414 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
Double plasma resonance instability as a source of solar zebra emission
The double plasma resonance (DPR) instability plays a basic role in the
generation of solar radio zebras. In the plasma, consisting of the loss-cone
type distribution of hot electrons and much denser and colder background
plasma, this instability generates the upper-hybrid waves, which are then
transformed into the electromagnetic waves and observed as radio zebras. In the
present paper we numerically study the double plasma resonance instability from
the point of view of the zebra interpretation. We use a 3-dimensional
electromagnetic particle-in-cell (3-D PIC) relativistic model. First using the
multi-mode model, we study details of the double plasma resonance instability.
We show how the distribution function of hot electrons changes during this
instability. Then we show that there is a very good agreement between results
obtained by the multi-mode and specific-mode models, which is caused by a
dominance of the wave with the maximal growth rate. Therefore, for computations
in a broad range of model parameters, we use the specific-mode model. We
compute the maximal growth rates of the double plasma resonance instability.
The results are compared with the analytical ones. We find a very good
agreement between numerical and analytical growth rates. We also compute
saturation energies of the upper-hybrid waves in a very broad range of
parameters. We find that the saturation energies of the upper-hybrid waves show
maxima and minima at almost the same values of
as the growth rates. Furthermore, we
find that the saturation energy of the upper-hybrid waves is proportional to
the density of hot electrons. The maximum saturated energy can be up to one
percent of the kinetic energy of hot electrons. All these findings can be used
in the interpretation of solar radio zebras.Comment: 8 pages, 12 figure
Pulse-beam heating of deep atmospheric layers triggering their oscillations and upwards moving shocks that can modulate the reconnection in solar flares
We study processes occurring after a sudden heating of the chromosphere at
the flare arcade footpoints which is assumed to be caused by particle beams.
For the numerical simulations we adopt a 2-D magnetohydrodynamic (MHD) model,
in which we solve a full set of the time-dependent MHD equations by means of
the FLASH code, using the Adaptive Mesh Refinement (AMR) method. In the initial
state we consider a model of the solar atmosphere with densities according to
the VAL-C model and the magnetic field arcade having the X-point structure
above, where the magnetic reconnection is assumed. We found that the sudden
pulse-beam heating of the chromosphere at the flare arcade footpoints generates
magnetohydrodynamic shocks, one propagating upwards and the second one
propagating downwards in the solar atmosphere. The downward moving shock is
reflected at deep and dense atmospheric layers and triggers oscillations of
these layers. These oscillations generate the upwards moving
magnetohydrodynamic waves that can influence the above located magnetic
reconnection in a quasi-periodic way. Because these processes require a sudden
heating in very localized regions in the chromosphere therefore they can be
also associated with seismic waves
Radio fiber bursts and fast magnetoacoustic wave trains
We present a model for dm-fiber bursts that is based on assuming fast sausage
magnetoacoustic wave trains that propagate along a dense vertical filament or
current sheet. Eight groups of dm-fiber bursts that were observed during solar
flares were selected and analyzed by the wavelet analysis method. To model
these fiber bursts we built a semi-empirical model. We also did
magnetohydrodynamic simulations of a propagation of the magnetoacoustic wave
train in a vertical and gravitationally stratified current sheet. In the
wavelet spectra of the fiber bursts computed at different radio frequencies we
found the wavelet tadpoles, whose head maxima have the same frequency drift as
the drift of fiber bursts. It indicates that the drift of these fiber bursts
can be explained by the propagating fast sausage magnetoacoustic wave train.
