groups
leads to the accumulation of magnetic energy in the corona. When
the magnetic field undergoes abrupt relaxation, the energy released
powers coronal mass ejections as well as heating plasma to temperatures
beyond tens of millions of kelvins. While recent work has shed light
on how magnetic energy and twist accumulate in the corona4
and on how three-dimensional magnetic reconnection allows for rapid
energy release5,6, a self-consistent model capturing how
such magnetic changes translate into observable diagnostics has remained
elusive. Here, we present a comprehensive radiative magnetohydrodynamics
simulation of a solar flare capturing the process from emergence to
eruption. The simulation has sufficient realism for the synthesis of
remote sensing measurements to compare with observations at visible,
ultraviolet and X-ray wavelengths. This unifying model allows us to
explain a number of well-known features of solar flares7,
including the time profile of the X-ray flux during flares, origin
and temporal evolution of chromospheric evaporation and condensation,
and sweeping of flare ribbons in the lower atmosphere. Furthermore,
the model reproduces the apparent non-thermal shape of coronal X-ray
spectra, which is the result of the superposition of multi-component
super-hot plasmas8 up to and beyond 100 million K.

---------------------------------------------------------
Title: Principles Of Heliophysics: a textbook on the universal
processes behind planetary habitability
Authors: Schrijver, Karel; Bagenal, Fran; Bastian, Tim; Beer,
Juerg; Bisi, Mario; Bogdan, Tom; Bougher, Steve; Boteler, David;
Brain, Dave; Brasseur, Guy; Brownlee, Don; Charbonneau, Paul; Cohen,
Ofer; Christensen, Uli; Crowley, Tom; Fischer, Debrah; Forbes, Terry;
Fuller-Rowell, Tim; Galand, Marina; Giacalone, Joe; Gloeckler, George;
Gosling, Jack; Green, Janet; Guetersloh, Steve; Hansteen, Viggo;
Hartmann, Lee; Horanyi, Mihaly; Hudson, Hugh; Jakowski, Norbert;
Jokipii, Randy; Kivelson, Margaret; Krauss-Varban, Dietmar; Krupp,
Norbert; Lean, Judith; Linsky, Jeff; Longcope, Dana; Marsh, Daniel;
Miesch, Mark; Moldwin, Mark; Moore, Luke; Odenwald, Sten; Opher, Merav;
Osten, Rachel; Rempel, Matthias; Schmidt, Hauke; Siscoe, George;
Siskind, Dave; Smith, Chuck; Solomon, Stan; Stallard, Tom; Stanley,
Sabine; Sojka, Jan; Tobiska, Kent; Toffoletto, Frank; Tribble, Alan;
Vasyliunas, Vytenis; Walterscheid, Richard; Wang, Ji; Wood, Brian;
Woods, Tom; Zapp, Neal
2019arXiv191014022S Altcode:
This textbook gives a perspective of heliophysics in a way that
emphasizes universal processes from a perspective that draws attention
to what provides Earth (and similar (exo-)planets) with a relatively
stable setting in which life as we know it can thrive. The book is
intended for students in physical sciences in later years of their
university training and for beginning graduate students in fields of
solar, stellar, (exo-)planetary, and planetary-system sciences.

---------------------------------------------------------
Title: Multi-component Decomposition of Astronomical Spectra by
Compressed Sensing
Authors: Cheung, Mark C. M.; De Pontieu, Bart; Martínez-Sykora,
Juan; Testa, Paola; Winebarger, Amy R.; Daw, Adrian; Hansteen, Viggo;
Antolin, Patrick; Tarbell, Theodore D.; Wuelser, Jean-Pierre; Young,
Peter; MUSE Team
2019ApJ...882...13C Altcode: 2019arXiv190203890C
The signal measured by an astronomical spectrometer may be due to
radiation from a multi-component mixture of plasmas with a range of
physical properties (e.g., temperature, Doppler velocity). Confusion
between multiple components may be exacerbated if the spectrometer
sensor is illuminated by overlapping spectra dispersed from different
slits, with each slit being exposed to radiation from a different
portion of an extended astrophysical object. We use a compressed sensing
method to robustly retrieve the different components. This method can
be adopted for a variety of spectrometer configurations, including
single-slit, multi-slit (e.g., the proposed MUlti-slit Solar Explorer
mission), and slot spectrometers (which produce overlappograms).

---------------------------------------------------------
Title: New View of the Solar Chromosphere
Authors: Carlsson, Mats; De Pontieu, Bart; Hansteen, Viggo H.
2019ARA&A..57..189C Altcode:
The solar chromosphere forms a crucial, yet complex and until recently
poorly understood, interface between the solar photosphere and the
heliosphere. Advances in high-resolution instrumentation, adaptive
optics, image reconstruction techniques, and space-based observatories
allow unprecedented high-resolution views of the finely structured and
highly dynamic chromosphere. Dramatic progress in numerical computations
allows 3D radiative magnetohydrodynamic forward models to take the place
of the previous generation of 1D semiempirical atmosphere models. These
new models provide deep insight into complex nonlocal thermodynamic
equilibrium chromospheric diagnostics and enable physics-based
interpretations of observations. This combination of modeling and
observations has led to new insights into the role of shock waves,
transverse magnetic waves, magnetic reconnection and flux emergence
in the chromospheric energy balance, the formation of spicules,
the impact of ion-neutral interactions, and the connectivity between
chromosphere and transition region. During the next few years, the
advent of new instrumentation (integral-field-unit spectropolarimetry)
and observatories (ALMA, DKIST), coupled with novel inversion codes and
expansion of existing numerical models to deal with ever more complex
physical processes (including multifluid approaches), is expected to
lead to major new insights into the dominant heating processes in the
chromosphere and beyond.

---------------------------------------------------------
Title: Radiative MHD Simulation of a Solar Flare
Authors: Cheung, Mark; Rempel, Matthias D.; Chintzoglou, Georgios;
Chen, Feng; Testa, Paola; Martinez-Sykora, Juan; Sainz Dalda, Alberto;
DeRosa, Marc L.; Malanushenko, Anna; Hansteen, Viggo; Carlsson, Mats;
De Pontieu, Bart; Gudiksen, Boris; McIntosh, Scott W.
2019AAS...23431005C Altcode:
We present a radiative MHD simulation of a solar flare. The
computational domain captures the near-surface layers of the convection
zone and overlying atmosphere. Inspired by the observed evolution of
NOAA Active Region (AR) 12017, a parasitic bipolar region is imposed
to emerge in the vicinity of a pre-existing sunspot. The emergence of
twisted magnetic flux generates shear flows that create a pre-existing
flux rope underneath the canopy field of the sunspot. Following erosion
of the overlying bootstrapping field, the flux rope erupts. Rapid
release of magnetic energy results in multi-wavelength synthetic
observables (including X-ray spectra, narrowband EUV images, Doppler
shifts of EUV lines) that are consistent with flare observations. This
works suggests the super-position of multi-thermal, superhot (up
to 100 MK) plasma may be partially responsible for the apparent
non-thermal shape of coronal X-ray sources in flares. Implications
for remote sensing observations of other astrophysical objects is also
discussed. This work is an important stepping stone toward high-fidelity
data-driven MHD models.

---------------------------------------------------------
Title: Ellerman bombs and UV bursts: transient events in chromospheric
current sheets
Authors: Hansteen, V.; Ortiz, A.; Archontis, V.; Carlsson, M.; Pereira,
T. M. D.; Bjørgen, J. P.
2019A&A...626A..33H Altcode: 2019arXiv190411524H
Context. Ellerman bombs (EBs), observed in the photospheric wings
of the Hα line, and UV bursts, observed in the transition region Si
IV line, are both brightenings related to flux emergence regions and
specifically to magnetic flux of opposite polarity that meet in the
photosphere. These two reconnection-related phenomena, nominally formed
far apart, occasionally occur in the same location and at the same
time, thus challenging our understanding of reconnection and heating
of the lower solar atmosphere. Aims: We consider the formation
of an active region, including long fibrils and hot and dense coronal
plasma. The emergence of a untwisted magnetic flux sheet, injected 2.5
Mm below the photosphere, is studied as it pierces the photosphere
and interacts with the preexisting ambient field. Specifically, we
aim to study whether EBs and UV bursts are generated as a result of
such flux emergence and examine their physical relationship. 
Methods: The Bifrost radiative magnetohydrodynamics code was used
to model flux emerging into a model atmosphere that contained a
fairly strong ambient field, constraining the emerging field to
a limited volume wherein multiple reconnection events occur as
the field breaks through the photosphere and expands into the outer
atmosphere. Synthetic spectra of the different reconnection events were
computed using the 1.5D RH code and the fully 3D MULTI3D code. Results: The formation of UV bursts and EBs at intensities and
with line profiles that are highly reminiscent of observed spectra
are understood to be a result of the reconnection of emerging flux
with itself in a long-lasting current sheet that extends over several
scale heights through the chromosphere. Synthetic spectra in the Hα
and Si IV 139.376 nm lines both show characteristics that are typical
of the observations. These synthetic diagnostics suggest that there
are no compelling reasons to assume that UV bursts occur in the
photosphere. Instead, EBs and UV bursts are occasionally formed at
opposite ends of a long current sheet that resides in an extended bubble
of cool gas. The movie associated to Fig. 3 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201935376/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: On the Origin of the Magnetic Energy in the Quiet Solar
Chromosphere
Authors: Martínez-Sykora, Juan; Hansteen, Viggo H.; Gudiksen, Boris;
Carlsson, Mats; De Pontieu, Bart; Gošić, Milan
2019ApJ...878...40M Altcode: 2019arXiv190404464M
The presence of magnetic field is crucial in the transport of energy
through the solar atmosphere. Recent ground-based and space-borne
observations of the quiet Sun have revealed that magnetic field
accumulates at photospheric heights, via a local dynamo or from
small-scale flux emergence events. However, most of this small-scale
magnetic field may not expand into the chromosphere due to the entropy
drop with height at the photosphere. Here we present a study that uses
a high-resolution 3D radiative MHD simulation of the solar atmosphere
with non-gray and non-LTE radiative transfer and thermal conduction
along the magnetic field to reveal that (1) the net magnetic flux
from the simulated quiet photosphere is not sufficient to maintain a
chromospheric magnetic field (on average), (2) processes in the lower
chromosphere, in the region dominated by magnetoacoustic shocks,
are able to convert kinetic energy into magnetic energy, (3) the
magnetic energy in the chromosphere increases linearly in time until
the rms of the magnetic field strength saturates at roughly 4-30 G
(horizontal average) due to conversion from kinetic energy, (4) and
that the magnetic features formed in the chromosphere are localized
to this region.

---------------------------------------------------------
Title: Multi-component Decomposition of Astronomical Spectra by
Compressed Sensing
Authors: Cheung, Mark; De Pontieu, Bart; Martinez-Sykora, Juan; Testa,
Paola; Winebarger, Amy R.; Daw, Adrian N.; Hansteen, Viggo; Antolin,
Patrick; Tarbell, Theodore D.; Wuelser, Jean-Pierre; Young, Peter R.
2019AAS...23411603C Altcode:
The signal measured by an astronomical spectrometer may be due to
radiation from a multi-component mixture of plasmas with a range of
physical properties (e.g. temperature, Doppler velocity). Confusion
between multiple components may be exacerbated if the spectrometer
sensor is illuminated by overlapping spectra dispersed from different
slits, with each slit being exposed to radiation from a different
portion of an extended astrophysical object. We use a compressed sensing
method to robustly retrieve the different components. This method can
be adopted for a variety of spectrometer configurations, including
single-slit, multi-slit (e.g., the proposed MUlti-slit Solar Explorer
mission; MUSE) and slot spectrometers (which produce overlappograms).

---------------------------------------------------------
Title: Linking the Sun to the heliosphere using composition data
and modelling: coronal jets as a test case
Authors: Wimmer-Schweingruber, Robert F.; Parenti, Susanna; Del Zanna,
G.; Edmondson, J.; Giunta, A.; Hansteen, V. H.; Higginson, A.; Lepri,
S.; Laming, M.; Lynch, B. J.; von Steiger, R. E.; Wiegelmann, T.;
Zambrana Prado, N.
2019shin.confE.231W Altcode:
Understanding the formation and evolution of the solar wind is still
a priority in the Solar and Heliospheric communities. We expect
a significant progress in terms of observations with the upcoming
Solar Orbiter mission (launch in 2020), which will provide detailed
in-situ measurements of the solar wind and several remote-sensing
observations. However, real progress will only be possible if we
improve our understanding of the physical link between what measured
in-situ and its source regions on the Sun. In this respect, the plasma
chemical and charge-state compositions are considered good diagnostic
tools. In this paper we present results obtained from an extensive team
work aiming at providing solid diagnostics for linking the in-situ and
the remote sensing measurements. For our test cases, we selected two
periods when a single active region produced, close to its sunspot,
jets which had a counterpart signature in the Heliosphere in the form
of type-III radio bursts. These jets therefore marked magnetically
open regions expanding in the heliosphere. Firstly, we looked for
signatures of the open field associated with the active regions in
in-situ data from ACE and WIND, finding potential tracers. Secondly,
we studied the magnetic topology of the full Sun and Heliosphere with
extrapolations of photospheric data and MHD modeling. We found that
the open field area is consistent with the source and evolution of the
jets, as observed with EUV imagers (SDO/AIA, STEREO/EUVI). Thirdly, we
analysed remote sensing EUV spectroscopic observations to measure the
plasma conditions (densities, temperatures and chemical composition)
whenever available. We then modeled the solar wind and charge
state evolution with the solar distance along the open fields to
establish a link between the in-situ signatures and the remote sensing
observations. We discuss the various difficulties associated with such
studies, and highlight how Solar Orbiter measurements can improve them.

---------------------------------------------------------
Title: Ellerman bombs and UV bursts: reconnection at different
atmospheric layers?
Authors: Hansteen, Viggo
2018csc..confE..71H Altcode:
The emergence of magnetic flux through the photosphere and into the
outer solar atmosphere produces, amongst many other phenomena, the
appearance of Ellerman bombs (EBs) in the photosphere. EBs are observed
in the wings of H(alpha) and are highly likely to be due to reconnection
in the photosphere, below the chromospheric canopy. But signs of the
reconnection process are also observed in several other spectral lines,
typical of the chromosphere or transition region. An example are the
UV bursts observed in the transition region lines of Si IV. In this
work we analyze high cadence coordinated observations between the 1-m
Swedish Solar Telescope and the IRIS spacecraft in order to study
the possible relationship between reconnection events at different
layers in the atmosphere, and in particular, the timing history between
them. High cadence, high resolution H-alpha images from the SST provide
us with the positions, timings and trajectories of Ellerman bombs in an
emerging flux region. Simultaneous co-aligned IRIS slit-jaw images at
1400 and 1330 A and detailed Si IV spectra from the fast spectrograph
raster allow us to study the possible transition region counterparts
of those photospheric Ellerman bombs. We complement these observations
with numerical models of Ellerman bombs and UV bursts. Our main goal is
to study whether there is a temporal and spatial relationship between
the appearance of an EB and the appearance of a UV burst.

---------------------------------------------------------
Title: Impact of Type II Spicules in the Corona: Simulations and
Synthetic Observables
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; De Moortel, Ineke;
Hansteen, Viggo H.; Carlsson, Mats
2018ApJ...860..116M Altcode: 2018arXiv180506475M
The role of type II spicules in the corona has been a much debated topic
in recent years. This paper aims to shed light on the impact of type
II spicules in the corona using novel 2.5D radiative MHD simulations,
including ion-neutral interaction effects with the Bifrost code. We
find that the formation of simulated type II spicules, driven by
the release of magnetic tension, impacts the corona in various
manners. Associated with the formation of spicules, the corona
exhibits (1) magneto-acoustic shocks and flows, which supply mass
to coronal loops, and (2) transversal magnetic waves and electric
currents that propagate at Alfvén speeds. The transversal waves and
electric currents, generated by the spicule’s driver and lasting
for many minutes, are dissipated and heat the associated loop. These
complex interactions in the corona can be connected with blueshifted
secondary components in coronal spectral lines (red-blue asymmetries)
observed with Hinode/EIS and SOHO/SUMER, as well as the EUV counterpart
of type II spicules and propagating coronal disturbances observed with
the 171 Å and 193 Å SDO/AIA channels.

---------------------------------------------------------
Title: Disentangling flows in the solar transition region
Authors: Zacharias, P.; Hansteen, V. H.; Leenaarts, J.; Carlsson,
M.; Gudiksen, B. V.
2018A&A...614A.110Z Altcode: 2018arXiv180407513Z
Context. The measured average velocities in solar and stellar spectral
lines formed at transition region temperatures have been difficult
to interpret. The dominant redshifts observed in the lower transition
region naturally leads to the question of how the upper layers of the
solar (and stellar) atmosphere can be maintained. Likewise, no ready
explanation has been made for the average blueshifts often found in
upper transition region lines. However, realistic three-dimensional
radiation magnetohydrodynamics (3D rMHD) models of the solar atmosphere
are able to reproduce the observed dominant line shifts and may thus
hold the key to resolve these issues. Aims: These new 3D rMHD
simulations aim to shed light on how mass flows between the chromosphere
and corona and on how the coronal mass is maintained. These simulations
give new insights into the coupling of various atmospheric layers
and the origin of Doppler shifts in the solar transition region and
corona. Methods: The passive tracer particles, so-called corks,
allow the tracking of parcels of plasma over time and thus the study of
changes in plasma temperature and velocity not only locally, but also
in a co-moving frame. By following the trajectories of the corks, we
can investigate mass and energy flows and understand the composition
of the observed velocities. Results: Our findings show that
most of the transition region mass is cooling. The preponderance of
transition region redshifts in the model can be explained by the higher
percentage of downflowing mass in the lower and middle transition
region. The average upflows in the upper transition region can be
explained by a combination of both stronger upflows than downflows
and a higher percentage of upflowing mass. The most common combination
at lower and middle transition region temperatures are corks that are
cooling and traveling downward. For these corks, a strong correlation
between the pressure gradient along the magnetic field line and the
velocity along the magnetic field line has been observed, indicating a
formation mechanism that is related to downward propagating pressure
disturbances. Corks at upper transition region temperatures are
subject to a rather slow and highly variable but continuous heating
process. Conclusions: Corks are shown to be an essential tool
in 3D rMHD models in order to study mass and energy flows. We have
shown that most transition region plasma is cooling after having been
heated slowly to upper transition region temperatures several minutes
before. Downward propagating pressure disturbances are identified as
one of the main mechanisms responsible for the observed redshifts at
transition region temperatures. The movie associated to Fig. 3
is available at <A href="https://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: Small-scale Magnetic Flux Emergence in the Quiet Sun
Authors: Moreno-Insertis, F.; Martinez-Sykora, J.; Hansteen, V. H.;
Muñoz, D.
2018ApJ...859L..26M Altcode: 2018arXiv180600489M
Small bipolar magnetic features are observed to appear in the interior
of individual granules in the quiet Sun, signaling the emergence of
tiny magnetic loops from the solar interior. We study the origin
of those features as part of the magnetoconvection process in the
top layers of the convection zone. Two quiet-Sun magnetoconvection
models, calculated with the radiation-magnetohydrodynamic (MHD)
Bifrost code and with domain stretching from the top layers of the
convection zone to the corona, are analyzed. Using 3D visualization
as well as a posteriori spectral synthesis of Stokes parameters,
we detect the repeated emergence of small magnetic elements in the
interior of granules, as in the observations. Additionally, we identify
the formation of organized horizontal magnetic sheets covering whole
granules. Our approach is twofold, calculating statistical properties
of the system, like joint probability density functions (JPDFs), and
pursuing individual events via visualization tools. We conclude that
the small magnetic loops surfacing within individual granules in the
observations may originate from sites at or near the downflows in the
granular and mesogranular levels, probably in the first 1 or 1.5 Mm
below the surface. We also document the creation of granule-covering
magnetic sheet-like structures through the sideways expansion of a
small subphotospheric magnetic concentration picked up and pulled out
of the interior by a nascent granule. The sheet-like structures that we
found in the models may match the recent observations of Centeno et al.

---------------------------------------------------------
Title: Small-scale heating events in the solar atmosphere: lifetime,
total energy and magnetic properties
Authors: Guerreiro, Nuno; Haberreiter, Margit; Hansteen, Viggo;
Schmutz, Werner
2018EGUGA..2014516G Altcode:
Small-scale heating events (SSHEs) are believed to play a fundamental
role in the heating of the solar corona, the pervading redshifts in
the transition region, and the acceleration of spicules. We present
a method to identify and track SSHEs over their lifetime and apply
it to two simulation models. We identify the locations where the
energy dissipation is maximum inside the SSHEs volume and we trace the
SSHEs by following the spatial and temporal evolution of the maximum
energy dissipation inside the SSHEs volume. The method is effective
in following the SSHE and allows us to determine their lifetime,
total energy and properties of the plasma as well as the magnetic
field orientation in the vicinity of the SSHEs. We conclude that
the SSHEs that have the potential to heat the corona live less than
4 minutes. Moreover, the typically energy release ranges from 1020
erg to 1024 erg. In addition, the directional change of the magnetic
field lines on both sides of the current sheet constituting the SSHEs
at the time of the absolute maximum energy dissipation ranges from 5
to 15 degree. This work is very relevant for the interpretation and
the potential observational evidence of the SSHEs from upcoming data
from the Spectral Imaging of the Coronal Environment instrument (SPICE)
and the Extreme UV imager (EUI) onboard the Solar Orbiter Mission.

---------------------------------------------------------
Title: Chromospheric counterparts of solar transition region
unresolved fine structure loops
Authors: Pereira, Tiago M. D.; Rouppe van der Voort, Luc; Hansteen,
Viggo H.; De Pontieu, Bart
2018A&A...611L...6P Altcode: 2018arXiv180304415P
Low-lying loops have been discovered at the solar limb in transition
region temperatures by the Interface Region Imaging Spectrograph
(IRIS). They do not appear to reach coronal temperatures, and it
has been suggested that they are the long-predicted unresolved fine
structures (UFS). These loops are dynamic and believed to be visible
during both heating and cooling phases. Making use of coordinated
observations between IRIS and the Swedish 1-m Solar Telescope, we study
how these loops impact the solar chromosphere. We show for the first
time that there is indeed a chromospheric signal of these loops, seen
mostly in the form of strong Doppler shifts and a conspicuous lack of
chromospheric heating. In addition, we find that several instances have
a inverse Y-shaped jet just above the loop, suggesting that magnetic
reconnection is driving these events. Our observations add several
puzzling details to the current knowledge of these newly discovered
structures; this new information must be considered in theoretical
models. Two movies associated to Fig. 1 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201832762/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: Three-dimensional modeling of the Ca II H and K lines in the
solar atmosphere
Authors: Bjørgen, Johan P.; Sukhorukov, Andrii V.; Leenaarts, Jorrit;
Carlsson, Mats; de la Cruz Rodríguez, Jaime; Scharmer, Göran B.;
Hansteen, Viggo H.
2018A&A...611A..62B Altcode: 2017arXiv171201045B
Context. CHROMIS, a new imaging spectrometer at the Swedish 1-m Solar
Telescope (SST), can observe the chromosphere in the H and K lines of
Ca II at high spatial and spectral resolution. Accurate modeling as
well as an understanding of the formation of these lines are needed to
interpret the SST/CHROMIS observations. Such modeling is computationally
challenging because these lines are influenced by strong departures from
local thermodynamic equilibrium, three-dimensional radiative transfer,
and partially coherent resonance scattering of photons. Aim. We aim to
model the Ca II H and K lines in 3D model atmospheres to understand
their formation and to investigate their diagnostic potential for
probing the chromosphere. Methods: We model the synthetic
spectrum of Ca II using the radiative transfer code Multi3D in three
different radiation-magnetohydrodynamic model atmospheres computed with
the Bifrost code. We classify synthetic intensity profiles according
to their shapes and study how their features are related to the
physical properties in the model atmospheres. We investigate whether
the synthetic data reproduce the observed spatially-averaged line
shapes, center-to-limb variation and compare this data with SST/CHROMIS
images. Results: The spatially-averaged synthetic line profiles
show too low central emission peaks, and too small separation between
the peaks. The trends of the observed center-to-limb variation of
the profiles properties are reproduced by the models. The Ca II H and
K line profiles provide a temperature diagnostic of the temperature
minimum and the temperature at the formation height of the emission
peaks. The Doppler shift of the central depression is an excellent
probe of the velocity in the upper chromosphere.

---------------------------------------------------------
Title: Ellerman bombs and UV bursts: reconnection at different
atmospheric layers?
Authors: Hansteen, V. H.; Ortiz-Carbonell, A. N.; Rouppe van der
Voort, L.
2017AGUFMSH43A2801H Altcode:
The emergence of magnetic flux through the photosphere and into the
outer solar atmosphere produces, amongst many other phenomena, the
appearance of Ellerman bombs (EBs) in the photosphere. EBs are observed
in the wings of H(alpha) and are highly likely to be due to reconnection
in the photosphere, below the chromospheric canopy. However, signs of
the reconnection process are also observed in several other spectral
lines, typical of the chromosphere or transition region. An example
are the UV bursts observed in the transition region lines of Si
IV. In this work we analyze high cadence coordinated observations
between the 1-m Swedish Solar Telescope and the IRIS spacecraft in
order to study the possible relationship between reconnection events
at different layers in the atmosphere, and in particular, the timing
history between them. High cadence, high resolution H-alpha images
from the SST provide us with the positions, timings and trajectories
of Ellerman bombs in an emerging flux region. Simultaneous co-aligned
IRIS slit-jaw images at 1400 and 1330 A and detailed Si IV spectra from
the fast spectrograph raster allow us to study the transition region
counterparts of those photospheric Ellerman bombs. Our main goal is to
study whether there is a temporal relationship between the appearance
of an EB and the appearance of a UV burst. Eventually we would like
to investigate whether reconnection happens at discrete heights,
or as a reconnection sheet spanning several layers at the same time.

---------------------------------------------------------
Title: Mass and energy flows between the Solar chromosphere,
transition region, and corona
Authors: Hansteen, V. H.
2017AGUFMSH41C..01H Altcode:
A number of increasingly sophisticated numerical simulations spanning
the convection zone to corona have shed considerable insight into the
role of the magnetic field in the structure and energetics of the Sun's
outer atmosphere. This development is strengthened by the wealth of
observational data now coming on-line from both ground based and space
borne observatories. We discuss what numerical models can tell us about
the mass and energy flows in the region of the upper chromosphere and
lower corona, using a variety of tools, including the direct comparison
with data and the use of passive tracer particles (so-called 'corks')
inserted into the simulated flows.

---------------------------------------------------------
Title: Constraints on active region coronal heating properties
from observations and modeling of chromospheric, transition region,
and coronal emission
Authors: Testa, P.; Polito, V.; De Pontieu, B.; Carlsson, M.; Reale,
F.; Allred, J. C.; Hansteen, V. H.
2017AGUFMSH43A2804T Altcode:
We investigate coronal heating properties in active region cores in
non-flaring conditions, using high spatial, spectral, and temporal
resolution chromospheric/transition region/coronal observations coupled
with detailed modeling. We will focus, in particular, on observations
with the Interface Region Imaging Spectrograph (IRIS), joint with
observations with Hinode (XRT and EIS) and SDO/AIA. We will discuss
how these observations and models (1D HD and 3D MHD, with the RADYN
and Bifrost codes) provide useful diagnostics of the coronal heating
processes and mechanisms of energy transport.

---------------------------------------------------------
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: Impact of Type II Spicules into the Corona
Authors: Martinez-Sykora, Juan; De Pontieu, Bart; Carlsson, Mats;
Hansteen, Viggo H.; Pereira, Tiago M. D.
2017SPD....4810403M Altcode:
In the lower solar atmosphere, the chromosphere is permeated by jets,
in which plasma is propelled at speeds of 50-150 km/s into the Sun’s
atmosphere or corona. Although these spicules may play a role in heating
the million-degree corona and are associated with Alfvén waves that
help drive the solar wind, their generation remains mysterious. We
implemented in the radiative MHD Bifrost code the effects of partial
ionization using the generalized Ohm’s law. This code also solves
the full MHD equations with non-grey and non-LTE radiative transfer
and thermal conduction along magnetic field lines. The ion-neutral
collision frequency is computed using recent studies that improved the
estimation of the cross sections under chromospheric conditions (Vranjes
&amp; Krstic 2013). Self-consistently driven jets (spicules type II)
in magnetohydrodynamic simulations occur ubiquitously when magnetic
tension is confined and transported upwards through interactions
between ions and neutrals, and impulsively released to drive flows,
heat plasma, generate Alfvén waves, and may play an important role in
maintaining the substructure of loop fans. This mechanism explains how
spicular plasma can be heated to millions of degrees and how Alfvén
waves are generated in the chromosphere.

---------------------------------------------------------
Title: Realistic radiative MHD simulation of a solar flare
Authors: Rempel, Matthias D.; Cheung, Mark; Chintzoglou, Georgios;
Chen, Feng; Testa, Paola; Martinez-Sykora, Juan; Sainz Dalda, Alberto;
DeRosa, Marc L.; Viktorovna Malanushenko, Anna; Hansteen, Viggo H.;
De Pontieu, Bart; Carlsson, Mats; Gudiksen, Boris; McIntosh, Scott W.
2017SPD....4840001R Altcode:
We present a recently developed version of the MURaM radiative
MHD code that includes coronal physics in terms of optically thin
radiative loss and field aligned heat conduction. The code employs
the "Boris correction" (semi-relativistic MHD with a reduced speed
of light) and a hyperbolic treatment of heat conduction, which allow
for efficient simulations of the photosphere/corona system by avoiding
the severe time-step constraints arising from Alfven wave propagation
and heat conduction. We demonstrate that this approach can be used
even in dynamic phases such as a flare. We consider a setup in which
a flare is triggered by flux emergence into a pre-existing bipolar
active region. After the coronal energy release, efficient transport
of energy along field lines leads to the formation of flare ribbons
within seconds. In the flare ribbons we find downflows for temperatures
lower than ~5 MK and upflows at higher temperatures. The resulting
soft X-ray emission shows a fast rise and slow decay, reaching a peak
corresponding to a mid C-class flare. The post reconnection energy
release in the corona leads to average particle energies reaching 50
keV (500 MK under the assumption of a thermal plasma). We show that
hard X-ray emission from the corona computed under the assumption of
thermal bremsstrahlung can produce a power-law spectrum due to the
multi-thermal nature of the plasma. The electron energy flux into the
flare ribbons (classic heat conduction with free streaming limit) is
highly inhomogeneous and reaches peak values of about 3x1011
erg/cm2/s in a small fraction of the ribbons, indicating
regions that could potentially produce hard X-ray footpoint sources. We
demonstrate that these findings are robust by comparing simulations
computed with different values of the saturation heat flux as well as
the "reduced speed of light".

---------------------------------------------------------
Title: Small-scale heating events in the solar
atmosphere. II. Lifetime, total energy, and magnetic properties
Authors: Guerreiro, N.; Haberreiter, M.; Hansteen, V.; Schmutz, W.
2017A&A...603A.103G Altcode:
Context. Small-scale heating events (SSHEs) are believed to play a
fundamental role in understanding the process responsible for heating
of the solar corona, the pervading redshifts in the transition region,
and the acceleration of spicules. Aims: We determine the
properties of the SSHEs and the atmospheric response to them in 3D
magnetohydrodynamics (3D-MHD) simulations of the solar atmosphere. Methods: We developed a method for identifying and following
SSHEs over their lifetime, and applied it to two simulation models. We
identified the locations where the energy dissipation is greatest inside
the SSHEs volume, and we traced the SSHEs by following the spatial
and temporal evolution of the maximum energy dissipation inside the
SSHEs volume. Results: The method is effective in following the
SSHEs. We can determine their lifetime, total energy, and properties of
the plasma, as well as the magnetic field orientation in the vicinity
of the SSHEs. Conclusions: We determine that the SSHEs that have
the potential to heat the corona live less than 4 min, and typically
the energy they release ranges from 1020 to 1024
erg. In addition, the directional change of the field lines on the two
sides of the current sheet constituting the SSHEs ranges from 5° to
15° at the moment of the absolute maximum energy dissipation.

---------------------------------------------------------
Title: On the generation of solar spicules and Alfvénic waves
Authors: Martínez-Sykora, J.; De Pontieu, B.; Hansteen, V. H.;
Rouppe van der Voort, L.; Carlsson, M.; Pereira, T. M. D.
2017Sci...356.1269M Altcode: 2017arXiv171007559M
In the lower solar atmosphere, the chromosphere is permeated by jets
known as spicules, in which plasma is propelled at speeds of 50 to
150 kilometers per second into the corona. The origin of the spicules
is poorly understood, although they are expected to play a role in
heating the million-degree corona and are associated with Alfvénic
waves that help drive the solar wind. We compare magnetohydrodynamic
simulations of spicules with observations from the Interface Region
Imaging Spectrograph and the Swedish 1-m Solar Telescope. Spicules
are shown to occur when magnetic tension is amplified and transported
upward through interactions between ions and neutrals or ambipolar
diffusion. The tension is impulsively released to drive flows, heat
plasma (through ambipolar diffusion), and generate Alfvénic waves.

---------------------------------------------------------
Title: Bombs and Flares at the Surface and Lower Atmosphere of the Sun
Authors: Hansteen, V. H.; Archontis, V.; Pereira, T. M. D.; Carlsson,
M.; Rouppe van der Voort, L.; Leenaarts, J.
2017ApJ...839...22H Altcode: 2017arXiv170402872H
A spectacular manifestation of solar activity is the appearance of
transient brightenings in the far wings of the Hα line, known as
Ellerman bombs (EBs). Recent observations obtained by the Interface
Region Imaging Spectrograph have revealed another type of plasma
“bombs” (UV bursts) with high temperatures of perhaps up to 8 ×
104 K within the cooler lower solar atmosphere. Realistic
numerical modeling showing such events is needed to explain
their nature. Here, we report on 3D radiative magnetohydrodynamic
simulations of magnetic flux emergence in the solar atmosphere. We
find that ubiquitous reconnection between emerging bipolar magnetic
fields can trigger EBs in the photosphere, UV bursts in the mid/low
chromosphere and small (nano-/micro-) flares (106 K) in
the upper chromosphere. These results provide new insights into the
emergence and build up of the coronal magnetic field and the dynamics
and heating of the solar surface and lower atmosphere.

---------------------------------------------------------
Title: Bombs, jets and flares at the surface and lower atmosphere
of the Sun
Authors: Hansteen, Viggo
2017psio.confE..58H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Fundamental Physics of the Slow Solar Wind - What do we Know?
Authors: Ofman, L.; Abbo, L.; Antiochos, S. K.; Hansteen, V. H.;
Harra, L.; Ko, Y. K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.;
von Steiger, R.; Wang, Y. M.
2016AGUFMSH42A..01O Altcode:
Fundamental physical properties of the slow solar wind (SSW), such
as density, temperature, outflow speed, heavy ion abundances and
charges states were obtained from in-situ measurements at 1AU in
the past from WIND, ACE, and other spacecraft. Plasma and magnetic
field measurement are available as close as 0.3 AU from Helios data,
Spektr-R, and MESSENGER spacecraft. Remote sensing spectroscopic
measurements are available in the corona and below from SOHO/UVCS,
Hinode, and other missions. One of the major objectives of the Solar
Orbiter and Solar Probe Plus missions is to study the sources of the
SSW close to the Sun. The present state of understanding of the physics
of the SSW is based on the combination of the existing observations,
theoretical and numerical 3D MHD and multi-fluid models, that connect
between the SSW sources in the corona and the heliosphere. Recently,
hybrid models that combine fluid electrons and kinetic ions of the
expanding solar wind were developed, and provide further insights of the
local SSW plasma heating processes that related to turbulent magnetic
fluctuations spectra and kinetic ion instabilities observed in the
SSW plasma. These models produce the velocity distribution functions
(VDFs) of the protons and heavier ions as well as the ion anisotropic
temperatures. I will discuss the results of the above observations
and models, and review the current status of our understanding of
the fundamental physics of the SSW. I will review the open questions,
and discuss how they could be addressed with near future observations
and models.

---------------------------------------------------------
Title: Slow Solar Wind: Observations and Modeling
Authors: Abbo, L.; Ofman, L.; Antiochos, S. K.; Hansteen, V. H.;
Harra, L.; Ko, Y. -K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.;
von Steiger, R.; Wang, Y. -M.
2016SSRv..201...55A Altcode: 2016SSRv..tmp...34A
While it is certain that the fast solar wind originates from coronal
holes, where and how the slow solar wind (SSW) is formed remains an
outstanding question in solar physics even in the post-SOHO era. The
quest for the SSW origin forms a major objective for the planned future
missions such as the Solar Orbiter and Solar Probe Plus. Nonetheless,
results from spacecraft data, combined with theoretical modeling, have
helped to investigate many aspects of the SSW. Fundamental physical
properties of the coronal plasma have been derived from spectroscopic
and imaging remote-sensing data and in situ data, and these results
have provided crucial insights for a deeper understanding of the origin
and acceleration of the SSW. Advanced models of the SSW in coronal
streamers and other structures have been developed using 3D MHD and
multi-fluid equations.

---------------------------------------------------------
Title: On the Misalignment between Chromospheric Features and the
Magnetic Field on the Sun
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Carlsson, Mats;
Hansteen, Viggo
2016ApJ...831L...1M Altcode: 2016arXiv160702551M
Observations of the upper chromosphere show an enormous amount of
intricate fine structure. Much of this comes in the form of linear
features, which are most often assumed to be well aligned with the
direction of the magnetic field in the low plasma β regime that is
thought to dominate the upper chromosphere. We use advanced radiative
magnetohydrodynamic simulations, including the effects of ion-neutral
interactions (using the generalized Ohm’s law) in the partially
ionized chromosphere, to show that the magnetic field is often not well
aligned with chromospheric features. This occurs where the ambipolar
diffusion is large, I.e., ions and neutral populations decouple as
the ion-neutral collision frequency drops, allowing the field to
slip through the neutral population; where currents perpendicular to
the field are strong; and where thermodynamic timescales are longer
than or similar to those of ambipolar diffusion. We find this often
happens in dynamic spicule or fibril-like features at the top of the
chromosphere. This has important consequences for field extrapolation
methods, which increasingly use such upper chromospheric features
to help constrain the chromospheric magnetic field: our results
invalidate the underlying assumption that these features are aligned
with the field. In addition, our results cast doubt on results from
1D hydrodynamic models, which assume that plasma remains on the same
field lines. Finally, our simulations show that ambipolar diffusion
significantly alters the amount of free energy available in the coronal
part of our simulated volume, which is likely to have consequences
for studies of flare initiation.

---------------------------------------------------------
Title: Ellerman Bombs and IRIS Bombs; In the photosphere and above
Authors: Hansteen, Viggo; Archontis, V.
2016usc..confE.118H Altcode:
The lower Solar atmosphere, consisting of the photosphere and
chromosphere, can occasionally show violent activity more often
associated with the magnetically dominated outer layers of the Sun;
the upper chromosphere, transition region and corona. However, in
regions of strong flux emergence, where Solar active regions are being
formed, one can see evidence of photospheric reconnection as the field
struggles to emerge through the non-buoyant photosphere and expand
into the atmosphere above. Ellerman bombs, short lived, brightness
enhancements in the outer wings of strong optical lines are thought to
be a result of such reconnection. Observations made with the NASA's
Interface Region Imaging Spectrograph, showed similar 'UV bursts' in
lines usually associated with the outer Solar atmosphere, while at the
same time clearly being situated below much cooler gas. We here present
a numerical model of flux emergence in which both Ellerman bombs and
perhaps IRIS bombs (UV bursts) are naturally and copiously produced.

