Flares on A-type Stars: Evidence for Heating of Solar Corona by Nanoflares? |
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Marian Karlický Submitted: 2016-11-14 02:42
We analyzed the occurrence rates of flares on stars of spectral types K, G, F, and A, observed by Kepler. We found that the histogram of occurrence frequencies of stellar flares is systematically shifted toward a high-energy tail for A-type stars compared to stars of cooler spectral types. We extrapolated the fitted power laws toward flares with smaller energies (nanoflares) and made estimates for total energy flux to stellar atmospheres by flares. We found that, for A-type stars, the total energy flux density was at least four-times smaller than for G stars. We speculate that this deficit in energy supply may explain the lack of hot coronae on A-type stars. Our results indicate the importance of nanoflares for heating and formation of the solar corona.
Authors: Švanda, M., Karlický, M.
Projects: None
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Publication Status: Published in ApJ 831, 9S (2016)
Last Modified: 2016-11-16 12:17
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Oscillation Maps in the Broadband Radio Spectrum of the 1 August 2010 Event |
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Marian Karlický Submitted: 2016-11-14 00:35
We search for indications of waves in the 25-2000 MHz radio spectrum of the 1 August 2010 event (SOL2010-08-01T08:57:00L075C013), where fast propagating waves in the solar corona with the periods of 181, 69, and 40 seconds were detected in UV observations. Using the wavelet technique we construct a new type of map of oscillations for selected periods in the whole domain of the radio spectrum. While the oscillation with the period of 181 seconds was recognized in the whole 25-2000 MHz radio spectrum, oscillations with periods of 69 and 40 seconds were confirmed only in the 250-870 MHz frequency range. In the 800-2000 MHz range we found periods of 50 and 80 seconds. Moreover, in the 250-870 MHz frequency range, the oscillation with the period of about 420 seconds was detected. We also made maps of phases of the 181-second oscillations in order to analyze their frequency drift. At the beginning of the radio event, in the 2000-500 MHz frequency range the phase of the 181-second oscillation drifts towards lower frequencies. On the other hand, at frequencies 25-500 MHz we found that the phase is nearly synchronous. While the phase drift at higher frequencies can be interpreted as being caused by the UV wave, the synchronization of the phase on lower frequencies is explained by the fast-electron beams, whose acceleration is modulated by the UV wave. Owing to this modulation, the electron beams are accelerated with the period of the UV wave (181 seconds). These beams propagate upwards through the solar corona and generate the 25-500 MHz radio emission with the 181-second period. Due to high beam velocity (~c/3, where c is the light speed) the 25-500 MHz radio emission, corresponding to a large interval of heights in the solar corona, is nearly synchronous.
Authors: Karlický, M., Rybak, J.
Projects: None
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Publication Status: Solar Physics (accepted)
Last Modified: 2016-11-16 12:17
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Thermal fronts in solar flares |
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Marian Karlický Submitted: 2015-12-07 06:50
We studied the formation of a thermal front during the expansion of hot plasma into colder plasma. We used a
three-dimensional electromagnetic particle-in-cell model that includes inductive effects. In early phases, in the area of the expanding hot plasma, we found several thermal fronts, which are defined as a sudden decrease of the local electron kinetic energy. The fronts formed a cascade. Thermal fronts with higher temperature contrast were located near plasma density depressions, generated during the hot plasma expansion. The formation of the main thermal front was associated with the return-current process induced by hot electron expansion and electrons backscattered at the front. A part of the hot plasma was trapped by the thermal front while another part, mainly with the most energetic electrons, escaped and generated Langmuir and electromagnetic waves in front of the thermal front, as shown by the dispersion diagrams. Considering all of these processes and those described in the literature, we show that anomalous electric resistivity is produced at the location of the thermal front. Thus, the thermal front can contribute to energy dissipation in the current-carrying loops of solar flares. We estimated the values of such anomalous resistivity in the solar atmosphere together with collisional resistivity and electric fields. We propose that the slowly drifting reverse drift bursts, observed at the beginning of some solar flares, could be signatures of the thermal front.