Using new semi-empirical and magnetohydrodynamic models with a simple radio
emission model we generated the artificial radio spectra of the fiber bursts,
which are similar to the observed ones.Comment: 7 pages, 10 figure
Radio bursts observed during solar eruptive flares and their schematic summary
In this review we summarize results of our analysis of the observations of
solar eruptive flares made by the Ond\v{r}ejov radiospectrograph for more than
twenty years. We also present some Potsdam-Tremsdorf radio spectra from our
common studies. Considering a 3-dimensional model of eruptive flares together
with the results of our magnetohydrodynamic and particle-in-cell simulations we
show an importance of decimetric radio bursts for understanding of plasma
processes in eruptive flares. We present drifting pulsation structures as
signatures of plasmoids, an unusual zebra pattern in the very early flare
stage, narrowband dm-spikes as the bursts generated in the reconnection plasma
outflows, radio bursts indicating a merging of plasmoids, pair of decimetric
type III bursts indicating the electron beams propagating upwards and downwards
in the solar atmosphere from the acceleration site, and a special decimetric
type III burst formed probably around the plasmoid. We present unusual radio
bursts connected with the rising magnetic rope at the very beginning of
eruptive flares. Furthermore, based on the analysis of decimetric continua we
estimated the level of the plasma turbulence in a vicinity of the flare
termination shock. Interpretations of all these bursts are based on models and
time coincidences with observations in X-ray, UV and optical ranges; in most
cases an information about positions of these radio sources is missing. To show
an importance of positional information, we present a rare example of
observations, where the drifting pulsation structure was observed
simultaneously with the observations made by the EOVSA radiointerferometer. All
the presented bursts are then summarized in a new scheme of bursts and compared
with the schema commonly used.Comment: 20 pages, 12 figure
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
Spontaneous current-layer fragmentation and cascading reconnection in solar flares: II. Relation to observations
In the paper by B\'arta et al. (arXive:astro-ph:/1011.4035, 2010) the authors
addressed some open questions of the CSHKP scenario of solar flares by means of
high-resolution MHD simulations. They focused, in particular, on the problem of
energy transfer from large to small scales in decaying flare current sheet
(CS). Their calculations suggest, that magnetic flux-ropes (plasmoids) are
formed in full range of scales by a cascade of tearing and coalescence
processes. Consequently, the initially thick current layer becomes highly
fragmented. Thus, the tearing and coalescence cascade can cause an effective
energy transfer across the scales. In the current paper we investigate whether
this mechanism actually applies in solar flares. We extend the MHD simulation
by deriving model-specific features that can be looked for in observations. The
results of the underlying MHD model showed that the plasmoid cascade creates a
specific hierarchical distribution of non-ideal/acceleration regions embedded
in the CS. We therefore focus on the features associated with the fluxes of
energetic particles, in particular on the structure and dynamics of emission
regions in flare ribbons. We assume that the structure and dynamics of
diffusion regions embedded in the CS imprint themselves into structure and
dynamics of flare-ribbon kernels by means of magnetic-field mapping. Using the
results of the underlying MHD simulation we derive the expected structure of
ribbon emission and we extract selected statistical properties of the modelled
bright kernels. Comparing the predicted emission and its properties with the
observed ones we obtain a good agreement of the two.Comment: 7 pages, 5 figure
Reconnection of a kinking flux rope triggering the ejection of a microwave and hard X-ray source. II. Numerical Modeling
Numerical simulations of the helical () kink instability of an
arched, line-tied flux rope demonstrate that the helical deformation enforces
reconnection between the legs of the rope if modes with two helical turns are
dominant as a result of high initial twist in the range . Such
reconnection is complex, involving also the ambient field. In addition to
breaking up the original rope, it can form a new, low-lying, less twisted flux
rope. The new flux rope is pushed downward by the reconnection outflow, which
typically forces it to break as well by reconnecting with the ambient field.
The top part of the original rope, largely rooted in the sources of the ambient
flux after the break-up, can fully erupt or be halted at low heights, producing
a "failed eruption." The helical current sheet associated with the instability
is squeezed between the approaching legs, temporarily forming a double current
sheet. The leg-leg reconnection proceeds at a high rate, producing sufficiently
strong electric fields that it would be able to accelerate particles. It may
also form plasmoids, or plasmoid-like structures, which trap energetic
particles and propagate out of the reconnection region up to the top of the
erupting flux rope along the helical current sheet. The kinking of a highly
twisted flux rope involving leg-leg reconnection can explain key features of an
eruptive but partially occulted solar flare on 18 April 2001, which ejected a
relatively compact hard X-ray and microwave source and was associated with a
fast coronal mass ejection.Comment: Solar Physics, in pres
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