141 research outputs found
Hydrogen Balmer line formation in solar flares affected by return currents
Aims. We investigate the effect of the electric return currents in solar
flares on the profiles of hydrogen Balmer lines. We consider the monoenergetic
approximation for the primary beam and runaway model of the neutralizing return
current. Methods. Propagation of the 10 keV electron beam from a coronal
reconnection site is considered for the semiempirical chromosphere model F1. We
estimate the local number density of return current using two approximations
for beam energy fluxes between and . Inelastic collisions of beam and return-current electrons
with hydrogen are included according to their energy distributions, and the
hydrogen Balmer line intensities are computed using an NLTE radiative transfer
approach. Results. In comparison to traditional NLTE models of solar flares
that neglect the return-current effects, we found a significant increase
emission in the Balmer line cores due to nonthermal excitation by return
current. Contrary to the model without return current, the line shapes are
sensitive to a beam flux. It is the result of variation in the return-current
energy that is close to the hydrogen excitation thresholds and the density of
return-current electrons.Comment: 4 pages, 3 figures, 1 table, accepted for publication in Astronomy
and Astrophysics Letter
Modifications of thick-target model: re-acceleration of electron beams by static and stochastic electric fields
We study two modifications of the collisional thick-target model (CTTM) based
on the global and local re-acceleration of non-thermal electrons by static and
stochastic electric fields during their transport from the coronal acceleration
site to the thick-target region in the chromosphere. We concentrate on a
comparison of the non-thermal electron distribution functions, chromospheric
energy deposits, and HXR spectra obtained for both considered modifications
with the CTTM itself. The results were obtained using a relativistic
test-particle approach. We simulated the transport of non-thermal electrons
with a power-law spectrum including the influence of scattering, energy losses,
magnetic mirroring, and also the effects of the electric fields corresponding
to both modifications of the CTTM. We show that both modifications of the CTTM
change the outcome of the chromospheric bombardment in several aspects. The
modifications lead to an increase in chromospheric energy deposit, change of
its spatial distribution, and a substantial increase in the corresponding HXR
spectrum intensity.Comment: 15 pages, 14 figures, 3 tables, to be published in Astronomy and
Astrophysic
Sunspot waves and flare energy release
We address a possibility of the flare process initiation and further
maintenance of its energy release due to a transformation of sunspot
longitudinal waves into transverse magnetic loop oscillations with initiation
of reconnection. This leads to heating maintaining after the energy release
peak and formation of a flat stage on the X-ray profile. We applied the
time-distance plots and pixel wavelet filtration (PWF) methods to obtain
spatio-temporal distribution of wave power variations in SDO/AIA data. To find
magnetic waveguides, we used magnetic field extrapolation of SDO/HMI
magnetograms. The propagation velocity of wave fronts was measured from their
spatial locations at specific times. In correlation curves of the 17 GHz (NoRH)
radio emission we found a monotonous energy amplification of 3-min waves in the
sunspot umbra before the 2012 June 7 flare. This dynamics agrees with an
increase in the wave-train length in coronal loops (SDO/AIA, 171 {\AA})
reaching the maximum 30 minutes prior to the flare onset. A peculiarity of this
flare time profile in soft X-rays (RHESSI, 3-25 keV) is maintaining the
constant level of the flare emission for 10 minutes after the short impulse
phase, which indicates at the energy release continuation. Throughout this
time, we found 30-sec period transverse oscillations of the flare loop in the
radio-frequency range (NoRH, 17 GHz). This periodicity is apparently related to
the transformation of propagating longitudinal 3-min waves from the sunspot
into the loop transverse oscillations. The magnetic field extrapolation showed
the existence of the magnetic waveguide (loop) connecting the sunspot with the
energy release region. A flare loop heating can be caused by the interaction
(reconnections) of this transversally oscillating waveguide with the underlying
twisted loops.Comment: 7 pages, 9 figure
A Systematic Examination of Particle Motion in a Collapsing Magnetic Trap Model for Solar Flares
Context. It has been suggested that collapsing magnetic traps may contribute
to accelerating particles to high energies during solar flares.
Aims. We present a detailed investigation of the energization processes of
particles in collapsing magnetic traps, using a specific model. We also compare
for the first time the energization processes in a symmetric and an asymmetric
trap model.
Methods. Particle orbits are calculated using guiding centre theory. We
systematically investigate the dependence of the energization process on
initial position, initial energy and initial pitch angle.