Authors: Karlický, M.
Projects: None
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Publication Status: ApJ 814, id. 153, 7 pp. (2015)
Last Modified: 2015-12-08 12:11
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Fragmentation of electric currents in the solar corona by plasma flows |
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Marian Karlický Submitted: 2013-10-03 23:37
Aims: We consider a magnetic configuration consisting of an arcade structure and a detached plasmoid, resulting from a magnetic reconnection process, as is typically found in connection with solar flares. We study spontaneous current fragmentation caused by shear and vortex plasma flows.
Methods: An exact analytical transformation method was applied to calculate self-consistent solutions of the nonlinear stationary magnetohydrodynamic equations. The assumption of incompressible field-aligned flows implies that both the Alfvén Mach number and the mass density are constant on field lines. We first calculated nonlinear magnetohydrostatic equilibria with the help of the Liouville method, emulating the scenario of a solar eruptive flare configuration with plasmoids (magnetic ropes or current-carrying loops in 3D) and flare arcade. Then a Mach number profile was constructed that describes the upflow along the open magnetic field lines and implements a vortex flow inside the plasmoid. This Mach number profile was used to map the magnetohydrostatic equilibrium to the stationary one.
Results: We find that current fragmentation takes place at different locations within our configuration. Steep gradients of the Alfvén Mach number are required, implying the strong influence of shear flows on current amplification and filamentation of the magnetohydrostatic current sheets. Crescent- or ring-like structures appear along the outer separatrix, butterfly structures between the upper and lower plasmoids, and strong current peaks close the lower boundary (photosphere). Furthermore, impressing an intrinsic small-scale structure on the upper plasmoid results in strong fragmentation of the plasmoid. Hence fragmentation of current sheets and plasmoids is an inherent property of magnetohydrodynamic theory.
Conclusions: Transformations from magnetohydrostatic into magnetohydrodynamic steady-states deliver fine-structures needed for plasma heating and acceleration of particles and bulk plasma flows in dissipative events that are typically connected to magnetic reconnection processes in flares and coronal mass ejections.
Authors: Nickeler, D. H., Karlický, M., Wiegelmann, T., Kraus, M.
Projects: None
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Publication Status: Published in Astronomy & Astrophysics, Volume 556, id.A61, 12 pp.
Last Modified: 2013-10-05 20:19
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Flare line impact polarization. Na D2 589 nm line polarization in the 2001 June 15 flare |
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Marian Karlický Submitted: 2013-10-03 23:32
Context. The impact polarization of optical chromospheric lines in solar flares is still being debated. For this reason, additional observations and improved flare atmosphere models are needed still.
Aims: The polarization-free telescope THEMIS used in multiline 2 MulTiRaies (MTR) mode allows accurate simultaneous linear polarization measurements in various spectral lines.
Methods: In the 2001 June 15 flare, Hα, Hβ, and Mg D2 lines linear impact polarization was reported as present in THEMIS 2 MTR observations. In this paper, THEMIS data analysis was extended to the Na D2 line. Sets of I ? U and I ? Q flare Stokes S 2D-spectra were corrected from dark-current, spectral-line curvature and from transmission differences. Then, we derived the linear polarization degree P and polarization orientation angle α 2D-spectra. No change in relative positioning could be found that would reduce the Stokes parameters U and Q values. No V and I crosstalks could explain our results either.
Results: The Na D2 line is linearly polarized with a polarization degree exceeding 5% at some locations. The polarization was found to be radial at outer ribbons edges, and tangential at their inner edges. This orientation change may be due to differences in electron distribution functions on the opposite borders of flare chromospheric ribbons. Electron beams propagating along magnetic field lines, together with return currents, could explain both radial and tangential polarization. At the inner ribbon edges, intensity profile-width enlargements and blueshifts in polarization profiles are observed. This suggests chromospheric evaporation.
