Author name code: ugarte-urra
ADS astronomy entries on 2022-09-14
author:Ugarte-Urra, I.
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Title: Properties of EUV Imaging Spectrometer (EIS) Slot Observations
Authors: Young, Peter R.; Ugarte-Urra, Ignacio
Bibcode: 2022SoPh..297...87Y
Altcode: 2022arXiv220314161Y
The Extreme ultraviolet Imaging Spectrometer (EIS) on board the Hinode
spacecraft has been operating since 2006, returning high-resolution data
in the 170 - 212 and 246 - 292 Å wavelength regions. EIS has four slit
options, with the narrow 1'' and 2'' slits used
for spectroscopy and the wide 40'' and 266''
slits used for monochromatic imaging. In this article several
properties of the 40'' slit (or slot) are measured using
the Fe XII 195.12 Å line, which is formed at 1.5 MK. The projected
width of the slot on the detector shows a small variation along the
slit with an average value of 40.949''. The slot image is
tilted on the detector and a quadratic formula is provided to describe
the tilt. The tilt corresponds to four pixels on the detector and the
slot centroid is offset mostly to the right (longer wavelengths) of the
1'' slit by up to four pixels. Measurement of the intensity
decrease at the edge of the slot leads to an estimate of the spatial
resolution of the images in the x -direction. The resolution varies
quadratically along the slot, with a minimum value of 2.9''
close to the detector center. Intensities measured from the slot
images are found to be on average 14% higher than those measured
from the 1'' slit at the same spatial location. Background
subtraction is necessary to derive accurate intensities in quiet-Sun
and coronal-hole regions. Prescriptions for deriving accurate slot
intensities for different types of slot datasets are presented.
Title: Geometric Assumptions in Hydrodynamic Modeling of Coronal
and Flaring Loops
Authors: Reep, Jeffrey W.; Ugarte-Urra, Ignacio; Warren, Harry P.;
Barnes, Will T.
Bibcode: 2022ApJ...933..106R
Altcode: 2022arXiv220304385R
In coronal loop modeling, it is commonly assumed that the loops
are semicircular with a uniform cross-sectional area. However,
observed loops are rarely semicircular, and extrapolations of the
magnetic field show that the field strength decreases with height,
implying that the cross-sectional area expands with height. We examine
these two assumptions directly, to understand how they affect the
hydrodynamic and radiative response of short, hot loops to strong,
impulsive electron beam heating events. Both the magnitude and rate
of area expansion impact the dynamics directly, and an expanding cross
section significantly lengthens the time for a loop to cool and drain,
increases upflow durations, and suppresses sound waves. The standard
T ~ n 2 relation for radiative cooling does not hold
with expanding loops, which cool with relatively little draining. An
increase in the eccentricity of loops, on the other hand, only increases
the draining timescale, and is a minor effect in general. Spectral
line intensities are also strongly impacted by the variation in the
cross-sectional area because they depend on both the volume of the
emitting region as well as the density and ionization state. With a
larger expansion, the density is reduced, so the lines at all heights
are relatively reduced in intensity, and because of the increase of
cooling times, the hottest lines remain bright for significantly
longer. Area expansion is critical to accurate modeling of the
hydrodynamics and radiation, and observations are needed to constrain
the magnitude, rate, and location of the expansion-or lack thereof.
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
Bibcode: 2022ApJ...926...53C
Altcode: 2021arXiv210615591C
Current state-of-the-art spectrographs cannot resolve the fundamental
spatial (subarcseconds) and temporal (less than a few tens of
seconds) scales of the coronal dynamics of solar flares and eruptive
phenomena. The highest-resolution coronal data to date are based on
imaging, which is blind to many of the processes that drive coronal
energetics and dynamics. As shown by the Interface Region Imaging
Spectrograph for the low solar atmosphere, we need high-resolution
spectroscopic measurements with simultaneous imaging to understand the
dominant processes. In this paper: (1) we introduce the Multi-slit Solar
Explorer (MUSE), a spaceborne observatory to fill this observational
gap by providing high-cadence (<20 s), subarcsecond-resolution
spectroscopic rasters over an active region size of the solar transition
region and corona; (2) using advanced numerical models, we demonstrate
the unique diagnostic capabilities of MUSE for exploring solar coronal
dynamics and for constraining and discriminating models of solar flares
and eruptions; (3) we discuss the key contributions MUSE would make
in addressing the science objectives of the Next Generation Solar
Physics Mission (NGSPM), and how MUSE, the high-throughput Extreme
Ultraviolet Solar Telescope, and the Daniel K Inouye Solar Telescope
(and other ground-based observatories) can operate as a distributed
implementation of the NGSPM. This is a companion paper to De Pontieu
et al., which focuses on investigating coronal heating with MUSE.
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
Bibcode: 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.
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
Bibcode: 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
(<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.
Title: Energetics and 3D Structure of Elementary Events in Solar
Coronal Heating
Authors: Einaudi, G.; Dahlburg, R. B.; Ugarte-Urra, I.; Reep, J. W.;
Rappazzo, A. F.; Velli, M.
Bibcode: 2021ApJ...910...84E
Altcode: 2021arXiv210313499E
Parker first proposed (1972) that coronal heating was the necessary
outcome of an energy flux caused by the tangling of coronal magnetic
field lines by photospheric flows. In this paper we discuss how
this model has been modified by subsequent numerical simulations
outlining in particular the substantial differences between the
"nanoflares" introduced by Parker and "elementary events," defined
here as small-scale spatially and temporally isolated heating
events resulting from the continuous formation and dissipation
of field-aligned current sheets within a coronal loop. We present
numerical simulations of the compressible 3D MHD equations using the
HYPERION code. We use two clustering algorithms to investigate the
properties of the simulated elementary events: an IDL implementation of
a density-based spatial clustering of applications with noise technique,
and our own physical distance clustering algorithm. We identify and
track elementary heating events in time, both in temperature and in
Joule heating space. For every event we characterize properties such
as density, temperature, volume, aspect ratio, length, thickness,
duration, and energy. The energies of the events are in the range
of 1018-1021 erg, with durations shorter
than 100 s. A few events last up to 200 s and release energies
up to 1023 erg. While high temperatures are typically
located at the flux tube apex, the currents extend all the way to
the footpoints. Hence, a single elementary event cannot at present
be detected. The observed emission is due to the superposition of
many elementary events distributed randomly in space and time within
the loop.
Title: Analysis of a long-duration AR throughout five solar rotations:
Magnetic properties and ejective events
Authors: Iglesias, Francisco; Mandrini, Cristina H.; Lopez Fuentes,
Marcelo; Cremades, Hebe; Lopez, Fernando M.; Merenda, Luciano A.;
Ugarte-Urra, Ignacio
Bibcode: 2021cosp...43E1757I
Altcode:
Coronal mass ejections (CMEs), which are among the most magnificent
solar eruptions, are a major driver of space weather and can thus affect
diverse human technologies. Different processes have been proposed to
explain the initiation and release of CMEs from solar active regions
(ARs), without reaching consensus on which is the predominant scenario,
and thus rendering impossible to accurately predict when a CME is going
to erupt from a given AR. To investigate AR magnetic properties that
favor CMEs production, we employ multi-spacecraft data to analyze a
long duration AR (NOAA 11089, 11100, 11106, 11112 and 11121) throughout
its complete lifetime, spanning five Carrington rotations from July
to November 2010. We use data from the Solar Dynamics Observatory
to study the evolution of the AR magnetic properties during the five
near-side passages, and a proxy to follow the magnetic flux changes
when no magnetograms are available, i.e. during far-side transits. The
ejectivity is studied by characterizing the angular widths, speeds
and masses of 108 CMEs that we associated to the AR, when examining
a 124-day period. Such an ejectivity tracking was possible thanks
to the multi-viewpoint images provided by the Solar-Terrestrial
Relations Observatory and Solar and Heliospheric Observatory in a
quasi-quadrature configuration. We also inspected the X-ray flares
registered by the GOES satellite and found 162 to be associated to
the AR under study. Given the substantial number of ejections studied,
we use a statistical approach instead of a single-event analysis. We
found three well defined periods of very high CMEs activity and two
periods with no mass ejections that are preceded or accompanied by
characteristic changes in the AR magnetic flux, free magnetic energy
and/or presence of electric currents. Our large sample of CMEs and
long term study of a single AR, provide further evidence relating AR
magnetic activity to CME and Flare production.
Title: Observation and Modeling of High-temperature Solar Active
Region Emission during the High-resolution Coronal Imager Flight of
2018 May 29
Authors: Warren, Harry P.; Reep, Jeffrey W.; Crump, Nicholas A.;
Ugarte-Urra, Ignacio; Brooks, David H.; Winebarger, Amy R.; Savage,
Sabrina; De Pontieu, Bart; Peter, Hardi; Cirtain, Jonathan W.; Golub,
Leon; Kobayashi, Ken; McKenzie, David; Morton, Richard; Rachmeler,
Laurel; Testa, Paola; Tiwari, Sanjiv; Walsh, Robert
Bibcode: 2020ApJ...896...51W
Altcode:
Excellent coordinated observations of NOAA active region 12712 were
obtained during the flight of the High-resolution Coronal Imager (Hi-C)
sounding rocket on 2018 May 29. This region displayed a typical active
region core structure with relatively short, high-temperature loops
crossing the polarity inversion line and bright "moss" located at the
footpoints of these loops. The differential emission measure (DEM) in
the active region core is very sharply peaked at about 4 MK. Further,
there is little evidence for impulsive heating events in the moss, even
at the high spatial resolution and cadence of Hi-C. This suggests that
active region core heating is occurring at a high frequency and keeping
the loops close to equilibrium. To create a time-dependent simulation of
the active region core, we combine nonlinear force-free extrapolations
of the measured magnetic field with a heating rate that is dependent
on the field strength and loop length and has a Poisson waiting time
distribution. We use the approximate solutions to the hydrodynamic
loop equations to simulate the full ensemble of active region core
loops for a range of heating parameters. In all cases, we find that
high-frequency heating provides the best match to the observed DEM. For
selected field lines, we solve the full hydrodynamic loop equations,
including radiative transfer in the chromosphere, to simulate transition
region and chromospheric emission. We find that for heating scenarios
consistent with the DEM, classical signatures of energy release,
such as transition region brightenings and chromospheric evaporation,
are weak, suggesting that they would be difficult to detect.
Title: Solar physics in the 2020s: DKIST, parker solar probe, and
solar orbiter as a multi-messenger constellation
Authors: Martinez Pillet, V.; Tritschler, A.; Harra, L.; Andretta, V.;
Vourlidas, A.; Raouafi, N.; Alterman, B. L.; Bellot Rubio, L.; Cauzzi,
G.; Cranmer, S. R.; Gibson, S.; Habbal, S.; Ko, Y. K.; Lepri, S. T.;
Linker, J.; Malaspina, D. M.; Matthews, S.; Parenti, S.; Petrie, G.;
Spadaro, D.; Ugarte-Urra, I.; Warren, H.; Winslow, R.
Bibcode: 2020arXiv200408632M
Altcode:
The National Science Foundation (NSF) Daniel K. Inouye Solar Telescope
(DKIST) is about to start operations at the summit of Haleakala
(Hawaii). DKIST will join the early science phases of the NASA
and ESA Parker Solar Probe and Solar Orbiter encounter missions. By
combining in-situ measurements of the near-sun plasma environment and
detail remote observations of multiple layers of the Sun, the three
observatories form an unprecedented multi-messenger constellation to
study the magnetic connectivity inside the solar system. This white
paper outlines the synergistic science that this multi-messenger
suite enables.
Title: Analysis of a long-duration AR throughout five solar rotations:
Magnetic properties and ejective events
Authors: Iglesias, Francisco A.; Cremades, Hebe; Merenda, Luciano A.;
Mandrini, Cristina H.; López, Fernando M.; López Fuentes, Marcelo
C.; Ugarte-Urra, Ignacio
Bibcode: 2020AdSpR..65.1641I
Altcode: 2019arXiv191101265I
Coronal mass ejections (CMEs), which are among the most magnificent
solar eruptions, are a major driver of space weather and can thus affect
diverse human technologies. Different processes have been proposed to
explain the initiation and release of CMEs from solar active regions
(ARs), without reaching consensus on which is the predominant scenario,
and thus rendering impossible to accurately predict when a CME is going
to erupt from a given AR. To investigate AR magnetic properties that
favor CMEs production, we employ multi-spacecraft data to analyze a
long duration AR (NOAA 11089, 11100, 11106, 11112 and 11121) throughout
its complete lifetime, spanning five Carrington rotations from July
to November 2010. We use data from the Solar Dynamics Observatory
to study the evolution of the AR magnetic properties during the five
near-side passages, and a proxy to follow the magnetic flux changes
when no magnetograms are available, i.e. during far-side transits. The
ejectivity is studied by characterizing the angular widths, speeds
and masses of 108 CMEs that we associated to the AR, when examining
a 124-day period. Such an ejectivity tracking was possible thanks
to the multi-viewpoint images provided by the Solar-Terrestrial
Relations Observatory and Solar and Heliospheric Observatory in a
quasi-quadrature configuration. We also inspected the X-ray flares
registered by the GOES satellite and found 162 to be associated to
the AR under study. Given the substantial number of ejections studied,
we use a statistical approach instead of a single-event analysis. We
found three well defined periods of very high CMEs activity and two
periods with no mass ejections that are preceded or accompanied by
characteristic changes in the AR magnetic flux, free magnetic energy
and/or presence of electric currents. Our large sample of CMEs and
long term study of a single AR, provide further evidence relating AR
magnetic activity to CME and Flare production.
Title: Further Evidence for Looplike Fine Structure inside
“Unipolar” Active Region Plages
Authors: Wang, Y. -M.; Ugarte-Urra, I.; Reep, J. W.