---------------------------------------------------------
Title: High Spatial Resolution Fe XII Observations of Solar Active
Regions
Authors: Testa, Paola; De Pontieu, Bart; Hansteen, Viggo
2016ApJ...827...99T Altcode: 2016arXiv160604603T
We use UV spectral observations of active regions with the Interface
Region Imaging Spectrograph (IRIS) to investigate the properties of
the coronal Fe xii 1349.4 Å emission at unprecedented high spatial
resolution (∼0.33″). We find that by using appropriate observational
strategies (I.e., long exposures, lossless compression), Fe xii emission
can be studied with IRIS at high spatial and spectral resolution, at
least for high-density plasma (e.g., post-flare loops and active region
moss). We find that upper transition region (TR; moss) Fe xii emission
shows very small average Doppler redshifts ({v}{{D}} ∼ 3
km s-1) as well as modest non-thermal velocities (with an
average of ∼24 km s-1 and the peak of the distribution at
∼15 km s-1). The observed distribution of Doppler shifts
appears to be compatible with advanced three-dimensional radiative
MHD simulations in which impulsive heating is concentrated at the TR
footpoints of a hot corona. While the non-thermal broadening of Fe
xii 1349.4 Å peaks at similar values as lower resolution simultaneous
Hinode Extreme Ultraviolet Imaging Spectrometer (EIS) measurements of
Fe xii 195 Å, IRIS observations show a previously undetected tail
of increased non-thermal broadening that might be suggestive of the
presence of subarcsecond heating events. We find that IRIS and EIS
non-thermal line broadening measurements are affected by instrumental
effects that can only be removed through careful analysis. Our results
also reveal an unexplained discrepancy between observed 195.1/1349.4
Å Fe xii intensity ratios and those predicted by the CHIANTI atomic
database.

---------------------------------------------------------
Title: Chromospheric and Coronal Wave Generation in a Magnetic
Flux Sheath
Authors: Kato, Yoshiaki; Steiner, Oskar; Hansteen, Viggo; Gudiksen,
Boris; Wedemeyer, Sven; Carlsson, Mats
2016ApJ...827....7K Altcode: 2016arXiv160608826K
Using radiation magnetohydrodynamic simulations of the solar
atmospheric layers from the upper convection zone to the lower corona,
we investigate the self-consistent excitation of slow magneto-acoustic
body waves (slow modes) in a magnetic flux concentration. We
find that the convective downdrafts in the close surroundings of
a two-dimensional flux slab “pump” the plasma inside it in
the downward direction. This action produces a downflow inside the
flux slab, which encompasses ever higher layers, causing an upwardly
propagating rarefaction wave. The slow mode, excited by the adiabatic
compression of the downflow near the optical surface, travels along the
magnetic field in the upward direction at the tube speed. It develops
into a shock wave at chromospheric heights, where it dissipates,
lifts the transition region, and produces an offspring in the form
of a compressive wave that propagates further into the corona. In the
wake of downflows and propagating shock waves, the atmosphere inside
the flux slab in the chromosphere and higher tends to oscillate with a
period of ν ≈ 4 mHz. We conclude that this process of “magnetic
pumping” is a most plausible mechanism for the direct generation
of longitudinal chromospheric and coronal compressive waves within
magnetic flux concentrations, and it may provide an important heat
source in the chromosphere. It may also be responsible for certain
types of dynamic fibrils.

---------------------------------------------------------
Title: The SPICE Spectral Imager on Solar Orbiter: Linking the Sun
to the Heliosphere
Authors: Fludra, Andrzej; Haberreiter, Margit; Peter, Hardi; Vial,
Jean-Claude; Harrison, Richard; Parenti, Susanna; Innes, Davina;
Schmutz, Werner; Buchlin, Eric; Chamberlin, Phillip; Thompson,
William; Gabriel, Alan; Morris, Nigel; Caldwell, Martin; Auchere,
Frederic; Curdt, Werner; Teriaca, Luca; Hassler, Donald M.; DeForest,
Craig; Hansteen, Viggo; Carlsson, Mats; Philippon, Anne; Janvier, Miho;
Wimmer-Schweingruber, Robert; Griffin, Douglas; Davila, Joseph; Giunta,
Alessandra; Waltham, Nick; Eccleston, Paul; Gottwald, Alexander;
Klein, Roman; Hanley, John; Walls, Buddy; Howe, Chris; Schuehle, Udo
2016cosp...41E.607F Altcode:
The SPICE (Spectral Imaging of the Coronal Environment) instrument is
one of the key remote sensing instruments onboard the upcoming Solar
Orbiter Mission. SPICE has been designed to contribute to the science
goals of the mission by investigating the source regions of outflows
and ejection processes which link the solar surface and corona to the
heliosphere. In particular, SPICE will provide quantitative information
on the physical state and composition of the solar atmosphere
plasma. For example, SPICE will access relative abundances of ions to
study the origin and the spatial/temporal variations of the 'First
Ionization Potential effect', which are key signatures to trace the
solar wind and plasma ejections paths within the heliosphere. Here we
will present the instrument and its performance capability to attain the
scientific requirements. We will also discuss how different observation
modes can be chosen to obtain the best science results during the
different orbits of the mission. To maximize the scientific return of
the instrument, the SPICE team is working to optimize the instrument
operations, and to facilitate the data access and their exploitation.

---------------------------------------------------------
Title: Solar abundances with the SPICE spectral imager on Solar
Orbiter
Authors: Giunta, Alessandra; Haberreiter, Margit; Peter, Hardi;
Vial, Jean-Claude; Harrison, Richard; Parenti, Susanna; Innes, Davina;
Schmutz, Werner; Buchlin, Eric; Chamberlin, Phillip; Thompson, William;
Bocchialini, Karine; Gabriel, Alan; Morris, Nigel; Caldwell, Martin;
Auchere, Frederic; Curdt, Werner; Teriaca, Luca; Hassler, Donald M.;
DeForest, Craig; Hansteen, Viggo; Carlsson, Mats; Philippon, Anne;
Janvier, Miho; Wimmer-Schweingruber, Robert; Griffin, Douglas; Baudin,
Frederic; Davila, Joseph; Fludra, Andrzej; Waltham, Nick; Eccleston,
Paul; Gottwald, Alexander; Klein, Roman; Hanley, John; Walls, Buddy;
Howe, Chris; Schuehle, Udo; Gyo, Manfred; Pfiffner, Dany
2016cosp...41E.681G Altcode:
Elemental composition of the solar atmosphere and in particular
abundance bias of low and high First Ionization Potential (FIP)
elements are a key tracer of the source regions of the solar wind. These
abundances and their spatio-temporal variations, as well as the other
plasma parameters , will be derived by the SPICE (Spectral Imaging
of the Coronal Environment) EUV spectral imager on the upcoming
Solar Orbiter mission. SPICE is designed to provide spectroheliograms
(spectral images) using a core set of emission lines arising from ions
of both low-FIP and high-FIP elements. These lines are formed over
a wide range of temperatures, enabling the analysis of the different
layers of the solar atmosphere. SPICE will use these spectroheliograms
to produce dynamic composition maps of the solar atmosphere to be
compared to in-situ measurements of the solar wind composition of
the same elements (i.e. O, Ne, Mg, Fe). This will provide a tool to
study the connectivity between the spacecraft (the Heliosphere) and
the Sun. We will discuss the SPICE capabilities for such composition
measurements.

---------------------------------------------------------
Title: Detection and characterization of small-scale heating events
in the solar atmosphere from 3D-MHD simulations and their potential
role in coronal heating
Authors: Guerreiro, Nuno; Haberreiter, Margit; Schmutz, Werner;
Hansteen, Viggo
2016cosp...41E.750G Altcode:
Aiming at better understanding the mechanism(s) responsible for the
coronal heating we focus on analyzing the properties of the magnetically
generated small-scale heating events (SSHEs) in the solar atmosphere. We
present a comprehensive method to detect and follow SSHEs over time
in 3D-MHD simulations of the solar atmosphere. Applying the method
we are able to better understand the properties of the SSHEs and how
the plasma in their vicinity respond to them. We study the lifetime,
energy and spectral signatures and show that the energy flux dissipated
by them is enough to heat the corona. Ultimately, these results will
be important for the coordinated scientific exploration of SPICE and
EUI along with other instruments on board solar orbiter.

---------------------------------------------------------
Title: Emergence of Granular-sized Magnetic Bubbles Through the
Solar Atmosphere. III. The Path to the Transition Region
Authors: Ortiz, Ada; Hansteen, Viggo H.; Bellot Rubio, Luis Ramón;
de la Cruz Rodríguez, Jaime; De Pontieu, Bart; Carlsson, Mats;
Rouppe van der Voort, Luc
2016ApJ...825...93O Altcode: 2016arXiv160400302O
We study, for the first time, the ascent of granular-sized magnetic
bubbles from the solar photosphere through the chromosphere into the
transition region and above. Such events occurred in a flux emerging
region in NOAA 11850 on 2013 September 25. During that time, the
first co-observing campaign between the Swedish 1-m Solar Telescope
(SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft
was carried out. Simultaneous observations of the chromospheric Hα
656.28 nm and Ca II 854.2 nm lines, plus the photospheric Fe I 630.25
nm line, were made with the CRISP spectropolarimeter at the Spitzer
Space Telescope (SST) reaching a spatial resolution of 0.″14. At
the same time, IRIS was performing a four-step dense raster of the
emerging flux region, taking slit jaw images at 133 (C II, transition
region), 140 (Si IV, transition region), 279.6 (Mg II k, core, upper
chromosphere), and 283.2 nm (Mg II k, wing, photosphere). Spectroscopy
of several lines was performed by the IRIS spectrograph in the far-
and near-ultraviolet, of which we have used the Si IV 140.3 and the
Mg II k 279.6 nm lines. Coronal images from the Atmospheric Imaging
Assembly of the Solar Dynamics Observatory were used to investigate
the possible coronal signatures of the flux emergence events. The
photospheric and chromospheric properties of small-scale emerging
magnetic bubbles have been described in detail in Ortiz et al. Here
we are able to follow such structures up to the transition region. We
describe the properties, including temporal delays, of the observed
flux emergence in all layers. We believe this may be an important
mechanism of transporting energy and magnetic flux from subsurface
layers to the transition region and corona.

---------------------------------------------------------
Title: Physics &amp; Diagnostics of the Drivers of Solar Eruptions
Authors: Cheung, Mark; Rempel, Matthias D.; Martinez-Sykora, Juan;
Testa, Paola; Hansteen, Viggo H.; Viktorovna Malanushenko, Anna;
Sainz Dalda, Alberto; DeRosa, Marc L.; De Pontieu, Bart; Carlsson,
Mats; Chen, Feng; McIntosh, Scott W.; Gudiksen, Boris
2016SPD....47.0607C Altcode:
We provide an update on our NASA Heliophysics Grand Challenges Research
(HGCR) project on the ‘Physics &amp; Diagnostics of the Drivers of
Solar Eruptions’. This presentation will focus on results from a
data-inspired, 3D radiative MHD model of a solar flare. The model
flare results from the interaction of newly emerging flux with a
pre-existing active region. Synthetic observables from the model
reproduce observational features compatible with actual flares. These
include signatures of coronal magnetic reconnection, chromospheric
evaporation, EUV flare arcades, sweeping motion of flare ribbons
and sunquakes.

---------------------------------------------------------
Title: Characterization of small-scale heating events in the solar
atmosphere from 3D-MHD simulations and their potential role in
coronal heating
Authors: Guerreiro, Nuno; Haberreiter, Margit; Hansteen, Viggo;
Schmutz, Werner
2016EGUGA..1814407G Altcode:
Aiming at better understanding the mechanism(s) responsible for
the coronal heating and the ubiquitous redshifts observed in the
lower transition region we focus on analyzing the properties of
small-scale heating events (SSHEs) in the solar atmosphere. We present
a comprehensive method to follow SSHEs over time in 3D-MHD simulations
of the solar atmosphere. Applying the method we are able to better
understand the properties of the SSHEs and how the plasma in their
vicinity respond to them. We present results for the lifetime, energy
and spectral signatures of the SSHEs. Ultimately, these results will
be important for the coordinated scientific exploration of SPICE and
EUI along with other interments on board solar orbiter.

---------------------------------------------------------
Title: Modeling Repeatedly Flaring δ Sunspots
Authors: Chatterjee, Piyali; Hansteen, Viggo; Carlsson, Mats
2016PhRvL.116j1101C Altcode: 2016arXiv160100749C
Active regions (ARs) appearing on the surface of the Sun are classified
into α , β , γ , and δ by the rules of the Mount Wilson Observatory,
California on the basis of their topological complexity. Amongst these,
the δ sunspots are known to be superactive and produce the most
x-ray flares. Here, we present results from a simulation of the Sun
by mimicking the upper layers and the corona, but starting at a more
primitive stage than any earlier treatment. We find that this initial
state consisting of only a thin subphotospheric magnetic sheet breaks
into multiple flux tubes which evolve into a colliding-merging system
of spots of opposite polarity upon surface emergence, similar to those
often seen on the Sun. The simulation goes on to produce many exotic δ
sunspot associated phenomena: repeated flaring in the range of typical
solar flare energy release and ejective helical flux ropes with embedded
cool-dense plasma filaments resembling solar coronal mass ejections.

---------------------------------------------------------
Title: A publicly available simulation of an enhanced network region
of the Sun
Authors: Carlsson, Mats; Hansteen, Viggo H.; Gudiksen, Boris V.;
Leenaarts, Jorrit; De Pontieu, Bart
2016A&A...585A...4C Altcode: 2015arXiv151007581C
Context. The solar chromosphere is the interface between the
solar surface and the solar corona. Modelling of this region is
difficult because it represents the transition from optically
thick to thin radiation escape, from gas-pressure domination to
magnetic-pressure domination, from a neutral to an ionised state,
from MHD to plasma physics, and from near-equilibrium (LTE) to
non-equilibrium conditions. Aims: Our aim is to provide the
community with realistic simulations of the magnetic solar outer
atmosphere. This will enable detailed comparison of existing and
upcoming observations with synthetic observables from the simulations,
thereby elucidating the complex interactions of magnetic fields and
plasma that are crucial for our understanding of the dynamic outer
atmosphere. Methods: We used the radiation magnetohydrodynamics
code Bifrost to perform simulations of a computational volume
with a magnetic field topology similar to an enhanced network
area on the Sun. Results: The full simulation cubes are
made available from the Hinode Science Data Centre Europe. The
general properties of the simulation are discussed, and limitations
are discussed. The Hinode Science Data Centre Europe (<A
href="http://www.sdc.uio.no/search/simulations">http://www.sdc.uio.no/search/simulations</A>).

---------------------------------------------------------
Title: Time Dependent Nonequilibrium Ionization of Transition Region
Lines Observed with IRIS
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo H.;
Gudiksen, Boris
2016ApJ...817...46M Altcode: 2015arXiv151200865M
The properties of nonstatistical equilibrium ionization of silicon
and oxygen ions are analyzed in this work. We focus on five solar
targets (quiet Sun; coronal hole; plage; quiescent active region,
AR; and flaring AR) as observed with the Interface Region Imaging
Spectrograph (IRIS). IRIS is best suited for this work owing to the
high cadence (up to 0.5 s), high spatial resolution (up to 0.″32),
and high signal-to-noise ratios for O IV λ1401 and Si IV λ1402. We
find that the observed intensity ratio between lines of three times
ionized silicon and oxygen ions depends on their total intensity
and that this correlation varies depending on the region observed
(quiet Sun, coronal holes, plage, or active regions) and on the
specific observational objects present (spicules, dynamic loops, jets,
microflares, or umbra). In order to interpret the observations, we
compare them with synthetic profiles taken from 2D self-consistent
radiative MHD simulations of the solar atmosphere, where the
statistical equilibrium or nonequilibrium treatment of silicon and
oxygen is applied. These synthetic observations show vaguely similar
correlations to those in the observations, I.e., between the intensity
ratios and their intensities, but only in the nonequilibrium case do
we find that (some of) the observations can be reproduced. We conclude
that these lines are formed out of statistical equilibrium. We use
our time-dependent nonequilibrium ionization simulations to describe
the physical mechanisms behind these observed properties.

---------------------------------------------------------
Title: Slow Solar Wind: Observable Characteristics for Constraining
Modelling
Authors: Ofman, L.; Abbo, L.; Antiochos, S. K.; Hansteen, V. H.;
Harra, L.; Ko, Y. K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.;
von Steiger, R.; Wang, Y. M.
2015AGUFMSH11F..03O Altcode:
The Slow Solar Wind (SSW) origin is an open issue in the post SOHO
era and forms a major objective for planned future missions such as
the Solar Orbiter and Solar Probe Plus.Results from spacecraft data,
combined with theoretical modeling, have helped to investigate many
aspects of the SSW. Fundamental physical properties of the coronal
plasma have been derived from spectroscopic and imaging remote-sensing
data and in-situ data, and these results have provided crucial insights
for a deeper understanding of the origin and acceleration of the
SSW.Advances models of the SSW in coronal streamers and other structures
have been developed using 3D MHD and multi-fluid equations.Nevertheless,
there are still debated questions such as:What are the source regions
of SSW? What are their contributions to the SSW?Which is the role
of the magnetic topology in corona for the origin, acceleration and
energy deposition of SSW?Which are the possible acceleration and heating
mechanisms for the SSW?The aim of this study is to present the insights
on the SSW origin and formationarisen during the discussions at the
International Space Science Institute (ISSI) by the Team entitled
”Slowsolar wind sources and acceleration mechanisms in the corona”
held in Bern (Switzerland) in March2014--2015. The attached figure will
be presented to summarize the different hypotheses of the SSW formation.

---------------------------------------------------------
Title: Properties of moss emission from joint FeXII IRIS and Hinode
observations of active region plasma
Authors: Testa, P.; De Pontieu, B.; Hansteen, V. H.
2015AGUFMSH31D..06T Altcode:
IRIS provides unprecedented high resolution observations of the solar
chromosphere and transition region. Joint with Hinode XRT and EIS,
and SDO/AIA, these observations cover from the upper photosphere to
the corona and provide tight constraints on the mechanisms of energy
transport and heating of the plasma to coronal temperatures. We present
new IRIS and Hinode coronal studies of the corona in non-flaring
conditions, and compare the spectral line properties of FeXII emission
observed with EIS and IRIS in active region moss. We will discuss
the implications for the heating of hot coronal loops in the core of
active regions.

---------------------------------------------------------
Title: Characterisation of small-scale heating events in the solar
atmosphere from 3D MHD simulations and their potential role in
coronal heating
Authors: Haberreiter, M.; Guerreiro, N.; Hansteen, V. H.; Schmutz,
W. K.
2015AGUFMSH31C2442H Altcode:
The physical mechanism that heats the solar corona is one of the still
open science questions in solar physics. One of the proposed mechanism
for coronal heating are nanoflares. To investigate their role in coronal
heating we study the properties of the small-scale heating events in
the solar atmosphere using 3D MHD simulations. We present a method
to identify and track these heating events in time which allows us to
study their life time, energy, and spectral signatures. These spectal
signatures will be compared with available spectrosopic observations
obtained with IRIS and SUMER. Ultimately, these results will be
important for the coordinated scientific exploitation of SPICE and
EUI along with other instruments onboard Solar Orbiter to address the
coronal heating problem.

---------------------------------------------------------
Title: Impact of the Ion-Neutral Interaction Effects in the Solar
Chromosphere
Authors: Martínez-Sykora, J.; De Pontieu, B.; Hansteen, V. H.;
Carlsson, M.
2015AGUFMSH31B2411M Altcode:
The complexity of the chromosphere is due to various regime changes
that take place across it. Consequently, the interpretation of
chromospheric observations is a challenging task. It is thus crucial
to combine these observations with advanced radiative-MHD numerical
modeling. Because the photosphere, chromosphere and transition region
are partially ionized, the interaction between ionized and neutral
particles has important consequences on the magneto-thermodynamics
of these regions. We implemented the effects of partial ionization
using generalized Ohm's law in the Bifrost code (Gudiksen et al. 2011)
which solves the full MHD equations with non-grey and non-LTE radiative
transfer and thermal conduction along magnetic field lines. We perform
2.5D simulations which combines large and small scales structures. This
leads to a highly dynamic chromosphere with large variety of physical
processes which have not been reproduced with smaller simulations. The
implementation of partial ionization effects impact our modeled
radiative-MHD atmosphere, such as producing chromospheric heating and
diffusion of photospheric magnetic field into the upper-chromosphere. We
will also focus on which observables of these processes can be revealed
with chromospheric observations.

---------------------------------------------------------
Title: Small-scale Heating Events in the Solar
Atmosphere. I. Identification, Selection, and Implications for
Coronal Heating
Authors: Guerreiro, N.; Haberreiter, M.; Hansteen, V.; Schmutz, W.
2015ApJ...813...61G Altcode:
We present a comprehensive method to analyze small-scale heating
events in detail in a 3D magnetohydrodynamics simulation for quiet-Sun
conditions. The method determines the number, volume, and some general
geometric properties of the small-scale heating events at different
instants in a simulation with a volume of 16 × 8 × 16 Mm3,
spanning from the top of the convection zone to the corona. We found
that there are about 104 small-scale heating events at
any instant above the simulated area of 128 Mm2. They
occur mainly at heights between 1.5 and 3.0 Mm. We determine the
average value of their projected vertical extent, which ranges from
375 to 519 km over time, and we show that height, volume, and energy
distribution of the events at any instant resemble power laws. Finally,
we demonstrate that larger heating structures are a combination of much
smaller heating events and that small-scale heating events dissipate
enough energy to maintain the coronal energetic balance at any instant.

---------------------------------------------------------
Title: Numerical Simulations of Coronal Heating through Footpoint
Braiding
Authors: Hansteen, V.; Guerreiro, N.; De Pontieu, B.; Carlsson, M.
2015ApJ...811..106H Altcode: 2015arXiv150807234H
Advanced three-dimensional (3D) radiative MHD simulations now reproduce
many properties of the outer solar atmosphere. When including a domain
from the convection zone into the corona, a hot chromosphere and corona
are self-consistently maintained. Here we study two realistic models,
with different simulated areas, magnetic field strength and topology,
and numerical resolution. These are compared in order to characterize
the heating in the 3D-MHD simulations which self-consistently
maintains the structure of the atmosphere. We analyze the heating
at both large and small scales and find that heating is episodic and
highly structured in space, but occurs along loop-shaped structures,
and moves along with the magnetic field. On large scales we find that
the heating per particle is maximal near the transition region and that
widely distributed opposite-polarity field in the photosphere leads
to a greater heating scale height in the corona. On smaller scales,
heating is concentrated in current sheets, the thicknesses of which are
set by the numerical resolution. Some current sheets fragment in time,
this process occurring more readily in the higher-resolution model
leading to spatially highly intermittent heating. The large-scale
heating structures are found to fade in less than about five minutes,
while the smaller, local, heating shows timescales of the order of two
minutes in one model and one minutes in the other, higher-resolution,
model.

---------------------------------------------------------
Title: Numerical Modeling of the Solar Chromosphere and Corona:
What Has Been Done? What Should Be Done?
Authors: Hansteen, V.; Carlsson, M.; Gudiksen, B.
2015ASPC..498..141H Altcode:
A number of increasingly sophisticated numerical simulations spanning
the solar atmosphere from below the photosphere in the convection
zone to far above in the corona have shed considerable insight into
the role of the magnetic field in the structure and energetics of the
Sun's outer layers. This development is strengthened by the wealth of
observational data now coming on-line from both ground and space based
observatories. In this talk we will concentrate on the successes and
failures of the modeling effort thus far and discuss the inclusion of
various effects not traditionally considered in the MHD description
such as time dependent ionization, non-LTE radiative transfer, and
generalized Ohm's law.

---------------------------------------------------------
Title: Emergence of Granular-sized Magnetic Bubbles through the
Solar Atmosphere. II. Non-LTE Chromospheric Diagnostics and Inversions
Authors: de la Cruz Rodríguez, Jaime; Hansteen, Viggo; Bellot-Rubio,
Luis; Ortiz, Ada
2015ApJ...810..145D Altcode: 2015arXiv150303846D
Magnetic flux emergence into the outer layers of the Sun is a
fundamental mechanism for releasing energy into the chromosphere and
the corona. In this paper, we study the emergence of granular-sized
flux concentrations and the structuring of the corresponding physical
parameters and atmospheric diagnostics in the upper photosphere and
in the chromosphere. We make use of a realistic 3D MHD simulation of
the outer layers of the Sun to study the formation of the Ca ii 8542
line. We also derive semi-empirical 3D models from non-LTE inversions of
our observations. These models contain information on the line-of-sight
stratifications of temperature, velocity, and the magnetic field. Our
analysis explains the peculiar Ca ii 8542 Å profiles observed in the
flux emerging region. Additionally, we derive detailed temperature
and velocity maps describing the ascent of a magnetic bubble from the
photosphere to the chromosphere. The inversions suggest that, in active
regions, granular-sized bubbles emerge up to the lower chromosphere
where the existing large-scale field hinders their ascent. We report
hints of heating when the field reaches the chromosphere.

---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. IV. The Mg II Triplet
Lines as a New Diagnostic for Lower Chromospheric Heating
Authors: Pereira, Tiago M. D.; Carlsson, Mats; De Pontieu, Bart;
Hansteen, Viggo
2015ApJ...806...14P Altcode: 2015arXiv150401733P
A triplet of subordinate lines of Mg ii exists in the region around
the h&amp;k lines. In solar spectra these lines are seen mostly
in absorption, but in some cases can become emission lines. The
aim of this work is to study the formation of this triplet, and
investigate any diagnostic value they can bring. Using 3D radiative
magnetohydrodynamic simulations of quiet Sun and flaring flux emergence,
we synthesize spectra and investigate how spectral features respond
to the underlying atmosphere. We find that emission in the lines
is rare and is typically caused by a steep temperature increase in
the lower chromosphere (above 1500 K, with electron densities above
1017 m-3). In both simulations the lines are
sensitive to temperature increases taking place at column masses ≳5
· 10-4 g cm-2. Additional information can
also be inferred from the peak-to-wing ratio and shape of the line
profiles. Using observations from NASA's Interface Region Imaging
Spectrograph we find both absorption and emission line profiles with
similar shapes to the synthetic spectra, which suggests that these lines
represent a useful diagnostic that complements the Mg ii h&amp;k lines.

---------------------------------------------------------
Title: IRIS observations and 3D `realistic' MHD models of the solar
chromosphere
Authors: Hansteen, V.; Carlsson, M.; Gudiksen, B.
2015hsa8.conf...19H Altcode:
The Interface Region Imaging Spectrograph (IRIS) is a NASA “Small
Explorer” mission. It was launched in late June 2013 and since then
it has obtained spectra and images from the outer solar atmosphere at
unprecedented spatial and temporal resolution. Its primary goal is to
probe the photosphere-corona interface: the source region of outer
atmosphere heating and dynamics and a region that has an extremely
complicated interplay between plasma, radiation and magnetic field. The
scientific justification for IRIS hinges on the capabilities of 3D
magnetohydrodynamic models to allow the confident interpretation of
observed data. The interplay between observations and modeling is
discussed, illustrated with examples from recent IRIS observations.

---------------------------------------------------------
Title: The role of partial ionization effects in the chromosphere
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo;
Carlsson, Mats
2015RSPTA.37340268M Altcode: 2015arXiv150302723M
The energy for the coronal heating must be provided from the
convection zone. However, the amount and the method by which this
energy is transferred into the corona depend on the properties of the
lower atmosphere and the corona itself. We review: (i) how the energy
could be built in the lower solar atmosphere, (ii) how this energy is
transferred through the solar atmosphere, and (iii) how the energy is
finally dissipated in the chromosphere and/or corona. Any mechanism of
energy transport has to deal with the various physical processes in the
lower atmosphere. We will focus on a physical process that seems to
be highly important in the chromosphere and not deeply studied until
recently: the ion-neutral interaction effects in the chromosphere. We
review the relevance and the role of the partial ionization in the
chromosphere and show that this process actually impacts considerably
the outer solar atmosphere. We include analysis of our 2.5D radiative
magnetohydrodynamic simulations with the Bifrost code (Gudiksen et
al. 2011 Astron. Astrophys. 531, A154 (doi:10.1051/0004-6361/201116520))
including the partial ionization effects on the chromosphere
and corona and thermal conduction along magnetic field lines. The
photosphere, chromosphere and transition region are partially ionized
and the interaction between ionized particles and neutral particles
has important consequences on the magneto-thermodynamics of these
layers. The partial ionization effects are treated using generalized
Ohm's law, i.e. we consider the Hall term and the ambipolar diffusion
(Pedersen dissipation) in the induction equation. The interaction
between the different species affects the modelled atmosphere as
follows: (i) the ambipolar diffusion dissipates magnetic energy and
increases the minimum temperature in the chromosphere and (ii) the
upper chromosphere may get heated and expanded over a greater range
of heights. These processes reveal appreciable differences between
the modelled atmospheres of simulations with and without ion-neutral
interaction effects.

---------------------------------------------------------
Title: Internetwork Chromospheric Bright Grains Observed With IRIS
and SST
Authors: Martínez-Sykora, Juan; Rouppe van der Voort, Luc; Carlsson,
Mats; De Pontieu, Bart; Pereira, Tiago M. D.; Boerner, Paul; Hurlburt,
Neal; Kleint, Lucia; Lemen, James; Tarbell, Ted D.; Title, Alan;
Wuelser, Jean-Pierre; Hansteen, Viggo H.; Golub, Leon; McKillop, Sean;
Reeves, Kathy K.; Saar, Steven; Testa, Paola; Tian, Hui; Jaeggli,
Sarah; Kankelborg, Charles
2015ApJ...803...44M Altcode: 2015arXiv150203490M
The Interface Region Imaging Spectrograph (IRIS) reveals small-scale
rapid brightenings in the form of bright grains all over coronal holes
and the quiet Sun. These bright grains are seen with the IRIS 1330,
1400, and 2796 Å slit-jaw filters. We combine coordinated observations
with IRIS and from the ground with the Swedish 1 m Solar Telescope
(SST) which allows us to have chromospheric (Ca ii 8542 Å, Ca ii H
3968 Å, Hα, and Mg ii k 2796 Å) and transition region (C ii 1334 Å,
Si iv 1403 Å) spectral imaging, and single-wavelength Stokes maps
in Fe i 6302 Å at high spatial (0\buildrel{\prime\prime}\over{.}
33), temporal, and spectral resolution. We conclude that the IRIS
slit-jaw grains are the counterpart of so-called acoustic grains,
i.e., resulting from chromospheric acoustic waves in a non-magnetic
environment. We compare slit-jaw images (SJIs) with spectra from the
IRIS spectrograph. We conclude that the grain intensity in the 2796
Å slit-jaw filter comes from both the Mg ii k core and wings. The
signal in the C ii and Si iv lines is too weak to explain the presence
of grains in the 1300 and 1400 Å SJIs and we conclude that the grain
signal in these passbands comes mostly from the continuum. Although
weak, the characteristic shock signatures of acoustic grains can often
be detected in IRIS C ii spectra. For some grains, a spectral signature
can be found in IRIS Si iv. This suggests that upward propagating
acoustic waves sometimes reach all the way up to the transition region.

---------------------------------------------------------
Title: Synthesized Spectra of Optically Thin Emission Lines
Authors: Olluri, K.; Gudiksen, B. V.; Hansteen, V. H.; De Pontieu, B.
2015ApJ...802....5O Altcode:
In recent years realistic 3D numerical models of the solar atmosphere
have become available. The models attempt to recreate the solar
atmosphere and mimic observations in the best way, in order to make it
possible to couple complicated observations with physical properties
such as the temperatures, densities, velocities, and magnetic fields. We
here present a study of synthetic spectra created using the Bifrost code
in order to assess how well they fit with previously taken solar data. A
study of the synthetic intensity, nonthermal line widths, Doppler
shifts, and correlations between any two of these three components of
the spectra first assuming statistical equilibrium is made, followed by
a report on some of the effects nonequilibrium ionization will have on
the synthesized spectra. We find that the synthetic intensities compare
well with the observations. The synthetic observations depend on the
assumed resolution and point-spread function (PSF) of the instrument,
and we find a large effect on the results, especially for intensity
and nonthermal line width. The Doppler shifts produce the reported
persistent redshifts for the transition region (TR) lines and blueshifts
for the upper TR and corona lines. The nonthermal line widths reproduce
the well-known turnoff point around (2-3) × 105 K, but
with much lower values than those observed. The nonthermal line widths
tend to increase with decreasing assumed instrumental resolution, also
when nonequilibrium ionization is included. Correlations between the
nonthermal line width of any two TR line studies as reported by Chae et
al. are reproduced, while the correlations of intensity to line width
are reproduced only after applying a PSF to the data. Doppler shift
correlations reported by Doschek for the TR lines and correlations of
Doppler shift to nonthermal line width of the Fe xii 19.5
line reported by Doschek et al. are reproduced.

---------------------------------------------------------
Title: Homologous Helical Jets: Observations By IRIS, SDO, and Hinode
and Magnetic Modeling With Data-Driven Simulations
Authors: Cheung, Mark C. M.; De Pontieu, B.; Tarbell, T. D.; Fu, Y.;
Tian, H.; Testa, P.; Reeves, K. K.; Martínez-Sykora, J.; Boerner,
P.; Wülser, J. P.; Lemen, J.; Title, A. M.; Hurlburt, N.; Kleint,
L.; Kankelborg, C.; Jaeggli, S.; Golub, L.; McKillop, S.; Saar, S.;
Carlsson, M.; Hansteen, V.
2015ApJ...801...83C Altcode: 2015arXiv150101593C
We report on observations of recurrent jets by instruments on board
the Interface Region Imaging Spectrograph, Solar Dynamics Observatory
(SDO), and Hinode spacecraft. Over a 4 hr period on 2013 July 21,
recurrent coronal jets were observed to emanate from NOAA Active Region
11793. Far-ultraviolet spectra probing plasma at transition region
temperatures show evidence of oppositely directed flows with components
reaching Doppler velocities of ±100 km s-1. Raster Doppler
maps using a Si iv transition region line show all four jets to have
helical motion of the same sense. Simultaneous observations of the
region by SDO and Hinode show that the jets emanate from a source
region comprising a pore embedded in the interior of a supergranule. The
parasitic pore has opposite polarity flux compared to the surrounding
network field. This leads to a spine-fan magnetic topology in the
coronal field that is amenable to jet formation. Time-dependent
data-driven simulations are used to investigate the underlying drivers
for the jets. These numerical experiments show that the emergence of
current-carrying magnetic field in the vicinity of the pore supplies
the magnetic twist needed for recurrent helical jet formation.

---------------------------------------------------------
Title: Heating Signatures in the Disk Counterparts of Solar Spicules
in Interface Region Imaging Spectrograph Observations
Authors: Rouppe van der Voort, L.; De Pontieu, B.; Pereira, T. M. D.;
Carlsson, M.; Hansteen, V.
2015ApJ...799L...3R Altcode: 2014arXiv1412.4531R
We use coordinated observations with the Interface Region Imaging
Spectrograph (IRIS) and the Swedish 1 m Solar Telescope to identify
the disk counterpart of type II spicules in upper-chromospheric and
transition region (TR) diagnostics. These disk counterparts were
earlier identified through short-lived asymmetries in chromospheric
spectral lines: rapid blue- or red-shifted excursions (RBEs or RREs). We
find clear signatures of RBEs and RREs in Mg II h &amp; k, often with
excursions of the central h3 and k3 absorption features in concert with
asymmetries in co-temporal and co-spatial Hα spectral profiles. We find
spectral signatures for RBEs and RREs in C II 1335 and 1336 Å and Si
IV 1394 and 1403 Å spectral lines and interpret this as a sign that
type II spicules are heated to at least TR temperatures, supporting
other recent work. These C II and Si IV spectral signals are weaker
for a smaller network region than for more extended network regions in
our data. A number of bright features around extended network regions
observed in IRIS slit-jaw imagery SJI 1330 and 1400, recently identified
as network jets, can be clearly connected to Hα RBEs and/or RREs in
our coordinated data. We speculate that at least part of the diffuse
halo around network regions in the IRIS SJI 1330 and 1400 images can
be attributed to type II spicules with insufficient opacity in the C
II and Si IV lines to stand out as single features in these passbands.

---------------------------------------------------------
Title: Observables of Ion-Neutral Interaction Effects in the Solar
Chromosphere
Authors: Martínez-Sykora, J.; De Pontieu, B.; Hansteen, V. H.;
Pereira, T. M. D.; Leenaarts, J.; Carlsson, M.
2014AGUFMSH51C4176M Altcode:
The chromosphere and transition region constitute the interface
between the solar surface and the corona and modulate the flow of
mass and energy into the upper atmosphere. IRIS was launched in 2013
to study the chromosphere and transition region. The complexity of the
chromosphere is due to various regime changes that take place across it,
like: Hydrogen goes from predominantly neutral to predominantly ionized;
the plasma behavior changes from collisional to collision-less; it goes
from gas-pressure dominated to magnetically driven, etc. Consequently,
the interpretation of chromospheric observations in general and those
from IRIS, in particular, is a challenging task. It is thus crucial
to combine IRIS observations with advanced radiative-MHD numerical
modeling. Because the photosphere, chromosphere and transition region
are partially ionized, the interaction between ionized and neutral
particles has important consequences on the magneto-thermodynamics of
these regions. We implemented the effects of partial ionization using
generalized Ohm's law in the Bifrost code (Gudiksen et al. 2011) which
solves the full MHD equations with non-grey and non-LTE radiative
transfer and thermal conduction along magnetic field lines. The
implementation of partial ionization effects impact our modeled
radiative-MHD atmosphere, such as producing chromospheric heating and
diffusion of photospheric magnetic field into the upper-chromosphere. We
will focus on which observables of these processes can be revealed
with IRIS.

---------------------------------------------------------
Title: Solar Flaring Activity and Coronal Heating.
Authors: Archontis, V.; Hansteen, V. H.
2014AGUFMSH53D..02A Altcode:
We report on the formation of small solar flares produced
by patchy magnetic reconnection between interacting magnetic
loops. Three-dimensional (3D) magnetohydrodynamic (MHD) numerical
experiments were performed, where a uniform magnetic flux sheet
was injected into a fully developed convective layer. The gradual
emergence of the field into the solar atmosphere results in a network
of magnetic loops, which interact dynamically forming current layers
at their interfaces. The formation and ejection of plasmoids out of
the current layers leads to patchy reconnection and the spontaneous
formation of several small (size ≈1-2 Mm) flares. We find that
these flares are short-lived (30 s-3 minutes) bursts of energy in the
range O(1025-1027) erg, which is basically the nanoflare-microflare
range. Their persistent formation and co-operative action and evolution
leads to recurrent emission of fast EUV/X-ray jets and considerable
plasma heating in the active corona.

---------------------------------------------------------
Title: Hot explosions in the cool atmosphere of the Sun
Authors: Peter, H.; Tian, H.; Curdt, W.; Schmit, D.; Innes, D.;
De Pontieu, B.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.;
Tarbell, T. D.; Wuelser, J. P.; Martínez-Sykora, Juan; Kleint,
L.; Golub, L.; McKillop, S.; Reeves, K. K.; Saar, S.; Testa, P.;
Kankelborg, C.; Jaeggli, S.; Carlsson, M.; Hansteen, V.
2014Sci...346C.315P Altcode: 2014arXiv1410.5842P
The solar atmosphere was traditionally represented with a simple
one-dimensional model. Over the past few decades, this paradigm shifted
for the chromosphere and corona that constitute the outer atmosphere,
which is now considered a dynamic structured envelope. Recent
observations by the Interface Region Imaging Spectrograph (IRIS) reveal
that it is difficult to determine what is up and down, even in the cool
6000-kelvin photosphere just above the solar surface: This region hosts
pockets of hot plasma transiently heated to almost 100,000 kelvin. The
energy to heat and accelerate the plasma requires a considerable
fraction of the energy from flares, the largest solar disruptions. These
IRIS observations not only confirm that the photosphere is more complex
than conventionally thought, but also provide insight into the energy
conversion in the process of magnetic reconnection.

