explanation blue bibcodes open ADS page with paths to full text
Author name code: rubiodacosta
ADS astronomy entries on 2022-09-14
author:"Rubio da Costa, Fatima"
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Title: Probing the Puzzle of Behind-the-limb γ-Ray Flares:
Data-driven Simulations of Magnetic Connectivity and CME-driven
Shock Evolution
Authors: Jin, Meng; Petrosian, Vahe; Liu, Wei; Nitta, Nariaki V.;
Omodei, Nicola; Rubio da Costa, Fatima; Effenberger, Frederic; Li,
Gang; Pesce-Rollins, Melissa; Allafort, Alice; Manchester, Ward, IV
2018ApJ...867..122J Altcode: 2018arXiv180701427J
Recent detections of high-energy γ-rays from behind-the-limb (BTL)
solar flares by the Fermi Gamma-ray Space Telescope pose a puzzle and
challenge on the particle acceleration and transport mechanisms. In
such events, the γ-ray emission region is located away from the BTL
flare site by up to tens of degrees in heliographic longitude. It
is thus hypothesized that particles are accelerated at the shock
driven by the coronal mass ejection (CME) and then travel from the
shock downstream back to the front side of the Sun to produce the
observed γ-rays. To test this scenario, we performed data-driven,
global magnetohydrodynamics simulations of the CME associated with a
well-observed BTL flare on 2014 September 1. We found that part of the
CME-driven shock develops magnetic connectivity with the γ-ray emission
region, facilitating transport of particles back to the Sun. Moreover,
the observed increase in γ-ray flux is temporally correlated with (1)
the increase of the shock compression ratio and (2) the presence of a
quasi-perpendicular shock over the area that is magnetically connected
to the γ-ray emitting region, both conditions favoring the diffusive
shock acceleration (DSA) of particles. These results support the above
hypothesis and can help resolve another puzzle, i.e., long-duration
(up to 20 hr) γ-rays flares. We suggest that, in addition to DSA,
stochastic acceleration by plasma turbulence may also play a role,
especially in the shock downstream region and during the early stage
when the shock Alfvén Mach number is small.
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Title: High-Energy Aspects of Solar Flares: Recent Advances in
Observations and Models
Authors: Liu, Wei; Effenberger, Frederic; Petrosian, Vahe; Nitta,
Nariaki; Omodei, Nicola; Rubio da Costa, Fatima; Pesce-Rollins,
Melissa; Jin, Meng; Allafort, Alice
2018cosp...42E2050L Altcode:
Solar flares are one of the most spectacular manifestations of
solar activity with significant space-weather impacts. They also
serve as a unique laboratory for probing the underlying physics of
wide-ranging processes, including magnetic reconnection, generation
of plasma turbulence and shocks, particle acceleration, and plasma
heating. Solar flares thus bear important implications for physically
similar phenomena elsewhere in the universe, such as laboratory plasmas,
planetary magnetospheres, and flares on other stars or near compact
objects. In this presentation, we review recent observational and
modeling advances in solar flare research, with a focus on high-energy
aspects. Special attention will be paid to imaging and spectroscopic
observations, e.g., of long-duration and behind-the-limb gamma-ray
flares detected by Fermi, of coronal reconnection sites seen by
RHESSI, SDO, Hinode, and IRIS, and of the flaring lower atmosphere
(chromosphere and transition region) seen by IRIS. We will discuss
broad physical connections of flares to other phenomena on the Sun
(to a much greater extent than conventionally thought), such as the
correlation between quasi-periodic flare pulsations (seen from radio to
hard X-rays) and various MHD waves (some are directly imaged). We will
also review relevant modeling efforts to explain these observations,
focusing on a hybrid model of stochastic (by turbulence) plus shock
acceleration of particles, as well as combined kinetic and radiative
hydrodynamic simulations.
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Title: Data-driven Simulations of Magnetic Connectivity in
Behind-the-Limb Gamma-ray Flares and Associated Coronal Mass Ejections
Authors: Jin, M.; Petrosian, V.; Liu, W.; Nitta, N.; Omodei, N.;
Rubio da Costa, F.; Effenberger, F.; Li, G.; Pesce-Rollins, M.
2017AGUFMSH44B..03J Altcode:
Recent Fermi detection of high-energy gamma-ray emission from the
behind-the-limb (BTL) solar flares pose a puzzle on the particle
acceleration and transport mechanisms in such events. Due to
the large separation between the flare site and the location of
gamma-ray emission, it is believed that the associated coronal
mass ejections (CMEs) play an important role in accelerating and
subsequently transporting particles back to the Sun to produce obseved
gamma-rays. We explore this scenario by simulating the CME associated
with a well-observed flare on 2014 September 1 about 40 degrees behind
the east solar limb and by comparing the simulation and observational
results. We utilize a data-driven global magnetohydrodynamics model
(AWSoM: Alfven-wave Solar Model) to track the dynamical evolution
of the global magnetic field during the event and investigate the
magnetic connectivity between the CME/CME-driven shock and the Fermi
emission region. Moreover, we derive the time-varying shock parameters
(e.g., compression ratio, Alfven Mach number, and ThetaBN) over the
area that is magnetically connected to the visible solar disk where
Fermi gamma-ray emission originates. Our simulation shows that the
visible solar disk develops connections both to the flare site and to
the CME-driven shock during the eruption, which indicate that the CME's
interaction with the global solar corona is critical for understanding
such Fermi BTL events and gamma-ray flares in general. We discuss
the causes and implications of Fermi BTL events, in the framework
of a potential shift of paradigm on particle acceleration in solar
flares/CMEs.
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Title: The connection between X-ray and coronal emission measure in
solar limb flares as a diagnostic of non-thermal particle acceleration
and heating processes
Authors: Rubio da Costa, F.; Effenberger, F.; Kleint, L.
2017AGUFMSH41A2747R Altcode:
Using RHESSI X-ray observations and EUV differential emission measures
(DEM) inferred from SDO/AIA observations, we investigate thermal and
non-thermal heating processes associated with coronal emission. We
focus on partially occulted flares located near the solar limb, without
contamination of the strong non-thermal footpoint emission, which allows
us to investigate non-thermal sources at/near the loop top.This study
allows us to temporally and spatially correlate the non-thermal hard
X-ray signatures with temperature dependent heating processes, with
the goal of constraining the physical processes of energy release in
the upper corona. This leads to a better understanding of the thermal
response of the upper atmosphere to non-thermal processes during solar
flares. Our preliminary results show that low coronal loops are denser
and cooler than higher coronal emissions. Higher coronal emissions are
associated to low energy (6-12 keV) thermal emission and lower loops,
to non-thermal (24-26 keV) emission.
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Title: The size of coronal hard X-ray sources in solar flares:
How big are they?
Authors: Effenberger, F.; Krucker, S.; Rubio da Costa, F.
2017AGUFMSH41A2746E Altcode:
Coronal hard X-ray sources are considered to be one of the key
signatures of non-thermal particle acceleration and heating during
the energy release in solar flares. In some cases, X-ray observations
reveal multiple components spatially located near and above the loop
top and even further up in the corona. Here, we combine a detailed
RHESSI imaging analysis of near-limb solar flares with occulted
footpoints and a multi-wavelength study of the flare loop evolution in
SDO/AIA. We connect our findings to different current sheet formation
and magnetic break-out scenarios and relate it to particle acceleration
theory. We find that the upper and usually fainter emission regions can
be underestimated in their size due to the majority of flux originating
from the lower loops.
