650 research outputs found
Interrogating Solar Flare Loop Models with IRIS Observations 2: Plasma Properties, Energy Transport, and Future Directions
During solar flares a tremendous amount of magnetic energy is released and
transported through the Sun's atmosphere and out into the heliosphere. Despite
over a century of study, many unresolved questions surrounding solar flares are
still present. Among those are how does the solar plasma respond to flare
energy deposition, and what are the important physical processes that transport
that energy from the release site in the corona through the transition region
and chromosphere? Attacking these questions requires the concert of advanced
numerical simulations and high spatial-, temporal-, and spectral- resolution
observations. While flares are 3D phenomenon, simulating the NLTE flaring
chromosphere in 3D and performing parameter studies of 3D models is largely
outwith our current computational capabilities. We instead rely on
state-of-the-art 1D field-aligned simulations to study the physical processes
that govern flares. Over the last decade, data from the Interface Region
Imaging Spectrograph (IRIS) have provided the crucial observations with which
we can critically interrogate the predictions of those flare loop models. Here
in Paper 2 of a two-part review of IRIS and flare loop models, I discuss how
forward modelling flares can help us understand the observations from IRIS, and
how IRIS can reveal where our models do well and where we are likely missing
important processes, focussing in particular on the plasma properties, energy
transport mechanisms, and future directions of flare modelling.Comment: Accepted for publication in Frontiers in Astronomy and Space Sciences
(Research Topic: Flare Observations in the IRIS Era: What Have we Learned,
and What's Next
Interrogating solar flare loop models with IRIS observations 2: Plasma properties, energy transport, and future directions
During solar flares a tremendous amount of magnetic energy is released and transported through the Sun’s atmosphere and out into the heliosphere. Despite over a century of study, many unresolved questions surrounding solar flares are still present. Among those are how does the solar plasma respond to flare energy deposition, and what are the important physical processes that transport that energy from the release site in the corona through the transition region and chromosphere? Attacking these questions requires the concert of advanced numerical simulations and high spatial-, temporal-, and spectral-resolution observations. While flares are 3D phenomenon, simulating the NLTE flaring chromosphere in 3D and performing parameter studies of 3D models is largely outwith our current computational capabilities. We instead rely on state-of-the-art 1D field-aligned simulations to study the physical processes that govern flares. Over the last decade, data from the Interface Region Imaging Spectrograph (IRIS) have provided the crucial observations with which we can critically interrogate the predictions of those flare loop models. Here in Paper 2 of a two-part review of IRIS and flare loop models, I discuss how forward modelling flares can help us understand the observations from IRIS, and how IRIS can reveal where our models do well and where we are likely missing important processes, focussing in particular on the plasma properties, energy transport mechanisms, and future directions of flare modelling
IRIS Observations of the Mg II h & k Lines During a Solar Flare
The bulk of the radiative output of a solar flare is emitted from the
chromosphere, which produces enhancements in the optical and UV continuum, and
in many lines, both optically thick and thin. We have, until very recently,
lacked observations of two of the strongest of these lines: the Mg II h & k
resonance lines. We present a detailed study of the response of these lines to
a solar flare. The spatial and temporal behaviour of the integrated
intensities, k/h line ratios, line of sight velocities, line widths and line
asymmetries were investigated during an M class flare (SOL2014-02-13T01:40).
Very intense, spatially localised energy input at the outer edge of the ribbon
is observed, resulting in redshifts equivalent to velocities of ~15-26km/s,
line broadenings, and a blue asymmetry in the most intense sources. The
characteristic central reversal feature that is ubiquitous in quiet Sun
observations is absent in flaring profiles, indicating that the source function
increases with height during the flare. Despite the absence of the central
reversal feature, the k/h line ratio indicates that the lines remain optically
thick during the flare. Subordinate lines in the Mg II passband are observed to
be in emission in flaring sources, brightening and cooling with similar
timescales to the resonance lines. This work represents a first analysis of
potential diagnostic information of the flaring atmosphere using these lines,
and provides observations to which synthetic spectra from advanced radiative
transfer codes can be compared.Comment: 12 pages, 14 figures, Accepted for publication in Astronomy and
Astrophysic
Modeling of the hydrogen Lyman lines in solar flares
The hydrogen Lyman lines (91.2 nm < λ < 121.6 nm) are significant contributors to the radiative losses of the solar chromosphere, and they are enhanced during flares. We have shown previously that the Lyman lines observed by the Extreme Ultraviolet Variability instrument onboard the Solar Dynamics Observatory exhibit Doppler motions equivalent to speeds on the order of 30 km s−1. However, contrary to expectations, both redshifts and blueshifts were present and no dominant flow direction was observed. To understand the formation of the Lyman lines, particularly their Doppler motions, we have used the radiative hydrodynamic code, RADYN, along with the radiative transfer code, RH, to simulate the evolution of the flaring chromosphere and the response of the Lyman lines during solar flares. We find that upflows in the simulated atmospheres lead to blueshifts in the line cores, which exhibit central reversals. We then model the effects of the instrument on the profiles, using the Extreme Ultraviolet Variability Experiment (EVE) instrument's properties. What may be interpreted as downflows (redshifted emission) in the lines, after they have been convolved with the instrumental line profile, may not necessarily correspond to actual downflows. Dynamic features in the atmosphere can introduce complex features in the line profiles that will not be detected by instruments with the spectral resolution of EVE, but which leave more of a signature at the resolution of the Spectral Investigation of the Coronal Environment instrument onboard the Solar Orbiter
A Possible Mechanism for "Late Phase" in Stellar White-Light Flares
M-dwarf flares observed by the \textit{Transiting Exoplanet Survey Satellite}
(\textit{TESS}) sometimes exhibit a "peak-bump" light-curve morphology,
characterized by a secondary, gradual peak well after the main, impulsive peak.
A similar "late phase" is frequently detected in solar flares observed in the
extreme-ultraviolet from longer hot coronal loops distinct from the impulsive
flare structures. White-light emission has also been observed in off-limb solar
flare loops. Here, we perform a suite of one-dimensional hydrodynamic loop
simulations for M-dwarf flares inspired by these solar examples. Our results
suggest that coronal plasma condensation following impulsive flare heating can
yield high electron number density in the loop, allowing it to contribute
significantly to the optical light curves via free-bound and free-free emission
mechanisms. Our simulation results qualitatively agree with \textit{TESS}
observations: the longer evolutionary time scale of coronal loops produces a
distinct, secondary emission peak; its intensity increases with the injected
flare energy. We argue that coronal plasma condensation is a possible mechanism
for the \textit{TESS} late-phase flares.Comment: 31 pages, 13 figures, accepted for publication in Ap
Simulations of the Mg II k and Ca II 8542 lines from an AlfvÉn Wave-heated Flare Chromosphere
We use radiation hydrodynamic simulations to examine two models of solar flare chromospheric heating:
Alfven wave dissipation and electron beam collisional losses. Both mechanisms are capable of strong chro- ´
mospheric heating, and we show that the distinctive atmospheric evolution in the mid-to-upper chromosphere
results in Mg ii k-line emission that should be observably different between wave-heated and beam-heated simulations.
We also present Ca ii 8542Ã… profiles which are formed slightly deeper in the chromosphere. The
Mg ii k-line profiles from our wave-heated simulation are quite different from those from a beam-heated model
and are more consistent with IRIS observations. The predicted differences between the Ca ii 8542Ã… in the two
models are small. We conclude that careful observational and theoretical study of lines formed in the mid-toupper
chromosphere holds genuine promise for distinguishing between competing models for chromospheric
heating in flares
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