---------------------------------------------------------
Title: The unresolved fine structure resolved: IRIS observations of
the solar transition region
Authors: Hansteen, V.; De Pontieu, B.; Carlsson, M.; Lemen, J.; Title,
A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.; Pereira,
T. M. D.; De Luca, E. E.; Golub, L.; McKillop, S.; Reeves, K.; Saar,
S.; Testa, P.; Tian, H.; Kankelborg, C.; Jaeggli, S.; Kleint, L.;
Martínez-Sykora, J.
2014Sci...346E.315H Altcode: 2014arXiv1412.3611H
The heating of the outer solar atmospheric layers, i.e., the transition
region and corona, to high temperatures is a long-standing problem
in solar (and stellar) physics. Solutions have been hampered by an
incomplete understanding of the magnetically controlled structure of
these regions. The high spatial and temporal resolution observations
with the Interface Region Imaging Spectrograph (IRIS) at the solar
limb reveal a plethora of short, low-lying loops or loop segments
at transition-region temperatures that vary rapidly, on the time
scales of minutes. We argue that the existence of these loops solves
a long-standing observational mystery. At the same time, based on
comparison with numerical models, this detection sheds light on a
critical piece of the coronal heating puzzle.

---------------------------------------------------------
Title: Evidence of nonthermal particles in coronal loops heated
impulsively by nanoflares
Authors: Testa, P.; De Pontieu, B.; Allred, J.; Carlsson, M.; Reale,
F.; Daw, A.; Hansteen, V.; Martinez-Sykora, J.; Liu, W.; DeLuca, E. E.;
Golub, L.; McKillop, S.; Reeves, K.; Saar, S.; Tian, H.; Lemen, J.;
Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.;
Kleint, L.; Kankelborg, C.; Jaeggli, S.
2014Sci...346B.315T Altcode: 2014arXiv1410.6130T
The physical processes causing energy exchange between the Sun’s
hot corona and its cool lower atmosphere remain poorly understood. The
chromosphere and transition region (TR) form an interface region between
the surface and the corona that is highly sensitive to the coronal
heating mechanism. High-resolution observations with the Interface
Region Imaging Spectrograph (IRIS) reveal rapid variability (~20 to
60 seconds) of intensity and velocity on small spatial scales (≲500
kilometers) at the footpoints of hot and dynamic coronal loops. The
observations are consistent with numerical simulations of heating by
beams of nonthermal electrons, which are generated in small impulsive
(≲30 seconds) heating events called “coronal nanoflares.” The
accelerated electrons deposit a sizable fraction of their energy
(≲1025 erg) in the chromosphere and TR. Our analysis
provides tight constraints on the properties of such electron beams
and new diagnostics for their presence in the nonflaring corona.

---------------------------------------------------------
Title: Prevalence of small-scale jets from the networks of the solar
transition region and chromosphere
Authors: Tian, H.; DeLuca, E. E.; Cranmer, S. R.; De Pontieu, B.;
Peter, H.; Martínez-Sykora, J.; Golub, L.; McKillop, S.; Reeves,
K. K.; Miralles, M. P.; McCauley, P.; Saar, S.; Testa, P.; Weber,
M.; Murphy, N.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.;
Tarbell, T. D.; Wuelser, J. P.; Kleint, L.; Kankelborg, C.; Jaeggli,
S.; Carlsson, M.; Hansteen, V.; McIntosh, S. W.
2014Sci...346A.315T Altcode: 2014arXiv1410.6143T
As the interface between the Sun’s photosphere and corona, the
chromosphere and transition region play a key role in the formation and
acceleration of the solar wind. Observations from the Interface Region
Imaging Spectrograph reveal the prevalence of intermittent small-scale
jets with speeds of 80 to 250 kilometers per second from the narrow
bright network lanes of this interface region. These jets have lifetimes
of 20 to 80 seconds and widths of ≤300 kilometers. They originate from
small-scale bright regions, often preceded by footpoint brightenings
and accompanied by transverse waves with amplitudes of ~20 kilometers
per second. Many jets reach temperatures of at least ~105
kelvin and constitute an important element of the transition region
structures. They are likely an intermittent but persistent source of
mass and energy for the solar wind.

---------------------------------------------------------
Title: On the prevalence of small-scale twist in the solar
chromosphere and transition region
Authors: De Pontieu, B.; Rouppe van der Voort, L.; McIntosh, S. W.;
Pereira, T. M. D.; Carlsson, M.; Hansteen, V.; Skogsrud, H.; Lemen,
J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser,
J. P.; De Luca, E. E.; Golub, L.; McKillop, S.; Reeves, K.; Saar,
S.; Testa, P.; Tian, H.; Kankelborg, C.; Jaeggli, S.; Kleint, L.;
Martinez-Sykora, J.
2014Sci...346D.315D Altcode: 2014arXiv1410.6862D
The solar chromosphere and transition region (TR) form an interface
between the Sun’s surface and its hot outer atmosphere. There,
most of the nonthermal energy that powers the solar atmosphere
is transformed into heat, although the detailed mechanism remains
elusive. High-resolution (0.33-arc second) observations with NASA’s
Interface Region Imaging Spectrograph (IRIS) reveal a chromosphere
and TR that are replete with twist or torsional motions on sub-arc
second scales, occurring in active regions, quiet Sun regions, and
coronal holes alike. We coordinated observations with the Swedish
1-meter Solar Telescope (SST) to quantify these twisting motions and
their association with rapid heating to at least TR temperatures. This
view of the interface region provides insight into what heats the low
solar atmosphere.

---------------------------------------------------------
Title: An Interface Region Imaging Spectrograph First View on Solar
Spicules
Authors: Pereira, T. M. D.; De Pontieu, B.; Carlsson, M.; Hansteen,
V.; Tarbell, T. D.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt,
N.; Wülser, J. P.; Martínez-Sykora, J.; Kleint, L.; Golub, L.;
McKillop, S.; Reeves, K. K.; Saar, S.; Testa, P.; Tian, H.; Jaeggli,
S.; Kankelborg, C.
2014ApJ...792L..15P Altcode: 2014arXiv1407.6360P
Solar spicules have eluded modelers and observers for decades. Since
the discovery of the more energetic type II, spicules have become
a heated topic but their contribution to the energy balance of the
low solar atmosphere remains unknown. Here we give a first glimpse of
what quiet-Sun spicules look like when observed with NASA's recently
launched Interface Region Imaging Spectrograph (IRIS). Using IRIS
spectra and filtergrams that sample the chromosphere and transition
region, we compare the properties and evolution of spicules as
observed in a coordinated campaign with Hinode and the Atmospheric
Imaging Assembly. Our IRIS observations allow us to follow the thermal
evolution of type II spicules and finally confirm that the fading
of Ca II H spicules appears to be caused by rapid heating to higher
temperatures. The IRIS spicules do not fade but continue evolving,
reaching higher and falling back down after 500-800 s. Ca II H type
II spicules are thus the initial stages of violent and hotter events
that mostly remain invisible in Ca II H filtergrams. These events
have very different properties from type I spicules, which show lower
velocities and no fading from chromospheric passbands. The IRIS spectra
of spicules show the same signature as their proposed disk counterparts,
reinforcing earlier work. Spectroheliograms from spectral rasters also
confirm that quiet-Sun spicules originate in bushes from the magnetic
network. Our results suggest that type II spicules are indeed the
site of vigorous heating (to at least transition region temperatures)
along extensive parts of the upward moving spicular plasma.

---------------------------------------------------------
Title: The Interface Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, B.; Title, A. M.; Lemen, J. R.; Kushner, G. D.;
Akin, D. J.; Allard, B.; Berger, T.; Boerner, P.; Cheung, M.; Chou,
C.; Drake, J. F.; Duncan, D. W.; Freeland, S.; Heyman, G. F.; Hoffman,
C.; Hurlburt, N. E.; Lindgren, R. W.; Mathur, D.; Rehse, R.; Sabolish,
D.; Seguin, R.; Schrijver, C. J.; Tarbell, T. D.; Wülser, J. -P.;
Wolfson, C. J.; Yanari, C.; Mudge, J.; Nguyen-Phuc, N.; Timmons,
R.; van Bezooijen, R.; Weingrod, I.; Brookner, R.; Butcher, G.;
Dougherty, B.; Eder, J.; Knagenhjelm, V.; Larsen, S.; Mansir, D.;
Phan, L.; Boyle, P.; Cheimets, P. N.; DeLuca, E. E.; Golub, L.;
Gates, R.; Hertz, E.; McKillop, S.; Park, S.; Perry, T.; Podgorski,
W. A.; Reeves, K.; Saar, S.; Testa, P.; Tian, H.; Weber, M.; Dunn, C.;
Eccles, S.; Jaeggli, S. A.; Kankelborg, C. C.; Mashburn, K.; Pust, N.;
Springer, L.; Carvalho, R.; Kleint, L.; Marmie, J.; Mazmanian, E.;
Pereira, T. M. D.; Sawyer, S.; Strong, J.; Worden, S. P.; Carlsson,
M.; Hansteen, V. H.; Leenaarts, J.; Wiesmann, M.; Aloise, J.; Chu,
K. -C.; Bush, R. I.; Scherrer, P. H.; Brekke, P.; Martinez-Sykora,
J.; Lites, B. W.; McIntosh, S. W.; Uitenbroek, H.; Okamoto, T. J.;
Gummin, M. A.; Auker, G.; Jerram, P.; Pool, P.; Waltham, N.
2014SoPh..289.2733D Altcode: 2014arXiv1401.2491D; 2014SoPh..tmp...25D
The Interface Region Imaging Spectrograph (IRIS) small explorer
spacecraft provides simultaneous spectra and images of the photosphere,
chromosphere, transition region, and corona with 0.33 - 0.4 arcsec
spatial resolution, two-second temporal resolution, and 1 km
s−1 velocity resolution over a field-of-view of up to
175 arcsec × 175 arcsec. IRIS was launched into a Sun-synchronous
orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a
19-cm UV telescope that feeds a slit-based dual-bandpass imaging
spectrograph. IRIS obtains spectra in passbands from 1332 - 1358 Å,
1389 - 1407 Å, and 2783 - 2834 Å, including bright spectral lines
formed in the chromosphere (Mg II h 2803 Å and Mg II k 2796 Å) and
transition region (C II 1334/1335 Å and Si IV 1394/1403 Å). Slit-jaw
images in four different passbands (C II 1330, Si IV 1400, Mg II k
2796, and Mg II wing 2830 Å) can be taken simultaneously with spectral
rasters that sample regions up to 130 arcsec × 175 arcsec at a variety
of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to
emission from plasma at temperatures between 5000 K and 10 MK and will
advance our understanding of the flow of mass and energy through an
interface region, formed by the chromosphere and transition region,
between the photosphere and corona. This highly structured and dynamic
region not only acts as the conduit of all mass and energy feeding
into the corona and solar wind, it also requires an order of magnitude
more energy to heat than the corona and solar wind combined. The
IRIS investigation includes a strong numerical modeling component
based on advanced radiative-MHD codes to facilitate interpretation of
observations of this complex region. Approximately eight Gbytes of data
(after compression) are acquired by IRIS each day and made available
for unrestricted use within a few days of the observation.

---------------------------------------------------------
Title: Detection of Supersonic Downflows and Associated Heating
Events in the Transition Region above Sunspots
Authors: Kleint, L.; Antolin, P.; Tian, H.; Judge, P.; Testa, P.;
De Pontieu, B.; Martínez-Sykora, J.; Reeves, K. K.; Wuelser, J. P.;
McKillop, S.; Saar, S.; Carlsson, M.; Boerner, P.; Hurlburt, N.; Lemen,
J.; Tarbell, T. D.; Title, A.; Golub, L.; Hansteen, V.; Jaeggli, S.;
Kankelborg, C.
2014ApJ...789L..42K Altcode: 2014arXiv1406.6816K
Interface Region Imaging Spectrograph data allow us to study the solar
transition region (TR) with an unprecedented spatial resolution of
0.”33. On 2013 August 30, we observed bursts of high Doppler shifts
suggesting strong supersonic downflows of up to 200 km s-1
and weaker, slightly slower upflows in the spectral lines Mg II h
and k, C II 1336, Si IV 1394 Å, and 1403 Å, that are correlated
with brightenings in the slitjaw images (SJIs). The bursty behavior
lasts throughout the 2 hr observation, with average burst durations
of about 20 s. The locations of these short-lived events appear to
be the umbral and penumbral footpoints of EUV loops. Fast apparent
downflows are observed along these loops in the SJIs and in the
Atmospheric Imaging Assembly, suggesting that the loops are thermally
unstable. We interpret the observations as cool material falling
from coronal heights, and especially coronal rain produced along the
thermally unstable loops, which leads to an increase of intensity
at the loop footpoints, probably indicating an increase of density
and temperature in the TR. The rain speeds are on the higher end of
previously reported speeds for this phenomenon, and possibly higher
than the free-fall velocity along the loops. On other observing days,
similar bright dots are sometimes aligned into ribbons, resembling
small flare ribbons. These observations provide a first insight into
small-scale heating events in sunspots in the TR.

---------------------------------------------------------
Title: IRIS Observations of Twist in the Low Solar Atmosphere
Authors: De Pontieu, Bart; Rouppe van der Voort, Luc; Pereira,
Tiago M. D.; Skogsrud, Haakon; McIntosh, Scott W.; Carlsson, Mats;
Hansteen, Viggo
2014AAS...22431302D Altcode:
The Interface Region Imaging Spectrograph (IRIS) small explorer
was launched in June 2013. IRIS’s high-resolution (0.33 arcsec),
high-cadence (2s) images and spectra reveal a solar chromosphere and
transition region that is riddled with twist. This is evidenced by the
presence of ubiquitous torsional motions on very small (subarcsec)
spatial scales. These motions occur in active regions, quiet Sun
and coronal holes on a variety of structures such as spicules at
the limb, rapid-blue/red-shifted events (RBEs and RREs) as well as
low-lying loops. We use IRIS data and observations from the Swedish
Solar Telescope (SST) in La Palma, Spain to describe these motions
quantitatively, study their propagation, and illustrate how such
strong twisting motions are often associated with significant and
rapid heating to at least transition region temperatures.

---------------------------------------------------------
Title: Diagnostics of coronal heating and mechanisms of energy
transport from IRIS and AIA observations of active region moss
Authors: Testa, Paola; De Pontieu, Bart; Allred, Joel C.; Carlsson,
Mats; Reale, Fabio; Daw, Adrian N.; Hansteen, Viggo
2014AAS...22431305T Altcode:
The variability of emission of the "moss", i.e., the upper transition
region (TR) layer of high pressure loops in active regions provides
stringent constraints on the characteristics of heating events. The
Interface Region Imaging Spectrograph (IRIS), launched in June
2013, provides imaging and spectral observations at high spatial
(0.166 arcsec/pix), and temporal (down to ~1s) resolution at FUV
and NUV wavelengths, and together with the high spatial and temporal
resolution observations of SDO/AIA, can provide important insights
into the coronal heating mechanisms. We present here an analysis of
the temporal variability properties of moss regions at the footpoints
of hot active region core loops undergoing heating, as observed by IRIS
and AIA, covering emission from the corona to the transition region and
the chromosphere. We model the observations using dynamic loop models
(the Palermo-Harvard code, and RADYN, which also includes the effects of
non-thermal particles) and discuss the implications on energy transport
mechanisms (thermal conduction vs beams of non-thermal particles).

---------------------------------------------------------
Title: Clusters of Small Eruptive Flares Produced by Magnetic
Reconnection in the Sun
Authors: Archontis, V.; Hansteen, V.
2014ApJ...788L...2A Altcode: 2014arXiv1405.6420A
We report on the formation of small solar flares produced by
patchy magnetic reconnection between interacting magnetic loops. A
three-dimensional (3D) magnetohydrodynamic (MHD) numerical experiment
was performed, where a uniform magnetic flux sheet was injected into
a fully developed convective layer. The gradual emergence of the
field into the solar atmosphere results in a network of magnetic
loops, which interact dynamically forming current layers at their
interfaces. The formation and ejection of plasmoids out of the
current layers leads to patchy reconnection and the spontaneous
formation of several small (size ≈1-2 Mm) flares. We find that
these flares are short-lived (30 s-3 minutes) bursts of energy in the
range O(1025-1027) erg, which is basically the
nanoflare-microflare range. Their persistent formation and co-operative
action and evolution leads to recurrent emission of fast EUV/X-ray
jets and considerable plasma heating in the active corona.

---------------------------------------------------------
Title: High-resolution Observations of the Shock Wave Behavior for
Sunspot Oscillations with the Interface Region Imaging Spectrograph
Authors: Tian, H.; DeLuca, E.; Reeves, K. K.; McKillop, S.; De Pontieu,
B.; Martínez-Sykora, J.; Carlsson, M.; Hansteen, V.; Kleint, L.;
Cheung, M.; Golub, L.; Saar, S.; Testa, P.; Weber, M.; Lemen, J.;
Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.;
Kankelborg, C.; Jaeggli, S.; McIntosh, S. W.
2014ApJ...786..137T Altcode: 2014arXiv1404.6291T
We present the first results of sunspot oscillations from observations
by the Interface Region Imaging Spectrograph. The strongly nonlinear
oscillation is identified in both the slit-jaw images and the
spectra of several emission lines formed in the transition region and
chromosphere. We first apply a single Gaussian fit to the profiles of
the Mg II 2796.35 Å, C II 1335.71 Å, and Si IV 1393.76 Å lines in the
sunspot. The intensity change is ~30%. The Doppler shift oscillation
reveals a sawtooth pattern with an amplitude of ~10 km s-1
in Si IV. The Si IV oscillation lags those of C II and Mg II by ~3 and
~12 s, respectively. The line width suddenly increases as the Doppler
shift changes from redshift to blueshift. However, we demonstrate
that this increase is caused by the superposition of two emission
components. We then perform detailed analysis of the line profiles at
a few selected locations on the slit. The temporal evolution of the
line core is dominated by the following behavior: a rapid excursion
to the blue side, accompanied by an intensity increase, followed by a
linear decrease of the velocity to the red side. The maximum intensity
slightly lags the maximum blueshift in Si IV, whereas the intensity
enhancement slightly precedes the maximum blueshift in Mg II. We find
a positive correlation between the maximum velocity and deceleration,
a result that is consistent with numerical simulations of upward
propagating magnetoacoustic shock waves.

---------------------------------------------------------
Title: Emergence of Granular-sized Magnetic Bubbles through the
Solar Atmosphere. I. Spectropolarimetric Observations and Simulations
Authors: Ortiz, Ada; Bellot Rubio, Luis R.; Hansteen, Viggo H.;
de la Cruz Rodríguez, Jaime; Rouppe van der Voort, Luc
2014ApJ...781..126O Altcode: 2013arXiv1312.5735O
We study a granular-sized magnetic flux emergence event that occurred
in NOAA 11024 in 2009 July. The observations were made with the CRISP
spectropolarimeter at the Swedish 1 m Solar Telescope achieving a
spatial resolution of 0.”14. Simultaneous full Stokes observations of
the two photospheric Fe I lines at 630.2 nm and the chromospheric Ca
II 854.2 nm line allow us to describe in detail the emergence process
across the solar atmosphere. We report here on three-dimensional
(3D) semi-spherical bubble events, where instead of simple magnetic
footpoints, we observe complex semi-circular feet straddling a few
granules. Several phenomena occur simultaneously, namely, abnormal
granulation, separation of opposite-polarity legs, and brightenings at
chromospheric heights. However, the most characteristic signature in
these events is the observation of a dark bubble in filtergrams taken
in the wings of the Ca II 854.2 nm line. There is a clear coincidence
between the emergence of horizontal magnetic field patches and the
formation of the dark bubble. We can infer how the bubble rises through
the solar atmosphere as we see it progressing from the wings to the
core of Ca II 854.2 nm. In the photosphere, the magnetic bubble shows
mean upward Doppler velocities of 2 km s-1 and expands at a
horizontal speed of 4 km s-1. In about 3.5 minutes it travels
some 1100 km to reach the mid chromosphere, implying an average ascent
speed of 5.2 km s-1. The maximum separation attained by the
magnetic legs is 6.”6. From an inversion of the observed Stokes spectra
with the SIR code, we find maximum photospheric field strengths of 480 G
and inclinations of nearly 90° in the magnetic bubble interior, along
with temperature deficits of up to 250 K at log τ = -2 and above. To
aid the interpretation of the observations, we carry out 3D numerical
simulations of the evolution of a horizontal, untwisted magnetic flux
sheet injected in the convection zone, using the Bifrost code. The
computational domain spans from the upper convection zone to the lower
corona. In the modeled chromosphere, the rising flux sheet produces a
large, cool, magnetized bubble. We compare this bubble with the observed
ones and find excellent agreement, including similar field strengths
and velocity signals in the photosphere and chromosphere, temperature
deficits, ascent speeds, expansion velocities, and lifetimes.

---------------------------------------------------------
Title: Realistic 3D simulations of a small flare resulting from
flux emergence
Authors: Hansteen, Viggo; Archontis, Vasilis
2014cosp...40E1151H Altcode:
We have performed three-dimensional (3d) magnetohydrodynamic simulations
of magnetic flux emergence in a model that spans the convection zone
and into the outer solar atmosphere with the Bifrost code. This is a
“realistic” model, in the sense that the parameters and physical
effects that control the atmosphere can be used to produce diagnostics
that can be directly compared with observations. The emerging flux
leads to the formation of several current sheets as it rises into
the modeled corona. Multiple plasmoids are ejected from the current
sheets. Reconnection occurs impulsively, producing heating and fast
outflows near or in the current sheet, arranged in a manner reminiscent
of the CSHKP flare model. This includes a cusp like arcade and a flux
rope in the lower atmospere underneath the current sheet. We discuss
the evolution of the model and several synthetic observables.

---------------------------------------------------------
Title: Diagnostics of coronal heating and mechanisms of energy
transport from IRIS and AIA observations of active region moss
Authors: Testa, Paola; Reale, Fabio; De Pontieu, Bart; Hansteen,
Viggo; Carlsson, Mats; Allred, Joel; Daw, Adrian
2014cosp...40E3323T Altcode:
The variability of emission of the "moss", i.e., the upper transition
region (TR) layer of high pressure loops in active regions provides
stringent constraints on the characteristics of heating events. The
Interface Region Imaging Spectrograph (IRIS), launched in June
2013, provides imaging and spectral observations at high spatial
(0.166 arcsec/pix), and temporal (down to ~1s) resolution at FUV
and NUV wavelengths, and together with the high spatial and temporal
resolution observations of SDO/AIA, can provide important insights
into the coronal heating mechanisms. We present here an analysis of
the temporal variability properties of moss regions at the footpoints
of hot active region core loops undergoing heating, as observed by IRIS
and AIA, covering emission from the corona to the transition region and
the chromosphere. We model the observations using dynamic loop models
(the Palermo-Harvard code, and RADYN, which also includes the effects of
non-thermal particles) and discuss the implications on energy transport
mechanisms (thermal conduction vs beams of non-thermal particles).

---------------------------------------------------------
Title: Identification and characterization of small-scale heating
events in the solar atmosphere from 3D MHD simulations
Authors: Guerreiro, Nuno; Haberreiter, Margit; Schmutz, Werner;
Hansteen, Viggo
2014cosp...40E1095G Altcode:
We studied the properties of small scale heating events (in the nano
and micro-flare regime) in the solar atmosphere using existing 3D
MHD simulations. We put forward a method for event identification
and categorization. We determine the spatial extend of the events
and their frequency and energy distributions. These results aim at
improving the understanding of small scale heating events and their
role for the heating of the solar corona.

---------------------------------------------------------
Title: Measuring energy flux of magneto-acoustic wave in the magnetic
elements by using IRIS
Authors: Kato, Yoshiaki; De Pontieu, Bart; Martinez-Sykora, Juan;
Hansteen, Viggo; Pereira, Tiago; Leenaarts, Jorritt; Carlsson, Mats
2014cosp...40E1423K Altcode:
NASA's Interface Region Imaging Spectrograph (IRIS) has opened a new
window to explore the chromospheric/coronal waves that potentially
energize the solar atmosphere. By using an imaging spectrograph covering
the Si IV and Mg II h&amp;k lines as well as a slit-jaw imager centered
at Si IV and Mg II k onboard IRIS, we can determine the nature of
propagating magneto-acoustic waves just below and in the transition
region. In this study, we compute the vertically emergent intensity of
the Si IV and Mg II h&amp;k lines from a time series of snapshots of
a magnetic element in a two-dimensional Radiative MHD simulation from
the Bifrost code. We investigate the synthetic line profiles to detect
the slow magneto-acoustic body wave (slow mode) which becomes a slow
shock at the lower chromosphere in the magnetic element. We find that
the Doppler shift of the line core gives the velocity amplitude of the
longitudinal magneto-acoustic body wave. The contribution function of
the line core indicates that the formation of Mg II h&amp;k lines is
associated with the propagating shocks and therefore the time evolution
of the line core intensity represents the propagating shocks projected
on the optical surface. We will report on measurement of the energy
flux of slow modes in the magnetic elements by using IRIS observations.

---------------------------------------------------------
Title: IRIS observations of transition region unresolved fine
structure
Authors: Hansteen, Viggo; De Pontieu, Bart
2014cosp...40E1152H Altcode:
The Interface Region Imaging Spectrograph was launched on 28-June-2013
and has been obtaining high resolution images and spectra in the
far and near ultraviolet since 17-July-2013 covering temperatures
from the photosphere into the corona. We analyze the presence of a
multitude of short, relatively cool transition region loops as visible
at the solar limb in slit jaw images dominated by C II 1335 Angstrom
and Si IV 1402 Angstrom emission. We study the dynamical nature and
temperature evolution of these loops and investigate how they relate to
the so-called "unresolved fine structure" (UFS) that has been proposed
as a dominant source of transition region emission, but that has not
yet been directly observed to date. We will also discuss the physical
nature of this emission.

---------------------------------------------------------
Title: Comparison between IRIS Data and Numerical Models
Authors: Carlsson, Mats; De Pontieu, Bart; Hansteen, Viggo; Pereira,
Tiago; Leenaarts, Jorritt
2014cosp...40E.458C Altcode:
The enigmatic chromosphere is the transition between the solar surface
and the eruptive outer solar atmosphere. The chromosphere harbours
and constrains the mass and energy loading processes that define the
heating of the corona, the acceleration and the composition of the solar
wind, and the energetics and triggering of solar outbursts (filament
eruptions, flares, coronal mass ejections). The chromosphere is arguably
the most difficult and least understood domain of solar physics. All
at once it represents the transition from optically thick to thin
radiation escape, from gas-pressure domination to magnetic-pressure
domination, from neutral to ionised state, from MHD to plasma physics,
and from near-equilibrium ("LTE") to non-equilibrium conditions. IRIS
provides a leap in observational capability of the chromospheric
plasma with an unprecedented combination of high spatial, temporal
and spectral resolution in lines with diagnostic information all the
way from the photosphere to the upper transition region. To fully
extract this information it is necessary to combine the observations
with numerical simulations that include a realistic description of the
complicated physics of the chromosphere. In this talk, we will present
such realistic simulations, spanning the solar atmosphere from the
convection zone to the corona, and synthetic observations calculated
from the simulations. These synthetic observations are compared with
observations from IRIS.

---------------------------------------------------------
Title: Impact of the Partial Ionization in the solar atmosphere
using 2.5D Radiative MHD Simulations
Authors: Martinez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo;
Carlsson, Mats
2014cosp...40E2019M Altcode:
The chromosphere/transition region constitute the interface between
the solar surface and the corona and modulate the flow of mass and
energy into the upper atmosphere. IRIS was launched in 2013 to study the
chromosphere and transition region. The complexity of the chromosphere
is due to various regime changes that take place across it, like:
Hydrogen goes from predominantly neutral to predominantly ionized;
the plasma behavior changes from collisional to collision-less; it goes
from gas-pressure dominated to magnetically driven, etc. Consequently,
the interpretation of chromospheric observations in general and those
from IRIS, in particular, is a challenging task. It is thus crucial
to combine IRIS observations with advanced radiative-MHD numerical
modeling. Because the photosphere, chromosphere and transition region
are partially ionized, the interaction between ionized and neutral
particles has important consequences on the magneto-thermodynamics
of these regions. We implemented the effects of partial ionization
using generalized Ohm's law in the Bifrost code (Gudiksen et al. 2011)
which includes full MHD equations with non-grey and non-LTE radiative
transfer and thermal conduction along magnetic field lines. I will
describe the importance and impact of taking into account partial
ionization effects in the modeled radiative-MHD atmosphere, such as
chromospheric heating, photospheric magnetic field diffused into the
upper-chromosphere which expands into the upper atmosphere filling
the corona with mass, magnetic flux, energy and current, etc.

---------------------------------------------------------
Title: Effects of flux emergence in the outer solar
atmosphere. Observational advances
Authors: Ortiz Carbonell, Ada; De Pontieu, Bart; Bellot Rubio, L. R.;
Hansteen, Viggo; Rouppe van der Voort, Luc; Carlsson, Mats
2014cosp...40E2387O Altcode:
We study granular sized magnetic flux emergence events that occur in
a flux emergence region in NOAA 11850 on September 25, 2013. During
that time, the first co-observing campaign between the Swedish 1 m
Solar Telescope and the IRIS spacecraft was carried out. Simultaneous
observations of the Halpha 656.28 nm and Ca II 854.2 nm chromospheric
lines, and the Fe I 630.25 nm photospheric line, were made with
the CRISP/SST spectropolarimeter reaching a spatial resolution of
0."14. At the same time, IRIS was performing a four-step dense raster
of the said emerging flux region, taking slit-jaw images at 133 (C II
transiti on region), 140 (Si IV, transition region), 279.6 (Mg II k,
core, upper chromosphere), and 283.2 nm (Mg II k, wing, photosphere),
obtaining thus the highest resolution images ever taken of the upper
chromosphere and transition region. The photospheric and chromospheric
properties of the emerging magnetic flux bubbles have been described
in detail in Ortiz et al. (2014). However, in the current work we are
able to follow such lower atmosphere observations of flux emergence
up to the transition region with unprecedented spatial and temporal
resolution. We describe the properties (size, time delays, lifetime,
velocities, temperature) of the observed signatures of flux emergence
in the transition region. We believe this may be an important mechanism
of transporting energy and magnetic flux to the upper layers of the
solar atmosphere, namely the transition region and corona, at least
in cases when active regions are formed by flux emerging through the
photosphere. * Ortiz et al. (2014) ApJ 781, 126

---------------------------------------------------------
Title: Detecting Nanoflare Heating Events in Subarcsecond Inter-moss
Loops Using Hi-C
Authors: Winebarger, Amy R.; Walsh, Robert W.; Moore, Ronald;
De Pontieu, Bart; Hansteen, Viggo; Cirtain, Jonathan; Golub, Leon;
Kobayashi, Ken; Korreck, Kelly; DeForest, Craig; Weber, Mark; Title,
Alan; Kuzin, Sergey
2013ApJ...771...21W Altcode:
The High-resolution Coronal Imager (Hi-C) flew aboard a NASA sounding
rocket on 2012 July 11 and captured roughly 345 s of high-spatial and
temporal resolution images of the solar corona in a narrowband 193 Å
channel. In this paper, we analyze a set of rapidly evolving loops that
appear in an inter-moss region. We select six loops that both appear in
and fade out of the Hi-C images during the short flight. From the Hi-C
data, we determine the size and lifetimes of the loops and characterize
whether these loops appear simultaneously along their length or
first appear at one footpoint before appearing at the other. Using
co-aligned, co-temporal data from multiple channels of the Atmospheric
Imaging Assembly on the Solar Dynamics Observatory, we determine the
temperature and density of the loops. We find the loops consist of
cool (~105 K), dense (~1010 cm-3)
plasma. Their required thermal energy and their observed evolution
suggest they result from impulsive heating similar in magnitude to
nanoflares. Comparisons with advanced numerical simulations indicate
that such dense, cold and short-lived loops are a natural consequence
of impulsive magnetic energy release by reconnection of braided magnetic
field at low heights in the solar atmosphere.

---------------------------------------------------------
Title: A Detailed Comparison between the Observed and Synthesized
Properties of a Simulated Type II Spicule
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Leenaarts, Jorrit;
Pereira, Tiago M. D.; Carlsson, Mats; Hansteen, Viggo; Stern, Julie
V.; Tian, Hui; McIntosh, Scott W.; Rouppe van der Voort, Luc
2013ApJ...771...66M Altcode: 2013arXiv1305.2397M
We have performed a three-dimensional radiative MHD simulation of the
solar atmosphere. This simulation shows a jet-like feature that shows
similarities to the type II spicules observed for the first time with
Hinode's Solar Optical Telescope. Rapid blueshifted events (RBEs) on the
solar disk are associated with these spicules. Observational results
suggest they may contribute significantly in supplying the corona
with hot plasma. We perform a detailed comparison of the properties
of the simulated jet with those of type II spicules (observed with
Hinode) and RBEs (with ground-based instruments). We analyze a wide
variety of synthetic emission and absorption lines from the simulations
including chromospheric (Ca II 8542 Å, Ca II H, and Hα) to transition
region and coronal temperatures (10,000 K to several million K). We
compare their synthetic intensities, line profiles, Doppler shifts,
line widths, and asymmetries with observations from Hinode/SOT and
EIS, SOHO/SUMER, the Swedish 1 m Solar Telescope, and SDO/AIA. Many
properties of the synthetic observables resemble the observations,
and we describe in detail the physical processes that lead to these
observables. Detailed analysis of the synthetic observables provides
insight into how observations should be analyzed to derive information
about physical variables in such a dynamic event. For example, we
find that line-of-sight superposition in the optically thin atmosphere
requires the combination of Doppler shifts and spectral line asymmetry
to determine the velocity in the jet. In our simulated type II spicule,
the lifetime of the asymmetry of the transition region lines is shorter
than that of the coronal lines. Other properties differ from the
observations, especially in the chromospheric lines. The mass density
of the part of the spicule with a chromospheric temperature is too low
to produce significant opacity in chromospheric lines. The synthetic
Ca II 8542 Å and Hα profiles therefore do not show signal resembling
RBEs. These and other discrepancies are described in detail, and we
discuss which mechanisms and physical processes may need to be included
in the MHD simulations to mimic the thermodynamic processes of the
chromosphere and corona, in particular to reproduce type II spicules.

---------------------------------------------------------
Title: The Interface Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, Bart; Title, A. M.; Lemen, J.; Wuelser, J.;
Tarbell, T. D.; Schrijver, C. J.; Golub, L.; Kankelborg, C.; Carlsson,
M.; Hansteen, V. H.; Worden, S.; IRIS Team
2013SPD....44...03D Altcode:
The solar chromosphere and transition region (TR) form a highly
structured and dynamic interface region between the photosphere and
the corona. This region not only acts as the conduit of all mass and
energy feeding into the corona and solar wind, it also requires an
order of magnitude more energy to heat than the corona. Nevertheless,
the chromosphere remains poorly understood, because of the complexity
of the required observational and analytical tools: the interface
region is highly complex with transitions from optically thick to
optically thin radiation, from pressure to magnetic field domination,
and large density and temperature contrasts on small spatial scales. The
Interface Region Imaging Spectrograph (IRIS) was selected for a NASA
SMEX mission in 2009 and is scheduled to launch on 26-June-2013 (with
first light scheduled for mid July). IRIS addresses critical questions:
(1) Which types of non-thermal energy dominate in the chromosphere and
beyond? (2) How does the chromosphere regulate mass and energy supply
to the corona and heliosphere? (3) How do magnetic flux and matter
rise through the lower atmosphere, and what role does flux emergence
play in flares and mass ejections? These questions are addressed with
a high-resolution near and far UV imaging spectrometer sensitive to
emission from plasma at temperatures between 5,000 K and 10 MK. IRIS
has a field-of-view of 120 arcsec, a spatial resolution of 0.4 arcsec,
and velocity resolution of 0.5 km/s. The IRIS investigation includes
a strong numerical modeling component based on advanced radiative MHD
codes to facilitate interpretation of observations. We describe the
IRIS instrumentation and numerical modeling, and present the plans for
observations, calibration and data distribution. We will highlight some
of the issues that IRIS observations can help resolve. More information
can be found at http://iris.lmsal.com

---------------------------------------------------------
Title: Observing Coronal Nanoflares in Active Region Moss
Authors: Testa, Paola; De Pontieu, Bart; Martínez-Sykora, Juan;
DeLuca, Ed; Hansteen, Viggo; Cirtain, Jonathan; Winebarger, Amy;
Golub, Leon; Kobayashi, Ken; Korreck, Kelly; Kuzin, Sergey; Walsh,
Robert; DeForest, Craig; Title, Alan; Weber, Mark
2013ApJ...770L...1T Altcode: 2013arXiv1305.1687T
The High-resolution Coronal Imager (Hi-C) has provided Fe XII 193Å
images of the upper transition region moss at an unprecedented spatial
(~0.”3-0.”4) and temporal (5.5 s) resolution. The Hi-C observations
show in some moss regions variability on timescales down to ~15 s,
significantly shorter than the minute-scale variability typically found
in previous observations of moss, therefore challenging the conclusion
of moss being heated in a mostly steady manner. These rapid variability
moss regions are located at the footpoints of bright hot coronal
loops observed by the Solar Dynamics Observatory/Atmospheric Imaging
Assembly in the 94 Å channel, and by the Hinode/X-Ray Telescope. The
configuration of these loops is highly dynamic, and suggestive of
slipping reconnection. We interpret these events as signatures of
heating events associated with reconnection occurring in the overlying
hot coronal loops, i.e., coronal nanoflares. We estimate the order
of magnitude of the energy in these events to be of at least a few
1023 erg, also supporting the nanoflare scenario. These
Hi-C observations suggest that future observations at comparable
high spatial and temporal resolution, with more extensive temperature
coverage, are required to determine the exact characteristics of the
heating mechanism(s).

---------------------------------------------------------
Title: The Cycling of Material between the Solar Corona and
Chromosphere
Authors: Guerreiro, N.; Hansteen, Viggo; De Pontieu, B.
2013ApJ...769...47G Altcode:
Observations of transition region emission lines reveal the presence
of redshifts in lines formed from the top of the chromosphere up
to temperatures of about 2.5 × 105 K and blueshifts for
temperatures above that. However, it is doubtful that the apparent large
downward flows in the lower transition region represents an emptying of
the corona, so some mechanism must be responsible for maintaining the
mass balance between the corona and the lower atmospheric layers. We use
a three-dimensional magnetohydrodynamics code to study the cycling of
mass between the corona, transition region, and chromosphere by adding
a tracer fluid to the simulation in various temperature intervals in the
transition region. We find that most of the material seen in transition
region emission lines formed at temperatures below 3 × 105 K
is material that has been rapidly heated from chromospheric temperatures
and thereafter is pushed down as it cools. This implies that the bulk
of transition region material resides in small loops. In these loops,
the density is high and radiative cooling is efficient.