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Title: A Parameter Study for Modeling Mg II h and k Emission during
Solar Flares
Authors: Rubio da Costa, Fatima; Kleint, Lucia
2017ApJ...842...82R Altcode: 2017arXiv170405874R
Solar flares show highly unusual spectra in which the thermodynamic
conditions of the solar atmosphere are encoded. Current models are
unable to fully reproduce the spectroscopic flare observations,
especially the single-peaked spectral profiles of the Mg II h and k
lines. We aim to understand the formation of the chromospheric and
optically thick Mg II h and k lines in flares through radiative
transfer calculations. We take a flare atmosphere obtained from
a simulation with the radiative hydrodynamic code RADYN as input
for a radiative transfer modeling with the RH code. By iteratively
changing this model atmosphere and varying thermodynamic parameters
such as temperature, electron density, and velocity, we study their
effects on the emergent intensity spectra. We reproduce the typical
single-peaked Mg II h and k flare spectral shape and approximate the
intensity ratios to the subordinate Mg II lines by increasing either
densities, temperatures, or velocities at the line core formation height
range. Additionally, by combining unresolved upflows and downflows up to
∼250 km s<SUP>-1</SUP> within one resolution element, we reproduce the
widely broadened line wings. While we cannot unambiguously determine
which mechanism dominates in flares, future modeling efforts should
investigate unresolved components, additional heat dissipation, larger
velocities, and higher densities and combine the analysis of multiple
spectral lines.
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Title: Hard X-Ray Emission from Partially Occulted Solar Flares:
RHESSI Observations in Two Solar Cycles
Authors: Effenberger, Frederic; Rubio da Costa, Fatima; Oka, Mitsuo;
Saint-Hilaire, Pascal; Liu, Wei; Petrosian, Vahé; Glesener, Lindsay;
Krucker, Säm
2017ApJ...835..124E Altcode: 2016arXiv161202856E
Flares close to the solar limb, where the footpoints are occulted,
can reveal the spectrum and structure of the coronal looptop source
in X-rays. We aim at studying the properties of the corresponding
energetic electrons near their acceleration site, without footpoint
contamination. To this end, a statistical study of partially occulted
flares observed with Reuven Ramaty High-Energy Solar Spectroscopic
Imager is presented here, covering a large part of solar cycles
23 and 24. We perform detailed spectra, imaging, and light curve
analyses for 116 flares and include contextual observations from
SDO and STEREO when available, providing further insights into flare
emission that were previously not accessible. We find that most spectra
are fitted well with a thermal component plus a broken power-law,
non-thermal component. A thin-target kappa distribution model gives
satisfactory fits after the addition of a thermal component. X-ray
imaging reveals small spatial separation between the thermal and
non-thermal components, except for a few flares with a richer coronal
source structure. A comprehensive light curve analysis shows a very good
correlation between the derivative of the soft X-ray flux (from GOES)
and the hard X-rays for a substantial number of flares, indicative of
the Neupert effect. The results confirm that non-thermal particles
are accelerated in the corona and estimated timescales support the
validity of a thin-target scenario with similar magnitudes of thermal
and non-thermal energy fluxes.
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Title: Fermi-LAT Observations of High-energy Behind-the-limb Solar
Flares
Authors: Ackermann, M.; Allafort, A.; Baldini, L.; Barbiellini, G.;
Bastieri, D.; Bellazzini, R.; Bissaldi, E.; Bonino, R.; Bottacini, E.;
Bregeon, J.; Bruel, P.; Buehler, R.; Cameron, R. A.; Caragiulo, M.;
Caraveo, P. A.; Cavazzuti, E.; Cecchi, C.; Charles, E.; Ciprini, S.;
Costanza, F.; Cutini, S.; D'Ammando, F.; de Palma, F.; Desiante, R.;
Digel, S. W.; Di Lalla, N.; Di Mauro, M.; Di Venere, L.; Drell, P. S.;
Favuzzi, C.; Fukazawa, Y.; Fusco, P.; Gargano, F.; Giglietto, N.;
Giordano, F.; Giroletti, M.; Grenier, I. A.; Guillemot, L.; Guiriec,
S.; Jogler, T.; Jóhannesson, G.; Kashapova, L.; Krucker, S.; Kuss,
M.; La Mura, G.; Larsson, S.; Latronico, L.; Li, J.; Liu, W.; Longo,
F.; Loparco, F.; Lubrano, P.; Magill, J. D.; Maldera, S.; Manfreda,
A.; Mazziotta, M. N.; Mitthumsiri, W.; Mizuno, T.; Monzani, M. E.;
Morselli, A.; Moskalenko, I. V.; Negro, M.; Nuss, E.; Ohsugi, T.;
Omodei, N.; Orlando, E.; Pal'shin, V.; Paneque, D.; Perkins, J. S.;
Pesce-Rollins, M.; Petrosian, V.; Piron, F.; Principe, G.; Rainò,
S.; Rando, R.; Razzano, M.; Reimer, O.; Rubio da Costa, F.; Sgrò,
C.; Simone, D.; Siskind, E. J.; Spada, F.; Spandre, G.; Spinelli,
P.; Tajima, H.; Thayer, J. B.; Torres, D. F.; Troja, E.; Vianello, G.
2017ApJ...835..219A Altcode: 2017arXiv170200577A
We report on the Fermi-LAT detection of high-energy emission from the
behind-the-limb (BTL) solar flares that occurred on 2013 October 11,
and 2014 January 6 and September 1. The Fermi-LAT observations are
associated with flares from active regions originating behind both
the eastern and western limbs, as determined by STEREO. All three
flares are associated with very fast coronal mass ejections (CMEs)
and strong solar energetic particle events. We present updated
localizations of the >100 MeV photon emission, hard X-ray (HXR)
and EUV images, and broadband spectra from 10 keV to 10 GeV, as well
as microwave spectra. We also provide a comparison of the BTL flares
detected by Fermi-LAT with three on-disk flares and present a study
of some of the significant quantities of these flares as an attempt
to better understand the acceleration mechanisms at work during these
occulted flares. We interpret the HXR emission to be due to electron
bremsstrahlung from a coronal thin-target loop top with the accelerated
electron spectra steepening at semirelativistic energies. The >100
MeV gamma-rays are best described by a pion-decay model resulting
from the interaction of protons (and other ions) in a thick-target
photospheric source. The protons are believed to have been accelerated
(to energies >10 GeV) in the CME environment and precipitate down to
the photosphere from the downstream side of the CME shock and landed
on the front side of the Sun, away from the original flare site and
the HXR emission.
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Title: Fermi Large Area Telescope Observations of High-Energy
Gamma-ray Emission From Behind-the-limb Solar Flares
Authors: Omodei, Nicola; Pesce-Rollins, Melissa; Petrosian, Vahe;
Liu, Wei; Rubio da Costa, Fatima; Golenetskii, Sergei; Kashapova,
Larisa; Krucker, Sam; Palshin, Valentin; Fermi Large Area Telescope
Collaboration
2017APS..APR.Y3005O Altcode:
Fermi LAT >30 MeV observations of the active Sun have increased
the number of detected solar flares by almost a factor of 10 with
respect to previous space observations. Of particular interest are the
recent detections of three solar flares whose position behind the limb
was confirmed by the STEREO-B spacecraft. These observations sample
flares from active regions originating from behind both the eastern
and western limbs and include an event associated with the second
ground level enhancement event (GLE) of the 24th Solar Cycle. While
gamma-ray emission up to tens of MeV resulting from proton interactions
has been detected before from occulted solar flares, the significance
of these particular events lies in the fact that these are the first
detections of >100 MeV gamma-ray emission from footpoint-occulted
flares. These detections present an unique opportunity to diagnose
the mechanisms of high-energy emission and particle acceleration and
transport in solar flares. We will present the Fermi-LAT, RHESSI and
STEREO observations of these flares and discuss the various emission
scenarios for these sources.
