explanation blue bibcodes open ADS page with paths to full text
Author name code: nakariakov
ADS astronomy entries on 2022-09-14
author:"Nakariakov, Valery"
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Title: Two-Spacecraft Detection of Short-period Decayless Kink
Oscillations of Solar Coronal Loops
Authors: Zhong, Sihui; Nakariakov, Valery M.; Kolotkov, Dmitrii Y.;
Verbeeck, Cis; Berghmans, David
2022arXiv220901917Z Altcode:
Decayless kink oscillations of an ensemble of loops are captured
simultaneously by the High Resolution Imager (HRI) of the Extreme
Ultraviolet Imager (EUI) and the Atmospheric Imaging Assembly (AIA)
from 22:58 UT on 5 November to 00:27 UT on 6 November 2021. Oscillations
are analysed by processing image sequences taken by the two instruments
with a motion magnification technique. The analysed loops are around 51
Mm in length, and oscillate with short periods of 1-3 min (1.6 min in
average) and displacement amplitudes of 27-83 km. The signals recorded
by AIA are delayed by 66 s as compared to HRI, which coincides with
the light travel time difference from the Sun to each instrument. After
correction of this time difference, the cross-correlation coefficient
between the signals from the two data varies from 0.82 to 0.97,
indicating that they are well consistent. This work confirms that HRI
sees the same oscillations as AIA, which is the necessary first step
before proceeding to the detection of shorter time scales by EUI. In
addition, our results indicate the robustness of the de-jittering
procedure in the study of kink oscillations with HRI.
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Title: Probing the Density Fine Structuring of the Solar Corona with
Comet Lovejoy
Authors: Nisticò, Giuseppe; Zimbardo, Gaetano; Perri, Silvia;
Nakariakov, Valery M.; Duckenfield, Timothy J.; Druckmueller, Miloslav
2022arXiv220904051N Altcode:
The passage of sungrazing comets in the solar corona can be a powerful
tool to probe the local plasma properties. Here, we carry out a
study of the striae pattern appearing in the tail of sungrazing Comet
Lovejoy, as observed by the Atmospheric Imaging Assembly (AIA) aboard
the Solar Dynamics Observatory (SDO) during the inbound and outbound
phases of the comet orbit. We consider the images in EUV in the 171
Å bandpass, where emission from oxygen ions O$^{4+}$ and O$^{5+}$
is found. The striae are described as due to a beam of ions injected
along the local magnetic field, with the initial beam velocity decaying
because of collisions. Also, ion collisional diffusion contributes to
ion propagation. Both the collision time for velocity decay and the
diffusion coefficient for spatial spreading depend on the ambient plasma
density. A probabilistic description of the ion beam density along the
magnetic field is developed, where the beam position is given by the
velocity decay and the spreading of diffusing ions is described by
a Gaussian probability distribution. Profiles of emission intensity
along the magnetic field are computed and compared with the profiles
along the striae observed by AIA, showing a good agreement for most
considered striae. The inferred coronal densities are then compared
with a hydrostatic model of the solar corona. The results confirm that
the coronal density is strongly spatially structured.
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Title: Seismology of the solar corona and extensions to stellar
coronae
Authors: Nakariakov, Valery
2022cosp...44.2498N Altcode:
Wave motions detected in the upper part of the solar atmosphere, the
corona, become a commonly used tool for probing physical conditions
in the waveguiding plasma structures. Properties of the waves are
sensitive to macroscopic parameters of the plasma, as well as transport
coefficients and geometry and directivity of the magnetic field. Waves
observed in the corona are of the magnetohydrodynamic (MHD) type, and
are well described by MHD wave theory. The combination of high-precision
observations with an array of spaceborne and ground-based instruments,
and state-of-the-art theoretical modelling gives us estimations of
physical parameters which are not open to direct observations, such
as the absolute value of the magnetic field and its twist and free
magnetic energy, electric currents, fine perpendicular structuring,
the coronal heating function, and channels of the slow solar wind. A
specific interest is paid to quasi-periodic (QPP) processes in
flaring energy releases, as, on one hand it is natural to search for
oscillatory processes in the vicinity of impulsive energy releases and,
on the other, the enhanced radiation power can significantly improve
the signal-to-noise ratio. We discuss possible links of QPP in solar
and stellar flares with MHD wave processes, and implications of these
links to plasma diagnostics.
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Title: Probing the solar corona with sungrazing comets: magnetic
field and coronal density estimates from the EUV striae in the tail
of Comet Lovejoy
Authors: Nisticò, Giuseppe; Nakariakov, Valery; Zimbardo, Gaetano;
Perri, Silvia; Duckenfield, Timothy; Druckmüller, Miloslav
2022cosp...44.2501N Altcode:
The transit of Comet Lovejoy C/2011 W3 across the solar atmosphere in
December 2011 offers us the opportunity to probe the physical conditions
of the solar corona at distances that are still inaccessible to current
spacecraft. The tail of Comet Lovejoy, as observed in EUV with the
Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory
(SDO), appeared in the form of striae caused by the emission of cometary
ions injected along the local coronal magnetic field lines intersected
by the comet. We studied the spatial and time evolution of such striae
as observed in the channel at 171 \AA~ of SDO/AIA to acquire information
on the local magnetic field and plasma density. The striae are modelled
as a beam of oxygen ions, with the injection beam velocity decaying
because of collisions. Also, oxygen ions spread in space because of
collisional diffusion. Both the collision time for velocity decay and
the diffusion coefficient depend on the ambient plasma density. We
developed a probabilistic description of the ion beam density along
the magnetic field. In such a model, the beam position is determined by
the velocity decay and the spreading of diffusing ions is described by
a Gaussian probability distribution. Profiles of the ion density along
the magnetic field are computed and conveniently compared with the 171
\AA~ intensity profiles along the striae to constrain the values of
the local plasma density. Furthermore, transverse displacements of the
striae observed in the data are interpreted in terms of propagating
kink waves, whose properties allow us to estimate the magnitude of
the local magnetic field.
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Title: A new look at the frequency-dependent damping of slow-mode
waves in the solar corona
Authors: Kolotkov, Dmitrii Y.; Nakariakov, Valery M.
2022MNRAS.514L..51K Altcode: 2022MNRAS.tmpL..51K; 2022arXiv220505346K
Being directly observed in the Doppler shift and imaging data and
indirectly as quasi-periodic pulsations in solar and stellar flares,
slow magnetoacoustic waves offer an important seismological tool
for probing many vital parameters of the coronal plasma. A recently
understood active nature of the solar corona for magnetoacoustic
waves, manifested through the phenomenon of wave-induced thermal
misbalance, led to the identification of new natural mechanisms for the
interpretation of observed properties of waves. A frequency-dependent
damping of slow waves in various coronal plasma structures
remains an open question, as traditional wave damping theories
fail to match observations. We demonstrate that accounting for the
back-reaction caused by thermal misbalance on the wave dynamics
leads to a modification of the relationship between the damping
time and oscillation period of standing slow waves, prescribed by
the linear theory. The modified relationship is not of a power-law
form and has the equilibrium plasma conditions and properties of the
coronal heating/cooling processes as free parameters. It is shown
to readily explain the observed scaling of the damping time with
period of standing slow waves in hot coronal loops. Functional forms
of the unknown coronal heating process, consistent with the observed
frequency-dependent damping, are seismologically revealed.
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Title: High frequency waves in chromospheric spicules
Authors: Bate, William; Nakariakov, Valery; Jafarzadeh, Shahin; Jess,
David; Stangalini, Marco; Grant, Samuel; Keys, Peter; Christian,
Damian; Keenan, Francis
2022cosp...44.2548B Altcode:
Using high cadence observations from the Hydrogen-alpha Rapid
Dynamics camera imaging system on the Dunn Solar Telescope, we
present an investigation of the statistical properties of transverse
oscillations in spicules captured above the solar limb. At five equally
separated atmospheric heights, spanning approximately $4900-7500$~km,
we have detected a total of $15{\,}959$ individual wave events,
with a mean displacement amplitude of $151\pm 124$~km, a mean
period of $54\pm 45$~s, and a mean projected velocity amplitude
of $21\pm 13$~km{\,}s$^{-1}$. We find that both the displacement
and velocity amplitudes increase with height above the solar limb,
ranging from $132\pm 111$~km and $17.7\pm 10.6$~km{\,}s$^{-1}$ at
$\approx4900$~km, and $168\pm 125$~km and $26.3\pm 14.1$~km{\,}s$^{-1}$
at $\approx7500$~km, respectively. Following the examination of
neighboring oscillations in time and space, we find 45% of the waves
to be upwardly propagating, 49% to be downwardly propagating, and 6%
to be standing, with mean absolute phase velocities for the propagating
waves on the order of $75-150$~km{\,}s$^{-1}$. While the energy flux
of the waves propagating downwards does not appear to depend on height,
we find the energy flux of the upwardly propagating waves decreases with
atmospheric height at a rate of $-13{\,}200\pm6500$~W{\,}m$^{-2}$/Mm. As
a result, this decrease in energy flux as the waves propagate upwards
may provide significant thermal input into the local plasma.
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Title: Long-term evolution of decayless kink oscillations of solar
coronal loops
Authors: Zhong, Sihui; Nakariakov, Valery M.; Kolotkov, Dmitrii Y.;
Anfinogentov, Sergey A.
2022MNRAS.513.1834Z Altcode: 2022MNRAS.tmp.1009Z
Long-term evolution of instantaneous parameters of decayless kink
oscillations of six solar coronal loops observed for longer than
2 h each is studied. The oscillations are analysed by processing
sequences of 171 Å images obtained with the Solar Dynamics Observatory
(SDO)/Atmospheric Imaging Assembly (AIA) in the time interval from 2020
December till 2021 June, with the motion magnification technique. It
is established that decayless kink oscillations could exist for
more than 30 or 40 oscillation cycles. Neither the loop brightness
nor instantaneous parameters of the oscillations show a monotonic
increase or decrease during the oscillation. The observed instantaneous
oscillation periods and amplitudes are found to vary randomly in time,
with distributions around the mean values that resemble Gaussian
profiles. Mean values of the oscillation periods and amplitudes are
consistent with previous observations of this phenomenon. A power-law
dependence of the oscillation period on the displacement amplitude
is found, with the power-law index of 0.41 and with the 95 per cent
confidence interval of [0.39, 0.71]. In general, we established the
lack of correlation between instantaneous oscillation parameters and
loop brightness. One exception is an event with relatively strong
anticorrelation of the amplitude and the loop's brightness, with
the cross-correlation coefficient of about -0.81, but this effect
requires a further study. Fourier power spectra of the envelopes of
the time-evolving instantaneous amplitudes and periods are white noise,
indicating that consecutive values of the instantaneous parameters are
independent of each other. The results obtained provide an empirical
ground for validating and comparing existing and future theoretical
models of decayless kink oscillations of coronal loops.
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Title: Kink Oscillation of a Flux Rope During a Failed Solar Eruption
Authors: Kumar, Pankaj; Nakariakov, Valery M.; Karpen, Judith T.;
Richard DeVore, C.; Cho, Kyung-Suk
2022ApJ...932L...9K Altcode: 2022arXiv220503480K
We report a decaying kink oscillation of a flux rope during a confined
eruptive flare, observed off the solar limb by the Solar Dynamics
Observatory's Atmospheric Imaging Assembly (AIA), which lacked a
detectable white-light coronal mass ejection. The erupting flux rope
underwent kinking, rotation, and apparent leg-leg interaction during the
event. The oscillations were observed simultaneously in multiple AIA
channels at 304, 171, and 193 Å, indicating that multithermal plasma
was entrained in the rope. After reaching the overlying loops in the
active region, the flux rope exhibited large-amplitude, decaying kink
oscillations with an apparent initial amplitude of 30 Mm, a period of
about 16 minutes, and a decay time of about 17 minutes. We interpret
these oscillations as a fundamental standing kink mode of the flux
rope. The oscillation polarization has a clear vertical component,
while the departure of the detected waveform from a sinusoidal signal
suggests that the oscillation could be circularly or elliptically
polarized. The estimated kink speed is 1080 km s<SUP>-1</SUP>,
corresponding to an Alfvén speed of about 760 km s<SUP>-1</SUP>. This
speed, together with the estimated electron density in the rope from our
differential emission measure analysis, n <SUB> e </SUB> ≍ (1.5-2.0)
× 10<SUP>9</SUP> cm<SUP>-3</SUP>, yields a magnetic-field strength of
about 15 G. To the best of our knowledge, decaying kink oscillations of
a flux rope with nonhorizontal polarization during a confined eruptive
flare have not been reported before. These oscillations provide unique
opportunities for indirect measurements of the magnetic-field strength
in low-coronal flux ropes during failed eruptions.
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Title: Slow Magnetoacoustic Oscillations in Stellar Coronal Loops
Authors: Lim, Daye; Nakariakov, Valery M.; Moon, Yong-Jae
2022ApJ...931...63L Altcode: 2022arXiv220511750L
Slow magnetoacoustic oscillations in stellar coronal loops with
gravitational stratification are analyzed with a numerical solution
of the boundary value problem for eigenvalues and eigenfunctions. In
this study, we only focus on the resonant periods. The effects of the
gravitational stratification, star mass, loop temperature, and loop
length on the properties of slow magnetoacoustic oscillations are
investigated. It is shown that the discrepancy between stratified
and nonstratified loops is higher in density perturbations than
in velocity perturbations. When the star has a larger mass, higher
coronal temperature, and longer loop, the density perturbations in
the stratified loop are significantly different from the harmonic
functions. The periods in the stratified loop are slightly longer than
in the nonstratified loop. The periods calculated in our model (14-644
minutes) are consistent with the periods of stellar quasi-periodic
pulsations observed in both soft X-rays (2-70 minutes) and white lights
(8-390 minutes).
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Title: High-frequency Waves in Chromospheric Spicules
Authors: Bate, W.; Jess, D. B.; Nakariakov, V. M.; Grant, S. D. T.;
Jafarzadeh, S.; Stangalini, M.; Keys, P. H.; Christian, D. J.; Keenan,
F. P.
2022ApJ...930..129B Altcode: 2022arXiv220304997B
Using high-cadence observations from the Hydrogen-alpha Rapid
Dynamics camera imaging system on the Dunn Solar Telescope, we
present an investigation of the statistical properties of transverse
oscillations in spicules captured above the solar limb. At five equally
separated atmospheric heights, spanning approximately 4900-7500 km,
we have detected a total of 15,959 individual wave events, with a
mean displacement amplitude of 151 ± 124 km, a mean period of 54
± 45 s, and a mean projected velocity amplitude of 21 ± 13 km
s<SUP>-1</SUP>. We find that both the displacement and velocity
amplitudes increase with height above the solar limb, ranging from
132 ± 111 km and 17.7 ± 10.6 km s<SUP>-1</SUP> at ≍4900 km,
and 168 ± 125 km and 26.3 ± 14.1 km s<SUP>-1</SUP> at ≍7500 km,
respectively. Following the examination of neighboring oscillations in
time and space, we find 45% of the waves to be upwardly propagating,
49% to be downwardly propagating, and 6% to be standing, with mean
absolute phase velocities for the propagating waves on the order of
75-150 km s<SUP>-1</SUP>. While the energy flux of the waves propagating
downwards does not appear to depend on height, we find the energy flux
of the upwardly propagating waves decreases with atmospheric height
at a rate of -13,200 ± 6500 W m<SUP>-2</SUP>/Mm. As a result, this
decrease in energy flux as the waves propagate upwards may provide
significant thermal input into the local plasma.
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Title: Editorial to the Topical Collection: Oscillatory Processes
in Solar and Stellar Coronae
Authors: Nakariakov, Valery M.; Banerjee, Dipankar; Li, Bo; Wang,
Tongjiang; Zimovets, Ivan V.; Falanga, Maurizio
2022SSRv..218...13N Altcode:
No abstract at ADS
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Title: The dynamics and observability of circularly polarized
kink waves
Authors: Magyar, N.; Duckenfield, T.; Van Doorsselaere, T.; Nakariakov,
V. M.
2022A&A...659A..73M Altcode: 2021arXiv211214951M
Context. Kink waves are routinely observed in coronal loops. Resonant
absorption is a well-accepted mechanism that extracts energy
from kink waves. Nonlinear kink waves are know to be affected by
the Kelvin-Helmholtz instability. However, all previous numerical
studies consider linearly polarized kink waves. <BR /> Aims: We study
the properties of circularly polarized kink waves on straight plasma
cylinders, for both standing and propagating waves, and we compare them
to the properties of linearly polarized kink waves. <BR /> Methods:
We used the code MPI-AMRVAC to solve the full 3D magnetohydrodynamic
equations for a straight magnetic cylinder, excited by both standing and
propagating circularly polarized kink (m = 1) modes. <BR /> Results: The
damping due to resonant absorption is independent of the polarization
state. The morphology or appearance of the induced resonant flow is
different for the two polarizations; however, there are essentially
no differences in the forward-modeled Doppler signals. For nonlinear
oscillations, the growth rate of small scales is determined by the total
energy of the oscillation rather than the perturbation amplitude. We
discuss possible implications and seismological relevance. <P
/>Movies associated to Figs. 1, 2, and 5 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202141945/olm">https://www.aanda.org</A>
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Title: Hot Jets in the Solar Corona: Creating a Catalogue of Events
Based on Multi-Instrumental Observations
Authors: Kaltman, T. I.; Stupishin, A. G.; Anfinogentov, S. A.;
Nakariakov, V. M.; Loukitcheva, M. A.; Shendrik, A. V.
2021Ge&Ae..61.1083K Altcode:
We present a catalogue of solar coronal plasma jets with a temperature
above 0.5 MK, which includes primary information about the events,
parameters of the diagnosed jets, as well as related eruptive
phenomena. The catalogue (https://solar.sao.ru/coronal-jets-catalog/)
contains data obtained using the spaceborne EUV high-precision
telescope SDO/AIA and ground-based radio telescopes and spectrometers,
including RATAN-600, SRH and NoRH. For a number of events data on
the reconstructed magnetic field is also presented. The purpose
of the catalogue is to provide summary information on coronal jets
for further statistical analysis, determination of characteristic
parameters of jets, and for in-depth study of the individual events
by all interested researchers.
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Title: Multiwavelength Quasi-periodic Pulsations in a Stellar
Superflare
Authors: Kolotkov, Dmitrii Y.; Nakariakov, Valery M.; Holt, Robin;
Kuznetsov, Alexey A.
2021ApJ...923L..33K Altcode: 2021arXiv211207734K
We present the first multiwavelength simultaneous detection of
quasi-periodic pulsations (QPPs) in a superflare (more than a thousand
times stronger than known solar flares) on a cool star, in soft X-rays
(SXRs, with XMM-Newton) and white light (WL, with Kepler). It allowed
for the first ever analysis of oscillatory processes in a stellar flare
simultaneously in thermal and nonthermal emissions, conventionally
considered to come from the corona and chromosphere of the star,
respectively. The observed QPPs have periods 1.5 ± 0.15 hr (SXR)
and 3 ± 0.6 hr (WL), and correlate well with each other. The unique
relationship between the observed parameters of QPPs in SXR and WL
allowed us to link them with oscillations of the electric current in the
flare loop, which directly affect the dynamics of nonthermal electrons
and indirectly (via ohmic heating) the thermal plasma. These findings
could be considered in favor of the equivalent LCR contour model of a
flare loop, at least in the extreme conditions of a stellar superflare.
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Title: The solar corona as an active medium for magnetoacoustic waves
Authors: Kolotkov, D. Y.; Zavershinskii, D. I.; Nakariakov, V. M.
2021PPCF...63l4008K Altcode: 2021arXiv211102370K
The presence and interplay of continuous cooling and heating processes
maintaining the corona of the Sun at the observed one million K
temperature were recently understood to have crucial effects on the
dynamics and stability of magnetoacoustic waves. These essentially
compressive waves perturb the coronal thermal equilibrium, leading to
the phenomenon of a wave-induced thermal misbalance. Representing an
additional natural mechanism for the exchange of energy between the
plasma and the wave, thermal misbalance makes the corona an active
medium for magnetoacoustic waves, so that the wave can not only lose
but also gain energy from the coronal heating source (similarly to
burning gases, lasers and masers). We review recent achievements in
this newly emerging research field, focussing on the effects that
slow-mode magnetoacoustic waves experience as a back-reaction of
this perturbed coronal thermal equilibrium. The new effects include
enhanced frequency-dependent damping or amplification of slow waves,
and effective, not associated with the coronal plasma non-uniformity,
dispersion. We also discuss the possibility to probe the unknown coronal
heating function by observations of slow waves and linear theory of
thermal instabilities. The manifold of the new properties that slow
waves acquire from a thermodynamically active nature of the solar corona
indicate a clear need for accounting for the effects of combined coronal
heating/cooling processes not only for traditional problems of the
formation and evolution of prominences and coronal rain, but also for
an adequate modelling and interpretation of magnetohydrodynamic waves.
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Title: The high-energy Sun - probing the origins of particle
acceleration on our nearest star
Authors: Matthews, S. A.; Reid, H. A. S.; Baker, D.; Bloomfield, D. S.;
Browning, P. K.; Calcines, A.; Del Zanna, G.; Erdelyi, R.; Fletcher,
L.; Hannah, I. G.; Jeffrey, N.; Klein, L.; Krucker, S.; Kontar, E.;
Long, D. M.; MacKinnon, A.; Mann, G.; Mathioudakis, M.; Milligan,
R.; Nakariakov, V. M.; Pesce-Rollins, M.; Shih, A. Y.; Smith, D.;
Veronig, A.; Vilmer, N.
2021ExA...tmp..135M Altcode:
As a frequent and energetic particle accelerator, our Sun provides
us with an excellent astrophysical laboratory for understanding
the fundamental process of particle acceleration. The exploitation
of radiative diagnostics from electrons has shown that acceleration
operates on sub-second time scales in a complex magnetic environment,
where direct electric fields, wave turbulence, and shock waves all
must contribute, although precise details are severely lacking. Ions
were assumed to be accelerated in a similar manner to electrons, but
γ-ray imaging confirmed that emission sources are spatially separated
from X-ray sources, suggesting distinctly different acceleration
mechanisms. Current X-ray and γ-ray spectroscopy provides only a basic
understanding of accelerated particle spectra and the total energy
budgets are therefore poorly constrained. Additionally, the recent
detection of relativistic ion signatures lasting many hours, without
an electron counterpart, is an enigma. We propose a single platform
to directly measure the physical conditions present in the energy
release sites and the environment in which the particles propagate and
deposit their energy. To address this fundamental issue, we set out
a suite of dedicated instruments that will probe both electrons and
ions simultaneously to observe; high (seconds) temporal resolution
photon spectra (4 keV - 150 MeV) with simultaneous imaging (1 keV -
30 MeV), polarization measurements (5-1000 keV) and high spatial and
temporal resolution imaging spectroscopy in the UV/EUV/SXR (soft X-ray)
regimes. These instruments will observe the broad range of radiative
signatures produced in the solar atmosphere by accelerated particles.
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Title: Motion Magnification in Solar Imaging Data Sequences in the
Sub-pixel Regime
Authors: Zhong, Sihui; Duckenfield, Timothy J.; Nakariakov, Valery M.;
Anfinogentov, Sergey A.
2021SoPh..296..135Z Altcode:
The capability of the motion-magnification technique for the detection
of transverse oscillations, such as kink oscillations of solar coronal
loops observed with an imaging telescope, in the sub-pixel regime is
investigated. The technique is applied to artificial-image sequences
imitating harmonic transverse displacements of the loop, observed
in the optically thin regime. Motion magnification is found to work
well on the analysis of sub-pixel, ≥0.01 pixel oscillations, and it
is characterised by linear scaling between the magnified amplitude
and input amplitude. Oscillations of loops with transverse density
profiles of different steepness are considered. After magnification,
the original transverse profiles are preserved sufficiently well. The
motion-magnification performance is found to be robust in noisy data,
for coloured noise with spectral indices ranging from 0 to 3, and
additional Poisson noise with a signal-to-background-noise ratio down to
unity. Our findings confirm the reliability of the motion-magnification
technique for applications in magnetohydrodynamic seismology of the
solar corona.
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Title: Kink Oscillations of Coronal Loops
Authors: Nakariakov, V. M.; Anfinogentov, S. A.; Antolin, P.; Jain, R.;
Kolotkov, D. Y.; Kupriyanova, E. G.; Li, D.; Magyar, N.; Nisticò, G.;
Pascoe, D. J.; Srivastava, A. K.; Terradas, J.; Vasheghani Farahani,
S.; Verth, G.; Yuan, D.; Zimovets, I. V.
2021SSRv..217...73N Altcode: 2021arXiv210911220N
Kink oscillations of coronal loops, i.e., standing kink waves, is
one of the most studied dynamic phenomena in the solar corona. The
oscillations are excited by impulsive energy releases, such as low
coronal eruptions. Typical periods of the oscillations are from a
few to several minutes, and are found to increase linearly with the
increase in the major radius of the oscillating loops. It clearly
demonstrates that kink oscillations are natural modes of the loops,
and can be described as standing fast magnetoacoustic waves with the
wavelength determined by the length of the loop. Kink oscillations are
observed in two different regimes. In the rapidly decaying regime,
the apparent displacement amplitude reaches several minor radii of
the loop. The damping time which is about several oscillation periods
decreases with the increase in the oscillation amplitude, suggesting a
nonlinear nature of the damping. In the decayless regime, the amplitudes
are smaller than a minor radius, and the driver is still debated. The
review summarises major findings obtained during the last decade,
and covers both observational and theoretical results. Observational
results include creation and analysis of comprehensive catalogues of
the oscillation events, and detection of kink oscillations with imaging
and spectral instruments in the EUV and microwave bands. Theoretical
results include various approaches to modelling in terms of the
magnetohydrodynamic wave theory. Properties of kink oscillations are
found to depend on parameters of the oscillating loop, such as the
magnetic twist, stratification, steady flows, temperature variations
and so on, which make kink oscillations a natural probe of these
parameters by the method of magnetohydrodynamic seismology.
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Title: Fast magnetoacoustic wave trains: from tadpoles to boomerangs
Authors: Kolotkov, Dmitrii Y.; Nakariakov, Valery M.; Moss, Guy;
Shellard, Paul
2021MNRAS.505.3505K Altcode: 2021arXiv210513696K
Rapidly propagating fast magnetoacoustic wave trains guided by
field-aligned plasma non-uniformities are confidently observed in the
Sun's corona. Observations at large heights suggest that fast wave
trains can travel long distances from the excitation locations. We
study characteristic time signatures of fully developed, dispersive fast
magnetoacoustic wave trains in field-aligned zero-β plasma slabs in the
linear regime. Fast wave trains are excited by a spatially localized
impulsive driver and propagate along the waveguide as prescribed by
the waveguide-caused dispersion. In slabs with steeper transverse
density profiles, developed wave trains are shown to consist of three
distinct phases: a long-period quasi-periodic phase with the oscillation
period shortening with time, a multiperiodic (peloton) phase in which
distinctly different periods co-exist, and a short-lived periodic Airy
phase. The appearance of these phases is attributed to a non-monotonic
dependence of the fast wave group speed on the parallel wavenumber
due to the waveguide dispersion, and is shown to be different for
axisymmetric (sausage) and non-axisymmetric (kink) modes. In wavelet
analysis, this corresponds to the transition from the previously known
tadpole shape to a new boomerang shape of the wave train spectrum, with
two well-pronounced arms at shorter and longer periods. We describe
a specific previously published radio observation of a coronal fast
wave train, highly suggestive of a change of the wavelet spectrum from
a tadpole to a boomerang, broadly consistent with our modelling. The
applicability of these boomerang-shaped fast wave trains for probing the
transverse structuring of the waveguiding coronal plasma is discussed.
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Title: Diagnostics of plasma jets in the solar corona
Authors: Anfinogentov, Sergey; Kaltman, Tatyana; Stupishin, Alexey;
Nakariakov, Valery; Lukitcheva, Maria
2021STP.....7b...3A Altcode:
The paper discusses the diagnostics of plasma jets in the solar corona
with the use of data from modern space- and ground-based telescopes
observing the Sun in the extreme ultraviolet (EUV) and micro- wave
bands. We examine observational parameters of EUV and radio emission
in events associated with plasma jets, depending on the mechanism
of formation, initiation conditions, and evolution of the jets. The
opportunities provided by the study of plasma jets, which relies on
simultaneous observations in different bands, are highlighted. For
a number of jets, we have measured their primary parameters; and in
this paper we present preliminary results of statistical processing
of the data obtained. Microwave observations of several specific
events, made by ground-based instruments RATAN-600, SRH, and Nobeyama
Radioheliograph, are considered in detail. The diagnostic capabilities
of these instruments for studying coronal jets are shown. To analyze
the three-dimensional structure of the coronal magnetic field, we
have used SDO/HMI data, which allowed for the reconstruction of the
field in the lower corona. The information gained is compared with the
results of diagnostics of the magnetic field at the base of the corona
according to RATAN-600 data. The purpose of the methods developed is
to determine the physical mechanisms responsible for the generation,
collimation, and dynamics of plasma jets in the solar atmosphere.
---------------------------------------------------------
Title: Spectroscopic Detection of Alfvénic Waves in the Chromosphere
of Sunspot Regions
Authors: Chae, Jongchul; Cho, Kyuhyoun; Nakariakov, Valery M.; Cho,
Kyung-Suk; Kwon, Ryun-Young
2021ApJ...914L..16C Altcode:
Transverse magnetohydrodynamic waves often called Alfvénic (or
kink) waves have been often theoretically put forward to solve the
outstanding problems of the solar corona like coronal heating, solar
wind acceleration, and chemical abundance enhancement. Here we report
the first spectroscopic detection of Alfvénic waves around a sunspot at
chromospheric heights. By analyzing the spectra of the Hα line and Ca
II 854.2 nm line, we determined line-of-sight velocity and temperature
as functions of position and time. As a result, we identified transverse
magnetohydrodynamic waves pervading the superpenumbral fibrils. These
waves are characterized by the periods of 2.5 to 4.5 minutes, and
the propagation direction parallel to the fibrils, the supersonic
propagation speeds of 45 to 145 km s<SUP>-1</SUP>, and the close
association with umbral oscillations and running penumbral waves in
sunspots. Our results support the notion that the chromosphere around
sunspots abounds with Alfvénic waves excited by the mode conversion
of the upward-propagating slow magnetoacoustic waves.
---------------------------------------------------------
Title: Could Switchbacks Originate in the Lower Solar
Atmosphere? II. Propagation of Switchbacks in the Solar Corona
Authors: Magyar, Norbert; Utz, Dominik; Erdélyi, Robertus; Nakariakov,
Valery M.
2021ApJ...914....8M Altcode: 2021arXiv210410126M
The magnetic switchbacks observed recently by the Parker Solar Probe
have raised the question about their nature and origin. One of the
competing theories of their origin is the interchange reconnection
in the solar corona. In this scenario, switchbacks are generated at
the reconnection site between open and closed magnetic fields, and
are either advected by an upflow or propagate as waves into the solar
wind. In this paper we test the wave hypothesis, numerically modeling
the propagation of a switchback, modeled as an embedded Alfvén wave
packet of constant magnetic field magnitude, through the gravitationally
stratified solar corona with different degrees of background magnetic
field expansion. While switchbacks propagating in a uniform medium with
no gravity are relatively stable, as reported previously, we find that
gravitational stratification together with the expansion of the magnetic
field act in multiple ways to deform the switchbacks. These include WKB
effects, which depend on the degree of magnetic field expansion, and
also finite-amplitude effects, such as the symmetry breaking between
nonlinear advection and the Lorentz force. In a straight or radially
expanding magnetic field the propagating switchbacks unfold into waves
that cause minimal magnetic field deflections, while a super-radially
expanding magnetic field aids in maintaining strong deflections. Other
important effects are the mass uplift the propagating switchbacks
induce and the reconnection and drainage of plasmoids contained within
the switchbacks. In the Appendix, we examine a series of setups with
different switchback configurations and parameters, which broaden the
scope of our study.
---------------------------------------------------------
Title: Probing the solar corona magnetic field with sungrazing comets
Authors: Nisticò, Giuseppe; Nakariakov, Valery M.; Duckenfield,
Timothy; Druckmüller, Miloslav; Zimbardo, Gaetano
2021EGUGA..2314653N Altcode:
Space telescopes of the SoHO, STEREO and SDO missions have occasionally
acquired observations of comets, providing an interesting opportunity
to investigate the structure and dynamics of the heliospheric
plasma. Cometary plasma tails exhibit a wave-like motion, which is
believed to be a response to the physical conditions of the local
interplanetary medium. Furthermore, sungrazing comets diving in the
solar atmosphere provide us with an unprecedented way to diagnose the
coronal plasma at distances which are unaccessible from the current
spacecraft. Here, we present observations of Comet Lovejoy C/2011 W3
from SDO/AIA, which was seen to cross the EUV solar corona in December
2011. The cometary ions produced by the sublimation of the comet
nucleus were channelled along the magnetic field lines forming some
filamented structures. Such structures appear to show small amplitude
kink oscillations, which are used to determine the magnitude of the
coronal magnetic field by coronal seismology.