---------------------------------------------------------
Title: Non-equilibrium Ionization Effects on the Density Line Ratio
Diagnostics of O IV
Authors: Olluri, K.; Gudiksen, B. V.; Hansteen, V. H.
2013ApJ...767...43O Altcode:
The dynamic timescales in the solar atmosphere are shorter than the
ionization and recombination times of many ions used for line ratio
diagnostics of the transition region and corona. The long ionization
and recombination times for these ions imply that they can be found far
from their equilibrium temperatures, and spectroscopic investigations
require more care before being trusted in giving correct information
on local quantities, such as density and temperature. By solving
the full time-dependent rate equations for an oxygen model atom
in the three-dimensional numerical model of the solar atmosphere
generated by the Bifrost code, we are able to construct synthetic
intensity maps and study the emergent emission. We investigate the
method of electron density diagnostics through line ratio analysis
of the O IV 140.1 nm to the 140.4 nm ratio, the assumptions made
in carrying out the diagnostics, and the different interpretations
of the electron density. The results show big discrepancies between
emission in statistical equilibrium and emission where non-equilibrium
(NEQ) ionization is treated. Deduced electron densities are up to an
order of magnitude higher when NEQ effects are accounted for. The
inferred electron density is found to be a weighted mean average
electron density along the line of sight and has no relation to the
temperature of emission. This study shows that numerical modeling is
essential for electron density diagnostics and is a valuable tool when
the ions used for such studies are expected to be out of ionization
equilibrium. Though this study has been performed on the O IV ion,
similar results are also expected for other transition region ions.

---------------------------------------------------------
Title: Numerical Simulations of Spicule Acceleration
Authors: Guerreiro, N.; Carlsson, M.; Hansteen, V.
2013ApJ...766..128G Altcode:
Observations in the Hα line of hydrogen and the H and K lines of singly
ionized calcium on the solar limb reveal the existence of structures
with jet-like behavior, usually designated as spicules. The driving
mechanism for such structures remains poorly understood. Sterling
et al. shed some light on the problem mimicking reconnection events
in the chromosphere with a one-dimensional code by injecting energy
with different spatial and temporal distributions and tracing the
thermodynamic evolution of the upper chromospheric plasma. They found
three different classes of jets resulting from these injections. We
follow their approach but improve the physical description by including
non-LTE cooling in strong spectral lines and non-equilibrium hydrogen
ionization. Increased cooling and conversion of injected energy into
hydrogen ionization energy instead of thermal energy both lead to weaker
jets and smaller final extent of the spicules compared with Sterling
et al. In our simulations we find different behavior depending on
the timescale for hydrogen ionization/recombination. Radiation-driven
ionization fronts also form.

---------------------------------------------------------
Title: Non-equilibrium Ionization in the Bifrost Stellar Atmosphere
Code
Authors: Olluri, K.; Gudiksen, B. V.; Hansteen, V. H.
2013AJ....145...72O Altcode:
The chromosphere and transition region have for the last 20 years been
known to be quite dynamic layers of the solar atmosphere, characterized
by timescales shorter than the ionization equilibrium timescales of
many of the ions dominating emission in these regions. Due to the
fast changes in the properties of the atmosphere, long ionization and
recombination times can lead these ions to being found far from their
equilibrium temperatures. A number of the spectral lines that we observe
can therefore not be expected a priori to reflect information about
local quantities such as the density or temperature, and interpreting
observations requires numerical modeling. Modeling the ionization
balance is computationally expensive and has earlier only been done
in one dimension. However, one-dimensional models can primarily be
used to investigate the possible importance of a physical effect, but
cannot verify or disprove the importance of that effect in the fully
three-dimensional solar atmosphere. Here, using the atomic database
package DIPER, we extend one-dimensional methods and implement a solver
for the rate equations of the full three-dimensional problem, using
the numerical code Bifrost. We present our implementation and report
on a few test cases. We also report on studies of the important C IV
and Fe XII ions in a semi-realistic two-dimensional solar atmosphere
model, focusing on differences between statistical equilibrium and
non-equilibrium ionization results.

---------------------------------------------------------
Title: Solar Wind Models from the Chromosphere to 1 AU
Authors: Hansteen, Viggo H.; Velli, Marco
2013mspc.book...89H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Interface Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, B.; Title, A. M.; Lemen, J. R.; Wuelser, J.;
Tarbell, T. D.; Schrijver, C.; Golub, L.; Kankelborg, C. C.; Hansteen,
V. H.; Carlsson, M.
2012AGUFMSH33D2256D Altcode:
The solar chromosphere and transition region (TR) form a highly
structured and dynamic interface region between the photosphere and
the corona. This region not only acts as the conduit of all mass and
energy feeding into the corona and solar wind, it also requires an
order of magnitude more energy to heat than the corona. Nevertheless,
the chromosphere remains poorly understood, because of the complexity
of the required observational and analytical tools: the interface
region is highly complex with transitions from optically thick to
optically thin radiation, from pressure to magnetic field domination,
and large density and temperature contrasts on small spatial scales. The
Interface Region Imaging Spectrograph (IRIS) was selected for a NASA
SMEX mission in 2009 and is scheduled to launch in early 2013. IRIS
addresses critical questions: (1) Which types of non-thermal energy
dominate in the chromosphere and beyond? (2) How does the chromosphere
regulate mass and energy supply to the corona and heliosphere? (3)
How do magnetic flux and matter rise through the lower atmosphere, and
what role does flux emergence play in flares and mass ejections? These
questions are addressed with a high-resolution near and far UV imaging
spectrometer sensitive to emission from plasma at temperatures between
5,000 K and 10 MK. IRIS has a field-of-view of 120 arcsec, a spatial
resolution of 0.4 arcsec, and velocity resolution of 0.5 km/s. The
IRIS investigation includes a strong numerical modeling component
based on advanced radiative MHD codes to facilitate interpretation of
observations. We will describe the IRIS instrumentation and numerical
modeling, and present the status of the IRIS observatory development. We
will highlight some of the issues that IRIS observations can help
resolve.

---------------------------------------------------------
Title: Solar Wind Models from the Chromosphere to 1 AU
Authors: Hansteen, Viggo H.; Velli, Marco
2012SSRv..172...89H Altcode: 2012SSRv..tmp...35H
Recent models of the fast solar wind are characterized by low coronal
electron temperatures while proton, α-particle, and minor ion
temperatures are expected to be quite high and generally anisotropic,
including large temperatures perpendicular to the magnetic field
and parallel beams. This entails that the electric field should be
relatively unimportant and that solar wind outflows with both high
asymptotic flow speeds but maintaining a low mass flux should be a
natural outcome of plasma expansion along open polar magnetic field
lines. In this chapter we will explain why such changes with respect
to the classical, electron thermally driven solar wind have come about
and outline the most important remaining concerning the astrophysics of
coronal winds. The progress we have seen in the last decade is largely
due observations made with instruments onboard Ulysses (McComas et
al. in Space Sci. Rev. 72:93, 1995) and SOHO (Fleck et al. in The SOHO
Mission, Kluwer, Dordrecht, 1995). These observations have spawned a
new understanding of solar wind energetics, and the consideration of
the chromosphere, corona, and solar wind as a unified system. We will
begin by giving our own, highly biased, "pocket history" of solar wind
theory highlighting the problems that had to be resolved in order to
make the original Parker formulation of thermally driven winds conform
with observational results. Central to this discussion are questions
of how the wind's asymptotic flow speed and mass flux are set, but we
will also touch upon higher order moments such as the ion and electron
temperatures and heat fluxes as well as the possible role of Alfvén
waves and particle effects in driving the solar wind outflow. Solar
wind scaling laws will be discussed in the context of the origin of
slow and fast wind streams.

---------------------------------------------------------
Title: Sources of Solar Wind at Solar Minimum: Constraints from
Composition Data
Authors: Zurbuchen, Thomas H.; von Steiger, Rudolf; Gruesbeck, Jacob;
Landi, Enrico; Lepri, Susan T.; Zhao, Liang; Hansteen, Viggo
2012SSRv..172...41Z Altcode: 2012SSRv..tmp...25Z
In this discussion of observational constraints on the source regions
and acceleration processes of solar wind, we will focus on the
ionic composition of the solar wind and the distribution of charge
states of heavy elements such as oxygen and iron. We first focus on
the now well-known bi-modal nature of solar wind, which dominates
the heliosphere at solar minimum: Compositionally cool solar wind
from polar coronal holes over-expands, filling a much larger solid
angle than the coronal holes on the Sun. We use a series of remote and
in-situ characteristics to derive a global geometric expansion factor of
∼5. Slower, streamer-associated wind is located near the heliospheric
current sheet with a width of 10-20°, but in a well-defined band with
a geometrically small transition width. We then compute charge states
under the assumption of thermal electron distributions and temperature,
velocity, and density profiles predicted by a recent solar wind model,
and conclude that the solar wind originates from a hot source at around
1 million K, characteristic of the closed corona.

---------------------------------------------------------
Title: Investigating the Reliability of Coronal Emission Measure
Distribution Diagnostics using Three-dimensional Radiative
Magnetohydrodynamic Simulations
Authors: Testa, Paola; De Pontieu, Bart; Martínez-Sykora, Juan;
Hansteen, Viggo; Carlsson, Mats
2012ApJ...758...54T Altcode: 2012arXiv1208.4286T
Determining the temperature distribution of coronal plasmas can provide
stringent constraints on coronal heating. Current observations with
the Extreme ultraviolet Imaging Spectrograph (EIS) on board Hinode
and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory provide diagnostics of the emission measure distribution
(EMD) of the coronal plasma. Here we test the reliability of temperature
diagnostics using three-dimensional radiative MHD simulations. We
produce synthetic observables from the models and apply the Monte
Carlo Markov chain EMD diagnostic. By comparing the derived EMDs with
the "true" distributions from the model, we assess the limitations
of the diagnostics as a function of the plasma parameters and the
signal-to-noise ratio of the data. We find that EMDs derived from
EIS synthetic data reproduce some general characteristics of the true
distributions, but usually show differences from the true EMDs that
are much larger than the estimated uncertainties suggest, especially
when structures with significantly different density overlap along
the line of sight. When using AIA synthetic data the derived EMDs
reproduce the true EMDs much less accurately, especially for broad
EMDs. The differences between the two instruments are due to the:
(1) smaller number of constraints provided by AIA data and (2) broad
temperature response function of the AIA channels which provide looser
constraints to the temperature distribution. Our results suggest that
EMDs derived from current observatories may often show significant
discrepancies from the true EMDs, rendering their interpretation
fraught with uncertainty. These inherent limitations to the method
should be carefully considered when using these distributions to
constrain coronal heating.

---------------------------------------------------------
Title: Implications for Coronal Heating from Coronal Rain
Authors: Antolin, P.; Shibata, K.; Carlsson, M.; Rouppe van der Voort,
L.; Vissers, G.; Hansteen, V.
2012ASPC..454..171A Altcode:
Coronal rain is a phenomenon above active regions in which cool plasma
condensations fall down from coronal heights. Numerical simulations of
loops have shown that such condensations can naturally form in the case
of footpoint concentrated heating through the “catastrophic cooling”
mechanism. In this work we analize high resolution limb observations in
Ca II H and Hα of coronal rain performed by Hinode/SOT and by Crisp of
SST and derive statistical properties. We further investigate the link
between coronal rain and the coronal heating mechanisms by performing
1.5-D MHD simulations of a loop subject to footpoint heating and to
Alfvén waves generated in the photosphere. It is found that if a loop
is heated predominantly from Alfvén waves coronal rain is inhibited
due to the characteristic uniform heating they produce. Hence coronal
rain can point both to the spatial distribution of the heating and to
the agent of the heating itself, thus acting as a marker for coronal
heating mechanisms.

---------------------------------------------------------
Title: Spicules type I and type II: numerical simulations and their
role in coronal heating
Authors: Hansteen, Viggo
2012cosp...39..719H Altcode: 2012cosp.meet..719H
When viewed on the limb the chromosphere shows a great amount of
structure, both horizontally and vertically: the chromospheric plasma
is clearly ordered by the magnetic field. This is most particularly true
for the upper chromosphere where one of the most characteristic features
are the so called spicules, thin jets of chromspheric plasma reaching
up to heights of 10,000 km or more above the photosphere. Though
spicules were recognized as early as 1877 by Angelo Secchi, progress on
understanding their physical basis has proved slow. Progress has been
hampered by their small physical scale, short lifetimes, and the large
number of superpositions when viewed on the limb. Recently however,
the launch of the Hinode satellite with its large optical telescope
SOT and the application of new reduction techniques such as MOMFBD
to ground based solar data, has led to a much improved observational
situation and thus revived the field. This is also true of numerical
simulations which finally are approaching a level of sophistication good
enough to study the coronal heating problem as well as spicule dynamics
and energetics. In this talk we will discuss recent observations and
simulations of spicules and comment on their relevance for coronal
heating and solar wind acceleration.

---------------------------------------------------------
Title: Two-dimensional Radiative Magnetohydrodynamic Simulations of
the Importance of Partial Ionization in the Chromosphere
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo
2012ApJ...753..161M Altcode: 2012arXiv1204.5991M
The bulk of the solar chromosphere is weakly ionized and interactions
between ionized particles and neutral particles likely have significant
consequences for the thermodynamics of the chromospheric plasma. We
investigate the importance of introducing neutral particles into the
MHD equations using numerical 2.5D radiative MHD simulations obtained
with the Bifrost code. The models span the solar atmosphere from the
upper layers of the convection zone to the low corona, and solve the
full MHD equations with non-gray and non-LTE radiative transfer, and
thermal conduction along the magnetic field. The effects of partial
ionization are implemented using the generalized Ohm's law, i.e.,
we consider the effects of the Hall term and ambipolar diffusion
in the induction equation. The approximations required in going
from three fluids to the generalized Ohm's law are tested in our
simulations. The Ohmic diffusion, Hall term, and ambipolar diffusion
show strong variations in the chromosphere. These strong variations
of the various magnetic diffusivities are absent or significantly
underestimated when, as has been common for these types of studies,
using the semi-empirical VAL-C model as a basis for estimates. In
addition, we find that differences in estimating the magnitude
of ambipolar diffusion arise depending on which method is used to
calculate the ion-neutral collision frequency. These differences
cause uncertainties in the different magnetic diffusivity terms. In
the chromosphere, we find that the ambipolar diffusion is of the same
order of magnitude or even larger than the numerical diffusion used
to stabilize our code. As a consequence, ambipolar diffusion produces
a strong impact on the modeled atmosphere. Perhaps more importantly,
it suggests that at least in the chromospheric domain, self-consistent
simulations of the solar atmosphere driven by magnetoconvection can
accurately describe the impact of the dominant form of resistivity,
i.e., ambipolar diffusion. This suggests that such simulations may
be more realistic in their approach to the lower solar atmosphere
(which directly drives the coronal volume) than previously assumed.

---------------------------------------------------------
Title: Ubiquitous Torsional Motions in Type II Spicules
Authors: De Pontieu, B.; Carlsson, M.; Rouppe van der Voort, L. H. M.;
Rutten, R. J.; Hansteen, V. H.; Watanabe, H.
2012ApJ...752L..12D Altcode: 2012arXiv1205.5006D
Spicules are long, thin, highly dynamic features that jut out
ubiquitously from the solar limb. They dominate the interface between
the chromosphere and corona and may provide significant mass and energy
to the corona. We use high-quality observations with the Swedish 1
m Solar Telescope to establish that so-called type II spicules are
characterized by the simultaneous action of three different types of
motion: (1) field-aligned flows of order 50-100 km s-1,
(2) swaying motions of order 15-20 km s-1, and (3) torsional
motions of order 25-30 km s-1. The first two modes have been
studied in detail before, but not the torsional motions. Our analysis
of many near-limb and off-limb spectra and narrowband images using
multiple spectral lines yields strong evidence that most, if not all,
type II spicules undergo large torsional modulation and that these
motions, like spicule swaying, represent Alfvénic waves propagating
outward at several hundred km s-1. The combined action
of the different motions explains the similar morphology of spicule
bushes in the outer red and blue wings of chromospheric lines, and
needs to be taken into account when interpreting Doppler motions to
derive estimates for field-aligned flows in spicules and determining
the Alfvénic wave energy in the solar atmosphere. Our results also
suggest that large torsional motion is an ingredient in the production
of type II spicules and that spicules play an important role in the
transport of helicity through the solar atmosphere.

---------------------------------------------------------
Title: Non-equilibrium ionization in 3D numerical models
Authors: Olluri, Kosovare; Gudiksen, Boris; Hansteen, Viggo
2012decs.confE.118O Altcode:
The dynamic timescales in the chromosphere and transition region have
been observed to be much smaller then the ionization equilibration
timescales of many ions found in the region. Due to the fast changes in
the properties of the atmosphere, long ionization- and recombination
times may lead to ions being found far from their equilibrium
temperatures. Spectroscopic investigations therefore needs to be
interpreted with the help of numerical modeling in order to produce
reliable results. By solving the rate equations within a realistic MHD
simulation of the solar atmosphere, we are able to follow the ionization
balance, and study the non equilibrium effects of the emitting gas. Due
top lack of computation power, this has previously been done in simple
1D, but because of the many free parameters in these models, their
conclusions are not free of uncertainties. The resent development in
computing technology and atmospheric modeling makes it possible to
study the full 3D effect of non equilibrium ionization. With the solar
atmosphere model Bifrost, we have a 3D platform for calculating and
following the ionization degree of important atoms of high abundances
in the solar atmosphere. We will present our implementation, and a
study of the carbon IV 1549 Å , Iron XII 195 Å, Oxygen IV 1399 Å
and 1401 Å lines in 2D.

---------------------------------------------------------
Title: Using 3D MHD realistic simulations of the solar corona to
test plasma diagnostics
Authors: Testa, P.; De Pontieu, B.; Martinez-Sykora, J.; Hansteen,
V.; Carlsson, M.
2012decs.confE..27T Altcode:
We synthesize coronal images and spectra from advanced 3D MHD
simulations obtained from the state-of-the art Bifrost code, and
explore how well they reproduce coronal observations with SDO/AIA and
Hinode/EIS. We apply standard diagnostic techniques (e.g., density, and
temperature diagnostics) to the synthetic observations and investigate
how accurately the derived physical information matches the plasma
parameters of the model. We discuss the limitations of the diagnostics
and their implications.

---------------------------------------------------------
Title: Importance of the partial ionization in the chromosphere
using 2D radiative-MHD simulations
Authors: Martinez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo H.
2012decs.confE..81M Altcode:
The bulk of the solar chromosphere is weakly ionized and interactions
between ionized particles and neutral particles will have significant
consequences for the thermodynamics of the chromospheric plasma. We
investigate the importance of introducing neutral particles into the
MHD equations using numerical 2.5D radiative MHD simulations obtained
with the Bifrost code. The models span the solar atmosphere from
upper layers of the convection zone to the low corona, and solve the
full MHD equations with non-grey and non-LTE radiative transfer and
thermal conduction along the magnetic field. The effects of partial
ionization are implemented using the generalized Ohm's law, i.e.,
we consider the effects of the Hall and ambipolar diffusion in the
induction equation. The ohmic, Hall, and ambipolar diffusivities show
variations of several orders of magnitude in the chromosphere. These
strong variations of the various magnetic diffusivities are absent
and significantly underestimated when using the semi-empirical VAL-C
model as a basis for estimates. We find that in the chromosphere,
the ambipolar diffusion is of the same order of magnitude or even
larger than the numerical diffusion used to stabilize our code. As
result of this, we can study the effects of it in the simulations. The
ambipolar diffusion produces strong impact on the chromosphere changing
the thermal properties, dynamics and magnetic field evolution.

---------------------------------------------------------
Title: The generation of shock waves traveling from the photosphere
to the transition region within network magnetic elements
Authors: Kato, Y.; Hansteen, V.; Steiner, O.; Carlsson, M.
2012decs.confE..54K Altcode:
We investigate the generation of shock waves near the photosphere by
convective downdrafts in the immediate surroundings of the magnetic
flux concentration, using radiation magnetohydrodynamic (RMHD) 2D
simulations of the solar atmosphere. The simulations comprise the layers
from the upper convection zone to the lower corona. We call this the
"magnetic pumping process". We find that the generated slow modes via
magnetic pumping travel upward along the magnetic flux concentration,
developing into a shock wave in chromospheric heights. The waves
continue to propagate further up through the transition region and into
the corona. In the course of propagation through the transition layer,
a small fraction of the longitudinal slow mode is converted into a
transverse wave mode. We report on how much energy is deposited by
propagating shock waves through the transition region and we discuss
the the dissipation process above the photosphere within the magnetic
flux concentration..

---------------------------------------------------------
Title: Overview of simulations and observations of the coupling
between solar regions
Authors: Hansteen, Viggo
2012decs.confE..30H Altcode:
We will discuss simulations and observations of the outer solar
atmosphere, in particular the chromosphere, transition region and
corona, as a coupled system. Focus will be on how the atmosphere is
energized, how mass and energy is transferred between various regions,
and what the mechanisms are that couple the various regions. Current
simulations are described with an eye towards the physics included,
and a discussion of what is missing, e.g. the topology of magnetic
fields studied, large physical scales, non-equilibrium ionization
and generalized Ohm's law. The impact of the physical assumptions is
assessed. Specific synthetic observables from the IRIS instrument are
presented and their diagnostic value is discussed.

---------------------------------------------------------
Title: Forward Modeling of Emission in Solar Dynamics
Observatory/Atmospheric Imaging Assembly Passbands from Dynamic
Three-dimensional Simulations
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Testa, Paola;
Hansteen, Viggo
2011ApJ...743...23M Altcode: 2011arXiv1109.0704M
It is typically assumed that emission in the passbands of the
Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory (SDO) is dominated by single or several strong lines
from ions that under equilibrium conditions are formed in a narrow
range of temperatures. However, most SDO/AIA channels also contain
contributions from lines of ions that have formation temperatures
that are significantly different from the "dominant" ion(s). We
investigate the importance of these lines by forward modeling the
emission in the SDO/AIA channels with three-dimensional radiative MHD
simulations of a model that spans the upper layer of the convection
zone to the low corona. The model is highly dynamic. In addition,
we pump a steadily increasing magnetic flux into the corona, in
order to increase the coronal temperature through the dissipation
of magnetic stresses. As a consequence, the model covers different
ranges of coronal temperatures as time progresses. The model covers
coronal temperatures that are representative of plasma conditions in
coronal holes and quiet Sun. The 131, 171, and 304 Å AIA passbands
are found to be the least influenced by the so-called non-dominant
ions, and the emission observed in these channels comes mostly from
plasma at temperatures near the formation temperature of the dominant
ion(s). On the other hand, the other channels are strongly influenced
by the non-dominant ions, and therefore significant emission in these
channels comes from plasma at temperatures that are different from the
"canonical" values. We have also studied the influence of non-dominant
ions on the AIA passbands when different element abundances are assumed
(photospheric and coronal), and when the effects of the electron
density on the contribution function are taken into account.

---------------------------------------------------------
Title: Wave Propagation and Jet Formation in the Chromosphere
Authors: Heggland, L.; Hansteen, V. H.; De Pontieu, B.; Carlsson, M.
2011ApJ...743..142H Altcode: 2011arXiv1112.0037H
We present the results of numerical simulations of wave propagation
and jet formation in solar atmosphere models with different magnetic
field configurations. The presence in the chromosphere of waves with
periods longer than the acoustic cutoff period has been ascribed to
either strong inclined magnetic fields, or changes in the radiative
relaxation time. Our simulations include a sophisticated treatment
of radiative losses, as well as fields with different strengths
and inclinations. Using Fourier and wavelet analysis techniques,
we investigate the periodicity of the waves that travel through the
chromosphere. We find that the velocity signal is dominated by waves
with periods around 5 minutes in regions of strong, inclined field,
including at the edges of strong flux tubes where the field expands,
whereas 3 minute waves dominate in regions of weak or vertically
oriented fields. Our results show that the field inclination is very
important for long-period wave propagation, whereas variations in the
radiative relaxation time have little effect. Furthermore, we find
that atmospheric conditions can vary significantly on timescales of
a few minutes, meaning that a Fourier analysis of wave propagation
can be misleading. Wavelet techniques take variations with time into
account and are more suitable analysis tools. Finally, we investigate
the properties of jets formed by the propagating waves once they reach
the transition region, and find systematic differences between the
jets in inclined-field regions and those in vertical field regions,
in agreement with observations of dynamic fibrils.

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Title: Generation and propagation of Alfvenic waves in spicules
Authors: De Pontieu, B.; Okamoto, T. J.; Rouppe van der Voort, L.;
Hansteen, V. H.; Carlsson, M.
2011AGUFMSH13B1956D Altcode:
Both spicules and Alfven waves have recently been implicated in
playing a role in the heating of the outer atmosphere. Yet we do
not know how spicules or Alfven waves are generated. Here we focus
on the properties of Alfvenic waves in spicules and their role in
forming spicules. We use high-resolution observations taken with the
Solar Optical Telescope onboard Hinode, and with the CRISP Fabry-Perot
Interferometer at the Swedish Solar Telescope (SST) in La Palma to study
the generation and propagation of Alfvenic waves in spicules and their
disk counterparts. Using automated detection algorithms to identify
propagating waves in limb spicules, we find evidence for both up-
and downward propagating as well as standing waves. Our data suggests
significant reflection of waves in and around spicules and provides
constraints for theoretical models of spicules and wave propagation
through the chromosphere. We also show observational evidence (using
SST data) of the generation of Alfven waves and the role they play in
forming spicules.

---------------------------------------------------------
Title: Testing coronal plasma diagnostics using 3D MHD models of
the solar atmosphere
Authors: Testa, P.; Martinez-Sykora, J.; Hansteen, V. H.; De Pontieu,
B.; Carlsson, M.
2011AGUFMSH53C..06T Altcode:
We synthesize coronal images and spectra from advanced 3D radiative
MHD simulations obtained from the state-of-the-art Bifrost code, and
explore how well they reproduce coronal observations with SDO/AIA
and Hinode/EIS and XRT. We apply standard diagnostic techniques
(e.g., density, temperature, abundance diagnostics) to the synthetic
observations and investigate how accurately the derived physical
information matches the plasma parameters of the model. We discuss
the limitations of the diagnostics and their implications.

---------------------------------------------------------
Title: On the Origin of the Type II Spicules: Dynamic
Three-dimensional MHD Simulations
Authors: Martínez-Sykora, Juan; Hansteen, Viggo; Moreno-Insertis,
Fernando
2011ApJ...736....9M Altcode: 2010arXiv1011.4703M
Recent high temporal and spatial resolution observations of the
chromosphere have forced the definition of a new type of spicule, "type
II's," that are characterized by rising rapidly, having short lives,
and by fading away at the end of their lifetimes. Here, we report on
features found in realistic three-dimensional simulations of the outer
solar atmosphere that resemble the observed type II spicules. These
features evolve naturally from the simulations as a consequence of
the magnetohydrodynamical evolution of the model atmosphere. The
simulations span from the upper layer of the convection zone to
the lower corona and include the emergence of a horizontal magnetic
flux. The state-of-art Oslo Staggered Code is used to solve the full
MHD equations with non-gray and non-LTE radiative transfer and thermal
conduction along the magnetic field lines. We describe in detail the
physics involved in a process which we consider a possible candidate
for the driver mechanism that produces type II spicules. The modeled
spicule is composed of material rapidly ejected from the chromosphere
that rises into the corona while being heated. Its source lies in
a region with large field gradients and intense electric currents,
which lead to a strong Lorentz force that squeezes the chromospheric
material, resulting in a vertical pressure gradient that propels the
spicule along the magnetic field, as well as Joule heating, which
heats the jet material, forcing it to fade.

---------------------------------------------------------
Title: Alfvénic waves with sufficient energy to power the quiet
solar corona and fast solar wind
Authors: McIntosh, Scott W.; de Pontieu, Bart; Carlsson, Mats;
Hansteen, Viggo; Boerner, Paul; Goossens, Marcel
2011Natur.475..477M Altcode:
Energy is required to heat the outer solar atmosphere to millions of
degrees (refs 1, 2) and to accelerate the solar wind to hundreds of
kilometres per second (refs 2-6). Alfvén waves (travelling oscillations
of ions and magnetic field) have been invoked as a possible mechanism
to transport magneto-convective energy upwards along the Sun's magnetic
field lines into the corona. Previous observations of Alfvénic waves
in the corona revealed amplitudes far too small (0.5kms-1)
to supply the energy flux (100-200Wm-2) required to
drive the fast solar wind or balance the radiative losses of the
quiet corona. Here we report observations of the transition region
(between the chromosphere and the corona) and of the corona that
reveal how Alfvénic motions permeate the dynamic and finely structured
outer solar atmosphere. The ubiquitous outward-propagating Alfvénic
motions observed have amplitudes of the order of 20kms-1 and
periods of the order of 100-500s throughout the quiescent atmosphere
(compatible with recent investigations), and are energetic enough to
accelerate the fast solar wind and heat the quiet corona.

---------------------------------------------------------
Title: Quiet-Sun imaging asymmetries in Na I D1 compared
with other strong Fraunhofer lines
Authors: Rutten, R. J.; Leenaarts, J.; Rouppe van der Voort, L. H. M.;
de Wijn, A. G.; Carlsson, M.; Hansteen, V.
2011A&A...531A..17R Altcode: 2011arXiv1104.4307R
Imaging spectroscopy of the solar atmosphere using the Na I
D1 line yields marked asymmetry between the blue and
red line wings: sampling a quiet-Sun area in the blue wing displays
reversed granulation, whereas sampling in the red wing displays normal
granulation. The Mg I b2 line of comparable strength does
not show this asymmetry, nor does the stronger Ca II 8542 Å line. We
demonstrate the phenomenon with near-simultaneous spectral images in
Na I D1, Mg I b2, and Ca II 8542 Å from the
Swedish 1-m Solar Telescope. We then explain it with line-formation
insights from classical 1D modeling and with a 3D magnetohydrodynamical
simulation combined with NLTE spectral line synthesis that permits
detailed comparison with the observations in a common format. The
cause of the imaging asymmetry is the combination of correlations
between intensity and Dopplershift modulation in granular overshoot
and the sensitivity to these of the steep profile flanks of the Na
I D1 line. The Mg I b2 line has similar core
formation but much wider wings due to larger opacity buildup and
damping in the photosphere. Both lines obtain marked core asymmetry
from photospheric shocks in or near strong magnetic concentrations,
less from higher-up internetwork shocks that produce similar asymmetry
in the spatially averaged Ca II 8542 Å profile.

---------------------------------------------------------
Title: The stellar atmosphere simulation code Bifrost. Code
description and validation
Authors: Gudiksen, B. V.; Carlsson, M.; Hansteen, V. H.; Hayek, W.;
Leenaarts, J.; Martínez-Sykora, J.
2011A&A...531A.154G Altcode: 2011arXiv1105.6306G
Context. Numerical simulations of stellar convection and photospheres
have been developed to the point where detailed shapes of observed
spectral lines can be explained. Stellar atmospheres are very complex,
and very different physical regimes are present in the convection zone,
photosphere, chromosphere, transition region and corona. To understand
the details of the atmosphere it is necessary to simulate the whole
atmosphere since the different layers interact strongly. These physical
regimes are very diverse and it takes a highly efficient massively
parallel numerical code to solve the associated equations. Aims:
The design, implementation and validation of the massively parallel
numerical code Bifrost for simulating stellar atmospheres from the
convection zone to the corona. Methods: The code is subjected
to a number of validation tests, among them the Sod shock tube test,
the Orzag-Tang colliding shock test, boundary condition tests and
tests of how the code treats magnetic field advection, chromospheric
radiation, radiative transfer in an isothermal scattering atmosphere,
hydrogen ionization and thermal conduction. Results.Bifrost completes
the tests with good results and shows near linear efficiency scaling
to thousands of computing cores.

---------------------------------------------------------
Title: On the minimum temperature of the quiet solar chromosphere
Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Gudiksen, B. V.
2011A&A...530A.124L Altcode: 2011arXiv1104.5081L
 Aims: We aim to provide an estimate of the minimum temperature
of the quiet solar chromosphere. Methods: We perform a 2D
radiation-MHD simulation spanning the upper convection zone to the
lower corona. The simulation includes non-LTE radiative transfer
and an equation-of-state that includes non-equilibrium ionization
of hydrogen and non-equilibrium H2 molecule formation. We
analyze the reliability of the various assumptions made in our model
in order to assess the realism of the simulation. Results:
Our simulation contains pockets of cool gas with down to 1660 K from
1 Mm up to 3.2 Mm height. It overestimates the radiative heating,
and contains non-physical heating below 1660 K. Therefore we conclude
that cool pockets in the quiet solar chromosphere might have even
lower temperatures than in the simulation, provided that there exist
areas in the chromosphere without significant magnetic heating. We
suggest off-limb molecular spectroscopy to look for such cool pockets
and 3D simulations including a local dynamo and a magnetic carpet to
investigate Joule heating in the quiet chromosphere.

---------------------------------------------------------
Title: What do Spectral Line Profile Asymmetries Tell us About the
Solar Atmosphere?
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo;
McIntosh, Scott W.
2011ApJ...732...84M Altcode:
Recently, analysis of solar spectra obtained with the EUV Imaging
Spectrograph (EIS) onboard the Hinode satellite has revealed the
ubiquitous presence of asymmetries in transition region (TR) and coronal
spectral line profiles. These asymmetries have been observed especially
at the footpoints of coronal loops and have been associated with strong
upflows that may play a significant role in providing the corona with
hot plasma. Here, we perform a detailed study of the various processes
that can lead to spectral line asymmetries, using both simple forward
models and state-of-the-art three-dimensional radiative MHD simulations
of the solar atmosphere using the Bifrost code. We describe a novel
technique to determine the presence and properties of faint secondary
components in the wings of spectral line profiles. This method is based
on least-squares fitting of observed so-called R(ed)B(lue) asymmetry
profiles with pre-calculated RB asymmetry profiles for a wide variety
of secondary component properties. We illustrate how this method could
be used to perform reliable double Gaussian fits that are not over- or
under-constrained. We also find that spectral line asymmetries appear
in TR and coronal lines that are synthesized from our three-dimensional
MHD simulations. Our models show that the spectral asymmetries are a
sensitive measure of the velocity gradient with height in the TR of
coronal loops. The modeled TR shows a large gradient of velocity that
increases with height: this occurs as a consequence of ubiquitous,
episodic heating at low heights in the model atmosphere. We show
that the contribution function of spectral lines as a function of
temperature is critical for sensitivity to velocity gradients and thus
line asymmetries: lines that are formed over a temperature range that
includes most of the TR are the most sensitive. As a result, lines from
lithium-like ions (e.g., O VI) are found to be the most sensitive to
line asymmetries. We compare the simulated line profiles directly with
line profiles observed in the quiet Sun with SOHO/SUMER and Hinode/EIS
and find that the shape of the profiles is very similar. In addition,
the simulated profiles with the strongest blueward asymmetry occur in
footpoint regions of coronal loops, which is similar to what we observe
with SUMER and EIS. There is however a significant discrepancy between
the simulations and observations: the simulated RB asymmetries are
an order of magnitude smaller than the observations. We discuss the
possible reasons for this discrepancy. In summary, our analysis shows
that observations of spectral line asymmetries can provide a powerful
new diagnostic to help constrain coronal heating models.

---------------------------------------------------------
Title: The Origins of Hot Plasma in the Solar Corona
Authors: De Pontieu, B.; McIntosh, S. W.; Carlsson, M.; Hansteen,
V. H.; Tarbell, T. D.; Boerner, P.; Martinez-Sykora, J.; Schrijver,
C. J.; Title, A. M.
2011Sci...331...55D Altcode:
The Sun's outer atmosphere, or corona, is heated to millions of degrees,
considerably hotter than its surface or photosphere. Explanations for
this enigma typically invoke the deposition in the corona of nonthermal
energy generated by magnetoconvection. However, the coronal heating
mechanism remains unknown. We used observations from the Solar Dynamics
Observatory and the Hinode solar physics mission to reveal a ubiquitous
coronal mass supply in which chromospheric plasma in fountainlike jets
or spicules is accelerated upward into the corona, with much of the
plasma heated to temperatures between ~0.02 and 0.1 million kelvin (MK)
and a small but sufficient fraction to temperatures above 1 MK. These
observations provide constraints on the coronal heating mechanism(s)
and highlight the importance of the interface region between photosphere
and corona.

---------------------------------------------------------
Title: Ubiquitous Alfvenic Motions in Quiet Sun, Coronal Hole and
Active Region Corona
Authors: McIntosh, S. W.; de Pontieu, B.; Carlsson, M.; Hansteen,
V. H.; Sdo/Aia Mission Team
2010AGUFMSH14A..01M Altcode:
We use observations with AIA onboard SDO and report the discovery of
ubiquitous Alfvenic oscillations in the corona of quiet Sun, active
regions and coronal holes. These Alfvenic oscillations have significant
power, and seem to be connected to the chromospheric Alfvenic
oscillations previously reported with Hinode. We use Monte Carlo
simulations to determine the strength and periods of the waves. Using
unique joint observations of Hinode, the Solar Dynamics Observatory, and
HAO's CoMP instrument we study the excitation of transverse oscillations
as a function of space, time, and temperature. We will discuss the
energetic impact and diagnostic capabilities of this ever-present
process and how it can be used to build a more self-consistent picture
of energy transport into the inner heliosphere. Transverse Oscillations
Observed Above the Solar North Pole in the He II 304Å (bottom) and Fe
IX 171Å (top) channels. Studying the progression of such points with
altitude yields important information about wave propagation into the
magnetically open corona.

---------------------------------------------------------
Title: The role of the chromosphere in filling the corona with hot
plasma (Invited)
Authors: de Pontieu, B.; McIntosh, S. W.; Carlsson, M.; Hansteen,
V. H.; Tarbell, T. D.; Boerner, P.; Martinez-Sykora, J.; Schrijver,
C. J.; Title, A. M.
2010AGUFMSH21C..03D Altcode:
We use coordinated observations from the Solar Dynamics Observatory
(SDO), Hinode and the Swedish Solar Telescope (SST) to show how
plasma is heated to coronal temperatures from its source in the
chromosphere. Our observations reveal a ubiquitous mass supply
for the solar corona in which chromospheric plasma is accelerated
upward into the corona with much of the plasma heated to transition
region temperatures, and a small, but significant fraction heated
to temperatures in excess of 1 million K. Our observations show,
for the first time, how chromospheric spicules, fountain-like jets
that have long been considered potential candidates for coronal
heating, are directly associated with heating of plasma to coronal
temperatures. These results provide strong physical constraints on
the mechanism(s) responsible for coronal heating and do not seem
compatible with current models. The association with chromospheric
spicules highlights the importance of the interface region between
the photosphere and corona to gain a full understanding of the coronal
heating problem.

---------------------------------------------------------
Title: Line profile asymmetries in the transition region: models
and observations
Authors: Martinez-Sykora, J.; de Pontieu, B.; Hansteen, V. H.;
McIntosh, S. W.
2010AGUFMSH31A1784M Altcode:
Asymmetries in spectral line profiles provide a wealth of
information on the properties of the emitting plasma along the
line-of-sight. Asymmetries can be produced by the superposition
of profiles with different line-of-sight velocities and/or widths
resulting from the variation of the velocity and/or temperature from
emission sources along the line of sight. Spectral line asymmetries
from synthetic transition region and coronal lines constructed
from realistic 3D models appear similar to those observed with
Hinode/EIS. The simulations span the upper layer of the convection zone
to the lower corona and include horizontal magnetic flux emergence. We
use the state of the art Bifrost code to solve the full MHD equations
with non-grey and non-LTE radiative transfer and thermal conduction
along the magnetic field line. Here, we perform a detailed study of
the various physical, dynamical and observational processes that can
lead to spectral line asymmetries at the transition region footpoints
of loops in 3D radiative MHD simulations of the solar atmosphere and
compare these with observations. Our models show that the spectral
asymmetries are a sensitive measure of the velocity gradient with
height in the transition region of coronal loops. In our models the
TR shows a large gradient of velocity that increases with height:
this occurs as a natural consequence of ubiquitous, episodic heating
at low heights in the model atmosphere.