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Title: Hard X-Ray Observations of Coronal Sources: Implications for
Particle Acceleration
Authors: Effenberger, F.; Rubio da Costa, F.; Oka, M.; Saint-Hilaire,
P.; Liu, W.; Krucker, S.; Glesener, L.; Petrosian, V.
2016AGUFMSH51E2634E Altcode:
The properties of hard X-ray emission from solar flares can provide
insight into particle acceleration and transport processes. Commonly,
at higher energies the bright footpoint emission from the flare loop
prevents a detailed analysis of the weaker loop-top source due to
the limited dynamic range. Thus, flares close to the solar limb,
where the footpoints are occulted, are interesting events to study
because they can reveal the coronal loop-top emission and thus the
electron properties at their acceleration site. We present results of
a survey study of partially occulted flares observed with the Reuven
Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). We found that
most of the flare spectra allow a fit to a thermal plus non-thermal
component, either with a broken power-law or a kappa function. The
spatial separation between the thermal and non-thermal component,
as derived from imaging, is usually small. The light curve analysis
shows for many flares a very good correlation between the derivative
of the soft X-ray flux and the hard X-rays. We discuss implications
for particle acceleration models that result from our study.
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Title: Particle Acceleration in High-Energy Solar Flares Detected
by the Fermi Large Area Telescope
Authors: Omodei, N.; Pesce-Rollins, M.; Petrosian, V.; Liu, W.;
Rubio da Costa, F.
2016AGUFMSH41D..02O Altcode:
The Fermi Large Area Telescope (LAT) is the most sensitive instrument
ever deployed in space for observing gamma-ray emission above 100
MeV. LAT observations of the active Sun have increased the number of
detected solar flares by almost a factor of 10 with respect to previous
space observations. We will present an overview of these observations,
which include detections of impulsive and sustained emission, extending
up to 20 hours in the case of the X-class flare occurred on 2012
March 7. Of particular interest is the first detection of >100 MeV
gamma-ray emission from three solar flares whose positions behind the
limb were confirmed by the STEREO spacecrafts. These observations sample
flares from active regions originating from behind both the eastern
and western limbs and present a unique opportunity to diagnose the
mechanisms of high-energy emission and particle acceleration in solar
flares. We will present the Fermi-LAT, RHESSI and STEREO observations
of these flares and discuss how these observations provide constrains
on different emission mechanisms.
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Title: Hard X-ray morphology of the X1.3 April 25, 2014 partially
occulted limb solar flare
Authors: Effenberger, Frederic; Rubio da Costa, Fátima; Petrosian,
Vahé
2016JPhCS.767a2005E Altcode: 2016arXiv160504858E
At hard X-ray energies, the bright footpoint emission from solar
flare loops often prevents a detailed analysis of the weaker loop-top
source morphology due to the limited dynamic range available for
X-ray imaging. Here, we study the X1.3 April 25, 2014 flare with the
Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). This
partially occulted limb flare allows the analysis of the loop-top
emission in isolation. We present results on the flare light curve
at different energies, the source morphology from X-ray imaging and
a detailed spectral analysis of the different source components by
imaging spectroscopy. The loop-top source, a likely site of particle
acceleration, shows a clear composition of different emission
components. The results indicate the opportunities that detailed
imaging of hard X-rays can provide to learn about particle acceleration,
transport and heating processes in solar flares.
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Title: Data-driven Radiative Hydrodynamic Modeling of the 2014 March
29 X1.0 Solar Flare
Authors: Rubio da Costa, Fatima; Kleint, Lucia; Petrosian, Vahé;
Liu, Wei; Allred, Joel C.
2016ApJ...827...38R Altcode: 2016arXiv160304951R; 2016ApJ...827...38D
Spectroscopic observations of solar flares provide critical diagnostics
of the physical conditions in the flaring atmosphere. Some key
features in observed spectra have not yet been accounted for in
existing flare models. Here we report a data-driven simulation of
the well-observed X1.0 flare on 2014 March 29 that can reconcile some
well-known spectral discrepancies. We analyzed spectra of the flaring
region from the Interface Region Imaging Spectrograph (IRIS) in Mg
II h&k, the Interferometric BIdimensional Spectropolarimeter at
the Dunn Solar Telescope (DST/IBIS) in Hα 6563 Å and Ca II 8542 Å,
and the Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI)
in hard X-rays. We constructed a multithreaded flare loop model and
used the electron flux inferred from RHESSI data as the input to
the radiative hydrodynamic code RADYN to simulate the atmospheric
response. We then synthesized various chromospheric emission lines
and compared them with the IRIS and IBIS observations. In general, the
synthetic intensities agree with the observed ones, especially near the
northern footpoint of the flare. The simulated Mg II line profile has
narrower wings than the observed one. This discrepancy can be reduced
by using a higher microturbulent velocity (27 km s<SUP>-1</SUP>) in a
narrow chromospheric layer. In addition, we found that an increase of
electron density in the upper chromosphere within a narrow height range
of ≈800 km below the transition region can turn the simulated Mg II
line core into emission and thus reproduce the single peaked profile,
which is a common feature in all IRIS flares.
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Title: Understanding the formation of the Mg II h&k lines during
solar flares
Authors: Rubio Da Costa, Fatima; Kleint, Lucia; Petrosian, Vahe';
Liu, Wei; Allred, Joel C.
2016SPD....4740304R Altcode:
The Mg II h&k lines are useful diagnostics for physical processes
in the solar chromosphere. Understanding the line formation is
crucial for the correct interpretation of spectral observations
and characteristics such as line asymmetries or how their central
reversals in the line cores disappear and turn into emission during
flares are manifestations of various physical processes.Focusing on
the well-observed X1.0 flare on 2014 March 29, we carried out a joint
observational and modeling study to analyze the Mg II h&k spectra
observed by IRIS. We constructed a multi-threaded flare loop model and
used the time-dependent electron flux inferred from the RHESSI hard
X-ray data as the input to the radiative hydrodynamic code RADYN to
simulate the atmospheric response. Using the RH code we conducted a
detailed modeling on line shape and evolution to derive how different
atmospheric parameters may affect the MgII line emission.We successfully
simulated the single-peaked Mg II h&k line profiles by increasing
electron density in the upper chromosphere within a narrow height range
of ≈ 800 km below the transition region. To our knowledge, this is the
first successful attempt in reproducing such line-profile shapes under
flaring conditions. We will discuss the implications of this result for
diagnosing atmospheric dynamics and energy transport in solar flares.
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Title: A self-consistent combined radiative transfer hydrodynamic and
particle acceleration model for the X1.0 class flare on March 29, 2014
Authors: Rubio da Costa, F.; Kleint, L.; Sainz Dalda, A.; Petrosian,
V.; Liu, W.
2015AGUFMSH31B2419R Altcode:
The X1.0 flare on March 29, 2014 was well observed, covering its
emission at several wavelengths from the photosphere to the corona. The
RHESSI spectra images allow us to estimate the temporal variation of
the electron spectra using regularized inversion techniques. Using
this as input for a combined particle acceleration and transport
(Stanford-Flare) and radiative transfer hydrodynamic (Radyn) code, we
calculate the response of the atmosphere to the electron heating. We
will present the evolution of the thermal continuum and several line
emissions. Comparing them with GOES soft X-ray and high resolution
observations from IRIS, SDO and DST/IBIS allows us to test the basic
mechanism(s) of acceleration and to constrain its characteristics. We
will also present perspectives on how to apply this methodology and
related diagnostics to other flares.