---------------------------------------------------------
Title: Kinematic Oscillations of Post-CME Blobs Detected by K-Cor
on 2017 September 10
Authors: Lee, Jae-Ok; Cho, Kyung-Suk; Nakariakov, Valery M.; Lee,
Harim; Kim, Rok-Soon; Jang, Soojeong; Yang, Heesu; Kim, Sujin; Kim,
Yeon-Han
2021JKAS...54...61L Altcode:
We investigate 20 post-coronal mass ejection (CME) blobs formed in
the post-CME current sheet (CS) that were observed by K-Cor on 2017
September 10. By visual inspection of the trajectories and projected
speed variations of each blob, we find that all blobs except one
show irregular "zigzag" trajectories resembling transverse oscillatory
motions along the CS, and have at least one oscillatory pattern in their
instantaneous radial speeds. Their oscillation periods are ranging from
30 to 91 s and their speed amplitudes from 128 to 902 km/s. Among 19
blobs, 10 blobs have experienced at least two cycles of radial speed
oscillations with different speed amplitudes and periods, while 9 blobs
undergo one oscillation cycle. To examine whether or not the apparent
speed oscillations can be explained by vortex shedding, we estimate
the quantitative parameter of vortex shedding, the Strouhal number,
by using the observed lateral widths, linear speeds, and oscillation
periods of the blobs. We then compare our estimates with theoretical
and experimental results from MHD simulations and fluid dynamics
experiments. We find that the observed Strouhal numbers range from 0.2
to 2.1, consistent with those (0.15-3.0) from fluid dynamics experiments
of bluff spheres, while they are higher than those (0.15-0.25) from MHD
simulations of cylindrical shapes. We thus find that blobs formed in
a post-CME CS undergo kinematic oscillations caused by fluid dynamic
vortex shedding. The vortex shedding is driven by the interaction
of the outward-moving blob having a bluff spherical shape with the
background plasma in the post-CME CS.
---------------------------------------------------------
Title: Could Switchbacks Originate in the Lower Solar
Atmosphere? I. Formation Mechanisms of Switchbacks
Authors: Magyar, Norbert; Utz, Dominik; Erdélyi, Robertus; Nakariakov,
Valery M.
2021ApJ...911...75M Altcode: 2021arXiv210303726M
The recent rediscovery of magnetic field switchbacks or deflections
embedded in the solar wind flow by the Parker Solar Probe mission lead
to a huge interest in the modeling of the formation mechanisms and
origin of these switchbacks. Several scenarios for their generation were
put forth, ranging from lower solar atmospheric origins by reconnection,
to being a manifestation of turbulence in the solar wind, and so
on. Here we study some potential formation mechanisms of magnetic
switchbacks in the lower solar atmosphere, using three-dimensional
magnetohydrodynamic (MHD) numerical simulations. The model is that of an
intense flux tube in an open magnetic field region, aiming to represent
a magnetic bright point opening up to an open coronal magnetic field
structure, e.g., a coronal hole. The model is driven with different
plasma flows in the photosphere, such as a fast up-shooting jet,
as well as shearing flows generated by vortex motions or torsional
oscillations. In all scenarios considered, we witness the formation
of magnetic switchbacks in regions corresponding to chromospheric
heights. Therefore, photospheric plasma flows around the foot-points
of intense flux tubes appear to be suitable drivers for the formation
of magnetic switchbacks in the lower solar atmosphere. Nevertheless,
these switchbacks do not appear to be able to enter the coronal heights
of the simulation in the present model. In conclusion, based on the
presented simulations, switchbacks measured in the solar wind are
unlikely to originate from photospheric or chromospheric dynamics.
---------------------------------------------------------
Title: On the Nature of Propagating Intensity Disturbances in Polar
Plumes during the 2017 Total Solar Eclipse
Authors: Cho, Kyung-Suk; Cho, Il-Hyun; Madjarska, Maria S.; Nakariakov,
Valery M.; Yang, Heesu; Choi, Seonghwan; Lim, Eun-Kyung; Lee,
Kyung-Sun; Seough, Jung-Jun; Lee, Jaeok; Kim, Yeon-Han
2021ApJ...909..202C Altcode: 2021arXiv210202085C
The propagating intensity disturbances (PIDs) in plumes are still
poorly understood, and their identity (magnetoacoustic waves or flows)
remains an open question. We investigate PIDs in five plumes located
in the northern polar coronal hole observed during the 2017 total solar
eclipse. Three plumes are associated with coronal bright points, jets,
and macrospicules at their base (active plumes), and the other two
plumes are not (quiet plumes). The electron temperature at the base of
the plumes is obtained from the filter ratio of images taken with the
X-ray Telescope on board Hinode and the passband ratio around 400 nm
from an eclipse instrument, the Diagnostic Coronagraph Experiment. The
phase speed (v<SUB>r</SUB>), frequency (ω), and wavenumber (k) of the
PIDs in the plumes are obtained by applying a Fourier transformation to
the spacetime (r - t plane) plots in images taken with the Atmospheric
Imaging Assembly (AIA) in three different wavelength channels (171,
193, and 211 Å). We found that the PIDs in the higher-temperature AIA
channels, 193 and 211 Å, are faster than that of the cooler AIA 171
Å channel. This tendency is more significant for the active plumes
than the quiet ones. The observed speed ratio (∼1.3) between the
AIA 171 and 193 Å channels is similar to the theoretical value (1.25)
of a slow magnetoacoustic wave. Our results support the idea that PIDs
in plumes represent a superposition of slow magnetoacoustic waves and
plasma outflows that consist of dense cool flows and hot coronal jets.
---------------------------------------------------------
Title: The effect of the magnetic field on the damping of slow waves
in the solar corona
Authors: Duckenfield, T. J.; Kolotkov, D. Y.; Nakariakov, V. M.
2021A&A...646A.155D Altcode: 2020arXiv201110437D
Context. Slow magnetoacoustic waves are routinely observed in
astrophysical plasma systems such as the solar corona, and they are
usually seen to damp rapidly. As a slow wave propagates through a
plasma, it modifies the equilibrium quantities of density, temperature,
and the magnetic field. In the corona and other plasma systems, the
thermal equilibrium is comprised of a balance between continuous
heating and cooling processes, the magnitudes of which vary with
density, temperature and the magnetic field. Thus the wave may induce
a misbalance between these competing processes. Its back reaction on
the wave has been shown to lead to dispersion, and amplification or
damping, of the wave. <BR /> Aims: This effect of heating and cooling
misbalance has previously been studied in the infinite magnetic field
approximation in a plasma whose thermal equilibrium is comprised of
optically thin radiative losses and field-aligned thermal conduction,
balanced by an (unspecified) heating process. In this work we extend
this analysis by considering a non-zero β plasma. The importance
of the effect of the magnetic field in the rapid damping of slow
waves in the solar corona is evaluated and compared to the effects of
thermal conduction. <BR /> Methods: A linear perturbation under the
thin flux tube approximation is considered, and a dispersion relation
describing the slow magnetoacoustic modes is found. The dispersion
relation's limits of strong non-adiabaticity and weak non-adiabaticity
are studied. The characteristic timescales were calculated for plasma
systems with a range of typical coronal densities, temperatures, and
magnetic field strengths. <BR /> Results: The number of timescales
characterising the effect of the misbalance is found to remain at
two, as with the infinite magnetic field case. In the non-zero β
case, these two timescales correspond to the partial derivatives of
the combined heating and cooling function with respect to constant
gas pressure and with respect to constant magnetic pressure. The
predicted damping times of slow waves from thermal misbalance in the
solar corona are found to be of the order of 10-100 min, coinciding
with the wave periods and damping times observed. Moreover, the slow
wave damping by thermal misbalance is found to be comparable to the
damping by field-aligned thermal conduction. The change in damping with
plasma-β is complex and depends on the coronal heating function's
dependence on the magnetic field in particular. Nonetheless, we show
that in the infinite field limit, the wave dynamics is insensitive to
the dependence of the heating function on the magnetic field, and this
approximation is found to be valid in the corona so long as the magnetic
field strength is greater than approximately 10 G for quiescent loops
and plumes, and 100 G for hot and dense loops. <BR /> Conclusions:
A thermal misbalance may damp slow magnetoacoustic waves rapidly in
much of the corona, and its inclusion in our understanding of slow
mode damping may resolve discrepancies between the observations and
theory relying on compressive viscosity and thermal conduction alone.
---------------------------------------------------------
Title: Three-dimensional Simulations of the Inhomogeneous Low
Solar Wind
Authors: Magyar, N.; Nakariakov, V. M.
2021ApJ...907...55M Altcode: 2020arXiv201200811M
In the near future, the Parker Solar Probe will put theories about
the dynamics and nature of the transition between the solar corona and
the solar wind to stringent tests. The most popular mechanism aimed to
explain the dynamics of the nascent solar wind, including its heating
and acceleration, is magnetohydrodynamic (MHD) turbulence. Most of the
previous models focused on nonlinear cascade induced by interactions
of outgoing Alfvén waves and their reflections, ignoring effects that
might be related to perpendicular structuring of the solar coronal
plasma, despite overwhelming evidence for it. In this paper, for the
first time, we analyze through 3D MHD numerical simulations the dynamics
of the perpendicularly structured solar corona and solar wind, from the
low corona to 15 R<SUB>⊙</SUB>. We find that background structuring
has a strong effect on the evolution of MHD turbulence, on much faster
timescales than in the perpendicularly homogeneous case. On timescales
shorter than nonlinear times, linear effects related to phase mixing
result in a 1/f perpendicular energy spectrum. As the turbulent cascade
develops, we observe a perpendicular (parallel) energy spectrum with a
power-law index of -3/2 or -5/3 (-2), a steeper perpendicular magnetic
field than velocity spectrum, and a strong build-up of negative
residual energy. We conclude that the turbulence is most probably
generated by the self-cascade of the driven transverse kink waves,
referred to previously as "uniturbulence," which might represent the
dominant nonlinear energy cascade channel in the pristine solar wind.
---------------------------------------------------------
Title: Solar Bayesian Analysis Toolkit—A New Markov Chain Monte
Carlo IDL Code for Bayesian Parameter Inference
Authors: Anfinogentov, Sergey A.; Nakariakov, Valery M.; Pascoe,
David J.; Goddard, Christopher R.
2021ApJS..252...11A Altcode: 2020arXiv200505365A
We present the Solar Bayesian Analysis Toolkit (SoBAT), which is a
new easy to use tool for Bayesian analysis of observational data,
including parameter inference and model comparison. SoBAT is aimed
(but not limited) to be used for the analysis of solar observational
data. We describe a new IDL code designed to facilitate the comparison
of a user-supplied model with data. Bayesian inference allows prior
information to be taken into account. The use of Markov Chain Monte
Carlo sampling allows efficient exploration of large parameter spaces
and provides reliable estimation of model parameters and their
uncertainties. The Bayesian evidence for different models can be
used for quantitative comparison. The code is tested to demonstrate
its ability to accurately recover a variety of parameter probability
distributions. Its application to practical problems is demonstrated
using studies of the structure and oscillation of coronal loops.
---------------------------------------------------------
Title: Effect of Higher Radial Harmonics in Sausage Oscillations in
Coronal Loops
Authors: Lim, D.; Nakariakov, V. M.; Moon, Y. J.; Yu, D. J.; Cho, I. H.
2020AGUFMSH0290006L Altcode:
Impulsively excited sausage oscillations of a plasma cylinder with a
smooth radial profile of Alfvén speed are analyzed with a numerical
solution of the initial-value problem for a partial differential
equation of the Klein-Gordon type, describing linear magnetoacoustic
oscillations with a fixed axial wavelength and an azimuthal mode
number. The range of analyzed ratios of Alfvén speeds outside and
inside the cylinder is from 2 to 10. Both trapped and leaky regimes
of the oscillations are considered. It is shown that even in the
long-wavelength limit, i.e., for axial wavenumbers much smaller than
the cutoff values, damping times of higher radial sausage harmonics
could be significantly greater than the oscillation periods, i.e.,
several oscillation cycles could be present in the signal. The quality
factors decrease with decreasing ratios of Alfvén speeds outside and
inside the cylinder. Oscillation periods of the second and third radial
harmonics remain practically independent of the axial wavelength even
when the wavelength is shorter than the radius of the cylinder. The
ratios of oscillation periods of fundamental and higher radial and axial
harmonics are found to be significantly different, up to a factor of
two in the long-wavelength limit. It is concluded that higher radial
harmonics could be responsible for the departure of observed sausage
oscillation signals from a harmonic shape, especially during the first
several cycles of the oscillation. Even in the absence of spatially
resolved data, higher axial and radial harmonics can be distinguished
from each other by the period ratios. <P />This work was supported by
Institute for Information & communications Technology Promotion
(IITP) grant funded by the Korea government (2018-0-01422, Study on
analysis and prediction technique of solar flares).
---------------------------------------------------------
Title: Seismological constraints on the solar coronal heating function
Authors: Kolotkov, D. Y.; Duckenfield, T. J.; Nakariakov, V. M.
2020A&A...644A..33K Altcode: 2020arXiv201003364K
<BR /> Aims: The hot solar corona exists because of the balance
between radiative and conductive cooling and some counteracting
heating mechanism that remains one of the major puzzles in solar
physics. <BR /> Methods: The coronal thermal equilibrium is perturbed
by magnetoacoustic waves, which are abundantly present in the corona,
causing a misbalance between the heating and cooling rates. As a
consequence of this misbalance, the wave experiences a back-reaction,
either losing or gaining energy from the energy supply that heats the
plasma, at timescales comparable to the wave period. <BR /> Results:
In particular, the plasma can be subject to wave-induced instability
or over-stability, depending on the specific choice of the coronal
heating function. In the unstable case, the coronal thermal equilibrium
would be violently destroyed, which does not allow for the existence
of long-lived plasma structures typical for the corona. Based on this,
we constrained the coronal heating function using observations of slow
magnetoacoustic waves in various coronal plasma structures.
---------------------------------------------------------
Title: Accelerating and Supersonic Density Disturbances in Solar
Polar Plumes
Authors: Cho, I. H.; Moon, Y. J.; Nakariakov, V. M.; Lee, J. Y.; Yu,
D. J.; Cho, K. S. F.; Yurchyshyn, V.; Lee, H.
2020AGUFMSH0290003C Altcode:
Propagating intensity disturbances in solar coronal holes are often
considered as wave propagations or mass flows. By applying the
differential emission measure technique for the extreme ultraviolet
images taken by the Atmospheric Imaging Assembly on board the Solar
Dynamics Observatory, we analyze the propagation speed of density
disturbances of plume structure in an off-limb coronal hole for a
given temperature. We construct the map of cross-correlation between
density profile for a given height and the profile at the height
of 50 Mm. The evolution of density disturbances is well fitted by
the second-order polynomial. The acceleration is calculated to be
36 m s<SUP>-2</SUP>. The initial speed is 134 km s<SUP>-1</SUP>
which is comparable with the sound speed given by the DEM-weighted
temperature. Hence, density disturbances are accelerating and supersonic
at around the base of the solar corona. The excess speed relative
to the sound speed is ∼ 30 km s<SUP>-1</SUP> at the height of 1.23
solar radii, which is consistent with the Doppler speeds and Doppler
dimming speeds observed by different instruments. The extrapolated
sonic distance of the excess speed is ∼ 2.16 solar radii which is
consistent with those of solar winds. The lower limit of the mass
flux corresponds to 7% of the global solar wind. Hence, we interpret
that the observed density disturbances are slow magnetoacoustic waves
propagating in subsonic and accelerating solar winds.
---------------------------------------------------------
Title: The Solar Orbiter magnetometer
Authors: Horbury, T. S.; O'Brien, H.; Carrasco Blazquez, I.; Bendyk,
M.; Brown, P.; Hudson, R.; Evans, V.; Oddy, T. M.; Carr, C. M.; Beek,
T. J.; Cupido, E.; Bhattacharya, S.; Dominguez, J. -A.; Matthews, L.;
Myklebust, V. R.; Whiteside, B.; Bale, S. D.; Baumjohann, W.; Burgess,
D.; Carbone, V.; Cargill, P.; Eastwood, J.; Erdös, G.; Fletcher,
L.; Forsyth, R.; Giacalone, J.; Glassmeier, K. -H.; Goldstein, M. L.;
Hoeksema, T.; Lockwood, M.; Magnes, W.; Maksimovic, M.; Marsch, E.;
Matthaeus, W. H.; Murphy, N.; Nakariakov, V. M.; Owen, C. J.; Owens,
M.; Rodriguez-Pacheco, J.; Richter, I.; Riley, P.; Russell, C. T.;
Schwartz, S.; Vainio, R.; Velli, M.; Vennerstrom, S.; Walsh, R.;
Wimmer-Schweingruber, R. F.; Zank, G.; Müller, D.; Zouganelis, I.;
Walsh, A. P.
2020A&A...642A...9H Altcode:
The magnetometer instrument on the Solar Orbiter mission is designed
to measure the magnetic field local to the spacecraft continuously
for the entire mission duration. The need to characterise not only
the background magnetic field but also its variations on scales from
far above to well below the proton gyroscale result in challenging
requirements on stability, precision, and noise, as well as magnetic
and operational limitations on both the spacecraft and other
instruments. The challenging vibration and thermal environment has
led to significant development of the mechanical sensor design. The
overall instrument design, performance, data products, and operational
strategy are described.
---------------------------------------------------------
Title: Accelerating and Supersonic Density Fluctuations in Coronal
Hole Plumes: Signature of Nascent Solar Winds
Authors: Cho, Il-Hyun; Nakariakov, Valery M.; Moon, Yong-Jae; Lee,
Jin-Yi; Yu, Dae Jung; Cho, Kyung-Suk; Yurchyshyn, Vasyl; Lee, Harim
2020ApJ...900L..19C Altcode: 2020arXiv200807848C
Slow magnetoacoustic waves in a static background provide a
seismological tool to probe the solar atmosphere in the analytic
frame. By analyzing the spatiotemporal variation of the electron
number density of plume structure in coronal holes above the limb for
a given temperature, we find that the density perturbations accelerate
with supersonic speeds in the distance range from 1.02 to 1.23 solar
radii. We interpret them as slow magnetoacoustic waves propagating at
about the sound speed with accelerating subsonic flows. The average
sonic height of the subsonic flows is calculated to be 1.27 solar
radii. The mass flux of the subsonic flows is estimated to be 44.1%
relative to the global solar wind. Hence, the subsonic flow is likely
to be the nascent solar wind. In other words, the evolution of the
nascent solar wind in plumes at the low corona is quantified for the
first time from imaging observations. Based on the interpretation,
propagating density perturbations present in plumes could be used as
a seismological probe of the gradually accelerating solar wind.
---------------------------------------------------------
Title: Magnetohydrodynamic Waves in the Solar Corona
Authors: Nakariakov, Valery M.; Kolotkov, Dmitrii Y.
2020ARA&A..58..441N Altcode:
The corona of the Sun is a unique environment in which
magnetohydrodynamic (MHD) waves, one of the fundamental processes of
plasma astrophysics, are open to a direct study. There is striking
progress in both observational and theoretical research of MHD wave
processes in the corona, with the main recent achievements summarized
as follows: Both periods and wavelengths of the principal MHD modes
of coronal plasma structures, such as kink, slow and sausage modes,
are confidently resolved. Scalings of various parameters of detected
waves and waveguiding plasma structures allow for the validation of
theoretical models. In particular, kink oscillation period scales
linearly with the length of the oscillating coronal loop, clearly
indicating that they are eigenmodes of the loop. Damping of decaying
kink and standing slow oscillations depends on the oscillation
amplitudes, demonstrating the importance of nonlinear damping. The
dominant excitation mechanism for decaying kink oscillations is
associated with magnetized plasma eruptions. Propagating slow waves are
caused by the leakage of chromospheric oscillations. Fast wave trains
could be formed by waveguide dispersion. The knowledge gained in the
study of coronal MHD waves provides ground for seismological probing
of coronal plasma parameters, such as the Alfvén speed, the magnetic
field and its topology, stratification, temperature, fine structuring,
polytropic index, and transport coefficients.
---------------------------------------------------------
Title: Excitation of Negative Energy Surface Magnetohydrodynamic
Waves in an Incompressible Cylindrical Plasma
Authors: Yu, D. J.; Nakariakov, V. M.
2020ApJ...896...21Y Altcode: 2020arXiv200413552Y
Negative energy wave (NEW) phenomena may appear in shear flows in the
presence of a wave decay mechanism and external energy supply. We
study the appearance of negative energy surface waves in a plasma
cylinder in the incompressible limit. The cylinder is surrounded by an
axial magnetic field and by a plasma of different density. Considering
flow inside and viscosity outside the flux tube, we derive dispersion
relations and obtain analytical solutions for the phase speed and growth
rate (increment) of the waves. It is found that the critical speed
shear for the occurrence of the dissipative instability associated
with NEWs and the threshold of Kelvin-Helmholtz instability (KHI)
depend on the axial wavelength. The critical shear for the appearance
of sausage NEW is lowest for the longest axial wavelengths, while
for kink waves the minimum value of the critical shear is reached for
the axial wavelength comparable to the diameter of the cylinder. The
range between the critical speed of the dissipative instability and
the KHI threshold is shown to depend on the difference of the Alfvén
speeds inside and outside of the cylinder. For all axial wavenumbers,
NEW appears for the shear flow speeds lower than the KHI threshold. It
is easier to excite NEW in an underdense cylinder than in an overdense
one. The negative energy surface waves can be effectively generated
for an azimuthal number m = 0 with a large axial wavenumber and for
higher modes (m > 0) with a small axial wavenumber.
---------------------------------------------------------
Title: Standing Kink Waves in Sigmoid Solar Coronal Loops:
Implications for Coronal Seismology
Authors: Magyar, N.; Nakariakov, V. M.
2020ApJ...894L..23M Altcode: 2020arXiv200414083M
Using full three-dimensional magnetohydrodynamic numerical simulations,
we study the effects of magnetic field sigmoidity or helicity on
the properties of the fundamental kink oscillation of solar coronal
loops. Our model consists of a single denser coronal loop, embedded
in a plasma with dipolar force-free magnetic field with a constant
α-parameter. For the loop with no sigmoidity, we find that the
numerically determined oscillation period of the fundamental kink
mode matches the theoretical period calculated using WKB theory. In
contrast, with increasing sigmoidity of the loop, the actual period is
increasingly smaller than the one estimated by WKB theory. Translated
through coronal seismology, increasing sigmoidity results in magnetic
field estimates that are increasingly shifting toward higher
values, and even surpassing the average value for the highest α
value considered. Nevertheless, the estimated range of the coronal
magnetic field value lies within the minimal/maximal limits, proving
the robustness coronal seismology. We propose that the discrepancy in
the estimations of the absolute value of the force-free magnetic field
could be exploited seismologically to determine the free energy of
coronal loops, if averages of the internal magnetic field and density
can be reliably estimated by other methods.
---------------------------------------------------------
Title: Quasi-Periodic Pulsations in Solar and Stellar Flares. Review
Authors: Kupriyanova, Elena; Kolotkov, Dmitrii; Nakariakov, Valery;
Kaufman, Anastasiia
2020STP.....6a...3K Altcode: 2020SZF.....6a...3K
This paper provides an overview of the state-of-the-art studies of
oscillatory processes in solar and stellar flares, based on modern
observational data from ground-based and space-borne instruments
with high temporal, spatial, and spectral resolution in different
bands of the electromagnetic spectrum. We examine the mecha-nisms
that generate flare emission and its quasi-periodic modulation. We
discuss similarities and differences between solar and stellar flares,
and address associated problems of superflares on the Sun and space
weather. Quasi-periodic pulsations (QPPs) of flare emission are shown to
be an effective tool for diagnosing both the flare processes themselves
and the parameters of flaring plasmas and accelerated particles. We
consider types of QPPs, their statistical properties, and methods
of analysis, taking into account the non-stationarity of the QPP'
parameters. We review the proposed mechanisms of QPPs and summarize
open questions.
---------------------------------------------------------
Title: Higher Radial Harmonics of Sausage Oscillations in Coronal
Loops
Authors: Lim, Daye; Nakariakov, Valery M.; Yu, Dae Jung; Cho, Il-Hyun;
Moon, Yong-Jae
2020ApJ...893...62L Altcode:
Impulsively excited sausage oscillations of a plasma cylinder with a
smooth radial profile of Alfvén speed are analyzed with a numerical
solution of the initial-value problem for a partial differential
equation of the Klein-Gordon type, describing linear magnetoacoustic
oscillations with a fixed axial wavelength and an azimuthal mode
number. The range of analyzed ratios of Alfvén speeds outside and
inside the cylinder is from 2 to 10. Both trapped and leaky regimes
of the oscillations are considered. It is shown that even in the
long-wavelength limit, I.e., for axial wavenumbers much smaller than
the cutoff values, damping times of higher radial sausage harmonics
could be significantly greater than the oscillation periods, I.e.,
several oscillation cycles could be present in the signal. The quality
factors decrease with decfreasing ratios of Alfvén speeds outside and
inside the cylinder. Oscillation periods of the second and third radial
harmonics remain practically independent of the axial wavelength even
when the wavelength is shorter than the radius of the cylinder. The
ratios of oscillation periods of fundamental and higher radial and axial
harmonics are found to be significantly different, up to a factor of
two in the long-wavelength limit. It is concluded that higher radial
harmonics could be responsible for the departure of observed sausage
oscillation signals from a harmonic shape, especially during the first
several cycles of the oscillation. Even in the absence of spatially
resolved data, higher axial and radial harmonics can be distinguished
from each other by the period ratios.
---------------------------------------------------------
Title: Magnetic Connectivity between the Light Bridge and Penumbra
in a Sunspot
Authors: Feng, Song; Miao, Yuhu; Yuan, Ding; Qu, Zhongquan; Nakariakov,
Valery M.
2020ApJ...893L...2F Altcode: 2020arXiv200303976F
A light bridge is a prominent structure commonly observed within a
sunspot. Its presence usually triggers a wealth of dynamics in a sunspot
and has a lasting impact on sunspot evolution. However, the fundamental
structure of light bridges is still not well understood. In this study,
we used the high-resolution spectropolarimetry data obtained by the
Solar Optical Telescope on board the Hinode satellite to analyze the
magnetic and thermal structure of a light bridge at AR 12838. We
also combined the high-cadence $1700\,\mathring{\rm A} $ channel
data provided by the Atmospheric Imaging Assembly on board the Solar
Dynamics Observatory to study the dynamics on this bridge. We found
a pair of blue and red Doppler shift patches at two ends of this
bridge; this pattern appears to be the convective motion directed
by the horizontal component of the magnetic field aligned with the
spine of the bridge. Paired upward and downward motions imply that
the light bridge could have a two-legged or undulating magnetic
field. Significant 4 minute oscillations in the emission intensity
of the $1700\,\mathring{\rm A} $ bandpass were detected at two ends,
which overlapped the paired blue- and redshift patches. The oscillatory
signals at the light bridge and the penumbra were highly correlated with
each other. Although they are separated in space at the photosphere,
the periodicity seems to have a common origin from underneath the
sunspot. Therefore, we infer that the light bridge and penumbra could
share a common magnetic source and become fragmented at the photosphere
by magnetoconvection.
---------------------------------------------------------
Title: Editorial: Magnetohydrodynamic Waves in the Solar Atmosphere:
Heating and Seismology
Authors: Van Doorsselaere, Tom; Nakariakov, Valery M.; Li, Bo;
Antolin, Patrick
2020FrASS...6...79V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Excitation of decay-less transverse oscillations of coronal
loops by random motions
Authors: Afanasyev, A. N.; Van Doorsselaere, T.; Nakariakov, V. M.
2020A&A...633L...8A Altcode: 2019arXiv191207980A
Context. The relatively large-amplitude decaying regime of transverse
oscillations of coronal loops has been known for two decades and
has been interpreted in terms of magnetohydrodynamic kink modes of
cylindrical plasma waveguides. In this regime oscillations decay
in several cycles. Recent observational analysis has revealed
so-called decay-less, small-amplitude oscillations, in which a
multi-harmonic structure has been detected. Several models have been
proposed to explain these oscillations. In particular, decay-less
oscillations have been described in terms of standing kink waves
driven with continuous mono-periodic motions of loop footpoints,
in terms of a simple oscillator model of forced oscillations due
to harmonic external force, and as a self-oscillatory process due
to the interaction of a loop with quasi-steady flows. However,
an alternative mechanism is needed to explain the simultaneous
excitation of several longitudinal harmonics of the oscillation. <BR
/> Aims: We study the mechanism of random excitation of decay-less
transverse oscillations of coronal loops. <BR /> Methods: With a
spatially one-dimensional and time-dependent analytical model taking
into account effects of the wave damping and kink speed variation
along the loop, we considered transverse loop oscillations driven by
random motions of footpoints. The footpoint motions were modelled by
broad-band coloured noise. <BR /> Results: We found the excitation of
loop eigenmodes and analysed their frequency ratios as well as the
spatial structure of the oscillations along the loop. The obtained
results successfully reproduce the observed properties of decay-less
oscillations. In particular, excitation of eigenmodes of a loop as
a resonator can explain the observed quasi-monochromatic nature of
decay-less oscillations and the generation of multiple harmonics
detected recently. <BR /> Conclusions: We propose a mechanism that
can interpret decay-less transverse oscillations of coronal loops in
terms of kink waves randomly driven at the loop footpoints.
---------------------------------------------------------
Title: Quasi-periodic Pulsations of Gamma-Ray Emissions from a Solar
Flare on 2017 September 6
Authors: Li, D.; Kolotkov, D. Y.; Nakariakov, V. M.; Lu, L.; Ning,
Z. J.
2020ApJ...888...53L Altcode: 2019arXiv191201145L
We investigate quasi-periodic pulsations (QPPs) of high-energy
nonthermal emissions from an X9.3 flare (SOL2017-Sep-06T11:53),
the most powerful flare since the beginning of solar cycle 24. The
QPPs are identified as a series of regular and repeating peaks in the
light curves in the γ- and hard X-ray (HXR) channels recorded by the
Konus-Wind, as well as the radio and microwave fluxes measured by the
CALLISTO radio spectrograph during the impulsive phase. The periods
are determined from the global wavelet and Fourier power spectra
as 24-30 s in the HXR and microwave channels, which are associated
with nonthermal electrons, and ∼20 s in the γ-ray band related to
nonthermal ions. Both nonthermal electrons and ions may be accelerated
by repetitive magnetic reconnection during the impulsive phase. However,
we could not rule out other mechanisms, such as magnetohydrodynamic
oscillation in a sausage mode. The QPP detected in this study is useful
for understanding the particle acceleration and dynamic process in
solar flares and also bridging the gap between stellar and solar flares
because the energy realm of the X9.3 solar flare is almost comparable
to a typical stellar flare.
---------------------------------------------------------
Title: Observational signatures of the third harmonic in a decaying
kink oscillation of a coronal loop
Authors: Duckenfield, T. J.; Goddard, C. R.; Pascoe, D. J.; Nakariakov,
V. M.
2019A&A...632A..64D Altcode:
<BR /> Aims: An observation of a coronal loop standing kink mode
is analysed to search for higher harmonics, aiming to reveal the
relation between different harmonics' quality factors. <BR /> Methods:
Observations of a coronal loop were taken by the Atmospheric Imaging
Assembly (AIA) of the Solar Dynamics Observatory (SDO). The loop's
axis was tracked at many spatial positions along the loop to generate
time series data. <BR /> Results: The distribution of spectral power
of the oscillatory transverse displacements throughout the loop
reveals the presence of two harmonics, a fundamental at a period of
∼8 min and its third harmonic at ∼2.6 min. The node of the third
harmonic is seen at approximately a third of the way along the length
of the loop, and cross correlations between the oscillatory motion
on opposing sides of the node show a change in phase behaviour. The
ratio of periods P<SUB>1</SUB>/3P<SUB>3</SUB> was found to be ∼0.87,
indicating a non-uniform distribution of kink speed through the
loop. The quality factor for the fundamental mode of oscillation
was measured to be ∼3.4. The quality factor of the third harmonic
was measured for each spatial location and, where data was reliable,
yielded a value of ∼3.6. For all locations, the quality factors for
the two harmonics were found to agree within error as expected from
1d resonant absorption theory. This is the first time a measurement
of the signal quality for a higher harmonic of a kink oscillation has
been reported with spatially resolved data.
---------------------------------------------------------
Title: A Blueprint of State-of-the-art Techniques for Detecting
Quasi-periodic Pulsations in Solar and Stellar Flares
Authors: Broomhall, Anne-Marie; Davenport, James R. A.; Hayes, Laura
A.; Inglis, Andrew R.; Kolotkov, Dmitrii Y.; McLaughlin, James A.;
Mehta, Tishtrya; Nakariakov, Valery M.; Notsu, Yuta; Pascoe, David J.;
Pugh, Chloe E.; Van Doorsselaere, Tom
2019ApJS..244...44B Altcode: 2019arXiv191008458B
Quasi-periodic pulsations (QPPs) appear to be a common feature observed
in the light curves of both solar and stellar flares. However, their
quasi-periodic nature, along with the fact that they can be small
in amplitude and short-lived, makes QPPs difficult to unequivocally
detect. In this paper, we test the strengths and limitations of
state-of-the-art methods for detecting QPPs using a series of
hare-and-hounds exercises. The hare simulated a set of flares,
both with and without QPPs of a variety of forms, while the hounds
attempted to detect QPPs in blind tests. We use the results of these
exercises to create a blueprint for anyone who wishes to detect QPPs
in real solar and stellar data. We present eight clear recommendations
to be kept in mind for future QPP detections, with the plethora of
solar and stellar flare data from new and future satellites. These
recommendations address the key pitfalls in QPP detection, including
detrending, trimming data, accounting for colored noise, detecting
stationary-period QPPs, detecting QPPs with nonstationary periods,
and ensuring that detections are robust and false detections are
minimized. We find that QPPs can be detected reliably and robustly
by a variety of methods, which are clearly identified and described,
if the appropriate care and due diligence are taken.
