1,940 research outputs found
Physical Properties of White-Light Sources in the 2011 Feb 15 Solar Flare
White light flares (WLFs) are observational rarities, making them
understudied events. However, optical emission is a significant contribution to
flare energy budgets and the emission mechanisms responsible could have
important implications for flare models. Using Hinode SOT optical continuum
data taken in broadband red, green and blue filters, we investigate white-light
emission from the X2.2 flare SOL2011-02-15T01:56:00. We develop a technique to
robustly identify enhanced flare pixels and, using a knowledge of the RGB
filter transmissions, determined the source color temperature and effective
temperature. We investigated two idealized models of WL emission - an optically
thick photospheric source, and an optically thin chromospheric slab. Under the
optically thick assumption, the color temperature and effective temperature of
flare sources in sunspot umbra and penumbra were determined as a function of
time and position. Values in the range of 5000-6000K were found, corresponding
to a blackbody temperature increase of a few hundred kelvin. The power emitted
in the optical was estimated at ergs s. In some of the
white-light sources the color and blackbody temperatures are the same within
uncertainties, consistent with a blackbody emitter. In other regions this is
not the case, suggesting that some other continuum emission process is
contributing. An optically thin slab model producing hydrogen recombination
radiation is also discussed as a potential source of WL emission; it requires
temperatures in the range 5,500 - 25,000K, and total energies of ergs s.Comment: Accepted for publication in the Astrophysical Journal, 15 pages, 15
figure
Observations and Modelling of Helium Lines in Solar Flares
We explore the response of the He II 304 Ã… and He I 584 Ã… line intensities to electron beam heating in solar flares using radiative hydrodynamic simulations. Comparing different electron beams parameters, we found that the intensities of both He lines are very sensitive to the energy flux deposited in the chromosphere, or more specifically to the heating rate, with He II 304 {\AA} being more sensitive to the heating than He I 584 {\AA}. Therefore, the He line ratio increases for larger heating rates in the chromosphere. A similar trend is found in observations, using SDO/EVE He irradiance ratios and estimates of the electron beam energy rate obtained from hard X-ray data. From the simulations, we also found that spectral index of the electrons can affect the He ratio but a similar effect was not found in the observations
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
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
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
May essential provisions of a contract be determined by one of the parties alone
When the Supreme Court of Appeal raises a question but does not answer it, what it says can be interpreted as an invitation to all those interested in the topic to discuss it. This note is a response to such an invitation in NBS Boland Bank v One Berg River Drive CC, Deeb v ABSA Bank Ltd, Friedman v Standard Bank of South Africa Ltd 1999 (4) SA 928 (SCA);[1999] 4 All SA 183, hereinafter referred to as the NBS Boland Bank case
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