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Title: Combined Modeling of Acceleration, Transport, and Hydrodynamic
Response in Solar Flares. II. Inclusion of Radiative Transfer
with RADYN
Authors: Rubio da Costa, Fatima; Liu, Wei; Petrosian, Vahé; Carlsson,
Mats
2015ApJ...813..133R Altcode: 2015arXiv150501549R
Solar flares involve complex processes that are coupled and span a
wide range of temporal, spatial, and energy scales. Modeling such
processes self-consistently has been a challenge in the past. Here we
present results from simulations that couple particle kinetics with
hydrodynamics (HD) of the atmospheric plasma. We combine the Stanford
unified Fokker-Planck code that models particle acceleration and
transport with the RADYN HD code that models the atmospheric response
to collisional heating by accelerated electrons through detailed
radiative transfer calculations. We perform simulations using two
different electron spectra, one an ad hoc power law and the other
predicted by the model of stochastic acceleration by turbulence or
plasma waves. Surprisingly, the later model, even with energy flux \ll
{10}<SUP>10</SUP> {erg} {{{s}}}<SUP>-1</SUP> {{cm}}<SUP>-2</SUP>, can
cause “explosive” chromospheric evaporation and drive stronger
up- and downflows (and HD shocks). This is partly because our
acceleration model, like many others, produces a spectrum consisting
of a quasi-thermal component plus a power-law tail. We synthesize
emission-line profiles covering different heights in the lower
atmosphere, including Hα 6563 Å, He ii 304 Å, Ca ii K 3934 Å, and
Si iv 1393 Å. One interesting result is the unusual high temperature
(up to a few times 10<SUP>5</SUP> K) of the formation site of He ii
304 Å, which is expected owing to photoionization-recombination under
flare conditions, compared to those in the quiet Sun dominated by
collisional excitation. When compared with observations, our results
can constrain the properties of nonthermal electrons and thus the
poorly understood particle acceleration mechanism.
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Title: Fermi Large Area Telescope observation of high-energy solar
flares: constraining emission scenarios
Authors: Omodei, Nicola; Pesce-Rollins, Melissa; Petrosian, Vahe;
Liu, Wei; Rubio da Costa, Fatima
2015IAUGA..2255415O Altcode:
The Fermi Large Area Telescope (LAT) is the most sensitive instrument
ever deployed in space for observing gamma-ray emission >100
MeV. This has also been demonstrated by its detection of quiescent
gamma-ray emission from pions produced by cosmic-ray protons
interacting in the solar atmosphere, and from cosmic-ray electron
interactions with solar optical photons. The Fermi LAT has also
detected high-energy gamma-ray emission associated with GOES M-class
and X-class X-ray flares, each accompanied by a coronal mass ejection
and a solar energetic particle event increasing the number of detected
solar flares by almost a factor of 10 with respect to previous space
observations. During the impulsive phase, gamma rays with energies up
to several hundreds of MeV have been recorded by the LAT. Emission up
to GeV energies lasting several hours after the flare has also been
recorded by the LAT. Of particular interest are the recent detections
of two solar flares whose position behind the limb was confirmed by
the STEREO-B satellite. While gamma-ray emission up to tens of MeV
resulting from proton interactions has been detected before from
occulted solar flares, the significance of these particular events
lies in the fact that these are the first detections of >100 MeV
gamma-ray emission from footpoint-occulted flares. We will present the
Fermi-LAT, RHESSI and STEREO observations of these flares and discuss
the various emission scenarios for these sources.
---------------------------------------------------------
Title: Fermi Large Area Telescope observations of high-energy
gamma-ray emission from behind-the-limb solar flares
Authors: Pesce-Rollins, M.; Omodei, N.; Petrosian, V.; Liu, W.;
Rubio da Costa, F.; Allafort, A.; Fermi-LAT Collaboration
2015ICRC...34..128P Altcode: 2015arXiv150704303P; 2015PoS...236..128P
Fermi-LAT >30 MeV observations have increased the number of detected
solar flares by almost a factor of 10 with respect to previous space
observations. These sample both the impulsive and long duration phases
of GOES M and X class flares. Of particular interest is the recent
detections of three solar flares whose position behind the limb was
confirmed by the STEREO-B spacecraft. While gamma-ray emission up to
tens of MeV resulting from proton interactions has been detected before
from occulted solar flares, the significance of these particular events
lies in the fact that these are the first detections of >100 MeV
gamma-ray emission from footpoint-occulted flares. We will present the
Fermi-LAT, RHESSI and STEREO observations of these flares and discuss
the various emission scenarios for these sources and implications for
the particle acceleration mechanisms.
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Title: First Detection of &gt100 MeV Gamma Rays Associated with
a Behind-the-limb Solar Flare
Authors: Pesce-Rollins, M.; Omodei, N.; Petrosian, V.; Liu, Wei;
Rubio da Costa, Fatima; Allafort, A.; Chen, Qingrong
2015ApJ...805L..15P Altcode: 2015arXiv150503480P
We report the first detection of >100 MeV gamma-rays associated with
a behind-the-limb solar flare, which presents a unique opportunity to
probe the underlying physics of high-energy flare emission and particle
acceleration. On 2013 October 11 a GOES M1.5 class solar flare occurred
∼9.°9 behind the solar limb as observed by STEREO-B. RHESSI observed
hard X-ray (HXR) emission above the limb, most likely from the flare
loop-top, as the footpoints were occulted. Surprisingly, the Fermi Large
Area Telescope (LAT) detected >100 MeV gamma-rays for ∼30 minutes
with energies up to 3 GeV. The LAT emission centroid is consistent
with the RHESSI HXR source, but its uncertainty does not constrain the
source to be located there. The gamma-ray spectra can be adequately
described by bremsstrahlung radiation from relativistic electrons having
a relatively hard power-law (PL) spectrum with a high-energy exponential
cutoff, or by the decay of pions produced by accelerated protons and
ions with an isotropic pitch-angle distribution and a PL spectrum with
a number index of ∼3.8. We show that high optical depths rule out
the gamma-rays originating from the flare site and a high-corona trap
model requires very unusual conditions, so a scenario in which some
of the particles accelerated by the CME shock travel to the visible
side of the Sun to produce the observed gamma-rays may be at work.
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Title: Solar Flare Chromospheric Line Emission: Comparison Between
IBIS High-resolution Observations and Radiative Hydrodynamic
Simulations
Authors: Rubio da Costa, Fatima; Kleint, Lucia; Petrosian, Vahé;
Sainz Dalda, Alberto; Liu, Wei
2015ApJ...804...56R Altcode: 2015ApJ...804...56D; 2014arXiv1412.1815R
Solar flares involve impulsive energy release, which results in enhanced
radiation over a broad spectral range and a wide range of heights. In
particular, line emission from the chromosphere can provide critical
diagnostics of plasma heating processes. Thus, a direct comparison
between high-resolution spectroscopic observations and advanced
numerical modeling results could be extremely valuable, but has not
yet been attempted. In this paper, we present such a self-consistent
investigation of an M3.0 flare observed by the Dunn Solar Telescope’s
Interferometric Bi-dimensional Spectrometer (IBIS) on 2011 September 24
which we have modeled using the radiative hydrodynamic code RADYN. We
obtained images and spectra of the flaring region with IBIS in Hα
6563 Å and Ca ii 8542 Å, and with RHESSI in X-rays. The latter
observations were used to infer the non-thermal electron population,
which was passed to RADYN to simulate the atmospheric response to
electron collisional heating. We then synthesized spectral lines and
compared their shapes and intensities to those observed by IBIS and
found a general agreement. In particular, the synthetic Ca ii 8542
Å profile fits well to the observed profile, while the synthetic Hα
profile is fainter in the core than for the observation. This indicates
that Hα emission is more responsive to the non-thermal electron flux
than the Ca ii 8542 Å emission. We suggest that it is necessary to
refine the energy input and other processes to resolve this discrepancy.