---------------------------------------------------------
Title: A New Type of Jet in a Polar Limb of the Solar Coronal Hole
Authors: Cho, Il-Hyun; Moon, Yong-Jae; Cho, Kyung-Suk; Nakariakov,
Valery M.; Lee, Jin-Yi; Kim, Yeon-Han
2019ApJ...884L..38C Altcode: 2019arXiv191009737C
A new type of chromospheric jet in a polar limb of a coronal hole is
discovered in the Ca II filtergram of the Solar Optical Telescope on
board the Hinode. We identify 30 jets in a filtered Ca II movie with
a duration of 53 minutes. The average speed at their maximum heights
is found to be 132 ± 44 km s<SUP>-1</SUP> ranging from 57 to 264 km
s<SUP>-1</SUP> along the propagation direction. The average lifetime
is 20 ± 6 ranging from 11 to 36 s. The speed and lifetime of the
jets are located at end-tails of those parameters determined for type
II spicules, hence implying a new type of jets. To confirm whether
these jets are different from conventional spicules, we construct
a time-height image averaged over a horizontal region of 1″, and
calculate lagged cross-correlations of intensity profiles at each height
with the intensity at 2 Mm. From this, we obtain a cross-correlation
map as a function of lag and height. We find that the correlation
curve as a function of lag time is well fitted into three different
Gaussian functions whose standard deviations of the lag time are 193,
42, and 17 s. The corresponding propagation speeds are calculated to be
9 km s<SUP>-1</SUP>, 67 km s<SUP>-1</SUP>, and 121 km s<SUP>-1</SUP>,
respectively. The kinematic properties of the former two components
seem to correspond to the 3-minute oscillations and type II spicules,
while the latter component to the jets is addressed in this study.
---------------------------------------------------------
Title: Magnetohydrodynamic Seismology of Quiet Solar Active Regions
Authors: Anfinogentov, Sergey A.; Nakariakov, V. M.
2019ApJ...884L..40A Altcode: 2019arXiv191003809A
The ubiquity of recently discovered low-amplitude decayless kink
oscillations of plasma loops allows for the seismological probing
of the corona on a regular basis. In particular, in contrast to
traditionally applied seismology that is based on the large-amplitude
decaying kink oscillations excited by flares and eruptions, decayless
oscillations can potentially provide the diagnostics necessary for
their forecasting. We analyzed decayless kink oscillations in several
distinct loops belonging to active region NOAA 12107 on 2010 July 10
during its quiet time period, when it was observed on the west limb in
extreme ultraviolet by the Atmospheric Imaging Assembly on board the
Solar Dynamics Observatory. The oscillation periods were estimated
with the use of the motion magnification technique. The lengths of
the oscillating loops were determined within the assumption of its
semicircular shape by measuring the position of their footpoints. The
density contrast in the loops was estimated from the observed intensity
contrast accounting for the unknown spatial scale of the background
plasma. The combination of those measurements allows us to determine
the distribution of kink and Alfvén speeds in the active region. Thus,
we demonstrate the possibility of obtaining seismological information
about coronal active regions during the quiet periods of time.
---------------------------------------------------------
Title: DTCWT based motion magnification v0.5.0
Authors: Anfinogentov, Sergey; Nakariakov, Valery; Kosak, Katie
2019zndo...3368774A Altcode:
This is the first release.
---------------------------------------------------------
Title: Formation of quasi-periodic slow magnetoacoustic wave trains
by the heating/cooling misbalance
Authors: Zavershinskii, D. I.; Kolotkov, D. Y.; Nakariakov, V. M.;
Molevich, N. E.; Ryashchikov, D. S.
2019PhPl...26h2113Z Altcode: 2019arXiv190708168Z
Slow magnetoacoustic waves are omnipresent in both natural and
laboratory plasma systems. The wave-induced misbalance between plasma
cooling and heating processes causes the amplification or attenuation,
and also dispersion, of slow magnetoacoustic waves. The wave dispersion
could be attributed to the presence of characteristic time scales in
the system, connected with the plasma heating or cooling due to the
competition of the heating and cooling processes in the vicinity of
thermal equilibrium. We analyzed linear slow magnetoacoustic waves in
a plasma in thermal equilibrium formed by a balance of optically thin
radiative losses, field-align thermal conduction, and an unspecified
heating process. The dispersion is manifested by the dependence of the
effective adiabatic index of the wave on the wave frequency, making the
phase and group speeds frequency-dependent. The mutual effect of the
wave amplification and dispersion is shown to result in the occurrence
of an oscillatory pattern in an initially broadband slow wave, with
the characteristic period determined by the thermal misbalance time
scales, i.e., by the derivatives of the combined radiation loss
and heating function with respect to the density and temperature,
evaluated at the equilibrium. This effect is illustrated by estimating
the characteristic period of the oscillatory pattern, appearing because
of thermal misbalance in the plasma of the solar corona. It is found
that by an order of magnitude, the period is about the typical periods
of slow magnetoacoustic oscillations detected in the corona.
---------------------------------------------------------
Title: Damping of slow magnetoacoustic oscillations by the misbalance
between heating and cooling processes in the solar corona
Authors: Kolotkov, D. Y.; Nakariakov, V. M.; Zavershinskii, D. I.
2019A&A...628A.133K Altcode: 2019arXiv190707051K
Context. Rapidly decaying slow magnetoacoustic waves are regularly
observed in the solar coronal structures, offering a promising tool
for a seismological diagnostics of the coronal plasma, including its
thermodynamical properties. <BR /> Aims: The effect of damping of
standing slow magnetoacoustic oscillations in the solar coronal loops
is investigated accounting for field-aligned thermal conductivity and a
wave-induced misbalance between radiative cooling and some unspecified
heating rates. <BR /> Methods: The non-adiabatic terms were allowed to
be arbitrarily large, corresponding to the observed values. The thermal
conductivity was taken in its classical form, and a power-law dependence
of the heating function on the density and temperature was assumed. The
analysis was conducted in the linear regime and in the infinite magnetic
field approximation. <BR /> Results: The wave dynamics is found to
be highly sensitive to the characteristic timescales of the thermal
misbalance. Depending on certain values of the misbalance, timescales
three regimes of the wave evolution were identified, namely the regime
of a suppressed damping, enhanced damping in which the damping rate
drops down to observational values, and acoustic over-stability. The
specific regime is determined by the dependences of the radiative
cooling and heating functions on thermodynamical parameters of the
plasma in the vicinity of the perturbed thermal equilibrium. <BR />
Conclusions: The comparison of the observed and theoretically derived
decay times and oscillation periods allows us to constrain the coronal
heating function. For typical coronal parameters, the observed
properties of standing slow magnetoacoustic oscillations could be
readily reproduced with a reasonable choice of the heating function.
---------------------------------------------------------
Title: Evidence for Vortex Shedding in the Sun's Hot Corona
Authors: Samanta, Tanmoy; Tian, Hui; Nakariakov, Valery M.
2019PhRvL.123c5102S Altcode: 2019arXiv190708930S
Vortex shedding is an oscillating flow that is commonly observed
in fluids due to the presence of a blunt body in a flowing
medium. Numerical simulations have shown that the phenomenon of
vortex shedding could also develop in the magnetohydrodynamic (MHD)
domain. The dimensionless Strouhal number, the ratio of the blunt body
diameter to the product of the period of vortex shedding and the speed
of a flowing medium, is a robust indicator for vortex shedding, and,
generally of the order of 0.2 for a wide range of Reynolds number. Using
an observation from the Atmospheric Imaging Assembly on board the
Solar Dynamics Observatory, we report a wavelike or oscillating plasma
flow propagating upward against the Sun's gravitational force. A newly
formed shrinking loop in the postflare region possibly generates the
oscillation of the upflow in the wake of the hot and dense loop through
vortex shedding. The computed Strouhal number is consistent with the
prediction from previous MHD simulations. Our observation suggests
the possibility of vortex shedding in the solar corona.
---------------------------------------------------------
Title: Oscillation of a Small Hα Surge in a Solar Polar Coronal Hole
Authors: Cho, Kyung-Suk; Cho, Il-Hyun; Nakariakov, V. M.; Yurchyshyn,
Vasyl B.; Yang, Heesu; Kim, Yeon-Han; Kumar, Pankaj; Magara, Tetsuya
2019ApJ...877L...1C Altcode:
Hα surges (i.e., cool/dense collimated plasma ejections) may act as
a guide for a propagation of magnetohydrodynamic waves. We report
a high-resolution observation of a surge observed with 1.6 m Goode
Solar Telescope (GST) on 2009 August 26, from 18:20 UT to 18:45
UT. Characteristics of plasma motions in the surge are determined
with the normalizing radial gradient filter and the Fourier motion
filter. The shape of the surge is found to change from a “C”
shape to an inverse “C” shape after a formation of a cusp, a
signature of reconnection. There are apparent upflows seen above
the cusp top and downflows below it. The upflows show rising and
rotational motions in the right-hand direction, with the rotational
speed decreasing with height. Near the cusp top, we find a transverse
oscillation of the surge, with the period of ∼2 minutes. There is no
change of the oscillation phase below the cusp top, but above the top
a phase change is identified, giving a vertical phase speed about 86
km s<SUP>-1</SUP>. As the height increases, the initial amplitude of
the oscillation increases, and the oscillation damping time decreases
from 5.13 to 1.18 minutes. We conclude that the oscillation is a
propagating kink wave that is possibly excited by the repetitive
spontaneous magnetic reconnection.
---------------------------------------------------------
Title: The Physical Nature of Spiral Wave Patterns in Sunspots
Authors: Kang, Juhyung; Chae, Jongchul; Nakariakov, Valery M.; Cho,
Kyuhyoun; Kwak, Hannah; Lee, Kyeore
2019ApJ...877L...9K Altcode: 2019arXiv190508908K
Recently, spiral wave patterns (SWPs) have been detected in 3 minute
oscillations of sunspot umbrae, but the nature of this phenomenon has
remained elusive. We present a theoretical model that interprets the
observed SWPs as the superposition of two different azimuthal modes
of slow magnetoacoustic waves driven below the surface in an untwisted
and non-rotating magnetic cylinder. We apply this model to SWPs of the
line-of-sight (LOS) velocity in a pore observed by the Fast Imaging
Solar Spectrograph installed at the 1.6 m Goode Solar Telescope. One-
and two-armed SWPs were identified in instantaneous amplitudes of
LOS Doppler velocity maps of 3 minute oscillations. The associated
oscillation periods are about 160 s, and the durations are about 5
minutes. In our theoretical model, the observed spiral structures are
explained by the superposition of non-zero azimuthal modes driven 1600
km below the photosphere in the pore. The one-armed SWP is produced
by the slow-body sausage (m = 0) and kink (m = 1) modes, and the
two-armed SWP is formed by the slow-body sausage (m = 0) and fluting
(m = 2) modes of the magnetic flux tube forming the pore.
---------------------------------------------------------
Title: Scaling laws of quasi-periodic pulsations in solar flares
Authors: Pugh, C. E.; Broomhall, A. -M.; Nakariakov, V. M.
2019A&A...624A..65P Altcode: 2019arXiv190209627P
Context. Quasi-periodic pulsations (QPPs) are a common feature of
solar flares, but there has previously been a lack of observational
evidence to support any of the theoretical models that might explain
the origin of these QPPs. <BR /> Aims: We aimed to determine if there
are any relationships between the QPP period and other properties
of the flaring region, using a previously assembled sample of flares
with QPPs. If any relationships exist, then these can be compared with
scaling laws for the theoretical QPP mechanisms. <BR /> Methods: To
obtain the flaring region properties, we made use of the Atmospheric
Imaging Assembly (AIA) 1600 Å and Helioseismic and Magnetic Imager
(HMI) data. The flare ribbons are visible in AIA 1600 Å images, and the
positive and negative magnetic polarity ribbons can be distinguished
and the magnetic properties determined in the HMI magnetograms. The
ribbon properties calculated in this study were the ribbon separation
distance, area, total unsigned magnetic flux, and average magnetic field
strength. Only the flares that occurred within ±60° of the solar
disc centre were included, which meant a sample of 20 flares with 22
QPP signals. <BR /> Results: Positive correlations were found between
the QPP period and the ribbon properties. The strongest correlations
were with the separation distance and magnetic flux. Because these
ribbon properties also correlate with the flare duration and because the
relationship between the QPP period and flare duration may be influenced
by observational bias, we also made use of simulated data to determine
whether artificial correlations were introduced. These simulations
show that although QPPs cannot be detected for certain combinations
of QPP period and flare duration, this does not introduce an apparent
correlation. <BR /> Conclusions: There is evidence of relationships
between the QPP period and flare ribbon properties, and in the future,
the derived scaling laws between these properties can be compared to
equivalent scaling laws for theoretical QPP mechanisms.
---------------------------------------------------------
Title: Catalog of Decaying Kink Oscillations of Coronal Loops in
the 24th Solar Cycle
Authors: Nechaeva, Alena; Zimovets, Ivan V.; Nakariakov, V. M.;
Goddard, C. R.
2019ApJS..241...31N Altcode:
A catalog of kink oscillations of solar coronal loops, which spans
during almost all of solar cycle 24, is presented. The catalog is
based on the observations made in the extreme ultraviolet band at
171 Å with Solar Dynamics Observatory/Atmospheric Imaging Assembly
and includes parameters of 223 oscillating loops in 96 oscillation
events. The catalog provides the information about the oscillation
locations, time, and dates of the events, associated flare, initial
displacement, oscillation period, exponential damping time, and
apparent amplitude. The vast majority of the oscillation detections,
84%, were made in the loops situated near or off the solar limb. The
oscillation periods are found to range from 1 to 28 minutes, with 74%
of the events that have the period in the range of 2-10 minutes. About
90% of the oscillations have the apparent amplitude in the range
of 1-10 Mm. The oscillating loop lengths are 70-600 Mm. The typical
apparent amplitude is about 1% of the loop length. The oscillation
period scales linearly with the loop length, and the damping time
scales linearly with the period, which confirm previous findings. The
oscillation quality factor scales with the amplitude to the power of
minus 0.7. No statistically significant evidence of correlation was
found between both the oscillation period and the mean sunspot number,
and the loop length and mean sunspot number. The catalog provides the
research community with the foundation for the further statistical
study of kink oscillations and their use for coronal seismology.
---------------------------------------------------------
Title: Fast magnetoacoustic wave trains with time-dependent drivers
Authors: Goddard, C. R.; Nakariakov, V. M.; Pascoe, D. J.
2019A&A...624L...4G Altcode:
Context. Frequent observations of quasi-periodic rapidly-propagating
wave trains in coronal structures have been made in the last decade. The
dispersive evolution of fast magnetohydrodynamic waves propagating in
coronal waveguides can provide a physical interpretation for many of
these observations. <BR /> Aims: Previous studies have considered the
generation of fast wave trains by impulsive drivers which deposit energy
instantaneously. The signatures of dispersively formed wave trains
must depend on the temporal nature of the driver. We investigate the
effect of varying the temporal width of the driving perturbation. <BR
/> Methods: 2D magnetohydrodynamic numerical simulations of
impulsively generated wave trains in a guiding field-aligned density
enhancement were performed with the novel addition of a time-dependant
driver. <BR /> Results: The final spatial and spectral signatures of
the guided wave trains are found to depend strongly on the temporal
duration of the initial perturbation. In particular, the wavelength
(or frequency) of highest spectral amplitude is found to increase
(decrease) with increasing temporal duration, whereas the spectral
width decreases. Additionally, the efficiency of generation of fast wave
trains is found to decrease strongly with increasing temporal width of
the driver, with a cut-off at twice the internal Alfvén crossing time.
---------------------------------------------------------
Title: Properties of Slow Magnetoacoustic Oscillations of Solar
Coronal Loops by Multi-instrumental Observations
Authors: Nakariakov, V. M.; Kosak, M. K.; Kolotkov, D. Y.;
Anfinogentov, S. A.; Kumar, P.; Moon, Y. -J.
2019ApJ...874L...1N Altcode:
Rapidly decaying oscillations of the thermal emission detected in the
decay phase of solar and stellar flares are usually interpreted as
standing or sloshing (reflecting) slow magnetoacoustic oscillations. We
determine the scalings of the oscillation periods, damping times, and
amplitudes with the temperature, considering both standing and sloshing
oscillations detected with different instruments. In addition, the time
evolution of different spatial harmonics of a sloshing oscillation is
considered. Parameters of slow oscillations observed in the EUV, X-ray,
and microwave bands, and published in the literature, are used. The
damping time of slow oscillations is found to scale almost linearly
with the oscillation period, as the period to 0.87 ± 0.1, giving the
average Q-factor determined as the ratio of the damping time to the
period, of about 1. The Q-factor is found to scale with the relative
amplitude to the power of {0.33}<SUB>-0.11</SUB><SUP>+0.10</SUP> with
95% confidence. The amplitudes of different spatial harmonics forming
a sloshing pulse show similar time evolution, suggesting that the
period-dependent dissipation is counteracted by another mechanism. The
results obtained indicate that the damping of slow oscillations depends
on the oscillation amplitude, and that the competition of nonlinear
and dissipative effects could allow for the existence of wave pulses
of a sustained shape.
---------------------------------------------------------
Title: Seismological Determination of the Alfvén Speed and Plasma
Beta in Solar Photospheric Bright Points
Authors: Cho, Il-Hyun; Moon, Yong-Jae; Nakariakov, Valery M.; Yu,
Dae Jung; Lee, Jin-Yi; Bong, Su-Chan; Kim, Rok-Soon; Cho, Kyung-Suk;
Kim, Yeon-Han; Lee, Jae-Ok
2019ApJ...871L..14C Altcode: 2019arXiv190104144C
The Alfvén speed and plasma beta in photospheric bright points
(BPs) observed by the Broadband Filter Imager (BFI) of the Solar
Optical Telescope on board the Hinode satellite are estimated
seismologically. The diagnostics is based on the theory of slow
magnetoacoustic waves in a non-isothermally stratified photosphere
with a uniform vertical magnetic field. We identify and track BPs in a
G-band movie by using the 3D region growing method, and align them with
blue continuum images to derive their brightness temperatures. From the
Fourier power spectra of 118 continuum light curves made in the BPs,
we find that light curves of 91 BPs have oscillations with properties
that are significantly different from oscillation in quiet regions,
with the periods ranging 2.2-16.2 minutes. We find that the model
gives a moderate value of the plasma beta when γ lies at around
5/3. The calculated Alfvén speed is 9.68 ± 2.02 km s<SUP>-1</SUP>,
ranging in 6.3-17.4 km s<SUP>-1</SUP>. The plasma beta is estimated
to be of 0.93 ± 0.36, ranging in 0.2-1.9.
---------------------------------------------------------
Title: Three-dimensional Oscillations of 21 Halo Coronal Mass
Ejections Using Multi-spacecraft Data
Authors: Lee, Harim; Moon, Y. -J.; Nakariakov, V. M.; Na, Hyeonock;
Cho, Il-Hyun; Park, Eunsu
2018ApJ...868...18L Altcode:
We investigate the 3D structure of kinematic oscillations of full halo
coronal mass ejections (FHCMEs) using multi-spacecraft coronagraph
data from two non-parallel lines of sight. For this, we consider 21
FHCMEs which are simultaneously observed by the Solar and Heliospheric
Observatory and the Solar TErrestrial RElations Observatory A or B,
from 2010 June to 2012 August when the spacecraft were roughly in
quadrature. Using sequences of running difference images, we estimate
the instantaneous projected speeds of the FHCMEs at 24 different
azimuthal angles in the planes of the sky of those coronagraphs. We
find that all these FHCMEs have experienced kinematic oscillations
characterized by quasi-periodic variations of the instantaneous
projected radial velocity with periods ranging from 24 to 48 min. The
oscillations detected in the analyzed events are found to show distinct
azimuthal wave modes. Thirteen events (about 62%) are found to oscillate
with the azimuthal wave number m = 1. The oscillating directions
of the nodes of the m = 1 mode for these FHCMEs are consistent with
those of their position angles (or the direction of eruption), with a
mean difference of about 23°. The oscillation amplitude is found to
correlate well with the projected radial speed of the CME. An estimation
of Lorentz accelerations shows that they are dominant over other forces,
implying that the magnetic force is responsible for the kinematic
oscillations of CMEs. However, we cannot rule out other possibilities:
a global layer of enhanced current around the CMEs or the nonlinear
nature of its driver, for example the effect of vortex shedding.
---------------------------------------------------------
Title: Sausage oscillations in a plasma cylinder with a surface
current
Authors: Lim, Daye; Nakariakov, Valery M.; Moon, Yong-Jae
2018JASTP.175...49L Altcode:
Linear sausage oscillations of a cylinder embedded in a plasma with
an azimuthal magnetic field, created by a current on the surface
of the cylinder, are studied. Such a plasma configuration could be
applied to modelling flaring loops, and magnetic ropes in coronal mass
ejections. The plasma is assumed to be cold everywhere. Dispersion
relations demonstrate that the lowest radial harmonic of the sausage
mode is in the trapped regime for all values of the parallel wave
number. In the long-wavelength limit, phase and group speeds of this
mode are equal to the Alfvén speed in the external medium. It makes
the oscillation period to be determined by the ratio of the parallel
wavelength, e.g. double the length of an oscillating loop, to the
external Alfvén speed, allowing for its seismological estimations. The
application of the results obtained to the interpretation of long-period
(longer than a minute) oscillations of emission intensity detected in
solar coronal structures, gives reasonable estimations of the external
Alfvén speed. Cutoff values of the parallel wavenumber for higher
radial harmonics are determined analytically. Implications of this
finding to the observational signatures of fast magnetoacoustic wave
trains guided by cylindrical plasma non-uniformities are discussed.
---------------------------------------------------------
Title: Magnetic structure of solar flare regions producing hard
X-ray pulsations
Authors: Zimovets, I. V.; Wang, R.; Liu, Y. D.; Wang, C.; Kuznetsov,
S. A.; Sharykin, I. N.; Struminsky, A. B.; Nakariakov, V. M.
2018JASTP.174...17Z Altcode: 2017arXiv170801869Z
We present analysis of the magnetic field in seven solar flare regions
accompanied by the pulsations of hard X-ray (HXR) emission. These
flares were studied by Kuznetsov et al. (2016) (Paper I), and chosen
here because of the availability of the vector magnetograms for their
parent active regions (ARs) obtained with the SDO/HMI data. In Paper I,
based on the observations only, it was suggested that a magnetic flux
rope (MFR) might play an important role in the process of generation
of the HXR pulsations. The goal of the present paper is to test this
hypothesis by using the extrapolation of magnetic field with the
non-linear force-free field (NLFFF) method. Having done this, we found
that before each flare indeed there was an MFR elongated along and
above a magnetic polarity inversion line (MPIL) on the photosphere. In
two flare regions the sources of the HXR pulsations were located at
the footpoints of different magnetic field lines wrapping around the
central axis, and constituting an MFR by themselves. In five other
flares the parent field lines of the HXR pulsations were not a part
of an MFR, but surrounded it in the form of an arcade of magnetic
loops. These results show that, at least in the analyzed cases, the
"single flare loop" models do not satisfy the observations and magnetic
field modeling, while are consistent with the concept that the HXR
pulsations are a consequence of successive episodes of energy release
and electron acceleration in different magnetic flux tubes (loops)
of a complex AR. An MFR could generate HXR pulsations by triggering
episodes of magnetic reconnection in different loops in the course of
its non-uniform evolution along an MPIL. However, since three events
studied here were confined flares, actual eruptions may not be required
to trigger sequential particle acceleration episodes in the magnetic
systems containing an MFR.
---------------------------------------------------------
Title: Oscillations of cometary tails: a vortex shedding phenomenon?
Authors: Nisticò, G.; Vladimirov, V.; Nakariakov, V. M.; Battams,
K.; Bothmer, V.
2018A&A...615A.143N Altcode: 2018arXiv180400997N
Context. During their journey to perihelion, comets may appear in the
field of view of space-borne optical instruments, showing in some
cases a nicely developed plasma tail extending from their coma and
exhibiting an oscillatory behaviour. <BR /> Aims: The oscillations of
cometary tails may be explained in terms of vortex shedding because of
the interaction of the comet with the solar wind streams. Therefore,
it is possible to exploit these oscillations in order to infer the
value of the Strouhal number S t, which quantifies the vortex shedding
phenomenon, and the physical properties of the local medium. <BR />
Methods: We used the Heliospheric Imager (HI) data of the Solar
TErrestrial Relations Observatory (STEREO) mission to study the
oscillations of the tails of comets 2P/Encke and C/2012 S1 (ISON) during
their perihelion in Nov 2013. We determined the corresponding Strouhal
numbers from the estimates of the halo size, the relative speed of the
solar wind flow, and the period of the oscillations. <BR /> Results:
We found that the estimated Strouhal numbers are very small, and the
typical value of S t 0.2 would be extrapolated for size of the halo
larger than 10<SUP>6</SUP> km. <BR /> Conclusions: Although the vortex
shedding phenomenon has not been unambiguously revealed, the findings
suggest that some kind of magnetohydrodynamic (MHD) instability process
is responsible for the observed behaviour of cometary tails, which
can be exploited for probing the physical conditions of the near-Sun
region. <P />The movies associated to Figs. 1 and 4 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201732474/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Two-Dimensional Solar Wind Speeds from 6 to 26 Solar Radii
in Solar Cycle 24 by Using Fourier Filtering
Authors: Cho, Il-Hyun; Moon, Yong-Jae; Nakariakov, Valery M.; Bong,
Su-Chan; Lee, Jin-Yi; Song, Donguk; Lee, Harim; Cho, Kyung-Suk
2018PhRvL.121g5101C Altcode: 2018arXiv180608540C
Measurement of the solar wind speed near the Sun is important for
understanding the acceleration mechanism of the solar wind. In this
Letter, we determine 2D solar wind speeds from 6 to 26 solar radii
by applying Fourier motion filters to SOHO/LASCO C3 movies observed
from 1999 to 2010. Our method successfully reproduces the original flow
speeds in the artificially generated data as well as streamer blobs. We
measure 2D solar wind speeds from one-day to one-year timescales and
their variation in solar cycle 24. We find that the solar wind speeds
at timescales longer than a month in the solar maximum period are
relatively uniform in the azimuthal direction, while they are clearly
bimodal in the minimum period, as expected from the Ulysses observations
and interplanetary radio scintillation reconstruction. The bimodal
structure appears at around 2006, becomes most distinctive in 2009,
and abruptly disappears in 2010. The radial evolution of the solar
wind speeds resembles the Parker's solar wind solution.
---------------------------------------------------------
Title: Probing the inner heliosphere with comets
Authors: Nisticò, Giuseppe; Vladimirov, Vangelis; Nakariakov, Valery
M.; Battams, Karl; Bothmer, Volker
2018shin.confE..41N Altcode:
Optical instruments aboard space missions have recently provided
us with exciting observations of comets in the vicinity of their
perihelion. At this stage, a tail of dust and ions from the comet
nucleus is formed, which interacts with the local solar wind flow and
exhibits an oscillatory dynamics. The observed phenomenon is attributed
to the formation of a Kármán vortex street in the wake of the cometary
coma, whose properties depend upon the characteristics of the comet
itself and the local medium. We present observations of the comets
Encke and ISON detected in 2013 with the Heliospheric Imager aboard
the STEREO spacecraft, and discuss the possibility to exploit comets
as natural probes of the inner heliosphere, by relating the physical
behaviour of cometary tails with the local conditions of the solar wind.
---------------------------------------------------------
Title: Origin of the Modulation of the Radio Emission from the Solar
Corona by a Fast Magnetoacoustic Wave
Authors: Kolotkov, Dmitrii Y.; Nakariakov, Valery M.; Kontar, Eduard P.
2018ApJ...861...33K Altcode: 2018arXiv180508282K
Observational detection of quasi-periodic drifting fine structures in a
type III radio burst associated with a solar flare SOL2015-04-16T11:22,
with the LOw Frequency ARray (LOFAR), is presented. Although similar
modulations of the type III emission have been observed before and
were associated with the plasma density fluctuations, the origin
of those fluctuations was unknown. Analysis of the striae of the
intensity variation in the dynamic spectrum allowed us to reveal two
quasi-oscillatory components. The shorter component has an apparent
wavelength of ∼2 Mm, phase speed of ∼657 km s<SUP>-1</SUP>, which
gives an oscillation period of ∼3 s, and a relative amplitude of
∼0.35%. The longer component has a wavelength of ∼12 Mm and relative
amplitude of ∼5.1%. The short frequency range of the detection does
not allow us to estimate its phase speed. However, the properties of
the shorter oscillatory component allowed us to interpret it as a fast
magnetoacoustic wave guided by a plasma nonuniformity along the magnetic
field outwards from the Sun. The assumption that the intensity of the
radio emission is proportional to the amount of plasma in the emitting
volume allowed us to show that the superposition of the plasma density
modulation by a fast wave and a longer-wavelength oscillation of an
unspecified nature could readily reproduce the fine structure of the
observed dynamic spectrum. The observed parameters of the fast wave
give an absolute value for the magnetic field in the emitting plasma
of ∼1.1 G, which is consistent with the radial magnetic field model.
---------------------------------------------------------
Title: Finite amplitude transverse oscillations of a magnetic rope
Authors: Kolotkov, Dmitrii Y.; Nisticò, Giuseppe; Rowlands, George;
Nakariakov, Valery M.
2018JASTP.172...40K Altcode: 2018arXiv180305195K
The effects of finite amplitudes on the transverse oscillations of a
quiescent prominence represented by a magnetic rope are investigated
in terms of the model proposed by Kolotkov et al. (2016). We consider
a weakly nonlinear case governed by a quadratic nonlinearity, and
also analyse the fully nonlinear equations of motion. We treat the
prominence as a massive line current located above the photosphere and
interacting with the magnetised dipped environment via the Lorentz
force. In this concept the magnetic dip is produced by two external
current sources located at the photosphere. Finite amplitude horizontal
and vertical oscillations are found to be strongly coupled between
each other. The coupling is more efficient for larger amplitudes and
smaller attack angles between the direction of the driver and the
horizontal axis. Spatial structure of oscillations is represented
by Lissajous-like curves with the limit cycle of a hourglass shape,
appearing in the resonant case, when the frequency of the vertical
mode is twice the horizontal mode frequency. A metastable equilibrium
of the prominence is revealed, which is stable for small amplitude
displacements, and becomes horizontally unstable, when the amplitude
exceeds a threshold value. The maximum oscillation amplitudes are
also analytically derived and analysed. Typical oscillation periods
are determined by the oscillation amplitude, prominence current,
its mass and position above the photosphere, and the parameters of
the magnetic dip. The main new effects of the finite amplitude are
the coupling of the horizontally and vertically polarised transverse
oscillations (i.e. the lack of a simple, elliptically polarised regime)
and the presence of metastable equilibria of prominences.
---------------------------------------------------------
Title: In Situ Generation of Transverse Magnetohydrodynamic Waves
from Colliding Flows in the Solar Corona
Authors: Antolin, Patrick; Pagano, Paolo; De Moortel, Ineke;
Nakariakov, Valery M.
2018ApJ...861L..15A Altcode: 2018arXiv180700395A
Transverse magnetohydrodynamic (MHD) waves permeate the solar
atmosphere and are a candidate for coronal heating. However, the
origin of these waves is still unclear. In this Letter, we analyze
coordinated observations from Hinode/Solar Optical Telescope (SOT) and
Interface Region Imaging Spectrograph ( IRIS) of a prominence/coronal
rain loop-like structure at the limb of the Sun. Cool and dense
downflows and upflows are observed along the structure. A collision
between a downward and an upward flow with an estimated energy flux
of 10<SUP>7</SUP>-10<SUP>8</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>
is observed to generate oscillatory transverse perturbations of the
strands with an estimated ≈40 km s<SUP>-1</SUP> total amplitude, and
a short-lived brightening event with the plasma temperature increasing
to at least 10<SUP>5</SUP> K. We interpret this response as sausage
and kink transverse MHD waves based on 2D MHD simulations of plasma
flow collision. The lengths, density, and velocity differences between
the colliding clumps and the strength of the magnetic field are major
parameters defining the response to the collision. The presence of
asymmetry between the clumps (angle of impact surface and/or offset
of flowing axis) is crucial for generating a kink mode. Using the
observed values, we successfully reproduce the observed transverse
perturbations and brightening, and show adiabatic heating to coronal
temperatures. The numerical modeling indicates that the plasma β
in this loop-like structure is confined between 0.09 and 0.36. These
results suggest that such collisions from counter-streaming flows can
be a source of in situ transverse MHD waves, and that for cool and dense
prominence conditions such waves could have significant amplitudes.
