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A parameter study for modeling MgII h and k emission during solar flares  

Fatima Rubio da Costa   Submitted: 2017-04-20 19:46

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 MgII h and k lines. We aim at understanding the formation of the chromospheric and optically thick MgII 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 velocities, we study their effects on the emergent intensity spectra. We can reproduce the typical single-peaked MgII h and k flare spectral shape and their approximate intensity ratios to the subordinate MgII lines by either increasing densities, temperatures or velocities at the line core formation height range. Additionally, by combining unresolved up- and downflows up to ~250 km s-1 within one resolution element, we also 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.

Authors: F. Rubio da Costa; L. Kleint
Projects: None

Publication Status: Accepted in ApJ
Last Modified: 2017-04-26 12:03
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Data-driven Radiative Hydrodynamic Modeling of the 2014 March 29 X1.0 Solar Flare  

Fatima Rubio da Costa   Submitted: 2016-03-15 22:23

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 MgII h&k, the Interferometric BIdimensional Spectropolarimeter at the Dunn Solar Telescope (DST/IBIS) in H𝛼 6563 Å and CaII 8542 Å, and the Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI) in hard X-rays. We constructed a multi-threaded 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 MgII 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 MgII line core into emission and thus reproduce the single peaked profile, which is a common feature in all IRIS flares

Authors: Fatima Rubio da Costa, Lucia Kleint, Vahé Petrosian, Wei Liu and Joel C. Allred
Projects: IRIS,RHESSI,DST/IBIS

Publication Status: Submitted to ApJ
Last Modified: 2016-03-16 11:28
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COMBINED MODELING OF ACCELERATION, TRANSPORT, AND HYDRODYNAMIC RESPONSE IN SOLAR FLARES. II. Inclusion of Radiative Transfer with RADYN  

Fatima Rubio da Costa   Submitted: 2015-05-20 12:15

Solar flares involve complex processes that are coupled together 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 such a model to simulate the coupling of high-energy particle kinetics with hydrodynamics of the atmospheric plasma. We combine the Stanford unified Fokker-Planck code that models particle acceleration, transport, and bremsstrahlung radiation with the RADYN hydrodynamic code that models the atmospheric response to collisional heating by non-thermal electrons through detailed radiative transfer calculations. We perform simulations using different injection electron spectra, including an ad hoc power law and more realistic spectra predicted by the stochastic acceleration model due to turbulence or plasma waves. Surprisingly, stochastically accelerated electrons, even with energy flux 1010 (erg/s/cm^2), cause "explosive" chromospheric evaporation and drive stronger up- and downflows (and hydrodynamic shocks). We synthesize emission line profiles covering different heights in the lower atmosphere, including Hα 6563 A, He II 304 Å, Ca II K 3934 A and Si IV 1393 A. One interesting result is the unusual high temperature (up to a few 105K) of the formation site of He II 304 Å, which is expected due to photonionization-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 non-thermal electrons and thus the poorly understood particle acceleration mechanism.

Authors: Fatima Rubio da Costa, Wei Liu, Vahé Petrosian, Mats Carlsson
Projects: Other

Publication Status: submitted
Last Modified: 2015-05-25 09:30
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Solar Flare Chromospheric Line Emission: Comparison Between IBIS High-resolution Observations and Radiative Hydrodynamic Simulations  

Fatima Rubio da Costa   Submitted: 2014-12-04 13:56

Solar flares involve impulsive energy release, which results in enhanced radiation in a broad spectral and at a wide height range. In particular, line emission from the chromosphere (lower atmosphere) can provide critical diagnostics of plasma heating processes. Thus, a direct comparison between high-resolution spectroscopic observations and advanced numerical modeling results can be extremely valuable, but has not been attempted so far. We present in this paper such a self-consistent investigation of an M3.0 flare observed by the Dunn Solar Telescope's (DST) Interferometric Bi-dimensional Spectrometer (IBIS) on 2011 September 24 that we have modeled with the radiative hydrodynamic code RADYN (Carlsson & Stein 1992, 1997; Abbett & Hawley 1999; Allred et al. 2005). We obtained images and spectra of the flaring region with IBIS in Hα 6563 Å and Ca II 8542 Å, and with the Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI)in X-rays. The latter was 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 with those observed by IBIS and found that they exhibit an agreement in general. 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 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 a refinement of the energy input and other processes is necessary to resolve this discrepancy.

Authors: Fatima Rubio da Costa, Lucia Kleint, Vahé Petrosian, Alberto Sainz Dalda, Wei Liu
Projects: None

Publication Status: submitted
Last Modified: 2014-12-05 13:41
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Observations of a solar flare and filament eruption in Lyman α and X-rays  

Fatima Rubio da Costa   Submitted: 2009-09-25 07:04

Context. L α 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 A 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 L α as well as by the Yohkoh Soft and Hard X-ray telescopes. We explore the flare energetics andits spatial and temporal evolution. We have in mind the fact that the L α 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 A, 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 L α signature. The L α 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 L α flare is characterised by strong, compact footpoints (smaller than the UV ribbons) which correlate well with HXR footpoints. The L α 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 L α line with the planned EUI and LYOT telescopes will provide valuable insight into solar flare evolution and energetics, especially when accompaniedby HXR imaging and spectroscopy.

Authors: F. Rubio da Costa, L. Fletcher, N. Labrosse, and F. Zuccarello
Projects: None

Publication Status: accepted
Last Modified: 2009-09-25 08:08
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Abstracts by Author
A parameter study for modeling MgII h and k emission during solar flares
Data-driven Radiative Hydrodynamic Modeling of the 2014 March 29 X1.0 Solar Flare
COMBINED MODELING OF ACCELERATION, TRANSPORT, AND HYDRODYNAMIC RESPONSE IN SOLAR FLARES. II. Inclusion of Radiative Transfer with RADYN
Solar Flare Chromospheric Line Emission: Comparison Between IBIS High-resolution Observations and Radiative Hydrodynamic Simulations
Observations of a solar flare and filament eruption in Lyman alpha and X-rays

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