---------------------------------------------------------
Title: Observations and Interpretation of Behind the Limb Solar
Flares Detected by Fermi-LAT and Other Instruments
Authors: Petrosian, Vahe; Omodei, Nicola; Pesce-Rollins, Melissa;
Rubio da Costa, Fatima; Liu, Wei
2015TESS....120505P Altcode:
The Fermi Large Area Telescope (LAT) is the most sensitive instrument
ever deployed in space for observing > 30 MeV gamma-rays. During the
past active period of the Sun the LAT has detected more than 40 flares
up to GeV energies some of which occur behind the limb as determined
by STEREO observations. We will present the observations on two such
flares with significant flux of > 100 MeV (and some indication of
1 to 10 MeV detected by Fermi-GBM) gamma-rays coming from the visible
disk while the flare and associated CMEs are initiated in active regions
tens of degrees behind the visible limb of the Sun. We will consider
acceleration of particles, their transport and radiative signatures, and
the transfer of these radiation in the solar atmosphere to distinguish
between (i) acceleration in the low corona, in a high corona trap,
and/or in the CME driven shock; (ii) between continuous and prompt
acceleration; and (iii) between electron bremsstrahlung and decay of
pions produced by accelerated ions.
---------------------------------------------------------
Title: Electron Acceleration and Radiative Hydrodynamic Simulations
of the 29 March 2014 X1.0 flare
Authors: Rubio da Costa, Fatima; Kleint, Lucia; Petrosian, Vahe
2015TESS....130205R Altcode:
The X1.0 flare on 29 March 2014 presents a unique opportunity to use
its observations to better understand the origin of the white light
emission and the evolution of the spectral line profiles. RHESSI
observed the whole flare including the impulsive phase, allowing us to
estimate the variation of the spectral parameters of the accelerated
electrons using the Stanford acceleration code. Using this as input
to the radiative RADYN code, we determine the hydrodynamic response
of the solar atmosphere and the spectrum of the continuum and line
emission. Using this self consistent results and observations we
constrain the characteristics of the acceleration mechanism.
---------------------------------------------------------
Title: Centre-to-limb properties of small, photospheric quiet-Sun jets
Authors: Rubio da Costa, F.; Solanki, S. K.; Danilovic, S.; Hizberger,
J.; Martínez-Pillet, V.
2015A&A...574A..95R Altcode: 2014arXiv1412.1620R
Context. Strongly Doppler-shifted Stokes V profiles have been detected
in the quiet Sun with the IMaX instrument on-board the SUNRISE
stratospheric balloon-borne telescope. High velocities are required
to produce such signals, hence these events have been interpreted as
jets, although other sources are also possible. <BR /> Aims: We aim
to characterize the variation of the main properties of these events
(occurrence rate, lifetime, size, and velocities) with their position on
the solar disk between disk centre and the solar limb. <BR /> Methods:
These events were identified in SUNRISE/IMaX data according to the same
objective criteria at all available positions on the solar disk. Their
properties were determined using standard techniques. <BR /> Results:
Our study yielded a number of new insights into this phenomenon. Most
importantly, the number density of these events is independent of
the heliocentric angle, meaning that the investigated supersonic
flows are nearly isotropically distributed. Size and lifetime are
also nearly independent of the heliocentric angle, while their
intensity contrast increases towards the solar limb. The Stokes V
jets are associated with upflow velocities deduced from Stokes I,
which are stronger towards the limb. Their intensity decreases with
time, while their line-of-sight velocity does not display a clear
temporal evolution. Their association with linear polarization signals
decreases towards the limb. <BR /> Conclusions: The density of events
appears to be independent of heliocentric angle, establishing that they
are directed nearly randomly. If these events are jets triggered by
magnetic reconnection between emerging magnetic flux and the ambient
field, then our results suggest that there is no preferred geometry
for the reconnection process.
---------------------------------------------------------
Title: Shedding new light on the Sun with the Fermi LAT
Authors: Omodei, N.; Petrosian, V.; Liu, W.; Rubio da Costa, F.;
Chen, Q.; Pesce-Rollins, M.; Grove, E.; Longo, F.
2015arXiv150203895O Altcode:
During its first six years of operation, the Fermi Large Area Telescope
(LAT) has detected >30 MeV gamma-ray emission from more than
40 solar flares, nearly a factor of 10 more than those detected by
EGRET. These include detections of impulsive and sustained emissions,
extending up to 20 hours in the case of the 2012 March 7 X-class
flares. We will present an overview of solar flare detections with
LAT, highlighting recent results and surprising features, including
the detection of >100 MeV emission associated with flares located
behind the limb. Such flares may shed new light on the relationship
between the sites of particle acceleration and gamma-ray emission.
---------------------------------------------------------
Title: Comparison between IBIS Observations and Radiative Transfer
Hydrodynamic Simulations of a Solar Flare
Authors: Rubio da Costa, F.; Kleint, L.; Liu, W.; Sainz Dalda, A.;
Petrosian, V.
2014AGUFMSH13B4104R Altcode:
High-resolution spectroscopic observations of solar flares are
rare but can provide valuable diagnostics. On September 24, 2011 an
M3.0 class flare was observed by the Interferometric BIdimensional
Spectropolarimeter (IBIS) in chromospheric Hα and CaII 8542 Å
lines and by the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) in X-rays. We fitted the RHESSI spectra at different times
with a power-law plus isothermal component. We then used the fitted
real-time spectral parameters of nonthermal electrons as the input to
the RADYN radiative hydrodynamic code (Carlsson et al, 1992, 1996;
Allred et al, 2005) to simulate the low-chromospheric response to
collisional heating by energetic electrons. We synthesized both the
Hα and CaII 8542 Å lines from the simulation results and compare
them with the IBIS observations. We discuss the constraints from this
comparison on particle acceleration mechanisms in solar flares.
---------------------------------------------------------
Title: Fermi Detection of Gamma-ray Emission from a Behind-the-limb
M1.5 Flare on 2013 October 11
Authors: Pesce-Rollins, Melissa; Omodei, Nicola; Petrosian, Vahe;
Liu, Wei; Chen, Qingrong; Rubio Da Costa, Fatima
2014shin.confE..91P Altcode:
On 2013 October 11 an M1.5 class solar flare erupted from the NOAA
active region 11868, which was then behind the solar limb. RHESSI images
reveal hard X-ray emission well above the limb, most likely from the
top of the flare loop whose footpoints were occulted. Surprisingly the
Fermi Large Area Telescope (LAT) detected gamma-rays up to 3 GeV for
30 minutes from this flare, making it the first behind-the-limb flare
observed by Fermi. The LAT gamma-ray emission centroid is consistent
with the vicinity of the RHESSI hard X-ray source. The gamma-ray
spectra can be adequately described by a power law with a high-energy
exponential cutoff, or as a result of the decay of pions produced by
accelerated protons and ions with an isotropic pitch angle distribution
and a power-law energy spectrum. The required proton spectrum would
have a number index of 3.8. STEREO-B detected enhanced electron,
proton, and other ion fluxes from this flare. We present the Fermi and
RHESSI observations together with STEREO and SDO data to explore the
various emission scenarios of this behind-the-limb flare, as well as
the possible correlation with the SEPs.