---------------------------------------------------------
Title: Quasi-periodic Pulsations in a Solar Microflare
Authors: Nakariakov, V. M.; Anfinogentov, S.; Storozhenko, A. A.;
Kurochkin, E. A.; Bogod, V. M.; Sharykin, I. N.; Kaltman, T. I.
2018ApJ...859..154N Altcode:
Irregular time evolution of the radio emission generated in a B2-class
microflare (SOL2017-01-25T10:15), occurring on 2017 January 25 in
active region 12,628, is studied. The microflare was apparently
initiated by an appearance of an s-shaped loop, observed in the EUV
band. The radio emission is associated with the nonthermal electrons
detected with Ramaty High Energy Solar Spectroscopic Imager, and
originates simultaneously from two opposite footpoints of a magnetic
fan structure beginning at a sunspot. According to the active region
geometry, the footpoints are situated in the meridional direction,
and hence are observed by RATAN-600 simultaneously. The radio emission
intensity signal, as well as the left-hand and right-hand circular
polarization signals in the low-frequency band (3-4 GHz) show good
correlation with each other, with the average characteristic time
of the variation 1.4 ± 0.3 s. The polarization signal shows a time
variation with the characteristic time of about 0.7 ± 0.2 s. The
irregular quasi-periodic pulsations of the radio emission are likely
to be caused by the superposition of the signals generated at the local
electron plasma frequencies by the interaction of nonthermal electrons
with the plasma at the footpoints. In this scenario, the precipitation
rate of the nonthermal electrons at the opposite footpoints could be
modulated by the superposition of fundamental and second harmonic
modes of sausage oscillations, resulting in the observed different
characteristic times of the intensity and polarization signals. However,
other mechanisms, e.g., the oscillatory regime of loop coalescence or
magnetic null point oscillation could not be rigorously excluded.
---------------------------------------------------------
Title: Spatiotemporal Analysis of Coronal Loops Using Seismology
of Damped Kink Oscillations and Forward Modeling of EUV Intensity
Profiles
Authors: Pascoe, D. J.; Anfinogentov, S. A.; Goddard, C. R.;
Nakariakov, V. M.
2018ApJ...860...31P Altcode:
The shape of the damping profile of kink oscillations in coronal loops
has recently allowed the transverse density profile of the loop to be
estimated. This requires accurate measurement of the damping profile
that can distinguish the Gaussian and exponential damping regimes,
otherwise there are more unknowns than observables. Forward modeling
of the transverse intensity profile may also be used to estimate the
width of the inhomogeneous layer of a loop, providing an independent
estimate of one of these unknowns. We analyze an oscillating loop for
which the seismological determination of the transverse structure is
inconclusive except when supplemented by additional spatial information
from the transverse intensity profile. Our temporal analysis describes
the motion of a coronal loop as a kink oscillation damped by resonant
absorption, and our spatial analysis is based on forward modeling the
transverse EUV intensity profile of the loop under the isothermal and
optically thin approximations. We use Bayesian analysis and Markov chain
Monte Carlo sampling to apply our spatial and temporal models both
individually and simultaneously to our data and compare the results
with numerical simulations. Combining the two methods allows both
the inhomogeneous layer width and density contrast to be calculated,
which is not possible for the same data when each method is applied
individually. We demonstrate that the assumption of an exponential
damping profile leads to a significantly larger error in the inferred
density contrast ratio compared with a Gaussian damping profile.
---------------------------------------------------------
Title: Quasi-periodic Pulsations in the Most Powerful Solar Flare
of Cycle 24
Authors: Kolotkov, Dmitrii Y.; Pugh, Chloe E.; Broomhall, Anne-Marie;
Nakariakov, Valery M.
2018ApJ...858L...3K Altcode: 2018arXiv180404955K
Quasi-periodic pulsations (QPPs) are common in solar flares and are
now regularly observed in stellar flares. We present the detection
of two different types of QPP signals in the thermal emission light
curves of the X9.3-class solar flare SOL2017-09-06T12:02, which is the
most powerful flare of Cycle 24. The period of the shorter-period QPP
drifts from about 12 to 25 s during the flare. The observed properties
of this QPP are consistent with a sausage oscillation of a plasma loop
in the flaring active region. The period of the longer-period QPP is
about 4 to 5 minutes. Its properties are compatible with standing slow
magnetoacoustic oscillations, which are often detected in coronal
loops. For both QPP signals, other mechanisms such as repetitive
reconnection cannot be ruled out, however. The studied solar flare
has an energy in the realm of observed stellar flares, and the fact
that there is evidence of a short-period QPP signal typical of solar
flares along with a long-period QPP signal more typical of stellar
flares suggests that the different ranges of QPP periods typically
observed in solar and stellar flares is likely due to observational
constraints, and that similar physical processes may be occurring in
solar and stellar flares.
---------------------------------------------------------
Title: Nonlinear waves in the terrestrial quasi-parallel foreshock
Authors: Hnat, Bogdan; Kolotkov, Dmitri; O'Connell, Daniel; Nakariakov,
Valery; Rowlands, George
2018EGUGA..20.4956H Altcode:
Upstream regions of quasiparallel collisionless astrophysical shocks are
among the most complex plasma systems. When the magnetic field direction
is quasiparallel to the shock normal, a fraction of incoming ions are
reflected upstream generating an extended and turbulent foreshock. We
provide strongly conclusive evidence that the cubic nonlinearity plays
an important part in the evolution of the large amplitude magnetic
structures in the terrestrial foreshock. Large amplitude nonlinear
wave trains at frequencies above the proton cyclotron frequency are
identified after non harmonic slow variations are filtered out by
applying the empirical mode decomposition. Numerical solutions of the
derivative nonlinear Schrodinger equation, predicted analytically by
the use of a pseudo-potential approach, are found to be consistent
with the observed wave forms. The approximate phase speed of these
nonlinear waves, indicated by the parameters of numerical solutions,
is of the order of the local Alfven speed. We suggest that the feedback
of the large amplitudec fluctuations on background plasma is reflected
in the evolution of the pseudo-potential.
---------------------------------------------------------
Title: Detection of the Second Harmonic of Decay-less Kink
Oscillations in the Solar Corona
Authors: Duckenfield, T.; Anfinogentov, S. A.; Pascoe, D. J.;
Nakariakov, V. M.
2018ApJ...854L...5D Altcode:
EUV observations of a multi-thermal coronal loop, taken by the
Atmospheric Imaging Assembly of the Solar Dynamics Observatory, which
exhibits decay-less kink oscillations are presented. The data cube of
the quiet-Sun coronal loop was passed through a motion magnification
algorithm to accentuate transverse oscillations. Time-distance maps are
made from multiple slits evenly spaced along the loop axis and oriented
orthogonal to the loop axis. Displacements of the intensity peak are
tracked to generate time series of the loop displacement. Fourier
analysis on the time series shows the presence of two periods within
the loop: {P}<SUB>1</SUB>={10.3}<SUB>-1.7</SUB><SUP>+1.5</SUP>
minutes and {P}<SUB>2</SUB>={7.4}<SUB>-1.3</SUB><SUP>+1.1</SUP>
minutes. The longer period component is greatest in amplitude at the
apex and remains in phase throughout the loop length. The shorter
period component is strongest further down from the apex on both legs
and displays an anti-phase behavior between the two loop legs. We
interpret these results as the coexistence of the fundamental and
second harmonics of the standing kink mode within the loop in the
decay-less oscillation regime. An illustration of seismological
application using the ratio P <SUB>1</SUB>/2P <SUB>2</SUB> ∼ 0.7
to estimate the density scale height is presented. The existence of
multiple harmonics has implications for understanding the driving and
damping mechanisms for decay-less oscillations and adds credence to
their interpretation as standing kink mode oscillations.
---------------------------------------------------------
Title: Modelling Quasi-Periodic Pulsations in Solar and Stellar Flares
Authors: McLaughlin, J. A.; Nakariakov, V. M.; Dominique, M.; Jelínek,
P.; Takasao, S.
2018SSRv..214...45M Altcode: 2018arXiv180204180M
Solar flare emission is detected in all EM bands and variations in flux
density of solar energetic particles. Often the EM radiation generated
in solar and stellar flares shows a pronounced oscillatory pattern, with
characteristic periods ranging from a fraction of a second to several
minutes. These oscillations are referred to as quasi-periodic pulsations
(QPPs), to emphasise that they often contain apparent amplitude and
period modulation. We review the current understanding of quasi-periodic
pulsations in solar and stellar flares. In particular, we focus on
the possible physical mechanisms, with an emphasis on the underlying
physics that generates the resultant range of periodicities. These
physical mechanisms include MHD oscillations, self-oscillatory
mechanisms, oscillatory reconnection/reconnection reversal, wave-driven
reconnection, two loop coalescence, MHD flow over-stability, the
equivalent LCR-contour mechanism, and thermal-dynamical cycles. We
also provide a histogram of all QPP events published in the literature
at this time. The occurrence of QPPs puts additional constraints on
the interpretation and understanding of the fundamental processes
operating in flares, e.g. magnetic energy liberation and particle
acceleration. Therefore, a full understanding of QPPs is essential in
order to work towards an integrated model of solar and stellar flares.
---------------------------------------------------------
Title: Corrugation Instability of a Coronal Arcade
Authors: Klimushkin, D. Y.; Nakariakov, V. M.; Mager, P. N.;
Cheremnykh, O. K.
2017SoPh..292..184K Altcode:
We analyse the behaviour of linear magnetohydrodynamic perturbations
of a coronal arcade modelled by a half-cylinder with an azimuthal
magnetic field and non-uniform radial profiles of the plasma
pressure, temperature, and the field. Attention is paid to the
perturbations with short longitudinal (in the direction along the
arcade) wavelengths. The radial structure of the perturbations, either
oscillatory or evanescent, is prescribed by the radial profiles of the
equilibrium quantities. Conditions for the corrugation instability of
the arcade are determined. It is established that the instability growth
rate increases with decreases in the longitudinal wavelength and the
radial wave number. In the unstable mode, the radial perturbations of
the magnetic field are stronger than the longitudinal perturbations,
creating an almost circularly corrugated rippling of the arcade in the
longitudinal direction. For coronal conditions, the growth time of the
instability is shorter than one minute, decreasing with an increase in
the temperature. Implications of the developed theory for the dynamics
of coronal active regions are discussed.
---------------------------------------------------------
Title: Properties of quasi-periodic pulsations in solar flares from
a single active region
Authors: Pugh, C. E.; Nakariakov, V. M.; Broomhall, A. -M.; Bogomolov,
A. V.; Myagkova, I. N.
2017A&A...608A.101P Altcode: 2017arXiv170909472P
Context. Quasi-periodic pulsations (QPPs) are a common feature of
solar and stellar flares, and so the nature of these pulsations should
be understood in order to fully understand flares. <BR /> Aims: We
investigate the properties of a set of solar flares originating from
a single active region (AR) that exhibit QPPs, and in particular
look for any indication of QPP periods relating to AR properties,
as might be expected if the characteristic timescale of the pulsations
corresponds to a characteristic length scale of the structure from which
the pulsations originate. The three AR properties used for this study
are the photospheric area, bipole separation distance, and average
magnetic field strength at the photosphere. The AR studied, known as
NOAA 12172/12192/12209, was unusually long-lived and persisted for over
three Carrington rotations between September and November 2014. During
this time a total of 181 flares were observed by GOES. <BR /> Methods:
Data from the GOES/XRS, SDO/EVE/ESP, Fermi/GBM, Vernov/DRGE and Nobeyama
Radioheliograph observatories were used to determine if QPPs were
present in the flares. For the soft X-ray GOES/XRS and EVE/ESP data,
the time derivative of the signal was used so that any variability
in the impulsive phase of the flare was emphasised. Periodogram power
spectra of the time series data, without any form of detrending, were
inspected and flares with a peak above the 95% confidence level in the
power spectrum were labelled as having candidate QPPs. The confidence
levels were determined taking full account of data uncertainties
and the possible presence of red noise. Active region properties
were determined using SDO/HMI line of sight magnetogram data. <BR
/> Results: A total of 37 flares, i.e. 20% of the sample, show good
evidence of having stationary or weakly non-stationary QPPs, and some
of the pulsations can be seen in data from multiple instruments and
in different wavebands. Because the detection method used was rather
conservative, this may be a lower bound for the true number of flares
with QPPs. The QPP periods were found to show a weak correlation
with the flare amplitude and duration, but this is likely due to an
observational bias. A stronger correlation was found between the QPP
period and duration of the QPP signal, which can be partially but not
entirely explained by observational constraints. No correlations were
found with the AR area, bipole separation distance, or average magnetic
field strength. <BR /> Conclusions: The fact that a substantial fraction
of the flare sample showed evidence of QPPs, using a strict detection
method with minimal processing of the data, demonstrates that these
QPPs are a real phenomenon that cannot be explained by the presence of
red noise or the superposition of multiple unrelated flares. The lack
of correlation between the QPP periods and AR properties implies that
the small-scale structure of the AR is important and/or that different
QPP mechanisms act in different cases.
---------------------------------------------------------
Title: Effect of Local Thermal Equilibrium Misbalance on
Long-wavelength Slow Magnetoacoustic Waves
Authors: Nakariakov, V. M.; Afanasyev, A. N.; Kumar, S.; Moon, Y. -J.
2017ApJ...849...62N Altcode:
Evolution of slow magnetoacoustic waves guided by a cylindrical
magnetic flux tube that represents a coronal loop or plume, is modeled
accounting for the effects of finite gas pressure, weak nonlinearity,
dissipation by thermal conduction and viscosity, and the misbalance
between the cooling by optically thin radiation and unspecified heating
of the plasma. An evolutionary equation of the Burgers-Malthus type is
derived. It is shown that the cooling/heating misbalance, determined by
the derivatives of the combined radiative cooling and heating function,
with respect to the density, temperature, and magnetic field at the
thermal equilibrium affect the wave rather strongly. This effect may
either cause additional damping, or counteract it, or lead to the
gradual amplification of the wave. In the latter case, the coronal
plasma acts as an active medium for the slow magnetoacoustic waves. The
effect of the cooling/heating misbalance could be important for coronal
slow waves, and could be responsible for certain discrepancies between
theoretical results and observations, in particular, the increased or
decreased damping lengths and times, detection of the waves at certain
heights only, and excitation of compressive oscillations. The results
obtained open up a possibility for the diagnostics of the coronal
heating function by slow magnetoacoustic waves.
---------------------------------------------------------
Title: Seismology of contracting and expanding coronal loops using
damping of kink oscillations by mode coupling
Authors: Pascoe, D. J.; Russell, A. J. B.; Anfinogentov, S. A.;
Simões, P. J. A.; Goddard, C. R.; Nakariakov, V. M.; Fletcher, L.
2017A&A...607A...8P Altcode:
<BR /> Aims: We extend recently developed seismological methods to
analyse oscillating loops which feature a large initial shift in the
equilibrium position and investigate additional observational signatures
related to the loop environment and oscillation driver. <BR /> Methods:
We model the motion of coronal loops as a kink oscillation damped by
mode coupling, accounting for any change in loop length and the possible
presence of parallel harmonics in addition to the fundamental mode. We
apply our model to a loop which rapidly contracts due to a post-flare
implosion (SOL2012-03-09) and a loop with a large lateral displacement
(SOL2012-10-20). <BR /> Results: The seismological method is used to
calculate plasma parameters of the oscillating loops including the
transverse density profile, magnetic field strength, and phase mixing
timescale. For SOL2012-03-09 the period of oscillation has a linear
correlation with the contracting motion and suggests the kink speed
remains constant during the oscillation. The implosion excitation
mechanism is found to be associated with an absence of additional
parallel harmonics. <BR /> Conclusions: The improved Bayesian
analysis of the coronal loop motion allows for accurate seismology
of plasma parameters, and the evolution of the period of oscillation
compared with the background trend can be used to distinguish between
loop motions in the plane of the loop and those perpendicular to
it. The seismologically inferred kink speed and density contrast
imply sub-Alfvénic (M<SUB>A</SUB> = 0.16 ± 0.03) propagation of the
magnetic reconfiguration associated with the implosion, as opposed to
triggering by a wave propagating at the Alfvén speed.
---------------------------------------------------------
Title: Dispersive Evolution of Nonlinear Fast Magnetoacoustic
Wave Trains
Authors: Pascoe, D. J.; Goddard, C. R.; Nakariakov, V. M.
2017ApJ...847L..21P Altcode:
Quasi-periodic rapidly propagating wave trains are frequently observed
in extreme ultraviolet observations of the solar corona, or are inferred
by the quasi-periodic modulation of radio emission. The dispersive
nature of fast magnetohydrodynamic waves in coronal structures
provides a robust mechanism to explain the detected quasi-periodic
patterns. We perform 2D numerical simulations of impulsively generated
wave trains in coronal plasma slabs and investigate how the behavior
of the trapped and leaky components depend on the properties of the
initial perturbation. For large amplitude compressive perturbations,
the geometrical dispersion associated with the waveguide suppresses
the nonlinear steepening for the trapped wave train. The wave train
formed by the leaky components does not experience dispersion once it
leaves the waveguide and so can steepen and form shocks. The mechanism
we consider can lead to the formation of multiple shock fronts by
a single, large amplitude, impulsive event and so can account for
quasi-periodic features observed in radio spectra.
---------------------------------------------------------
Title: Toward a Next Generation Solar Coronagraph: Development of
a Compact Diagnostic Coronagraph on the ISS
Authors: Cho, K. -S.; Bong, S. -C.; Choi, S.; Yang, H.; Kim, J.;
Baek, J. -H.; Park, J.; Lim, E. -K.; Kim, R. -S.; Kim, S.; Kim,
Y. -H.; Park, Y. -D.; Clarke, S. W.; Davila, J. M.; Gopalswamy, N.;
Nakariakov, V. M.; Li, B.; Pinto, R. F.
2017JKAS...50..139C Altcode:
The Korea Astronomy and Space Science Institute plans to develop
a coronagraph in collaboration with National Aeronautics and Space
Administration (NASA) and to install it on the International Space
Station (ISS). The coronagraph is an externally occulted one-stage
coronagraph with a field of view from 3 to 15 solar radii. The
observation wavelength is approximately 400 nm, where strong Fraunhofer
absorption lines from the photosphere experience thermal broadening and
Doppler shift through scattering by coronal electrons. Photometric
filter observations around this band enable the estimation of
2D electron temperature and electron velocity distribution in the
corona. Together with a high time cadence (<12 min) of corona images
used to determine the geometric and kinematic parameters of coronal
mass ejections, the coronagraph will yield the spatial distribution
of electron density by measuring the polarized brightness. For the
purpose of technical demonstration, we intend to observe the total
solar eclipse in August 2017 with the filter system and to perform a
stratospheric balloon experiment in 2019 with the engineering model
of the coronagraph. The coronagraph is planned to be installed on the
ISS in 2021 for addressing a number of questions (e.g., coronal heating
and solar wind acceleration) that are both fundamental and practically
important in the physics of the solar corona and of the heliosphere.
---------------------------------------------------------
Title: A statistical study of the inferred transverse density profile
of coronal loop threads observed with SDO/AIA
Authors: Goddard, C. R.; Pascoe, D. J.; Anfinogentov, S.; Nakariakov,
V. M.
2017A&A...605A..65G Altcode:
<BR /> Aims: We carry out a statistical study of the inferred coronal
loop cross-sectional density profiles using extreme ultraviolet (EUV)
imaging data from the Atmospheric Imaging Assembly (AIA) on board
the Solar Dynamics Observatory (SDO). <BR /> Methods: We analysed 233
coronal loops observed during 2015/2016. We consider three models for
the density profile; the step function (model S), the linear transition
region profile (model L), and a Gaussian profile (model G). Bayesian
inference is used to compare the three corresponding forward modelled
intensity profiles for each loop. These are constructed by integrating
the square of the density from a cylindrical loop cross-section
along the line of sight, assuming an isothermal cross-section, and
applying the instrumental point spread function. <BR /> Results:
Calculating the Bayes factors for comparisons between the models,
it was found that in 47% of cases there is very strong evidence for
model L over model S and in 45% of cases very strong evidence for
model G over S. Using multiple permutations of the Bayes factor the
favoured density profile for each loop was determined for multiple
evidence thresholds. There were a similar number of cases where model
L or G are favoured, showing evidence for inhomogeneous layers and
constantly varying density cross-sections, subject to our assumptions
and simplifications. <BR /> Conclusions: For sufficiently well
resolved loop threads with no visible substructure it has been shown
that using Bayesian inference and the observed intensity profile we
can distinguish between the proposed density profiles at a given AIA
wavelength and spatial resolution. We have found very strong evidence
for inhomogeneous layers, with model L being the most general, and a
tendency towards thicker or even continuous layers.
---------------------------------------------------------
Title: Quasi-periodic Radio Bursts Associated with Fast-mode Waves
near a Magnetic Null Point
Authors: Kumar, Pankaj; Nakariakov, Valery M.; Cho, Kyung-Suk
2017ApJ...844..149K Altcode: 2017arXiv170609988K
This paper presents an observation of quasi-periodic rapidly propagating
waves observed in the Atmospheric Image Assembly (AIA) 171/193 Å
channels during the impulsive phase of an M1.9 flare that occurred on
2012 May 7. The instant period was found to decrease from 240 to 120
s, and the speed of the wavefronts was in the range of ∼664-1416
km s<SUP>-1</SUP>. Almost simultaneously, quasi-periodic bursts with
similar instant periods, ∼70 and ∼140 s, occur in the microwave
emission and in decimetric type IV and type III radio bursts, and in
the soft X-ray emission. The magnetic field configuration of the flare
site was consistent with a breakout topology, I.e., a quadrupolar
field along with a magnetic null point. The quasi-periodic rapidly
propagating wavefronts of the EUV emission are interpreted as a fast
magnetoacoustic wave train. The observations suggest that the fast-mode
waves are generated during the quasi-periodic magnetic reconnection
in the cusp region above the flare arcade loops. For the first time,
we provide evidence of a tadpole wavelet signature at about 70-140
s in decimetric (245/610 MHz) radio bursts, along with the direct
observation of a coronal fast-mode wave train in EUV. In addition, at
AIA 131/193 Å we observed quasi-periodic EUV disturbances with periods
of 95 and 240 s propagating downward at apparent speeds of 172-273 km
s<SUP>-1</SUP>. The nature of these downward propagating disturbances
is not revealed, but they could be connected to magnetoacoustic waves
or periodically shrinking loops.
---------------------------------------------------------
Title: Significance testing for quasi-periodic pulsations in solar
and stellar flares
Authors: Pugh, C. E.; Broomhall, A. -M.; Nakariakov, V. M.
2017A&A...602A..47P Altcode: 2017arXiv170307294P
The robust detection of quasi-periodic pulsations (QPPs) in solar and
stellar flares has been the topic of recent debate. In light of this,
we have adapted a method described by Vaughan (2005, A&A, 431,
391) to aid with the search for QPPs in flare time series data. The
method identifies statistically significant periodic signals in
power spectra, and properly accounts for red noise as well as the
uncertainties associated with the data. We show how the method
can be further developed to be used with rebinned power spectra,
allowing us to detect QPPs whose signal is spread over more than one
frequency bin. An advantage of these methods is that there is no need
to detrend the data prior to creating the power spectrum. Examples are
given where the methods have been applied to synthetic data, as well
as real flare time series data with candidate QPPs from the Nobeyama
Radioheliograph. These show that, despite the transient nature of QPPs,
peaks corresponding to the QPPs can be seen at a significant level in
the power spectrum without any form of detrending or other processing
of the original time series data, providing the background trends are
not too steep.
---------------------------------------------------------
Title: Nonlinear Evolution of Short-wavelength Torsional Alfvén Waves
Authors: Shestov, S. V.; Nakariakov, V. M.; Ulyanov, A. S.; Reva,
A. A.; Kuzin, S. V.
2017ApJ...840...64S Altcode: 2017arXiv170502790S
We analyze nonlinear evolution of torsional Alfvén waves in a straight
magnetic flux tube filled in with a low-β plasma, and surrounded with
a plasma of lower density. Such magnetic tubes model, in particular,
a segment of a coronal loop or a polar plume. The wavelength is taken
comparable to the tube radius. We perform a numerical simulation of
the wave propagation using ideal magnetohydrodynamics. We find that a
torsional wave nonlinearly induces three kinds of compressive flows:
the parallel flow at the Alfvén speed, which constitutes a bulk
plasma motion along the magnetic field, the tube wave, and also
transverse flows in the radial direction, associated with sausage
fast magnetoacoustic modes. In addition, the nonlinear torsional
wave steepens and its propagation speed increases. The latter effect
leads to the progressive distortion of the torsional wave front, I.e.,
nonlinear phase mixing. Because of the intrinsic non-uniformity of the
torsional wave amplitude across the tube radius, the nonlinear effects
are more pronounced in regions with higher wave amplitudes. They
are always absent at the axes of the flux tube. In the case of a
linear radial profile of the wave amplitude, the nonlinear effects
are localized in an annulus region near the tube boundary. Thus,
the parallel compressive flows driven by torsional Alfvén waves in
the solar and stellar coronae, are essentially non-uniform in the
perpendicular direction. The presence of additional sinks for the
wave energy reduces the efficiency of the nonlinear parallel cascade
in torsional Alfvén waves.
---------------------------------------------------------
Title: Multi-instrument observations of a failed flare eruption
associated with MHD waves in a loop bundle
Authors: Nisticò, G.; Polito, V.; Nakariakov, V. M.; Del Zanna, G.
2017A&A...600A..37N Altcode: 2016arXiv161202077N
Context. We present observations of a B7.9-class flare that occurred on
the 24th January, 2015, using the Atmospheric Imaging Assembly (AIA)
of the Solar Dynamics Observatory (SDO), the EUV Imaging Spectrometer
(EIS) and the X-Ray Telescope of Hinode. The flare triggers the eruption
of a dense cool plasma blob as seen in AIA 171 Å, which is unable
to completely break out and remains confined within a local bundle
of active region loops. During this process, transverse oscillations
of the threads are observed. The cool plasma is then observed to
descend back to the chromosphere along each loop strand. At the same
time, a larger diffuse co-spatial loop observed in the hot wavebands
of SDO/AIA and Hinode/XRT is formed, exhibiting periodic intensity
variations along its length. <BR /> Aims: The formation and evolution
of magnetohydrodynamic (MHD) waves depend upon the values of the local
plasma parameters (e.g. density, temperature and magnetic field),
which can hence be inferred by coronal seismology. In this study we
aim to assess how the observed MHD modes are affected by the variation
of density and temperature. <BR /> Methods: We combined analysis
of EUV/X-ray imaging and spectroscopy using SDO/AIA, Hinode/EIS
and XRT. <BR /> Results: The transverse oscillations of the cool
loop threads are interpreted in terms of vertically polarised kink
oscillations. The fitting procedure applied to the loop displacement
time series gives a period of 3.5 to 4 min, and an amplitude of 5
Mm. The oscillations are strongly damped showing very low quality
factor (1.5-2), which is defined as the ratio of the damping time and
the oscillation period. The weak variation of the period of the kink
wave, which is estimated from the fitting analysis, is in agreement
with the density variations due to the presence of the plasma blob
inferred from the intensity light curve at 171 Å. The coexisting
intensity oscillations along the hot loop are interpreted as a slow
MHD wave with a period of 10 min and phase speed of approximately 436
km s<SUP>-1</SUP>. Comparison between the fast and slow modes allows
for the determination of the Alfvén speed, and consequently magnetic
field values. The plasma-β inferred from the analysis is estimated to
be approximately 0.1-0.3. <BR /> Conclusions: We show that the evolution
of the detected waves is determined by the temporal variations of the
local plasma parameters, caused by the flare heating and the consequent
cooling. We apply coronal seismology to both waves obtaining estimates
of the background plasma parameters. <P />Movies are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201629324/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Coronal loop seismology using damping of standing kink
oscillations by mode coupling. II. additional physical effects and
Bayesian analysis
Authors: Pascoe, D. J.; Anfinogentov, S.; Nisticò, G.; Goddard,
C. R.; Nakariakov, V. M.
2017A&A...600A..78P Altcode:
Context. The strong damping of kink oscillations of coronal loops can
be explained by mode coupling. The damping envelope depends on the
transverse density profile of the loop. Observational measurements of
the damping envelope have been used to determine the transverse loop
structure which is important for understanding other physical processes
such as heating. <BR /> Aims: The general damping envelope describing
the mode coupling of kink waves consists of a Gaussian damping
regime followed by an exponential damping regime. Recent observational
detection of these damping regimes has been employed as a seismological
tool. We extend the description of the damping behaviour to account
for additional physical effects, namely a time-dependent period of
oscillation, the presence of additional longitudinal harmonics, and
the decayless regime of standing kink oscillations. <BR /> Methods:
We examine four examples of standing kink oscillations observed by
the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory (SDO). We use forward modelling of the loop position
and investigate the dependence on the model parameters using Bayesian
inference and Markov chain Monte Carlo (MCMC) sampling. <BR /> Results:
Our improvements to the physical model combined with the use of Bayesian
inference and MCMC produce improved estimates of model parameters and
their uncertainties. Calculation of the Bayes factor also allows us to
compare the suitability of different physical models. We also use a new
method based on spline interpolation of the zeroes of the oscillation to
accurately describe the background trend of the oscillating loop. <BR
/> Conclusions: This powerful and robust method allows for accurate
seismology of coronal loops, in particular the transverse density
profile, and potentially reveals additional physical effects.
---------------------------------------------------------
Title: Coronal loop density profile estimated by forward modelling
of EUV intensity
Authors: Pascoe, D. J.; Goddard, C. R.; Anfinogentov, S.; Nakariakov,
V. M.
2017A&A...600L...7P Altcode:
<BR /> Aims: The transverse density structuring of coronal loops was
recently calculated for the first time using the general damping
profile for kink oscillations. This seismological method assumes
a density profile with a linear transition region. We consider to
what extent this density profile accounts for the observed intensity
profile of the loop, and how the transverse intensity profile may
be used to complement the seismological technique. <BR /> Methods:
We use isothermal and optically transparent approximations for which
the intensity of extreme ultraviolet (EUV) emission is proportional to
the square of the plasma density integrated along the line of sight. We
consider four different models for the transverse density profile; the
generalised Epstein profile, the step function, the linear transition
region profile, and a Gaussian profile. The effect of the point spread
function is included and Bayesian analysis is used for comparison of
the models. <BR /> Results: The two profiles with finite transitions are
found to be preferable to the step function profile, which supports the
interpretation of kink mode damping as being due to mode coupling. The
estimate of the transition layer width using forward modelling is
consistent with the seismological estimate. <BR /> Conclusions: For
wide loops, that is those observed with sufficiently high spatial
resolution, this method can provide an independent estimate of density
profile parameters for comparison with seismological estimates. In
the ill-posed case of only one of the Gaussian or exponential damping
regimes being observed, it may provide additional information to allow
a seismological inversion to be performed. Alternatively, it may be
used to obtain structuring information for loops that do not oscillate.
---------------------------------------------------------
Title: Determination of the Alfvén Speed and Plasma-beta Using the
Seismology of Sunspot Umbra
Authors: Cho, I. -H.; Cho, K. -S.; Bong, S. -C.; Moon, Y. -J.;
Nakariakov, V. M.; Park, J.; Baek, J. -H.; Choi, S.; Kim, Y. -H.;
Lee, J.
2017ApJ...837L..11C Altcode:
For 478 centrally located sunspots observed in the optical continuum
with Solar Dynamics Observatory/Helioseismic Magnetic Imager,
we perform seismological diagnostics of the physical parameters of
umbral photospheres. The new technique is based on the theory of slow
magnetoacoustic waves in a non-isothermally stratified photosphere
with a uniform vertical magnetic field. We construct a map of the
weighted frequency of three-minute oscillations inside the umbra
and use it for the estimation of the Alfvén speed, plasma-beta,
and mass density within the umbra. We find the umbral mean Alfvén
speed ranges between 10.5 and 7.5 km s<SUP>-1</SUP> and is negatively
correlated with magnetic field strength. The umbral mean plasma-beta
is found to range approximately between 0.65 and 1.15 and does not
vary significantly from pores to mature sunspots. The mean density
ranges between (1-6) × 10<SUP>-4</SUP> kg m<SUP>-3</SUP> and shows
a strong positive correlation with magnetic field strength.
---------------------------------------------------------
Title: Long-period quasi-periodic oscillations of a small-scale
magnetic structure on the Sun
Authors: Kolotkov, D. Y.; Smirnova, V. V.; Strekalova, P. V.;
Riehokainen, A.; Nakariakov, V. M.