---------------------------------------------------------
Title: Forward modeling of emission in AIA passbands from advanced
radiative MHD simulations
Authors: de Pontieu, B.; Martinez-Sykora, J.; Hansteen, V. H.
2010AGUFMSH11A1597D Altcode:
The emission from many of the passbands observed with the Atmospheric
Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO)
is dominated by single or several lines from ions that are formed in a
narrow range of temperatures (under equilibrium conditions). However,
most AIA passbands contain contributions from lines of ions that
have formation temperatures that are significantly different from the
dominant ion. We investigate the importance of these lines by forward
modeling of the AIA passband emission from advanced radiative 3D MHD
simulations calculated with the state of the art Bifrost code. We
use simulations that span the upper layer of the convection zone to
the low corona and solve the full magnetohydrodynamic equations with
non-grey and non-LTE radiative transfer and thermal conduction along the
magnetic field lines. We find that several of the AIA passbands often
include significant contributions from plasma at different temperatures
than the canonical temperature values. We describe under which solar
conditions in the simulations these discrepancies can typically be
expected to occur.

---------------------------------------------------------
Title: Modeling of EIS Spectrum Drift from Instrumental Temperatures
Authors: Kamio, S.; Hara, H.; Watanabe, T.; Fredvik, T.; Hansteen,
V. H.
2010SoPh..266..209K Altcode: 2010SoPh..tmp..137K; 2010arXiv1003.3540K
An empirical model has been developed to reproduce the drift of the
spectrum recorded by the EIS on Hinode using instrumental temperatures
and relative motion of the spacecraft. The EIS spectrum shows an
artificial drift in wavelength dimension in sync with the revolution of
the spacecraft, which is caused by temperature variations inside the
spectrometer. The drift amounts to 70 km s−1 in Doppler
velocity and introduces difficulties in velocity measurements. An
artificial neural network is incorporated to establish a relationship
between the instrumental temperatures and the spectral drift. This
empirical model reproduces observed spectrum shift with an rms error
of 4.4 km s−1. This procedure is robust and applicable to
any spectrum obtained with EIS, regardless of the observing field. In
addition, spectral curvatures and spatial offset in the north - south
direction are determined to compensate for instrumental effects.

---------------------------------------------------------
Title: On Redshifts and Blueshifts in the Transition Region and Corona
Authors: Hansteen, V. H.; Hara, H.; De Pontieu, B.; Carlsson, M.
2010ApJ...718.1070H Altcode: 2010arXiv1001.4769H
Emission lines formed in the transition region (TR) of the Sun have long
been known to show pervasive redshifts. Despite a variety of proposed
explanations, these TR downflows (and the slight upflows in the low
corona) remain poorly understood. We present results from comprehensive
three-dimensional MHD models that span the upper convection zone up to
the corona, 15 Mm above the photosphere. The TR and coronal heating
in these models is caused by the stressing of the magnetic field by
photospheric and convection "zone dynamics," but also in some models by
the injection of emerging magnetic flux. We show that rapid, episodic
heating, at low heights of the upper chromospheric plasma to coronal
temperatures naturally produces downflows in TR lines, and slight
upflows in low coronal lines, with similar amplitudes to those observed
with EUV/UV spectrographs. We find that TR redshifts naturally arise
in episodically heated models where the average volumetric heating
scale height lies between that of the chromospheric pressure scale
height of 200 km and the coronal scale height of 50 Mm.

---------------------------------------------------------
Title: Radiative transfer with scattering for domain-decomposed 3D
MHD simulations of cool stellar atmospheres. Numerical methods and
application to the quiet, non-magnetic, surface of a solar-type star
Authors: Hayek, W.; Asplund, M.; Carlsson, M.; Trampedach, R.; Collet,
R.; Gudiksen, B. V.; Hansteen, V. H.; Leenaarts, J.
2010A&A...517A..49H Altcode: 2010arXiv1007.2760H
 Aims: We present the implementation of a radiative
transfer solver with coherent scattering in the new BIFROST
code for radiative magneto-hydrodynamical (MHD) simulations of
stellar surface convection. The code is fully parallelized using
MPI domain decomposition, which allows for large grid sizes and
improved resolution of hydrodynamical structures. We apply the code
to simulate the surface granulation in a solar-type star, ignoring
magnetic fields, and investigate the importance of coherent scattering
for the atmospheric structure. Methods: A scattering term
is added to the radiative transfer equation, requiring an iterative
computation of the radiation field. We use a short-characteristics-based
Gauss-Seidel acceleration scheme to compute radiative flux divergences
for the energy equation. The effects of coherent scattering are
tested by comparing the temperature stratification of three 3D
time-dependent hydrodynamical atmosphere models of a solar-type star:
without scattering, with continuum scattering only, and with both
continuum and line scattering. Results: We show that continuum
scattering does not have a significant impact on the photospheric
temperature structure for a star like the Sun. Including scattering in
line-blanketing, however, leads to a decrease of temperatures by about
350 K below log10 τ5000 ⪉ -4. The effect is
opposite to that of 1D hydrostatic models in radiative equilibrium,
where scattering reduces the cooling effect of strong LTE lines in
the higher layers of the photosphere. Coherent line scattering also
changes the temperature distribution in the high atmosphere, where
we observe stronger fluctuations compared to a treatment of lines as
true absorbers.

---------------------------------------------------------
Title: Numerical Simulations of the Chromosphere
Authors: Hansteen, Viggo H.
2010AAS...21630505H Altcode:
The solar chromosphere forms the all important link between the
photosphere's and convection zone's excess mechanical energy and
the magnetically dominated corona and the solar wind. We still do
not know which modes of nonthermal energy power the chromosphere
and overlying layers. We know that waves, electrical currents,
and magnetic reconnection all may release substantial energy, and
that non-thermal particles, resistive dissipation, and wave damping
occur. Shedding light on these issues requires that one use a variety
of approaches, both theoretical and observational. In this presentation
we will focus on sol called `realistic' numerical modelling and on
comparing the results of numerical modelling with present and upcoming
observations. To model the chromosphere with neighboring regions
from the convection zone to the corona in a realistic manner a 3D
radiation magnetohydrodynamic code `Bifrost' has been developend using
a sixth-order finite difference compact scheme. Radiation is treated
with multi-group opacities. Conduction along the magnetic field is
treated implicitly using a multi-grid approach. Novel aspects of the
code are the extension of the multi-group opacity method to include
scattering and the treatment of the radiative exchange in strong lines
in the middle and upper chromosphere. We have there used the detailed
1D radiation-hydrodynamic simulations to develop recipes that contain
the essentials of the physics while keeping the computational expenses
at tractable levels. Currently we are extending the code to include
the effects of partial time-dependent ionization of Hydrogen and a
generalized Ohm's law.

---------------------------------------------------------
Title: Comparison Of Observations And Advanced Numerical Simulations
Of Type II Spicules
Authors: Martinez-Sykora, Juan; De Pontieu, B.; Hansteen, V.;
Moreno-Insertis, F.
2010AAS...21640306M Altcode: 2010BAAS...41..878M
We have performed realistic 3D radiation MHD simulations of the
solar atmosphere. These simulations show jet-like features that
are similar to the type II spicules discovered with Hinode's Solar
Optical Telescope. These type II spicules have been associated with
so-called rapid blueshifted events (RBE's) on the solar disk, and with
significant blueward asymmetries in transition region and coronal
lines at the footpoints of coronal loops (discovered with Hinode's
EIS). These observational results and their ubiquity suggest they may
play a significant role in providing the corona with hot plasma. We
will present a detailed comparison of the properties of the simulated
jets, with those of type II spicules (observed with Hinode) and RBE's
(with ground-based instruments). We will present analysis of a wide
variety of synthetic emission lines from the simulations covering
temperatures from 10,000 K to several million K, and compare their
intensities, velocities, line widths and asymmetry with those of the
observed phenomena. We will also show how the formation mechanism of
these jets (reconnection at tangential discontinuities) complicates
efforts to establish a firm link between observations of magnetic
fields and of chromospheric flows, and suggests that magnetic field
observations at chromospheric heights may be crucial to establish from
observations how these jets are formed.

---------------------------------------------------------
Title: The Quiet Solar Atmosphere Observed and Simulated in Na
I D1
Authors: Leenaarts, J.; Rutten, R. J.; Reardon, K.; Carlsson, M.;
Hansteen, V.
2010ApJ...709.1362L Altcode: 2009arXiv0912.2206L
The Na I D1 line in the solar spectrum is sometimes
attributed to the solar chromosphere. We study its formation in
quiet-Sun network and internetwork. We first present high-resolution
profile-resolved images taken in this line with the imaging
spectrometer Interferometric Bidimensional Spectrometer at the Dunn
Solar Telescope and compare these to simultaneous chromospheric images
taken in Ca II 8542 Å and Hα. We then model Na I D1
formation by performing three-dimensional (3D) non-local
thermodynamic equilibrium profile synthesis for a snapshot from a
3D radiation-magnetohydrodynamics simulation. We find that most Na I
D1 brightness is not chromospheric but samples the magnetic
concentrations that make up the quiet-Sun network in the photosphere,
well below the height where they merge into chromospheric canopies,
with aureoles from 3D resonance scattering. The line core is sensitive
to magneto-acoustic shocks in and near magnetic concentrations, where
shocks occur deeper than elsewhere, and may provide evidence of heating
deep within magnetic concentrations.

---------------------------------------------------------
Title: On red-shifts in the transition region and corona .
Authors: Hansteen, V. H.; Hara, H.; de Pontieu, B.; Carlsson, M.
2010MmSAI..81..729H Altcode:
We present evidence that transition region red-shifts are naturally
produced in episodically heated models where the average volumetric
heating scale height lies between that of the chromospheric
pressure scale height of 200 km and the coronal scale height of
50 Mm. In order to do so we present results from 3d MHD models
spanning the upper convection zone up to the corona, 15 Mm above the
photosphere. Transition region and coronal heating in these models
is due both the stressing of the magnetic field by photospheric and
convection `zone dynamics, but also in some models by the injection
of emerging magnetic flux.

---------------------------------------------------------
Title: Chromospheric heating and structure as determined from high
resolution 3D simulations .
Authors: Carlsson, M.; Hansteen, V. H.; Gudiksen, B. V.
2010MmSAI..81..582C Altcode: 2010arXiv1001.1546C
We have performed 3D radiation MHD simulations extending from the
convection zone to the corona covering a box 16 Mm3 at 32
km spatial resolution. The simulations show very fine structure in
the chromosphere with acoustic shocks interacting with the magnetic
field. Magnetic flux concentrations have a temperature lower than the
surroundings in the photosphere but higher in the low chromosphere. The
heating is there mostly through ohmic dissipation preferentially at
the edges of the flux concentrations. The magnetic field is often
wound up around the flux concentrations. When acoustic waves travel
up along the field this topology leads to swirling motions seen in
chromospheric diagnostic lines such as the calcium infrared triplet.

---------------------------------------------------------
Title: Observing the Roots of Coronal Heating - in the Chromosphere
Authors: McIntosh, S. W.; de Pontieu, B.; Hansteen, V. H.; Schrjver, K.
2009AGUFMSH44A..01M Altcode:
I will discuss recent results using Hinode/SOT-EIS-XRT, SOHO/SUMER,
CRISP (at the Swedish Solar Telescope) and TRACE that provide a
direct connection between coronal dynamics and those of the lower
atmosphere. We use chromospheric measurements (H-alpha and Ca II
8542 spectral imaging, and Ca II H images), as well as UV spectra
(EIS and SUMER), and EUV/X-ray images (XRT and TRACE) to show that
faint, high-speed upflows at velocities of 50-100 km/s across a wide
range of temperatures from chromospheric (10,000 K), through lower
and upper transition region (0.1 to 0.7 MK) and coronal temperatures
(2 MK) are associated with significant mass-loading of the corona with
hot plasma. Our observations are incompatible with current models in
which coronal heating occurs as a result of nanoflares at coronal
heights. Instead we suggest that a significant fraction of heating
of plasma to coronal temperatures may occur at chromospheric heights
in association with jets driven from below (the recently discovered
type II spicules). Illustrating the mass and energy transport between
the chromosphere, transition region and corona, as deduced from Hinode
observations. Convective flows and oscillations in the convection zone
and photosphere of the Sun buffet the magnetic field of the Sun. This
leads to at least two different kinds of jets in the chromosphere:
Type I, and II spicules. Type II spicules drive matter upward violently
and likely form when magnetic field reconnects because of stresses
introduced by convective flows. A significant fraction of the plasma
in type II spicules is heated to coronal temperatures (&gt;1MK),
providing the corona with hot plasma. The correlation between the
chromospheric and coronal parts of the spicules depends greatly on the
viewing angle between the line-of-sight and the direction of the upward
flows. Order of magnitude estimates indicate that the mass supplied
by type II spicules plays a significant role in supplying the corona
with hot plasma.

---------------------------------------------------------
Title: Twisted Flux Tube Emergence from the Convection Zone to the
Corona. II. Later States
Authors: Martínez-Sykora, Juan; Hansteen, Viggo; Carlsson, Mats
2009ApJ...702..129M Altcode: 2009arXiv0906.5464M
Three-dimensional numerical simulations of magnetic flux emergence
are carried out in a computational domain spanning the upper layers
of the convection zone to the lower corona. We use the Oslo Staggered
Code to solve the full magnetohydrodynamic equations with non-gray
and non-local thermodynamic equilibrium radiative transfer and thermal
conduction along the magnetic field lines. In this paper, we concentrate
on the later stages of the simulations and study the evolution of the
structure of the rising flux in the upper chromosphere and corona, the
interaction between the emerging flux and the weak coronal magnetic
field initially present, and the associated dynamics. The flux tube
injected at the bottom boundary rises to the photosphere where it
largely remains. However, some parts of the flux tube become unstable
and expand in patches into the upper chromosphere. The flux rapidly
expands toward the corona, pushing the coronal and transition region
material aside, and lifting and maintaining the transition region at
heights greater than 5 Mm above the photosphere for extensive periods
of time. The pre-existing magnetic field in the corona and transition
region is perturbed by the incoming flux and reoriented by a series of
high Joule heating events. Low-density structures form in the corona,
while at later times a high-density filamentary structure appears in
the lower part of the expanding flux. The dynamics of these and other
structures is discussed. While Joule heating due to the expanding flux
is episodic, it increases in relative strength as fresh magnetic field
rises and becomes energetically important in the upper chromosphere and
corona at later times. Chromospheric, transition region, and coronal
lines are computed and their response to the perturbation caused by
the expanding emerging flux is discussed.

---------------------------------------------------------
Title: Estimating the Chromospheric Absorption of Transition Region
Moss Emission
Authors: De Pontieu, Bart; Hansteen, Viggo H.; McIntosh, Scott W.;
Patsourakos, Spiros
2009ApJ...702.1016D Altcode: 2009arXiv0907.1883D
Many models for coronal loops have difficulty explaining the observed
EUV brightness of the transition region, which is often significantly
less than theoretical models predict. This discrepancy has been
addressed by a variety of approaches including filling factors and
time-dependent heating, with varying degrees of success. Here, we
focus on an effect that has been ignored so far: the absorption of
EUV light with wavelengths below 912 Å by the resonance continua
of neutral hydrogen and helium. Such absorption is expected to occur
in the low-lying transition region of hot, active region loops that
is colocated with cool chromospheric features and called "moss" as a
result of the reticulated appearance resulting from the absorption. We
use cotemporal and cospatial spectroheliograms obtained with the Solar
and Heliospheric Observatory/SUMER and Hinode/EIS of Fe XII 1242 Å,
195 Å, and 186.88 Å, and compare the density determination from
the 186/195 Å line ratio to that resulting from the 195/1242 Å line
ratio. We find that while coronal loops have compatible density values
from these two line pairs, upper transition region moss has conflicting
density determinations. This discrepancy can be resolved by taking
into account significant absorption of 195 Å emission caused by
the chromospheric inclusions in the moss. We find that the amount of
absorption is generally of the order of a factor of 2. We compare to
numerical models and show that the observed effect is well reproduced
by three-dimensional radiative MHD models of the transition region
and corona. We use STEREO A/B data of the same active region and find
that increased angles between line of sight and local vertical cause
additional absorption. Our determination of the amount of chromospheric
absorption of TR emission can be used to better constrain coronal
heating models.

---------------------------------------------------------
Title: Observational Signatures of Simulated Reconnection Events in
the Solar Chromosphere and Transition Region
Authors: Heggland, L.; De Pontieu, B.; Hansteen, V. H.
2009ApJ...702....1H Altcode: 2009arXiv0902.0977H
We present the results of numerical simulations of wave-induced magnetic
reconnection in a model of the solar atmosphere. In the magnetic field
geometry we study in this paper, the waves, driven by a monochromatic
piston and a driver taken from Hinode observations, induce periodic
reconnection of the magnetic field, and this reconnection appears to
help drive long-period chromospheric jets. By synthesizing spectra
for a variety of wavelengths that are sensitive to a wide range of
temperatures, we shed light on the often confusing relationship between
the plethora of jet-like phenomena in the solar atmosphere, e.g.,
explosive events, spicules, and other phenomena thought to be caused by
reconnection. Our simulations produce spicule-like jets with lengths
and lifetimes that match observations, and the spectral signatures of
several reconnection events are similar to observations of explosive
events. We also find that in some cases, absorption from overlying
neutral hydrogen can hide emission from matter at coronal temperatures.

---------------------------------------------------------
Title: Spicule-Like Structures Observed in Three-Dimensional Realistic
Magnetohydrodynamic Simulations
Authors: Martínez-Sykora, Juan; Hansteen, Viggo; De Pontieu, Bart;
Carlsson, Mats
2009ApJ...701.1569M Altcode: 2009arXiv0906.4446M
We analyze features that resemble type I spicules in two different
three-dimensional numerical simulations in which we include horizontal
magnetic flux emergence in a computational domain spanning the
upper layers of the convection zone to the lower corona. The two
simulations differ mainly in the pre-existing ambient magnetic field
strength and in the properties of the inserted flux tube. We use the
Oslo Staggered Code to solve the full magnetohydrodynamic equations
with nongray and non-LTE radiative transfer and thermal conduction
along the magnetic field lines. We find a multitude of features that
show a spatiotemporal evolution that is similar to that observed in
type I spicules, which are characterized by parabolic height versus
time profiles, and are dominated by rapid upward motion at speeds
of 10-30 km s-1, followed by downward motion at similar
velocities. We measured the parameters of the parabolic profile of the
spicules and find similar correlations between the parameters as those
found in observations. The values for height (or length) and duration
of the spicules found in the simulations are more limited in range than
those in the observations. The spicules found in the simulation with
higher pre-existing ambient field have shorter length and smaller
velocities. From the simulations, it appears that these kinds of
spicules can, in principle, be driven by a variety of mechanisms that
include p-modes, collapsing granules, magnetic energy release in the
photosphere and lower chromosphere, and convective buffeting of flux
concentrations.

---------------------------------------------------------
Title: Stellar winds and magnetic fields
Authors: Hansteen, Viggo H.
2009hppl.book..225H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Observing the Roots of Solar Coronal Heating—in the
Chromosphere
Authors: De Pontieu, Bart; McIntosh, Scott W.; Hansteen, Viggo H.;
Schrijver, Carolus J.
2009ApJ...701L...1D Altcode: 2009arXiv0906.5434D
The Sun's corona is millions of degrees hotter than its 5000 K
photosphere. This heating enigma is typically addressed by invoking
the deposition at coronal heights of nonthermal energy generated
by the interplay between convection and magnetic field near the
photosphere. However, it remains unclear how and where coronal heating
occurs and how the corona is filled with hot plasma. We show that energy
deposition at coronal heights cannot be the only source of coronal
heating by revealing a significant coronal mass supply mechanism that
is driven from below, in the chromosphere. We quantify the asymmetry
of spectral lines observed with Hinode and SOHO and identify faint
but ubiquitous upflows with velocities that are similar (50-100 km
s-1) across a wide range of magnetic field configurations and
for temperatures from 100,000 to several million degrees. These upflows
are spatiotemporally correlated with and have similar upward velocities
as recently discovered, cool (10,000 K) chromospheric jets or (type II)
spicules. We find these upflows to be pervasive and universal. Order
of magnitude estimates constrained by conservation of mass and observed
emission measures indicate that the mass supplied by these spicules can
play a significant role in supplying the corona with hot plasma. The
properties of these events are incompatible with coronal loop models
that include only nanoflares at coronal heights. Our results suggest
that a significant part of the heating and energizing of the corona
occurs at chromospheric heights, in association with chromospheric jets.

---------------------------------------------------------
Title: The solar atmosphere
Authors: Hansteen, Viggo H.
2009hppl.book..195S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Evidence from the Extreme-Ultraviolet Imaging Spectrometer
for Axial Filament Rotation before a Large Flare
Authors: Williams, David R.; Harra, Louise K.; Brooks, David H.;
Imada, Shinsuke; Hansteen, Viggo H.
2009PASJ...61..493W Altcode:
In this article, we present observations made with the
Extreme-ultraviolet Imaging Spectrometer on-board the Hinode solar
satellite, of an active region filament in the HeII emission line at
256.32Å. The host active region AR 10930 produces an X-class flare
during these observations. We measure Doppler shifts with apparent
velocities of up to 20km s-1, which are antisymmetric about
the filament length and occur several minutes before the flare's
impulsive phase. This is indicative of a rotation of the filament,
which is in turn consistent with expansion of a twisted flux rope due
to the MHD helical kink instability. This is the first time that such
an observation has been possible in this transition-region line, and we
note that the signature observed occurs before the first indications of
pre-flare activity in the GOES solar soft X-ray flux, suggesting that
the filament begins to destabilise in tandem with a reorganization of
the local magnetic field. We suggest that this expansion is triggered
by the decrease of magnetic tension around, and/or total pressure above,
the filament.

---------------------------------------------------------
Title: Observing the Roots of Solar Coronal Heating in the
Chromosphere
Authors: McIntosh, Scott W.; De Pontieu, B.; Hansteen, V.; Schrijver,
C. J.
2009SPD....40.2602M Altcode:
The Sun's atmosphere or corona is millions of degrees hotter than
its 5,000 K surface or photosphere. This heating enigma is typically
addressed by invoking the deposition at coronal heights of non-thermal
energy generated by the interplay between convection and magnetic field
near the photosphere. However, it remains unclear how and where coronal
heating occurs and how the corona is filled with hot plasma. Here,
we show that energy deposition at coronal heights cannot be the only
source of coronal heating, by revealing a significant coronal mass
supply mechanism that is driven from below, in the chromosphere, the
interface between photosphere and corona. We quantify the asymmetry
of spectral lines observed with Hinode and SOHO and identify faint
but ubiquitous upflows with velocities that are similar (50-100
km/s) across a wide range of magnetic field configurations and for
temperatures from 100,000 to several million degrees. These upflows
are correlated with and have similar upward velocities as the very fine
and dynamic chromospheric jets, or spicules, discovered by Hinode. As
these phenomena are incompatible with models of coronal loops that
only include nanoflare heating at coronal heights, we conclude that
a significant fraction of the energy needed to heat coronal plasma is
deposited at chromospheric heights in association with spicular jets
driven from below.

---------------------------------------------------------
Title: Three-Dimensional Non-LTE Radiative Transfer Computation of
the CA 8542 Infrared Line From a Radiation-MHD Simulation
Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Rouppe van der
Voort, L.
2009ApJ...694L.128L Altcode: 2009arXiv0903.0791L
The interpretation of imagery of the solar chromosphere in the
widely used Ca II 854.2 nm infrared line is hampered by its complex,
three-dimensional, and non-LTE formation. Forward modeling is
required to aid understanding. We use a three-dimensional non-LTE
radiative transfer code to compute synthetic Ca II 854.2 nm images
from a radiation-MHD simulation of the solar atmosphere spanning
from the convection zone to the corona. We compare the simulation
with observations obtained with the CRISP filter at the Swedish 1 m
Solar Telescope. We find that the simulation reproduces dark patches
in the blue line wing caused by Doppler shifts, brightenings in the
line core caused by upward-propagating shocks, and thin dark elongated
structures in the line core that form the interface between upward and
downward gas motion in the chromosphere. The synthetic line core is
narrower than the observed one, indicating that the Sun exhibits both
more vigorous large-scale dynamics as well as small scale motions that
are not resolved within the simulation, presumably owing to a lack of
spatial resolution.

---------------------------------------------------------
Title: Advancing our understanding of the chromosphere
Authors: Hansteen, V. H.; Carlsson, M.; de Pontieu, B.
2008AGUFMSH51C..01H Altcode:
Recent progress has shown the solar chromosphere to be fundamentally
dynamic, where non-linear techniques must be used to understand
its nature. It is also the region where the magnetic field grows
to dominate the plasma and where the coupling between radiation and
matter becomes becomes quite tenuous. Understanding the workings of the
chromosphere is vital if one is to understand the flow of energy between
the solar surface and its outer atmosphere and wind. Recent numerical
developments have shown that it is feasible to model the chromosphere,
even to the extent that newly available high resolution observations
sometimes can be reproduced in detail. We will discuss the challenges
facing numerical chromospheric models and the observations needed to
validate or refute them.

---------------------------------------------------------
Title: Dynamics of the upper chromosphere
Authors: de Pontieu, B.; Carlsson, M.; McIntosh, S.; Hansteen, V.;
Tarbell, T.
2008AGUFMSH51C..05D Altcode:
In the past few years, high-resolution observations with ground-based
telescopes and the Broadband Filter Imager (BFI) and Narrowband
Filter Imager (NFI) of the Solar Optical Telescope onboard Hinode
have revolutionized our view of the dynamics and energetics of
the chromosphere. We review some of these results, including the
discovery of two different types of spicules and the finding that the
chromosphere is riddled with strong Alfvenic waves. We describe how
these observations, when combined with advanced numerical simulations,
can help address important unresolved issues regarding the connection
between the photosphere and corona, such as the role of waves and
of reconnection in driving the dynamics and energetics of the upper
chromosphere, and how chromospheric dynamics impact the transition
region and corona.

---------------------------------------------------------
Title: What do Spicules Tell us About the Chromosphere?
Authors: de Pontieu, B.; Carlsson, M.; McIntosh, S.; Hansteen, V.;
Tarbell, T.
2008ESPM...12.2.15D Altcode:
In the past few years, high-resolution observations with ground-based
telescopes and the Broadband Filter Imager (BFI) and Narrowband
Filter Imager (NFI) of the Solar Optical Telescope onboard
Hinode have revolutionized our view of spicules and their role in
the chromosphere. We review some of these results, including the
discovery of two different types of spicules with different dynamics
and formation mechanisms, as well as the finding that the chromosphere
is riddled with strong Alfvenic waves. In an effort to determine
the formation mechanism of spicules and their impact on the outer
atmosphere, we further focus on the thermal evolution and velocities
developed by spicules. We use Dopplergrams made in the Na D 589.6 nm,
H-alpha 656.3 nm and Mg B 517.3 nm passbands, as well as filtergrams in
the Ca H 396.8 nm passband to study the spatio-temporal relationship
between the various spicular features. We compare those findings with
synthesized images based on line profiles computed from high-resolution
3D MHD numerical simulations from the University of Oslo. We also use
the Dopplergram data to investigate the velocities that develop in
the two types of spicules that were reported previously. We perform
statistical analysis of apparent velocities in the plane of the sky
and line-of-sight velocities derived from Dopplergrams to disentangle
the superposition of Alfvenic wave amplitudes and field-aligned
flows. We study these properties for a variety of magnetic field
configurations (coronal holes, quiet Sun, active region). Finally,
we focus on the formation mechanism of spicules by analyzing spicular
features in Dopplergrams on the disk that were taken simultaneously
with SP magnetograms.

---------------------------------------------------------
Title: Search for High Velocities in the Disk Counterpart of Type
II Spicules
Authors: Langangen, Ø.; De Pontieu, B.; Carlsson, M.; Hansteen,
V. H.; Cauzzi, G.; Reardon, K.
2008ApJ...679L.167L Altcode: 2008arXiv0804.3256L
Recently, De Pontieu and coworkers discovered a class of spicules
that evolve more rapidly than previously known spicules, with rapid
apparent motions of 50-150 km s-1, thickness of a few 100
km, and lifetimes of order 10-60 s. These so-called type II spicules
have been difficult to study because of limited spatiotemporal and
thermal resolution. Here we use the IBIS instrument to search for the
high velocities in the disk counterpart of type II spicules. We have
detected rapidly evolving events, with lifetimes that are less than a
minute and often equal to the cadence of the instrument (19 s). These
events are characterized by a Doppler shift that only appears in the
blue wing of the Ca II IR line. Furthermore, the spatial extent,
lifetime, and location near network all suggest a link to type II
spicules. However, the magnitude of the measured Doppler velocity is
significantly lower than the apparent motions seen at the limb. We
use Monte Carlo simulations to show that this discrepancy can be
explained by a forward model in which the visibility on the disk of
the high-velocity flows in these events is limited by a combination
of line-of-sight projection and reduced opacity in upward propelled
plasma, especially in reconnection driven jets that are powered by a
roughly constant energy supply.

---------------------------------------------------------
Title: Twisted Flux Tube Emergence From the Convection Zone to
the Corona
Authors: Martínez-Sykora, Juan; Hansteen, Viggo; Carlsson, Mats
2008ApJ...679..871M Altcode: 2007arXiv0712.3854M
Three-dimensional numerical simulations of a horizontal magnetic flux
tube emergence with different twist are carried out in a computational
domain spanning the upper layers of the convection zone to the lower
corona. We use the Oslo Stagger Code to solve the full MHD equations
with non-gray, non-LTE radiative transfer and thermal conduction along
the magnetic lines. A magnetic flux tube is input at the bottom boundary
into a weakly magnetized atmosphere. The photospheric and chromospheric
response is described with magnetograms and synthetic continuum as
well as Ca II H line images and velocity field distributions. In the
photosphere the granular size increases when the flux tube approaches
from below, as has been reported previously in the literature. In
the convective overshoot region, some 200 km above the photosphere,
adiabatic expansion produces cooling, darker regions with the structure
of granulation cells. We also find evidence of collapsed granulation
at the boundaries of the rising flux tube. Once the flux tube has
crossed the photosphere, bright points related to concentrated magnetic
field, vorticity, high vertical velocities, and heating by compressed
material are found at heights up to 500 km above the photosphere. At
greater heights, in the magnetized chromosphere, the rising flux tube
produces a large, cool, magnetized bubble that tends to expel the
usual chromospheric oscillations. In addition, the rising flux tube
dramatically increases the chromospheric scale height, pushing the
transition region and corona aside, such that the chromosphere extends
up to 6 Mm above the photosphere. We find flux tube emergence through
the photosphere to the lower corona to be a relatively slow process,
taking of order 1 hr.

---------------------------------------------------------
Title: Velocities and thermal evolution of chromospheric spicules
Authors: de Pontieu, B.; McIntosh, S. W.; Tarbell, T.; Carlsson,
M. P.; Hansteen, V. H.
2008AGUSMSP53A..06D Altcode:
We use the Broadband Filter Imager (BFI) and Narrowband Filter
Imager (NFI) of the Solar Optical Telescope on Hinode to study the
thermal evolution and velocities developed by chromospheric plasma in
spicules. We use Dopplergrams made in the Na D 589.6 nm, Hα 656.3 nm
and Mg B 517.3 nm passbands, as well as filtergrams in the Ca H 396.8 nm
passband to study the spatio-temporal relationship between the various
spicular features. We compare those findings with synthesized images
based on line profiles computed from high-resolution 3D MHD numerical
simulations from the University of Oslo. We also use the Dopplergram
data to investigate the velocities that develop in the two types of
spicules that were reported previously. We perform statistical analysis
of apparent velocities in the plane of the sky and line-of-sight
velocities derived from Dopplergrams to disentangle the superposition
of Alfvenic wave amplitudes and field-aligned flows. We study these
properties for a variety of magnetic field configurations (coronal
holes, quiet Sun, active region). Finally, we focus on the formation
mechanism of spicules by analyzing spicular features in Dopplergrams
on the disk that were taken simultaneously with SP magnetograms. This
work was supported by NASA contract NNM07AA01C. The Hinode mission is
operated by ISAS/JAXA, NAOJ, NASA, STFC, ESA and NSC.

---------------------------------------------------------
Title: Chromospheric Flows in the Vicinity of Magnetic Features in
the Quiet Sun Observed with Hinode SOT
Authors: Tarbell, T.; de Pontieu, B.; Carlsson, M.; Hansteen, V.;
McIntosh, S.; Ichimoto, K.
2008AGUSMSP41B..02T Altcode:
The Narrowband Filter Imager of the Solar Optical Telescope on Hinode
can measure Doppler shifts and line-of- sight magnetic fields in two
lines with contributions from the low chromosphere: Na D 589.6 nm and
Mg b 517.3 nm. The SOT Spectro-Polarimeter also measures very accurate
vector magnetic fields and Doppler velocities in the photosphere. These
observations have diffraction-limited spatial resolution and superb
stability. We present examples of these measurements in quiet sun
at various disk positions. In addition to the expected granulation
and f- and p-modes, conspicuous longer-lived downflows are seen near
strong network flux elements. Transient upflows are also detected,
presumably the base of flows seen in spicules at the limb and H-alpha
mottles on the disk. Velocity features associated with emerging and
cancelling magnetic features are also described. The observations are
compared with synthesized images made from line profiles computed from
the University of Oslo 3-D MHD simulations. This work was supported by
NASA contract NNM07AA01C. The Hinode mission is operated by ISAS/JAXA,
NAOJ, NASA, STFC, ESA and NSC.

---------------------------------------------------------
Title: Spectroscopic Measurements of Dynamic Fibrils in the Ca II
λ8662 Line
Authors: Langangen, Øystein; Carlsson, Mats; Rouppe van der Voort,
Luc; Hansteen, Viggo; De Pontieu, Bart
2008ApJ...673.1194L Altcode: 2007arXiv0710.0247L
We present high spatial resolution spectroscopic measurements of dynamic
fibrils (DFs) in the Ca II λ8662 line. These data show clear Doppler
shifts in the identified DFs, which demonstrates that at least a subset
of DFs are actual mass motions in the chromosphere. A statistical
analysis of 26 DFs reveals a strong and statistically significant
correlation between the maximal velocity and the deceleration. The
range of the velocities and the decelerations are substantially lower,
about a factor 2, in our spectroscopic observations compared to the
earlier results based on proper motion in narrowband images. There
are fundamental differences in the different observational methods;
when DFs are observed spectroscopically, the measured Doppler shifts
are a result of the atmospheric velocity, weighted with the response
function to velocity over an extended height. When the proper motion
of DFs is observed in narrowband images, the movement of the top
of the DF is observed. This point is sharply defined because of the
high contrast between the DF and the surroundings. The observational
differences between the two methods are examined by several numerical
experiments using both numerical simulations and a time series of
narrowband Hα images. With basis in the simulations we conclude that
the lower maximal velocity is explained by the low formation height of
the Ca IR line. We conclude that the present observations support the
earlier result that DFs are driven by magnetoacoustic shocks excited
by convective flows and p-modes.

---------------------------------------------------------
Title: Chromospheric Alfvénic Waves Strong Enough to Power the
Solar Wind
Authors: De Pontieu, B.; McIntosh, S. W.; Carlsson, M.; Hansteen,
V. H.; Tarbell, T. D.; Schrijver, C. J.; Title, A. M.; Shine, R. A.;
Tsuneta, S.; Katsukawa, Y.; Ichimoto, K.; Suematsu, Y.; Shimizu, T.;
Nagata, S.
2007Sci...318.1574D Altcode:
Alfvén waves have been invoked as a possible mechanism for the heating
of the Sun's outer atmosphere, or corona, to millions of degrees and
for the acceleration of the solar wind to hundreds of kilometers per
second. However, Alfvén waves of sufficient strength have not been
unambiguously observed in the solar atmosphere. We used images of
high temporal and spatial resolution obtained with the Solar Optical
Telescope onboard the Japanese Hinode satellite to reveal that the
chromosphere, the region sandwiched between the solar surface and
the corona, is permeated by Alfvén waves with strong amplitudes on
the order of 10 to 25 kilometers per second and periods of 100 to
500 seconds. Estimates of the energy flux carried by these waves and
comparisons with advanced radiative magnetohydrodynamic simulations
indicate that such Alfvén waves are energetic enough to accelerate
the solar wind and possibly to heat the quiet corona.

---------------------------------------------------------
Title: Hinode and the Corona's Lower Boundary: Spicules and Alfven
Waves
Authors: de Pontieu, B.; McIntosh, S. W.; Hansteen, V.; Carlsson, M. P.
2007AGUFMSH52C..08D Altcode:
The lower boundary of the corona, or chromosphere, requires of order
100 times more energy than the corona itself, and provides the mass
to fill coronal loops. Yet the chromosphere and its coupling to the
corona is often overlooked. Recently, observations with the Solar
Optical Telescope (SOT) onboard Hinode and ground-based telescopes
combined with advanced numerical simulations have provided us with
unprecedented views and a better understanding of the (spicular)
dynamics of the chromosphere and how the lower boundary couples to
the corona and solar wind. We analyze high-resolution, high-cadence
Ca II and Hα observations of the solar chromosphere and find that the
dynamics of the magnetized chromosphere are dominated by at least two
different types of spicules. We show that the first type involves up-
and downward motion that is driven by shock waves that form when global
oscillations and convective flows leak into the chromosphere along
magnetic field lines on on 3-7 minute timescales. The second type of
spicules is much more dynamic: they form rapidly (in ~10s), are very
thin (&lt;200km wide), have lifetimes of 10-150s (at any one height) and
seem to be rapidly heated to (at least) transition region temperatures,
sending material through the chromosphere at speeds of order 50-150
km/s. The properties of Type II spicules suggest a formation process
that is a consequence of magnetic reconnection. We discuss the impact of
both spicules types on the coronal mass and energy balance. Our analysis
of Hinode data also indicates that the chromosphere is permeated by
strong Alfvén waves. Both types of spicules are observed to carry
these Alfvén waves, which have significant amplitudes of order 20 km/s,
transverse displacements of order 500-1,000 km and periods of 150-400
s. Estimates of the energy flux carried by these Alfvén waves and
comparisons to advanced radiative MHD simulations indicate that these
waves most likely play a significant role in the acceleration of the
solar wind, and possibly the heating of the quiet Sun corona. We will
discuss the implications of these waves on the energy balance of the
lower atmosphere.