---------------------------------------------------------
Title: Fermi Detection of Gamma-ray Emission from a Behind-the-limb
M1.5 Flare on 2013 October 11
Authors: Pesce-Rollins, Melissa; Omodei, Nicola; Petrosian, Vahe;
Liu, Wei; Chen, Qingrong; Rubio Da Costa, Fatima
2014AAS...22441404P Altcode:
On 2013 October 11 an M1.5 class solar flare erupted from the NOAA
active region 11868, which was then behind the solar limb. RHESSI images
reveal hard X-ray emission well above the limb, most likely from the
top of the flare loop whose footpoints were occulted. Surprisingly
the Fermi Large Area Telescope (LAT) detected gamma-rays up to 3 GeV
for ~30 minutes from this flare, making it the first behind-the-limb
flare observed by Fermi. The LAT gamma-ray emission centroid is
consistent with the vicinity of the RHESSI hard X-ray source. The
gamma-ray spectra can be adequately described by a power law with a
high-energy exponential cutoff, or as a result of the decay of pions
produced by accelerated protons and ions with an isotropic pitch angle
distribution and a power-law energy spectrum. The required proton
spectrum would have a number index of ~3.8. We present the Fermi and
RHESSI observations together with STEREO and SDO data to explore the
various emission scenarios of this behind-the-limb flare.
---------------------------------------------------------
Title: Hybrid Kinetic and Radiative Hydrodynamic Simulations of
Solar Flares and Comparison With Multiwavelength Observations
Authors: Rubio Da Costa, Fatima; Petrosian, Vahe; Liu, Wei; Carlsson,
Mats; Kleint, Lucia
2014AAS...22440906R Altcode:
We present a unified simulation which combines two physical processes:
how the particles are accelerated and the energy is transported along
a coronal loop, and how the atmosphere responds. The “flare”
code from Stanford University (Petrosian et al, 2001) models the
stochastic acceleration and transport of particles and radiation of
solar flares. It includes pitch angle diffusion and energy loss, and
computes collisional heating to the background plasma and bremsstrahlung
emission along the loop. The radiative hydrodynamic RADYN Code
(Carlsson et al, 1992, 1996; Allred et al, 2005) computes the energy
transport by the injected non-thermal electrons at the top of a 1D
coronal loop. Recently, we have combined the two codes by updating the
non-thermal heating in the RADYN code from the "flare" code, allowing us
to develop a self-consistent simulation. In addition, we can now model
more realistically the multi-wavelength emission of solar flares and
compare it with observations, e.g., at optical wavelengths from IBIS at
the Dunn Solar Telescope and in X-rays from RHESSI. The high resolution
UV observations from the recently launched IRIS imaging spectrograph
will be particularly useful in this regard. These will allow us to
compare numerically modeled and observed emissions of solar flares in
several lines using more robust simulations than possible before.
---------------------------------------------------------
Title: A new approach to model particle acceleration and energy
transfer in solar flares
Authors: Rubio Da Costa, Fatima; Zuccarello, F.; Fletcher, L.;
Labrosse, N.; Kasparova, J.; Prosecký, T.; Carlsson, M.; Petrosian,
V.; Liu, W.
2013SPD....4440401R Altcode:
Motivated by available observations of two different flares in Lyα and
Hα, we model the conditions of the solar atmosphere using a radiation
hydrodynamics code (RADYN, Carlsson & Stein, 1992) and analyze the
energy transport carried by a beam of non-thermal electrons injected
at the top of a 1D coronal loop. The numerical Lyα and Hα intensities
match with the observations. The electron energy distribution is assumed
to follow a power law of the form (E/E<SUP>c</SUP> )<SUB>-δ</SUB> for
energies greater than a cutoff value of E<SUP>c</SUP>. Abbett &
Hawley (1999) and Allred et al. (2005) assumed that the non-thermal
electrons flux injected at the top of a flaring loop, the cut-off energy
and the power law index are constant over time. An improvement was
achieved by Allred & Hawley (2006), who modified the RADYN code
in such a way that the input parameters were time dependent. Their
inputs were based on observations of a flare obtained with RHESSI. By
combining RADYN with the “flare” code from Stanford University
which models the acceleration and transport of particles and radiation
of solar flares in non-LTE regime, we can calculate the non-thermal
electrons flux, the cut-off energy and the power law index at every
simulated time step. The atmospheric parameters calculated by RADYN
could in turn be used as updated inputs for "flare", providing several
advantages over the results from Liu et al. (2009), who combined the
particle acceleration code with a 1-D hydrodynamic code, improving
the atmospheric conditions.
---------------------------------------------------------
Title: The role of filament activation in a solar eruption
Authors: Rubio da Costa, F.; Zuccarello, F.; Fletcher, L.; Romano,
P.; Labrosse, N.
2012A&A...539A..27R Altcode: 2014arXiv1412.1858R
Context. Observations show that the mutual relationship between
filament eruptions and solar flares cannot be described in terms of an
unique scenario. In some cases, the eruption of a filament appears to
trigger a flare, while in others the observations are more consistent
with magnetic reconnection that produces both the flare observational
signatures (e.g., ribbons, plasma jets, post-flare loops, etc.) and
later the destabilization and eruption of a filament. <BR /> Aims:
Contributing to a better comprehension of the role played by filament
eruptions in solar flares, we study an event which occurred in NOAA
8471, where a flare and the activation of (at least) two filaments
were observed on 28 February 1999. <BR /> Methods: By using imaging
data acquired in the 1216, 1600, 171 and 195 Å TRACE channels and by
BBSO in the continnum and in the Hα line, a morphological study of
the event is carried out. Moreover, using TRACE 1216 and 1600 Å data,
an estimate of the "pure" Lyα power is obtained. The extrapolation of
the magnetic field lines is done using the SOHO/MDI magnetograms and
assuming a potential field. <BR /> Results: Initially an area hosting a
filament located over a δ spot becomes brighter than the surroundings,
both in the chromosphere and in the corona. This area increases in
brightness and extension, eventually assuming a two-ribbon morphology,
until it reaches the eastern part of the active region. Here a second
filament becomes activated and the brightening propagates to the south,
passing over a large supergranular cell. The potential magnetic field
extrapolation indicates that the field line connectivity changes
after the flare. <BR /> Conclusions: The event is triggered by the
destabilization of a filament located between the two polarities
of a δ spot. This destabilization involves the magnetic arcades
of the active region and causes the eruption of a second filament,
that gives rise to a CME and to plasma motions over a supergranular
cell. We conclude that in this event the two filaments play an active
and decisive role, albeit in different stages of the phenomenon,
in fact the destabilization of one filament causes brightenings,
reconnection and ribbons, while the second one, whose eruption is caused
by the field reconfiguration resulting from the previous reconnection,
undergoes the greatest changes and causes the CME.
---------------------------------------------------------
Title: Relationship between an M6.6 solar flare and subsequent
filament activations.
Authors: Rubio da Costa, F.; Zuccarello, F.; Romano, P.; Fletcher,
L.; Labrosse, N.