2017A&A...598L...2K Altcode:
<BR /> Aims: Long-period quasi-periodic variations of the average
magnetic field in a small-scale magnetic structure on the Sun are
analysed. The structure is situated at the photospheric level and is
involved in a facula formation in the chromosphere. <BR /> Methods:
The observational signal obtained from the SDO/HMI line-of-sight
magnetograms of the target structure has a non-stationary behaviour,
and is therefore processed with the Hilbert-Huang Transform spectral
technique. <BR /> Results: The empirical decomposition of the original
signal and subsequent testing of the statistical significance of
its intrinsic modes reveal the presence of the white and pink noisy
components for the periods shorter and longer than 10 min, respectively,
and a significant oscillatory mode. The oscillation is found to have
a non-stationary period growing from approximately 80 to 230 min and
an increasing relative amplitude, while the mean magnetic field in the
oscillating structure is seen to decrease. The observed behaviour could
be interpreted either by the dynamical interaction of the structure
with the boundaries of supergranula cells in the region of interest
or in terms of the vortex shedding appearing during the magnetic
flux emergence.
---------------------------------------------------------
Title: Observation of a Short Period Quasi-periodic Pulsation in
Solar X-Ray, Microwave, and EUV Emissions
Authors: Kumar, Pankaj; Nakariakov, Valery M.; Cho, Kyung-Suk
2017ApJ...836..121K Altcode: 2017arXiv170102159K
This paper presents the multiwavelength analysis of a 13 s
quasi-periodic pulsation (QPP) observed in hard X-ray (12-300 keV)
and microwave (4.9-34 GHz) emissions during a C-class flare that
occurred on 2015 September 21. Atmospheric Image Assembly (AIA) 304
and 171 Å images show an emerging loop/flux tube (L1) moving radially
outward, which interacts with the preexisting structures within the
active region (AR). The QPP was observed during the expansion of and
rising motion of L1. The Nobeyama Radioheliograph microwave images in
17/34 GHz channels reveal a single radio source that was co-spatial
with a neighboring loop (L2). In addition, using AIA 304 Å images,
we detected intensity oscillations in the legs of L2 with a period
of about 26 s. A similar oscillation period was observed in the GOES
soft X-ray flux derivative. This oscillation period seems to increase
with time. We suggest that the observed QPP is most likely generated
by the interaction between L2 and L3 observed in the AIA hot channels
(131 and 94 Å). The merging speed of loops L2 and L3 was ∼35 km
s<SUP>-1</SUP>. L1 was destroyed possibly by its interaction with
preexisting structures in the AR, and produced a cool jet with the
speed of ∼106-118 km s<SUP>-1</SUP> associated with a narrow CME
(∼770 km s<SUP>-1</SUP>). Another mechanism of the QPP in terms of
a sausage oscillation of the loop (L2) is also possible.
---------------------------------------------------------
Title: Diagnostics of Coronal Heating in Active-region Loops
Authors: Fludra, A.; Hornsey, C.; Nakariakov, V. M.
2017ApJ...834..100F Altcode:
Understanding coronal heating remains a central problem in solar
physics. Many mechanisms have been proposed to explain how energy
is transferred to and deposited in the corona. We summarize past
observational studies that attempted to identify the heating mechanism
and point out the difficulties in reproducing the observations of
the solar corona from the heating models. The aim of this paper is
to study whether the observed extreme ultraviolet (EUV) emission in
individual coronal loops in solar active regions can provide constraints
on the volumetric heating function, and to develop a diagnostic for
the heating function for a subset of loops that are found close to
static thermal equilibrium. We reconstruct the coronal magnetic field
from Solar Dynamics Observatory/HMI data using a nonlinear force-free
magnetic field model. We model selected loops using a one-dimensional
stationary model, with a heating rate dependent locally on the magnetic
field strength along the loop, and we calculate the emission from
these loops in various EUV wavelengths for different heating rates. We
present a method to measure a power index β defining the dependence
of the volumetric heating rate E<SUB>H</SUB> on the magnetic field,
{E}<SUB>H</SUB>\propto {B}<SUP>β </SUP>, and controlling also
the shape of the heating function: concentrated near the loop top,
uniform and concentrated near the footpoints. The diagnostic is based
on the dependence of the electron density on the index β. This method
is free from the assumptions of the loop filling factor but requires
spectroscopic measurements of the density-sensitive lines. The range of
applicability for loops of different length and heating distributions
is discussed, and the steps to solving the coronal heating problem
are outlined.
---------------------------------------------------------
Title: Preface to Topical Issue: Waves in the Solar Corona: From
Microphysics to Macrophysics
Authors: Nakariakov, V. M.; Pascoe, D. J.; Sych, R.; van
Driel-Gesztelyi, L.
2016SoPh..291.3139N Altcode: 2016SoPh..tmp..187N
No abstract at ADS
---------------------------------------------------------
Title: Motion Magnification in Coronal Seismology
Authors: Anfinogentov, Sergey; Nakariakov, Valery M.
2016SoPh..291.3251A Altcode: 2016SoPh..tmp..180A; 2016arXiv161101790A
We introduce a new method for the investigation of low-amplitude
transverse oscillations of solar plasma non-uniformities, such
as coronal loops, individual strands in coronal arcades, jets,
prominence fibrils, polar plumes, and other contrast features that
have been observed with imaging instruments. The method is based on
the two-dimensional dual-tree complex wavelet transform (DTℂWT). It
allows us to magnify transverse, in the plane-of-the-sky, quasi-periodic
motions of contrast features in image sequences. The tests performed
on the artificial data cubes that imitated exponentially decaying,
multi-periodic and frequency-modulated kink oscillations of coronal
loops showed the effectiveness, reliability, and robustness of this
technique. The algorithm was found to give linear scaling of the
magnified amplitudes with the original amplitudes, provided these are
sufficiently small. In addition, the magnification is independent of
the oscillation period in a broad range of the periods. The application
of this technique to SDO/AIA EUV data cubes of a non-flaring active
region allowed for the improved detection of low-amplitude decay-less
oscillations in the majority of loops.
---------------------------------------------------------
Title: Quasi-periodic Acceleration of Electrons in the Flare on 2012
July 19
Authors: Huang, Jing; Kontar, Eduard P.; Nakariakov, Valery M.;
Gao, Guannan
2016ApJ...831..119H Altcode:
Quasi-periodic pulsations (QPPs) of nonthermal emission in an M7.7
class flare on 2012 July 19 are investigated with spatially resolved
observations at microwave and HXR bands and with spectral observations
at decimetric, metric waves. Microwave emission at 17 GHz of two
footpoints, HXR emission at 20-50 keV of the north footpoint and
loop top, and type III bursts at 0.7-3 GHz show prominent in-phase
oscillations at 270 s. The microwave emission of the loop leg has less
pulsation but stronger emission. Through the estimation of plasma
density around the loop top from EUV observations, we find that the
local plasma frequency would be 1.5 GHz or even higher. Thus, type
III bursts at 700 MHz originate above the loop top. Quasi-periodic
acceleration or injection of energetic electrons is proposed to
dominate these in-phase QPPs of nonthermal emission from footpoints,
loop top, and above. In the overlying region, drifting pulsations
(DPS) at 200-600 MHz oscillate at a distinct period (200 s). Its global
structure drifts toward lower frequency, which is closely related to
upward plasmoids observed simultaneously from EUV emission. Hence,
nonthermal emission from overlying plasmoids and underlying flaring
loops show different oscillating periods. Two individual systems of
quasi-periodic acceleration of electrons are proposed to coincide in
the bi-direction outflows from the reconnection region.
---------------------------------------------------------
Title: Dependence of Occurrence Rates of Solar Flares and Coronal
Mass Ejections on the Solar Cycle Phase and the Importance of
Large-scale Connectivity
Authors: Lee, Kangjin; Moon, Y. -J.; Nakariakov, V. M.
2016ApJ...831..131L Altcode:
We investigate the dependence of the occurrence rates of major solar
flares (M- and X-class) and front-side halo coronal mass ejections
(FHCMEs), observed from 1996 to 2013, on the solar cycle (SC) phase
for six active McIntosh sunspot group classes: Fkc, Ekc, Dkc, Fki,
Eki, and Dki. We classify SC phases as follows: (1) ascending phase
of SC 23 (1996-1999), (2) maximum phase of SC 23 (2000-2002), (3)
descending phase of SC 23 (2003-2008), and (4) ascending phase of SC
24 (2009-2013). We find that the occurrence rates of major flares and
FHCMEs during the descending phase are noticeably higher than those
during the other phases for most sunspot group classes. For the most
active sunspot group class, Fkc, the occurrence rate of FHCMEs during
the descending phase of SC 23 is three times as high as that during
the ascending phase of SC 23. The potential of each McIntosh sunspot
group class to produce major flares or FHCMEs is found to depend on
the SC phase. The occurrence rates (R) of major flares and FHCMEs
are strongly anti-correlated with the annual average latitude of the
sunspot groups (L): R∼ {L}<SUP>-2.07</SUP> for major flares and R∼
{L}<SUP>-2.42</SUP> for FHCMEs. This finding indicates the possible
role of large-scale coronal connectivity, e.g., trans-equatorial loops,
in powerful energy releases. Interestingly, this relationship is very
similar to that between the volumetric coronal heating rate and X-ray
loop lengths, indicating common energy release mechanisms.
---------------------------------------------------------
Title: Observation of quasi-periodic solar radio bursts associated
with propagating fast-mode waves
Authors: Goddard, C. R.; Nisticò, G.; Nakariakov, V. M.; Zimovets,
I. V.; White, S. M.
2016A&A...594A..96G Altcode: 2016arXiv160804232G
<BR /> Aims: Radio emission observations from the Learmonth and Bruny
Island radio spectrographs are analysed to determine the nature of
a train of discrete, periodic radio "sparks" (finite-bandwidth,
short-duration isolated radio features) which precede a type II
burst. We analyse extreme ultraviolet (EUV) imaging from SDO/AIA
at multiple wavelengths and identify a series of quasi-periodic
rapidly-propagating enhancements, which we interpret as a fast
wave train, and link these to the detected radio features. <BR />
Methods: The speeds and positions of the periodic rapidly propagating
fast waves and the coronal mass ejection (CME) were recorded using
running-difference images and time-distance analysis. From the frequency
of the radio sparks the local electron density at the emission location
was estimated for each. Using an empirical model for the scaling of
density in the corona, the calculated electron density was used to
obtain the height above the surface at which the emission occurs, and
the propagation velocity of the emission location. <BR /> Results: The
period of the radio sparks, δt<SUB>r</SUB> = 1.78 ± 0.04 min, matches
the period of the fast wave train observed at 171 Å, δt<SUB>EUV</SUB>
= 1.7 ± 0.2 min. The inferred speed of the emission location of the
radio sparks, 630 km s<SUP>-1</SUP>, is comparable to the measured
speed of the CME leading edge, 500 km s<SUP>-1</SUP>, and the speeds
derived from the drifting of the type II lanes. The calculated height
of the radio emission (obtained from the density) matches the observed
location of the CME leading edge. From the above evidence we propose
that the radio sparks are caused by the quasi-periodic fast waves, and
the emission is generated as they catch up and interact with the leading
edge of the CME. <P />The movie associated to Fig. 2 is available at <A
href="http://www.aanda.org/10.1051/0004-6361/201628478/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Comparison of Damped Oscillations in Solar and Stellar
X-Ray flares
Authors: Cho, I. -H.; Cho, K. -S.; Nakariakov, V. M.; Kim, S.;
Kumar, P.
2016ApJ...830..110C Altcode:
We explore the similarity and difference of the quasi-periodic
pulsations (QPPs) observed in the decay phase of solar and stellar
flares at X-rays. We identified 42 solar flares with pronounced QPPs,
observed with RHESSI, and 36 stellar flares with QPPs, observed
with XMM-Newton. The empirical mode decomposition (EMD) method and
least-squares fit by a damped sine function were applied to obtain
the periods (P) and damping times (τ) of the QPPs. We found that (1)
the periods and damping times of the stellar QPPs are 16.21 ± 15.86
minutes and 27.21 ± 28.73 minutes, while those of the solar QPPs are
0.90 ± 0.56 and 1.53 ± 1.10 minutes, respectively; (2) the ratios of
the damping times to the periods (τ /P) observed in the stellar QPPs
(1.69 ± 0.56) are statistically identical to those of solar QPPs
(1.74 ± 0.77) and (3) the scalings of the QPP damping time with the
period are well described by the power law in both solar and stellar
cases. The power indices of the solar and stellar QPPs are 0.96 ±
0.10 and 0.98+/- 0.05, respectively. This scaling is consistent with
the scalings found for standing slow magnetoacoustic and kink modes in
solar coronal loops. Thus, we propose that the underlying mechanism
responsible for the stellar QPPs is the natural magnetohydrodynamic
oscillation in the flaring or adjacent coronal loops, as in the case
of solar flares.
---------------------------------------------------------
Title: Spatially resolved observation of the fundamental and second
harmonic standing kink modes using SDO/AIA
Authors: Pascoe, D. J.; Goddard, C. R.; Nakariakov, V. M.
2016A&A...593A..53P Altcode:
<BR /> Aims: We consider a coronal loop kink oscillation observed by
the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory
(SDO) which demonstrates two strong spectral components. The period of
the lower frequency component being approximately twice that of the
shorter frequency component suggests the presence of harmonics. <BR
/> Methods: We examine the presence of two longitudinal harmonics
by investigating the spatial dependence of the loop oscillation. The
time-dependent displacement of the loop is measured at 15 locations
along the loop axis. For each position the displacement is fitted as
the sum of two damped sinusoids, having periods P<SUB>1</SUB> and
P<SUB>2</SUB>, and a damping time τ. The shorter period component
exhibits anti-phase oscillations in the loop legs. <BR /> Results:
We interpret the observation in terms of the first (global or
fundamental) and second longitudinal harmonics of the standing kink
mode. The strong excitation of the second harmonic appears connected
to the preceding coronal mass ejection (CME) which displaced one of
the loop legs. The oscillation parameters found are P<SUB>1</SUB>
= 5.00±0.62 min, P<SUB>2</SUB> = 2.20±0.23 min, P<SUB>1</SUB>/
2P<SUB>2</SUB> = 1.15±0.22, and τ/P = 3.35 ± 1.45. <P />A
movie associated to Fig. 5 is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201628784/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Standing sausage modes in curved coronal slabs
Authors: Pascoe, D. J.; Nakariakov, V. M.
2016A&A...593A..52P Altcode:
Context. Magnetohydrodynamic waveguides such as dense coronal loops
can support standing modes. The ratios of the periods of oscillations
for different longitudinal harmonics depend on the dispersive
nature of the waveguide and so may be used as a seismological tool to
determine coronal parameters. <BR /> Aims: We extend models of standing
sausage modes in low β coronal loops to include the effects of loop
curvature. The behaviour of standing sausage modes in this geometry is
used to explain the properties of observed oscillations that cannot be
accounted for using straight loop models. <BR /> Methods: We perform
2D numerical simulations of an oscillating coronal loop, modelled
as a dense slab embedded in a potential magnetic field. The loop is
field-aligned and so experiences expansion with height in addition
to being curved. Standing sausage modes are excited by compressive
perturbations of the loop and their properties are studied. <BR />
Results: The spatial profiles of standing sausage modes are found to be
modified by the expanding loop geometry typical for flaring loops and
modelled by a potential magnetic field in our simulations. Longitudinal
harmonics of order n > 1 have anti-nodes that are shifted towards
the loop apex and the amplitude of anti-nodes near the loop apex is
smaller than those near the loop footpoints. <BR /> Conclusions: We
find that the observation of standing sausage modes by the Nobeyama
Radioheliograph in a flaring coronal loop on 12 January 2000 is
consistent with interpretation in terms of the global mode (n = 1)
and third harmonic (n = 3). This interpretation accounts for the period
ratio and spatial structure of the observed oscillations.
---------------------------------------------------------
Title: Empirical mode decomposition analysis of random processes in
the solar atmosphere
Authors: Kolotkov, D. Y.; Anfinogentov, S. A.; Nakariakov, V. M.
2016A&A...592A.153K Altcode:
Context. Coloured noisy components with a power law spectral energy
distribution are often shown to appear in solar signals of various
types. Such a frequency-dependent noise may indicate the operation
of various randomly distributed dynamical processes in the solar
atmosphere. <BR /> Aims: We develop a recipe for the correct usage of
the empirical mode decomposition (EMD) technique in the presence of
coloured noise, allowing for clear distinguishing between quasi-periodic
oscillatory phenomena in the solar atmosphere and superimposed random
background processes. For illustration, we statistically investigate
extreme ultraviolet (EUV) emission intensity variations observed
with SDO/AIA in the coronal (171 Å), chromospheric (304 Å), and
upper photospheric (1600 Å) layers of the solar atmosphere, from
a quiet sun and a sunspot umbrae region. <BR /> Methods: EMD has
been used for analysis because of its adaptive nature and essential
applicability to the processing non-stationary and amplitude-modulated
time series. For the comparison of the results obtained with EMD,
we use the Fourier transform technique as an etalon. <BR /> Results:
We empirically revealed statistical properties of synthetic coloured
noises in EMD, and suggested a scheme that allows for the detection of
noisy components among the intrinsic modes obtained with EMD in real
signals. Application of the method to the solar EUV signals showed that
they indeed behave randomly and could be represented as a combination
of different coloured noises characterised by a specific value of
the power law indices in their spectral energy distributions. On
the other hand, 3-min oscillations in the analysed sunspot were
detected to have energies significantly above the corresponding noise
level. <BR /> Conclusions: The correct accounting for the background
frequency-dependent random processes is essential when using EMD for
analysis of oscillations in the solar atmosphere. For the quiet sun
region the power law index was found to increase with height above
the photosphere, indicating that the higher frequency processes are
trapped deeper in the quiet sun atmosphere. In contrast, lower levels
of the sunspot umbrae were found to be characterised by higher values
of the power law index, meaning the domination of lower frequencies
deep inside the sunspot atmosphere. Comparison of the EMD results with
those obtained with the Fourier transform showed good consistency,
justifying the applicability of EMD.
---------------------------------------------------------
Title: Statistically Determined Dispersion Relations of Magnetic
Field Fluctuations in the Terrestrial Foreshock
Authors: Hnat, B.; O'Connell, D.; Nakariakov, V. M.; Sundberg, T.
2016ApJ...827...91H Altcode: 2016arXiv160108054H
We obtain dispersion relations of magnetic field fluctuations for two
crossings of the terrestrial foreshock by Cluster spacecraft. These
crossings cover plasma conditions that differ significantly in their
plasma β and in the density of the reflected ion beam, but not in the
properties of the encountered ion population, both showing shell-like
distribution function. Dispersion relations are reconstructed using
two-point instantaneous wave number estimations from pairs of Cluster
spacecraft. The accessible range of wave vectors, limited by the
available spacecraft separations, extends to ≈2 × 10<SUP>4</SUP>
km. Results show multiple branches of dispersion relations, associated
with different powers of magnetic field fluctuations. We find that
sunward propagating fast magnetosonic waves and beam resonant modes
are dominant for the high plasma β interval with a dense beam, while
the dispersions of the interval with low beam density include Alfvén
and fast magnetosonic modes propagating sunward and anti-sunward.
---------------------------------------------------------
Title: Statistical properties of quasi-periodic pulsations in
white-light flares observed with Kepler
Authors: Pugh, C. E.; Armstrong, D. J.; Nakariakov, V. M.; Broomhall,
A. -M.
2016MNRAS.459.3659P Altcode: 2016arXiv160403018P; 2016MNRAS.tmp..660P
We embark on a study of quasi-periodic pulsations (QPPs) in the decay
phase of white-light stellar flares observed by Kepler. Out of the
1439 flares on 216 different stars detected in the short-cadence data
using an automated search, 56 flares are found to have pronounced
QPP-like signatures in the light curve, of which 11 have stable
decaying oscillations. No correlation is found between the QPP period
and the stellar temperature, radius, rotation period and surface
gravity, suggesting that the QPPs are independent of global stellar
parameters. Hence they are likely to be the result of processes
occurring in the local environment. There is also no significant
correlation between the QPP period and flare energy, however there is
evidence that the period scales with the QPP decay time for the Gaussian
damping scenario, but not to a significant degree for the exponentially
damped case. This same scaling has been observed for MHD oscillations
on the Sun, suggesting that they could be the cause of the QPPs in
those flares. Scaling laws of the flare energy are also investigated,
supporting previous reports of a strong correlation between the flare
energy and stellar temperature/radius. A negative correlation between
the flare energy and stellar surface gravity is also found.
---------------------------------------------------------
Title: Effect of a Sausage Oscillation on Radio Zebra-pattern
Structures in a Solar Flare
Authors: Yu, Sijie; Nakariakov, V. M.; Yan, Yihua
2016ApJ...826...78Y Altcode: 2016arXiv160804289Y
Sausage modes that are axisymmetric fast magnetoacoustic oscillations
of solar coronal loops are characterized by variation of the plasma
density and magnetic field, and hence cause time variations of
the electron plasma frequency and cyclotron frequency. The latter
parameters determine the condition for the double plasma resonance
(DPR), which is responsible for the appearance of zebra-pattern (ZP)
structures in time spectra of solar type IV radio bursts. We perform
numerical simulations of standing and propagating sausage oscillations
in a coronal loop modeled as a straight, field-aligned plasma slab, and
determine the time variation of the DPR layer locations. Instant values
of the plasma density and magnetic field at the DPR layers allowed us
to construct skeletons of the time variation of ZP stripes in radio
spectra. In the presence of a sausage oscillation, the ZP structures are
shown to have characteristic wiggles with the time period prescribed by
the sausage oscillation. Standing and propagating sausage oscillations
are found to have different signatures in ZP patterns. We conclude
that ZP wiggles can be used for the detection of short-period sausage
oscillations and the exploitation of their seismological potential.
---------------------------------------------------------
Title: Effect of a Radiation Cooling and Heating Function on Standing
Longitudinal Oscillations in Coronal Loops
Authors: Kumar, S.; Nakariakov, V. M.; Moon, Y. -J.
2016ApJ...824....8K Altcode: 2016arXiv160308335K
Standing long-period (with periods longer than several minutes)
oscillations in large, hot (with a temperature higher than 3 MK)
coronal loops have been observed as the quasi-periodic modulation of
the EUV and microwave intensity emission and the Doppler shift of
coronal emission lines, and they have been interpreted as standing
slow magnetoacoustic (longitudinal) oscillations. Quasi-periodic
pulsations of shorter periods, detected in thermal and non-thermal
emissions in solar flares could be produced by a similar mechanism. We
present theoretical modeling of the standing slow magnetoacoustic
mode, showing that this mode of oscillation is highly sensitive
to peculiarities of the radiative cooling and heating function. We
generalized the theoretical model of standing slow magnetoacoustic
oscillations in a hot plasma, including the effects of the radiative
losses and accounting for plasma heating. The heating mechanism is
not specified and taken empirically to compensate the cooling by
radiation and thermal conduction. It is shown that the evolution of
the oscillations is described by a generalized Burgers equation. The
numerical solution of an initial value problem for the evolutionary
equation demonstrates that different dependences of the radiative
cooling and plasma heating on the temperature lead to different regimes
of the oscillations, including growing, quasi-stationary, and rapidly
decaying. Our findings provide a theoretical foundation for probing the
coronal heating function and may explain the observations of decayless
long-period, quasi-periodic pulsations in flares. The hydrodynamic
approach employed in this study should be considered with caution in
the modeling of non-thermal emission associated with flares, because
it misses potentially important non-hydrodynamic effects.
---------------------------------------------------------
Title: Undamped transverse oscillations of coronal loops as a
self-oscillatory process
Authors: Nakariakov, V. M.; Anfinogentov, S. A.; Nisticò, G.; Lee,
D. -H.
2016A&A...591L...5N Altcode:
Context. Standing transverse oscillations of coronal loops are
observed to operate in two regimes: rapidly decaying, large amplitude
oscillations and undamped small amplitude oscillations. In the
latter regime the damping should be compensated by energy supply,
which allows the loop to perform almost monochromatic oscillations
with almost constant amplitude and phase. Different loops oscillate
with different periods. The oscillation amplitude does not show
dependence on the loop length or the oscillation period. <BR /> Aims:
We aim to develop a low-dimensional model explaining the undamped kink
oscillations as a self-oscillatory process caused by the effect of
negative friction. The source of energy is an external quasi-steady
flow, for example, supergranulation motions near the loop footpoints
or external flows in the corona. <BR /> Methods: We demonstrate that
the interaction of a quasi-steady flow with a loop can be described
by a Rayleigh oscillator equation that is a non-linear ordinary
differential equation, with the damping and resonant terms determined
empirically. <BR /> Results: Small-amplitude self-oscillatory solutions
to the Rayleigh oscillator equation are harmonic signals of constant
amplitude, which is consistent with the observed properties of undamped
kink oscillations. The period of self-oscillations is determined by
the frequency of the kink mode. The damping by dissipation and mode
conversion is compensated by the continuous energy deposition at the
frequency of the natural oscillation. <BR /> Conclusions: We propose
that undamped kink oscillations of coronal loops may be caused by
the interaction of the loops with quasi-steady flows, and hence are
self-oscillations, which is analogous to producing a tune by moving
a bow across a violin string.
---------------------------------------------------------
Title: Coronal loop seismology using damping of standing kink
oscillations by mode coupling
Authors: Pascoe, D. J.; Goddard, C. R.; Nisticò, G.; Anfinogentov,
S.; Nakariakov, V. M.
2016A&A...589A.136P Altcode:
Context. Kink oscillations of solar coronal loops are frequently
observed to be strongly damped. The damping can be explained by mode
coupling on the condition that loops have a finite inhomogeneous layer
between the higher density core and lower density background. The
damping rate depends on the loop density contrast ratio and
inhomogeneous layer width. <BR /> Aims: The theoretical description
for mode coupling of kink waves has been extended to include the
initial Gaussian damping regime in addition to the exponential
asymptotic state. Observation of these damping regimes would provide
information about the structuring of the coronal loop and so provide
a seismological tool. <BR /> Methods: We consider three examples
of standing kink oscillations observed by the Atmospheric Imaging
Assembly (AIA) of the Solar Dynamics Observatory (SDO) for which
the general damping profile (Gaussian and exponential regimes) can be
fitted. Determining the Gaussian and exponential damping times allows us
to perform seismological inversions for the loop density contrast ratio
and the inhomogeneous layer width normalised to the loop radius. The
layer width and loop minor radius are found separately by comparing
the observed loop intensity profile with forward modelling based on
our seismological results. <BR /> Results: The seismological method
which allows the density contrast ratio and inhomogeneous layer width
to be simultaneously determined from the kink mode damping profile has
been applied to observational data for the first time. This allows the
internal and external Alfvén speeds to be calculated, and estimates
for the magnetic field strength can be dramatically improved using the
given plasma density. <BR /> Conclusions: The kink mode damping rate
can be used as a powerful diagnostic tool to determine the coronal loop
density profile. This information can be used for further calculations
such as the magnetic field strength or phase mixing rate.
---------------------------------------------------------
Title: Transverse oscillations and stability of prominences in a
magnetic field dip
Authors: Kolotkov, D. Y.; Nisticò, G.; Nakariakov, V. M.
2016A&A...590A.120K Altcode:
<BR /> Aims: We developed an analytical model of the global transverse
oscillations and mechanical stability of a quiescent prominence in
the magnetised environment with a magnetic field dip that accounts
for the mirror current effect. <BR /> Methods: The model is based on
the interaction of line currents through the Lorentz force. Within
this concept the prominence is treated as a straight current-carrying
wire, and the magnetic dip is provided by two photospheric current
sources. <BR /> Results: Properties of both vertical and horizontal
oscillations are determined by the value of the prominence current,
its density and height above the photosphere, and the parameters of
the magnetic dip. The prominence can be stable in both horizontal
and vertical directions simultaneously when the prominence current
dominates in the system and its height is less than the half-distance
between the photospheric sources.
---------------------------------------------------------
Title: Dependence of kink oscillation damping on the amplitude
Authors: Goddard, C. R.; Nakariakov, V. M.
2016A&A...590L...5G Altcode:
Context. Kink oscillations of coronal loops are one of the most
intensively studied oscillatory phenomena in the solar corona. In the
large-amplitude rapidly damped regime, these oscillations are observed
to have a low quality factor with only a few cycles of oscillation
detected before they are damped. The specific mechanism responsible
for rapid damping is commonly accepted to be associated with the
linear coupling between collective kink oscillations and localised
torsional oscillations, the phenomenon of resonant absorption of the
kink mode. The role of finite amplitude effects, however, is still not
clear. <BR /> Aims: We investigated the empirical dependence of the
kink oscillation damping time and its quality factor, which is defined
as the ratio of damping time to oscillation period, on the oscillation
amplitude. <BR /> Methods: We analysed decaying kink oscillation events
detected previously with TRACE, SDO/AIA and and STEREO/EUVI in the
extreme ultraviolet (EUV) 171 Å band. <BR /> Results: We found that
the ratio of the kink oscillation damping time to the oscillation period
systematically decreases with the oscillation amplitude. We approximated
the quality factor dependence on the oscillation displacement amplitude
via the power-law dependence with the exponent of -1/2, however we
stress that this is a by-eye estimate, and a more rigorous estimation
of the scaling law requires more accurate measurements and increased
statistics. We conclude that damping of kink oscillations of coronal
loops depends on the oscillation amplitude, indicating the possible
role of non-linear mechanisms for damping.
---------------------------------------------------------
Title: Observation of a Quasiperiodic Pulsation in Hard X-Ray, Radio,
and Extreme-ultraviolet Wavelengths
Authors: Kumar, Pankaj; Nakariakov, Valery M.; Cho, Kyung-Suk
2016ApJ...822....7K Altcode: 2016arXiv160303121K
We present a multiwavelength analysis of a quasiperiodic pulsation (QPP)
observed in the hard X-ray (HXR), radio, and extreme-ultraviolet (EUV)
channels during an M1.9 flare that occurred on 2011 September 23-24. The
nonthermal HXR emission in 25-50 keV observed by RHESSI shows five
distinct impulsive peaks of decaying amplitude with a period of about 3
minutes. A similar QPP was observed in the microwave emission recorded
by the Nobeyama Radioheliograph and Polarimeter in the 2, 3.75, 9.4,
and 17 GHz channels. Interestingly, the 3-minute QPP was also observed
in the metric and decimetric radio frequencies (25-180, 245, 610 MHz) as
repetitive type III bursts. Multiwavelength observations from the Solar
Dynamics Observatory/Atmospheric Image Assembly, Hinode/SOT, and Solar
TErrestrial RElations Observatory/SECCHI suggest a fan-spine topology
at the eruption site, associated with the formation of a quasi-circular
ribbon during the flare. A small filament was observed below the fan
loops before the flare onset. The filament rose slowly and interacted
with the ambient field. This behavior was followed by an untwisting
motion of the filament. Two different structures of the filament
showed an approximately 3-minute periodic alternate rotation in the
clockwise and counterclockwise directions. The 3-minute QPP was found
to highly correlate with 3-minute oscillations in a nearby sunspot. We
suggest that the periodic reconnection (modulated either by a sunspot
slow-mode wave or by an untwisting filament) at a magnetic null point
most likely causes the repetitive particle acceleration, generating
the QPP observed in HXR, microwave, and type III radio bursts.
---------------------------------------------------------
Title: Solar Science with the Atacama Large Millimeter/Submillimeter
Array—A New View of Our Sun
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.;
Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu,
B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin,
P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz,
A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.;
Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary,
D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van
der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.;
Selhorst, C. L.; Barta, M.
2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W
The Atacama Large Millimeter/submillimeter Array (ALMA) is a new
powerful tool for observing the Sun at high spatial, temporal, and
spectral resolution. These capabilities can address a broad range
of fundamental scientific questions in solar physics. The radiation
observed by ALMA originates mostly from the chromosphere—a complex
and dynamic region between the photosphere and corona, which plays a
crucial role in the transport of energy and matter and, ultimately,
the heating of the outer layers of the solar atmosphere. Based on
first solar test observations, strategies for regular solar campaigns
are currently being developed. State-of-the-art numerical simulations
of the solar atmosphere and modeling of instrumental effects can help
constrain and optimize future observing modes for ALMA. Here we present
a short technical description of ALMA and an overview of past efforts
and future possibilities for solar observations at submillimeter and
millimeter wavelengths. In addition, selected numerical simulations
and observations at other wavelengths demonstrate ALMA's scientific
potential for studying the Sun for a large range of science cases.
---------------------------------------------------------
Title: Magnetohydrodynamic Oscillations in the Solar Corona and
Earth's Magnetosphere: Towards Consolidated Understanding
Authors: Nakariakov, V. M.; Pilipenko, V.; Heilig, B.; Jelínek,
P.; Karlický, M.; Klimushkin, D. Y.; Kolotkov, D. Y.; Lee, D. -H.;
Nisticò, G.; Van Doorsselaere, T.; Verth, G.; Zimovets, I. V.
2016SSRv..200...75N Altcode: 2016SSRv..tmp....2N
Magnetohydrodynamic (MHD) oscillatory processes in different
plasma systems, such as the corona of the Sun and the Earth's
magnetosphere, show interesting similarities and differences, which
so far received little attention and remain under-exploited. The
successful commissioning within the past ten years of THEMIS, Hinode,
STEREO and SDO spacecraft, in combination with matured analysis of data
from earlier spacecraft (Wind, SOHO, ACE, Cluster, TRACE and RHESSI)
makes it very timely to survey the breadth of observations giving
evidence for MHD oscillatory processes in solar and space plasmas,
and state-of-the-art theoretical modelling. The paper reviews several
important topics, such as Alfvénic resonances and mode conversion;
MHD waveguides, such as the magnetotail, coronal loops, coronal
streamers; mechanisms for periodicities produced in energy releases
during substorms and solar flares, possibility of Alfvénic resonators
along open field lines; possible drivers of MHD waves; diagnostics of
plasmas with MHD waves; interaction of MHD waves with partly-ionised
boundaries (ionosphere and chromosphere). The review is mainly oriented
to specialists in magnetospheric physics and solar physics, but not
familiar with specifics of the adjacent research fields.
