75 research outputs found
Parameterizations of Chromospheric Condensations in dG and dMe Model Flare Atmospheres
The origin of the near-ultraviolet and optical continuum radiation in flares
is critical for understanding particle acceleration and impulsive heating in
stellar atmospheres. Radiative-hydrodynamic simulations in 1D have shown that
high energy deposition rates from electron beams produce two flaring layers at
T~10^4 K that develop in the chromosphere: a cooling condensation (downflowing
compression) and heated non-moving (stationary) flare layers just below the
condensation. These atmospheres reproduce several observed phenomena in flare
spectra, such as the red wing asymmetry of the emission lines in solar flares
and a small Balmer jump ratio in M dwarf flares. The high beam flux simulations
are computationally expensive in 1D, and the (human) timescales for completing
NLTE models with adaptive grids in 3D will likely be unwieldy for a time to
come. We have developed a prescription for predicting the approximate evolved
states, continuum optical depth, and the emergent continuum flux spectra of
radiative-hydrodynamic model flare atmospheres. These approximate prescriptions
are based on an important atmospheric parameter: the column mass (m_ref) at
which hydrogen becomes nearly completely ionized at the depths that are
approximately in steady state with the electron beam heating. Using this new
modeling approach, we find that high energy flux density (>F11) electron beams
are needed to reproduce the brightest observed continuum intensity in IRIS data
of the 2014-Mar-29 X1 solar flare and that variation in m_ref from 0.001 to
0.02 g/cm2 reproduces most of the observed range of the optical continuum flux
ratios at the peaks of M dwarf flares.Comment: 29 pages, 9 figures, accepted for publication in the Astrophysical
Journa
Using Models for How Energetic Electrons Heat the Atmosphere During Flares
Using models for how energetic electrons heat the atmosphere during flares, we simulate the radiative-hydrodynamic response of the lower solar atmosphere to flare heating. The simulations account for much of the non-LTE, optically thick radiative transfer that occurs in the chromosphere. Our models predict an increase in white light continuum during the flare on the order of 20%, but this is highly sensitive to the electron beam flux used in the simulation. We find that a majority of the white light continuum originates in the chromosphere as a result of Balmer and Paschen recombinations, but a significant portion also forms in the photosphere which has been heated by radiative backwarming
Models of the Solar Atmospheric Response to Flare Heating
I will present models of the solar atmospheric response to flare heating. The models solve the equations of non-LTE radiation hydrodynamics with an electron beam added as a flare energy source term. Radiative transfer is solved in detail for many important optically thick hydrogen and helium transitions and numerous optically thin EUV lines making the models ideally suited to study the emission that is produced during flares. I will pay special attention to understanding key EUV lines as well the mechanism for white light production. I will also present preliminary results of how the model solar atmosphere responds to Fletcher & Hudson type flare heating. I will compare this with the results from flare simulations using the standard thick target model
A Unified Computational Model for Solar and Stellar Flares
We present a unified computational framework which can be used to describe
impulsive flares on the Sun and on dMe stars. The models assume that the flare
impulsive phase is caused by a beam of charged particles that is accelerated in
the corona and propagates downward depositing energy and momentum along the
way. This rapidly heats the lower stellar atmosphere causing it to explosively
expand and dramatically brighten. Our models consist of flux tubes that extend
from the sub-photosphere into the corona. We simulate how flare-accelerated
charged particles propagate down one-dimensional flux tubes and heat the
stellar atmosphere using the Fokker-Planck kinetic theory. Detailed radiative
transfer is included so that model predictions can be directly compared with
observations. The flux of flare-accelerated particles drives return currents
which additionally heat the stellar atmosphere. These effects are also included
in our models. We examine the impact of the flare-accelerated particle beams on
model solar and dMe stellar atmospheres and perform parameter studies varying
the injected particle energy spectra. We find the atmospheric response is
strongly dependent on the accelerated particle cutoff energy and spectral