---------------------------------------------------------
Title: The Structure and Dynamics of the Quiet Corona from
Observations with the Extreme ultraviolet Imaging Spectrometer
Authors: Dere, K. P.; Doschek, G. A.; Mariska, J. T.; Harra, L. K.;
Matsuzaki, K.; Hansteen, V.; Thomas, R. J.
2007AGUFMSH53A1046D Altcode:
The goal of the Extreme-ultraviolet Imaging Spectrometer (EIS) on the
Hinode satellite is to measure such physical parameters as the velocity
and density of the solar corona in order to provide an observational
basis to understand how coronal plasmas are heated and accelerated. On
2007 January 20, EIS performed a raster of a 128 x 512 arc-sec. area
of a quiet region near Sun center. The observing program recorded
spectra of He II λ256, formed at 9 × 104 K, and lines of Fe VIII-XV,
formed at temperatures spanning the range from 5 × 105 through 2 × 106
K. Maps of intensities, velocities and electron densities derived from
these observations are presented and discussed. Intensity maps in He II
λ256 show the chromospheric network. Line intensities of Fe X-XIV show
small-scale bright points and more extended structures. The intensity
map of Fe VIII shows a transition between the two temperatures. The
coronal lines reveal regions of high outflow velocities on the
order of 100 km s-1 in a compact region and 12 km s-1 in an extended
region. The presence of these high velocities in the quiet corona
is an entirely new and unexpected result. Electron densities derived
from density sensitive line ratios of Fe XII and XIII are typically
about 3 - 20×108 cm-3. The highest densities are found in bright,
compact areas. For the first time, explosive events in the quiet sun
have been observed in the extreme-ultraviolet in He II λ256 profiles
and have properties similar to those previously reported.

---------------------------------------------------------
Title: The Structure and Dynamics of the Quiet Corona from
Observations with the Extreme Ultraviolet Imaging Spectrometer
on Hinode
Authors: Dere, Kenneth P.; Doschek, George A.; Mariska, John T.;
Hansteen, Viggo H.; Harra, Louise K.; Matsuzaki, Keiichi; Thomas,
Roger J.
2007PASJ...59S.721D Altcode:
The goal of the Hinode mission is to provide an observational basis for
understanding the heating and acceleration of coronal plasmas. On 2007
January 20, the Extreme ultraviolet Imaging Spectrometer performed
a raster of a quiet region near Sun center. Maps of intensities,
velocities, and electron densities derived from these observations are
presented and discussed. Intensity maps in HeII λ 256, formed at 9
× 104 K, show the chromospheric network. Line intensities
of FeX-XIV, formed at temperatures from 1-2 × 106 K, show
small-scale bright points and more extended structures. The intensity
map of FeVIII shows a transition between the two temperatures. The
coronal lines reveal regions of high outflow velocities on the order
of 100kms-1 in a compact region and 12kms-1 in
an extended region. The presence of such high velocities in the quiet
corona is an entirely new and unexpected result. Electron densities
derived from density sensitive line ratios of FeXII and XIII are
typically 3-20 × 108cm-3. The highest densities
are found in bright, compact areas. For the first time, explosive
events in the quiet sun have been observed in the extreme-ultraviolet
in HeII λ 256 profiles.

---------------------------------------------------------
Title: Hinode EUV Study of Jets in the Sun's South Polar Corona
Authors: Culhane, Len; Harra, Louise K.; Baker, Deborah; van
Driel-Gesztelyi, Lidia; Sun, Jian; Doschek, George A.; Brooks, David
H.; Lundquist, Loraine L.; Kamio, Suguru; Young, Peter R.; Hansteen,
Viggo H.
2007PASJ...59S.751C Altcode:
A number of coronal bright points and associated plasma jet features
were seen in an observation of the South polar coronal hole during
2007 January. The 40" wide slot was used at the focus of the Hinode
EUV Imaging Spectrometer to provide spectral images for two of these
events. Light curves are plotted for a number of emission lines that
include He II 256Å (0.079MK) and cover the temperature interval from
0.4MK to 5.0MK. Jet speed measurements indicate values less than the
escape velocity. The light curves show a post-jet enhancement in a
number of the cooler coronal lines indicating that after a few minutes
cooling, the plasma fell back to its original acceleration site. This
behavior has not been previously observed by e.g., the Yohkoh Soft
X-ray Telescope due to the comparatively high temperature cut-off
in its response. The observations are consistent with the existing
models that involve magnetic reconnection between emerging flux and the
ambient open field lines in the polar coronal hole. However we do not
have sufficient coverage of lines from lower temperature ion species
to register the Hα-emitting surge material that is associated with
some of these models.

---------------------------------------------------------
Title: On Connecting the Dynamics of the Chromosphere and Transition
Region with Hinode SOT and EIS
Authors: Hansteen, Viggo H.; de Pontieu, Bart; Carlsson, Mats;
McIntosh, Scott; Watanabe, Tetsuya; Warren, Harry P.; Harra, Louise K.;
Hara, Hirohisa; Tarbell, Theodore D.; Shine, Dick; Title, Alan M.;
Schrijver, Carolus J.; Tsuneta, Saku; Katsukawa, Yukio; Ichimoto,
Kiyoshi; Suematsu, Yoshinori; Shimizu, Toshifumi
2007PASJ...59S.699H Altcode: 2007arXiv0711.0487H
We use coordinated Hinode SOT/EIS observations that include
high-resolution magnetograms, chromospheric, and transition region
(TR) imaging, and TR/coronal spectra in a first test to study how
the dynamics of the TR are driven by the highly dynamic photospheric
magnetic fields and the ubiquitous chromospheric waves. Initial
analysis shows that these connections are quite subtle and require a
combination of techniques including magnetic field extrapolations,
frequency-filtered time-series, and comparisons with synthetic
chromospheric and TR images from advanced 3D numerical simulations. As a
first result, we find signatures of magnetic flux emergence as well as
3 and 5mHz wave power above regions of enhanced photospheric magnetic
field in both chromospheric, transition region, and coronal emission.

---------------------------------------------------------
Title: Can High Frequency Acoustic Waves Heat the Quiet Sun
Chromosphere?
Authors: Carlsson, Mats; Hansteen, Viggo H.; de Pontieu, Bart;
McIntosh, Scott; Tarbell, Theodore D.; Shine, Dick; Tsuneta, Saku;
Katsukawa, Yukio; Ichimoto, Kiyoshi; Suematsu, Yoshinori; Shimizu,
Toshifumi; Nagata, Shin'ichi
2007PASJ...59S.663C Altcode: 2007arXiv0709.3462C
We use Hinode/SOT Ca II H-line and blue continuum broadband observations
to study the presence and power of high frequency acoustic waves at
high spatial resolution. We find that there is no dominant power at
small spatial scales; the integrated power using the full resolution of
Hinode (0.05” pixels, 0.16” resolution) is larger than the power in
the data degraded to 0.5” pixels (TRACE pixel size) by only a factor
of 1.2. At 20 mHz the ratio is 1.6. Combining this result with the
estimates of the acoustic flux based on TRACE data of Fossum &amp;
Carlsson (2006), we conclude that the total energy flux in acoustic
waves of frequency 5-40 mHz entering the internetwork chromosphere of
the quiet Sun is less than 800 W m$^{-2}$, inadequate to balance the
radiative losses in a static chromosphere by a factor of five.

---------------------------------------------------------
Title: A Tale of Two Spicules: The Impact of Spicules on the Magnetic
Chromosphere
Authors: de Pontieu, Bart; McIntosh, Scott; Hansteen, Viggo H.;
Carlsson, Mats; Schrijver, Carolus J.; Tarbell, Theodore D.; Title,
Alan M.; Shine, Richard A.; Suematsu, Yoshinori; Tsuneta, Saku;
Katsukawa, Yukio; Ichimoto, Kiyoshi; Shimizu, Toshifumi; Nagata,
Shin'ichi
2007PASJ...59S.655D Altcode: 2007arXiv0710.2934D
We use high-resolution observations of the Sun in CaIIH (3968Å)
from the Solar Optical Telescope on Hinode to show that there are
at least two types of spicules that dominate the structure of the
magnetic solar chromosphere. Both types are tied to the relentless
magnetoconvective driving in the photosphere, but have very different
dynamic properties. “Type-I” spicules are driven by shock waves
that form when global oscillations and convective flows leak into
the upper atmosphere along magnetic field lines on 3--7minute
timescales. “Type-II” spicules are much more dynamic: they form
rapidly (in ∼ 10s), are very thin (≤ 200 km wide), have lifetimes
of 10-150s (at any one height), and seem to be rapidly heated to
(at least) transition region temperatures, sending material through
the chromosphere at speeds of order 50--150kms-1. The
properties of Type II spicules suggest a formation process that is
a consequence of magnetic reconnection, typically in the vicinity
of magnetic flux concentrations in plage and network. Both types of
spicules are observed to carry Alfvén waves with significant amplitudes
of order 20kms-1.

---------------------------------------------------------
Title: Waves and Shocks in the Solar Atmosphere
Authors: Hansteen, V. H.
2007ASPC..369..193H Altcode:
There is a large volume of observations showing wave phenomena in the
solar chromosphere and corona. These waves affect the very structure
of the atmosphere, and may contribute to the heating of both the
chromosphere, transition region and corona. They also provide powerful
diagnostics using seismological techniques. In this talk we will review
the observations of waves and compare these with simulations of wave
excitation in the solar convection zone and of wave propagation and
mode conversion in realistic magnetic field topologies.

---------------------------------------------------------
Title: Non-equilibrium hydrogen ionization in 2D simulations of the
solar atmosphere
Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Rutten, R. J.
2007A&A...473..625L Altcode: 2007arXiv0709.3751L
Context: The ionization of hydrogen in the solar chromosphere and
transition region does not obey LTE or instantaneous statistical
equilibrium because the timescale is long compared with important
hydrodynamical timescales, especially of magneto-acoustic shocks. Since
the pressure, temperature, and electron density depend sensitively on
hydrogen ionization, numerical simulation of the solar atmosphere
requires non-equilibrium treatment of all pertinent hydrogen
transitions. The same holds for any diagnostic application employing
hydrogen lines. Aims: To demonstrate the importance and to
quantify the effects of non-equilibrium hydrogen ionization, both
on the dynamical structure of the solar atmosphere and on hydrogen
line formation, in particular Hα. Methods: We implement an
algorithm to compute non-equilibrium hydrogen ionization and its
coupling into the MHD equations within an existing radiation MHD code,
and perform a two-dimensional simulation of the solar atmosphere from
the convection zone to the corona. Results: Analysis of the
simulation results and comparison to a companion simulation assuming
LTE shows that: a) non-equilibrium computation delivers much smaller
variations of the chromospheric hydrogen ionization than for LTE. The
ionization is smaller within shocks but subsequently remains high in
the cool intershock phases. As a result, the chromospheric temperature
variations are much larger than for LTE because in non-equilibrium,
hydrogen ionization is a less effective internal energy buffer. The
actual shock temperatures are therefore higher and the intershock
temperatures lower. b) The chromospheric populations of the hydrogen
n = 2 level, which governs the opacity of Hα, are coupled to the
ion populations. They are set by the high temperature in shocks
and subsequently remain high in the cool intershock phases. c)
The temperature structure and the hydrogen level populations differ
much between the chromosphere above photospheric magnetic elements
and above quiet internetwork. d) The hydrogen n = 2 population and
column density are persistently high in dynamic fibrils, suggesting
that these obtain their visibility from being optically thick in Hα
also at low temperature. Movie and Appendix A are only available
in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: Numerical Simulations of Shock Wave-driven Chromospheric Jets
Authors: Heggland, L.; De Pontieu, B.; Hansteen, V. H.
2007ApJ...666.1277H Altcode: 2007astro.ph..3498H
We present the results of numerical simulations of shock wave-driven
jets in the solar atmosphere. The dependence of observable quantities
such as maximum velocity and deceleration on parameters such as the
period and amplitude of initial disturbances and the inclination of
the magnetic field is investigated. Our simulations show excellent
agreement with observations, and shed new light on the correlation
between velocity and deceleration and on the regional differences
found in observations.

---------------------------------------------------------
Title: The EUV Imaging Spectrometer for Hinode
Authors: Culhane, J. L.; Harra, L. K.; James, A. M.; Al-Janabi, K.;
Bradley, L. J.; Chaudry, R. A.; Rees, K.; Tandy, J. A.; Thomas, P.;
Whillock, M. C. R.; Winter, B.; Doschek, G. A.; Korendyke, C. M.;
Brown, C. M.; Myers, S.; Mariska, J.; Seely, J.; Lang, J.; Kent,
B. J.; Shaughnessy, B. M.; Young, P. R.; Simnett, G. M.; Castelli,
C. M.; Mahmoud, S.; Mapson-Menard, H.; Probyn, B. J.; Thomas, R. J.;
Davila, J.; Dere, K.; Windt, D.; Shea, J.; Hagood, R.; Moye, R.; Hara,
H.; Watanabe, T.; Matsuzaki, K.; Kosugi, T.; Hansteen, V.; Wikstol, Ø.
2007SoPh..243...19C Altcode:
The EUV Imaging Spectrometer (EIS) on Hinode will observe solar corona
and upper transition region emission lines in the wavelength ranges
170 - 210 Å and 250 - 290 Å. The line centroid positions and profile
widths will allow plasma velocities and turbulent or non-thermal line
broadenings to be measured. We will derive local plasma temperatures and
densities from the line intensities. The spectra will allow accurate
determination of differential emission measure and element abundances
within a variety of corona and transition region structures. These
powerful spectroscopic diagnostics will allow identification
and characterization of magnetic reconnection and wave propagation
processes in the upper solar atmosphere. We will also directly study
the detailed evolution and heating of coronal loops. The EIS instrument
incorporates a unique two element, normal incidence design. The optics
are coated with optimized multilayer coatings. We have selected highly
efficient, backside-illuminated, thinned CCDs. These design features
result in an instrument that has significantly greater effective area
than previous orbiting EUV spectrographs with typical active region
2 - 5 s exposure times in the brightest lines. EIS can scan a field
of 6×8.5 arc min with spatial and velocity scales of 1 arc sec and
25 km s−1 per pixel. The instrument design, its absolute
calibration, and performance are described in detail in this paper. EIS
will be used along with the Solar Optical Telescope (SOT) and the X-ray
Telescope (XRT) for a wide range of studies of the solar atmosphere.

---------------------------------------------------------
Title: High-Resolution Observations and Numerical Simulations of
Chromospheric Fibrils and Mottles
Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.;
van Noort, M.; Carlsson, M.
2007ASPC..368...65D Altcode:
With the recent advent of the Swedish 1-m Solar Telescope (SST),
advanced image processing techniques, as well as numerical simulations
that provide a more realistic view of the chromosphere, a comprehensive
understanding of chromospheric jets such as spicules, mottles and
fibrils is now within reach. In this paper, we briefly summarize results
from a recent analysis of dynamic fibrils, short-lived jet-like features
that dominate the chromosphere (as imaged in Hα) above and about active
region plage. Using extremely high-resolution observations obtained
at the SST, and advanced numerical 2D radiative MHD simulations, we
show that fibrils are most likely formed by chromospheric shock waves
that occur when convective flows and global oscillations leak into the
chromosphere along the field lines of magnetic flux concentrations. In addition, we present some preliminary observations of quiet Sun
jets or mottles. We find that the mechanism that produces fibrils
in active regions is most likely also at work in quiet Sun regions,
although it is modified by the weaker magnetic field and the presence of
more mixed-polarity. A comparison with numerical simulations suggests
that the weaker magnetic field in quiet Sun allows for significantly
stronger (than in active regions) transverse motions that are
superposed on the field-aligned, shock-driven motions. This leads
to a more dynamic, and much more complex environment than in active
region plage. In addition, our observations of the mixed polarity
environment in quiet Sun regions suggest that other mechanisms, such
as reconnection, may well play a significant role in the formation of
some quiet Sun jets. Simultaneous high-resolution magnetograms (such
as those provided by Hinode), as well as numerical simulations that
take into account a whole variety of different magnetic configurations,
will be necessary to determine the relative importance in quiet Sun of,
respectively, the fibril-mechanism and reconnection.

---------------------------------------------------------
Title: Non-equilibrium Hydrogen Ionization in the Solar Atmosphere
Authors: Leenaarts, J.; Wedemeyer-Böhm, S.; Carlsson, M.; Hansteen,
V. H.
2007ASPC..368..103L Altcode:
The assumption of statistical equilibrium for atomic level populations
of hydrogen does not hold under the conditions of the chromosphere
due to the low density and the short dynamic timescale. In order to
calculate the hydrogen ionization balance and the electron density one
has to solve the time-dependent rate equations. We present results
from 2D and 3D radiation-magneto-hydrodynamics simulations of the
solar atmosphere incorporating the time-dependent rate equations
for hydrogen. Both the hydrogen ionization degree and the electron
density in our models are much more constant than LTE and statistical
equilibrium theory predict. These simulations provide multi-dimensional
model atmospheres with realistic electron densities and hydrogen level
populations that can be used in detailed radiative transfer modeling.

---------------------------------------------------------
Title: Chromospheric Spectrometry at High Spatial Resolution
Authors: Langangen, Ø.; Carlsson, M.; Rouppe van der Voort, L.;
Hansteen, V. H.; de Pontieu, B.
2007ASPC..368..145L Altcode:
In this summary we present spectrograms and images of the
chromosphere obtained in a co-observation campaign with the SST and
the DOT. The data are used to identify and measure the Doppler shifts
of dynamic fibrils. Quantitative comparison with the results of \citep{ol-2006ApJ...647L..73H} requires compensation for several
observational issues.

---------------------------------------------------------
Title: Hinode Euv Study Of Jets In The Sun’s South Polar Corona
Authors: Culhane, J. L.; Brooks, D. H.; Doschek, G. A.; Harra, L. K.;
van Driel-Gesztelyi, L.; Baker, D.; Lundquist, L. L.; Hansteen, V. H.;
Kamio, S.
2007AAS...210.7201C Altcode: 2007BAAS...39..178C
Using the Hinode EUV Imaging Spectrometer coronal jets were observed
on 20-JAN-2007 over a range of emission lines and corresponding plasma
temperatures using the 40 arc sec wide slot images. In this preliminary
analysis, jet plasma temperature and emissivity have been estimated
while, based on assumptions about the jet morphology, electron density
estimates are given and jet velocity measured. The evolution of the
jets will be followed in a number of different EUV emission lines and
jet energy input as a function of time will be assessed with reference
to the magnetic field topologies involved.

---------------------------------------------------------
Title: 3D Numerical Models of the Chromosphere, Transition Region,
and Corona
Authors: Hansteen, V. H.; Carlsson, M.; Gudiksen, B.
2007ASPC..368..107H Altcode: 2007arXiv0704.1511H
A major goal in solar physics has during the last five decades
been to find how energy flux generated in the solar convection zone
is transported and dissipated in the outer solar layers. Progress
in this field has been slow and painstaking. However, advances in
computer hardware and numerical methods, vastly increased observational
capabilities and growing physical insight seem finally to be leading
towards understanding. Here we present exploratory numerical MHD models
that span the entire solar atmosphere from the upper convection zone
to the lower corona. These models include non-grey, non-LTE radiative
transport in the photosphere and chromosphere, optically thin radiative
losses as well as magnetic field-aligned heat conduction in the
transition region and corona.

---------------------------------------------------------
Title: Observational Evidence For The Ubiquity Of Strong Alfven
Waves In The Magnetized Chromosphere
Authors: De Pontieu, Bart; McIntosh, S. W.; Carlsson, M.; Hansteen,
V. H.; Schrijver, C. J.; Tarbell, T. D.; Title, A.; SOT Team
2007AAS...210.9415D Altcode: 2007BAAS...39R.219D
Hinode/SOT Ca II broadband observations show that Alfven waves with
significant amplitudes of order 10-20 km/s and periods of 150-300 s are
extremely ubiquitous in the magnetized middle to upper chromosphere. Our
observations focus on spicules at the limb, and straw-like features
associated with network and plage on the disk. We find that the
weak straw-like features and most spicules all undergo significant
transverse motions that are driven by Alfven waves. These waves are
seen to propagate both up- and downward, and may carry an energy flux
that is significant compared to both the local, coronal and solar wind
energy balance. We will provide estimates of the energy flux carried
by these waves, and will compare our observations with Alfven waves
that are observed in 3D numerical simulations that include advanced
radiative transfer treatment for the chromosphere. This work was
supported by NASA contract NNM07AA01C.

---------------------------------------------------------
Title: Connecting The Dynamics Of The Chromosphere And Transition
Region With Hinode/sot And Eis
Authors: Hansteen, Viggo H.; McIntosh, S.; De Pontieu, B.; Carlsson,
M.; SOT Team
2007AAS...210.9430H Altcode: 2007BAAS...39..222H
We will use coordinated Hinode SOT/EIS observations that include
high-resolution magnetograms, chromospheric and TR imaging and
TR/coronal spectra to study how the dynamics of the TR are driven by
the higly dynamic photospheric magnetic fields and the ubiquitous
chromospheric waves. Using travel time analysis, magnetic field
extrapolations, frequency filtered timeseries and comparisons with
synthetic chromospheric and TR images from advanced 3D numerical
simulations, we will study and establish how the dynamics of the
photosphere, chromosphere and TR are connected.

---------------------------------------------------------
Title: Magneto-acoustic Waves And Their Role In The Energetics And
Dynamics Of The Solar Chromosphere
Authors: Jefferies, Stuart; De Pontieu, B.; McIntosh, S.; Hansteen,
V. H.
2007AAS...21012004J Altcode: 2007BAAS...39..245J
We analyze a diverse set of observations obtained with SOHO and
TRACE, as well as with MOTH and the Swedish 1 m Solar Telescope to
show that sound waves play an important role in shaping the structure
and energetics of the magnetized chromosphere. Travel time analysis
of TRACE, MOTH and SST observations and comparisons with numerical
simulations show that normally evanescent 5 minute p-mode oscillations
leak into the chromosphere along flux tubes that are inclined with
the vertical. Comparisons of SST data of fibril-like jets above
active region plage and quiet Sun mottles with advanced radiative
MHD simulations show how these oscillations develop into slow mode
magnetoacoustic shocks that drive spicule-like chromospheric jets up
to coronal heights. The leaking waves not only drive much of the
dynamics of the magnetized chromosphere: Doppler measurements from the
MOTH instrument at several heights in the atmosphere show that the total
energy flux carried by these leaking waves may play a significant role
in the energy balance of the magnetized chromosphere. We describe
first approaches to determine more precisely how and where the wave
energy is deposited in the low atmosphere.

---------------------------------------------------------
Title: Magnetoacoustic Shocks as a Driver of Quiet-Sun Mottles
Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Hansteen,
V. H.; Carlsson, M.; van Noort, M.
2007ApJ...660L.169R Altcode: 2007astro.ph..3535R
We present high spatial and high temporal resolution observations of
the quiet Sun in Hα obtained with the Swedish 1-m Solar Telescope on
La Palma. We observe that many mottles, jetlike features in the quiet
Sun, display clear up- and downward motions along their main axis. In
addition, many mottles show vigorous transverse displacements. Unique
identification of the mottles throughout their lifetime is much harder
than for their active region counterpart, dynamic fibrils. This is
because many seem to lack a sharply defined edge at their top, and
significant fading often occurs throughout their lifetime. For those
mottles that can be reliably tracked, we find that the mottle tops
often undergo parabolic paths. We find a linear correlation between
the deceleration these mottles undergo and the maximum velocity they
reach, similar to what was found earlier for dynamic fibrils. Combined
with an analysis of oscillatory properties, we conclude that at least
part of the quiet-Sun mottles are driven by magnetoacoustic shocks. In
addition, the mixed-polarity environment and vigorous dynamics suggest
that reconnection may play a significant role in the formation of some
quiet-Sun jets.

---------------------------------------------------------
Title: A Tale of Two Spicules
Authors: McIntosh, Scott W.; De Pontieu, B.; Carlsson, M.; Hansteen,
V. H.; Schrijver, C. J.; Tarbell, T. D.; Title, A. M.; SOT Team
2007AAS...210.9414M Altcode: 2007BAAS...39..219M
Hinode/SOT Ca II broadband images and movies show that there are
several different types of spicules at the limb. These different
types are distinguished by dynamics on different timescales. The
first type involves up- and downward motion on timescales of 3-5
minutes. The dynamics of these spicules are very similar to those of
fibrils and mottles as observed on the disk. Recent work suggests that
these are driven by slow-mode magnetoacoustic shocks that form when
convective flows and global oscillations leak into the chromosphere
along magnetic flux tubes. The second type is much more dynamic with
typical lifetimes of 10-60 s. These spicules are characterized by
sudden appearance and disappearance that may be indicative of rapid
heating to TR temperatures. We will describe the properties of these
spicules in various magnetic environments (coronal hole, quiet Sun,
active region) and study the possible role of reconnection in driving
the second type of spicules. In addition, we will perform detailed
comparisons of these different types of jets with synthetic Ca images
derived from advanced 3D numerical simulations that encompass the
convection zone up through the corona.

---------------------------------------------------------
Title: Can High Frequency Acoustic Waves Heat the Quiet Sun
Chromosphere?
Authors: Carlsson, Mats P. O.; De Pontieu, B.; Tarbell, T.; Hansteen,
V. H.; McIntosh, S.; SOT Team
2007AAS...210.6306C Altcode: 2007BAAS...39..172C
We use Hinode/SOT Ca II, G-band and blue continuum broadband
observations to study the presence and power of high frequency acoustic
waves at high spatial resolution. Previous observations with TRACE,
which were limited by the 1 arcsec resolution, and 1D numerical
simulations (Fossum &amp; Carlsson, 2005) have been used to constrain
the possible role of high frequency waves in the heating of the quiet
Sun chromosphere. We will use the higher spatial resolution Hinode
data and comparisons with both 1D and 3D numerical models to study
the amount of high frequency power at smaller scales, and whether that
power is sufficient to heat the quiet Sun chromosphere.

---------------------------------------------------------
Title: Observations and Simulations of Fibrils and Mottles
Authors: De Pontieu, Bart; Hansteen, Viggo H.; Rouppe van der Voort,
Luc; van Noort, Michiel; Carlsson, Mats
2007astro.ph..2081D Altcode:
With the recent advent of the Swedish 1-m Solar Telescope (SST),
advanced image processing techniques, as well as numerical simulations
that provide a more realistic view of the chromosphere, a comprehensive
understanding of chromospheric jets such as spicules, mottles and
fibrils is now within reach. In this paper, we briefly summarize
results from a recent analysis of dynamic fibrils, short-lived
jet-like features that dominate the chromosphere (as imaged in
H-alpha) above and about active region plage. Using extremely
high-resolution observations obtained at the SST, and advanced
numerical 2D radiative MHD simulations, we show that fibrils are most
likely formed by chromospheric shock waves that occur when convective
flows and global oscillations leak into the chromosphere along the
field lines of magnetic flux concentrations. In addition, we present
some preliminary observations of quiet Sun jets or mottles. We find
that the mechanism that produces fibrils in active regions is most
likely also at work in quiet Sun regions, although it is modified by
the weaker magnetic field and the presence of more mixed-polarity. A
comparison with numerical simulations suggests that the weaker magnetic
field in quiet Sun allows for significantly stronger (than in active
regions) transverse motions that are superposed on the field-aligned,
shock-driven motions. This leads to a more dynamic, and much more
complex environment than in active region plage. In addition, our
observations of the mixed polarity environment in quiet Sun regions
suggest that other mechanisms, such as reconnection, may well play a
significant role in the formation of some quiet Sun jets.

---------------------------------------------------------
Title: High-Resolution Observations and Modeling of Dynamic Fibrils
Authors: De Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.;
van Noort, M.; Carlsson, M.
2007ApJ...655..624D Altcode: 2007astro.ph..1786D
We present unprecedented high-resolution Hα observations, obtained
with the Swedish 1 m Solar Telescope, that, for the first time,
spatially and temporally resolve dynamic fibrils in active regions on
the Sun. These jetlike features are similar to mottles or spicules in
quiet Sun. We find that most of these fibrils follow almost perfect
parabolic paths in their ascent and descent. We measure the properties
of the parabolic paths taken by 257 fibrils and present an overview
of the deceleration, maximum velocity, maximum length, and duration,
as well as their widths and the thickness of a bright ring that often
occurs above dynamic fibrils. We find that the observed deceleration
of the projected path is typically only a fraction of solar gravity
and incompatible with a ballistic path at solar gravity. We report on
significant differences of fibril properties between those occurring
above a dense plage region and those above a less dense plage region
where the magnetic field seems more inclined from the vertical. We
compare these findings to advanced numerical two-dimensional radiative
MHD simulations and find that fibrils are most likely formed by
chromospheric shock waves that occur when convective flows and global
oscillations leak into the chromosphere along the field lines of
magnetic flux concentrations. Detailed comparison of observed and
simulated fibril properties shows striking similarities of the values
for deceleration, maximum velocity, maximum length, and duration. We
compare our results with observations of mottles and find that a
similar mechanism is most likely at work in the quiet Sun.

---------------------------------------------------------
Title: High Spatial Resolution Observations of Solar Magnetic
Structures
Authors: Rouppe van der Voort, L.; van Noort, M.; Carlsson, M.;
Hansteen, V.
2006ASPC..354...37R Altcode:
We present observations of the dynamic evolution of photospheric
magnetic structures in the G-band, continuum, magnetograms and
Dopplergrams. The observations were obtained with the Swedish one-m
Solar Telescope on La Palma. Using adaptive optics and the Multi-Object
Multi-Frame Blind Deconvolution image restoration technique, we obtained
several datasets at close to the diffraction limit of the telescope
(0.1 arcsec) over long periods of time. We show examples of the
dynamical evolution of different magnetic structures: the advection
of individual bright points by the granular flow, the formation and
fragmentation of flux sheets, and the continuous transition between
micro-pores, elongated ribbons and more circular “flowers”. Narrow
sheets with downdrafts are found right at the edges of magnetic field
concentrations.

---------------------------------------------------------
Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks
Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.;
van Noort, M.; Carlsson, M.
2006AGUFMSH23B0359D Altcode:
With the recent advent of the Swedish 1-m Solar Telescope (SST),
advanced image processing techniques, as well as numerical simulations
that provide a more realistic view of the chromosphere, a comprehensive
understanding of chromospheric jets such as spicules, mottles and
fibrils is now within reach. We will present results from a recent
analysis of dynamic fibrils, short-lived jet-like features that
dominate the chromosphere (as imaged in Hα) above and about active
region plage. These jets are similar to mottles and spicules in quiet
Sun. Our analysis is based on a time series of extremely high-resolution
(120 km) images taken in Hα linecenter at 1 second cadence, obtained by
the Oslo group at the SST in October 2005. The 78 min long time series
for the first time, spatially and temporally resolves dynamic fibrils
in active regions. Our analysis shows that most of the fibrils follow
almost perfect parabolic paths in their ascent and descent. We measure
the properties of the parabolic paths taken by 257 different dynamic
fibrils, and find that the observed deceleration of the projected
path is typically only a fraction of solar gravity, and incompatible
with a ballistic path at solar gravity. We report on significant
differences of measured fibril properties between those occurring in
association with a dense plage region, and those above a less dense
plage region where the magnetic field seems more inclined away from
the vertical. We compare these observational findings to advanced
numerical 2D radiative MHD simulations, and find that fibrils are most
likely formed by chromospheric shock waves that occur when convective
flows and global oscillations leak into the chromosphere along the
field lines of magnetic flux concentrations. Detailed comparison
of the properties of fibrils found in our observations and those in
our numerical simulations shows striking similarities of the values
for deceleration, maximum velocity, maximum length and duration. The
numerical simulations also reproduce the correlations we observe between
various fibrils properties, as well as the regional differences, taking
into account the different magnetic configuration for the various
regions. We compare our results with observations of mottles and find
that a similar mechanism is most likely at work in the quiet Sun.

---------------------------------------------------------
Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks
Authors: Hansteen, V. H.; De Pontieu, B.; Rouppe van der Voort, L.;
van Noort, M.; Carlsson, M.
2006ApJ...647L..73H Altcode: 2006astro.ph..7332H
The formation of jets such as dynamic fibrils, mottles, and spicules
in the solar chromosphere is one of the most important, but also
most poorly understood, phenomena of the Sun's magnetized outer
atmosphere. We use extremely high resolution observations from the
Swedish 1 m Solar Telescope combined with advanced numerical modeling
to show that in active regions these jets are a natural consequence of
upwardly propagating slow-mode magnetoacoustic shocks. These shocks
form when waves generated by convective flows and global p-mode
oscillations in the lower lying photosphere leak upward into the
magnetized chromosphere. We find excellent agreement between observed
and simulated jet velocities, decelerations, lifetimes, and lengths. Our
findings suggest that previous observations of quiet-Sun spicules and
mottles may also be interpreted in light of a shock-driven mechanism.

---------------------------------------------------------
Title: Radiative transfer aspects of the magnetic coupling
Authors: Hansteen, V. H.
2006cosp...36.3543H Altcode: 2006cosp.meet.3543H
We present numerical simulations of the quiet sun chromosphere
and corona These models span the entire solar atmosphere from the
upper convection zone to the lower corona Included are non-grey
non-lte radiative transport in the photosphere and lower chromosphere
effectively thin radiative losses in the upper chromosphere transition
region and corona as well as magnetic field-aligned heat conduction
Coronal heating is effected through the dissipation of currents
generated by photospheric motions We will discuss radiative diagnostics
of various phenomena in the chromosphere transition region and corona
as they occur in the simulations and compare these with observations

---------------------------------------------------------
Title: a Multi-Wavelength View on Coronal Rain
Authors: Müller, D. A. N.; de Groof, A.; de Pontieu, B.; Hansteen,
V. H.
2005ESASP.600E..30M Altcode: 2005dysu.confE..30M; 2005ESPM...11...30M
No abstract at ADS

---------------------------------------------------------
Title: Chromospheric Waves
Authors: Carlsson, M.; Hansteen, V.
2005ESASP.600E..16C Altcode: 2005dysu.confE..16C; 2005ESPM...11...16C
No abstract at ADS

---------------------------------------------------------
Title: a Multi-Wavelength View on Coronal Rain
Authors: Müller, D. A. N.; de Groof, A.; de Pontieu, B.; Hansteen,
V. H.
2005ESASP.596E..37M Altcode: 2005ccmf.confE..37M
No abstract at ADS

---------------------------------------------------------
Title: Chromospheric Waves
Authors: Carlsson, M.; Hansteen, V.
2005ESASP.596E..39C Altcode: 2005ccmf.confE..39C
No abstract at ADS

---------------------------------------------------------
Title: 3D Numerical Models of Quiet Sun Coronal Heating
Authors: Hansteen, V. H.; Gudiksen, B.
2005ESASP.592..483H Altcode: 2005soho...16E..87H; 2005ESASP.592E..87H
No abstract at ADS

---------------------------------------------------------
Title: High-speed coronal rain
Authors: Müller, D. A. N.; De Groof, A.; Hansteen, V. H.; Peter, H.
2005A&A...436.1067M Altcode:
At high spatial and temporal resolution, coronal loops are observed to
have a highly dynamic nature. Recent observations with SOHO and TRACE
frequently show localized brightenings "raining" down towards the solar
surface. What is the origin of these features? Here we present for
the first time a comparison of observed intensity enhancements from an
EIT shutterless campaign with non-equilibrium ionization simulations
of coronal loops in order to reveal the physical processes governing
fast flows and localized brightenings. We show that catastrophic cooling
around the loop apex as a consequence of footpoint-concentrated heating
offers a simple explanation for these observations. An advantage of
this model is that no external driving mechanism is necessary as the
dynamics result entirely from the non-linear character of the problem.

---------------------------------------------------------
Title: Heating of the Quiet Solar Corona
Authors: Hansteen, V. H.; Gudiksen, B. W.
2005AGUSMSP41A..01H Altcode:
A major goal in solar physics has during the last five decades
been to find how energy flux generated in the solar convection zone
is transported and dissipated in the outer solar layers. Progress
in this field has been slow and painstaking. However, advances in
computer hardware and numerical methods, vastly increased observational
capababilities and growing physical insight seem finally to be leading
towards understanding. We present numerical simulations of quiet
sun heating that span the entire solar atmosphere from the upper
convection zone to the lower corona. These models include non-grey,
non-lte radiative transport in the photosphere and chromosphere,
optically thin radiative losses as well as magnetic field-aligned heat
conduction in the transition region and corona. The relation between
the mean magnetic field strength and structure and the heating of the
corona is discussed.

---------------------------------------------------------
Title: Release of Helium from Closed-Field Regions of the Sun
Authors: Endeve, Eirik; Lie-Svendsen, Øystein; Hansteen, Viggo H.;
Leer, Egil
2005ApJ...624..402E Altcode:
Using a numerical model that extends from the chromosphere into the
supersonic solar wind, we study the dynamics of previously closed
coronal flux tubes that open, allowing plasma to be expelled from
the corona. In particular, we study whether the opening of flux tubes
may provide a source of helium-rich material for the solar wind. We
use higher order moment fluid equations to describe the plasma,
which consists of hydrogen (neutral and protons), helium (neutral,
singly ionized, and α-particles), and electrons. The helium abundance
decreases rapidly with altitude in a closed flux tube, caused by the
collisional coupling between α-particles and protons leading to a
small α-particle scale height. When the flux tube is rapidly opened,
protons escape from the Sun immediately. The coronal α-particles
leave the corona only much later, 10-20 hr after the protons, when the
collisional coupling to protons has eased, allowing their temperature
to become sufficiently high for them to escape.

---------------------------------------------------------
Title: Solar magnetic elements at 0.1 arcsec resolution. II. Dynamical
evolution
Authors: Rouppe van der Voort, L. H. M.; Hansteen, V. H.; Carlsson,
M.; Fossum, A.; Marthinussen, E.; van Noort, M. J.; Berger, T. E.
2005A&A...435..327R Altcode:
Small magnetic structures can be seen in G-band filtergrams as isolated
bright points, strings of bright points and dark micro-pores. At a
resolution of 0.1 arcsec, new forms of magnetic structures are found
in strong field areas: elongated “ribbons” and more circular
“flowers”. In this paper we study the temporal evolution of
these small scale magnetic structures. In strong-field regions the
time-evolution is more that of a magnetic fluid than that of collections
of flux-tubes that keep their identity. We find that the granular flow
concentrates the magnetic field into flux sheets that are visible as
thin bright features in the filtergrams. Weak upflows are found in
the flux sheets and downflows in the immediate surroundings. The flux
sheets often become unstable to a fluting instability and the edges
buckle. The sheets tend to break up into strings of bright points,
still with weak upflows in the magnetic elements and zero velocity or
downflows between them. Where there are larger flux concentrations
we find ribbons, flowers and micro-pores. There is a continuous
transition between these forms and they evolve from one form to
another. The appearance is mostly determined by the horizontal size
- larger structures are dark (micro-pores), narrower structures are
ribbon shaped and the flowers are the smallest in extent. All these
structures have darker inner parts and a bright edge. The plasma is
found to be at rest in the ribbons, with small concentrations of weak
upflow sites. Narrow sheets with downdrafts are found right at the
edges of the magnetic field concentrations.

