166 research outputs found
Numerical simulations of coronal particle trapping
In this paper the trapping of high energy particles in solar coronal loops is addressed. Using simulations, the time evolution of electrons and protons trapped in a magnetic bottle is calculated under various scattering conditions and the results compared with loss-cone analysis. Thereafter the case of time-dependent injection into a magnetic loop is addressed, and the results compared with previous analytic work on X and γ-ray delay times
Generation of solar Hα impact polarization by fragmented evaporative upflows
In this paper a novel mechanism is proposed for the generation of Hα impact polarization observed during some solar flares. Rather than being generated by the primary particle beams transporting energy from the chromosphere to the corona, we suggest that following heating, the solar chromosphere evaporates in a fragmented manner, and that impact excitations in the regions of interaction of hot evaporating and cool non-evaporating material locally generates impact-polarized Hα emission. This thermal upflow model is more consistent with the large areas and times over which polarization is observed than are beam models. A simple model for the process is given, and the resulting polarization is calculated and compared with observations, under two assumptions about the number density of neutral particles in the interaction regions
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
The Impulsive Phase in Solar Flares: Recent Multi-wavelength Results and their Implications for Microwave Modeling and Observations
This short paper reviews several recent key observations of the processes
occurring in the lower atmosphere (chromosphere and photosphere) during flares.
These are: evidence for compact and fragmentary structure in the flare
chromosphere, the conditions in optical flare footpoints, step-like variations
in the magnetic field during the flare impulsive phase, and hot, dense
'chromospheric' footpoints. The implications of these observations for
microwaves are also discussed.Comment: 6 pages, 5 figures, presented at 'Solar Physics with Radio
Observations' Symposium, November 2012, Nagoya, Japa
Cycle 23 Variation in Solar Flare Productivity
The NOAA listings of solar flares in cycles 21-24, including the GOES soft
X-ray magnitudes, enable a simple determination of the number of flares each
flaring active region produces over its lifetime. We have studied this measure
of flare productivity over the interval 1975-2012. The annual averages of flare
productivity remained approximately constant during cycles 21 and 22, at about
two reported M or X flares per region, but then increased significantly in the
declining phase of cycle 23 (the years 2004-2005). We have confirmed this by
using the independent RHESSI flare catalog to check the NOAA events listings
where possible. We note that this measure of solar activity does not correlate
with the solar cycle. The anomalous peak in flare productivity immediately
preceded the long solar minimum between cycles 23 and 24
Determining Energy Balance in the Flaring Chromosphere from Oxygen V Line Ratios
The impulsive phase of solar flares is a time of rapid energy deposition and
heating in the lower solar atmosphere, leading to changes in the temperature
and density structure of the region. We use an O V density diagnostic formed of
the 192 to 248 line ratio, provided by Hinode EIS, to determine the density of
flare footpoint plasma, at O V formation temperatures of 250,000 K, giving a
constraint on the properties of the heated transition region. Hinode EIS
rasters from 2 small flare events in December 2007 were used. Raster images
were co-aligned to identify and establish the footpoint pixels,
multiple-component Gaussian line fitting of the spectra was carried out to
isolate the diagnostic pair, and the density was calculated for several
footpoint areas. The assumptions of equilibrium ionization and optically thin
radiation for the O V lines were found to be acceptable. Properties of the
electron distribution, for one event, were deduced from earlier RHESSI hard
X-ray observations and used to calculate the plasma heating rate, delivered by
an electron beam adopting collisional thick-target assumptions, for 2 model
atmospheres. Electron number densities of at least log n = 12.3 cm-3 were
measured during the flare impulsive phase, far higher than previously expected.
For one footpoint, the radiative loss rate for this plasma was found to exceed
that which can be delivered by an electron beam implied by the RHESSI data.
However, when assuming a completely ionised target atmosphere the heating rate
exceeded the losses. A chromospheric thickness of 70-700 km was found to be
required to balance a conductive input to the O V-emitting region with
radiative losses. The analysis shows that for heating by collisional electrons,
it is difficult, or impossible to raise the temperature of the chromosphere to
explain the observed densities without assuming a completely ionised
atmosphere.Comment: Accepted to A&A 14th September 201
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