Bibcode: 2019ApJ...885...34W
Altcode: 2021arXiv210406633W
Earlier studies using extreme-ultraviolet images and line-of-sight
magnetograms from the Solar Dynamics Observatory (SDO) have suggested
that active region (AR) plages and strong network concentrations
often have small, looplike features embedded within them, even
though no minority-polarity flux is visible in the corresponding
magnetograms. Because of the unexpected nature of these findings, we
have searched the SDO database for examples of inverted-Y structures
rooted inside “unipolar” plages, with such jetlike structures
being interpreted as evidence for magnetic reconnection between small
bipoles and the dominant-polarity field. Several illustrative cases are
presented from the period of 2013-2015, all of which are associated with
transient outflows from AR “moss.” The triangular or dome-shaped
bases have horizontal dimensions of ∼2-4 Mm, corresponding to ∼1-3
granular diameters. We also note that the spongy-textured Fe IX 17.1 nm
moss is not confined to plages, but may extend into regions where the
photospheric field is relatively weak or even has mixed polarity. We
again find a tendency for bright coronal loops seen in the 17.1,
19.3, and 21.1 nm passbands to show looplike fine structure and
compact brightenings at their footpoints. These observations provide
further confirmation that present-day magnetograms are significantly
underrepresenting the amount of minority-polarity flux inside AR plages
and again suggest that footpoint reconnection and small-scale flux
cancellation may play a major role in coronal heating, both inside
and outside ARs.
Title: Achievements of Hinode in the first eleven years
Authors: Hinode Review Team; Al-Janabi, Khalid; Antolin, Patrick;
Baker, Deborah; Bellot Rubio, Luis R.; Bradley, Louisa; Brooks,
David H.; Centeno, Rebecca; Culhane, J. Leonard; Del Zanna, Giulio;
Doschek, George A.; Fletcher, Lyndsay; Hara, Hirohisa; Harra,
Louise K.; Hillier, Andrew S.; Imada, Shinsuke; Klimchuk, James A.;
Mariska, John T.; Pereira, Tiago M. D.; Reeves, Katharine K.; Sakao,
Taro; Sakurai, Takashi; Shimizu, Toshifumi; Shimojo, Masumi; Shiota,
Daikou; Solanki, Sami K.; Sterling, Alphonse C.; Su, Yingna; Suematsu,
Yoshinori; Tarbell, Theodore D.; Tiwari, Sanjiv K.; Toriumi, Shin;
Ugarte-Urra, Ignacio; Warren, Harry P.; Watanabe, Tetsuya; Young,
Peter R.
Bibcode: 2019PASJ...71R...1H
Altcode:
Hinode is Japan's third solar mission following Hinotori (1981-1982)
and Yohkoh (1991-2001): it was launched on 2006 September 22 and is in
operation currently. Hinode carries three instruments: the Solar Optical
Telescope, the X-Ray Telescope, and the EUV Imaging Spectrometer. These
instruments were built under international collaboration with the
National Aeronautics and Space Administration and the UK Science and
Technology Facilities Council, and its operation has been contributed
to by the European Space Agency and the Norwegian Space Center. After
describing the satellite operations and giving a performance evaluation
of the three instruments, reviews are presented on major scientific
discoveries by Hinode in the first eleven years (one solar cycle long)
of its operation. This review article concludes with future prospects
for solar physics research based on the achievements of Hinode.
Title: The Magnetic Properties of Heating Events on High-temperature
Active-region Loops
Authors: Ugarte-Urra, Ignacio; Crump, Nicholas A.; Warren, Harry P.;
Wiegelmann, Thomas
Bibcode: 2019ApJ...877..129U
Altcode: 2019arXiv190411976U
Understanding the relationship between the magnetic field and coronal
heating is one of the central problems of solar physics. However,
studies of the magnetic properties of impulsively heated loops have
been rare. We present results from a study of 34 evolving coronal loops
observed in the Fe XVIII line component of 94 Å filter images obtained
by the Atmospheric Imaging Assembly (AIA)/Solar Dynamics Observatory
(SDO) from three active regions with different magnetic conditions. We
show that the peak intensity per unit cross section of the loops depends
on their individual magnetic and geometric properties. The intensity
scales proportionally to the average field strength along the loop (B
avg) and inversely with the loop length (L) for a combined
dependence of {({B}avg}/L)}0.52+/- 0.13. These
loop properties are inferred from magnetic extrapolations of the
photospheric Helioseismic and Magnetic Imager (HMI)/SDO line-of-sight
and vector magnetic field in three approximations: potential and two
nonlinear force-free (NLFF) methods. Through hydrodynamic modeling
(enthalpy-based thermal evolution loop (EBTEL) model) we show that
this behavior is compatible with impulsively heated loops with a
volumetric heating rate that scales as {ɛ }{{H}}∼
{B}avg}0.3+/- 0.2/{L}0.2{+/-
0.10.2}.
Title: Advancing the Advective Flux Transport Model
Authors: Upton, Lisa; Ugarte-Urra, Ignacio; Warren, Harry
Bibcode: 2019AAS...23411802U
Altcode:
The Advective Flux Transport (AFT) model has proven to be a reliable
surface flux transport model for describing the evolution of the global
magnetic field, accurately reproducing the evolution of the polar
field. AFT has also been shown to accurately (within a factor of 2)
reproduce the evolution of the total unsigned flux of simple active
regions over the course of their lifetimes. Here we will discuss the
work being done to validate and advance the AFT model. We will discuss
the ability of AFT to reproduce other active region properties, such
as tilt angles, polarity separation, area expansion and magnetic
elements size distribution, for simple and more complex active
regions. Currently, AFT uses data assimilation to incorporate the
magnetic field from magnetograms from the Earth's vantage point. We will
also discuss the work that is being done to develop an automated process
for adding in far-side active regions observed by STEREO in 304 Å.
Title: Dependence of Coronal Loop Temperature on Loop Length and
Magnetic Field Strength
Authors: Dahlburg, R. B.; Einaudi, G.; Ugarte-Urra, I.; Rappazzo,
A. F.; Velli, M.
Bibcode: 2018ApJ...868..116D
Altcode:
The temperature characteristics of solar coronal loops over a wide
range of lengths and magnetic field strengths are investigated by
means of numerical simulations. A very high correlation between
magnetic field strength (B 0) and maximum temperature
(T max) is found. Shorter loops rooted at stronger fields
are those that reach higher maximum temperatures. High temperatures
constitute a small part of the loop volume. For loops of equal length,
those with stronger magnetic fields have broader emission measure
distributions. The conditions underlying the variety of loops observed
in the solar corona are discussed, an explanation of why both cold
and hot loops exist is provided, and suggestions are given as to
what observations need to be made to confirm the results. Data in
the analysis are provided by numerical simulations using HYPERION,
an explicit massively parallel Fourier collocation-finite-difference
code. In the simulations footpoints are convected with a randomized
large-scale flow. This produces a Poynting flux which leads to the
buildup of magnetic energy in the loop. The magnetic energy is then
transformed into thermal energy by a magnetic reconnection process
occurring within current sheets formed locally by an energy cascade
toward small scales.
Title: Toward a Quantitative Comparison of Magnetic Field
Extrapolations and Observed Coronal Loops
Authors: Warren, Harry P.; Crump, Nicholas A.; Ugarte-Urra, Ignacio;
Sun, Xudong; Aschwanden, Markus J.; Wiegelmann, Thomas
Bibcode: 2018ApJ...860...46W
Altcode: 2018arXiv180500281W
It is widely believed that loops observed in the solar atmosphere
trace out magnetic field lines. However, the degree to which magnetic
field extrapolations yield field lines that actually do follow loops
has yet to be studied systematically. In this paper, we apply three
different extrapolation techniques—a simple potential model, a
nonlinear force-free (NLFF) model based on photospheric vector data,
and an NLFF model based on forward fitting magnetic sources with
vertical currents—to 15 active regions that span a wide range of
magnetic conditions. We use a distance metric to assess how well each
of these models is able to match field lines to the 12202 loops traced
in coronal images. These distances are typically 1″-2″. We also
compute the misalignment angle between each traced loop and the local
magnetic field vector, and find values of 5°-12°. We find that the
NLFF models generally outperform the potential extrapolation on these
metrics, although the differences between the different extrapolations
are relatively small. The methodology that we employ for this study
suggests a number of ways that both the extrapolations and loop
identification can be improved.
Title: The Magnetic Properties of High-Temperature Active Region Loops
Authors: Ugarte-Urra, Ignacio; Crump, Nicholas A.; Warren, Harry
Bibcode: 2018tess.conf22206U
Altcode:
Understanding the relationship between the magnetic field and coronal
heating is one of the central problems of solar physics. However,
studies of the magnetic properties of impulsively heated loops have
been rare. We present results from a study of 34 coronal loops observed
in the in the Fe XVIII line component of AIA/SDO 94 Å filter images
from three active regions with different magnetic conditions. We
show that the peak radiance per unit volume of the Fe XVIII loops is
correlated to their individual magnetic and geometric properties, namely
field strength (B) and length (L). These are inferred from magnetic
extrapolations of the photospheric field, in three approximations
(potential and two NLFF methods), thus providing an uncertainty in
our estimate of those quantities. Our results provide support, for
the first time at the scale of individual loops, to the B/L scaling
in the heating that has been successful in modeling full active regions.
Title: Spectroscopic Observations of Current Sheet Formation and
Evolution
Authors: Warren, Harry; Brooks, David; Ugarte-Urra, Ignacio; Crump,
Nicholas A.; Doschek, George A.; Stenborg, Guillermo; Reep, Jeffrey W.
Bibcode: 2018tess.conf31904W
Altcode:
<span class="s1" We report on the structure and evolution of
a current sheet that formed in the wake of an eruptive X8.3 flare
observed at the west limb of the Sun on September 10, 2017. Using
observations from the Hinode/EIS and SDO/AIA, we find that plasma
in the current sheet reaches temperatures of about 20MK and that the
range of temperatures is relatively narrow. The highest temperatures
occur at the base of the current sheet, in the region near the top
of the post-flare loop arcade. The broadest high temperature line
profiles, in contrast, occur at the largest observed heights. Further,
line broadening is strong very early in the flare and diminishes over
time. The current sheet can be observed in the AIA 211 and 171 channels,
which have a considerable contribution from thermal bremsstrahlung
at flare temperatures. Comparisons of the emission measure in these
channels with other EIS wavelengths and AIA channels dominated by
Fe line emission indicate a coronal composition and suggest that
the current sheet is formed by the heating of plasma already in the
corona. Finally, we also investigate the structure in the current sheet
as imaged by AIA and find clear evidence for collapsing loops. Taken
together, these observations suggest that some flare heating occurs
in the current sheet while additional energy is released as newly
reconnected field lines relax and become more dipolar.
Title: Spectroscopic Observations of Current Sheet Formation and
Evolution
Authors: Warren, Harry P.; Brooks, David H.; Ugarte-Urra, Ignacio;
Reep, Jeffrey W.; Crump, Nicholas A.; Doschek, George A.
Bibcode: 2018ApJ...854..122W
Altcode: 2017arXiv171110826W
We report on the structure and evolution of a current sheet that formed
in the wake of an eruptive X8.3 flare observed at the west limb of
the Sun on 2017 September 10. Using observations from the EUV Imaging
Spectrometer (EIS) on Hinode and the Atmospheric Imaging Assembly
(AIA) on the Solar Dynamics Observatory, we find that plasma in the
current sheet reaches temperatures of about 20 MK and that the range
of temperatures is relatively narrow. The highest temperatures occur
at the base of the current sheet, in the region near the top of the
post-flare loop arcade. The broadest high temperature line profiles,
in contrast, occur at the largest observed heights. Furthermore,
line broadening is strong very early in the flare and diminishes over
time. The current sheet can be observed in the AIA 211 and 171 channels,
which have a considerable contribution from thermal bremsstrahlung
at flare temperatures. Comparisons of the emission measure in these
channels with other EIS wavelengths and AIA channels dominated by
Fe line emission indicate a coronal composition and suggest that
the current sheet is formed by the heating of plasma already in the
corona. Taken together, these observations suggest that some flare
heating occurs in the current sheet, while additional energy is released
as newly reconnected field lines relax and become more dipolar.
Title: Signatures Of Coronal Heating Driven By Footpoint Shuffling:
Closed and Open Structures.
Authors: Velli, M. C. M.; Rappazzo, A. F.; Dahlburg, R. B.; Einaudi,
G.; Ugarte-Urra, I.
Bibcode: 2017AGUFMSH41D..01V
Altcode:
We have previously described the characteristic state of the confined
coronal magnetic field as a special case of magnetically dominated
magnetohydrodynamic (MHD) turbulence, where the free energy in
the transverse magnetic field is continuously cascaded to small
scales, even though the overall kinetic energy is small. This coronal
turbulence problem is defined by the photospheric boundary conditions:
here we discuss recent numerical simulations of the fully compressible
3D MHD equations using the HYPERION code. Loops are forced at their
footpoints by random photospheric motions, energizing the field to
a state with continuous formation and dissipation of field-aligned
current sheets: energy is deposited at small scales where heating
occurs. Only a fraction of the coronal mass and volume gets heated
at any time. Temperature and density are highly structured at scales
that, in the solar corona, remain observationally unresolved: the
plasma of simulated loops is multithermal, where highly dynamical
hotter and cooler plasma strands are scattered throughout the loop at
sub-observational scales. We will also compare Reduced MHD simulations
with fully compressible simulations and photospheric forcings with
different time-scales compared to the Alfv'en transit time. Finally,
we will discuss the differences between the closed field and open field
(solar wind) turbulence heating problem, leading to observational
consequences that may be amenable to Parker Solar Probe and Solar
Orbiter.