2012MSAIS..19..113R Altcode:
We study an event which occurred in NOAA 8471, where an M6.6 flare
and the activation of two filaments were observed on 28 February
1999. A multi-wavelength study allows us to investigate the behavior
of the several features observed at different atmospheric levels, that
might be used to answer to the question whether and in what conditions
the eruption of filaments can play an active or a passive role in the
flare occurrence. Imaging data were acquired by BBSO in the Halpha line
and by TRACE in the 1216, 1600, 171 and 195 Å channels, allowing us
to deduce the morphology and temporal evolution of the event and to
estimate the Ly-alpha power. Moreover, in order to study the magnetic
topology, the extrapolation of the photospheric magnetic field lines
was done assuming potential field and using SOHO/MDI magnetograms.
---------------------------------------------------------
Title: Solar flares in Halpha and Ly-alpha : observations vs
simulations.
Authors: Rubio da Costa, F.; Zuccarello, F.; Fletcher, L.; Labrosse,
N.; Prosecký, T.; Kašparová, J.
2012MSAIS..19..117R Altcode:
In order to study the properties of faint, moderate and bright flares,
we simulate the conditions of the solar atmosphere using a radiative
hydrodynamic model \citep{2005ApJ...630..573A}. A constant beam of
non-thermal electrons is injected at the apex of a 1D coronal loop
and heating from thermal soft X-ray and UV emission is included. We
study the contribution of different processes to the total intensity of
different lines at different atmospheric layers. We obtain the total
integrated intensity of different lines and we compare those of the
Ly-alpha and Halpha lines with the observational values for Ly-alpha
(using TRACE 1216 and 1600 Å data and estimating the “pure” Ly-alpha
emission) and Halpha (using data from the Ondřejov Observatory). We
inferred from the analysis of the values obtained by simulation that
the X-ray energy of the different kind of flares does not strongly
affect the Ly-alpha results; the Halpha results are comparable to the
observed ones, concluding that the simulated solar atmosphere fits
better at lower layers of the chromosphere than at upper layers.
---------------------------------------------------------
Title: Detection of Active Regions in Solar Images Using Visual
Attention
Authors: Cannavo, Flavio; Spampinato, Concetto; Giordano, Daniela;
Rubio da Costa, Fatima; Nunnari, Silvia
2011dict.book..231C Altcode:
This paper deals with the problem of processing solar images using
a visual saliency based approach. The system consists of two main
parts: 1) a pre-processing part carried out by using an enhancement
method that aims at highlighting the Sun in solar images and 2) a
visual saliency based approach that detects active regions (events
of interest) on the pre-processed images. Experimental results show
that the proposed approach exhibits a precision index of about of 70%
and thus it is, to some extent, suitable to allow detection of active
regions, without human assistance, mainly in massive processing of
solar images. However, the recall performance points out that at the
current stage of development the method has room for improvements
in detecting some active areas, as shown the F-score index that at
presently is about 60%.
---------------------------------------------------------
Title: Solar flares: observations vs simulations
Authors: Rubio da Costa, Fatima; Zuccarello, Francesca; Labrosse,
Nicolas; Fletcher, Lyndsay; Prosecký, Tomáš; Kašparová, Jana
2011IAUS..274..182R Altcode:
In order to study the properties of faint, moderate and bright flares,
we simulate the conditions of the solar atmosphere using a radiative
hydrodynamic model (Abbett & Hawley, 1999). A constant beam of
non-thermal electrons is injected at the apex of a 1D coronal loop and
heating from thermal soft X-ray emission is included. We compare the
results with some observational data in Ly-α (using TRACE 1216 and
1600 Å data and estimating the “pure” Ly-α emission) and in Hα
(data taken with a Multichannel Flare Spectrograph, at the Ondrejov
Observatory).
---------------------------------------------------------
Title: Solar Chromospheric Flares: Observations in Ly-lpha and Hlpha
and Radiative Hydrodynamic Simulations
Authors: Rubio da Costa, Fatima
2011PhDT........11R Altcode:
This thesis is divided into two main parts: a multiwavelength
observational study of solar flares, focusing mainly in the chromosphere
in Ly-α and Hα, and an application of a radiative transfer code
and a radiative hydrodynamic code, to compare the results obtained
by observations with the simulated ones. The Ly-α emission is a very
interesting line because it is a natural tracer of the solar activity
in the chromosphere. The Transition Region And Coronal Explorer
satellite observed a small number of flares in the Ly-α passband,
but apart from this, these events have not often been observed in this
strong chromospheric line. Because TRACE has a broad Ly-α channel,
in order to estimate the "pure" Lyα emission, we had to apply an
empirical correction. We found that there is a reasonable coverage
in TRACE 1216 A and the TRACE 1600 A for two different flares: on 8
September 1999 and on 28 February 1999. Studying them we estimated,
for the first time, the pure Ly-α flare signature, being on the order
of 10^25 erg/s at the flare peak. The study of the first flare gave us
the possibility to calculate the electron energy budget using the X-ray
data from Yohkoh/HXT in the context of the collisional thick target
model, finding that the Ly-α power is less than 10% of the power
inferred by the electrons. The morphology and evolution of the second
flare were described in different wavelengths by using imaging data
acquired by TRACE and by BBSO in white light and in Hα. We studied
the magnetic topology using the magnetic field provided by SOHO/MDI,
extrapolating the photospheric magnetic field lines, assuming a
potential field. We found different morphologies in the magnetic
configuration before and after the flare, confirming the occurrence of
a reconnection process. The Hα line is the most important line in the
chromosphere. We studied the Hα emission of a flare which occurred on
3 July 2002 using some spectroscopical observations from the Ondrejov
Observatory. Analyzing the available data in other wavelengths, we made
a morphological study of the active region from three hours before the
flare to seven hours after it. The results obtained by observations,
both in the form of integrated intensity as a function of time, and
detailed line profiles, motivated the second part of the thesis. In
this, we used a radiative transfer code (Gouttebroze et al. 1978)
applying different atmospheric models as input parameters in order to
compute the hydrogen spectral lines and study how they are affected
by the temperature and microturbulent stratification. In particular,
the intensity of the Ly-α and Hα lines is related to the temperature
stratification of the atmospheric model, the position of the transition
region being a key factor. The variation of the microturbulent
velocity does not significantly affect the resulting intensities,
but we observed that an increase of the microturbulent velocity
broadens the line profiles. The RADYN Radiative HydroDynamic code
(Allred et al. 2005) was applied to solar flares, modelling a flare
loop from its footpoints in the photosphere to its apex in the corona
by adding non-thermal heating at the lower atmosphere and soft X-ray
irradiation. The majority of this work was to deal with investigating
the dynamical response of the solar chromosphere to energy injected in
the form of non-thermal electrons during solar flares. We studied the
flare energy transport and radiation production in the chromosphere as
well as the Hα and Ly-α emission. The Ly-α intensity is affected
by the flux of the initial beam of electrons injected at the top
of the loop, while the Hα intensity appears to be less affected by
the flare model. Comparing the observational results in Lyα and Hα
with the computed ones from the radiative code and the RADYN code,
we found that the RADYN code fits better the Hα intensities to the
observations than the Lyα intensities, concluding that the code gives
a better description of processes in the lower chromosphere than those
in the upper layers.
---------------------------------------------------------
Title: Integrated Ly-alpha intensity emission in ribbon flares
Authors: Rubio da Costa, F.; Fletcher, L.; Labrosse, N.; Zuccarello, F.
2010MSAIS..14..193R Altcode:
We have analyzed two flares observed by TRACE in Ly alpha (on 8th
September 1999 and 28th February 1999) in order to deduce their
morphology, temporal evolution, radiative outputs and compare these
results with data obtained in the X-range (SXT and HXT on Yohkoh) and
with magnetograms (MDI/SOHO). These observational data and the results
obtained by a theoretical study of the intensity of the radiation
emitted by hydrogen lines, contribute to construct semi-empirical and
theoretical models of the chromospheric emission during flares. Future
observations by the planned Extreme Ultraviolet Imager selected for
the Solar Orbiter mission -which will have a Lyman alpha channel-
and this work, can help in designing observational flare studies.