Authors: Henoux, J.C., Karlický, M.
Projects: None
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Publication Status: Published in Astronomy & Astrophysics, Volume 556, id.A95, 8 pp
Last Modified: 2013-10-05 20:19
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Radio continua modulated by waves: Zebra patterns in solar and pulsar radio spectra? |
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Marian Karlický Submitted: 2013-04-09 02:03
We aim to answer the question how waves with plasma density variations affect the radio continua generated by the plasma emission mechanism. We built a simple semi-empirical model of the radio continuum modulation. Assuming that the waves with density variations are in the source of this radio continuum, we modeled the artificial radio spectrum, which we compared with observed spectra. We show that the waves with density variations modulate the radio continua
generated by the plasma emission mechanism. Considering a single slow magnetoacoustic wave, we model the radio spectra, which resemble solar zebra patterns. We show that this modulation generates zebra effects even when the
radio continuum is composed of many spiky bursts. Generalizing from one single wave to a wave turbulence we find that the computed radio spectrum is similar to so-called lace bursts. Finally, using the same procedure, but for fast magnetoacoustic waves, we modeled the radio spectrum similar to that observed during the interpulse phase of the radio emission of the Crab Nebula pulsar.
Authors: M. Karlický
Projects: None
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Publication Status: published in Astronomy & Astrophysics, Volume 552, id.A90, 6 pp
Last Modified: 2013-04-09 16:47
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Fragmentation during merging of plasmoids in the magnetic field reconnection |
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Marian Karlický Submitted: 2012-05-09 06:33
Application of the magnetic-reconnection theory onto large-scale events, such as solar flares, requires formation of very thin (kinetic-scale) current sheets within the rather thick flare current layer. Hence, some fragmentation/filamentation mechanisms has to be in action.
We aim at identifying fragmentation mechanisms for magnetic field and current density structures. Namely, we focus at detailed study of the processes during the merging of plasmoids that had been formed in the current layer.
A 2.5-D electromagnetic Particle-In-Cell model is used and its results analysed.
It is shown that the merging process of plasmoids is not a simple process as presented in some previous studies. On the contrary, this process leads to a complex fragmentation. We found two types of fragmentation processes: a) fragmentation in the current sheet generated between the merging plasmoids and b) fragmentation at the boundary of plasma outflow from the reconnection between
these plasmoids. While the first type of fragmentation is generated by the tearing-mode (plasmoid) instability of the secondary current sheet, the second one looks to
be connected with an increase of the plasma beta parameter during these processes. Thus, sheared high-beta plasma flows produce this additional fragmentation.
The fragmentation and energy transport from large to small scales in a large-scale magnetic reconnection seem to be the result of interplay and positive feedback between instabilities driven by high gradients in both
magnetic (intense current density) and velocity (high vorticity) fields.
Authors: M. Karlický, M. Barta, D. Nickeler
Projects: None
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Publication Status: published in Astronomy & Astrophysics, Volume 541, id.A86
Last Modified: 2012-05-09 12:08
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On the physical meaning of n-distributions in solar flares |
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Marian Karlický Submitted: 2011-12-29 00:28
Aims: We investigate the physical meaning of the n-distributions detected in solar flares. Methods: We consider a Maxwellian velocity distribution with a velocity drift. This distribution is analytically integrated to obtain the energy distribution, and its stability is investigated numerically using a fully electromagnetic particle-in-cell code. Results: It is shown that the derived moving Maxwellian energy distribution is very similar to the n-distribution, especially in their high-energy parts. Both these distributions are mutually fitted and a relation between their parameters found. Contrary to the n-distribution, the moving Maxwellian distribution has a simple physical meaning, e.g., the electron component of the return current in the beam-plasma system. However, for high drift velocities of such a component, the moving Maxwellian distribution is unstable. Therefore to keep the form of this distribution similar to the n-distribution, some stabilization processes are necessary. If so, then the high intensities of the Si xiid 5.56 Å and 5.82 Å satellite lines and their evolution in solar flares can be explained by moving Maxwellian distributions instead of the n-distributions. Thus, our previous results connected with the n-distributions can be understood in a new, physically profound way.