Title: Modeling Coronal Response in Decaying Active Regions with
Magnetic Flux Transport and Steady Heating
Authors: Ugarte-Urra, Ignacio; Warren, Harry P.; Upton, Lisa A.;
Young, Peter R.
Bibcode: 2017ApJ...846..165U
Altcode: 2017arXiv170804324U
We present new measurements of the dependence of the extreme ultraviolet
(EUV) radiance on the total magnetic flux in active regions as obtained
from the Atmospheric Imaging Assembly (AIA) and the Helioseismic
and Magnetic Imager on board the Solar Dynamics Observatory. Using
observations of nine active regions tracked along different stages of
evolution, we extend the known radiance—magnetic flux power-law
relationship (I\propto {{{Φ }}}α ) to the AIA 335
Å passband, and the Fe xviii 93.93 Å spectral line in the 94 Å
passband. We find that the total unsigned magnetic flux divided by the
polarity separation ({{Φ }}/D) is a better indicator of radiance for
the Fe xviii line with a slope of α =3.22+/- 0.03. We then use these
results to test our current understanding of magnetic flux evolution
and coronal heating. We use magnetograms from the simulated decay of
these active regions produced by the Advective Flux Transport model
as boundary conditions for potential extrapolations of the magnetic
field in the corona. We then model the hydrodynamics of each individual
field line with the Enthalpy-based Thermal Evolution of Loops model with
steady heating scaled as the ratio of the average field strength and the
length (\bar{B}/L) and render the Fe xviii and 335 Å emission. We find
that steady heating is able to partially reproduce the magnitudes and
slopes of the EUV radiance—magnetic flux relationships and discuss
how impulsive heating can help reconcile the discrepancies. This
study demonstrates that combined models of magnetic flux transport,
magnetic topology, and heating can yield realistic estimates for the
decay of active region radiances with time.
Title: Modeling Active Region Evolution - at the Sun’s Surface
and into the Corona
Authors: Upton, Lisa; Ugarte-Urra, Ignacio; Warren, Harry; Young,
Peter R.
Bibcode: 2017SPD....4840502U
Altcode:
The STEREO mission provides the first opportunity to track the long-term
evolution of Active Regions over multiple rotations. The Advective Flux
Transport (AFT) model is a state of the art Surface Flux Transport
model, which simulates the observed near-surface flows to model
the transport of magnetic flux over the entire Sun. Combining STEREO
observations with AFT has allowed us to characterize the flux-luminosity
relationship for He 304 Å and to validate the far-side evolution of
individual active regions produced with AFT. Here, we present recent
results in which we extend this radiance - magnetic flux power-law
relationship to the AIA 335 Å passband, and the Fe XVIII 93.93 Å
spectral line in the 94 Å passband. We use these results to test
our current understanding of magnetic flux evolution and coronal
heating by modeling the hydrodynamics of individual field lines with
the Enthalpy-based Thermal Evolution of Loops (EBTEL) model including
steady heating scaled as the ratio of the average field strength and
the length (B/L). We find that steady heating is able to partially
reproduce the EUV radiance - magnetic flux relationships and their
observed temporal evolution. We also discuss how time-dependent
heating may be able to explain the remaining discrepancies. This
study demonstrates that combined models of magnetic flux transport,
magnetic topology and heating can yield realistic estimates for the
decay of active region radiances with time.
Title: A study of the long term evolution in active region upflows
Authors: Harra, Louise K.; Ugarte-Urra, Ignacio; De Rosa, Marc;
Mandrini, Cristina; van Driel-Gesztelyi, Lidia; Baker, Deborah;
Culhane, J. Leonard; Démoulin, Pascal
Bibcode: 2017PASJ...69...47H
Altcode:
Since their discovery, upflows at the edges of active regions have
attracted a lot of interest, primarily as they could potentially
contribute to the slow solar wind. One aspect that has not been studied
yet is how the long term evolution of active regions impacts the
upflows. In this work, we analyze one active region that survives three
solar rotations. We track how the flows change with time. We use local
and global modeling of the decaying active region to determine how the
age of the active region will impact the extent of the open magnetic
fields, and then how some of the upflows could become outflows. We
finish with a discussion of how these results, set in a broader context,
can be further developed with the Solar Orbiter mission.
Title: Advancing our Understanding of Active Region Evolution and
Surface Flux Transport Using Far Side Imaging from STEREO 304
Authors: Upton, L.; Ugarte-Urra, I.; Warren, H. P.; Hathaway, D. H.
Bibcode: 2016AGUFMSH42B..02U
Altcode:
The STEREO mission, combined with SDO, provides a unique opportunity
to view the solar surface continuously. These continuous observations
provide the first opportunity to track the long-term evolution of Active
Regions over multiple rotations. We present recent results in which we
illustrate how He 304 Å images can be used as a proxies for magnetic
flux measurements. We will present the long-term evolution of select
isolated Active Regions as seen in He 304 Å. These data are then
used to validate the far-side evolution of individual active regions
produced with our Advective Flux Transport model - AFT. The AFT model
is a state of the art Surface Flux Transport model, which simulates
the observed near-surface flows (including an evolving convective flow
velocity field) to model the transport of magnetic flux over the entire
Sun. Finally, we will show that when new flux emergence occurs on the
far-side of the Sun, 304 Å images can provide sufficient information
about the active region to predict its evolution. These far-side Active
Regions have a substantial impact on the coronal and interplanetary
field configuration used for space weather predictions.
Title: Observational Signatures of Coronal Heating
Authors: Dahlburg, R. B.; Einaudi, G.; Ugarte-Urra, I.; Warren, H. P.;
Rappazzo, A. F.; Velli, M.; Taylor, B.
Bibcode: 2016AGUFMSH42A..06D
Altcode:
Recent research on observational signatures of turbulent heating of
a coronal loop will be discussed. The evolution of the loop is is
studied by means of numericalsimulations of the fully compressible
three-dimensionalmagnetohydrodynamic equations using the HYPERION
code. HYPERION calculates the full energy cycle involving footpoint
convection, magnetic reconnection,nonlinear thermal conduction and
optically thin radiation.The footpoints of the loop magnetic field
are convected by random photospheric motions. As a consequence
the magnetic field in the loop is energized and develops turbulent
nonlinear dynamics characterized by the continuous formation and
dissipation of field-aligned current sheets: energy is deposited
at small scales where heating occurs. Dissipation is non-uniformly
distributed so that only a fraction of thecoronal mass and volume gets
heated at any time. Temperature and density are highly structured at
scales which, in the solar corona, remain observationally unresolved:
the plasma of the simulated loop is multi-thermal, where highly
dynamical hotter and cooler plasma strands arescattered throughout
the loop at sub-observational scales. Typical simulated coronal loops
are 50000 km length and have axial magnetic field intensities ranging
from 0.01 to 0.04 Tesla.To connect these simulations to observations
the computed numberdensities and temperatures are used to synthesize
the intensities expected inemission lines typically observed with
the Extreme ultraviolet Imaging Spectrometer(EIS) on Hinode. These
intensities are then employed to compute differentialemission measure
distributions, which are found to be very similar to those derivedfrom
observations of solar active regions.
Title: Using STEREO/SECCHI EUV Far Side Solar Images as Proxies of
Magnetic Fields
Authors: Ugarte-Urra, I.
Bibcode: 2016AGUFMSH43A2552U
Altcode:
In a recent study we showed that 304 A images can be used as a proxy of
the total magnetic flux when magnetic field data is not available. We
took advantage of our ability to obtain 360 degree EUV maps of the Sun,
resulting from a combination of the multiple perspectives of SDO/AIA and
STEREO EUVI A and B images, and tracked active regions over long periods
of time that we then compared to a magnetic flux transport model. Here
we simplify the scenario to show how to use single perspective far
side 304 images to extract magnetic field information of active regions.
Title: Correlation of Coronal Plasma Properties and Solar Magnetic
Field in a Decaying Active Region
Authors: Ko, Yuan-Kuen; Young, Peter R.; Muglach, Karin; Warren,
Harry P.; Ugarte-Urra, Ignacio
Bibcode: 2016ApJ...826..126K
Altcode:
We present the analysis of a decaying active region observed by
the EUV Imaging Spectrometer on Hinode during 2009 December 7-11. We
investigated the temporal evolution of its structure exhibited by plasma
at temperatures from 300,000 to 2.8 million degrees, and derived the
electron density, differential emission measure, effective electron
temperature, and elemental abundance ratios of Si/S and Fe/S (as a
measure of the First Ionization Potential (FIP) Effect). We compared
these coronal properties to the temporal evolution of the photospheric
magnetic field strength obtained from the Solar and Heliospheric
Observatory Michelson Doppler Imager magnetograms. We find that, while
these coronal properties all decreased with time during this decay
phase, the largest change was at plasma above 1.5 million degrees. The
photospheric magnetic field strength also decreased with time but
mainly for field strengths lower than about 70 Gauss. The effective
electron temperature and the FIP bias seem to reach a “basal” state
(at 1.5 × 106 K and 1.5, respectively) into the quiet Sun
when the mean photospheric magnetic field (excluding all areas <10 G)
weakened to below 35 G, while the electron density continued to decrease
with the weakening field. These physical properties are all positively
correlated with each other and the correlation is the strongest in
the high-temperature plasma. Such correlation properties should be
considered in the quest for our understanding of how the corona is
heated. The variations in the elemental abundance should especially
be considered together with the electron temperature and density.
Title: Numerical Simulation of DC Coronal Heating
Authors: Dahlburg, Russell B.; Einaudi, G.; Taylor, Brian D.;
Ugarte-Urra, Ignacio; Warren, Harry; Rappazzo, A. F.; Velli, Marco
Bibcode: 2016SPD....47.0305D
Altcode:
Recent research on observational signatures of turbulent heating of
a coronal loop will be discussed. The evolution of the loop is is
studied by means of numerical simulations of the fully compressible
three-dimensional magnetohydrodynamic equations using the HYPERION
code. HYPERION calculates the full energy cycle involving footpoint
convection, magnetic reconnection, nonlinear thermal conduction
and optically thin radiation. The footpoints of the loop magnetic
field are convected by random photospheric motions. As a consequence
the magnetic field in the loop is energized and develops turbulent
nonlinear dynamics characterized by the continuous formation and
dissipation of field-aligned current sheets: energy is deposited
at small scales where heating occurs. Dissipation is non-uniformly
distributed so that only a fraction of thecoronal mass and volume gets
heated at any time. Temperature and density are highly structured at
scales which, in the solar corona, remain observationally unresolved:
the plasma of the simulated loop is multi thermal, where highly
dynamical hotter and cooler plasma strands are scattered throughout
the loop at sub-observational scales. Typical simulated coronal loops
are 50000 km length and have axial magnetic field intensities ranging
from 0.01 to 0.04 Tesla. To connect these simulations to observations
the computed number densities and temperatures are used to synthesize
the intensities expected in emission lines typically observed with
the Extreme ultraviolet Imaging Spectrometer (EIS) on Hinode. These
intensities are then employed to compute differential emission measure
distributions, which are found to be very similar to those derived
from observations of solar active regions.
Title: Observational Signatures of Coronal Loop Heating and Cooling
Driven by Footpoint Shuffling
Authors: Dahlburg, R. B.; Einaudi, G.; Taylor, B. D.; Ugarte-Urra,
I.; Warren, H. P.; Rappazzo, A. F.; Velli, M.
Bibcode: 2016ApJ...817...47D
Altcode: 2015arXiv151203079D
The evolution of a coronal loop is studied by means of
numerical simulations of the fully compressible three-dimensional
magnetohydrodynamic equations using the HYPERION code. The footpoints
of the loop magnetic field are advected by random motions. As a
consequence, the magnetic field in the loop is energized and develops
turbulent nonlinear dynamics characterized by the continuous formation
and dissipation of field-aligned current sheets: energy is deposited
at small scales where heating occurs. Dissipation is nonuniformly
distributed so that only a fraction of the coronal mass and volume gets
heated at any time. Temperature and density are highly structured at
scales that, in the solar corona, remain observationally unresolved:
the plasma of our simulated loop is multithermal, where highly dynamical
hotter and cooler plasma strands are scattered throughout the loop at
sub-observational scales. Numerical simulations of coronal loops of
50,000 km length and axial magnetic field intensities ranging from 0.01
to 0.04 T are presented. To connect these simulations to observations,
we use the computed number densities and temperatures to synthesize
the intensities expected in emission lines typically observed with the
Extreme Ultraviolet Imaging Spectrometer on Hinode. These intensities
are used to compute differential emission measure distributions using
the Monte Carlo Markov Chain code, which are very similar to those
derived from observations of solar active regions. We conclude that
coronal heating is found to be strongly intermittent in space and time,
with only small portions of the coronal loop being heated: in fact,
at any given time, most of the corona is cooling down.
Title: Magnetic Flux Transport and the Long-term Evolution of Solar
Active Regions
Authors: Ugarte-Urra, Ignacio; Upton, Lisa; Warren, Harry P.; Hathaway,
David H.
Bibcode: 2015ApJ...815...90U
Altcode: 2015arXiv151104030U
With multiple vantage points around the Sun, Solar Terrestrial Relations
Observatory (STEREO) and Solar Dynamics Observatory imaging observations
provide a unique opportunity to view the solar surface continuously. We
use He ii 304 Å data from these observatories to isolate and track
ten active regions and study their long-term evolution. We find
that active regions typically follow a standard pattern of emergence
over several days followed by a slower decay that is proportional in
time to the peak intensity in the region. Since STEREO does not make
direct observations of the magnetic field, we employ a flux-luminosity
relationship to infer the total unsigned magnetic flux evolution. To
investigate this magnetic flux decay over several rotations we use
a surface flux transport model, the Advective Flux Transport model,
that simulates convective flows using a time-varying velocity field
and find that the model provides realistic predictions when information
about the active region's magnetic field strength and distribution at
peak flux is available. Finally, we illustrate how 304 Å images can
be used as a proxy for magnetic flux measurements when magnetic field
data is not accessible.