---------------------------------------------------------
Title: Solar magnetic elements at 0.1 arcsec resolution. General
appearance and magnetic structure
Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Löfdahl,
M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.;
Title, A.; Scharmer, G.
2004A&A...428..613B Altcode:
New observations of solar magnetic elements in a remnant active region
plage near disk center are presented. The observations were obtained at
the recently commissioned Swedish 1-m Solar Telescope on La Palma. We
examine a single 430.5 nm G-band filtergram that resolves ∼70 km
(0.1 arcsec) structures and find new forms of magnetic structures
in this particular region. A cotemporal Ca II H-line image is used
to examine the low-chromosphere of network elements. A cotemporal Fe
I 630.25 nm magnetogram that resolves structures as small as 120 km
(0.18 arcsec) FWHM with a flux sensitivity of approximately 130 Mx
cm-2 quantifies the magnetic structure of the region. A
Ni I 676.8 nm Dopplergram establishes relative velocity patterns
associated with the network features with an accuracy of about 300 m
s-1. We find that magnetic flux in this region as seen in
both the magnetogram and the G-band image is typically structured into
larger, amorphous, “ribbons” which are not resolved into individual
flux tubes. The measured magnetic flux density in the ribbon structures
ranges from 300 to 1500 Mx cm-2, the higher values occurring
at localized concentrations embedded within the ribbons. The Dopplergram
indicates relative downflows associated with all magnetic elements
with some indication that higher downflows occur adjacent to the peak
magnetic flux location. The mean absolute magnetic flux density of the
remnant plage network is about 130 Mx cm-2; in the lowest
flux regions of the field-of-view, the mean absolute flux density is
approximately 60 Mx cm-2. Within these quiet regions we do
not find evidence of pervasive kilo-gauss strength magnetic elements
as seen in recent high resolution internetwork studies. In general,
the observations confirm recent 3-dimensional numerical simulations
which show that the magnetic field in high-density regions such as
plage is concentrated in complex structures that are not generally
composed of discrete magnetic flux tubes. Appendices are only
available in electronic form at http://www.edpsciences.org

---------------------------------------------------------
Title: Mode Conversion in Magneto-Atmospheres
Authors: Bogdan, T. J.; Carlsson, M.; Hansteen, V.; Heggland, L.;
Leer, E.; McMurry, A. D.; Stein, R. F.
2004AGUFMSH13A1162B Altcode:
Numerical simulations of wave propagation in a simple magneto-atmosphere
are employed to illustrate the complex nature of wave transformation
and conversion taking place in solar and stellar atmospheres. An
isothermal atmosphere threaded by a potential poloidal magnetic
field, and a superposed uniform toroidal field, is treated in a local
cartesian approximation. Spatial variations are restricted to the
two poloidal dimensions, but the toroidal field ensures that all
three MHD waves are present in the simulation. As in our previous
purely two-dimensional simulations (Bogdan et al. ApJ 599, 626-60,
2003), mode mixing and transformation take place at surfaces where
the magnetic and thermal pressures are equal. In the present case,
the upward propagating acoustic-gravity (MAG) wave is converted into
roughly equal parts transmitted fast, intermediate (Alfven), and
slow magneto-acoustic-gravity waves in passing through this mixing
layer. Unlike the fast and slow waves, the Alfven wave is weakly
damped, and is able to deposit its energy and momentum in the upper
chromosphere and corona. The fast and slow MAG waves are decoupled
on either side of mixing layer owing to their disparate propagation
speeds. Under certain fortuitous circumstances, the Alfven wave also
decouples from the fast and slow MAG waves.

---------------------------------------------------------
Title: Thermal Instability as the Origin of High Speed Coronal Rain
Authors: Müller, D. A. N.; de Groof, A.; Hansteen, V. H.; Peter, H.
2004ESASP.575..291M Altcode: 2004soho...15..291M
No abstract at ADS

---------------------------------------------------------
Title: Dynamics of solar coronal loops. II. Catastrophic cooling
and high-speed downflows
Authors: Müller, D. A. N.; Peter, H.; Hansteen, V. H.
2004A&A...424..289M Altcode: 2004astro.ph..5538M
This work addresses the problem of plasma condensation and
“catastrophic cooling” in solar coronal loops. We have carried out
numerical calculations of coronal loops and find several classes of
time-dependent solutions (static, periodic, irregular), depending on
the spatial distribution of a temporally constant energy deposition
in the loop. Dynamic loops exhibit recurrent plasma condensations,
accompanied by high-speed downflows and transient brightenings of
transition region lines, in good agreement with features observed with
TRACE. Furthermore, these results also offer an explanation for the
recent EIT observations of \cite{DeGroof+al2004AA} of moving bright
blobs in large coronal loops. In contrast to earlier models, we suggest
that the process of catastrophic cooling is not initiated by a drastic
decrease of the total loop heating but rather results from a loss
of equilibrium at the loop apex as a natural consequence of heating
concentrated at the footpoints of the loop, but constant in time.

---------------------------------------------------------
Title: Observations of solar magnetic elements with 0.1" resolution
Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Lofdahl,
M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.;
Title, A. M.; Scharmer, G.
2004AAS...204.2005B Altcode: 2004BAAS...36..686B
New observations of solar magnetic elements in a remnant active region
plage near disk center are presented. The observations were taken with
the Swedish 1-meter Solar Telescope on La Palma. We examine a single
430.5 nm G-band filtergram that resolves ∼70 km (0.”1) structures
and find new forms of magnetic structures in this particular region. A
simultaneous Ca II H-line image is used to examine the low-chromosphere
of network elements. A simultaneous Fe I 630.25 nm magnetogram
that resolves structures as small as 120 km (0.”18) FWHM with a
flux sensitivity of approximately 130 Mx cm-2 quantifies
the magnetic structure of the region. A Ni I 676.8 nm Dopplergram
establishes relative velocity patterns associated with the network
features with an accuracy of about 300 m s-1. Magnetic flux
in this region as seen in both the magnetogram and the G-band image
is typically structured into larger, amorphous, “ribbons” with a
wide range of flux density values, rather than isolated kilogauss
flux tubes. We also present filtergrams and magnetograms of magnetic
elements at the solar limb showing that solar faculae are resolved
into bright granular walls that appear to project 350 to 500 km above
the photosphere.

---------------------------------------------------------
Title: Observations at 0{. ^{primeprime}}1 Resolution of the Dynamic
Evolution of Magnetic Elements
Authors: Carlsson, Mats; Rouppe van der Voort, Luc; Hansteen, Viggo
2004IAUS..223..207C Altcode: 2005IAUS..223..207C
We present observations of the dynamic evolution of photospheric
magnetic elements in the G-band, magnetograms and Dopplergrams. The
observations were obtained with the Swedish 1m Solar Telescope on La
Palma at close to the diffraction limit of 0{. ^{primeprime}}1. In
the most quiet regions we observe individual bright points in the
G-band with corresponding magnetic signal in the magnetograms. Where
the filling factor of the magnetic field is larger, the bright points
interact when advected by the granular and super-granular flow-fields,
flux sheets form and fragment. The plage region of the decaying active
region is filled with more complex topologies like ribbon structures
with darker interior and bright, knotted edges. These change into
flower-like shape when small in extent and into micro-pores when
the flux region is larger in extent. The magnetic elements in the
plage region are associated with upflows with strong downflows in the
immediate vicinity in the low-field region.

---------------------------------------------------------
Title: Thermal non-equilibrium in coronal loops: A road to complex
evolution
Authors: Müller, Daniel; de Groof, A.; Hansteen, V. H.; Peter, H.
2004IAUS..223..289M Altcode: 2005IAUS..223..289M
At high spatial and temporal resolution, coronal loops are observed to
have a highly dynamic nature. Recent observations with SOHO and TRACE
frequently show localized brightening "raining" down towards the solar
surface. What is the origin of these features? Here we present for
the first time a comparison of observed intensity enhancements from an
EIT shutterless campaign with non-equilibrium ionization simulations
of coronal loops in order to reveal the physical processes governing
fast flows and localized brightening. We show that catastrophic cooling
around the loop apex as a consequence of footpoint-concentrated heating
offers a simple explanation for these observations. An advantage of
this model is that no external driving mechanism is necessary as the
dynamics result entirely from the non-linear character of the system.

---------------------------------------------------------
Title: Catastrophic Cooling and High-Speed Downflows in Solar
Coronal Loops
Authors: Müller, D.; Peter, H.; Hansteen, V.
2004IAUS..219..765M Altcode: 2003IAUS..219E..48M
We report numerical simulations of the condensation of plasma in
short solar coronal loops which has several interesting physical
consequences. Firstly we propose a connection between small cool loops
which presumably constitute the solar transition region and prominences
in the sense that the same physical mechanism governs their dynamics
namely the onset of instability and runaway cooling due to strong
radiative losses. Secondly we show that the temporal evolution of
these loop models exhibit a cyclic pattern of chromospheric evaporation
condensation formation motion of the condensation region to either side
of the loop and finally loop reheating with a period of 4000 - 6000
s for a loop of 10 Mm length. Thirdly we have synthesized transition
region lines from these simulations which show strong periodic intensity
variations making condensation processes in loops a candidate to
account for the observed temporal variability of these lines.

---------------------------------------------------------
Title: Plasma Condensation in Solar Coronal Loops: II. "Catastrophic
Cooling" and High-Speed Downflows
Authors: Müller, D. A. N.; Peter, H.; Hansteen, V. H.
2004ESASP.547..199M Altcode: 2004soho...13..199M
The second part of this work focuses on the application of the
concept of plasma condensation to large coronal loops. In contrast
to the short loops analyzed in Müller et al. (2003a), these models
can more easily be compared to SOHO and TRACE observations. From our
numerical calculations of coronal loops we find several classes of
time-dependent solutions (static, periodic, irregular), depending on
the spatial dependence of a temporally constant energy deposition
in the loop. One of these classes is in remarkably close agreement
with the features observed with TRACE, described by Schrijver (2001):
Emission in C IV (154.8 nm), developing initially near the loop tops,
cool plasma sliding down on both sides of the loop, downflow velocities
of up to 100 km/s, and a downward acceleration which is substantially
reduced with respect to the solar surface gravity. Furthermore, these
results also offer an explanation for the observations of De Groof
et al. (2003a,b). In contrast to earlier models, we suggest that the
process of catastrophic cooling does not have to be initiated by a
drastic decrease of the loop heating. It can also result from a loss
of equilibrium at the loop apex which is a natural consequence if the
loop is heated predominantly at the footpoints, but constant in time.

---------------------------------------------------------
Title: Initial simulations spanning the upper convection zone to
the corona
Authors: Hansteen, Viggo H.
2004IAUS..223..385H Altcode: 2005IAUS..223..385H
A major goal in solar physics has during the last five decades
been to find how energy flux generated in the solar convection zone
is transported and dissipated in the outer solar layers. Progress
in this field has been slow and painstaking. However, advances in
computer hardware and numerical methods, vastly increased observational
capababilities and growing physical insight seem finally to be leading
towards understanding. Here we present exploratory numerical MHD models
that span the entire solar atmosphere from the upper convection zone
to the lower corona. These models include non-grey, non-lte radiative
transport in the photosphere and chromosphere, optically thin radiative
losses as well as magnetic field-aligned heat conduction in the
transition region and corona.

---------------------------------------------------------
Title: Theory and Simulations of Solar Atmosphere Dynamics
Authors: Stein, R. F.; Bogdan, T. J.; Carlsson, M.; Hansteen, V.;
McMurry, A.; Rosenthal, C. S.; Nordlund, Å.
2004ESASP.547...93S Altcode: 2004soho...13...93S
Numerical simulations are used to study the generation and propagation
of waves in the solar atmosphere. Solar p-mode oscillations are excited
by turbulent pressure work and entropy fluctuations (non-adiabatic gas
pressure work) near the solar surface. Interactions between short and
long period waves and radiative energy transfer control the formation of
shocks. The magnetic structure of the atmosphere induces coupling among
various MHD wave modes, with intense coupling and wave transformation
at the beta equal one surface, which likely is the location of the
so-called "magnetic canopy".

---------------------------------------------------------
Title: Plasma Condensation in Solar Coronal Loops -- I. Basic
Processes
Authors: Müller, D. A. N.; Hansteen, V. H.; Peter, H.
2004ESASP.547..285M Altcode: 2004soho...13..285M
In the first part of this work, we report numerical calculations
of the condensation of plasma in short coronal loops, which has
several interesting physical consequences. We propose a connection
between small, cool loops, which presumably constitute the solar
transition region, and prominences in the sense that the same physical
mechanism governs their dynamics, namely the onset of instability
and runaway cooling due to strong radiative losses. We show that the
temporal evolution of these loop models exhibits a cyclic pattern of
chromospheric evaporation, condensation, motion of the condensation
region to either side of the loop, and finally loop reheating with
a period of 4000 - 8000 s for a loop of 10 Mm length. Furthermore,
we have synthesized transition region lines from these calculations
which show strong periodic intensity variations, making condensation
in loops a candidate to account for observed transient brightenings of
solar transition region lines. Remarkably, all these dynamic processes
take place for a heating function which is constant in time and has a
simple exponential height dependence. In the second part of this work
(Müller et al., 2003b), we apply this concept to large coronal loops.

---------------------------------------------------------
Title: Waves in the Magnetized Solar Atmosphere. II. Waves from
Localized Sources in Magnetic Flux Concentrations
Authors: Bogdan, T. J.; Carlsson, M.; Hansteen, V. H.; McMurry, A.;
Rosenthal, C. S.; Johnson, M.; Petty-Powell, S.; Zita, E. J.; Stein,
R. F.; McIntosh, S. W.; Nordlund, Å.
2003ApJ...599..626B Altcode:
Numerical simulations of wave propagation in a two-dimensional
stratified magneto-atmosphere are presented for conditions that
are representative of the solar photosphere and chromosphere. Both
the emergent magnetic flux and the extent of the wave source are
spatially localized at the lower photospheric boundary of the
simulation. The calculations show that the coupling between the
fast and slow magneto-acoustic-gravity (MAG) waves is confined to
thin quasi-one-dimensional atmospheric layers where the sound speed
and the Alfvén velocity are comparable in magnitude. Away from this
wave conversion zone, which we call the magnetic canopy, the two MAG
waves are effectively decoupled because either the magnetic pressure
(B2/8π) or the plasma pressure (p=NkBT)
dominates over the other. The character of the fluctuations observed
in the magneto-atmosphere depend sensitively on the relative location
and orientation of the magnetic canopy with respect to the wave source
and the observation point. Several distinct wave trains may converge
on and simultaneously pass through a given location. Their coherent
superposition presents a bewildering variety of Doppler and intensity
time series because (1) some waves come directly from the source while
others emerge from the magnetic canopy following mode conversion, (2)
the propagation directions of the individual wave trains are neither
co-aligned with each other nor with the observer's line of sight, and
(3) the wave trains may be either fast or slow MAG waves that exhibit
different characteristics depending on whether they are observed in
high-β or low-β plasmas (β≡8πp/B2). Through the
analysis of four numerical experiments a coherent and physically
intuitive picture emerges of how fast and slow MAG waves interact
within two-dimensional magneto-atmospheres.

---------------------------------------------------------
Title: MHD Waves in Magnetic Flux Concentrations
Authors: Bogdan, T. J.; Carlsson, M.; Hansteen, V.; Zita, E. J.;
Stein, R. F.; McIntosh, S. W.
2003AGUFMSH42B0535B Altcode:
Results from 2D MHD simulations of waves in a stratified isothermal
atmosphere will be presented and analyzed. The waves are generated
by a localized piston source situated on the lower, photospheric,
boundary of the computational domain. A combination of fast and slow
magneto-atmospheric waves propagates with little mutual interaction
until they encounter the surface where the sound speed and the Alfven
speed are comparable in magnitude. The waves couple strongly in this
region and emerge with different amplitudes and phases. Owing to
this mode mixing and the large variation in the Alfven speed in the
magneto-atmosphere, the fluctuations observed at a given location are
often a superposition of both fast and slow waves which have traversed
different paths and have undergone different transformations during
their journies.

---------------------------------------------------------
Title: Dynamics of solar coronal loops. I. Condensation in cool
loops and its effect on transition region lines
Authors: Müller, D. A. N.; Hansteen, V. H.; Peter, H.
2003A&A...411..605M Altcode:
We report numerical calculations of the condensation of plasma
in short coronal loops, which have several interesting physical
consequences. Firstly, we propose a connection between small, cool
loops (T &lt; 106 K), which constitute one of the basic
components of the solar transition region, and prominences, in the
sense that the same physical mechanism governs their dynamics: Namely
the onset of instability and runaway cooling due to strong radiative
losses. Secondly, we show that the temporal evolution of these
loop models exhibits a cyclic pattern of chromospheric evaporation,
condensation, motion of the condensation region to either side of the
loop, and finally loop reheating with a period of 4000-8000 s for a loop
of 10 Mm length. Thirdly, we have synthesized transition region lines
from these calculations which show strong periodic intensity variations,
making condensation in loops a candidate to account for observed
transient brightenings of solar transition region lines. Remarkably,
all these dynamic processes take place for a heating function which
is constant in time and has a simple exponential height dependence.

---------------------------------------------------------
Title: Helium Abundance in the Corona and Solar Wind: Gyrotropic
Modeling from the Chromosphere to 1 AU
Authors: Lie-Svendsen, Øystein; Hansteen, Viggo H.; Leer, Egil
2003ApJ...596..621L Altcode:
We have developed a solar wind model including helium that extends
from the chromosphere to 1 AU. The model is based on the gyrotropic
approximation to the 16-moment set of fluid transport equations,
which allows it to accommodate temperature anisotropies, as well as
nonclassical heat transport. We find that, as in a pure electron-proton
solar wind, the flow geometry close to the Sun also has a large
impact on helium. In a radially expanding flow, downward proton heat
conduction from the corona leads to a high transition region pressure
and a large thermal force that pulls helium ions into the corona. In
this case α-particles may easily become the dominant species in the
corona, resulting in a polar wind type of solar wind in which the
light protons are accelerated outward in the electric field set up by
the α-particles and electrons. By contrast, applying the same form
for the coronal heating in a rapidly expanding geometry intended to
simulate a coronal hole, protons become collisionless closer to the
Sun, and therefore the downward proton heat flux is smaller, resulting
in a lower transition region pressure and a lower thermal force on
helium. In this case the helium abundance is low everywhere and helium
is unimportant for the acceleration of the solar wind. For the low
coronal proton and α-particle densities found in the rapidly expanding
flow, where asymptotic flow speeds are typically significantly higher
than the gravitational escape speed at the solar surface, the solar
wind helium mass flux is determined by the amount of helium available
at the top of the chromosphere. In the radially expanding flow,
with asymptotic flow speeds lower than the escape speed, the helium
mass flux depends on the amount of energy available in the corona
to lift helium out of the gravitational potential. In both cases the
frictional coupling between helium and hydrogen in the chromosphere,
using currently accepted elastic cross sections, is too weak to pull
a sufficient number of helium atoms up to the top of the chromosphere
and thus obtain a mass flux in agreement with observations. A better
understanding of the chromosphere is therefore called for.

---------------------------------------------------------
Title: Thermal forces and the coronal helium abundance
Authors: Hansteen, V. H.; Lie-Svendsen, Ø.; Leer, E.
2003AIPC..679..620H Altcode:
The interaction between protons and minor ions in the
chromosphere-corona transition region produces an upward force on the
minor ions and an enhanced coronal abundance. In this presentation we
compare a “classical” hydrodynamical model of a hydrogen - helium
solar wind and a model based on a 16-moment fluid description where
the heat flux is treated in a self-consistent manner.

---------------------------------------------------------
Title: The effect of time-dependent coronal heating on the solar
wind from coronal holes
Authors: Lie-Svendsen, Øystein; Hansteen, Viggo H.; Leer, Egil
2003AIPC..679..299L Altcode:
We have modelled the solar wind response to a time-dependent
energy input in the corona. The model, which extends from the upper
chromosphere to 1 AU, solves the time-dependent transport equations
based on the gyrotropic approximation to the 16-moment set of transport
equations, which allow for temperature anisotropies. Protons are heated
perpendicularly to the magnetic field, assuming a coronal heating
function that varies sinusoidally in time. We find that heating with
periods less than about 3 hours does not leave visible manifestations
in the solar wind (the oscillations are efficiently damped near the
Sun) heating with periods of order 10 hours leads to perturbations
comparable to Ulysses observations; while heating with periods of
order 100 hours results in a series of steady-state solutions. Mass
flux perturbations tend to be larger than perturbations in wind
speed. Heating in coronal holes with periods of order 30 hours leads
to large mass flux perturbations near Earth, even when the amplitude
of the change in heating rate in the corona is small.

---------------------------------------------------------
Title: Dynamics of Coronal loops: "Catastrophic Cooling" and
High-speed Downflows
Authors: Müller, D. A. N.; Hansteen, V. H.; Peter, H.
2003ANS...324...13M Altcode: 2003ANS...324..B09M
No abstract at ADS

---------------------------------------------------------
Title: Condensation in Cool Coronal Loops and its Effect on Transition
Region Lines
Authors: Müller, D. A. N.; Hansteen, V. H.; Peter, H.
2003ANS...324..108M Altcode: 2003ANS...324..P14M
No abstract at ADS

---------------------------------------------------------
Title: On the nature of the 3 minute oscillations above sunspots
Authors: Hansteen, V. H.; Wikstøl, Ø.; Brynildsen, N.; Maltby, P.;
Fredvik, T.; Kjeldseth-Moe, O.
2002ESASP.505..183H Altcode: 2002IAUCo.188..183H; 2002solm.conf..183H
Oscillations in the sunspot transition region and corona are observed
simultaneously with the Coronal Diagnostic Spectrometer - CDS on SOHO
and the Transition Region And Coronal Explorer - TRACE. Observations
of thirteen sunspots in 2001 and 2002 show that the amplitude in
the relative integrated line intensity increases with increasing
temperature, reaches a maximum for emission lines formed close to
1-2×105K, and decreases at higher temperatures. Part of the
wave energy penetrates into the corona, channeled into small areas that
appear to coincide with the endpoints of sunspot coronal loops. The
observed power spectra show one dominating peak close to 6 mHz. To
explore the nature of the waves we calculate upwardly propagating
acoustic waves confined to a magnetic flux tube and compare the results
with the observations.

---------------------------------------------------------
Title: Effect of time-dependent coronal heating on the solar wind
Authors: Lie-Svendsen, Øystein; Hansteen, Viggo H.; Leer, Egil
2002JGRA..107.1325L Altcode:
We have used a higher-order fluid model to study the effect of
time-dependent coronal heating of protons on the solar wind, both for
the fast wind from rapidly expanding coronal holes and for slower
winds from a radially expanding geometry. The model extends from
the chromosphere to 1 AU in order to capture the coupling between the
chromosphere and corona and therefore the supply of plasma to the solar
wind. The protons are heated in the perpendicular direction (relative
to the magnetic field), assuming a simple sinusoidal variation with
time. With a short time period for the heating, less than about 3 hours,
the oscillations are efficiently damped in the inner part of the solar
wind, even when the amplitude of the change in heating rate is large,
leading to essentially steady state solutions near Earth. Heating
with a period of 10 hours or so leads to perturbations in the wind
near Earth that are comparable to Ulysses observations. Heating with a
period of order 100 hours leads to essentially a series of steady state
solutions. The mass flux perturbations are in general larger than the
perturbations in wind speed, and for heating of coronal holes with
periods of order 30 hours the mass flux perturbations are amplified
in the wind by the interaction between the fast and slow components
of the wind. In this case even moderate changes in the heating rate
in the corona can produce solar wind interaction regions with forward
and reversed shocks and large mass flux perturbations near Earth.

---------------------------------------------------------
Title: Neutral Atom Diffusion in a Partially Ionized Prominence Plasma
Authors: Gilbert, Holly R.; Hansteen, Viggo H.; Holzer, Thomas E.
2002ApJ...577..464G Altcode:
The support of solar prominences is normally described in terms of
a magnetic force on the prominence plasma that balances the solar
gravitational force. Because the prominence plasma is only partially
ionized, this support needs to be understood in terms of the frictional
coupling between the neutral and ionized components of the prominence
plasma, the efficacy of which depends directly on the ion density. More
specifically, the frictional force is proportional to the relative
flow of neutral and ion species, and for a plasma with a sufficiently
small vertical ion column density, this flow must be relatively large
to produce a frictional force that balances gravity. A large relative
flow, of course, implies significant draining of neutral particles from
the prominence. We evaluate the importance of this draining effect for
a hydrogen-helium plasma and consider the variation of the draining
with a variety of prominence parameters. Our calculations show that
the loss timescale for hydrogen is much longer than that for helium,
which for typical prominence parameters is about one day.

---------------------------------------------------------
Title: Waves in magnetic flux concentrations: The critical role of
mode mixing and interference
Authors: Bogdan, T. J.; Rosenthal, C. S.; Carlsson, M.; Hansteen, V.;
McMurry, A.; Zita, E. J.; Johnson, M.; Petty-Powell, S.; McIntosh,
S. W.; Nordlund, Å.; Stein, R. F.; Dorch, S. B. F.
2002AN....323..196B Altcode:
Time-dependent numerical simulations of nonlinear wave propagation
in a two-dimensional (slab) magnetic field geometry show wave mixing
and interference to be important aspects of oscillatory phenomena in
starspots and sunspots. Discrete sources located within the umbra
generate both fast and slow MHD waves. The latter are compressive
acoustic waves which are guided along the magnetic field lines and
steepen into N-waves with increasing height in the spot atmosphere. The
former are less compressive, and accelerate rapidly upward through the
overlying low-beta portion of the umbral photosphere and chromosphere
(beta equiv 8pi p/ B2). As the fast wave fronts impinge
upon the beta ~ 1 penumbral “magnetic canopy" from above, they
interfere with the outward-propagating field-guided slow waves, and
they also mode convert to (non-magnetic) acoustic-gravity waves as
they penetrate into the weak magnetic field region which lies between
the penumbral canopy and the base of the surrounding photosphere. In
a three-dimensional situation, one expects additional generation,
mixing and interference with the remaining torsional Alfvén waves.

---------------------------------------------------------
Title: The Effect of Transition Region Heating on the Solar Wind
from Coronal Holes
Authors: Lie-Svendsen, Øystein; Hansteen, Viggo H.; Leer, Egil;
Holzer, Thomas E.
2002ApJ...566..562L Altcode:
Using a 16 moment solar wind model extending from the chromosphere to
1 AU, we study how the solar wind is affected by direct deposition
of energy in the transition region, in both radially expanding
geometries and rapidly expanding coronal holes. Energy is required
in the transition region to lift the plasma up to the corona, where
additional coronal heating takes place. The amount of energy deposited
determines the transition region pressure and the number of particles
reaching the corona and, hence, how the solar wind energy flux is
divided between gravitational potential and kinetic energy. We find that
when only protons are heated perpendicularly to the magnetic field in a
rapidly expanding coronal hole, the protons quickly become collisionless
and therefore conduct very little energy into the transition region,
leading to a wind much faster than what is observed. Only by additional
deposition of energy in the transition region can a reasonable mass flux
and flow speed at 1 AU be obtained. Radiative loss in the transition
region is negligible in these low-mass flux solutions. In a radially
expanding geometry the same form of coronal heating results in a
downward heat flux to the transition region substantially larger
than what is needed to heat the upwelling plasma, resulting in a
higher transition region pressure, a slow, massive solar wind, and
radiative loss playing a dominant role in the transition region energy
budget. No additional energy input is needed in the transition region
in this case. In the coronal hole geometry the solar wind response to
transition region heating is highly nonlinear, and even a tiny input of
energy can have a very large influence on the asymptotic properties of
the wind. By contrast, the radially expanding wind is quite insensitive
to additional deposition of energy in the transition region.

---------------------------------------------------------
Title: Waves in the Magnetized Solar Atmosphere. I. Basic Processes
and Internetwork Oscillations
Authors: Rosenthal, C. S.; Bogdan, T. J.; Carlsson, M.; Dorch,
S. B. F.; Hansteen, V.; McIntosh, S. W.; McMurry, A.; Nordlund, Å.;
Stein, R. F.
2002ApJ...564..508R Altcode:
We have modeled numerically the propagation of waves through magnetic
structures in a stratified atmosphere. We first simulate the propagation
of waves through a number of simple, exemplary field geometries in
order to obtain a better insight into the effect of differing field
structures on the wave speeds, amplitudes, polarizations, direction
of propagation, etc., with a view to understanding the wide variety of
wavelike and oscillatory processes observed in the solar atmosphere. As
a particular example, we then apply the method to oscillations in the
chromospheric network and internetwork. We find that in regions where
the field is significantly inclined to the vertical, refraction by
the rapidly increasing phase speed of the fast modes results in total
internal reflection of the waves at a surface whose altitude is highly
variable. We conjecture a relationship between this phenomenon and the
observed spatiotemporal intermittancy of the oscillations. By contrast,
in regions where the field is close to vertical, the waves continue
to propagate upward, channeled along the field lines but otherwise
largely unaffected by the field.

---------------------------------------------------------
Title: Key issues of the solar transition region to be addressed by
the Solar Orbiter
Authors: Hansteen, V. H.
2001ESASP.493...51H Altcode: 2001sefs.work...51H
The transition region is the thermal contact between the dense and
cold chromosphere and the hot and tenuous corona. As such, the spectral
emission lines formed in the region potentially contain information on
the phenomena occurring in both. This diagnostic potential is increased
since the transition region is geometrically quite small and since the
radiative transfer is relatively simple for these lines. The transition
region thus presents us with the possibility of making observations that
can elucidate the still not understood energetics and dynamics of the
corona and the chromosphere. On the other hand, the nearly discontinuous
rise in temperature and the fact that plasma beta passes through β =
1 close to the transition region practically ensure that wave phenomena
such as reflection and refraction will occur. It is also certain that
time-dependent phenomena and the topology of the magnetic field are
factors that contribute to the complications in unraveling the nature
of the dynamics of these regions. There is a large contrast between
network and internetwork emission throughout the outer layers of the Sun
(indeed it is not certain that any material at coronal temperatures
is in thermal contact with the internetwork at all), while at any
given location the emission varies with time on several different time
scales. In the internetwork there is, perhaps, general agreement that
acoustic waves play an important role in powering the emission stemming
from the upper chromosphere. In the network the acoustic power seems
suppressed and in the upper regions of the solar atmosphere magnetic
phenomena, such as rapid reconnection or the dissipation of high
frequency Alfvén waves surely contribute to the heating. We will
discuss the various phenomena expected in the transition region in
the context of observations possible with the Solar Orbiter.

---------------------------------------------------------
Title: Neutral Atom Diffusion in a Paritially Ionized Prominence
Plasma
Authors: Gilbert, H. R.; Hansteen, V. H.; Holzer, T. E.
2001AGUSM..SP61A09G Altcode:
The support of solar prominences is normally described in terms of
a magnetic force on the prominence plasma that balances the solar
gravitational force. Because the prominence plasma is only partially
ionized, it is necessary to consider in addition the support of
the neutral component of the prominence plasma. This support is
accomplished through a frictional interaction between the neutral and
ionized components of the plasma, and its efficacy depends strongly
on the degree of ionization of the plasma. More specifically, the
frictional force is proportional to the relative flow of neutral and ion
species, and for a sufficiently weakly ionized plasma, this flow must be
relatively large to produce a frictional force that balances gravity. A
large relative flow, of course, implies significant draining of neutral
particles from the prominence. We evaluate the importance of this
draining effect for a hydrogen-helium plasma, and consider the variation
of the draining with the degree of heat input to the prominence plasma,
which determines the ionization state of the prominence.

---------------------------------------------------------
Title: Coupling Between Chromosphere and Corona: Why it Matters for
the Solar Wind
Authors: Lie-Svendsen, {; Hansteen, V. H.; Leer, E.
2001AGUSM..SH21B02L Altcode:
The solar wind is driven by energy input which must be deposited mainly
in the corona. In some sense, therefore, the solar wind “starts”
in the corona, and most solar wind models have their lower boundary
here. However, the underlying chromosphere and transition region is
not only a “passive” supplier of solar wind plasma. Energy must
be supplied as well in the upper chromosphere and transition region
to ionize and heat the outflowing gas from chromospheric to coronal
temperatures. While this energy input is usually small compared to the
energy deposited in the corona, it nevertheless can have a crucial
influence on the solar wind because the amount of energy deposited
in the transition region limits the mass flux of the wind. Using a
higher-order (16-moment) fluid solar wind model that extends from the
chromosphere to 1~AU, we find that when protons are heated in the
corona they may quickly become collisionless so that the heat flux
deviates strongly from classical heat conduction, making it difficult
to transport sufficient amounts of energy downwards to the transition
region. The problem is most acute in the rapidly expanding magnetic
field of coronal holes, where we find that adding even a small energy
input in the transition region therefore can increase the solar wind
mass flux by a factor ten or more, and not doing so leads to a wind
much faster than what is observed even in high-speed streams.

---------------------------------------------------------
Title: Wave Propagation in a Magnetized Atmosphere
Authors: Bogdan, T. J.; Rosenthal, C. S.; Carlsson, M.; McIntosh,
S.; Dorch, S.; Hansteen, V.; McMurry, A.; Nordlund, Å; Stein, R. F.
2001AGUSM..SH41A01B Altcode:
Numerical simulations of MHD wave propagation in plane-parallel
atmospheres threaded by non-trivial potential magnetic fields will be
presented, and their implications for understanding distinctions between
intranetwork and internetwork oscillations will be discussed. Our
findings basically confirm the conjecture of McIntosh et al. (2001,
ApJ 548, L237), that the two-dimensional surface where the Alfvén
and sound speeds coincide (i.e., where the plasma-β , the ratio of
gas to magnetic pressure, is of order unity) plays a fundamental
role in mediating the conversion between the fast-, intermediate-
(Alfvén), and slow-Magneto-Atmospheric-Gravity (MAG) waves. For
example, upward-propagating acoustic waves generated at the base of
the internetwork photosphere suffer significant downward reflection
when they encounter this β ≈ 1 surface. Close to the network, this
surface descends from the upper chromosphere and low corona (which
pertains in the internetwork cell interiors) down into the photosphere,
and so chromospheric oscillation `shadows' are predicted to surround
the network. In the network, strong vertical magnetic fields further
depress the β ≈ 1 surface below the surface layers where the
(magnetic field-aligned) acoustic waves (i.e., slow MAG-waves) are
generated. For frequencies in excess of the cutoff frequency, these
acoustic waves suffer little reflection from the overlying atmosphere
and they steepen as they progress upward.

---------------------------------------------------------
Title: A 16-moment solar wind model: From the chromosphere to 1 AU
Authors: Lie-Svendsen, Øystein; Leer, Egil; Hansteen, Viggo H.
2001JGR...106.8217L Altcode:
We present a solar wind fluid model extending from the chromosphere
to Earth. The model is based on the gyrotropic approximation to
the 16-moment set of transport equations, in which we solve for the
density, drift speed, temperature parallel and perpendicular to the
magnetic field, and transport of parallel and perpendicular thermal
energy along the magnetic field (heat flux). The solar wind plasma is
created dynamically through (photo) ionization in the chromosphere,
and the plasma density in the transition region and corona is computed
dynamically, dependent on the type of coronal heating applied, rather
than being set arbitrarily. The model improves the description of
proton energy transport in the transition region, where classical heat
conduction is only retrieved in the collision-dominated limit. This
model can serve as a “test bed” for any coronal heating mechanism. We
consider heating of protons by a turbulent cascade of Alfvén waves
in rapidly expanding coronal holes. The resulting high coronal proton
temperatures lead to a downward proton energy flux from the corona
which is much smaller than what classical transport theory predicts,
causing a very low coronal density and an extremely fast solar wind
with a small mass flux. Only when some of the wave energy is forcibly
deposited in the lower transition region can a realistic solar wind
be obtained. Because of the poor proton heat transport, in order
to produce a realistic solar wind any viable heating mechanism must
deposit some energy in the transition region, either directly or via
explicit heating of coronal electrons.

---------------------------------------------------------
Title: An Observational Manifestation of Magnetoatmospheric Waves
in Internetwork Regions of the Chromosphere and Transition Region
Authors: McIntosh, S. W.; Bogdan, T. J.; Cally, P. S.; Carlsson, M.;
Hansteen, V. H.; Judge, P. G.; Lites, B. W.; Peter, H.; Rosenthal,
C. S.; Tarbell, T. D.
2001ApJ...548L.237M Altcode:
We discuss an observational signature of magnetoatmospheric waves in
the chromosphere and transition region away from network magnetic
fields. We demonstrate that when the observed quantity, line or
continuum emission, is formed under high-β conditions, where β is
the ratio of the plasma and magnetic pressures, we see fluctuations in
intensity and line-of-sight (LOS) Doppler velocity consistent with the
passage of the magnetoatmospheric waves. Conversely, if the observations
form under low-β conditions, the intensity fluctuation is suppressed,
but we retain the LOS Doppler velocity fluctuations. We speculate that
mode conversion in the β~1 region is responsible for this change in
the observational manifestation of the magnetoatmospheric waves.

---------------------------------------------------------
Title: Waves in the Magnetised Solar Atmosphere
Authors: Rosenthal, C. S.; Carlsson, M.; Hansteen, V.; McMurry,
A.; Bogdan, T. J.; McIntosh, S.; Nordlund, A.; Stein, R. F.; Dorch,
S. B. F.
2001IAUS..203..170R Altcode:
We have simulated the propagation of magneto-acoustic disturbances
through various magneto-hydrostatic structures constructed to mimic
the solar magnetic field. As waves propagate from regions of strong
to weak magnetic field and vice-versa different types of wave modes
(transverse and longitudinal) are coupled. In closed-field geometries
we see the trapping of wave energy within loop-like structures. In
open-field regions we see wave energy preferentially focussed away
from strong-field regions. We discuss these oscillations in terms
of various wave processes seen on the Sun - umbral oscillations,
penumbral running waves, internetwork oscillations etc.

---------------------------------------------------------
Title: Transition Region Models
Authors: Hansteen, V.
2000eaa..bookE2267H Altcode:
The SOLAR TRANSITION REGION comprises the PLASMA between the
CHROMOSPHERE and the CORONA. In both of these regions the temperature
is fairly uniform. The transition region, by contrast, is believed to
be characterized by a very steep temperature rise from a chromospheric
temperature of slightly less than 104 K to coronal temperatures on
the order of 106 K. The goal of modeling the transition regi...

---------------------------------------------------------
Title: Rapid intensity and velocity variations in solar transition
region lines
Authors: Hansteen, V. H.; Betta, R.; Carlsson, M.
2000A&A...360..742H Altcode:
We have obtained short exposure (3 s) time series of strong upper
chromospheric and transition region emission lines from the quiet Sun
with the SUMER instrument onboard SOHO during two 1 hour periods in
1996. With a Nyqvist frequency of 167 mHz and relatively high count
rates the dataset is uniquely suited for searching for high frequency
variations in intensity and Doppler velocity. From Monte-Carlo
experiments taking into account the photon-counting statistics we
estimate our detection limit to correspond to a wave-packet of four
periods coherent over 3” with a Doppler-shift amplitude
of 2.5km s-1 in the darkest internetwork areas observed in
C III. In the network the detection limit is estimated to be 1.5km
s-1. Above 50 mHz we detect wave-packet amplitudes above
3km s-1 less than 0.5% of the time. Between 20 and 50 mHz
we detect some wave-packets with a typical duration of four periods
and amplitudes up to 8km s-1. At any given internetwork
location these wave-packets are present 1% of the time. In the
10-20 mHz range we see amplitudes above 3km s-1 12% of
the time. At lower frequencies our dataset is consistent with other
SUMER datasets reported in the literature. The chromospheric 3-7 mHz
signal is discernible in the line emission. In the internetwork this
is the dominant oscillation frequency but higher frequencies (7-10
mHz) are often present and appear coherent in Doppler velocity over
large spatial regions (≍ 40"). Wavelet analysis implies that these
oscillations have typical durations of 1000s. The network emission also
shows a 5 mHz signal but is dominated by low frequency variations (of
&lt; 4 mHz) in both intensity and velocity. The oscillations show less
power in intensity than in velocity. We find that while both red and
blue shifted emission is observed, the transition region lines are on
average red shifted between 5-10km s-1 in the network. A net
red shift is also found in the internetwork emission but it is smaller
(&lt; 4km s-1). The line widths do not differ much between
the internetwork and network, the non-thermal line widths increase
with increasing temperature of line formation from 30km s-1
for the C II 1334 Å line to 45km s-1 for the O VI 1032
Å line. By constructing scatterplots of velocity versus intensity we
find that in the network a mean redshift is correlated with a high mean
intensity. In the internetwork regions we do not find any correlation
between the intensity and the Doppler velocity.