---------------------------------------------------------
Title: Low-Frequency Waves in Space Plasmas
Authors: Keiling, Andreas; Lee, Dong-Hun; Nakariakov, Valery
2016GMS...216.....K Altcode:
Low-frequency waves in space plasmas have been studied for several
decades, and our knowledge gain has been incremental with several
paradigm-changing leaps forward. In our solar system, such waves
occur in the ionospheres and magnetospheres of planets, and around our
Moon. They occur in the solar wind, and more recently, they have been
confirmed in the Sun's atmosphere as well. The goal of wave research
is to understand their generation, their propagation, and their
interaction with the surrounding plasma. Low-frequency Waves in Space
Plasmas presents a concise and authoritative up-to-date look on where
wave research stands: What have we learned in the last decade? What
are unanswered questions? <P />While in the past waves in different
astrophysical plasmas have been largely treated in separate books,
the unique feature of this monograph is that it covers waves in many
plasma regions, including: <P />Waves in geospace, including ionosphere
and magnetosphere <P />Waves in planetary magnetospheres <P />Waves
at the Moon <P />Waves in the solar wind <P />Waves in the solar
atmosphere <P />Because of the breadth of topics covered, this volume
should appeal to a broad community of space scientists and students,
and it should also be of interest to astronomers/astrophysicists who
are studying space plasmas beyond our Solar System.
---------------------------------------------------------
Title: A statistical study of decaying kink oscillations detected
using SDO/AIA
Authors: Goddard, C. R.; Nisticò, G.; Nakariakov, V. M.; Zimovets,
I. V.
2016A&A...585A.137G Altcode: 2015arXiv151103558G
Context. Despite intensive studies of kink oscillations of coronal
loops in the last decade, a large-scale statistically significant
investigation of the oscillation parameters has not been made using
data from the Solar Dynamics Observatory (SDO). <BR /> Aims: We carry
out a statistical study of kink oscillations using extreme ultraviolet
imaging data from a previously compiled catalogue. <BR /> Methods:
We analysed 58 kink oscillation events observed by the Atmospheric
Imaging Assembly (AIA) on board SDO during its first four years of
operation (2010-2014). Parameters of the oscillations, including the
initial apparent amplitude, period, length of the oscillating loop,
and damping are studied for 120 individual loop oscillations. <BR />
Results: Analysis of the initial loop displacement and oscillation
amplitude leads to the conclusion that the initial loop displacement
prescribes the initial amplitude of oscillation in general. The
period is found to scale with the loop length, and a linear fit of
the data cloud gives a kink speed of C<SUB>k</SUB> = (1330 ± 50) km
s<SUP>-1</SUP>. The main body of the data corresponds to kink speeds in
the range C<SUB>k</SUB> = (800-3300) km s<SUP>-1</SUP>. Measurements
of 52 exponential damping times were made, and it was noted that
at least 21 of the damping profiles may be better approximated by a
combination of non-exponential and exponential profiles rather than a
purely exponential damping envelope. There are nine additional cases
where the profile appears to be purely non-exponential and no damping
time was measured. A scaling of the exponential damping time with the
period is found, following the previously established linear scaling
between these two parameters.
---------------------------------------------------------
Title: ALMA Observations of the Sun in Cycle 4 and Beyond
Authors: Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.;
Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.;
Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson,
M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.;
Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov,
V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões,
P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig,
A.; White, S.; Yagoubov, P.; Zaqarashvili, T.
2016arXiv160100587W Altcode:
This document was created by the Solar Simulations for the Atacama
Large Millimeter Observatory Network (SSALMON) in preparation of
the first regular observations of the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA), which are anticipated to start
in ALMA Cycle 4 in October 2016. The science cases presented here
demonstrate that a large number of scientifically highly interesting
observations could be made already with the still limited solar
observing modes foreseen for Cycle 4 and that ALMA has the potential
to make important contributions to answering long-standing scientific
questions in solar physics. With the proposal deadline for ALMA Cycle
4 in April 2016 and the Commissioning and Science Verification campaign
in December 2015 in sight, several of the SSALMON Expert Teams composed
strategic documents in which they outlined potential solar observations
that could be feasible given the anticipated technical capabilities
in Cycle 4. These documents have been combined and supplemented
with an analysis, resulting in recommendations for solar observing
with ALMA in Cycle 4. In addition, the detailed science cases also
demonstrate the scientific priorities of the solar physics community
and which capabilities are wanted for the next observing cycles. The
work on this White Paper effort was coordinated in close cooperation
with the two international solar ALMA development studies led by
T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be
further updated until the beginning of Cycle 4 in October 2016. In
particular, we plan to adjust the technical capabilities of the solar
observing modes once finally decided and to further demonstrate the
feasibility and scientific potential of the included science cases by
means of numerical simulations of the solar atmosphere and corresponding
simulated ALMA observations.
---------------------------------------------------------
Title: Damping profile of standing kink oscillations observed
by SDO/AIA
Authors: Pascoe, D. J.; Goddard, C. R.; Nisticò, G.; Anfinogentov,
S.; Nakariakov, V. M.
2016A&A...585L...6P Altcode:
<BR /> Aims: Strongly damped standing and propagating kink oscillations
are observed in the solar corona. This can be understood in terms
of mode coupling, which causes the wave energy to be converted from
the bulk transverse oscillation to localised, unresolved azimuthal
motions. The damping rate can provide information about the loop
structure, and theory predicts two possible damping profiles. <BR />
Methods: We used the recently compiled catalogue of decaying standing
kink oscillations of coronal loops to search for examples with high
spatial and temporal resolution and sufficient signal quality to allow
the damping profile to be examined. The location of the loop axis
was tracked, detrended, and fitted with sinusoidal oscillations with
Gaussian and exponential damping profiles. <BR /> Results: Using the
highest quality data currently available, we find that for the majority
of our cases a Gaussian profile describes the damping behaviour at
least as well as an exponential profile, which is consistent with the
recently developed theory for the damping profile due to mode coupling.
---------------------------------------------------------
Title: Solar cycle variations in the powers and damping rates of
low-degree solar acoustic oscillations
Authors: Broomhall, A. -M.; Pugh, C. E.; Nakariakov, V. M.
2015AdSpR..56.2706B Altcode:
Helioseismology uses the Sun's natural resonant oscillations to study
the solar interior. The properties of the solar oscillations are
sensitive to the Sun'2019;s magnetic activity cycle. Here we examine
variations in the powers, damping rates, and energy supply rates of the
most prominent acoustic oscillations in unresolved, Sun-as-a-star data,
obtained by the Birmingham Solar Oscillations Network (BiSON) during
solar cycles 22, 23, and the first half of 24. The variations in the
helioseismic parameters are compared to the 10.7 cm flux, a well-known
global proxy of solar activity. As expected the oscillations are most
heavily damped and the mode powers are at a minimum at solar activity
maximum. The 10.7 cm flux was linearly regressed using the fractional
variations of damping rates and powers observed during cycle 23. In
general, good agreement is found between the damping rates and the 10.7
cm flux. However, the linearly regressed 10.7 cm flux and fractional
variation in powers diverge in cycles 22 and 24, indicating that
the relationship between the mode powers and the 10.7 cm flux is not
consistent from one cycle to the next. The energy supply rate of the
oscillations, which is usually approximately constant, also decreases
at this time. We have determined that this discrepancy is not because
of the first-order bias introduced by an increase in the level of
background noise or gaps in the data. Although we cannot categorically
rule out an instrumental origin, the divergence observed in cycle 24,
when the data were of high quality and the data coverage was over 80%,
raises the possibility that the effect may be solar in origin.
---------------------------------------------------------
Title: SSALMON - The Solar Simulations for the Atacama Large
Millimeter Observatory Network
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Hudson,
H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E.; De Pontieu,
B.; Tiwari, S.; Kato, Y.; Soler, R.; Yagoubov, P.; Black, J. H.;
Antolin, P.; Gunár, S.; Labrosse, N.; Benz, A. O.; Nindos, A.;
Steffen, M.; Scullion, E.; Doyle, J. G.; Zaqarashvili, T.; Hanslmeier,
A.; Nakariakov, V. M.; Heinzel, P.; Ayres, T.; Karlicky, M.
2015AdSpR..56.2679W Altcode: 2015arXiv150205601W
The Solar Simulations for the Atacama Large Millimeter Observatory
Network (SSALMON) was initiated in 2014 in connection with two ALMA
development studies. The Atacama Large Millimeter/submillimeter Array
(ALMA) is a powerful new tool, which can also observe the Sun at
high spatial, temporal, and spectral resolution. The international
SSALMONetwork aims at co-ordinating the further development of solar
observing modes for ALMA and at promoting scientific opportunities
for solar physics with particular focus on numerical simulations,
which can provide important constraints for the observing modes and
can aid the interpretation of future observations. The radiation
detected by ALMA originates mostly in the solar chromosphere - a
complex and dynamic layer between the photosphere and corona, which
plays an important role in the transport of energy and matter and the
heating of the outer layers of the solar atmosphere. Potential targets
include active regions, prominences, quiet Sun regions, flares. Here,
we give a brief overview over the network and potential science cases
for future solar observations with ALMA.
---------------------------------------------------------
Title: Preface: Advances in solar physics
Authors: Georgoulis, Manolis K.; Nakariakov, Valery M.
2015AdSpR..56.2677G Altcode:
The idea for this special issue of Advances in Space Research (ASR)
was formulated during the 14th European Solar Physics Meeting (ESPM-14)
that took place in Dublin, Ireland in September 2014. Since ASR does not
publish conference proceedings, it was decided to extend a general call
to the international solar-physics community for manuscripts pertinent
to the following thematic areas: <P />New and upcoming heliospheric
observational and data assimilation facilities.
---------------------------------------------------------
Title: Fast Magnetoacoustic Wave Trains of Sausage Symmetry in
Cylindrical Waveguides of the Solar Corona
Authors: Shestov, S.; Nakariakov, V. M.; Kuzin, S.
2015ApJ...814..135S Altcode: 2015arXiv151007908S
Fast magnetoacoustic waves guided along the magnetic field by plasma
non-uniformities, in particular coronal loops, fibrils, and plumes,
are known to be highly dispersive, which lead to the formation of
quasi-periodic wave trains excited by a broadband impulsive driver,
e.g., a solar flare. We investigated the effects of cylindrical
geometry on the fast sausage wave train formation. We performed
magnetohydrodynamic numerical simulations of fast magnetoacoustic
perturbations of a sausage symmetry, propagating from a localized
impulsive source along a field-aligned plasma cylinder with a smooth
radial profile of the fast speed. The wave trains are found to have
pronounced period modulation, with the longer instant period seen
in the beginning of the wave train. The wave trains also have a
pronounced amplitude modulation. Wavelet spectra of the wave trains
have characteristic tadpole features, with the broadband large-amplitude
heads preceding low-amplitude quasi-monochromatic tails. The mean period
of the wave train is about the transverse fast magnetoacoustic transit
time across the cylinder. The mean parallel wavelength is about the
diameter of the wave-guiding plasma cylinder. Instant periods are longer
than the sausage wave cutoff period. The wave train characteristics
depend on the fast magnetoacoustic speed in both the internal and
external media, the smoothness of the transverse profile of the
equilibrium quantities, and also the spatial size of the initial
perturbation. If the initial perturbation is localized at the axis
of the cylinder, the wave trains contain higher radial harmonics that
have shorter periods.
---------------------------------------------------------
Title: North-south asymmetry in the magnetic deflection of polar
coronal hole jets
Authors: Nisticò, G.; Zimbardo, G.; Patsourakos, S.; Bothmer, V.;
Nakariakov, V. M.
2015A&A...583A.127N Altcode: 2015arXiv150801072N
Context. Measurements of the sunspots area, of the magnetic field in
the interplanetary medium, and of the heliospheric current sheet (HCS)
position, reveal a possible north-south (N-S) asymmetry in the magnetic
field of the Sun. This asymmetry could cause the bending of the HCS of
the order of 5-10 deg in the southward direction, and it appears to
be a recurrent characteristic of the Sun during the minima of solar
activity. <BR /> Aims: We study the N-S asymmetry as inferred from
measurements of the deflection of polar coronal hole jets when they
propagate throughout the corona. <BR /> Methods: Since the corona is
an environment where the magnetic pressure is greater than the kinetic
pressure (β ≪ 1), we can assume that the magnetic field controls the
dynamics of plasma. On average, jets follow magnetic field lines during
their propagation, highlighting their local direction. We measured
the position angles at 1 R<SUB>⊙</SUB> and at 2 R<SUB>⊙</SUB> of
79 jets, based on the Solar TErrestrial RElations Observatory (STEREO)
ultraviolet and white-light coronagraph observations during the solar
minimum period March 2007-April 2008. The average jet deflection is
studied both in the plane perpendicular to the line of sight and, for
a reduced number of jets, in 3D space. The observed jet deflection is
studied in terms of an axisymmetric magnetic field model comprising
dipole (g<SUB>1</SUB>), quadrupole (g<SUB>2</SUB>), and esapole
(g<SUB>3</SUB>) moments. <BR /> Results: We found that the propagation
of the jets is not radial, which is in agreement with the deflection
due to magnetic field lines. Moreover, the amount of the deflection is
different between jets over the north and those from the south pole. A
comparison of jet deflections and field line tracing shows that a ratio
g<SUB>2</SUB>/g<SUB>1</SUB> ≃ -0.5 for the quadrupole and a ratio
g<SUB>3</SUB>/g<SUB>1</SUB> ≃ 1.6-2.0 for the esapole can describe
the field. The presence of a non-negligible quadrupole moment confirms
the N-S asymmetry of the solar magnetic field for the considered
period. <BR /> Conclusions: We find that the magnetic deflection of
jets is larger in the north than in the south of the order of 25-40%,
with an asymmetry that is consistent with a southward deflection of
the heliospheric current sheet of the order of 10 deg, consistent with
that inferred from other independent datasets and instruments.
---------------------------------------------------------
Title: A Comparison Between Global Proxies of the Sun's Magnetic
Activity Cycle: Inferences from Helioseismology
Authors: Broomhall, A. -M.; Nakariakov, V. M.
2015SoPh..290.3095B Altcode: 2015SoPh..tmp...82B; 2015arXiv150702854B
The last solar minimum was, by recent standards, unusually deep and
long. We are now close to the maximum of the subsequent solar cycle,
which is relatively weak. In this article we make comparisons between
different global (unresolved) measures of the Sun's magnetic activity
to investigate how they are responding to this weak-activity epoch. We
focus on helioseismic data, which are sensitive to conditions,
including the characteristics of the magnetic field, in the solar
interior. Also considered are measures of the magnetic field in
the photosphere (sunspot number and sunspot area), the chromosphere
and corona (10.7 cm radio flux and 530.3 nm green coronal index),
and two measures of the Sun's magnetic activity closer to Earth
(the interplanetary magnetic field and the galactic cosmic-ray
intensity). Scaled versions of the activity proxies diverge from the
helioseismic data around 2000, indicating a change in relationship
between the proxies. The degree of divergence varies from proxy to
proxy, with sunspot area and 10.7 cm flux showing only small deviations,
while sunspot number, coronal index, and the two interplanetary proxies
show much larger departures. In Cycle 24 the deviations in the solar
proxies and the helioseismic data decrease, raising the possibility
that the deviations observed in Cycle 23 are just symptomatic of a
22-year Hale cycle. However, the deviations in the helioseismic data
and the interplanetary proxies increase in Cycle 24. Interestingly,
the divergence in the solar proxies and the helioseismic data are
not reflected in the shorter-term variations (often referred to as
quasi-biennial oscillations) observed on top of the dominant 11-year
solar cycle. However, despite being highly correlated in Cycle 22,
the short-term variations in the interplanetary proxies show very
little correlation with the helioseismic data during Cycles 23 and 24.
---------------------------------------------------------
Title: A Multi-period Oscillation in a Stellar Superflare
Authors: Pugh, C. E.; Nakariakov, V. M.; Broomhall, A. -M.
2015ApJ...813L...5P Altcode: 2015arXiv151003613P
Flares that are orders of magnitude larger than the most energetic
solar flares are routinely observed on Sun-like stars, raising the
question of whether the same physical processes are responsible for both
solar and stellar flares. In this Letter, we present a white-light
stellar superflare on the star KIC 9655129, observed by NASA’s
Kepler mission, with a rare multi-period quasi-periodic pulsation
(QPP) pattern. Two significant periodic processes were detected using
the wavelet and autocorrelation techniques, with periods of 78 ± 12
minutes and 32 ± 2 minutes. By comparing the phases and decay times
of the two periodicities, the QPP signal was found to most likely
be linear, suggesting that the two periodicities are independent,
possibly corresponding either to different magnetohydrodynamic (MHD)
modes of the flaring region or different spatial harmonics of the same
mode. The presence of multiple periodicities is a good indication
that the QPPs were caused by MHD oscillations and suggests that the
physical processes in operation during stellar flares could be the
same as those in solar flares.
---------------------------------------------------------
Title: Decayless low-amplitude kink oscillations: a common phenomenon
in the solar corona?
Authors: Anfinogentov, S. A.; Nakariakov, V. M.; Nisticò, G.
2015A&A...583A.136A Altcode: 2015arXiv150905519A
Context. We investigate the decayless regime of coronal kink
oscillations recently discovered in the Solar Dynamics Observatory
(SDO)/AIA data. In contrast to decaying kink oscillations that are
excited by impulsive dynamical processes, this type of transverse
oscillations is not connected to any external impulsive impact, such as
a flare or coronal mass ejection, and does not show any significant
decay. Moreover the amplitude of these decayless oscillations is
typically lower than that of decaying oscillations. <BR /> Aims:
The aim of this research is to estimate the prevalence of this
phenomenon and its characteristic signatures. <BR /> Methods: We
analysed 21 active regions (NOAA 11637-11657) observed in January
2013 in the 171 Å channel of SDO/AIA. For each active region we
inspected six hours of observations, constructing time-distance
plots for the slits positioned across pronounced bright loops. The
oscillatory patterns in time-distance plots were visually identified
and the oscillation periods and amplitudes were measured. We also
estimated the length of each oscillating loop. <BR /> Results:
Low-amplitude decayless kink oscillations are found to be present
in the majority of the analysed active regions. The oscillation
periods lie in the range from 1.5 to 10 min. In two active regions
with insufficient observation conditions we did not identify any
oscillation patterns. The oscillation periods are found to increase
with the length of the oscillating loop. <BR /> Conclusions: The
considered type of coronal oscillations is a common phenomenon in the
corona. The established dependence of the oscillation period on the loop
length is consistent with their interpretation in terms of standing
kink waves. <P />Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201526195/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Cut-off period for slow magnetoacoustic waves in coronal
plasma structures
Authors: Afanasyev, A. N.; Nakariakov, V. M.
2015A&A...582A..57A Altcode:
Context. There is abundant observational evidence of longitudinal
compressive waves in plasma structures of the solar corona, which are
confidently interpreted in terms of slow magnetoacoustic waves. The
uses of coronal slow waves in plasma diagnostics, as well as analysis
of their possible contribution to coronal heating and the solar
wind acceleration, require detailed theoretical modelling. <BR />
Aims: We investigate the effects of obliqueness, magnetic field, and
non-uniformity of the medium on the evolution of long-wavelength slow
magnetoacoustic waves guided by field-aligned plasma non-uniformities,
also called tube waves. Special attention is paid to the cut-off effect
due to the gravity stratification of the coronal plasma. <BR /> Methods:
We study the behaviour of linear tube waves in a vertical untwisted
straight field-aligned isothermal plasma cylinder. We apply the thin
flux tube approximation, taking into account effects of stratification
caused by gravity. The dispersion due to the finite radius of the flux
tube is neglected. We analyse the behaviour of the cut-off period
for an exponentially divergent magnetic flux tube filled in with a
stratified plasma. The results obtained are compared with the known
cases of the constant Alfven speed and the pure acoustic wave. <BR />
Results: We derive the wave equation for tube waves and reduce it to
the form of the Klein-Gordon equation with varying coefficients, which
explicitly contains the cut-off frequency. The cut-off period is found
to vary with height, decreasing significantly in the low-beta plasma and
in the plasma with the beta of the order of unity. The depressions in
the cut-off period profiles can affect the propagation of longitudinal
waves along coronal plasma structures towards the higher corona and
can form coronal resonators.
---------------------------------------------------------
Title: Coexisting fast and slow propagating waves of the extreme-UV
intensity in solar coronal plasma structures
Authors: Zhang, Yuzong; Zhang, Jun; Wang, Jingxiu; Nakariakov,
Valery M.
2015A&A...581A..78Z Altcode:
Context. From 06:15 UT to 08:15 UT on 2011 June 2, a toroidal
filament located at the joint of two active regions, 11226 and 11227,
appeared to perform two eruptions. During this phenomenon, fast and
slow magnetoacoustic waves were detected to propagate simultaneously
along a funnel coronal loop system of AR 11227. <BR /> Aims: We aim
to understand the relationship between fast and slow magnetoacoustic
waves during their propagations and measure their properties, such
as the propagating speed, path, amplitude, and period observed in the
extreme ultraviolet (EUV) wavebands. <BR /> Methods: We analyse time
sequences of EUV images acquired by the Atmospheric Imaging Assembly
onboard the Solar Dynamics Observatory. By creating time-distance maps
along selected directions, we measure the speeds and localisation of EUV
intensity waves in different EUV wavebands. We determine the periods
of the waves with wavelet analysis. <BR /> Results: The fast and slow
magnetoacoustic waves, apparently propagating along the same path,
are found to have different properties. Their apparent propagation
speeds, travel distances, and periods are about 900 km s<SUP>-1</SUP>
and 100 km s<SUP>-1</SUP>, 145 Mm and 36 Mm, and 2 min and 3 min,
respectively. <P />The movie is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201525621/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Hilbert-Huang transform analysis of periodicities in the last
two solar activity cycles
Authors: Kolotkov, D. Y.; Broomhall, A. -M.; Nakariakov, V. M.
2015MNRAS.451.4360K Altcode:
We investigated periodicities associated with the last two-and-a-half
solar activity cycles with the novel Hilbert-Huang transform (HHT)
method. Raw data signals of five different observational proxies:
the 10.7 cm radio flux intensity, the helioseismic frequency shift,
and the sunspot area signals recorded from the whole solar disc, and
separately from the Northern and Southern hemispheres, were expanded
into a set of intrinsic modes with the ensemble empirical mode
decomposition technique. Instant and mean periods of each empirical
mode were obtained with the use of the Hilbert transform applied
independently to each separate mode. The periodicities were allocated to
three distinct groups: short-term variations (with periods shorter than
0.5 yr), quasi-biennial oscillations (with typical periods from 0.5 yr
to 3.9 yr), and longer periodicities, e.g. such as the 11 yr cycle. All
periodicities detected in the examined solar cycles 22-24 are consistent
with the well-known results found in the earlier solar epochs. We have
demonstrated that the HHT method is a good tool for characterizing
periodicities in the helioseismic data, which are necessarily relatively
limited in terms of their time resolution. Periodicities obtained
using the helioseismic data are, nevertheless, consistent with those
found in other proxies. Since helioseismic oscillations are sensitive
to the solar interior, this indicates that the behaviour of surface
and atmospheric magnetic activity reflects that of the Sun's internal
magnetic field. All identified intrinsic modes are seen to have clear
amplitude modulation highly correlated with the 11 yr cycle. This
amplitude modulation is most pronounced in the short-period modes. The
short-term periodicities were found to be multiples of the shortest
period, of 25 d. This ordering of the short-term periodicities is
consistent with the previous findings. Signatures of the north-south
asymmetry were detected in the individual hemisphere sunspot area
indices. Furthermore, evidence of the last `extended' solar minimum
was detected too.
---------------------------------------------------------
Title: Oscillations in stellar superflares
Authors: Balona, L. A.; Broomhall, A. -M.; Kosovichev, A.; Nakariakov,
V. M.; Pugh, C. E.; Van Doorsselaere, T.
2015MNRAS.450..956B Altcode: 2015arXiv150401491B
Two different mechanisms may act to induce quasi-periodic pulsations
(QPP) in whole-disc observations of stellar flares. One mechanism may be
magnetohydromagnetic forces and other processes acting on flare loops
as seen in the Sun. The other mechanism may be forced local acoustic
oscillations due to the high-energy particle impulse generated by the
flare (known as `sunquakes' in the Sun). We analyse short-cadence Kepler
data of 257 flares in 75 stars to search for QPP in the flare decay
branch or post-flare oscillations which may be attributed to either
of these two mechanisms. About 18 per cent of stellar flares show a
distinct bump in the flare decay branch of unknown origin. The bump
does not seem to be a highly damped global oscillation because the
periods of the bumps derived from wavelet analysis do not correlate
with any stellar parameter. We detected damped oscillations covering
several cycles (QPP), in seven flares on five stars. The periods of
these oscillations also do not correlate with any stellar parameter,
suggesting that these may be a due to flare loop oscillations. We
searched for forced global oscillations which might result after
a strong flare. To this end, we investigated the behaviour of the
amplitudes of solar-like oscillations in eight stars before and after
a flare. However, no clear amplitude change could be detected. We
also analysed the amplitudes of the self-excited pulsations in two δ
Scuti stars and one γ Doradus star before and after a flare. Again,
no clear amplitude changes were found. Our conclusions are that a new
process needs to be found to explain the high incidence of bumps in
stellar flare light curves, that flare loop oscillations may have been
detected in a few stars and that no conclusive evidence exists as yet
for flare induced global acoustic oscillations (starquakes).
---------------------------------------------------------
Title: X-Ray and EUV Observations of Simultaneous Short and Long
Period Oscillations in Hot Coronal Arcade Loops
Authors: Kumar, Pankaj; Nakariakov, Valery M.; Cho, Kyung-Suk
2015ApJ...804....4K Altcode: 2015arXiv150207117K
We report decaying quasi-periodic intensity oscillations in the X-ray
(6-12 keV) and extreme-ultraviolet (EUV) channels (131, 94, 1600, 304
Å) observed by the Fermi Gamma-ray Burst Monitor and Solar Dynamics
Observatory/Atmospheric Imaging Assembly (AIA), respectively, during a
C-class flare. The estimated periods of oscillation and decay time in
the X-ray channel (6-12 keV) were about 202 and 154 s, respectively. A
similar oscillation period was detected at the footpoint of the
arcade loops in the AIA 1600 and 304 Å channels. Simultaneously,
AIA hot channels (94 and 131 Å) reveal propagating EUV disturbances
bouncing back and forth between the footpoints of the arcade loops. The
period of the oscillation and decay time were about 409 and 1121 s,
respectively. The characteristic phase speed of the wave is about 560
km s<SUP>-1</SUP> for about 115 Mm of loop length, which is roughly
consistent with the sound speed at the temperature about 10-16 MK
(480-608 km s<SUP>-1</SUP>). These EUV oscillations are consistent with
the Solar and Heliospheric Observatory/Solar Ultraviolet Measurement
of Emitted Radiation Doppler-shift oscillations interpreted as the
global standing slow magnetoacoustic wave excited by a flare. The
flare occurred at one of the footpoints of the arcade loops, where the
magnetic topology was a 3D fan-spine with a null-point. Repetitive
reconnection at this footpoint could have caused the periodic
acceleration of non-thermal electrons that propagated to the opposite
footpoint along the arcade and that are precipitating there, causing
the observed 202 s periodicity. Other possible interpretations, e.g.,
the second harmonics of the slow mode, are also discussed.
---------------------------------------------------------
Title: Excitation of kink oscillations of coronal loops: statistical
study
Authors: Zimovets, I. V.; Nakariakov, V. M.
2015A&A...577A...4Z Altcode:
Context. Solar flares are often accompanied by kink (transverse)
oscillations of coronal loops. Despite intensive study of these
oscillations in recent years, the mechanisms that excite them are still
not known. <BR /> Aims: We aim to clarify the excitation mechanisms for
these kink oscillations of coronal loops. <BR /> Methods: We analysed
58 kink-oscillation events observed by the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO) during its first
four years (2010-2014) with the use of the JHelioviewer. Association
of these oscillation events with flares, lower coronal (r ≲ 1.4
R<SUB>⊙</SUB>) eruptions and plasma ejections, coronal mass ejections
(CMEs), and coronal Type-II radio bursts is studied. <BR /> Results:
We find that 44 of these 58 oscillation events (76%) were associated
with CMEs observed in the white light emission. Moreover, 57 events
(98%) were accompanied by lower coronal eruptions/ejections (LCEs)
observed in the extreme-ultraviolet band in the parental active
regions. In the remaining event an LCE was not clearly seen, but it was
definitely associated with a CME too. The main observational finding
is that the kink oscillations were excited by the deviation of loops
from their equilibria by a nearby LCE in 55 events (95%). In three
remaining events, it was difficult to reliably determine the cause of
the oscillations because of limitations in the observational data. We
also found that 53 events (91%) were associated with flares. In
five remaining events, the parental active regions were behind the
limb and we could not directly see flare sites. It indicates that
there is a close relationship between these two kinds of solar
activity. However, the estimated speeds of a hypothetical driver
of kink oscillations by flares were found to be lower than 500 km
s<SUP>-1</SUP> in 80% of the cases. Such low speeds do not favour
the association of the oscillation excitation with a shock wave,
as usually assumed. That only 23 (40%) of the oscillation events
were found to be associated with coronal Type-II radio bursts also
goes against the shock wave mechanism for the excitation of kink
oscillations. <BR /> Conclusions: The statistical analysis shows
that the most probable mechanism for exciting the kink oscillations
of coronal loops is the deviation of loops from their equilibrium by
nearby eruptions or plasma ejections rather than a blast shock wave
ignited by a flare. <P />Table 1 is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201424960/olm">http://www.aanda.org</A>
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Title: Diagnostics of Coronal Heating in Solar Active Regions
Authors: Fludra, Andrzej; Hornsey, Christopher; Nakariakov, Valery
2015TESS....120305F Altcode:
We aim to develop a diagnostic method for the coronal heating mechanism
in active region loops. Observational constraints on coronal heating
models have been sought using measurements in the X-ray and EUV
wavelengths. Statistical analysis, using EUV emission from many active
regions, was done by Fludra and Ireland (2008) who studied power-law
relationships between active region integrated magnetic flux and
emission line intensities. A subsequent study by Fludra and Warren
(2010) for the first time compared fully resolved images in an EUV
spectral line of OV 63.0 nm with the photospheric magnetic field,
leading to the identification of a dominant, ubiquitous variable
component of the transition region EUV emission and a discovery
of a steady basal heating, and deriving the dependence of the basal
heating rate on the photospheric magnetic flux density. In this study,
we compare models of single coronal loops with EUV observations. We
assess to what degree observations of individual coronal loops made
in the EUV range are capable of providing constraints on the heating
mechanism. We model the coronal magnetic field in an active region using
an NLFF extrapolation code applied to a photospheric vector magnetogram
from SDO/HMI and select several loops that match an SDO/AIA 171 image of
the same active region. We then model the plasma in these loops using
a 1D hydrostatic code capable of applying an arbitrary heating rate as
a function of magnetic field strength along the loop. From the plasma
parameters derived from this model, we calculate the EUV emission along
the loop in AIA 171 and 335 bands, and in pure spectral lines of Fe
IX 17.1 nm and Fe XVI 33.5 nm. We use different spatial distributions
of the heating function: concentrated near the loop top, uniform and
concentrated near the footpoints, and investigate their effect on the
modelled EUV intensities. We find a diagnostics based on the dependence
of the total loop intensity on the shape of the heating function and
discuss its range of applicability for loops of different length.
---------------------------------------------------------
Title: Solar and Heliospheric Physics with the Square Kilometre Array
Authors: Nakariakov, V.; Bisi, M. M.; Browning, P. K.; Maia,
D.; Kontar, E. P.; Oberoi, D.; Gallagher, P. T.; Cairns, I. H.;
Ratcliffe, H.
2015aska.confE.169N Altcode: 2015PoS...215E.169N; 2015arXiv150700516N
The fields of solar radiophysics and solar system radio physics,
or radio heliophysics, will benefit immensely from an instrument
with the capabilities projected for SKA. Potential applications
include interplanetary scintillation (IPS), radio-burst tracking,
and solar spectral radio imaging with a superior sensitivity. These
will provide breakthrough new insights and results in topics of
fundamental importance, such as the physics of impulsive energy
releases, magnetohydrodynamic oscillations and turbulence, the
dynamics of post-eruptive processes, energetic particle acceleration,
the structure of the solar wind and the development and evolution of
solar wind transients at distances up to and beyond the orbit of the
Earth. The combination of the high spectral, time and spatial resolution
and the unprecedented sensitivity of the SKA will radically advance
our understanding of basic physical processes operating in solar and
heliospheric plasmas and provide a solid foundation for the forecasting
of space weather events.