Title: On the long-term evolution of solar active regions from full
Sun observations, magnetic flux transport and hydrodynamic modeling
Authors: Ugarte-Urra, Ignacio; Upton, Lisa; Warren, Harry; Hathaway,
David H.
Bibcode: 2015TESS....120104U
Altcode:
With their multiple vantage points around the Sun, STEREO and SDO
observations provide a unique opportunity to view the solar surface
continuously. We use data from these observatories to study the
long-term evolution of solar active regions in He II 304 A. We
show that active regions follow a universal pattern of emergence
over several days followed by a decay that is proportional to the
peak intensity in the region. We find that magnetic surface flux
transport simulations are able to reproduce this evolution. Since
STEREO does not make direct observations of the magnetic field, we use
the flux-luminosity relationship to infer the total unsigned magnetic
flux from the He 304 A images. We also illustrate the use of far-side
imaging to introduce solar active regions into magnetic surface flux
transport simulations. We finally show how these models can be used to
determine the long-term coronal emission evolution in active regions
by coupling extrapolations of the magnetic flux transport simulations
field with EBTEL solutions to the hydrodynamic loop equations.
Title: Full-Sun observations for identifying the source of the slow
solar wind
Authors: Brooks, David H.; Ugarte-Urra, Ignacio; Warren, Harry P.
Bibcode: 2015NatCo...6.5947B
Altcode: 2016arXiv160509514B; 2015NatCo...6E5947B
Fast (>700 km s-1) and slow
(~400 km s-1) winds stream from the Sun, permeate
the heliosphere and influence the near-Earth environment. While the
fast wind is known to emanate primarily from polar coronal holes,
the source of the slow wind remains unknown. Here we identify possible
sites of origin using a slow solar wind source map of the entire Sun,
which we construct from specially designed, full-disk observations
from the Hinode satellite, and a magnetic field model. Our map
provides a full-Sun observation that combines three key ingredients
for identifying the sources: velocity, plasma composition and magnetic
topology and shows them as solar wind composition plasma outflowing on
open magnetic field lines. The area coverage of the identified sources
is large enough that the sum of their mass contributions can explain
a significant fraction of the mass loss rate of the solar wind.
Title: The VAULT2.0 Observing Campaign: A Comprehensive Investigation
of the Chromosphere-Corona Interface at Sub-arcsecond scales
Authors: Vourlidas, A.; Korendyke, C.; Tun-Beltran, S. D.; Ugarte-Urra,
I.; Morrill, J. S.; Warren, H. P.; Young, P.; De Pontieu, B.; Gauzzi,
G.; Reardon, K.
Bibcode: 2014AGUFMSH41C4155V
Altcode:
We report the first results from an observing campaign in support of
the VAULT2.0 sounding rocket launch on September 30, 2014. VAULT2.0
is a Lya (1216Å) spectroheliograph capable of 0.3" (~250 km) spatial
resolution. The objective of the VAULT2.0 project is the study of
the chromosphere-corona interface. This interface has acquired renewed
emphasis over the last few years, thanks to high-resolution observations
from Hinode/SOT and EIS instruments and the Lya imaging from the two
VAULT flights. The observations have shown that the upper chromosphere
may play a more important role in heating the corona and in affecting
EUV observations that previously thought: (1) by supplying the mass
via Type-II spicules and, (2) by absorbing coronal emission. Many of
the required clues for further progress are located in sub-arcsecond
structures with temperatures between 10000 and 50000 K, a regime not
accessible by Hinode or SDO. Lyman-alpha observations are, therefore,
ideal, for filling in this gap. The observing campaign in support of
the VAULT2.0 is closely coordinated with the Hinode and IRIS missions
to study the mass/energy flow from the chromosphere to the corona with
joint observations of type-II spicules, and the magnetic connectivity
of coronal loops using the full imaging and spectral capabilities of
IRIS, Hinode and SDO. Several ground-based observatories also provide
important observations (IBIS, BBSO, SOLIS). The VAULT2.0 project is
funded by the NASA LCAS program.
Title: VizieR Online Data Catalog: UV spectrum of the quiet Sun
above the limb (Warren+, 2014)
Authors: Warren, H. P.; Ugarte-Urra, I.; Landi, E.
Bibcode: 2014yCat..22130011W
Altcode:
First, we compare full-disk mosaics constructed by scanning the EIS slot
over the Sun with irradiance observations made by the EUV Variability
Experiment (EVE; Woods et al. 2012SoPh..275..115W) on the Solar
Dynamics Observatory (SDO) mission. These comparisons provide a means
of establishing the absolute calibration for EIS. Second, we combine
extended EIS observations from above the limb in the quiet Sun with a
simple temperature model to simultaneously determine the differential
emission measure (DEM) distribution and the time-dependent changes
to the effective areas that best fit all of the available spectral
lines. In Figure 2 we show the average spectrum from an observation
of seven consecutive runs of ELFULLCCDWSUMER. The
observations began on 2007 November 4 19:12 and ended on the same date
at 23:51 UT. The EIS field of view was centered at (990", -50") about
22" above the limb of the Sun. The central 129 pixels along the slit
have been averaged over 38 exposures (11 exposures were corrupted in
transmission to the ground) for a total of 4902 intensity measurements
at each wavelength. Since each exposure is 300s, the spectrum represents
1470600 pixels of effective exposure time and allows weak lines at
the ends of the detector to be measured. (1 data file).
Title: The Absolute Calibration of the EUV Imaging Spectrometer
on Hinode
Authors: Warren, Harry P.; Ugarte-Urra, Ignacio; Landi, Enrico
Bibcode: 2014ApJS..213...11W
Altcode: 2013arXiv1310.5324W
We investigate the absolute calibration of the EUV Imaging Spectrometer
(EIS) on Hinode by comparing EIS full-disk mosaics with irradiance
observations from the EUV Variability Experiment on the Solar Dynamics
Observatory. We also use extended observations of the quiet corona above
the limb combined with a simple differential emission measure model
to establish new effective area curves that incorporate information
from the most recent atomic physics calculations. We find that changes
to the EIS instrument sensitivity are a complex function of both time
and wavelength. We find that the sensitivity is decaying exponentially
with time and that the decay constants vary with wavelength. The EIS
short wavelength channel shows significantly longer decay times than
the long wavelength channel.
Title: The Hydrodynamics of High Temperature Plasma: Reproducing
the Properties of High Temperature Emission in Solar Active Regions
Authors: Ugarte-Urra, Ignacio; Warren, Harry
Bibcode: 2014AAS...22431205U
Altcode:
The launch of Hinode and SDO have revolutionized our ability to
measure the plasma properties of the solar corona. Many studies have
documented both the temperature structure of the corona as well as
its temporal variability. Of particular interest is the behavior
of high temperature loops that are typically found in the core of
an active region. Temperature distributions in these regions are
often sharply peaked near 4 MK but rapidly evolving loops are also
observed. In this talk we will present results from our effort to
perform hydrodynamic simulations of 15 solar active regions that cover
a wide range of solar conditions and to reconcile these simulations
with observations. In this work we have coupled non-linear force
free extrapolations with solutions to the hydrodynamic loop equations
approximated by EBTEL. Using relatively simple heating scenarios we are
able to reproduce three important properties of the observations: the
dependance of the observed intensity on magnetic flux, the sharply
peaked emission measure distributions for large regions, and the
general frequency distribution of the observed events. Our current
simulations, however, suggest much stronger 1MK emission near the
neutral line than is observed, indicating the heating of small loops
is not well understood. We also do not properly reproduce the relative
distribution of large and small events in these active regions.
Title: Determining Heating Timescales in Solar Active Region Cores
from AIA/SDO Fe XVIII Images
Authors: Ugarte-Urra, Ignacio; Warren, Harry P.
Bibcode: 2014ApJ...783...12U
Altcode: 2013arXiv1311.6346U
We present a study of the frequency of transient brightenings in the
core of solar active regions as observed in the Fe XVIII line component
of AIA/SDO 94 Å filter images. The Fe XVIII emission is isolated using
an empirical correction to remove the contribution of "warm" emission to
this channel. Comparing with simultaneous observations from EIS/Hinode,
we find that the variability observed in Fe XVIII is strongly correlated
with the emission from lines formed at similar temperatures. We examine
the evolution of loops in the cores of active regions at various stages
of evolution. Using a newly developed event detection algorithm,
we characterize the distribution of event frequency, duration, and
magnitude in these active regions. These distributions are similar for
regions of similar age and show a consistent pattern as the regions
age. This suggests that these characteristics are important constraints
for models of solar active regions. We find that the typical frequency
of the intensity fluctuations is about 1400 s for any given line of
sight, i.e., about two to three events per hour. Using the EBTEL 0D
hydrodynamic model, however, we show that this only sets a lower limit
on the heating frequency along that line of sight.
Title: High Spatial Resolution Observations of Loops in the Solar
Corona
Authors: Brooks, David H.; Warren, Harry P.; Ugarte-Urra, Ignacio;
Winebarger, Amy R.
Bibcode: 2013ApJ...772L..19B
Altcode: 2013arXiv1305.2246B
Understanding how the solar corona is structured is of fundamental
importance to determine how the Sun's upper atmosphere is heated to
high temperatures. Recent spectroscopic studies have suggested that an
instrument with a spatial resolution of 200 km or better is necessary
to resolve coronal loops. The High Resolution Coronal Imager (Hi-C)
achieved this performance on a rocket flight in 2012 July. We use Hi-C
data to measure the Gaussian widths of 91 loops observed in the solar
corona and find a distribution that peaks at about 270 km. We also
use Atmospheric Imaging Assembly data for a subset of these loops and
find temperature distributions that are generally very narrow. These
observations provide further evidence that loops in the solar corona
are often structured at a scale of several hundred kilometers, well
above the spatial scale of many proposed physical mechanisms.
Title: Heating Frequency in the core of Active Regions
Authors: Ugarte-Urra, Ignacio; Warren, H.
Bibcode: 2013SPD....4430502U
Altcode:
We present a study of the frequency and duration of brightenings in the
core of solar active regions as observed in the Fe XVIII line component
of AIA/SDO 94 A filter images. The Fe XVIII emission was isolated by
removing the "warm" emission contribution using as proxy the emission
from the AIA 193 and 171 channels. We examined the evolution of loop
in cores of several active regions that span a wide range of total
magnetic field strengths and at various stages of evolution. Using
a newly developed event detector algorithm we find that the typical
frequency of occurrence of detectable brightness enhancements is in
the order of 20 minutes. Using EBTEL, a 0D hydrodynamical model, we
show that a single loop heated a that frequency would be experiencing
effectively steady heating. Then we evaluate different heating scenarios
with multiple loops along the line-of-sight. Finally, we report on our
preliminary efforts to reproduce those characteristic timescales on
full active region models where field lines from a non-linear force
free extrapolation are populated with EBTEL solutions.
Title: Heating frequency in active region cores as observed in AIA
Fe XVIII images
Authors: Ugarte-Urra, I.; Warren, H. P.
Bibcode: 2013enss.confE..85U
Altcode:
We present a study of the frequency and duration of brightenings in the
core of solar active regions as observed in the Fe XVIII line component
of AIA/SDO 94 A filter images. The Fe XVIII emission was isolated by
removing the "warm" emission contribution using as proxy the emission
from the AIA 193 and 171 channels. We examined the evolution of loop
in cores of several active regions that span a wide range of total
magnetic field strengths and at various stages of evolution. Using
a newly developed event detector algorithm we find that the typical
frequency of occurrence of brightness enhancements is in the order
of tens of minutes. We then use those values to evaluate different
scenarios of heating frequency using 1D hydrodynamical models of loops.
Title: Is Active Region Core Variability Age Dependent?
Authors: Ugarte-Urra, Ignacio; Warren, Harry P.
Bibcode: 2012ApJ...761...21U
Altcode:
The presence of both steady and transient loops in active region cores
has been reported from soft X-ray and extreme-ultraviolet observations
of the solar corona. The relationship between the different loop
populations, however, remains an open question. We present an
investigation of the short-term variability of loops in the core of
two active regions in the context of their long-term evolution. We
take advantage of the nearly full Sun observations of STEREO and Solar
Dynamics Observatory spacecraft to track these active regions as they
rotate around the Sun multiple times. We then diagnose the variability
of the active region cores at several instances of their lifetime
using EIS/Hinode spectral capabilities. We inspect a broad range of
temperatures, including for the first time spatially and temporally
resolved images of Ca XIV and Ca XV lines. We find that the active
region cores become fainter and steadier with time. The significant
emission measure at high temperatures that is not correlated with a
comparable increase at low temperatures suggests that high-frequency
heating is viable. The presence, however, during the early stages,
of an enhanced emission measure in the "hot" (3.0-4.5 MK) and "cool"
(0.6-0.9 MK) components suggests that low-frequency heating also plays
a significant role. Our results explain why there have been recent
studies supporting both heating scenarios.