---------------------------------------------------------
Title: Spectral diagnostics and transition region structure
Authors: Hansteen, V. H.
2000SPD....31.1202H Altcode: 2000BAAS...32..844H
Observations of spectral lines formed in the upper chromosphere,
transition region, and corona paint a complicated picture open to
several interpretations. It seems certain that time-dependent phenomena
and the topology of the magnetic field are factors that contribute
to the complications: There is a large contrast between network and
internetwork emission throughout the region, while at any given location
the emission varies with time on several different timescales. There is,
perhaps, general agreement that acoustic waves play an important role in
powering the upper chromosphere while in the upper regions of the Solar
atmosphere magnetic phenomena also contribute to the heating. Given
this framework we suggest that the best method of progressing towards
untangling the complicated picture drawn by the observations is to
proceed by comparing the results of simple physical models with the
observations. We will present various models to support this point of
view. This work has been supported by the Norwegian Research Coucil's
grant 121076/420, "Modelling of Astrophysical Plasmas"

---------------------------------------------------------
Title: Chromospheric and Transition Region Internetwork Oscillations:
A Signature of Upward-propagating Waves
Authors: Wikstøl, Ø.; Hansteen, V. H.; Carlsson, M.; Judge, P. G.
2000ApJ...531.1150W Altcode:
We analyze spectral time series obtained on 1997 April 25 with
the SUMER instrument on SOHO. Line and continuum data near 1037
Å were acquired at a cadence of 16 s. This spectral region was
chosen because it contains strong emission lines of C II, formed
in the upper chromosphere/lower transition region; O VI, formed
in the upper transition region; and neighboring continuum emission
formed in the middle chromosphere. The time series reveal oscillatory
behavior. Subsonic (3-5 km s-1 amplitude) Doppler velocity
oscillations in the C II and O VI lines, with periods between 120
and 200 s, are prominent. They are seen as large-scale coherent
oscillations, typically of 3-7 Mm length scale, occasionally approaching
15 Mm, visible most clearly in internetwork regions. The Doppler
velocity oscillations are related to oscillations seen in the continuum
intensity, which precede upward velocity in C II by 40-60 s. The C II
Doppler shift precedes the O VI Doppler shift by 3-10 s. Oscillations
are also present in the line intensities, but the intensity amplitudes
associated with the oscillations are small. The continuum intensity
precedes the C II intensity by 30-50 s. Phase difference analysis
shows that there is a preponderance of upward-propagating waves in the
upper chromosphere that drive an oscillation in the transition region
plasma, thus extending the evidence for upward-propagating waves from
the photosphere up to the base of the corona.

---------------------------------------------------------
Title: Advances in Modelling the Fast Solar Wind
Authors: Hansteen, V. H.; Leer, E.; Lie-Svendsen, Ø.
1999ESASP.448.1091H Altcode: 1999mfsp.conf.1091H; 1999ESPM....9.1091H
No abstract at ADS

---------------------------------------------------------
Title: Transition Region Oscillations
Authors: Hansteen, V. H.; Wisktol, O.; Carlsson, M.; Judge, P. G.
1999ESASP.446..351H Altcode: 1999soho....8..351H
We analyze time series data obtained April 25th 1997 with the SUMER
instrument on SOHO. Line and continuum data near 1037 were acquired at
a cadence of 16s. This spectral region was chosen because it contains
strong emission lines of C II, formed in the upper chromosphere/lower
transition region, O VI, formed in the upper transition region, as well
as neighboring continuum emission formed in the middle chromosphere. The
dataset reveals oscillatory behavior containing valuable information
on the physical structure of the chromosphere and transition
region. Prominent in the data are subsonic (3-5 km/s amplitude)
velocity oscillations with periods between 120 and 200 sec. They
are seen as large scale coherent oscillations, typically of 3--7 Mm
length scale but sometimes approaching 15Mm, visible most clearly
in internetwork regions. The oscillations are present in C II and O
VI velocities, as well as in the continuum intensity. The continuum
intensity precedes upward velocity in the C II line by 40-60s and the C
III velocity precedes the O VI velocity by 3-10s. The oscillations are
also present in the intensities of the two lines, but the intensity
amplitudes associated with the oscillations are small. We find that
the continuum intensity precedes the C II intensity by 30-50s. These
phase shifts indicate that there are upward propagating waves in the
upper chromosphere that drive an oscillation in the transition region
plasma. The oscillations seem to be present in most internetwork areas
at any time, thus they are the dominant resolvable dynamical feature
of the internetwork chromosphere and transition region.

---------------------------------------------------------
Title: The origin of the high speed solar wind
Authors: Hansteen, Viggo H.; Leer, Egil; Holzer, Thomas E.
1999AIPC..471...17H Altcode: 1999sowi.conf...17H
The outflow of coronal plasma into interplanetary space is a
consequence of the coronal heating process. Therefore the formation
of the corona and the acceleration of the solar wind should be
treated as a single problem. The deposition of energy into the corona
through some mechanical or electromagnetic energy flux is balanced
by the various sinks available to the corona, and the sum of these
processes determines the coronal structure, i.e. its temperature and
density. Heating of the extended solar corona leads to high proton and
ion temperatures and relatively low electron temperatures. This is due
to the low heat conductivity in the proton (ion) gas as compared to
the electrons. To a fairly good approximation we can say that most of
the energy flux deposited in the protons and ions is lost as kinetic
and gravitational energy flux in the solar wind flow, whereas a large
fraction of the energy flux added to the electrons is conducted back
into the transition region and lost as radiation. In order to drive
high speed wind most of the energy must be deposited in the ions.

---------------------------------------------------------
Title: Evidence in Support of the “Nanoflare” Picture of Coronal
Heating from SUMER Data
Authors: Judge, P. G.; Hansteen, V.; Wikstøl, Ø.; Wilhelm, K.;
Schühle, U.; Moran, T.
1998ApJ...502..981J Altcode:
We study high signal-to-noise profiles of O IV emission lines obtained
using the SUMER instrument on SOHO. Data for the quiet Sun obtained
close to the disk center and at the solar limb were acquired. After
careful data processing in which disk data were analyzed differentially
against limb data, we find a systematic correlation between a
density-sensitive emission-line ratio and Doppler shift across the
same emission-line profiles. While unidentified blended lines cannot be
completely discounted, the data suggest that the effects of such blends
are small. Based on theoretical results in an earlier paper, we argue
that if wave motions are responsible for the observed behavior, then
the data reveal evidence for compressive waves propagating downward
from the corona to the chromosphere. This analysis naturally lends
support to the dominance of the “nanoflare” mechanism for coronal
heating over other theories that invoke upward wave propagation,
but other mechanisms capable of generating downward-propagating waves
cannot be discounted. If, instead, steady flows are the cause of the
observed behavior, such as return flows from spicules, then they must
be such that the density is higher in the downflowing plasma. While
these particular data do not allow us to discriminate between waves
or steady flows, additional data from SOHO should be able to address
this problem. This work required and achieved very accurate wavelength
calibrations (better than 1/5 of a pixel on the detectors), taking
SUMER close to its observational capabilities. We therefore present
the elements of the analysis and calibration of SUMER data that may
be of interest to other users.

---------------------------------------------------------
Title: On Inferring the Properties of Dynamic Plasmas from Their
Emitted Spectra: The Case of the Solar Transition Region
Authors: Wikstøl, Øivind; Judge, P. G.; Hansteen, Viggo
1998ApJ...501..895W Altcode:
We reexamine the issue of inferring physical properties of solar
plasmas using EUV and UV observations. We focus on the question of
whether one can determine if typical structures seen as bright in
typical “transition-region” lines are formed in the thermal interface
between the coronal and chromospheric plasmas. Since 1983, Feldman and
colleagues have proposed, based upon Skylab and other data, that much
of the transition-region emission is formed in so-called unresolved
fine structures (UFS) that are magnetically and thermally disconnected
from the corona. This has led others to consider theoretical models of
the transition region that differ from classical models. We examine the
evidence cited in support of the UFS picture, specifically by relaxing
the implicit assumption of a static atmospheric structure. Noting
that observational data alone do not contain the information necessary
to infer essential properties of the emitting plasmas, we argue that
additional information must be added through forward calculations using
physical models. MHD models of coronal flux tubes are then examined
with explicit assumptions and boundary conditions, not as an attempt to
“fit” observed data, but in order to study the formation of emission
lines in dynamically evolving plasmas that are unresolved in space
and time. We show that incorrect conclusions can be drawn by applying
reasonable and traditional diagnostic methods to spectral data when
unresolved dynamic evolution of the emitting plasma is important but
not accounted for. In the particular case of the transition region,
we show that the UFS interpretation is not unique, and is likely to
be incorrect in the presence of unresolved dynamics. Most or all of
the evidence for UFS is amenable to a different, equally reasonable
interpretation, in which the transition-region emission is at all times
formed in the time-varying thermal interface between the corona and the
chromosphere. This work is likely to be important for a wider range of
astrophysical plasmas than simply in the solar transition region. At
stake is our basic ability to correctly diagnose physical conditions
of plasmas for which heating mechanisms are not yet understood, but
which are likely to be time dependent.

---------------------------------------------------------
Title: Acceleration of the Solar Wind: A New View
Authors: Hansteen, V. H.; Leer, E.
1998HiA....11..838H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: SUMER observations in transition region lines
Authors: Betta, R.; Hansteen, V.; Carlsson, M.; Wilhelm, K.
1998MmSAI..69..699B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Understanding the Solar Wind
Authors: Leer, E.; Hansteen, V. H.; Holzer, T. E.
1998cvsw.conf..263L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Signatures of Acoustic and Magnetic Waves in Solar and
Stellar Coronae
Authors: Wikstøl, Øivind; Judge, Philip G.; Hansteen, Viggo H.
1997ApJ...483..972W Altcode:
Time-dependent dynamical calculations of the radiating gas in solar
coronal flux tubes are used to identify features of UV spectral-line
profiles that can reveal the direction in which wave energy flows
through the solar transition region. The profile features survive
spatial and temporal averaging through nonlinear dependencies of
line emission coefficients on thermal properties of the plasma
that are correlated with fluid velocities. This approach can be
applied to stellar and other unresolved sources, as well as the
solar atmosphere. It can be regarded as a new angle of attack on the
long-standing problem of determining coronal heating mechanisms. The
approach requires low noise data of high spectral resolution. Therefore,
it can take advantage of some unique properties of the SUMER instrument
on SOHO. We make specific predictions for SUMER data that, in principle,
can test whether energy propagates upward or downward in coronal flux
tubes, thus allowing one to discriminate between competing theories of
coronal heating. We are acquiring SUMER data in an attempt to do this.

---------------------------------------------------------
Title: The Role of Helium in the Outer Solar Atmosphere
Authors: Hansteen, V. H.; Leer, E.; Holzer, T. E.
1997ApJ...482..498H Altcode:
We construct models of the outer solar atmosphere comprising the region
from the mid-chromosphere and into the solar wind in order to study
the force and energy balance in models with a significant helium
abundance. The corona is created by dissipation of an energy flux
from the Sun. The energy flux is lost as radiation from the top of the
chromosphere and as gravitational and kinetic solar wind energy flux. We
find that in models with significant ion heating of the extended corona
most of the energy flux is lost in the solar wind. The ion temperatures
are higher than the electron temperature in these models, and the
α-particle temperature is much higher than the proton temperature,
so there is energy transfer from the α-particle fluid to the protons
and electrons, but this energy exchange between the different species
is relatively small. To a fairly good approximation we can say that the
energy flux deposited in the protons and α-particles is lost as kinetic
and gravitational energy flux in the proton and α-particle flow. How
this energy flux is divided between gravitational and kinetic energy
flux (i.e., how large the particle fluxes and flow speeds are) depends
upon details of the heating process. We also find that mixing processes
in the chromosphere play an important role in determining the coronal
helium abundance and the relative solar wind proton and α-particle
fluxes. Roughly speaking, we find that the relative α-particle and
proton fluxes are set by the degree of chromospheric mixing, while
the speeds are set by the details of the coronal heating process.

---------------------------------------------------------
Title: Evidence in Support of the “Nanoflare Picture” of Coronal
Heating from SUMER Data
Authors: Judge, P.; Wikstol, O.; Hansteen, V.
1997SPD....28.0505J Altcode: 1997BAAS...29..909J
We study high signal\--to\--noise profiles of O 4 emission lines
obtained using the SUMER instrument on SoHO. Data for the quiet Sun
obtained close to disk center and at the solar limb were acquired. After
careful data processing in which disk data were analyzed differentially
against limb data, we find a systematic correlation between a
density sensitive emission line ratio and Doppler shift across the
same emission line profiles. Based upon theoretical results in an
earlier paper, we argue that if wave motions are responsible for
the observed behavior, then the data reveal evidence for compressive
waves propagating downwards from the corona to the chromosphere. This
analysis therefore lends support to the dominance of the “nanoflare”
mechanism for coronal heating (\cite{Parker1988}) over other theories
that invoke upward wave propagation. If instead steady flows are the
cause of the observed behavior, such as return flows from spicules,
then they must be such that the density is higher in the downflowing
plasma. We present time series data of other transition region lines
from SoHO that suggest that episodic heating such as may occur through
nanoflares is a more reasonable picture of heating mechanisms than
models based upon steady flows.

---------------------------------------------------------
Title: The Coronal Helium Abundance and the Solar Wind
Authors: Hansteen, Viggo H.; Hassler, Donald M.; Leer, Egil; Holzer,
Thomas E.; Woods, Thomas N.
1997SPD....28.0154H Altcode: 1997BAAS...29Q.889H
The coronal Helium abundance depends on the rate Helium is brought into
the corona via the transition region from the chromosphere and on the
rate that Helium is removed from the corona in the Solar wind. Recent
multi-fluid models of the combined chromosphere, corona, solar wind
system show that the corona may have a significant Helium abundance;
perhaps even exceeding 50% of the Hydrogen number density. These models
also indicate that in order to reproduce the Solar wind at 1AU ions
(alpha -particles as well as protons) may be required to be the most
important recipient of the coronal heating process. In these models the
role of electrons in the energetics of the Solar wind is much reduced
compared to the standard thermally driven winds. A measurement of the
coronal Helium abundance will serve to fix these theoretical ideas and
may give important clues as to mixing processes in the chromosphere as
well as to coronal heating processes. Consequently, we will discuss the
current state of observations (i.e. SOHO), and their limitations, as
well as plans for future observations (i.e. sounding rocket, Spartan).

---------------------------------------------------------
Title: Kinetic electrons in high-speed solar wind streams: Formation
of high-energy tails
Authors: Lie-Svendsen, Øystein; Hansteen, Viggo H.; Leer, Egil
1997JGR...102.4701L Altcode:
We study the evolution of the electron velocity distribution function
in high-speed solar wind streams from the collision-dominated corona
and into the collisionless interplanetary space. The model we employ
solves the kinetic transport equation with the Fokker-Planck collision
operator to describe Coulomb collisions between electrons. We use
a test particle approach, where test electrons are injected into
a prescribed solar wind background. The density, temperature, and
electric field associated with the background are computed from
fluid models. The test electrons are in thermal equilibrium with
the background at the base of the corona, and we study the evolution
of the velocity distribution of the test electrons as a function of
altitude. We find that velocity filtration, due to the energy dependence
of the Coulomb cross section, is a small effect and is not capable
of producing significant beams in the distribution or a temperature
moment that increases with altitude. The distribution function is
mainly determined by the electric field and the expanding geometry and
consists of a population with an almost isotropic core which is bound
in the electrostatic potential and a beam-like high-energy tail which
escapes. The trapped electrons contribute significantly to the even
moments of the distribution function but almost nothing to the odd
moments; the drift speed and energy flux moments are carried solely by
the tail. In order to describe the high-speed solar wind observed near
0.3 AU by the Helios spacecraft, we use a multifluid model where ions
are heated preferentially. The resulting test electron distribution
at 0.3 AU, in this background, is in very good agreement with the
velocity distributions observed by the Helios spacecraft.

---------------------------------------------------------
Title: Waves, Shocks and Non-Stationary Phenomena in the Outer
Solar Atmosphere
Authors: Hansteen, V. H.
1997ESASP.404...45H Altcode: 1997soho....5...45H; 1997cswn.conf...45H
No abstract at ADS

---------------------------------------------------------
Title: SUMER Observations Detecting Downward Propagating Waves in
the Solar Transition Region
Authors: Wiskstøl, Ø.; Judge, P. G.; Hansteen, V.; Wilhelm, K.;
Schühle, U.; Moran, T.
1997ESASP.404..731W Altcode: 1997cswn.conf..731W
No abstract at ADS

---------------------------------------------------------
Title: Temporal Variability in the Quiet Sun Transition Region
Authors: Wikstoøl, Ø.; Hansteen, V. H.; Brynildsen, N.; Maltby,
P.; Kyeldseth-Moe, O.; Harrison, R. A.; Wilhelm, K.; Tarbell, T. D.;
Scherrer, P. H.
1997ESASP.404..733W Altcode: 1997cswn.conf..733W
No abstract at ADS

---------------------------------------------------------
Title: Intensity and Velocity Variations in Transition Region Lines
Observed with SUMER
Authors: Betta, R.; Hansteen, V.; Carlsson, M.; Wilhelm, K.
1997ESASP.404..205B Altcode: 1997cswn.conf..205B
No abstract at ADS

---------------------------------------------------------
Title: Transition region lineshifts and nanoflare heating of the
corona.
Authors: Hansteen, V. H.
1997smf..conf...87H Altcode:
After briefly reviewing previous data and new observations taken by
the SUMER instrument aboard the SOHO spacecraft the author pursues
the interpretation that the observed pervasive transition region line
shifts are caused by MHD waves propagating along the magnetic field
lines down from the corona towards the chromosphere. He argues that a
likely source of such coronally generated MHD waves are the episodic
magnetic reconection events that are believed to heat the corona. He
also presents an alternate method of observation based on densitive
sensitive line pairs that may give further evidence of the processes
heating the corona.

---------------------------------------------------------
Title: Solar magnetic fields
Authors: Hansteen, V. H.
1997smf..conf.....H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Helium in the outer solar atmosphere
Authors: Hansteen, V. H.; Leer, E.; Holzer, T. E.
1997AIPC..385..197H Altcode: 1997recs.conf..197H
We construct models of the outer solar atmosphere comprising the
region from the mid chromosphere and into the solar wind in order
to study the force and energy balance in models with a significant
helium abundance. The corona is created by dissipation of an energy
flux from the Sun. The energy flux is lost as radiation from the
top of the chromosphere and as gravitational and kinetic solar wind
energy flux. We find that in models with significant ion heating of the
extended corona most of the energy flux is lost in the solar wind. The
ion temperatures are higher than the electron temperature in these
models, and the α-particle temperature is much higher than the proton
temperature. Roughly speaking we find that the relative α-particle
and proton fluxes are set by the degree of chromospheric mixing while
the speeds are set by the details of the coronal heating process.

---------------------------------------------------------
Title: Acceleration of the Solar Wind
Authors: Holzer, T. E.; Hansteen, V. H.; Leer, E.
1997cwh..conf..239H Altcode: 2006mslp.conf..239H
No abstract at ADS

---------------------------------------------------------
Title: Observational signatures of nanoflare heated solar stellar
coronae
Authors: Wikstøl, Ø.; Judge, P. G.; Hansteen, V. H.
1997AdSpR..20.2289W Altcode:
Using time-dependent dynamical models of the radiating gas in coronal
flux tubes, we identify features in UV spectral line profiles
that can reveal the direction in which energy flows through the
solar transition region, in observations without temporal or spatial
resolution. The profile features survive spatial and temporal averaging
through non-linear dependencies of the line emission coefficients
on thermal properties of the plasma that are correlated with the
material velocity. This approach requires only low noise data of high
spectral resolution and could naturally be applied to stars as well as
the solar corona. We make predictions for the SUMER instrument that
can in principle test whether energy propagates upwards or downwards
in coronal flux tubes, suggesting a new angle of attack on the long
standing problem of determining coronal heating mechanisms.

---------------------------------------------------------
Title: Outflow of He+ from the polar ionosphere: Comparison
of hydrodynamic and kinetic descriptions
Authors: Leer, Egil; Lie-Svendsen, Øystein; Olsen, Espen Lyngdal;
Hansteen, Viggo H.
1996JGR...10117207L Altcode:
Singly ionized helium, a minor species in the polar ionosphere, is
being propelled out along open magnetic field lines by an outward
polarization electric field. In the present study we compare the
kinetic and hydrodynamic descriptions of this transonic outflow
treating the singly ionized helium as a test particle population in a
static background of singly ionized oxygen and electrons. We find that
the resultant He+ particle fluxes are equal in both the
eight-moment hydrodynamic description and in the kinetic description
based on the Fokker-Planck equation. A five-moment hydrodynamic
description gives a flux that is some 40% lower. The increase of
the He+ energy flux with altitude is also equal in the
eight-moment hydrodynamic and in the kinetic descriptions.

---------------------------------------------------------
Title: Solar wind theory and modeling
Authors: Hansteen, Viggo H.
1996AIPC..382...44H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Are the Redshifts Observed in Transition Region Lines Caused
by Magnetic Reconnection?
Authors: Hansteen, Viggo; Maltby, Per; Malagoli, Andrea
1996ASPC..111..116H Altcode: 1997ASPC..111..116H
The detection of differential redshift between transition region and
chromospheric line emission, both in spectra of late type stars and in
solar spectra has been a puzzle for two decades. The fact that solar
observations are inconsistent with a continuous downflow calls for
another interpretation. The authors pursue the interpretation that
the observed pervasive transition region line shifts are caused by
MHD waves propagating along the magnetic field lines from the corona
downward towards the chromosphere.

---------------------------------------------------------
Title: Non-Equilibrium Effects on the Optically Thin Radiative
Loss Function
Authors: Wikstol, O.; Hansteen, V. H.
1996mpsa.conf..465W Altcode: 1996IAUCo.153..465W
No abstract at ADS

---------------------------------------------------------
Title: Coronal heating, densities, and temperatures and solar wind
acceleration
Authors: Hansteen, Viggo H.; Leer, Egil
1995JGR...10021577H Altcode:
The outflow of coronal plasma into interplanetary space is a
consequence of the coronal heating process. Therefore the formation
of the corona and the acceleration of the solar wind should be treated
as a single problem. The deposition of energy into the corona through
some “mechanical” energy flux is balanced by the various energy sinks
available to the corona, and the sum of these processes determines the
coronal structure, i.e., its temperature and density. The corona loses
energy through heat conduction into the transition region and through
the gravitational potential energy and kinetic energy put into the solar
wind. We show from a series of models of the chromosphere-transition
region-corona-solar wind system that most of the energy deposited in
a magnetically open region goes into the solar wind. The transition
region pressures and the coronal density and temperature structure may
vary considerably with the mode and location of energy deposition,
but the solar wind mass flux is relatively insensitive to these
variations; it is determined by the amplitude of the energy flux. In
these models the transition region pressure decreases in accordance
with the increasing coronal density scale height such that the solar
wind mass loss is consistent with the energy flux deposited in the
corona. On the basis of the present study we can conclude that the
exponential increase of solar wind mass flux with coronal temperature,
found in most thermally driven solar wind models, is a consequence of
fixing the transition region pressure.

---------------------------------------------------------
Title: Solar wind theory and modelling
Authors: Hansteen, Viggo H.
1995sowi.conf...29H Altcode:
The outflow of coronal plasma into interplanetary space is a consequence
of the coronal heating process. Therefore the formation of the corona
and the acceleration of the solar wind should be treated as a single
problem. Traditionally the mass or particle flux emanating from the
extended corona has been thought of as being determined by the coronal
temperature or scale height and the coronal (base) density. This
argument follows from considerations of the momentum balance of
the corona-wind system from which one obtains models of a close to
hydrostatic corona out to the critical point where the flow becomes
supersonic. With this approach to the acceleration of the wind is has
been difficult to reconcile the relatively small variation observed in
the proton flux at 1 AU with the predicted exponential dependence of the
proton flux on the coronal temperature. In this talk we would like to
emphasize another approach in which coronal energetics play the primary
role. The deposition of energy into the corona through some 'mechanical'
energy flux is balanced by the various energy sinks available to the
corona and the sum of these processes determine the coronal structure,
i.e. its temperature and density. The corona loses energy through
heat conduction into the transition region, through radiative losses,
and through the gravitational potential energy and kinetic energy put
into the solar wind itself. We will show from a series of models of
the chromosphere transition region-corona-solar wind system that most
of the energy deposited in a magnetically open region will go into
the solar wind, with roughly half going into kinetic energy and half
into lifting the plasma out of the solar gravity field. The coronal
base density will adjust itself in such a way that the heat conductive
flux flowing into the transition region is radiated away in the upper
chromosphere. The coronal temperature is set by the requirements that
most of the deposited energy goes into accelerating the solar wind;
the coronal scale height will adjust itself so that the solar wind
energy losses conform to the amplitude of the input energy. These
processes are modified by the 'mode' of energy deposition, and we
will show the effects on coronal structure of changing the parameters
describing coronal heating as well as the effects of including a helium
fluid in the models. However, the location, scale height and/or form
of the energy deposition (i.e. heating or direct acceleration) are not
too important for the solar wind, the coronal density and temperature
structure will vary with the 'mode' of energy deposition, but the
solar wind mass flux depends mainly on the amplitude of the energy flux.

---------------------------------------------------------
Title: On the Relation between Coronal Heating, Flux Tube Divergence,
and the Solar Wind Proton Flux and Flow Speed
Authors: Sandbaek, Onulf; Leer, Egil; Hansteen, Viggo H.
1994ApJ...436..390S Altcode:
A one-fluid solar wind model is used to investigate some relations
between coronal heating, the flux tube divergence near the Sun, and the
solar wind proton flux and flow speed. The effects of energy addition
to the supersonic region of the flow are also studied. We allow for
a mechanical energy flux that heats the corona, and an Alfven wave
energy flux that adds energy, mainly to the supersonic flow, both
as momentum and as heat. We find that the mechanical energy flux
determines the solar wind mass flux, and in order to keep an almost
constant proton flux at the orbit of Earth with changing flow geometry,
that the mechanical energy flux must vary linearly with the magnetic
field in the inner corona. This thermally driven wind generally has a
low asymptotic flow speed. When Alfven waves are added to the thermally
driven flow, the asymptotic flow speed is increased and is determined
by the ratio of the Alfven wave and the mechanical energy fluxes at
the coronal base. Flow speeds characteristic of recurrent high-speed
solar wind streams can be obtained only when the Alfven wave energy
flux, deposited in the supersonic flow, is larger than the mechanical
energy flux heating the corona.

---------------------------------------------------------
Title: Transition region lineshifts in the rebound shock spicule
model.
Authors: Hansteen, V. H.; Wikstol, O.
1994A&A...290..995H Altcode:
Spectral emission lines created in the solar chromosphere-corona
transition region show Doppler shifts indicating widespread plasma
down-flows. It has been proposed that this may be the result of the
return of spicular material. We simulate a spicule numerically using
the rebound shock model and find that the resulting hydrodynamic
evolution leads to a perceived up-flow in transition region spectral
lines even though the average velocity in the line forming region is
directed downward. The explanation for this apparent paradox is found
in the correlation between density and velocity in the waves generated
by the rebound shock `spicule'.

---------------------------------------------------------
Title: Transition region line-shifts in the rebound shock spicule
model
Authors: Hansteen, V. H.; Wikstøl, Ø.
1994SSRv...70..103H Altcode:
Spectral emission lines created in the solar chromosphere — corona
transition region show net red-shifts. It has been proposed that this
may be the result of the return of spicular material. We simulate
a spicule numerically using the rebound shock model and find that
the resulting hydrodynamic evolution leads to a perceived up-flow
in transition region spectral lines even though the average velocity
in the line forming region is directed downward. The explanation for
this apparent paradox is found in the correlation between density and
velocity in the waves generated by the rebound shock ‘spicule’.

---------------------------------------------------------
Title: Coupling of the coronal He abundance to the solar wind
Authors: Hansteen, V. H.; Leer, E.; Holzer, T. E.
1994SSRv...70..347H Altcode:
Models of the transition region — corona — solar wind system are
investigated in order to find the coronal helium abundance and to
study the role played by coronal helium in controlling the the solar
wind proton flux. The thermal force on α-particles in the transition
region sets the flow of helium into the corona. The frictional coupling
between α-particles and protons and/or the electric polarization field
determines the proton flux in the solar wind as well as the fate of
the coronal helium content.

---------------------------------------------------------
Title: Coupling of the Coronal Helium Abundance to the Solar Wind
Authors: Hansteen, Viggo H.; Leer, Egil; Holzer, Thomas E.
1994ApJ...428..843H Altcode:
Models of the transition region-corona-solar wind system are
investigated in order to find the coronal helium abundance and to
study the role played by coronal helium in controlling the solar wind
proton flux. The thermal force on alpha-particles in the transition
region sets the flow of helium into the corona. The frictional coupling
between alpha-particles and protons and/or the electric polarization
field determines the proton flux in the solar wind as well as the fate
of the coronal helium content. The models are constructed by solving
the time-dependent population and momentum equations for all species
of hydrogen and helium in an atmosphere with a given temperature
profile. Several temperature profiles are considered in order to very
the roles of frictional coupling and electric polarization field in the
solar wind, and the thermal force in the transition region. Steady-state
solutions are found for coronae with a hydrogen flux at 1 AU of 1.0
x 109/cm2/sec or larger. For coronae with
lower hydrogen fluxes, the helium flux into the corona is larger
than the flux 'pulled out' by the solar wind protons, and solutions
with increasing coronal helium content are found. The timescale for
forming a helium-filled corona, that may allow for a steady outflow,
is long compared to the mixing time for the corona.

---------------------------------------------------------
Title: Redshifted transition region lines explained
Authors: Hansteen, V.; Maltby, P.
1994AdSpR..14d..57H Altcode: 1994AdSpR..14...57H
The discovery of differential redshift between transition region and
chromospheric line emissions, both in the spectra of late-type stars
and in the solar spectra has been a puzzle for more than a decade. The
fact that the solar observations appear to be inconsistent with a
continuous downflow calls for another interpretation. We propose that
this pervasive redshift observed in transition region spectral lines
is caused by conduction modified MHD waves propagating along the
magnetic field lines from the corona towards the chromosphere. The
waves are assumed generated in the corona by nanoflares, i.e.by an
episodic heating mechanism. The calculations for acoustic waves show
line profiles with many of the same characteristics as the observations.

---------------------------------------------------------
Title: Observational signatures of acoustic wave propagation in the
solar transition region
Authors: Wikstøl, Ø.; Hansteen, V. H.
1994chdy.conf...91W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Wind Acceleration (Invited)
Authors: Hansteen, V. H.
1994scs..conf..453H Altcode: 1994IAUCo.144..453H
The general aspects of solar wind acceleration are well described
by considering the thermally driven outflow from an electron-proton
corona. However, two puzzling observations remain to be explained: 1)
The predicted asymptotic flow velocity is much lower than that observed
in high speed streams, and 2) The proton flux observed at 1 AU varies
considerably less than expected when considering the sensitivity of the
proton flux to the coronal temperature predicted by thermally driven
models. The solution of the first problem rests upon finding a mechanism
which can deposit energy and/or momentum beyond the critical point of
the flow. The invariance of the proton flux requires that a mechanism
for maintaining a relatively constant proton density scale height in
the subsonic region of the flow is found. One such possibility lies
in considering the effects of an enhanced coronal helium abundance on
the force balance of the subsonic flow. This scenario is discussed in
some depth.

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Title: A New Interpretation of the Redshift Observed in Optically
Thin Transition Region Lines
Authors: Hansteen, Viggo
1993ApJ...402..741H Altcode:
It is proposed that the pervasive redshift observed in transition region
spectral lines is caused by downward propagating acoustic waves. The
dynamic response of a coronal loop to energy released as heat near the
loop apex is examined by solving the hydrodynamic equations numerically,
consistently including the effects of nonequilibrium ionization on
the radiative losses and on the internal energy. It is found that
the radiative loss curve may change by a factor of 2 during the loop
evolution as a result of flows and waves. A simple analytical analysis
is performed to isolate the physical effects relevant to the line
formation process. The amplitude of the line shift is found to depend
on the characteristic time scale for ionization of the radiating ion as
well as the global loop parameters, the time scales for loop cooling,
the maximum temperature, and the base pressure.

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Title: Diffusion Effects on the Helium Abundance of the Solar
Transition Region and Corona
Authors: Hansteen, Viggo H.; Holzer, Thomas E.; Leer, Egil
1993ApJ...402..334H Altcode:
The diffusion of helium in the solar transition region is
studied by solving the mass and momentum conservation equations
for a hydrogen-helium plasma given a representative temperature
profile. Steady state solutions show that two distinct atmospheres
may result. In cases where the thermal force on alpha-particles is
balanced by the partial pressure gradient force, helium is the dominant
coronal species. On the other hand, if it is the frictional force
between protons and alpha-particles which balances the thermal force on
alpha-particles then hydrogen is the major coronal component. In order
to explore which of these solutions are attainable within reasonable
time scales, the time-dependent equations are solved, starting from
an initial state with a uniform helium abundance of 10 percent. The
atmosphere as a whole is close to hydrostatic equilibrium, but due the
thermal forces the individual elements are not. This force inbalance
leads to a differential flow between species. It is found that this
differential flow leads to a significant enhancement of the coronal
helium abundance. Even for the relatively shallow temperature gradient
used the helium abundance in the lower corona increases to 30 percent
over a 24 hr period.

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Title: On Redshifts in Stellar Ultraviolet Emission Lines
Authors: Hansteen, V.; Maltby, P.
1992ComAp..16..137H Altcode:
No abstract at ADS

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Title: Signatures of episodic coronal heating in solar transition
region spectral lines.
Authors: Hansteen, V.; Maltby, P.
1992mrpa.work..127H Altcode:
A brief introduction to the heating mechanisms in the solar atmosphere
and to the observed redshifts in transition region lines is given. The
authors discuss a new interpretation of the observed redshift and
present calculations that support the new explanation.

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Title: The Effect of Waves on Optically Thin Transition Region Lines
(With 2 Figures)
Authors: Hansteen, V.
1991mcch.conf..347H Altcode:
No abstract at ADS

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Title: Diffusion effects on the helium abundance of the solar
transition region and corona.
Authors: Hansteen, V.
1991cwlt.conf..171H Altcode:
The mass and momentum conservation equations for a hydrogen-helium
plasma are solved, given representative temperature profiles of the
transition region and corona. Steady state solutions show that two
distinct atmospheres may result depending on which term of the momentum
equation balances the thermal force. If the partial pressure gradient
dominates a helium filled corona occurs, while if the frictional
force is dominant hydrogen is the major coronal constituent. The
time-dependent problem is then solved, from an initially uniform
helium abundance corona, to explore which of these solutions are
attainable within reasonable timescales. The author has found that,
even for the relatively shallow temperature gradients given by the
observed emission measure, a significant enhancement of the coronal
helium abundance occurs.

---------------------------------------------------------
Title: The Effect of MHD-Waves on Transition Region Spectral Lines
Authors: Hansteen, V.; Maltby, P.
1990BAAS...22.1234H Altcode:
No abstract at ADS

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Title: An Ultraviolet Spectral Atlas of a Prominence
Authors: Engvold, Oddbjorn; Hansteen, V.; Kjeldseth-Moe, O.; Brueckner,
G. E.
1990Ap&SS.170..179E Altcode:
The ultraviolet spectrum of a large prominence has been observed with
the High Resolution Telescope and Spectrograph (HRTS) on SPACELAB 2,
5 August, 1985. The spectrum covers the wavelength range λλ1335 1670
Å and shows numerous emission lines from gas at chromospheric and
transition region temperatures. An Atlas of the prominence spectrum
has been made showing absolute calibrated intensities on an accurate
wavelength scale. The Atlas includes for comparison the corresponding
UV-spectrum from an average quiet solar region.

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Title: Propagation of magnetohydrodynamic (MHD) waves in the solar
transition region and corona.
Authors: Leer, E.; Hansteen, V.
1990ppst.conf...81L Altcode:
The propagation of MHD waves in the solar transition region and the
corona is discussed. It is shown that the non-compressive Alfvén mode
is the best candidate for energy transport into the corona and in the
solar wind.

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Title: The Prominence/corona Transition Region Analyzed from SL-2 HRTS
Authors: Engvold, O.; Hansteen, V.; Kjeldseth-Moe, O.; Brueckner, G. E.
1990LNP...363..250E Altcode: 1990IAUCo.117..250E; 1990doqp.coll..250E
The ultraviolet spectrum of a large prominence has been observed with
the High Resolution Telescope and Spectrograph (HRTS) on Spacelab 2
August 5, 1985. The spectrum covers the wavelength range 1335-1670Å and
shows numerous emission lines from gas at chromospheric and transition
region temperatures. A spectral atlas of these data is available. The
data reveals a variation with height of the line intensities. The
prominence becomes hotter with height. A value of ~ 0.12 dyn cm-2 for
the gas pressure in the prominence-corona transition region is obtained
from line ratios. The resolved fine structure of the He II 1640.400Å
line indicates that a major part of this emission comes from cold gas. A
broad Fe XI 1467.080Å suggests high velocities in the coronal cavity
region. The Fe XI line in the cavity region is a factor ~ 5 less bright
in the normal corona at the same height. Assuming that the temperature
is the same in the two regions the present obervations suggest that
the pressure in the cavity region is lower by a similar factor.

---------------------------------------------------------
Title: The effect of waves on optically thin transition region lines.
Authors: Hansteen, V.
1990ppst.conf..185H Altcode:
Numerical simulations of acoustic waves passing through plasma at
transition region temperature and density are performed. The resultant
effects on optically thin resonance line profiles are computed. The
ionization balance of the emitting ion is solved consistently with
the hydrodynamic equations. The results of the calculations show that
acoustic waves can produce average line shifts on the order of the
amplitude of the wave velocity.

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Title: The ionization and energy balance of the chromosphere-corona
transition region.
Authors: Hansteen, V.
1989ftsa.conf...35H Altcode:
The hypothesis of heating the lower transition region by thermal
conduction from the corona has been shown to contradict the
observations. Emission measure analysis implies that heating and
radiative losses are nearly in balance for the lower transition region,
explaining this balance may require time dependent modeling. The
geometry of the transition region is not known, though theoretical
and observational arguments imply that it is fine structured, and
further that not all gas at transition region temperatures is in
thermal contact with the corona. Observations show that flows are
prevalent throughout the transition region. This is important to
the energy balance through the enthalpy flux. Furthermore, flows
will impact the ionization balance and therefore the energy balance
through the radiative loss rate. The ionization balance should also
be considered in emission line diagnostics when flows are present,
the author has shown the contribution function for emission lines to
be sensitive to variations in the ionization balance.

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Title: The limb effect of the K i resonance line, 769.9 n m
Authors: Andersen, B. N.; Barth, S.; Hansteen, V.; Leifsen, T.; Lilje,
P. B.; Vikanes, F.
1985SoPh...99...17A Altcode:
Low-noise observations have been obtained to search for a possible limb
effect in the K I 769.9 nm resonance line. The observations were carried
out along the north/south diameter of the solar disc. The data were
individually corrected for the effects of straylight on the velocity
measurements. A small, but significant limb effect is detected. The
total shift in the line core from center to limb corresponds to 125
m s−1 with an uncertainty of &lt; 30 m s−1.

---------------------------------------------------------
Title: Zwei Schreiben des Herrn Professors Hansteen an den Herausgeber
Authors: Hansteen
1848AN.....27..161H Altcode:
No abstract at ADS