---------------------------------------------------------
Title: Observations of a solar flare and filament eruption in Lyman
α and X-rays
Authors: Rubio da Costa, F.; Fletcher, L.; Labrosse, N.; Zuccarello, F.
2009A&A...507.1005R Altcode: 2009arXiv0909.4705R
Context: Lα is a strong chromospheric emission line, which has been
relatively rarely observed in flares. The Transition Region and Coronal
Explorer (TRACE) has a broad “Lyman α” channel centered at 1216
Å used primarily at the beginning of the mission. A small number of
flares were observed in this channel. <BR />Aims: We aim to characterise
the appearance and behaviour of a flare and filament ejection which
occurred on 8th September 1999 and was observed by TRACE in Lα, as well
as by the Yohkoh Soft and Hard X-ray telescopes. We explore the flare
energetics and its spatial and temporal evolution. We have in mind the
fact that the Lα line is a target for the Extreme Ultraviolet Imaging
telescope (EUI) which has been selected for the Solar Orbiter mission,
as well as the LYOT telescope on the proposed SMESE mission. <BR
/>Methods: We use imaging data from the TRACE 1216 Å, 1600 Å and
171 Å channels, and the Yohkoh hard and soft X-ray telescopes. A
correction is applied to the TRACE data to obtain a better estimate of
the pure Lα signature. The Lα power is obtained from a knowledge of
the TRACE response function, and the flare electron energy budget is
estimated by interpreting Yohkoh/HXT emission in the context of the
collisional thick target model. <BR />Results: We find that the Lα
flare is characterised by strong, compact footpoints (smaller than the
UV ribbons) which correlate well with HXR footpoints. The Lα power
radiated by the flare footpoints can be estimated, and is found to be
on the order of 10<SUP>26</SUP> erg s<SUP>-1</SUP> at the peak. This is
less than 10% of the power inferred for the electrons which generate
the co-spatial HXR emission, and can thus readily be provided by
them. The early stages of the filament eruption that accompany the
flare are also visible, and show a diffuse, roughly circular spreading
sheet-like morphology, with embedded denser blobs. <BR />Conclusions:
On the basis of this observation, we conclude that flare and filament
observations in the Lα line with the planned EUI and LYOT telescopes
will provide valuable insight into solar flare evolution and energetics,
especially when accompanied by HXR imaging and spectroscopy.
---------------------------------------------------------
Title: Evolution of an eruptive flare loop system
Authors: Romano, P.; Zuccarello, F.; Fletcher, L.; Rubio da Costa,
F.; Bain, H. M.; Contarino, L.
2009A&A...498..901R Altcode:
Context: Flares, eruptive prominences and coronal mass ejections are
phenomena where magnetic reconnection plays an important role. However,
the location and the rate of the reconnection, as well as the mechanisms
of particle interaction with ambient and chromospheric plasma are still
unclear. <BR />Aims: In order to contribute to the comprehension of the
above mentioned processes we studied the evolution of the eruptive flare
loop system in an active region where a flare, a prominence eruption
and a CME occurred on August 24, 2002. <BR />Methods: We measured the
rate of expansion of the flare loop arcade using TRACE 195 Å images
and determined the rising velocity and the evolution of the low and high
energy hard X-ray sources using RHESSI data. We also fitted HXR spectra
and considered the radio emission at 17 and 34 GHZ. <BR />Results:
We observed that the top of the eruptive flare loop system initially
rises with a linear behavior and then, after 120 mn from the start of
the event registered by GOES at 1-8 Å, it slows down. We also observed
that the heating source (low energy X-ray) rises faster than the top
of the loops at 195 Å and that the high energy X-ray emission (30-40
keV) changes in time, changing from footpoint emission at the very
onset of the flare to being coincident during the flare peak with the
whole flare loop arcade. <BR />Conclusions: The evolution of the loop
system and of the X-ray sources allowed us to interpret this event in
the framework of the Lin & Forbes model (2000), where the absolute
rate of reconnection decreases when the current sheet is located at an
altitude where the Alfvén speed decreases with height. We estimated
that the lower limit for the altitude of the current sheet is 6 ×
10<SUP>4</SUP> km. Moreover, we interpreted the unusual variation of
the high energy HXR emission as a manifestation of the non thermal
coronal thick-target process which appears during the flare in a manner
consistent with the inferred increase in coronal column density.
---------------------------------------------------------
Title: Near-infrared sky background fluctuations at mid- and low
latitudes
Authors: Moreels, G.; Clairemidi, J.; Faivre, M.; Pautet, D.; Rubio
da Costa, F.; Rousselot, P.; Meriwether, J. W.; Lehmacher, G. A.;
Vidal, E.; Chau, J. L.; Monnet, G.
2008ExA....22...87M Altcode: 2008ExA...tmp....6M
The emission of the upper atmosphere introduces an additional
variable component into observations of astronomical objects in
the NIR 700 3,000 nm range. The subtraction of this component is
not easy because it varies during the night by as much as 100% and
it is not homogeneous over the sky. A program aimed at measuring and
understanding the main characteristics of the atmospheric NIR emission
was undertaken. A 512 × 512 CCD camera equipped with a RG780/2 mm
filter is used to obtain images of the sky in a 36° × 36° field
of view. The intensities of a given star and of the nearby region
devoid of star in a 439 arcmin<SUP>2</SUP> area are monitored during
periods of time of several hours. The sky intensity measured in the
754 900 nm bandpass, reduced to zenith and zero airmass is comprised
between mag20 and mag18.5 per arcsecond<SUP>2</SUP>. A diminution by
a factor of two during the night is frequently observed. Intensity
fluctuations having an amplitude of 15% and periods of 5 40 min are
present in the images with a structure of regularly spaced stripes. The
fluctuations of the NIR sky background intensity are due to (1)
the chemical evolution of the upper atmosphere composition during
the night and (2) dynamical processes such as tides with periods of
3 6 h or gravity waves with periods of several tens of minutes. We
suggest that a monitoring of the sky background intensity could be
set up when quantitative observations of astronomical objects require
exposure times longer than ~10 min. The publication is illustrated
with several video films accessible on the web site <ExternalRef>
<RefSource>http://www.obs-besancon.fr/nirsky/</RefSource>
<RefTarget Address="http://www.obs-besancon.fr/nirsky/"
TargetType="URL"/> </ExternalRef>. Enter username: nirsky
and password: skynir.
---------------------------------------------------------
Title: Investigation of Lyman <alpha> Emission in a Solar Flare
Authors: Rubio da Costa, F.; Fletcher, L.; Labrosse, N.; Zuccarello, F.
2008ESPM...12.2.64R Altcode:
The TRACE satellite observed a small number of solar flares in the
Lyman alpha channel, which have until now not been analysed. We look at
a well-observed flare on 8th September 1999 to investigate different
topics. We carry out a study of the spatial and temporal evolution of
the flare and associated filament eruption in Lyman alpha, hard X-rays
and soft X-rays, and examine the flare energetics using the hard X-rays
(in the collisional thick target approximation) to estimate the energy
flux carried by electrons, and TRACE Lyman alpha/1600 Å channels to
estimate the temperature and radiative power in UV. We will use these
observations to anticipate what can be observed in the future by the
proposed SMESE satellite mission, and to compare with predictions of
semi-empirical and theoretical models of the flare chromosphere.