Title: Solar Coronal Loops Resolved by Hinode and the Solar Dynamics
Observatory
Authors: Brooks, David H.; Warren, Harry P.; Ugarte-Urra, Ignacio
Bibcode: 2012ApJ...755L..33B
Altcode:
Despite decades of studying the Sun, the coronal heating problem remains
unsolved. One fundamental issue is that we do not know the spatial scale
of the coronal heating mechanism. At a spatial resolution of 1000 km or
more, it is likely that most observations represent superpositions of
multiple unresolved structures. In this Letter, we use a combination
of spectroscopic data from the Hinode EUV Imaging Spectrometer and
high-resolution images from the Atmospheric Imaging Assembly on the
Solar Dynamics Observatory to determine the spatial scales of coronal
loops. We use density measurements to construct multi-thread models of
the observed loops and confirm these models using the higher spatial
resolution imaging data. The results allow us to set constraints on the
number of threads needed to reproduce a particular loop structure. We
demonstrate that in several cases million degree loops are revealed to
be single monolithic structures that are fully spatially resolved by
current instruments. The majority of loops, however, must be composed
of a number of finer, unresolved threads, but the models suggest that
even for these loops the number of threads could be small, implying
that they are also close to being resolved. These results challenge
heating models of loops based on the reconnection of braided magnetic
fields in the corona.
Title: Solar Coronal Loops Resolved by Hinode and SDO
Authors: Brooks, David H.; Warren, Harry P.; Ugarte-Urra, Ignacio
Bibcode: 2012arXiv1205.5814B
Altcode:
Despite decades of studying the Sun, the coronal heating problem remains
unsolved. One fundamental issue is that we do not know the spatial scale
of the coronal heating mechanism. At a spatial resolution of 1000 km or
more it is likely that most observations represent superpositions of
multiple unresolved structures. In this letter, we use a combination
of spectroscopic data from the Hinode EUV Imaging Spectrometer (EIS)
and high resolution images from the Atmospheric Imaging Assembly
(AIA) on the Solar Dynamics Observatory to determine the spatial
scales of coronal loops. We use density measurements to construct
multi-thread models of the observed loops and confirm these models
using the higher spatial resolution imaging data. The results allow
us to set constraints on the number of threads needed to reproduce
a particular loop structure. We demonstrate that in several cases
million degree loops are revealed to be single monolithic structures
that are fully spatially resolved by current instruments. The majority
of loops, however, must be composed of a number of finer, unresolved
threads; but the models suggest that even for these loops the number
of threads could be small, implying that they are also close to being
resolved. These results challenge heating models of loops based on
the reconnection of braided magnetic fields in the corona.
Title: Can We Resolve Coronal Loops with Hinode and SDO?
Authors: Ugarte-Urra, Ignacio; Brooks, D. H.; Warren, H. P.
Bibcode: 2012AAS...22030903U
Altcode:
A combination of spectral data from the Hinode EUV Imaging Spectrometer
(EIS) and high resolution imaging from the Solar Dynamics Observatory
(SDO) Atmospheric Imaging Assembly (AIA) are used to investigate
the fundamental spatial scales of coronal loops. We construct
multi-isothermal thread models and find that we are able to successfully
reproduce the cross-loop intensity profiles observed by EIS and AIA. The
models allow us to set constraints on the number of threads needed to
reproduce a particular loop structure, and the results suggest that
although most coronal loops remain unresolved, current instruments
are close to resolving them. We discuss implications for future high
resolution EUV spectral imaging instruments.
Title: Filament destabilization and CME release during a long
duration flare
Authors: Zuccarello, F.; Contarino, L.; Farnik, F.; Karlicky, M.;
Romano, P.; Ugarte-Urra, I.
Bibcode: 2011A&A...533A.100Z
Altcode:
Context. During complex and long duration solar flares, several filament
destabilizations or eruptions can occur that are often related to
coronal mass ejections (CMEs).
Aims: We describe the study of
an X3.8 long duration event (LDE) that occurred in NOAA 10720 on 17
January 2005 and was characterized by three filament destabilizations
and two CMEs.
Methods: Using multi-wavelength data provided
by both ground-based instruments and satellites, in addition to MDI
magnetograms, we investigated the morphological and magnetic evolution
of the active region before and during the LDE.
Results: Our
analysis of Hα and 1600 Å images showed that initially a
two-ribbon structure developed in the central part of the active region,
where a filament was previously observed. At a later time, two bright
ribbons (in the most eastern side) and a strong brightness increase
(at the western outskirt of the active region) were simultaneously
observed. In a subsequent time interval, a new pair of ribbons was
observed in the western side of the active region. Moreover, a linear
force-free field extrapolation helped identify a null point in the
central part of the active region.
Conclusions: The initial
filament destabilization that occurred in the central part of NOAA
10720 was probably due to magnetic flux emergence and photospheric
shearing motions, which caused a slow tether-cutting process beneath
the filament. The rearrangement of the magnetic field configuration,
occurring in the same area as the location of the null point, changed
the magnetic field connectivity in the active region, triggering two
filament eruptions in the eastern and western part of the active region
and two halo CMEs, in a kind of domino effect.
Title: A Standard-to-blowout Jet
Authors: Liu, Chang; Deng, Na; Liu, Rui; Ugarte-Urra, Ignacio; Wang,
Shuo; Wang, Haimin
Bibcode: 2011ApJ...735L..18L
Altcode: 2011arXiv1105.3244L
The commonly observed jets provide critical information on the
small-scale energy release in the solar atmosphere. We report a near
disk-center jet on 2010 July 20, observed by the Solar Dynamics
Observatory. In this event, the standard interchange magnetic
reconnection between an emerging flux spanning 9 × 103
km and ambient open fields is followed by a blowout-like eruption. In
the "standard" stage, as the emerging negative element approached the
nearby positive network fields, a jet with a dome-like base in EUV grew
for 30 minutes before the jet spire began to migrate laterally with
enhanced flux emergence. In the "blowout" stage, the above converging
fields collided and the subsequent cancellation produced a UV microflare
lasting seven minutes, in which the dome of the jet seemed to be blown
out as (1) the spire swung faster and exhibited an unwinding motion
before vanishing, (2) a rising loop and a blob erupted leaving behind
cusped structures, with the blob spiraling outward in acceleration
after the flare maximum, and (3) ejecting material with a curtain-like
structure at chromospheric to transition-region temperatures also
underwent a transverse motion. It is thus suggested that the flare
reconnection rapidly removes the outer fields of the emerging flux to
allow its twisted core field to erupt, a scenario favoring the jet-scale
magnetic breakout model as recently advocated by Moore et al. in 2010.
Title: A Standard-to-blowout Jet
Authors: Liu, Chang; Deng, N.; Liu, R.; Ugarte-Urra, I.; Wang, S.;
Wang, H.
Bibcode: 2011SPD....42.1734L
Altcode: 2011BAAS..43S.1734L
The commonly observed jets provide critical information on the
small-scale energy release in the solar atmosphere. We report a
near disk-center jet on 2010 July 20, observed by the Solar Dynamics
Observatory. In this jet, the standard interchange magnetic reconnection
between an emerging flux spanning 9x103 km and ambient
open fields is followed by a blowout-like eruption. In the "standard"
stage, as the emerging negative element approached the nearby positive
network fields, a jet with a dome-like base in EUV grew for 30 minutes
before the jet spire began to migrate laterally with enhanced flux
emergence. In the "blowout" stage, the above converging fields collided
and the subsequent cancellation produced an A6 microflare visible
in 1700 A images. In the latter stage that lasted seven minutes, the
dome of the jet seemed to be blown out as (1) the spire swung faster
and exhibited an unwinding motion before vanishing, (2) a rising loop
and a blob erupted leaving behind cusped structures, with the blob
spiraling outward in acceleration after the flare maximum, and (3)
ejecting material that formed a curtain-like structure at chromospheric
to transition-region temperatures also underwent a transverse motion. It
is thus suggested that the flare reconnection rapidly removes the outer
fields of the emerging flux to allow its twisted core field to erupt,
a scenario favoring the jet-scale magnetic breakout model as recently
advocated by Moore et al. in 2010.
Title: Temporal Variability of Active Region Outflows
Authors: Ugarte-Urra, Ignacio; Warren, Harry P.
Bibcode: 2011ApJ...730...37U
Altcode: 2010arXiv1008.4730U
Recent observations from the Extreme-ultraviolet Imaging Spectrometer
(EIS) on board Hinode have shown that low-density areas on the
periphery of active regions are characterized by strong blueshifts in
the emission of spectral lines formed at 1 MK. These Doppler shifts have
been associated with outward propagating disturbances observed with
extreme-ultraviolet and soft X-ray imagers. Since these instruments
can have broad temperature responses, we investigate these intensity
fluctuations using the monochromatic imaging capabilities of the EIS
wide slit (slot) and confirm their 1 MK nature. We also look into
their spectral temporal variability using narrow slit observations and
present the first Doppler movies of the outflow regions. We find that
the Fe XII 195.119 Å blueshifted spectral profiles at their footpoints
exhibit transient blue wing enhancements on timescales as short as the 5
minute cadence. We have also looked at the fan peripheral loops observed
at 0.6 MK in Si VII 275.368 Å in those regions and find no sign of
the recurrent outward propagating disturbances with velocities of
40-130 km s-1 seen in Fe XII. We do observe downward trends
(15-20 km s-1) consistent with the characteristic redshifts
measured at their footpoints. We, therefore, find no evidence that the
structures at these two temperatures and the intensity fluctuations
they exhibit are related to one another.
Title: The Temperature Dependence of Solar Active Region Outflows
Authors: Warren, Harry P.; Ugarte-Urra, Ignacio; Young, Peter R.;
Stenborg, Guillermo
Bibcode: 2011ApJ...727...58W
Altcode: 2010arXiv1008.2696W
Spectroscopic observations with the EUV Imaging Spectrometer (EIS)
on Hinode have revealed large areas of high-speed outflows at the
periphery of many solar active regions. These outflows are of interest
because they may connect to the heliosphere and contribute to the
solar wind. In this paper, we use slit rasters from EIS in combination
with narrowband slot imaging to study the temperature dependence and
morphology of an outflow region and show that it is more complicated
than previously thought. Outflows are observed primarily in emission
lines from Fe XI to Fe XV. Observations at lower temperatures (Si VII),
in contrast, show bright fan-like structures that are dominated by
inflows. These data also indicate that the morphology of the outflows
and the fans is different, outflows are observed in regions where
there is no emission in Si VII. This suggests that the fans, which
are often associated with outflows in studies involving imaging data,
are not directly related to the active region outflows.
Title: Science Objectives for an X-Ray Microcalorimeter Observing
the Sun
Authors: Laming, J. Martin; Adams, J.; Alexander, D.; Aschwanden, M;
Bailey, C.; Bandler, S.; Bookbinder, J.; Bradshaw, S.; Brickhouse,
N.; Chervenak, J.; Christe, S.; Cirtain, J.; Cranmer, S.; Deiker, S.;
DeLuca, E.; Del Zanna, G.; Dennis, B.; Doschek, G.; Eckart, M.; Fludra,
A.; Finkbeiner, F.; Grigis, P.; Harrison, R.; Ji, L.; Kankelborg,
C.; Kashyap, V.; Kelly, D.; Kelley, R.; Kilbourne, C.; Klimchuk, J.;
Ko, Y. -K.; Landi, E.; Linton, M.; Longcope, D.; Lukin, V.; Mariska,
J.; Martinez-Galarce, D.; Mason, H.; McKenzie, D.; Osten, R.; Peres,
G.; Pevtsov, A.; Porter, K. Phillips F. S.; Rabin, D.; Rakowski, C.;
Raymond, J.; Reale, F.; Reeves, K.; Sadleir, J.; Savin, D.; Schmelz,
J.; Smith, R. K.; Smith, S.; Stern, R.; Sylwester, J.; Tripathi, D.;
Ugarte-Urra, I.; Young, P.; Warren, H.; Wood, B.
Bibcode: 2010arXiv1011.4052L
Altcode:
We present the science case for a broadband X-ray imager with
high-resolution spectroscopy, including simulations of X-ray spectral
diagnostics of both active regions and solar flares. This is part of
a trilogy of white papers discussing science, instrument (Bandler et
al. 2010), and missions (Bookbinder et al. 2010) to exploit major
advances recently made in transition-edge sensor (TES) detector
technology that enable resolution better than 2 eV in an array that
can handle high count rates. Combined with a modest X-ray mirror, this
instrument would combine arcsecondscale imaging with high-resolution
spectra over a field of view sufficiently large for the study of
active regions and flares, enabling a wide range of studies such as
the detection of microheating in active regions, ion-resolved velocity
flows, and the presence of non-thermal electrons in hot plasmas. It
would also enable more direct comparisons between solar and stellar
soft X-ray spectra, a waveband in which (unusually) we currently have
much better stellar data than we do of the Sun.
Title: An Argument for Two Coronal Heating Models
Authors: Winebarger, Amy R.; Warren, H. P.; Brooks, D. H.; Ugarte-Urra,
I.
Bibcode: 2010AAS...21630002W
Altcode:
There are several different models for the time and spatial scale
of the energy release in the corona. Over the past several years,
several different attempts at reconciling model predictions with
observations have been made with varying degrees of success. In this
talk, we review three popular heating models and discuss the expected
observables associated with each model. We then show observations
and compare them to the predictions. We find that observations of
loops with a peak temperature near 1 MK are consistent with impulsive
heating while observations of high temperature loops rooted in the
moss are consistent with a steady heating model. This indicates the
time scale of the heating, and potentially the heating mechanism,
are markedly different in the two structures.
Title: Variability of Hot Plasma in Solar Active Regions.
Authors: Ugarte-Urra, Ignacio; Warren, H. P.
Bibcode: 2010AAS...21640712U
Altcode: 2010BAAS...41..861U
The core of a solar active region is generally dominated by hot,
high density, slowly evolving loops that appear to be consistent with
steady heating. However, these loops are generally studied using
instruments with a broad temperature response, which may mask some
of the variability. Here we investigate the evolution of coronal
loops emitting at temperatures above 3MK in fast scans taken with
the Extreme-ultraviolet Imaging Spectrometer on board Hinode. We show
evidence for short term variability at these temperatures and present
the differential emission measure changes associated with them.
