52 research outputs found
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
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
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
Solar Radio Bursts with Spectral Fine Structures in Preflares
A good observation of preflare activities is important for us to understand
the origin and triggering mechanism of solar flares, and to predict the
occurrence of solar flares. This work presents the characteristics of microwave
spectral fine structures as preflare activities of four solar flares observed
by Ond\v{r}ejov radio spectrograph in the frequency range of 0.8--2.0 GHz. We
found that these microwave bursts which occurred 1--4 minutes before the onset
of flares have spectral fine structures with relatively weak intensities and
very short timescales. They include microwave quasi-periodic pulsations (QPP)
with very short period of 0.1-0.3 s and dot bursts with millisecond timescales
and narrow frequency bandwidths. Accompanying these microwave bursts, there are
filament motions, plasma ejection or loop brightening on the EUV imaging
observations and non-thermal hard X-ray emission enhancements observed by
RHESSI. These facts may reveal certain independent non-thermal energy releasing
processes and particle acceleration before the onset of solar flares. They may
be conducive to understand the nature of solar flares and predict their
occurrence
Torsional oscillations and observed rotational period variations in early-type stars
Some chemically peculiar stars in the upper main sequence show rotational period variations of unknown origin. We propose that these variations are a consequence of the propagation of internal waves in magnetic rotating stars that lead to the torsional oscillations of the star. We simulate the magnetohydrodynamic waves and calculate resonant frequencies for two stars that show rotational variations: CU Vir and HD 37776. We provide updated analyses of rotational period variations in these stars and compare our results with numerical models. For CU Vir, the length of the observed rotational period cycle, | | yr, can be well reproduced by the models, which predict a cycle length of 51 yr. However, for HD 37776, the observed lower limit of the cycle length, | | yr, is significantly longer than the numerical models predict. We conclude that torsional oscillations provide a reasonable explanation at least for the observed period variations in CU Vir
RADIO EVIDENCE OF BREAK-OUT RECONNECTION?
We reconsider the 2003 October 28 X17 flare/coronal mass ejection (CME), studying the five minutes immediately before the impulsive flare phase (not discussed in previous work). To this aim we examine complementary dynamic radio spectrograms, single frequency polarimeter records, radio images, space-based longitudinal field magnetograms, and ultraviolet images. We find widely distributed faint and narrowband meter wave radio sources located outside active regions but associated with the boundaries of magnetic flux connectivity cells, inferred from the potential extrapolation of the observed photospheric longitudinal field as a model for coronal magnetic field structures. The meter wave radio sources occur during the initial decimeter wave effects, which are well known to be associated with filament destabilization in the flaring active region (here NOAA 10486). Antiochos et al. predict in their break-out model for CME initiation that "... huge phenomena ... may be controlled by detailed plasma processes that occur in relatively tiny regions." They suggest that the expected faint energy release "... on long field lines far away from any neutral line ... may be detectable in radio/microwave emission from nonthermal particles..." In this paper, we describe meter wave sources whose properties correctly coincide with the quoted predictions of the break-out reconnection model of the CME initiation
Successive Solar Flares and Coronal Mass Ejections on 2005 September 13 from Noaa Ar 10808
We present a multiwavelength study of the 2005 September 13 eruption from
NOAA 10808 that produced total four flares and two fast coronal mass ejections
(CMEs) within 1.5 hours. Our primary attention is paid to the fact that these
eruptions occurred in close succession in time, and that all of them were
located along an S-shaped magnetic polarity inversion line (PIL) of the active
region. In our analysis, (1) the disturbance created by the first flare
propagated southward along the PIL to cause a major filament eruption that led
to the first CME and the associated second flare underneath. (2) The first CME
partially removed the overlying magnetic fields over the northern Delta spot to
allow the third flare and the second CME. (3) The ribbon separation during the
fourth flare would indicate reclosing of the overlying field lines opened by
the second CME. It is thus concluded that this series of flares and CMEs are
interrelated to each other via magnetic reconnections between the expanding
magnetic structure and the nearby magnetic fields. These results complement
previous works made on this event with the suggested causal relationship among
the successive eruptions.Comment: 11 pages, 8 figures, 2 tables, accepted to The Astrophysical Journa
Multi-wavelength analysis of high energy electrons in solar flares: a case study of August 20, 2002 flare
A multi-wavelength spatial and temporal analysis of solar high energy
electrons is conducted using the August 20, 2002 flare of an unusually flat
(gamma=1.8) hard X-ray spectrum. The flare is studied using RHESSI, Halpha,
radio, TRACE, and MDI observations with advanced methods and techniques never
previously applied in the solar flare context. A new method to account for
X-ray Compton backscattering in the photosphere (photospheric albedo) has been
used to deduce the primary X-ray flare spectra. The mean electron flux
distribution has been analysed using both forward fitting and model independent
inversion methods of spectral analysis. We show that the contribution of the
photospheric albedo to the photon spectrum modifies the calculated mean
electron flux distribution, mainly at energies below 100 keV. The positions of
the Halpha emission and hard X-ray sources with respect to the current-free
extrapolation of the MDI photospheric magnetic field and the characteristics of
the radio emission provide evidence of the closed geometry of the magnetic
field structure and the flare process in low altitude magnetic loops. In
agreement with the predictions of some solar flare models, the hard X-ray
sources are located on the external edges of the Halpha emission and show
chromospheric plasma heated by the non-thermal electrons. The fast changes of
Halpha intensities are located not only inside the hard X-ray sources, as
expected if they are the signatures of the chromospheric response to the
electron bombardment, but also away from them.Comment: 26 pages, 9 figures, accepted to Solar Physic
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