---------------------------------------------------------
Title: Radial and Azimuthal Oscillations in Halo Coronal Mass
Ejections
Authors: Lee, Harim; Moon, Yong-Jae; Nakariakov, Valery
2015TESS....131002L Altcode:
We present the first observational detection of radial and azimuthal
oscillations in full halo coronal mass ejections (HCMEs). We analyse
nine HCMEs well-observed by the Large Angle and Spectrometric
Coronagraph (LASCO) from February to June, 2011. Using the LASCO C3
running difference images, we estimated the instantaneous apparent
speeds of the HCMEs in different radial directions from the solar disk
centre. We find that the development of all these HCMEs is accompanied
with quasi-periodic variations of the instantaneous radial velocity
with the periods ranging from 24 to 48 minutes. The amplitudes of the
instant speed variations reach about a half of the projected speeds. The
amplitudes are found to anti-correlate with the periods and correlate
with the HCME speed, indicating the nonlinear nature of the process. The
oscillations have a clear azimuthal structure in the heliocentric polar
coordinate system. The oscillations in seven events are found to be
associated with distinct azimuthal wave modes with the azimuthal wave
number m=1 for six events and m=2 for one event. The polarisation of
the oscillations in these seven HCMEs is broadly consistent with those
of their position angles with the mean difference of 42.5 degree. The
oscillations may be connected with natural oscillations of the plasmoids
around a dynamical equilibrium, or self-oscillatory processes, e.g. the
periodic shedding of Alfvénic vortices. Our results indicate the need
for advanced theory of oscillatory processes in CMEs.
---------------------------------------------------------
Title: Radial and Azimuthal Oscillations of Halo Coronal Mass
Ejections in the Sun
Authors: Lee, Harim; Moon, Y. -J.; Nakariakov, V. M.
2015ApJ...803L...7L Altcode:
We present the first observational detection of radial and azimuthal
oscillations in full halo coronal mass ejections (HCMEs). We analyze
nine HCMEs well-observed by the Large Angle and Spectrometric
Coronagraph (LASCO) from 2011 February to June. Using the LASCO C3
running difference images, we estimated the instantaneous apparent
speeds of the HCMEs in different radial directions from the solar disk
center. We find that the development of all these HCMEs is accompanied
by quasi-periodic variations of the instantaneous radial velocity
with the periods ranging from 24 to 48 minutes. The amplitudes of
the instant speed variations reach about a half of the projected
speeds. The amplitudes are found to anti-correlate with the periods
and correlate with the HCME speed, indicating the nonlinear nature
of the process. The oscillations have a clear azimuthal structure in
the heliocentric polar coordinate system. The oscillations in seven
events are found to be associated with distinct azimuthal wave modes
with the azimuthal wave number m = 1 for six events and m = 2 for one
event. The polarization of the oscillations in these seven HCMEs is
broadly consistent with those of their position angles with the mean
difference of 43°. The oscillations may be connected with natural
oscillations of the plasmoids around a dynamical equilibrium, or
self-oscillatory processes, e.g., the periodic shedding of Alfvénic
vortices. Our results indicate the need for an advanced theory of
oscillatory processes in coronal mass ejections.
---------------------------------------------------------
Title: Intensity and Doppler Velocity Oscillations in Pore Atmospheres
Authors: Cho, K. -S.; Bong, S. -C.; Nakariakov, V. M.; Lim, E. -K.;
Park, Y. -D.; Chae, J. C.; Yang, H. -S.; Park, H. -M.; Yurchyshyn, V.
2015ApJ...802...45C Altcode:
We have investigated chromospheric traveling features running across two
merged pores from their centers at speeds of about 55 km s<SUP>-1</SUP>,
in the active region AR 11828. The pores were observed on 2013 August
24 by using high-time, spatial, and spectral resolution data from the
Fast Imaging Solar Spectrograph of the 1.6 m New Solar Telescope. We
infer a line-of-sight (LOS) velocity by applying the lambdameter method
to the Ca ii 8542 Å band and Hα band, and investigate intensity and
LOS velocity changes at different wavelengths and different positions
at the pores. We find that they have three-minute oscillations, and
the intensity oscillation from the line center (0.0 \overset{\circ}A
) is preceded by that from the core (-0.3 \overset{\circ}A ) of the
bands. There is no phase difference between the intensity and the
LOS velocity oscillations at a given wavelength. The amplitude of LOS
velocity from the near core spectra ({Δ }λ =0.10-0.21 \overset{\circ}A
) is greater than that from the far core spectra ({Δ }λ =0.24-0.36
\overset{\circ}A ). These results support the interpretation of the
observed wave as a slow magnetoacoustic wave propagating along the
magnetic field lines in the pores. The apparent horizontal motion and
a sudden decrease of its speed beyond the pores can be explained by
the projection effect caused by inclination of the magnetic field with
a canopy structure. We conclude that the observed wave properties of
the pores are quite similar to those from the sunspot observations.
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Title: Multi-mode quasi-periodic pulsations in a solar flare
Authors: Kolotkov, D. Y.; Nakariakov, V. M.; Kupriyanova, E. G.;
Ratcliffe, H.; Shibasaki, K.
2015A&A...574A..53K Altcode:
Context. Quasi-periodic pulsations (QPP) of the electromagnetic
radiation emitted in solar and stellar flares are often detected
in microwave, white light, X-ray, and gamma-ray bands. Mechanisms
for QPP are intensively debated in the literature. Previous studies
revealed that QPP may manifest non-linear, non-stationary and, perhaps,
multi-modal processes operating in flares. <BR /> Aims: We study QPP
of the microwave emission generated in an X3.2-class solar flare on
14 May, 2013, observed with the Nobeyama Radioheliograph (NoRH),
aiming to reveal signatures of the non-linear, non-stationary,
and multi-modal processes in the signal. <BR /> Methods: The NoRH
correlation signal obtained at the 17 GHz intensity has a clear QPP
pattern. The signal was analysed with the Hilbert-Huang transform
(HHT) that allows one to determine its instant amplitude and frequency,
and their time variation. <BR /> Results: It was established that the
QPP consists of at least three well-defined intrinsic modes, with the
mean periods of 15, 45, and 100 s. All the modes have quasi-harmonic
behaviour with different modulation patterns. The 100 s intrinsic
mode is a decaying oscillation, with the decay time of 250 s. The 15
s intrinsic mode shows a similar behaviour, with the decay time of
90 s. The 45 s mode has a wave-train behaviour. <BR /> Conclusions:
Dynamical properties of detected intrinsic modes indicate that the
100 s and 15 s modes are likely to be associated with fundamental
kink and sausage modes of the flaring loop, respectively. The 100 s
oscillation could also be caused by the fundamental longitudinal mode,
while this interpretation requires the plasma temperature of about 30
million K and hence is not likely. The 45 s mode could be the second
standing harmonics of the kink mode.
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Title: Evolution of Fast Magnetoacoustic Pulses in Randomly Structured
Coronal Plasmas
Authors: Yuan, D.; Pascoe, D. J.; Nakariakov, V. M.; Li, B.;
Keppens, R.
2015ApJ...799..221Y Altcode: 2014arXiv1411.4152Y
We investigate the evolution of fast magnetoacoustic pulses in randomly
structured plasmas, in the context of large-scale propagating waves in
the solar atmosphere. We perform one-dimensional numerical simulations
of fast wave pulses propagating perpendicular to a constant magnetic
field in a low-β plasma with a random density profile across the
field. Both linear and nonlinear regimes are considered. We study
how the evolution of the pulse amplitude and width depends on their
initial values and the parameters of the random structuring. Acting
as a dispersive medium, a randomly structured plasma causes amplitude
attenuation and width broadening of the fast wave pulses. After the
passage of the main pulse, secondary propagating and standing fast
waves appear. Width evolution of both linear and nonlinear pulses can
be well approximated by linear functions; however, narrow pulses may
have zero or negative broadening. This arises because narrow pulses are
prone to splitting, while broad pulses usually deviate less from their
initial Gaussian shape and form ripple structures on top of the main
pulse. Linear pulses decay at an almost constant rate, while nonlinear
pulses decay exponentially. A pulse interacts most efficiently with a
random medium with a correlation length of about half of the initial
pulse width. This detailed model of fast wave pulses propagating in
highly structured media substantiates the interpretation of EIT waves
as fast magnetoacoustic waves. Evolution of a fast pulse provides us
with a novel method to diagnose the sub-resolution filamentation of
the solar atmosphere.
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Title: A Combined Analysis of the Observational Aspects of the
Quasi-biennial Oscillation in Solar Magnetic Activity
Authors: Bazilevskaya, G.; Broomhall, A. -M.; Elsworth, Y.; Nakariakov,
V. M.
2015sac..book..359B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Nonlinear slow magnetoacoustic waves in coronal plasma
structures
Authors: Afanasyev, A. N.; Nakariakov, V. M.
2015A&A...573A..32A Altcode:
Context. There is abundant observational evidence of longitudinal
waves in the plasma structures of the solar corona. These essentially
compressive waves are confidently interpreted as slow magnetoacoustic
waves. The use of the slow waves in plasma diagnostics and estimating
their possible contribution to plasma heating and acceleration
require detailed theoretical modelling. <BR /> Aims: We investigate
the role of obliqueness and magnetic effects in the evolution of slow
magnetoacoustic waves, also called tube waves, in field-aligned plasma
structures. Special attention is paid to the wave damping caused
by nonlinear steepening. <BR /> Methods: We considered an untwisted
straight axisymmetric field-aligned plasma cylinder and analysed the
behaviour of the slow magnetoacoustic waves that are guided by this
plasma structure. We adopted a thin flux tube approximation. We took
into account dissipation caused by viscosity, resistivity and thermal
conduction, and nonlinearity. Effects of stratification and dispersion
caused by the finite radius of the flux tube were neglected. <BR />
Results: We derive the Burgers-type evolutionary equation for tube
waves in a uniform plasma cylinder. Compared with a plane acoustic
wave, the formation of shock fronts in tube waves is found to occur at
a larger distance from the source. In addition, tube waves experience
stronger damping. These effects are most pronounced in plasmas with
the parameter β at about or greater than unity. In a low-β plasma,
the evolution of tube waves can satisfactorily be described with the
Burgers equation for plane acoustic waves. <BR /> Conclusions:
---------------------------------------------------------
Title: Vertical Kink Oscillation of a Magnetic Flux Rope Structure
in the Solar Corona
Authors: Kim, S.; Nakariakov, V. M.; Cho, K. -S.
2014ApJ...797L..22K Altcode:
Vertical transverse oscillations of a coronal magnetic rope, observed
simultaneously in the 171 Å and 304 Å bandpasses of the Atmospheric
Imaging Assembly on board the Solar Dynamics Observatory (SDO), are
detected. The oscillation period is about 700 s and the displacement
amplitude is about 1 Mm. The oscillation amplitude remains constant
during the observation. Simultaneous observation of the rope in the
bandpasses corresponding to the coronal and chromospheric temperatures
suggests that it has a multi-thermal structure. Oscillatory patterns
in 171 Å and 304 Å are coherent, which indicates that the observed
kink oscillation is collective, in which the rope moves as a single
entity. We interpret the oscillation as a fundamental standing
vertically polarized kink mode of the rope, while the interpretation in
terms of a perpendicular fast wave could not be entirely ruled out. In
addition, the arcade situated above the rope and seen in the 171 Å
bandpass shows an oscillatory motion with the period of about 1000 s.
---------------------------------------------------------
Title: A Combined Analysis of the Observational Aspects of the
Quasi-biennial Oscillation in Solar Magnetic Activity
Authors: Bazilevskaya, G.; Broomhall, A. -M.; Elsworth, Y.; Nakariakov,
V. M.
2014SSRv..186..359B Altcode: 2014SSRv..tmp...34B
Solar quasi-biennial oscillations (QBOs) with the time scale of 0.6-4
yrs appear to be a basic feature of the Sun's activity. Observational
aspects of QBOs are reviewed on the basis of recent publications. Solar
QBOs are shown to be ubiquitous and very variable. We demonstrate
that many features of QBOs are common to different observations. These
features include variable periodicity and intermittence with signs of
stochastisity, a presence at all levels of the solar atmosphere and
even in the convective zone, independent development in the northern
and southern solar hemispheres, most pronounced amplitudes during
the maximum phase of the 11-yr cycle and the transition of QBOs into
interplanetary space. Temporal weakening of solar activity around the
maximum of the 11-yr cycle (Gnevyshev Gap) can be considered an integral
part of QBOs. The exact mechanism by which the solar QBO is produced is
poorly understood. We describe some of the most plausible theoretical
mechanisms and discuss observational features that support/contradict
the theory. QBOs have an important meaning as a benchmark of solar
activity, not only for investigation of the solar dynamo but also in
terms of space weather.
---------------------------------------------------------
Title: Dynamics of a multi-thermal loop in the solar corona
Authors: Nisticò, G.; Anfinogentov, S.; Nakariakov, V. M.
2014A&A...570A..84N Altcode:
Context. We present an observation of a long-living multi-thermal
coronal loop, visible in different extreme ultra-violet wavebands of
SDO/AIA in a quiet-Sun region close to the western solar limb. <BR />
Aims: Analysis of persistent kink displacements of the loop seen in
different bandpasses that correspond to different temperatures of the
plasma allows sub-resolution structuring of the loop to be revealed. <BR
/> Methods: A vertically oriented slit is taken at the loop top, and
time-distance maps are made from it. Loop displacements in time-distance
maps are automatically tracked with the Gaussian fitting technique and
fitted with a sinusoidal function that is "guessed". Wavelet transforms
are further used in order to quantify the periodicity variation in
time of the kink oscillations. <BR /> Results: The loop strands are
found to oscillate with the periods ranging between 3 and 15 min. The
oscillations are observed in intermittent regime with temporal changes
in the period and amplitude. The oscillations are different at three
analysed wavelengths. <BR /> Conclusions: This finding suggests that
the loop-like threads seen at different wavelengths are not co-spatial,
hence that the loop consists of several multi-thermal strands. The
detected irregularity of the oscillations can be associated with
a stochastic driver acting at the footpoints of the loop. <P />A
movie associated to Fig. 1 is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201423970/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Oscillations in a Sunspot with Light Bridges
Authors: Yuan, Ding; Nakariakov, Valery M.; Huang, Zhenghua; Li, Bo;
Su, Jiangtao; Yan, Yihua; Tan, Baolin
2014ApJ...792...41Y Altcode: 2014arXiv1407.1544Y
The Solar Optical Telescope on board Hinode observed a sunspot (AR
11836) with two light bridges (LBs) on 2013 August 31. We analyzed
a two-hour Ca II H emission intensity data set and detected strong
five-minute oscillation power on both LBs and in the inner penumbra. The
time-distance plot reveals that the five-minute oscillation phase
does not vary significantly along the thin bridge, indicating that
the oscillations are likely to originate from underneath it. The slit
taken along the central axis of the wide LB exhibits a standing wave
feature. However, at the center of the wide bridge, the five-minute
oscillation power is found to be stronger than at its sides. Moreover,
the time-distance plot across the wide bridge exhibits a herringbone
pattern that indicates a counter-stream of two running waves, which
originated at the bridge's sides. Thus, the five-minute oscillations
on the wide bridge also resemble the properties of running penumbral
waves. The five-minute oscillations are suppressed in the umbra, while
the three-minute oscillations occupy all three cores of the sunspot's
umbra, separated by the LBs. The three-minute oscillations were found
to be in phase at both sides of the LBs. This may indicate that either
LBs do not affect umbral oscillations, or that umbral oscillations
at different umbral cores share the same source. It also indicates
that LBs are rather shallow objects situated in the upper part of the
umbra. We found that umbral flashes (UFs) follow the life cycles of
umbral oscillations with much larger amplitudes. They cannot propagate
across LBs. UFs dominate the three-minute oscillation power within
each core; however, they do not disrupt the phase of umbral oscillation.
---------------------------------------------------------
Title: Observation of a high-quality quasi-periodic rapidly
propagating wave train using SDO/AIA
Authors: Nisticò, G.; Pascoe, D. J.; Nakariakov, V. M.
2014A&A...569A..12N Altcode:
Context. We present a new event of quasi-periodic wave trains observed
in EUV wavebands that rapidly propagate away from an active region
after a flare. <BR /> Aims: We measured the parameters of a wave
train observed on 7 December 2013 after an M1.2 flare, such as
the phase speeds, periods and wavelengths, in relationship to the
local coronal environment and the energy sources. <BR /> Methods:
We compared our observations with a numerical simulation of fast
magnetoacoustic waves that undergo dispersive evolution and leakage
in a coronal loop embedded in a potential magnetic field. <BR />
Results: The wave train is observed to propagate as several arc-shaped
intensity disturbances for almost half an hour, with a speed greater
than 1000 km s<SUP>-1</SUP> and a period of about 1 min. The wave
train followed two different patterns of propagation, in accordance
with the magnetic structure of the active region. The oscillatory
signal is found to be of high-quality, i.e. there is a large number
(10 or more) of subsequent wave fronts observed. The observations
are found to be consistent with the numerical simulation of a fast
wave train generated by a localised impulsive energy release. <BR />
Conclusions: Transverse structuring in the corona can efficiently
create and guide high-quality quasi-periodic propagating fast wave
trains. <P />The movies are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201423763/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Wave dynamics in a sunspot umbra
Authors: Sych, R.; Nakariakov, V. M.
2014A&A...569A..72S Altcode: 2014arXiv1409.4530S
Context. Sunspot oscillations are one of the most frequently studied
wave phenomena in the solar atmosphere. Understanding the basic physical
processes responsible for sunspot oscillations requires detailed
information about their fine structure. <BR /> Aims: We aim to reveal
the relationship between the fine horizontal and vertical structure,
time evolution, and the fine spectral structure of oscillations in a
sunspot umbra. <BR /> Methods: The high spatial and time resolution data
obtained with SDO/AIA for the sunspot in active region NOAA 11131 on 08
December 2010 were analysed with the time-distance plot technique and
the pixelised wavelet filtering method. Different levels of the sunspot
atmosphere were studied from the temperature minimum to the corona. <BR
/> Results: Oscillations in the 3 min band dominate in the umbra. The
integrated spectrum of umbral oscillations contains distinct narrowband
peaks at 1.9 min, 2.3 min, and 2.8 min. The power significantly varies
in time, forming distinct 12-20 min oscillation trains. The oscillation
power distribution over the sunspot in the horizontal plane reveals
that the enhancements of the oscillation amplitude, or wave fronts,
have a distinct structure consisting of an evolving two-armed spiral
and a stationary circular patch at the spiral origin, situated near
the umbra centre. This structure is seen from the temperature minimum
at 1700 Å to the 1.6 MK corona at 193 Å. In time, the spiral rotates
anti-clockwise. The wave front spirality is most pronounced during the
maximum amplitude phases of the oscillations, and in the bandpasses
where umbral oscillations have the highest power, 304 Å and 171 Å. In
the low-amplitude phases the spiral breaks into arc-shaped patches. The
2D cross-correlation function shows that the oscillations at higher
atmospheric levels occur later than at lower layers. The phase speed
is estimated to be about 100 km s<SUP>-1</SUP>. The fine spectral
analysis shows that the central patch corresponds to the high-frequency
oscillations, while the spiral arms highlight the lower-frequency
oscillations in the 3 min band. <BR /> Conclusions: The vertical and
horizontal radial structure of the oscillations is consistent with the
model that interprets umbral oscillations as slow magnetoacoustic waves
filtered by the atmospheric temperature non-uniformity in the presence
of the magnetic field inclination from the vertical. The mechanism
for the polar-angle structure of the oscillations, in particular the
spirality of the wave fronts, needs to be revealed.
---------------------------------------------------------
Title: Erratum: "Three-minute Oscillations above Sunspot Umbra
Observed with the Solar Dynamics Observatory/Atmospheric
Imaging Assembly and Nobeyama Radioheliograph" <A
href="/abs/2012ApJ...746..119R">(2012, ApJ, 746, 119)</A>
Authors: Reznikova, V. E.; Shibasaki, K.; Sych, R. A.; Nakariakov,
V. M.
2014ApJ...792...81R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Fast magnetoacoustic wave trains in coronal holes
Authors: Pascoe, D. J.; Nakariakov, V. M.; Kupriyanova, E. G.
2014A&A...568A..20P Altcode:
Context. Rapidly propagating coronal EUV disturbances recently
discovered in the solar corona are interpreted in terms of guided
fast magnetoacoustic waves. Fast magnetoacoustic waves experience
geometric dispersion in waveguides, which causes localised, impulsive
perturbations to develop into quasi-periodic wave trains. <BR /> Aims:
We consider the formation of fast wave trains in a super-radially
expanding coronal hole modelled by a magnetic funnel with a
field-aligned density profile that is rarefied in comparison to the
surrounding plasma. This kind of structure is typical of coronal
holes, and it forms a fast magnetoacoustic anti-waveguide as a local
maximum in the Alfvén speed. <BR /> Methods: We performed 2D MHD
numerical simulations for impulsively generated perturbations to
the system. Both sausage and kink perturbations are considered and
the role of the density contrast ratio investigated. <BR /> Results:
The anti-waveguide funnel geometry refracts wave energy away from the
structure. However, in this geometry the quasi-periodic fast wave
trains are found to appear, too, and so can be associated with the
observed rapidly propagating coronal EUV disturbances. The wave trains
propagate along the external edge of the coronal hole. The fast wave
trains generated in coronal holes exhibit less dispersive evolution than
in the case of a dense waveguide. <BR /> Conclusions: We conclude that
an impulsive energy release localised in a coronal plasma inhomogeneity
develops into a fast wave train for both kink and sausage disturbances
and for both waveguide and anti-waveguide transverse plasma profiles.
---------------------------------------------------------
Title: Editorial: solar radiophysics — recent results on
observations and theories
Authors: Nakariakov, Valery M.; Kashapova, Larisa K.; Yan, Yi-Hua
2014RAA....14....1N Altcode:
Solar radiophysics is a rapidly developing branch of solar physics
and plasma astrophysics. Solar radiophysics has the goal of analyzing
observations of radio emissions from the Sun and understanding basic
physical processes operating in quiet and active regions of the solar
corona. In the near future, the commissioning of a new generation
of solar radio observational facilities, which include the Chinese
Spectral Radio Heliograph (CSRH) and the upgrade of the Siberian
Solar Radio Telescope (SSRT), and the beginning of solar observations
with the Atacama Large Millimeter/submillimeter Array (ALMA), is
expected to bring us new breakthrough results of a transformative
nature. The Marie-Curie International Research Staff Exchange (MC
IRSES) “RadioSun” international network aims to create a solid
foundation for the successful exploitation of upcoming solar radio
observational facilities, as well as intensive use of the existing
observational tools, advanced theoretical modeling of relevant physical
processes and observables, and training a new generation of solar
radio physicists. The RadioSun network links research teams from China,
Czech Republic, Poland, Russia and the UK. This mini-volume presents
research papers based on invited reviews and contributed talks at
the 1st RadioSun workshop in China. These papers cover a broad range
of research topics and include recent observational and theoretical
advances in solar radiophysics, MHD seismology of the solar corona,
physics of solar flares, generation of radio emission, numerical
modeling of MHD and plasma physics processes, charged-particle
acceleration and novel instrumentation.
---------------------------------------------------------
Title: Sausage oscillations of coronal plasma slabs
Authors: Hornsey, C.; Nakariakov, V. M.; Fludra, A.
2014A&A...567A..24H Altcode:
Context. Sausage oscillations are observed in plasma non-uniformities
of the solar corona as axisymmetric perturbations of the
non-uniformity. Often, these non-uniformities can be modelled as
field-aligned slabs of the density enhancement. <BR /> Aims: We
perform parametric studies of sausage oscillations of plasma slabs,
aiming to determine the dependence of the oscillation period on
its parameters, and the onset of leaky and trapped regimes of the
oscillations. <BR /> Methods: Slabs with smooth transverse profiles
of the density of a zero-beta plasma are perturbed by an impulsive
localised perturbation of the sausage symmetry. In particular, the
slab can contain an infinitely thin current sheet in its centre. The
initial value problem is then solved numerically. The numerical
results are subject to spectral analysis. The results are compared
with analytical solutions for a slab with a step-function profile and
also with sausage oscillations of a plasma cylinder. <BR /> Results: We
established that sausage oscillations in slabs generally have the same
properties as in plasma cylinders. In the trapped regime, the sausage
oscillation period increases with the increase in the longitudinal
wavelength. In the leaky regime, the dependence of the period on the
wavelength experiences saturation, and the period becomes independent
of the wavelength in the long-wavelength limit. In the leaky regime
the period is always longer than in the trapped regime. The sausage
oscillation period in a slab is always longer than in a cylinder with
the same transverse profile. In slabs with steeper transverse profiles,
sausage oscillations have longer periods. The leaky regime occurs at
shorter wavelengths in slabs with smoother profiles.
---------------------------------------------------------
Title: Temperature Anisotropy in the Presence of Ultra Low Frequency
Waves in the Terrestrial Foreshock
Authors: Selzer, L. A.; Hnat, B.; Osman, K. T.; Nakariakov, V. M.;
Eastwood, J. P.; Burgess, D.
2014ApJ...788L...5S Altcode:
We report the first study of the correlation between elevated solar wind
core plasma temperatures and temperature anisotropy in the terrestrial
foreshock. Plasma temperature is enhanced near the fire hose marginal
stability threshold in the presence of ultra low frequency (ULF)
large amplitude magnetic perturbations, which are intrinsically
right-hand circularly polarized. Direct comparison of contemporaneous
anisotropic temperatures in the upstream solar wind and the foreshock
suggests that the net heating of plasma is mediated via increase of the
parallel temperature in the foreshock region where the ULF waves are
present. We consider the possibility that a mechanism based on Landau
damping, where solar wind plasma temperature parallel to the background
magnetic field is increased by interaction with oblique compressible
fast magneto-acoustic ULF waves, influences temperature anisotropy.
---------------------------------------------------------
Title: Fast magnetoacoustic wave trains in magnetic funnels of the
solar corona
Authors: Pascoe, David; Nakariakov, Valery; Kupriyanova, Elena
2014cosp...40E2453P Altcode:
Fast magnetoacoustic waves are highly dispersive in waveguides and
so can generate quasi-periodic wave trains if a localised, impulsive
driver is applied, such as flaring energy release. Wave trains have
been observed propagating 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. We extend existing models
of magnetoacoustic waveguides to consider the effects of an expanding
magnetic field with a field-aligned density structure, responding to the
recent discovery of fast wave trains of EUV intensity. We investigate
the dependence of the wave train properties on variables such as the
density contrast ratio, density stratification, and the spectral profile
of the driver. The funnel geometry leads to novel wave behaviour, such
as the formation of additional wave trains that propagate outside but
along the density structure due to leakage and refraction. The results
of our numerical simulations demonstrate properties similar to those
of wave trains observed in the solar corona.
---------------------------------------------------------
Title: Decaying long-period oscillations in flaring coronal loops
Authors: Nakariakov, Valery
2014cosp...40E2227N Altcode:
Quasi-periodic rapidly-decaying variations of the Doppler shift of the
emission lines associated with the hot plasma were detected in solar
flares about ten years ago with the SUMER spectrograph operating
in EUV. Later, similar field-aligned flows of the hot plasma in
flaring loops were found in the data of Yohkoh/BCS. The oscillations
characterised by relatively long periods, in the range 10-20 min,
and very short decay times, 15-30 min, are known as “SUMER”
oscillations. We present observations of SUMER oscillations in the
microwave band with the Nobeyama Radioheliograph and SDO/AIA. Analysis
of the microwave data, obtained in the 17 GHz channel during an
M1.6 flare revealed the presence of 12.6-min oscillations of the
emitting plasma density. The oscillations were seen to decay with the
characteristic time of about 15 min. Simultaneously, these oscillations
were detected in the variation of the EUV emission intensity measured
in the 335A channel of SDO/AIA. Our observational findings support the
interpretation of SUMER oscillations in terms of impulsively excited
standing acoustic oscillations in flaring loops or arcades, based upon
numerical radiative MHD simulations. Moreover, very recently a similar
dynamical pattern was detected in the light-curve of a megaflare on
the dM4.5e star YZ CMi in the white light band. This result indicates
striking similarities between dynamical processes in moderate solar
flares and stellar megaflares.
---------------------------------------------------------
Title: Spatial Structure of Multimode Oscillations in a Solar Flare
on 14 May 2013 in EUV and Radio Bands
Authors: Kolotkov, Dmitry; Nakariakov, Valery; Nisticò, Giuseppe;
Shibasaki, Kiyoto; Kupriyanova, Elena
2014cosp...40E1558K Altcode:
Quasi-periodic pulsations and coronal loop oscillations in an X-class
solar flare on 14 May 2013 are considered. Rapidly decaying kink
oscillations of coronal loops with periods of several minutes in the
flaring active region detected in the EUV band with SDO/AIA after
the impulsive phase of the flare. Oscillations of neighbouring loops
are excited simultaneously, but get rapidly out of phase. In the
impulsive phase, observations in the radio band with the Nobeyama
Radioheliograph and Radiopolarimeter show quasi-periodic pulsations
that are most pronounced in the 17 GHz band. In the correlation plots
and the integrated flux the pulsations have a symmetric triangular
shape. The period of pulsations is about 1 min. Analysis of the spatial
locations of the radio sources reveal that the triangularity is likely
to be caused by superposition of several harmonic modes.
---------------------------------------------------------
Title: Multi-height observations of magnetoacoustic cut-off frequency
in a sunspot atmosphere
Authors: Yuan, D.; Sych, R.; Reznikova, V. E.; Nakariakov, V. M.
2014A&A...561A..19Y Altcode: 2012arXiv1211.5196Y
Context. The cut-off frequency of magnetoacoustic gravity (MAG) waves
could be decreased by the inclined magnetic field, and therefore,
low-frequency waves could penetrate into the upper atmosphere. <BR
/> Aims: We observe the distribution of the cut-off frequency of
compressive waves at various heights and reconstruct the magnetic
field inclination, according to the MAG wave theory in a stratified
atmosphere permeated by a uniform magnetic field. <BR /> Methods:
We analysed the emission intensity oscillations of sunspot AR11131
(08 Dec. 2010) observed at the 1700 Å, 1600 Å, and 304 Å bandpasses
of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory (SDO), and computed the narrow-band power maps with the
pixelised wavelet filtering method. The distribution of the cut-off
frequency was defined as the median contour in the azimuthally-averaged
oscillation power. The magnetic field inclination was estimated with the
local cut-off frequency according to the MAG wave theory in the low-β
limit and was compared to the potential field extrapolation. <BR />
Results: Shorter period oscillations dominate in the sunspot umbra,
while longer period oscillations form an annular shape approximately
concentric with the sunspot. Oscillations with longer periods are
distributed further away from the sunspot centre. The 5 min oscillations
appear to originate at or lower than the photosphere. The magnetic
field inclinations determined with the cut-off frequency theory are
about 30-40% larger than the values obtained by the potential field
extrapolation. <BR /> Conclusions: The oscillation power distribution
in a sunspot atmosphere reflects its magnetic and thermal structure. The
cut-off frequency could be used to probe the magnetic field inclination,
however, other factors have to be included to fully understand this
phenomenon. The existence of return magnetic flux at the outer penumbra
was evidenced by the cut-off frequency distribution.
---------------------------------------------------------
Title: Observations of decay-less low-amplitude kink oscillations
of EUV coronal loops
Authors: Nisticò, Giuseppe; Nakariakov, Valery; Anfinogentov, Sergey
2014cosp...40E2294N Altcode:
The high spatial and temporal resolution observations at Extreme
Ultra-Violet (EUV) wavelengths from the Atmospheric Imaging Assembly
(AIA) of the Solar Dynamics Observatory (SDO) reveal new features in
kink oscillations of coronal loops. We show that, in addition to the
well-known rapidly decaying oscillations, a new type of kink waves is
present, characterized by low-amplitude and undamped oscillations,
that we define as decay-less. Typical periods range from 2.5 to 12
min in both regimes and are different for different loops, increasing
with the loop length. Estimates of the loop lengths are supported
by three dimensional reconstruction of the loop geometry. The
amplitude for the decay-less regime is about 1 Mm, close to the
spatial resolution of the AIA instruments. The oscillation phase,
measured by the cross-correlation method, is found to be constant
along each analysed loop, and the spatial structure of the phase of the
oscillations corresponds to the fundamental standing kink mode. We show
that the observed behaviours are consistent with the empirical model
of a damped linear oscillator excited by a continuous low-amplitude
harmonic driver, in addition to an eventual impulsive high-amplitude
driver. The observed life-time of the oscillations is likely to be
determined by the observational conditions rather than any physical
damping. However, the balance between the driving and damping is a
necessary ingredient of this model. The properties of this type of
transverse oscillations make them interesting object of study in the
framework of resonant absorption theory and coronal heating process.