Title: Modeling Evolving Coronal Loops with Observations from Stereo,
Hinode, and Trace
Authors: Warren, Harry P.; Kim, David M.; DeGiorgi, Amanda M.;
Ugarte-Urra, Ignacio
Bibcode: 2010ApJ...713.1095W
Altcode: 2009arXiv0904.3920W
The high densities, long lifetimes, and narrow emission measure
distributions observed in coronal loops with apex temperatures
near 1 MK are difficult to reconcile with physical models of the
solar atmosphere. It has been proposed that the observed loops are
actually composed of sub-resolution "threads" that have been heated
impulsively and are cooling. We apply this heating scenario to nearly
simultaneous observations of an evolving post-flare loop arcade
observed with EUVI/STEREO, EIS/Hinode, XRT/Hinode, and TRACE. We
find that it is possible to reproduce the extended loop lifetime,
high electron density, and the narrow differential emission measure
with a multi-thread hydrodynamic model provided that the timescale
for the energy release is sufficiently short. The model, however,
does not reproduce the evolution of the very high temperature emission
observed with XRT. In XRT the emission appears diffuse and it may be
that this discrepancy is simply due to the difficulty of isolating
individual loops at these temperatures. This discrepancy may also
reflect fundamental problems with our understanding of post-reconnection
dynamics during the conductive cooling phase of loop evolution.
Title: The Temperature Structure of Active Region Loops
Authors: Warren, H. P.; Ugarte-Urra, I.; Degiorgi, A.
Bibcode: 2009ASPC..415..303W
Altcode:
Previous solar observations have shown that coronal loops near 1 MK
are difficult to reconcile with simple heating models. These loops
have lifetimes that are long relative to a radiative cooling time and
densities that are large relative to thermodynamic equilibrium. Models
proposed to explain these properties generally rely on the existence of
small scale filaments that are in various stages of heating and cooling,
suggesting a distribution of temperatures within a loop. We present
the observation of an evolving coronal loop observed with EIS/Hinode
and TRACE. This loop has a high density, a narrow distribution of
temperatures, and a lifetime that is long relative to a radiative
cooling time. These properties will be difficult to reconcile with
physical models of coronal loops.
Title: Hinode Coronal Loop Observations
Authors: Ugarte-Urra, I.; Warren, H. P.; Brooks, D. H.
Bibcode: 2009ASPC..415..241U
Altcode:
Coronal loops are the building blocks of the solar
atmosphere. Understanding their mechanism of formation means
understanding the mechanism responsible for heating the solar
corona. The properties of coronal loops are only partially
established. Several issues such as the relationship among structures
seen at different temperatures, the temperature distribution of the
emission, or the degree of filamentation within a volume remain under
discussion. Hinode, as the new generation solar observatory, provides
the best suited diagnostics to address some of these issues. We
demonstrate that this is the case using a case study, AR 10978,
and encourage systematic studies of larger samples as solar activity
increases. Results for AR 10978 are consistent with loops rooted in
highly dynamic unipolar magnetic field areas made of multiple strands
that get heated to at least 2.5 MK, and cool down rather coherently
to transition region temperatures.
Title: X-Ray Jets in Coronal Holes: Numerical Simulation and Hinode
Observations
Authors: Moreno-Insertis, F.; Galsgaard, K.; Ugarte-Urra, I.
Bibcode: 2009ASPC..415...51M
Altcode:
We report on our recent 3D numerical models of the launching of hot,
high-speed jets in a coronal hole following the emergence of magnetized
plasma from the solar interior. As part of the same research, we have
also analyzed Hinode (EIS and XRT) and Soho-MDI observational data
of an actual process of flux emergence followed by jet launching in
a coronal hole. From the observations, we reconstruct the magnetic
topology at the emergence site and calculate velocity and further
physical properties of the observed event. The 3D model was calculated
for realistic conditions in a coronal hole, including, in particular,
a low-density (108 particles cm-3), high Alfven
speed plasma prior to the emergence. After emergence, a ribbon-like
current sheet is created at the site of collision of the emerging and
preexisting magnetic systems. Field line reconnection ensues, which
leads to the ejection of the X-Ray jet. We analyze the global magnetic
topology, and the temperature, velocity and current distribution in
the 3D experiment. The numerical results provide a good match to the
observed features of the coronal hole jets. This is meant regarding
both our own observational results as well as the ranges and average
values of the statistical study by Savcheva et al. (2007).
Title: Exploiting EIS/Hinode Imaging Diagnostic Capabilities
Authors: Ugarte-Urra, Ignacio; Warren, H. P.
Bibcode: 2009SPD....40.1219U
Altcode:
Using a wide slit, also called slot, the Extreme-ultraviolet Imaging
Spectrometer, on-board Hinode, is capable of obtaining relatively
fast (1-3 min) simultaneous monochromatic images of various spectral
lines with different formation temperatures ranging 0.4-3 MK. This
mode allows us to study morphology and dynamics of solar coronal
and transition region structures across the temperature spectrum in
a similar way to an EUV imager. This is achieved at the expense of
spectral resolution. In this paper we investigate the plasma diagnostic
capabilities of these spectrally pure images.Wide slit images can be
interpreted as a superposition of simultaneous narrow slit spectra
from adjacent solar positions. From the comparison of consecutive
narrow slit rasters and wide slit images, we demonstrate that by
making simple assumptions it is possible to extract the narrow slit
spectra out of the slot images. This encouraging result opens up the
door for plasma diagnostics, like electron density from spectral line
ratios and differential emission measure analysis, for solar dynamic
events. Various examples, as well as the limitations and validity of
the assumptions, are discussed.
Title: Active Region Transition Region Loop Populations and Their
Relationship to the Corona
Authors: Ugarte-Urra, Ignacio; Warren, Harry P.; Brooks, David H.
Bibcode: 2009ApJ...695..642U
Altcode: 2009arXiv0901.1075U
The relationships among coronal loop structures at different
temperatures are not settled. Previous studies have suggested that
coronal loops in the core of an active region (AR) are not seen cooling
through lower temperatures and therefore are steadily heated. If loops
were cooling, the transition region would be an ideal temperature regime
to look for a signature of their evolution. The Extreme-ultraviolet
Imaging Spectrometer on Hinode provides monochromatic images of the
solar transition region and corona at an unprecedented cadence and
spatial resolution, making it an ideal instrument to shed light on
this issue. Analysis of observations of AR 10978 taken in 2007 December
8-19 indicates that there are two dominant loop populations in the AR:
(1) core multitemperature loops that undergo a continuous process of
heating and cooling in the full observed temperature range 0.4-2.5
MK and even higher as shown by the X-Ray Telescope and (2) peripheral
loops which evolve mostly in the temperature range 0.4-1.3 MK. Loops
at transition region temperatures can reach heights of 150 Mm in the
corona above the limb and develop downflows with velocities in the
range of 39-105 km s-1.
Title: The X17.2 flare occurred in NOAA 10486: an example of filament
destabilization caused by a domino effect
Authors: Zuccarello, F.; Romano, P.; Farnik, F.; Karlicky, M.;
Contarino, L.; Battiato, V.; Guglielmino, S. L.; Comparato, M.;
Ugarte-Urra, I.
Bibcode: 2009A&A...493..629Z
Altcode:
Context: It is now possible to distinguish between two main models
describing the mechanisms responsible for eruptive flares : the standard
model, which assumes that most of the energy is released, by magnetic
reconnection, in the region hosting the core of a sheared magnetic
field, and the breakout model, which assumes reconnection occurs
at first in a magnetic arcade overlaying the eruptive features.
Aims: We analyze the phenomena observed in NOAA 10486 before and
during an X17.2 flare that occurred on 2003 October 28, to study the
relationship between the pre-flare and flare phases and determine which
model is the most suitable for interpreting this event.
Methods:
We performed an analysis of multiwavelength data set available for
the event using radio data (0.8-4.5 GHz), images in the visible range
(WL and Hα), EUV images (1600 and 195 Å), and X-ray data, as well
as MDI longitudinal magnetograms. We determined the temporal sequence
of events occurring before and during the X17.2 flare and the magnetic
field configuration in the linear force-free field approximation.
Results: The active region was characterized by a multiple arcade
configuration and the X17.2 flare was preceded, by ~2 h, by the partial
eruption of one filament. This eruption caused reconnection at null
points located in the low atmosphere and a decrease in magnetic tension
in the coronal field lines overlaying other filaments present in the
active region. As a consequence, these filaments were destabilized and
the X17.2 flare occurred.
Conclusions: The phenomena observed
in NOAA 10486 before and during the X17.2 flare cannot be explained
by a simple scenario such as the standard or breakout model, but
instead in terms of a so-called domino effect, involving a sequence
of destabilizing processes that triggered the flare.
Title: The Role of Transient Brightenings in Heating the Solar Corona
Authors: Brooks, David H.; Ugarte-Urra, Ignacio; Warren, Harry P.
Bibcode: 2008ApJ...689L..77B
Altcode:
Nanoflare reconnection events have been proposed as a mechanism for
heating the corona. Parker's original suggestion was that frequent
reconnection events occur in coronal loops due to the braiding of the
magnetic field. Many observational studies, however, have focused on the
properties of isolated transient brightenings unassociated with loops,
but their cause, role, and relevance for coronal heating have not
yet been established. Using Hinode SOT magnetograms and high-cadence
EIS spectral data we study the relationship between chromospheric,
transition region, and coronal emission and the evolution of the
magnetic field. We find that hot, relatively steadily emitting coronal
loops and isolated transient brightenings are both associated with
magnetic flux regions that are highly dynamic. An essential difference,
however, is that brightenings are typically found in regions of flux
collision and cancellation whereas coronal loops are generally rooted
in magnetic field regions that are locally unipolar with unmixed
flux. This suggests that the type of heating (transient vs. steady) is
related to the structure of the magnetic field, and that the heating
in transient events may be fundamentally different than in coronal
loops. This implies that they do not play an important role in heating
the "quiescent" corona.
Title: Observations of Active Region Loops with the EUV Imaging
Spectrometer on Hinode
Authors: Warren, Harry P.; Ugarte-Urra, Ignacio; Doschek, George A.;
Brooks, David H.; Williams, David R.
Bibcode: 2008ApJ...686L.131W
Altcode: 2008arXiv0808.3227W
Previous solar observations have shown that coronal loops near 1 MK
are difficult to reconcile with simple heating models. These loops have
lifetimes that are long relative to a radiative cooling time, suggesting
quasi-steady heating. The electron densities in these loops, however,
are too high to be consistent with thermodynamic equilibrium. Models
proposed to explain these properties generally rely on the existence
of smaller scale filaments within the loop that are in various stages
of heating and cooling. Such a framework implies that there should be
a distribution of temperatures within a coronal loop. In this paper
we analyze new observations from the EUV Imaging Spectrometer (EIS)
on Hinode. EIS is capable of observing active regions over a wide range
of temperatures (Fe VIII-Fe XVII) at relatively high spatial resolution
(1''). We find that most isolated coronal loops that are bright in Fe
XII generally have very narrow temperature distributions (σT
lesssim 3 × 105 K), but are not isothermal. We also derive
volumetric filling factors in these loops of approximately 10%. Both
results lend support to the filament models.
Title: The Role of Isolated EUV Brightenings in Heating the Corona
Authors: Brooks, D. H.; Warren, H. P.; Ugarte-Urra, I.
Bibcode: 2008AGUSMSP43C..04B
Altcode:
Nanoflare reconnection events have been proposed as a mechanism for
heating the solar corona. Parker's original suggestion was that frequent
reconnection events occur in coronal loops due to the twisting and
braiding of the magnetic field. Many observational studies, however,
have focused on the radiating properties of isolated brightening
events, but their cause, role, and relevance for coronal heating
has not yet been established. Using Hinode Solar Optical Telescope
(SOT) magnetograms and high cadence EUV Imaging Spectrometer (EIS)
slot rasters we study the relationship between transition region and
coronal emission and the evolution of the magnetic field. We find that
hot, relatively steadily emitting coronal loops are generally rooted in
magnetic field regions that are locally unipolar yet highly dynamic,
whereas detailed analysis shows that ubiquitous EUV brightenings are
found in regions of magnetic flux cancellation in the photosphere. This
suggests that the heating in transient events may be fundamentally
different than the heating in coronal loops and that they play no
direct role in the heating of the quiescent corona.
Title: EIS: a new view of active region transition region loops
Authors: Ugarte-Urra, I.; Warren, H. P.; Brooks, D. H.
Bibcode: 2008AGUSMSP41C..03U
Altcode:
The EUV Imaging Spectrometer (EIS) on board Hinode is providing
unprecedented diagnostics of solar coronal plasmas. One of its less
exploited capabilities is the ability to make instantaneous spectrally
pure images with the 40'' slot. Simultaneous transition region (Mg
VI, Mg VII, Si VII) and coronal (Fe XI - Fe XVI) images allow us
to observe active region loops as we have not been able to before,
given the spatial resolution (1arcsec pixels), cadence (70s) and,
most importantly, the broad temperature coverage. Under this scrutiny
two distinct populations of active region transition region loops can
be differentiated: core loops that result from the cooling of several
million degree plasma; and fan structures with their main contribution
in the 0.6-1 MK temperature range. These results suggest that the cores
of active regions are not as steady as commonly assumed and reinforce
the idea of coexistance of differentiated loop populations within the
active region topology. We present the properties of the loops and we
discuss the implications that these new observations have for current
transition region and coronal models.
Title: Jets in Coronal Holes: Hinode Observations and
Three-dimensional Computer Modeling
Authors: Moreno-Insertis, F.; Galsgaard, K.; Ugarte-Urra, I.
