Author name code: rubiodacosta ADS astronomy entries on 2022-09-14 author:"Rubio da Costa, Fatima" ------------------------------------------------------------------------ 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 Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. Title: A Parameter Study for Modeling Mg II h and k Emission during Solar Flares Authors: Rubio da Costa, Fatima; Kleint, Lucia Bibcode: 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-1 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. 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 Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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-1) 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. 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. Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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}10 {erg} {{{s}}}-1 {{cm}}-2, 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 105 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. 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 Bibcode: 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 Bibcode: 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. 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 Bibcode: 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. 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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.
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.
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.
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.
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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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/Ec )-δ for energies greater than a cutoff value of Ec. 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. Bibcode: 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.
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.
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.
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.
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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
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.
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 1026 erg s-1 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.
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. Bibcode: 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.
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.
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.
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.
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 × 104 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. Bibcode: 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 arcmin2 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 arcsecond2. 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. Bibcode: 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.