Authors: M. Karlický, E. Dzifcakova, J. Dudik
Projects:
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Publication Status: publushed in 2012, A&A 537, id. A36
Last Modified: 2011-12-29 14:02
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Electron beam-plasma interaction and return-current formation |
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Marian Karlický Submitted: 2009-01-28 00:37
Using a 3-D electromagnetic particle-in-cell model, the plasma systems with
electron beams are studied. The model parameters are chosen relevant to solar
flares. To show the complexity of the problem studied, in the first model, we
present an evolution of two oppositely propagating electron beams that escape
from the localized acceleration region. Then due to the limitations of this
model, in other models, we consider a plasma system with one spatially
homogeneous beam and a neutralizing return current. The models without the
initial magnetic field revealed an importance of the Weibel instability that
very efficiently transferred the beam energy into the heating of plasma
electrons in the direction perpendicular to that of the beam propagation. In
this case the return current evolved from the initially shifted Maxwell
distribution to the distribution with a broad flat maximum. On the other hand,
in the models with a sufficiently strong magnetic field or those with shorter
sizes of the numerical box (effectively the 1-D case), the Weibel instability
was reduced, and the electron distribution function evolved mainly in the
beam-propagation direction and the return-current was given by a shift of some
bulk electrons and an extended tail.
Authors: Marian Karlický
Projects:
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Publication Status: Published in ApJ 690, 189-197, 2009
Last Modified: 2009-01-28 13:30
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Particle-in-cell simulations of return current in solar flares |
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Marian Karlický Submitted: 2008-09-28 23:49
We numerically study a formation of the return current generated in solar flares. For simulations of the return current in the beam-plasma system,
a 3-D particle-in-cell electromagnetic code is used.
In conditions of solar flares with the electron beam fluxes of E_F = 9.1 x 109 - 4.55 x 1010 ergs s-1 cm-2, the
beam-plasma interaction with the return current is studied. We found that the electron beam relaxes to the plateau distribution function as known from electrostatic simulations. Simultaneously, due to electromagnetic effects and the Buneman instability of the prescribed Maxwell-shifted return current, the electron distribution function evolves to a new stationary state with a new
form of the return current. In this final state the return current is formed not only by electrons in the bulk of the electron distribution function, but also by electrons in the extended tail. We use the results of simulations to
estimate the critical beam fluxes for the processes under study in the low corona, the transition region and the upper chromosphere.
Authors: Karlický, M., Nickeler, D.H., Barta, M.
Projects: None
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Publication Status: published in A&A 486, 325-329 (2008)
Last Modified: 2008-09-29 11:14
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High-frequency slowly drifting structures in solar flares |
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Marian Karlický Submitted: 2002-12-05 05:16
Radio emission of four solar flares with high-frequency slowly drifting structures is presented. Three sub-classes of these structures were recognized. It is shown that the April 15, 2001 X14.4 flare started with the slowly drifting structure associated with a plasmoid ejection observed by TRACE in the 171 Å line. The August 18, 1998 event presents an example of the drifting pulsation structure (DPS) which is well limited in frequency extent at both sides. A further example of the DPS, but followed by clouds of the narrowband dm-spikes, was observed during the November 23, 2001 flare. Finally, in the case of the April 12, 2001 flare, the drifting pulsation-continuum structure was recorded at the same time as the metric type II radio burst, i.e. in different frequency ranges. The slowly drifting structures were analyzed and in two cases their relation to hard X-ray emission was studied. Possible underlying physical processes are discussed assuming the plasmoid ejection model of eruptive solar flares.
Authors: M. Karlický, F. Farnik, H. Meszarosova
Projects:
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Publication Status: Published in Astron.Astrophys. 395, 677-683
Last Modified: 2002-12-05 05:16
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