Bibcode: 2008ApJ...673L.211M
Altcode: 2007arXiv0712.1059M
Recent observations of coronal hole areas with the XRT and
EIS instruments on board the Hinode satellite have shown with
unprecedented detail the launching of fast, hot jets away from the
solar surface. In some cases these events coincide with episodes
of flux emergence from beneath the photosphere. In this Letter we
show results of a three-dimensional numerical experiment of flux
emergence from the solar interior into a coronal hole and compare
them with simultaneous XRT and EIS observations of a jet-launching
event that accompanied the appearance of a bipolar region in MDI
magnetograms. The magnetic skeleton and topology that result in the
experiment bear a strong resemblance to linear force-free extrapolations
of the SOHO/MDI magnetograms. A thin current sheet is formed at the
boundary of the emerging plasma. A jet is launched upward along the
open reconnected field lines with values of temperature, density,
and velocity in agreement with the XRT and EIS observations. Below the
jet, a split-vault structure results with two chambers: a shrinking one
containing the emerged field loops and a growing one with loops produced
by the reconnection. The ongoing reconnection leads to a horizontal
drift of the vault-and-jet structure. The timescales, velocities, and
other plasma properties in the experiment are consistent with recent
statistical studies of this type of event made with Hinode data.
Title: Observations of Transient Active Region Heating with Hinode
Authors: Warren, Harry P.; Ugarte-Urra, Ignacio; Brooks, David H.;
Cirtain, Jonathan W.; Williams, David R.; Hara, Hirohisa
Bibcode: 2007PASJ...59S.675W
Altcode: 2007arXiv0711.0357W
We present observations of transient active region heating events
observed with the Extreme Ultraviolet Imaging Spectrometer (EIS) and
X-ray Telescope (XRT) on Hinode. This initial investigation focuses
on NOAA active region 10940 as observed by Hinode on 2007 February 1
between 12 and 19UT. In these observations we find numerous examples
of transient heating events within the active region. The high spatial
resolution and broad temperature coverage of these instruments allows
us to track the evolution of coronal plasma. The evolution of the
emission observed with XRT and EIS during these events is generally
consistent with loops that have been heated and are cooling. We have
analyzed the most energetic heating event observed during this period,
a small GOES B-class flare, in some detail and present some of the
spectral signatures of the event, such as relative Doppler shifts at
one of the loop footpoints and enhanced line widths during the rise
phase of the event. While the analysis of these transient events has
the potential to yield insights into the coronal heating mechanism,
these observations do not rule out the possibility that there is a
strong steady heating level in the active region. Detailed statistical
analysis will be required to address this question definitively.
Title: Hinode EUV Imaging Spectrometer Observations of Solar Active
Region Dynamics
Authors: Mariska, John T.; Warren, Harry P.; Ugarte-Urra, Ignacio;
Brooks, David H.; Williams, David R.; Hara, Hirohisa
Bibcode: 2007PASJ...59S.713M
Altcode: 2007arXiv0708.4309M
The EUV Imaging Spectrometer (EIS) on the Hinode satellite is capable of
measuring emission line center positions for Gaussian line profiles to a
fraction of a spectral pixel, resulting in relative solar Doppler-shift
measurements with an accuracy of a less than a km s-1 for
strong lines. We show an example of the application of that capability
to an active region sit-and-stare observation in which the EIS slit
is placed at one location on the Sun and many exposures are taken
while the spacecraft tracking keeps the same solar location within
the slit. For the active region examined (NOAA10930), we find that
significant intensity and Doppler-shift fluctuations as a function of
time are present at a number of locations. These fluctuations appear
to be similar to those observed in high-temperature emission lines
with other space-borne spectroscopic instruments. With its increased
sensitivity over earlier spectrometers and its ability to image many
emission lines simultaneously, EIS should provide significant new
constraints on Doppler-shift oscillations in the corona.
Title: Hinode EUV Imaging Spectrometer Observations of Active Region
Loop Morphology: Implications for Static Heating Models of Coronal
Emission
Authors: Brooks, David H.; Warren, Harry P.; Ugarte-Urra, Ignacio;
Matsuzaki, Keiichi; Williams, David R.
Bibcode: 2007PASJ...59S.691B
Altcode:
Theoretically, magnetic fields are expected to expand as they rise
above the photosphere and into the corona, so the apparent uniform
cross-sections of active region loops are difficult to understand. There
has been some debate as to whether coronal loops really have constant
cross-sections, or are actually unresolved and composed of expanding
threads within the constant cross-section envelopes. Furthermore, loop
expansion is critical to the success or failure of hydrostatic models
in reproducing the intensities and morphology of observed emission. We
analyze Hinode EIS (EUV Imaging Spectrometer) observations of loops
in active region 10953 and detect only moderate apex width expansion
over a broad range of temperatures from log Te / K = 5.6
to 6.25. The expansion is less than required by steady-state heating
models of coronal emission suggesting that such models will have
difficulty reproducing both low and high temperature loop emission
simultaneously. At higher temperatures (> log Te /
K = 6.3) the apex widths increase substantially, but the emission
at these temperatures likely comes from a combination of multiple
loops. These observations demonstrate the advantage of EIS over previous
instruments. For the first time, active region loops can be examined
over a broad temperature range with high temperature fidelity and the
same spatial resolution. The results therefore provide further clues
to the coronal heating timescale and thus have implications for the
direction of future modeling efforts.
Title: The Magnetic Topology of Coronal Mass Ejection Sources
Authors: Ugarte-Urra, Ignacio; Warren, Harry P.; Winebarger, Amy R.
Bibcode: 2007ApJ...662.1293U
Altcode: 2007astro.ph..3049U
In an attempt to test current initiation models of coronal mass
ejections (CMEs), with an emphasis on the magnetic breakout model, we
inspect the magnetic topology of the sources of 26 CME events in the
context of their chromospheric and coronal response in an interval
of approximately 9 hr around the eruption onset. First we perform
current-free (potential) extrapolations of photospheric magnetograms
to retrieve the key topological ingredients, such as coronal magnetic
null points. Then we compare the reconnection signatures observed in
the high-cadence and high spatial resolution Transition Region and
Coronal Explorer (TRACE) images with the location of the relevant
topological features. The comparison reveals that only seven events
can be interpreted in terms of the breakout model, which requires
a multipolar topology with preeruption reconnection at a coronal
null. We find, however, that a larger number of events (12) cannot
be interpreted in those terms. No magnetic null is found in six of
them. Seven other cases remain difficult to interpret. We also show
that there are no systematic differences between the CME speed and
flare energies of events under different interpretations.
Title: EIS/Hinode Look At Active Region Dynamics
Authors: Ugarte-Urra, Ignacio; Warren, H. P.; Brooks, D. H.; Williams,
D. R.; Cirtain, J. W.; McKenzie, D. E.; Weber, M.; Hara, H.; Harra,
L. K.
Bibcode: 2007AAS...210.9429U
Altcode: 2007BAAS...39..222U
We present some initial results from the Hinode EUV Imaging Spectrometer
(EIS) on the heating and cooling of active region loops. The events
we observe are part of the short term active region evolution within
the span of 50 hours of sit-and-stare observations. We investigate
the temporal evolution of the intensity and Doppler shift of spectral
lines formed at different temperatures. We then determine time lags
and cooling times at various temperature regimes and discuss the
results in the context of previous results provided by earlier space
missions. This effort is considered a first step into the hydrodynamic
modeling of the loop structures. The morphology of the structures is
obtained from X-Ray Telescope (XRT) and TRACE images.
Title: Determining the Chirality Of Filaments Associated with CMEs
Authors: Mulu, Fana; Winebarger, A.; Ugarte-Urra, I.; Warren, H.
Bibcode: 2007AAS...210.2914M
Altcode: 2007BAAS...39..139M
There is currently much debate over the initiation mechanism of coronal
mass ejections. One distinction in the various models is the degree
of magnetic complexity in the pre-CME active region. For instance,
the magnetic breakout model requires a null point in the coronal
field and for reconnection to occur at that null point. Recently,
Ugarta-Urra et al. (2007) investigated the magnetic topology of the
sources of 26 CME events using extrapolations of photospheric fields to
find the location of coronal magnetic null points, if any, and hence
test the validity of the breakout model. Seven of the 26 CMEs studied
supported the breakout model, 12 did not support the model, and 7 were
difficult to interpret. In this poster, we present preliminary results
from a continuing investigation these 26 events. First we determine
if an Halpha filament was associated with the eruption. If a filament
was present, we find the chirality (handedness) of the filaments using
the method described by Martin (1998). We will categorize the filament
properties in terms of Ugarte-Urra's findings to determine if certain
filament properties are associated with a breakout or non-breakout CMEs.
Title: An Investigation into the Variability of Heating in a Solar
Active Region
Authors: Ugarte-Urra, Ignacio; Winebarger, Amy R.; Warren, Harry P.
Bibcode: 2006ApJ...643.1245U
Altcode:
Previous studies have indicated that both steady and impulsive heating
mechanisms play a role in active region heating. In this paper,
we present a study of 20 hours of soft X-ray and EUV observations
of solar active region NOAA AR 8731. We examine the evolution of six
representative loop structures that brighten and fade first from X-ray
images and subsequently from the EUV images. We determine their lifetime
and the delay between their appearance in the different filters. We
find that the lifetime in the EUV filters is much longer than expected
for a single cooling loop. We also notice that the delay in the loops'
appearance in the X-ray and EUV filters is proportional to the loop
length. We model one of the loops using a hydrodynamic model with both
impulsive and quasi-steady heating functions and find that neither of
these simple heating functions can well reproduce the observed loop
characteristics in both the X-ray and EUV images. Hence, although this
active region is dominated by variable emission and the characteristics
of the observed loops are qualitatively consistent with a cooling loop,
the timescale of the heating in this active region remains unknown.
Title: The Magnetic Topology of Coronal Mass Ejection Sources
Authors: Ugarte-Urra, Ignacio; Warren, H. P.
Bibcode: 2006SPD....37.2205U
Altcode: 2006BAAS...38..249U
We present results from the study of the sources of 15 coronal mass
ejections (CME) observed with TRACE. These events are a subset of
the preliminary 48 catalogued CME events with sources that fall
within ±400 arcsec of disk center and were observed with TRACE, in
any of its UV or EUV bandpasses, during the period 1998--2003. The
evolution of the eruptions is analyzed in the context of the magnetic
topology given by a potential field extrapolation of a photospheric
magnetogram. The extrapolations are used to determine the presence of
nulls and quasi-separatrix layers. We discuss the results in the context
of the current CME models and their observational predictions. The
breakout model for CMEs, for example, requires the presence of a null
encompassing the sheared neutral line. Pre-eruption reconnection is
expected to take place at the null. We find magnetic nulls in many
of the events. For most of them, however, we do not see compelling
evidence for pre-eruption reconnection.This research was supported by
the NASA Living With a Star Program.
Title: Optical and EUV observations of solar flare kernels
Authors: García-Alvarez, D.; Johns-Krull, C. M.; Doyle, J. G.;
Ugarte-Urra, I.; Madjarska, M. S.; Butler, C. J.
Bibcode: 2005A&A...444..593G
Altcode:
We present high-resolution spectral observations, covering the entire
optical region (3800-9000 Å), of a solar flare observed during
a multi-wavelength campaign. The flare, recorded on 2002 January
11, was a medium solar flare event (GOES class C7.5). The spectral
observations were carried out using the Hamilton echelle spectrograph on
the coudé auxiliary telescope at Lick Observatory and with the Coronal
Diagnostic Spectrometer (CDS) on board SoHO. The high signal-to-noise
optical spectra are analysed using the same techniques as we applied to
stellar flare data. Hα images obtained at Big Bear Solar Observatory
(BBSO), plus magnetograms obtained with the Michelson Doppler Imager
(MDI) on board SoHO and Transition Region And Coronal Explorer (TRACE)
1600 Å were used in the flare analysis. We observe stellar-like
behaviour in the main solar chromospheric activity indicators, which
show either filling-in or emission during the flare. We find that the
Balmer and Ca II lines show asymmetric profiles, with red-shifted
wings and blue-shifted cores. This behaviour could be explained by
material expanding. During the flare, the Mg i and Fe i lines show
a filling-in of the line profile indicating that the flare affected
the lower atmosphere. There is some evidence for pre-flare heating
as seen in Fe xix 592 Å. Furthermore, O v 629 Å shows an increase
in flux some 10 min. before the coronal lines, perhaps indicating
particle beam heating in the initial stages of the flare. We have
also determined the main physical parameters at flare maximum. The
electron densities and electron temperatures found for the flare imply
that the Balmer emitting plasma originates in the chromosphere. The
physical parameters obtained for the modelled flare are consistent
with previously derived values for solar flares.