index.Comment: Accepted for publication by the Astrophysical Journa
Complementing Büchi automata with a subset-tuple construction
Complementation of Büchi automata is well known for being difficult. In the worst case, a state-space growth of (0:76n)n is unavoidable. Recent studies suggest that “simpler” algorithms perform better than more involved ones on practical cases. In this paper, we present a simple “direct” algorithm for complementing Büchi automata. It involves a structured subset construction (using tuples of subsets of states) that produces a deterministic automaton. This construction leads to a complementation procedure that resembles the straightforward complementation algorithm for deterministic Büchi automata, the latter algorithm actually being a special case of our construction
Data-driven Radiative Hydrodynamic Modeling of the 2014 March 29 X1.0 Solar Flare
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 H 6563 \AA\ and CaII 8542 \AA, 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) 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.Comment: 14 pages, 18 figures, accepted in Ap
Modeling Mg II h, k and Triplet Lines at Solar Flare Ribbons
Observations from the \textit{Interface Region Imaging Spectrograph}
(\textsl{IRIS}) often reveal significantly broadened and non-reversed profiles
of the Mg II h, k and triplet lines at flare ribbons. To understand the
formation of these optically thick Mg II lines, we perform plane parallel
radiative hydrodynamics modeling with the RADYN code, and then recalculate the
Mg II line profiles from RADYN atmosphere snapshots using the radiative
transfer code RH. We find that the current RH code significantly underestimates
the Mg II h \& k Stark widths. By implementing semi-classical perturbation
approximation results of quadratic Stark broadening from the STARK-B database
in the RH code, the Stark broadenings are found to be one order of magnitude
larger than those calculated from the current RH code. However, the improved
Stark widths are still too small, and another factor of 30 has to be multiplied
to reproduce the significantly broadened lines and adjacent continuum seen in
observations. Non-thermal electrons, magnetic fields, three-dimensional effects
or electron density effect may account for this factor. Without modifying the
RADYN atmosphere, we have also reproduced non-reversed Mg II h \& k profiles,
which appear when the electron beam energy flux is decreasing. These profiles
are formed at an electron density of
and a temperature of K, where the source function slightly
deviates from the Planck function. Our investigation also demonstrates that at
flare ribbons the triplet lines are formed in the upper chromosphere, close to
the formation heights of the h \& k lines
Radiative hydrodynamic modelling and observations of the X-class solar flare on 2011 March 9
We investigated the response of the solar atmosphere to non-thermal electron
beam heating using the radiative transfer and hydrodynamics modelling code
RADYN. The temporal evolution of the parameters that describe the non-thermal
electron energy distribution were derived from hard X-ray observations of a
particular flare, and we compared the modelled and observed parameters. The
evolution of the non-thermal electron beam parameters during the X1.5 solar
flare on 2011 March 9 were obtained from analysis of RHESSI X-ray spectra. The
RADYN flare model was allowed to evolve for 110 seconds, after which the
electron beam heating was ended, and was then allowed to continue evolving for
a further 300s. The modelled flare parameters were compared to the observed
parameters determined from extreme-ultraviolet spectroscopy. The model produced
a hotter and denser flare loop than that observed and also cooled more rapidly,
suggesting that additional energy input in the decay phase of the flare is
required. In the explosive evaporation phase a region of high-density cool
material propagated upward through the corona. This material underwent a rapid
increase in temperature as it was unable to radiate away all of the energy
deposited across it by the non-thermal electron beam and via thermal
conduction. A narrow and high-density ( cm) region at
the base of the flare transition region was the source of optical line emission
in the model atmosphere. The collision-stopping depth of electrons was
calculated throughout the evolution of the flare, and it was found that the
compression of the lower atmosphere may permit electrons to penetrate farther
into a flaring atmosphere compared to a quiet Sun atmosphere.Comment: 12 pages, 12 figure
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