Results. We find that in our symmetric trap model particles can gain up to
about 50 times their initial energy, but that for most initial conditions the
energy gain is more moderate. Particles with an initial position in the weak
field region of the collapsing trap and with pitch angles around 90 degrees
achieve the highest energy gain, with betatron acceleration of the
perpendicular energy the dominant energization mechanism. For particles with
smaller initial pitch angle, but still outside the loss cone, we find the
possibility of a significant increase in parallel energy. This increase in
parallel energy can be attributed to the curvature term in the parallel
equation of motion and the associated energy gain happens in the center of the
trap where the field line curvature has its maximum. We find qualitatively
similar results for the asymmetric trap model, but with smaller energy gains
and a larger number of particles escaping from the trap.Comment: 11 pages, 13 figures. To be published in Astronomy and Astrophysic
Fast magnetoacoustic wave trains in magnetic funnels of the solar corona
Context: Fast magneto-acoustic waves are highly dispersive in waveguides, so they can generate quasi-periodic wave trains if a localised, impulsive driver is applied. Such wave trains have been observed in the solar corona and may be of use as a seismological tool since they depend upon the plasma structuring perpendicular to the direction of propagation.
Aims. We extend existing models of magnetoacoustic waveguides to consider the effects of an expanding magnetic field. The funnel geometry employed includes a field-aligned density structure.
Methods: We performed 2D numerical simulations of impulsively generated fast magneto-acoustic perturbations. The effects of the density contrast ratio, density stratification, and spectral profile of the driver upon the excited wave trains were investigated.
Results: The density structure acts as a dispersive waveguide for fast magneto-acoustic waves and generates a quasi-periodic wave train similar to previous models. The funnel geometry leads to generating additional wave trains that propagate outside the density structure. These newly discovered wave trains are formed by the leakage of transverse perturbations, but they propagate upwards owing to the refraction caused by the magnetic funnel.
Conclusions: The results of our funnel model may be applicable to wave trains observed propagating in the solar corona. They demonstrate similar properties to those found in our simulations
Response of optical hydrogen lines to beam heating: I. Electron beams
We investigate the role of non-thermal electrons in the formation regions of
Halpha, Hbeta, and Hgamma lines in order to unfold their influence on the
formation of these lines. We concentrate on pulse-beam heating varying on a
subsecond timescale. Furthermore, we theoretically explore possibility that a
new diagnostic tool exists indicating the presence of non-thermal electrons in
the flaring chromosphere based on observations of optical hydrogen lines. To
model the evolution of the flaring atmosphere and the time-dependent hydrogen
excitation and ionisation, we used a 1-D radiative hydrodynamic code combined
with a test-particle code that simulates the propagation, scattering, and
thermalisation of a power-law electron beam in order to obtain the flare
heating and the non-thermal collisional rates due to the interaction of the
beam with the hydrogen atoms. All calculated models have shown a
time-correlated response of the modelled Balmer line intensities on a subsecond
timescale, with a subsecond timelag behind the beam flux. Depending on the beam
parameters, both line centres and wings can show pronounced intensity
variations. The non-thermal collisional rates generally result in an increased
emission from a secondary region formed in the chromosphere.Comment: 13 pages, 11 figures, accepted to Astronomy and Astrophysic
Kappa distribution and hard X-ray emission of solar flares
We investigate whether the so-called kappa distribution, often used to fit
electron distributions detected in-situ in the solar wind, can describe
electrons producing the hard X-ray emission in solar flares. Using Ramaty High
Energy Solar Spectroscopic imager (RHESSI) flare data we fit spatially- and
feature-integrated spectra, assuming kappa distribution for the mean electron
flux spectrum. We show that a single kappa distribution generally cannot
describe spatially integrated X-ray emission composed of both footpoint and
coronal sources. In contrast, the kappa distribution is consistent with mean
electron spectra producing hard X-ray emission in some coronal sources.Comment: 4 pages, 4 figures, changed content, accepted to A&
Slipping magnetic reconnection during an X-Class solar flare observed by SDO/AIA
We present SDO/AIA observations of an eruptive X-class flare of July 12,
2012, and compare its evolution with the predictions of a 3D numerical
simulation. We focus on the dynamics of flare loops that are seen to undergo
slipping reconnection during the flare. In the AIA 131A observations, lower
parts of 10 MK flare loops exhibit an apparent motion with velocities of
several tens of km/s along the developing flare ribbons. In the early stages of
the flare, flare ribbons consist of compact, localized bright transition-region
emission from the footpoints of the flare loops. A DEM analysis shows that the
flare loops have temperatures up to the formation of Fe XXIV. A series of very
long, S-shaped loops erupt, leading to a CME observed by STEREO. The observed
dynamics are compared with the evolution of magnetic structures in the
"standard solar flare model in 3D". This model matches the observations well,
reproducing both the apparently slipping flare loops, S-shaped erupting loops,
and the evolution of flare ribbons. All of these processes are explained via 3D
reconnection mechanisms resulting from the expansion of a torus-unstable flux
rope. The AIA observations and the numerical model are complemented by radio
observations showing a noise storm in the metric range. Dm-drifting pulsation
structures occurring during the eruption indicate plasmoid ejection and
enhancement of reconnection rate. The bursty nature of radio emission shows
that the slipping reconnection is still intermittent, although it is observed
to persist for more than an hour
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