Title: Evidence for explosive event activity originating in the
chromosphere
Authors: Doyle, J. G.; Ishak, B.; Ugarte-Urra, I.; Bryans, P.; Summers,
H. P.
Bibcode: 2005A&A...439.1183D
Altcode:
We report on a joint SUMER, CDS, TRACE study, concentrating on
a region which shows prolonged EUV explosive event (EE) activity
in the transition region line N v 1238 Å, yet little evidence of
such activity in another transition region line O v 629 Å (formed
at a similar temperature) which was observed simultaneously. A
possible explanation for the lack of major activity in the O v line
in several explosive events could be that they originate in the lower
chromosphere. This is consistent with the enhancements in the C i 1249
Å line and with the findings of another study which reported time
delays between the chromospheric and transition region lines in some
EE's using high cadence observations (10 s exposure time) obtained
with the SUMER spectrometer in H i Ly 6 (20 000 K) and S vi (200 000
K). Using the generalized collisional-radiative picture, including
the population of metastable levels, we derive the density dependent
contribution function for both N v 1238 and O v 629 for four values
of the electron density; 106 cm-3 representing
the low density limit, 109 cm-3 for a typical
quiet Sun electron density plus 1011 cm-3 and
1012 cm-3 for an active region. These calculations
show that with increasing electron density, both lines shift to slightly
lower temperatures. However, the major difference is in the relative
increase in the line flux with increasing density. For N v, increasing
the density to 1011 cm-3 results in a 60%
increase in the line flux, while O v shows a 30% decrease. Increasing
the electron density to 1012 cm-3 results in a
factor of two decrease in the O v flux, thus making it difficult to
detect explosive event activity in this line if the event is formed
in the chromosphere. Other explosive events which show simultaneous
activity in both lines are probably formed in the transition region. In
one such event, activity is observed in both N v and O v, yet nothing
in C i. In this event we also observe an increase in the TRACE 173
emission, delayed by ≈40 s compared to the transition region lines.
Title: Electron density along a coronal loop observed with CDS/SOHO
Authors: Ugarte-Urra, I.; Doyle, J. G.; Walsh, R. W.; Madjarska, M. S.
Bibcode: 2005A&A...439..351U
Altcode:
The analysis of a coronal loop observed by CDS and EIT on board SOHO
is presented. The loop was situated above the North-East limb at
a latitude of ~48°, being clearly visible in the hottest lines of
the dataset, Fe xvi 360.76 Å, i.e. greater than 2 000 000 K. The
cooler lines in the sample (i.e. O v 629.73 Å and He i 584.35 Å)
showed only a brightening at the footpoints location. Based on
the Fe xiv 353.84/334.17 line ratio, the electron density along
the loop was determined following three different approaches for
the background subtraction. No differences, within the error bars,
can be found between the three methods. At the apex, the density is
0.9×109 cm-3, while at the footpoint it is
50% greater, i.e. 1.4×109 cm-3. The inferred
filling factor values along the loop, at the formation temperature
of the lines, are in the range 0.2-0.9. One dimensional hydrodynamic
modelling of the loop along a given field line, gravity neglected,
was performed. A minimum χ2 analysis results in a best fit
case where the total energy input is directed preferentially to the loop
footpoint (the heating rate is three times larger at the base than at
the apex). An isochoric solution can not be ruled out completely. The
exercise illustrates the necessity of accurate spectral diagnostics
in order to derive definite conclusions from theoretical models and
suggests the need for simultaneous density and temperature diagnostics.
Title: Comparison Between Explosive Events of Two Transition Region
Lines of Similar Temperatures
Authors: Ishak, B.; Doyle, J. G.; Ugarte-Urra, I.
Bibcode: 2005PADEU..15...21I
Altcode:
We present the difference in behavior of two transition region lines at
very close temperatures, observed with SUMER/SoHO. N v 1238.82 A shows
a series of explosive events with broadenings mostly shifted to the
blue. This is not seen in O v 629.73 A, the behavior of which remains
"quiet" throughout most of the time series.
Title: Electron densities in EUV coronal bright points
Authors: Ugarte-Urra, I.; Doyle, J. G.; Del Zanna, G.
Bibcode: 2005A&A...435.1169U
Altcode:
Electron density measurements of six coronal bright points have been
obtained using line ratio diagnostics of four ions in the temperature
range 1.3-2.0 × 106 K. The results suggest that BP plasma
has more similarities to active region plasma than to quiet Sun
plasma. Nevertheless, they do not show the exact same behaviour: the
increase of electron density at temperatures over log~Te ~
6.2, observed in the core of active regions, has not been detected. The
Fe XII results based on new atomic data, although in better agreement
decreasing more than a factor of two the densities obtained with older
calculations, are still in some instances higher than those from Si
X. This could be a consequence of the inhomogeneity of the plasma in
the observed volume. New measurements with a better constriction of
the volumetric properties seem necessary to rule out the influence of
other factors and confirm whether there is an inconsistency between
the ions, perhaps due to line blending problems in the Fe XII lines.
Title: Dissertation Talk: Brightness and Magnetic Evolution of Solar
Coronal Bright Points
Authors: Ugarte-Urra, I.
Bibcode: 2005AGUSMSP22A..01U
Altcode:
Dissertation: A study of the brightness and magnetic evolution of
several Extreme ultraviolet (EUV) coronal bright points (hereafter BPs)
using CDS, EIT, MDI (onboard SOHO) and TRACE is presented. The results
confirm that, down to 1" resolution, BPs are made of small loops with
lengths of ~6 Mm and cross-sections of ~2 Mm. The loops are very
dynamic, evolving in time scales as short as 1 -- 2 minutes. This
is reflected in a highly variable EUV response with fluctuations
highly correlated in spectral lines at transition region temperatures
(in the range 3.2×104 -- 3.5×105 K). A wavelet analysis of the
intensity variations reveals, for the first time, the existence of
quasi-periodic oscillations with periods ranging 400--1000 s, in the
range of periods characteristic of the chromospheric network. The
link between BPs and network bright points is discussed. Co-spatial
and co-temporal TRACE and MDI images also reveal the signature of
heating events that produce sudden EUV brightenings simultaneous to
magnetic flux cancellations. This is interpreted in terms of magnetic
reconnection events.
Title: Brightness and magnetic evolution of solar coronal bright
points
Authors: Ugarte Urra, Ignacio
Bibcode: 2005PhDT.........3U
Altcode:
This thesis presents a study of the brightness and magnetic evolution
of several Extreme ultraviolet (EUV) coronal bright points (hereafter
BPs). The study was carried out using several instruments on board the
Solar and Heliospheric Observatory, supported by the high resolution
imaging from the Transition Region And Coronal Explorer. The
results confirm that, down to 1" resolution, BPs are made of small
loops with lengths of [approximate]6 Mm and cross-sections of ≈2
Mm. The loops are very dynamic, evolving in time scales as short as
1 - 2 minutes. This is reflected in a highly variable EUV response
with fluctuations highly correlated in spectral lines at transition
region temperatures, but not always at coronal temperatures. A
wavelet analysis of the intensity variations reveals the existence
of quasi-periodic oscillations with periods ranging 400--1000s, in
the range of periods characteristic of the chromospheric network. The
link between BPs and network bright points is discussed, as well as
the interpretation of the oscillations in terms of global acoustic
modes of closed magnetic structures. A comparison of the magnetic
flux evolution of the magnetic polarities to the EUV flux changes is
also presented. Throughout their lifetime, the intrinsic EUV emission
of BPs is found to be dependent on the total magnetic flux of the
polarities. In short time scales, co-spatial and co-temporal coronal
images and magnetograms, reveal the signature of heating events
that produce sudden EUV brightenings simultaneous to magnetic flux
cancellations. This is interpreted in terms of magnetic reconnection
events. Finally, a electron density study of six coronal bright
points produces values of ≈1.6×10 9 cm -3 , closer to active region
plasma than to quiet Sun. The analysis of a large coronal loop (half
length of 72 Mm) introduces the discussion on the prospects of future
plasma diagnostics of BPs with forthcoming solar missions.
Title: Joint Observing Program 166: Multi-Instrumental Study of
Extreme-Ultraviolet Coronal Bright Points
Authors: Ugarte-Urra, I.; Doyle, J. G.
Bibcode: 2004ESASP.575..535U
Altcode: 2004soho...15..535U
No abstract at ADS
Title: Determining the Temperature Profile Along a Plasma Loop Iii:
Spectrometers Verus Imagers
Authors: Walsh, R. W.; Ugarte-Urra, I.; Doyle, J. G.; Noglik, J. B.
Bibcode: 2004ESASP.575..567W
Altcode: 2004soho...15..567W
No abstract at ADS
Title: Brightness and magnetic evolution of solar coronal bright
points
Authors: Ugarte-Urra, I.
Bibcode: 2004PhDT.........1U
Altcode:
This thesis presents a study of the brightness and magnetic evolution
of several Extreme ultraviolet (EUV) coronal bright points (hereafter
BPs). BPs are loop-like features of enhanced emission in the coronal
EUV and X-ray images of the Sun, that are associated to the interaction
of opposite photospheric magnetic polarities with magnetic fluxes of
≈1018 - 1019 Mx. The study was carried out using
several instruments on board the Solar and Heliospheric Observatory
(SOHO): the Extreme Ultraviolet Imager (EIT), the Coronal Diagnostic
Spectrometer (CDS) and the Michelson Doppler Imager (MDI), supported
by the high resolution imaging from the Transition Region And Coronal
Explorer (TRACE). The results confirm that, down to 1'' (i.e. ~715
km) resolution, BPs are made of small loops with lengths of ~6 Mm and
cross-sections of ~2 Mm. The loops are very dynamic, evolving in time
scales as short as 1 - 2 minutes. This is reflected in a highly variable
EUV response with fluctuations highly correlated in spectral lines at
transition region temperatures (in the range 3.2x10^4 - 3.5x10^5 K),
but not always at coronal temperatures. A wavelet analysis of the
intensity variations reveals, for the first time, the existence of
quasi-periodic oscillations with periods ranging 400 -- 1000 s, in
the range of periods characteristic of the chromospheric network. The
link between BPs and network bright points is discussed, as well as
the interpretation of the oscillations in terms of global acoustic
modes of closed magnetic structures. A comparison of the magnetic
flux evolution of the magnetic polarities to the EUV flux changes is
also presented. Throughout their lifetime, the intrinsic EUV emission
of BPs is found to be dependent on the total magnetic flux of the
polarities. In short time scales, co-spatial and co-temporal TRACE and
MDI images, reveal the signature of heating events that produce sudden
EUV brightenings simultaneous to magnetic flux cancellations. This is
interpreted in terms of magnetic reconnection events. Finally,
a electron density study of six coronal bright points produces values
of ~1.6x109 cm-3, closer to active region plasma
than to quiet Sun. The analysis of a large coronal loop (half length
of 72 Mm) introduces the discussion on the prospects of future plasma
diagnostics of BPs with forthcoming solar missions like Solar-B.
Title: CDS wide slit time-series of EUV coronal bright points
Authors: Ugarte-Urra, I.; Doyle, J. G.; Nakariakov, V. M.; Foley, C. R.
Bibcode: 2004A&A...425.1083U
Altcode:
Wide slit (90''×240'') movies of four Extreme
Ultraviolet coronal bright points (BPs) obtained with the Coronal
Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric
Observatory (SoHO) have been inspected. The wavelet analysis of the He
I 584.34 Å, O V 629.73 Å and Mg VII/IX 368 Å time-series confirms
the oscillating nature of the BPs, with periods ranging between
600 and 1100 s. In one case we detect periods as short as 236 s. We
suggest that these oscillations are the same as those seen in the
chromospheric network and that a fraction of the network bright points
are most likely the cool footpoints of the loops comprising coronal
bright points. These oscillations are interpreted in terms of global
acoustic modes of the closed magnetic structures associated with BPs.
Title: Signature of oscillations in coronal bright points
Authors: Ugarte-Urra, I.; Doyle, J. G.; Madjarska, M. S.; O'Shea, E.
Bibcode: 2004A&A...418..313U
Altcode:
A detailed study of two consecutive bright points observed
simultaneously with the Coronal Diagnostic Spectrometer (CDS),
the Extreme ultraviolet Imaging Telescope (EIT) and the Michelson
Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory
(SOHO) is presented. The analysis of the evolution of the photospheric
magnetic features and their coronal counterpart shows that there is
a linear dependence between the EIT Fe XII 195 Å flux and the total
magnetic flux of the photospheric bipolarity. The appearance of the
coronal emission is associated with the emergence of new magnetic
flux and the disappearance of coronal emission is associated with
the cancellation of one of the polarities. In one of the cases the
disappearance takes place ∼3-4 h before the full cancellation of
the weakest polarity. The spectral data obtained with CDS show
that one of the bright points experienced short time variations in
the flux on a time scale of 420-650 s, correlated in the transition
region lines (O V 629.73 Å and O III 599.60 Å) and also the He I
584.34 Å line. The coronal line (Mg IX 368.07 Å) undergoes changes
as well, but on a longer scale. The wavelet analysis of the temporal
series reveals that many of these events appear in a random fashion and
sometimes after periods of quietness. However, we have found two cases
of an oscillatory behaviour. A sub-section of the O V temporal series
of the second bright point shows a damped oscillation of five cycles
peaking in the wavelet spectrum at 546 s, but showing in the latter few
cycles a lengthening of that period. The period compares well with that
detected in the S VI 933.40 Å oscillations seen in another bright point
observed with the Solar Ultraviolet Measurements of Emitted Radiation
(SUMER) spectrometer, which has a period of 491 s. The derived electron
density in the transition region was 3×1010 cm-3
with some small variability, while the coronal electron density was
5×108 cm-3.
Title: Oscillations in Coronal Bright Points
Authors: Ugarte-Urra, I.; Doyle, J. G.; Madjarska, M. S.; O'Shea, E.
Bibcode: 2004ESASP.547..329U
Altcode: 2004soho...13..329U
No abstract at ADS
Title: Electron density variation in off-limb solar corona structures
Authors: Ugarte-Urra, I.; Doyle, J. G.; Madjarska, M. S.
Bibcode: 2002ESASP.505..595U
Altcode: 2002IAUCo.188..595U; 2002solm.conf..595U
We present a study of the electron density variation for off
limb observations with the Coronal Diagnostic Spectrometer (CDS)
on board SoHO using the density sensitive coronal line ratio Si X
(356.0/347.4). The analysis is divided into three parts: the latitudinal
variation from 1.01 < r/Rsolar < 1.09, the temporal
variation in successive rasters, and the electron density along a hot
coronal loop found in one of the rasters.