---------------------------------------------------------
Title: Three-dimensional geometry of coronal loops inferred by the
Principal Component Analysis
Authors: Nisticò, Giuseppe; Nakariakov, Valery
2014cosp...40E2293N Altcode:
We propose a new method for the determination of the three dimensional
(3D) shape of coronal loops from stereoscopy. The common approach
requires to find a 1D geometric curve, as circumference or ellipse,
that best-fits the 3D tie-points which sample the loop shape
in a given coordinate system. This can be easily achieved by the
Principal Component (PC) analysis. It mainly consists in calculating
the eigenvalues and eigenvectors of the covariance matrix of the 3D
tie-points: the eigenvalues give a measure of the variability of the
distribution of the tie-points, and the corresponding eigenvectors
define a new cartesian reference frame directly related to the loop. The
eigenvector associated with the smallest eigenvalues defines the normal
to the loop plane, while the other two determine the directions of
the loop axes: the major axis is related to the largest eigenvalue,
and the minor axis with the second one. The magnitude of the axes is
directly proportional to the square roots of these eigenvalues. The
technique is fast and easily implemented in some examples, returning
best-fitting estimations of the loop parameters and 3D reconstruction
with a reasonable small number of tie-points. The method is suitable
for serial reconstruction of coronal loops in active regions, providing
a useful tool for comparison between observations and theoretical
magnetic field extrapolations from potential or force-free fields.
---------------------------------------------------------
Title: Collective responses of coronal loops and CMEs to the eruptions
of a filament
Authors: Zhang, Yuzong; Nakariakov, Valery; Wang, Jingxu; Zhang, Jun
2014cosp...40E3810Z Altcode:
With the extreme ultraviolet data acquired by the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory (SDO), we
study the dramatic and multiple activities of coronal loops of the
active regions (ARs) 11226 and 11227 during the double eruption event
of a toroidal filament located at the joint of the two ARs on 2011
June 2. It is found that 11 of 21 identified coronal loop systems
demonstrated collective responses in five different ways, i.e., 1)
shrinkage; 2) oscillation; 3) expansion; 4) eruption; 5) sideward
motion, observed in six coronal wavebands of AIA. Moreover, an EUV
wave triggered by the first filament eruption was seen to reflect back
and also contributed to the shrinkage of the coronal loop systems in
the two ARs. The different responding ways are closely related to the
relative positions and the directions of the filament eruptions and
the driven EUV wave propagation. Also, for the first time, both fast
and slow magnetoacoustic waves are observed to propagate simultaneously
along the same loop systems, with the speeds of ∼1,000 and tens of km
s-1 , respectively. The slow mode waves were found to be unaffected by
the fast ones. Furthermore, we shift to a more global scale to discuss
the waves and some of the responses of the loops in conjunction with
the CMEs.
---------------------------------------------------------
Title: Fast magnetoacoustic wave trains in magnetic funnels of the
solar corona
Authors: Pascoe, D. J.; Nakariakov, V. M.; Kupriyanova, E. G.
2013A&A...560A..97P Altcode:
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. <BR /> 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. <BR /> 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. <BR /> 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. <BR
/> 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.
---------------------------------------------------------
Title: Long-Period Oscillations of Sunspots by NoRH and SSRT
Observations
Authors: Bakunina, Irina A.; Abramov-maximov, Vladimir E.; Nakariakov,
Valery M.; Lesovoy, Sergei V.; Soloviev, Alexander A.; Tikhomirov,
Yurii V.; Melnikov, Victor F.; Shibasaki, Kiyoto; Nagovitsyn, Yurii
A.; Averina, Elena L.
2013PASJ...65S..13B Altcode:
Long-term oscillations of microwave emission generated in sunspot
magnetospheres are detected with the Nobeyama Radioheliograph (NoRH)
at a frequency of 17 GHz, and the Siberian Solar Radio Telescope
(SSRT) at 5.7 GHz. Significant periodicities in the range of 22-170
min are found in the variation of the emission intensity, polarisation
and the degree of circular polarisation. Periods of the oscillations
are not stable: they are different in different sunspots and in the
same sunspot on different days. A cross-correlation analysis shows the
presence of common significant periods in both NoRH and SSRT data. The
cross-correlation coefficients are typically lower than 0.5, which
can be attributed to the different heights of the emission formation,
and different mechanisms for the emission generation (gyroresonance
and thermal bremstrahlung at 17 GHz, and pure gyroresonance at 5.7
GHz). The observational results are consistent with the global sunspot
oscillation model.
---------------------------------------------------------
Title: Decay-less kink oscillations in coronal loops
Authors: Anfinogentov, S.; Nisticò, G.; Nakariakov, V. M.
2013A&A...560A.107A Altcode:
Context. Kink oscillations of coronal loops in an off-limb active
region are detected with the Imaging Assembly Array (AIA) instruments
of the Solar Dynamics Observatory (SDO) at 171 Å. <BR /> Aims: We aim
to measure periods and amplitudes of kink oscillations of different
loops and to determinate the evolution of the oscillation phase along
the oscillating loop. <BR /> Methods: Oscillating coronal loops were
visually identified in the field of view of SDO/AIA and STEREO/EUVI-A:
the loop length was derived by three-dimensional analysis. Several
slits were taken along the loops to assemble time-distance maps. We
identified oscillatory patterns and retrieved periods and amplitudes
of the oscillations. We applied the cross-correlation technique to
estimate the phase shift between oscillations at different segments of
oscillating loops. <BR /> Results: We found that all analysed loops
show low-amplitude undamped transverse oscillations. Oscillation
periods of loops in the same active region range from 2.5 to 11 min,
and are different for different loops. The displacement amplitude is
lower than 1 Mm. The oscillation phase is constant along each analysed
loop. The spatial structure of the phase of the oscillations corresponds
to the fundamental standing kink mode. We conclude that the observed
behaviour is consistent with the empirical model in terms of a damped
harmonic resonator affected by a non-resonant continuously operating
external force. <P />A movie is available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Quasi-periodic Wiggles of Microwave Zebra Structures in a
Solar Flare
Authors: Yu, Sijie; Nakariakov, V. M.; Selzer, L. A.; Tan, Baolin;
Yan, Yihua
2013ApJ...777..159Y Altcode: 2013arXiv1309.5777Y
Quasi-periodic wiggles of microwave zebra pattern (ZP) structures with
periods ranging from about 0.5 s to 1.5 s are found in an X-class
solar flare on 2006 December 13 at the 2.6-3.8 GHz with the Chinese
Solar Broadband Radio Spectrometer (SBRS/Huairou). Periodogram and
correlation analysis show that the wiggles have two to three significant
periodicities and are almost in phase between stripes at different
frequencies. The Alfvén speed estimated from the ZP structures is about
700 km s<SUP>-1</SUP>. We find the spatial size of the wave-guiding
plasma structure to be about 1 Mm with a detected period of about 1
s. This suggests that the ZP wiggles can be associated with the fast
magnetoacoustic oscillations in the flaring active region. The lack of
a significant phase shift between wiggles of different stripes suggests
that the ZP wiggles are caused by a standing sausage oscillation.
---------------------------------------------------------
Title: 3D Reconstruction of Coronal Loops by the Principal Component
Analysis
Authors: Nisticò, Giuseppe; Verwichte, Erwin; Nakariakov, Valery
2013Entrp..15.4520N Altcode:
Knowing the three dimensional structure of plasma filaments in the
uppermost part of the solar atmosphere, known as coronal loops, and
especially their length, is an important parameter in the wave-based
diagnostics of this part of the Sun. The combination of observations of
the Sun from different points of observations in space, thanks to the
most recent missions, including the Solar Dynamics Observatory (SDO)
and the Solar TErrestrial RElations Observatory (STEREO), allows us
to infer information about the geometrical shape of coronal loops in
3D space. Here, we propose a new method to reconstruct the loop shape
starting from stereoscopically determined 3D points, which sample the
loop length, by principal component analysis. This method is shown
to retrieve in an easy way the main parameters that define the loop,
e.g., the minor and major axes, the loop plane, the azimuthal and
inclination angles, for the special case of a coplanar loop.
---------------------------------------------------------
Title: The Decaying Long-period Oscillation of a Stellar Megaflare
Authors: Anfinogentov, S.; Nakariakov, V. M.; Mathioudakis, M.;
Van Doorsselaere, T.; Kowalski, A. F.
2013ApJ...773..156A Altcode:
We analyze and interpret the oscillatory signal in the decay phase of
the U-band light curve of a stellar megaflare observed on 2009 January
16 on the dM4.5e star YZ CMi. The oscillation is well approximated
by an exponentially decaying harmonic function. The period of the
oscillation is found to be 32 minutes, the decay time about 46 minutes,
and the relative amplitude 15%. As this observational signature is
typical of the longitudinal oscillations observed in solar flares at
extreme ultraviolet and radio wavelengths, associated with standing
slow magnetoacoustic waves, we suggest that this megaflare may be of a
similar nature. In this scenario, macroscopic variations of the plasma
parameters in the oscillations modulate the ejection of non-thermal
electrons. The phase speed of the longitudinal (slow magnetoacoustic)
waves in the flaring loop or arcade, the tube speed, of about 230
km s<SUP>-1</SUP> would require a loop length of about 200 Mm. Other
mechanisms, such as standing kink oscillations, are also considered.
---------------------------------------------------------
Title: Distinct propagating fast wave trains associated with flaring
energy releases
Authors: Yuan, D.; Shen, Y.; Liu, Y.; Nakariakov, V. M.; Tan, B.;
Huang, J.
2013A&A...554A.144Y Altcode:
Context. Large-scale fast waves with perturbation of the EUV emission
intensity are well resolved in both temporal and spatial scale by
SDO/AIA. These waves are prone to propagate along the magnetic field
line. <BR /> Aims: We aim to probe the link between propagating
fast wave trains and flaring energy releases. By measuring the wave
parameters, we reveal their nature and investigate the potential
to diagnose the energy source and waveguide. <BR /> Methods: The
spatial and temporal evolution of the wave amplitude and propagating
speed are studied. The correlation of individual wave trains with
flare-generated radio bursts is tested. <BR /> Results: The propagating
wave pattern comprises distinct wave trains with varying periods
and wavelengths. This characteristic signature is consistent with
the patterns formed by waveguide dispersion, when different spectral
components propagate at different phase and group speeds. The wave
train releases are found to be highly correlated in start time with
the radio bursts emitted by the non-thermal electrons that were
accelerated in bursty energy releases. The wave amplitude is seen to
reach the maximum midway during its course. This can be caused by a
combined effect of the waveguide spread in the transverse direction
and density stratification. The transverse amplitude distribution
perpendicular to the wave vector is found to follow approximately a
Gaussian profile. The spatial structure is consistent with the kink
mode that is polarised along the line-of-sight. The propagating speed
is subject to deceleration from ~735-845 km s<SUP>-1</SUP> to ~600
km s<SUP>-1</SUP>. This could be caused by the decrease in the local
Alfvén speed and/or the projection effect.
---------------------------------------------------------
Title: Decaying and decayless transverse oscillations of a coronal
loop
Authors: Nisticò, G.; Nakariakov, V. M.; Verwichte, E.
2013A&A...552A..57N Altcode:
<BR /> Aims: We investigate kink oscillations of loops observed in an
active region with the Atmospheric Imaging Assembly (AIA) instrument on
board the Solar Dynamics Observatory (SDO) spacecraft before and after
a flare. <BR /> Methods: The oscillations were depicted and analysed
with time-distance maps, extracted from the cuts taken parallel or
perpendicular to the loop axis. Moving loops were followed in time
with steadily moving slits. The period of oscillations and its time
variation were determined by best-fitting harmonic functions. <BR />
Results: We show that before and well after the occurrence of the flare,
the loops experience low-amplitude decayless oscillations. The flare
and the coronal mass ejection associated to it trigger large-amplitude
oscillations that decay exponentially in time. The periods of the kink
oscillations in both regimes (about 240 s) are similar. An empirical
model of the phenomenon in terms of a damped linear oscillator
excited by a continuous low-amplitude harmonic driver and by an
impulsive high-amplitude driver is found to be consistent with the
observations. <P />Two movies are available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Sausage Oscillations of Coronal Plasma Structures
Authors: Nakariakov, V. M.; Hornsey, C.; Melnikov, V. F.
2012ApJ...761..134N Altcode:
The dependence of the period of sausage oscillations of coronal loops
on length together with the depth and steepness of the radial profile
are determined. We performed a parametric study of linear axisymmetric
fast magnetoacoustic (sausage) oscillations of coronal loops modeled
as a field-aligned low-β plasma cylinder with a smooth inhomogeneity
of the plasma density in the radial direction. The density decreases
smoothly in the radial direction. Sausage oscillations are impulsively
excited by a perturbation of the radial velocity, localized at the
cylinder axis and with a harmonic dependence on the longitudinal
coordinate. The initial perturbation results in either a leaky or a
trapped sausage oscillation, depending upon whether the longitudinal
wavenumber is smaller or greater than a cutoff value, respectively. The
period of the sausage oscillations was found to always increase with
increasing longitudinal wavelength, with the dependence saturating in
the long-wavelength limit. Deeper and steeper radial profiles of the
Alfvén speed correspond to more efficient trapping of sausage modes:
the cutoff value of the wavelength increases with the steepness and
the density (or Alfvén speed) contrast ratio. In the leaky regime,
the period is always longer than the period of a trapped mode of a
shorter wavelength in the same cylinder. For shallow density profiles
and shorter wavelengths, the period increases with wavelength. In the
long-wavelength limit, the period becomes independent of the wavelength
and increases with the depth and steepness of the radial profile of
the Alfvén speed.
---------------------------------------------------------
Title: Preface
Authors: Nakariakov, V. M.; Georgoulis, M. K.; Poedts, S.; van
Driel-Gesztelyi, L.; Mandrini, C. H.; Leibacher, J.
2012SoPh..280..295N Altcode: 2012SoPh..tmp..226N
No abstract at ADS
---------------------------------------------------------
Title: Slow Magnetoacoustic Oscillations in the Microwave Emission
of Solar Flares
Authors: Kim, S.; Nakariakov, V. M.; Shibasaki, K.
2012ApJ...756L..36K Altcode: 2013arXiv1310.2796K
Analysis of the microwave data, obtained in the 17 GHz channel of the
Nobeyama Radioheliograph during the M1.6 flare on 2010 November 4,
revealed the presence of 12.6 minute oscillations of the emitting
plasma density. The oscillations decayed with the characteristic time
of about 15 minutes. Similar oscillations with the period of about
13.8 minutes and the decay time of 25 minutes are also detected in the
variation of EUV emission intensity measured in the 335 Å channel
of the Solar Dynamics Observatory/Atmospheric Imaging Assembly. The
observed properties of the oscillations are consistent with the
oscillations of hot loops observed by the Solar and Heliospheric
Observatory/Solar Ultraviolet Measurement of Emitted Radiation (SUMER)
in the EUV spectra in the form of periodic Doppler shift. Our analysis
presents the first direct observations of the slow magnetoacoustic
oscillations in the microwave emission of a solar flare, complementing
accepted interpretations of SUMER hot loop oscillations as standing
slow magnetoacoustic waves.
---------------------------------------------------------
Title: Nonlinear evolution of torsional Alfvén waves
Authors: Vasheghani Farahani, S.; Nakariakov, V. M.; Verwichte, E.;
Van Doorsselaere, T.
2012A&A...544A.127V Altcode:
<BR /> Aims: We study the efficiency of the energy transfer to shorter
scales in the field-aligned direction - the parallel nonlinear cascade
- that accompanies the propagation of torsional Alfvén waves along
open magnetic fields in the solar and stellar coronae, and compare it
with the same effects for the shear Alfvén wave. The evolution of the
torsional Alfvén wave is caused by the back reaction of nonlinearly
induced compressive perturbations on the Alfvén wave. <BR /> Methods:
The evolution of upwardly propagating torsional Alfvén waves is
considered in terms of the second-order thin flux-tube approximation
in a straight untwisted and non-rotating magnetic flux-tube. The
Cohen-Kulsrud equation for weakly nonlinear torsional waves is
derived. In the model, the effect of the cubic nonlinearity on the
propagation of long-wavelength axisymmetric torsional waves is compared
with the similar effect that accompanies the propagation of plane
linearly-polarised (shear) Alfvén waves of small amplitude. <BR />
Results: The solution to the Cohen-Kulsrud type equation for torsional
waves shows that their evolution is independent of the plasma-β,
which is in contrast to the shear Alfvén wave. In a finite-β plasma,
the nonlinear evolution of torsional Alfvén waves is slower and the
parallel nonlinear cascade is less efficient than those of shear Alfvén
waves. These results have important implications for the analysis of
possible heating of the plasma and its acceleration in the upper layers
of solar and stellar coronae. In particular, one-dimensional models of
coronal heating and wave acceleration, which use shear Alfvén waves
instead of torsional Alfvén waves, over-estimate the efficiency of
these processes.
---------------------------------------------------------
Title: Magnetohydrodynamic vortices in the solar corona
Authors: Nakariakov, Valery
2012cosp...39.1329N Altcode: 2012cosp.meet.1329N
Interaction of coronal mass ejections with the plasma of the solar
corona is accompanied with the generation of sharp shear flows that
cause the induction of Alfvenic vortices. Numerical simulations revealed
that for a broad range of parameters, the vortices are essentially
compressible, with the perturbations of the density and the absolute
value of the magnetic field in the vortex arms reaching 50% of the
background density. The typical size of the vortex is about the size
of the obstacle. The frequency of the vortex shedding is controlled
by a dimensionless parameter known as the Strouhal number. We found
that in collisionless low-beta plasma this number is about 0.2. Recent
imaging observations with SDO/AIA revealed the generation of Alfvenic
vortices at the flanks of emerging plasmoids. The vortices introduce
a frictionless aerodynamic drag force, applied to the interacting
plasmas. Implication of these findings for the excitation of kink
oscillations of coronal loops and CME kinematics is discussed.
---------------------------------------------------------
Title: Intensity variations associated with fast sausage modes
Authors: Gruszecki, M.; Nakariakov, V. M.; Van Doorsselaere, T.
2012A&A...543A..12G Altcode:
<BR /> Aims: We determine the dependence of the observed properties of
fast magnetoacoustic axisymmetric oscillations (the sausage mode) of a
thick and dense flaring coronal loop, modelled by a magnetic cylinder,
on the parameters of the equilibrium plasma configuration. The plasma
inside and outside the cylinder is of low-beta, and penetrated by a
straight magnetic field. The plasma density has a smooth profile across
the magnetic field. <BR /> Methods: We use three-dimensional ideal
magnetohydrodynamic equations to model numerically the development
of the perturbations of the cylindrical equilibrium, considering
both leaky and trapped regimes. <BR /> Results: Short-period sausage
oscillations, trapped by the cylinder, are qualitatively consistent
with the analytical results obtained in the models of a plasma slab
or a cylinder with a step-function transverse profile. The period of
trapped sausage oscillations is determined by the ratio of the phase
speed, with the value between the internal and external Alfvén speeds,
to the wavelength. Longer-period sausage oscillations are leaky, and
their decay times are longer for higher density contrasts between the
internal and external media. Leaky sausage oscillations have longer
periods than trapped sausage oscillations of the same cylinder. In the
coronal conditions, sausage oscillations are essentially compressible
and transverse, hence produce modulation of the thermal optically
thin emission intensity and periodic Doppler broadening of emission
lines. However, if the oscillating plasma non-uniformity is poorly
spatially resolved, the variation in the emission intensity is weak and
proportional to the actual amplitude of the oscillation squared. The
latter variation property is connected with the transverse nature of
the oscillation, causing the conservation of mass in the transverse
cross-section of the oscillating plasma structure.
---------------------------------------------------------
Title: Slow magnetoacoustic waves in the solar corona
Authors: Nakariakov, Valery
2012cosp...39.1330N Altcode: 2012cosp.meet.1330N
Quasi-monochromatic propagating disturbances of EUV intensity emission
are often detected in fan-like structures and in polar plumes of the
solar corona. We demonstrate that the observed characteristics of these
propagating disturbances, including the apparent blue-wind asymmetry of
the emission line, are consistent with their interpretation in terms of
slow magnetoacoustic waves. The propagating waves are well-described by
a one-dimensional evolutionary equation that accounts for the effects
of thermal conduction and stratification. Moreover, the waves are shown
to be a genuine counterpart of 3-min oscillations in the chromospheric
slow-magnetoacoustic resonator. It is also found that the leaky slow
magnetoacoustic waves provide a link between oscillations in sunspots
and quasi-periodic pulsations in a solar flare occurring nearby,
suggesting that the waves can play a role of external triggering of
the energy release.
---------------------------------------------------------
Title: Measuring the apparent phase speed of propagating EUV
disturbances
Authors: Yuan, D.; Nakariakov, V. M.
2012A&A...543A...9Y Altcode:
Context. Propagating disturbances of the EUV emission intensity are
commonly observed over a variety of coronal structures. Parameters of
these disturbances, particularly the observed apparent (image-plane
projected) propagation speed, are important tools for MHD coronal
seismology. <BR /> Aims: We design and test tools to reliably measure
the apparent phase speed of propagating disturbances in imaging data
sets. <BR /> Methods: We designed cross-fitting technique (CFT), 2D
coupled fitting (DCF) and best similarity match (BSM) to measure the
apparent phase speed of propagating EUV disturbances in the running
differences of time-distance plots (R) and background-removed and
normalised time-distance plots (D). <BR /> Results: The methods were
applied to the analysis of quasi-periodic EUV disturbances propagating
at a coronal fan-structure of active region NOAA11330 on 27 Oct. 2011,
observed with the Atmospheric Imaging Assembly (AIA) on SDO in the
171 Å bandpass. The noise propagation in the AIA image processing was
estimated, resulting in the preliminary estimation of the uncertainties
in the AIA image flux. This information was used in measuring the
apparent phase speed of the propagating disturbances with the CFT, DCF
and BSM methods, which gave consistent results. The average projected
speed is measured at 47.6 ± 0.6 km s<SUP>-1</SUP> and 49.0 ± 0.7 km
s<SUP>-1</SUP> for R and D, with the corresponding periods at 179.7 ±
0.2 s and 179.7 ± 0.3 s, respectively. We analysed the effects of the
lag time and the detrending time in the running difference processing
and the background-removed plot, on the measurement of the speed,
and found that they are fairly weak. <BR /> Conclusions: The CFT,
DCF and BSM methods are found to be reliable techniques for measuring
the apparent (projected) phase speed. The samples of larger effective
spatial length are more suitable for these methods. Time-distance
plots with background removal and normalisation allow for more robust
measurements, with little effect of the choice of the detrending
time. Cross-fitting technique provides reliable measurements on
good samples (e.g. samples with large effective detection length and
recurring features). 2D coupled-fitting is found to be sensitive to the
initial guess for parameters of the 2D fitting function. Thus DCF is
only optimised in measuring one of the parameters (the phase speed in
our application), while the period is poorly measured. Best similarity
measure is robust for all types of samples and very tolerant to image
pre-processing and regularisation (smoothing).
---------------------------------------------------------
Title: Nonlinear long-wavelength torsional Alfven waves
Authors: Nakariakov, Valery
2012cosp...39.1331N Altcode: 2012cosp.meet.1331N
Long-wavelength torsional (Alfven) waves in solar and stellar coronal
structures experience geometrical amplification with height and hence
are subject to nonlinear effects. We analyse this regime in frames
the second order thin flux-tube approximation of Zhugzhda, which
describes axisymmetric (sausage) magnetohydrodynamic perturbations
of a straight untwisted and non-rotating magnetic flux-tube,
representing e.g. a polar plume or a jet, or a coronal loop or a
prominence filament. Attention is paid to the compressible motions
nonlinearly induced by long-wavelength torsional waves of small, but
finite amplitude. We obtained that propagating torsional waves induce
compressible perturbations oscillating with double the frequency
of the torsional waves. In contrast with plane shear Alfven waves,
the amplitude of compressible perturbations is independent of the
plasma-beta. Moreover, nonlinear evolution of torsional waves is not
affected by the singularity appearing at the height when the local
Alfven speed is equal to the sound speed. This result significantly
reduces the efficiency of nonlinear cascade, and hence suggests that the
present theories of the solar and stellar wind heating and acceleration
by Alfven waves, based upon the plane wave theory, require modification.
---------------------------------------------------------
Title: Magnetohydrodynamic waves and coronal seismology: an overview
of recent results
Authors: De Moortel, I.; Nakariakov, V. M.
2012RSPTA.370.3193D Altcode: 2012arXiv1202.1944D
Recent observations have revealed that MHD waves and oscillations are
ubiquitous in the solar atmosphere, with a wide range of periods. We
give a brief review of some aspects of MHD waves and coronal seismology
which have recently been the focus of intense debate or are newly
emerging. In particular, we focus on four topics: (i) the current
controversy surrounding propagating intensity perturbations along
coronal loops, (ii) the interpretation of propagating transverse
loop oscillations, (iii) the ongoing search for coronal (torsional)
Alfven waves and (iv) the rapidly developing topic of quasi-periodic
pulsations (QPP) in solar flares.
---------------------------------------------------------
Title: Solar Particle Acceleration Radiation and Kinetics (SPARK). A
mission to understand the nature of particle acceleration
Authors: Matthews, Sarah A.; Williams, David R.; Klein, Karl-Ludwig;
Kontar, Eduard P.; Smith, David M.; Lagg, Andreas; Krucker, Sam;
Hurford, Gordon J.; Vilmer, Nicole; MacKinnon, Alexander L.; Zharkova,
Valentina V.; Fletcher, Lyndsay; Hannah, Iain G.; Browning, Philippa
K.; Innes, Davina E.; Trottet, Gerard; Foullon, Clare; Nakariakov,
Valery M.; Green, Lucie M.; Lamoureux, Herve; Forsyth, Colin; Walton,
David M.; Mathioudakis, Mihalis; Gandorfer, Achim; Martinez-Pillet,
Valentin; Limousin, Olivier; Verwichte, Erwin; Dalla, Silvia; Mann,
Gottfried; Aurass, Henri; Neukirch, Thomas
2012ExA....33..237M Altcode: 2011ExA...tmp..124M
Energetic particles are critical components of plasma populations
found throughout the universe. In many cases particles are accelerated
to relativistic energies and represent a substantial fraction of
the total energy of the system, thus requiring extremely efficient
acceleration processes. The production of accelerated particles
also appears coupled to magnetic field evolution in astrophysical
plasmas through the turbulent magnetic fields produced by diffusive
shock acceleration. Particle acceleration is thus a key component
in helping to understand the origin and evolution of magnetic
structures in, e.g. galaxies. The proximity of the Sun and the range
of high-resolution diagnostics available within the solar atmosphere
offers unique opportunities to study the processes involved in particle
acceleration through the use of a combination of remote sensing
observations of the radiative signatures of accelerated particles, and
of their plasma and magnetic environment. The SPARK concept targets the
broad range of energy, spatial and temporal scales over which particle
acceleration occurs in the solar atmosphere, in order to determine how
and where energetic particles are accelerated. SPARK combines highly
complementary imaging and spectroscopic observations of radiation from
energetic electrons, protons and ions set in their plasma and magnetic
context. The payload comprises focusing-optics X-ray imaging covering
the range from 1 to 60 keV; indirect HXR imaging and spectroscopy
from 5 to 200 keV, γ-ray spectroscopic imaging with high-resolution
LaBr<SUB>3</SUB> scintillators, and photometry and source localisation
at far-infrared wavelengths. The plasma environment of the regions
of acceleration and interaction will be probed using soft X-ray
imaging of the corona and vector magnetography of the photosphere
and chromosphere. SPARK is designed for solar research. However,
in addition it will be able to provide exciting new insights into the
origin of particle acceleration in other regimes, including terrestrial
gamma-ray flashes (TGF), the origin of γ-ray bursts, and the possible
existence of axions.
---------------------------------------------------------
Title: Coronal Seismology: Waves and Oscillations in Stellar Coronae
Flare Plasma
Authors: Stepanov, Alexander V.; Zaitsev, Valery V.; Nakariakov,
Valery M.
2012cswo.book.....S Altcode:
This concise and systematic account of the current state of this new
branch of astrophysics presents the theoretical foundations of plasma
astrophysics, magneto-hydrodynamics and coronal magnetic structures,
taking into account the full range of available observation techniques
-- from radio to gamma. <P />The book discusses stellar loops during
flare energy releases, MHD waves and oscillations, plasma instabilities
and heating and charged particle acceleration. Current trends and
developments in MHD seismology of solar and stellar coronal plasma
systems are also covered, while recent progress is presented in the
observational study of quasi-periodic pulsations in solar and stellar
flares with radio, optical, X and gamma rays. In addition, the authors
investigate the origin of coherent radio emission from stellar loops,
paying special attention to their fine structure. <P />For advanced
students and specialists in astronomy, as well as theoretical and
plasma physics.
---------------------------------------------------------
Title: Frequency drifts of 3-min oscillations in microwave and EUV
emission above sunspots
Authors: Sych, R.; Zaqarashvili, T. V.; Nakariakov, V. M.;
Anfinogentov, S. A.; Shibasaki, K.; Yan, Y.
2012A&A...539A..23S Altcode: 2011arXiv1110.2556S
<BR /> Aims: We analysed 3-min oscillations of microwave and extreme
ultraviolet (EUV) emission generated at different heights of a sunspot
atmosphere, studied the amplitude and frequency modulation of the
oscillations, and its relationship with the variation of the spatial
structure of the oscillations. <BR /> Methods: High-resolution data
obtained with the Nobeyama Radioheliograph, TRACE and SDO/AIA were
analysed with pixelised wavelet filtering (PWF) and wavelet skeleton
techniques. <BR /> Results: Three-minute oscillations in sunspots
appear in the form of recurring trains of 8-20 min duration (13 min
in average). The typical interval between the trains is 30-50 min. The
oscillation trains are transient in frequency and power. The relative
amplitude of 3-min oscillations was about 3-8% and sometimes reached
17%. Recurring frequency drifts of 3-min oscillations were detected
during the development of individual trains, with the period varying
in the range 90-240 s. A wavelet analysis showed that there are three
types of oscillation trains: with positive drifts (to high frequencies),
negative drifts, and without a drift. Negative drifts, i.e., when
the 3-min oscillation period gradually increases, were found to occur
more often. The start and end of the drifts coincides with the start
time and end of the train. Sometimes two drifts co-exist, i.e. during
the end of the previous drift, a new drift appears near 160 s, when
the frequency is in the low-frequency part of the 3-min spectrum,
near 200 s. This behaviour is seen at all levels of the sunspot
atmosphere. The speed of the drift is 4-5 mHz/h in the photosphere,
5-8 mHz/h in the chromosphere, and 11-13 mHz/h in the corona. There
were also low-frequency peaks in the spectrum, corresponding to the
periods of 10-20 min, and 30-60 min. The comparative study of the
spatial structure of 3-min oscillations in microwave and EUV shows
the appearance of new sources of the sunspot oscillations during the
development of the trains. <BR /> Conclusions: These structures can
be interpreted as waveguides that channel upward propagating waves,
which in turn are responsible for the 3-min oscillations. A possible
explanation of the observed properties are two simultaneously operating
factors: dispersive evolution of the upward propagating wave pulses
and the non-uniformity of the oscillation power distribution over
the sunspot umbra with different wave sources that correspond to
different magnetic flux tubes with different physical conditions and
line-of-sight angles.
---------------------------------------------------------
Title: Three-minute Oscillations above Sunspot Umbra Observed with
the Solar Dynamics Observatory/Atmospheric Imaging Assembly and
Nobeyama Radioheliograph
Authors: Reznikova, V. E.; Shibasaki, K.; Sych, R. A.; Nakariakov,
V. M.
2012ApJ...746..119R Altcode: 2011arXiv1109.5434R
Three-minute oscillations over a sunspot's umbra in AR 11131 were
observed simultaneously in UV/EUV emission by the Solar Dynamics
Observatory (SDO)/Atmospheric Imaging Assembly (AIA) and in radio
emission by the Nobeyama Radioheliograph (NoRH). We use 24 hr series of
SDO and 8 hr series of NoRH observations to study spectral, spatial,
and temporal variations of pulsations in the 5-9 mHz frequency range
at different layers of the solar atmosphere. High spatial and temporal
resolution of SDO/AIA in combination with long-duration observations
allowed us to trace the variations of the cutoff frequency and
spectrum of oscillations across the umbra. We found that higher
frequency oscillations are more pronounced closer to the umbra's
center, while the lower frequencies concentrate on the peripheral
parts. We interpreted this discovery as a manifestation of variation
of the magnetic field inclination across the umbra at the level of
temperature minimum. Possible implications of this interpretation for
the diagnostics of sunspot atmospheres are discussed.
---------------------------------------------------------
Title: Slow magnetacoustic waves in magnetic arcades
Authors: Gruszecki, M.; Nakariakov, V. M.
2011A&A...536A..68G Altcode:
<BR /> Aims: We investigate two-dimensional effects on the evolution of
impulsively-generated slow magnetoacoustic waves in magnetic arcades
of the solar corona. <BR /> Methods: We used a two-dimensional box
model of a coronal arcade, neglecting the effects of gravity and
magnetic curvature. Ideal finite-β magnetohydrodynamic equations were
employed. The plasma was taken to be uniform, penetrated by a straight
and uniform magnetic field. We applied line-tying boundary conditions
at the magnetic footpoints. <BR /> Results: Running and standing slow
magnetoacoustic waves develop across the magnetic field due to the
reflection from the footpoints in the arcade. The perpendicular group
speed is lower than both sound and Alfvén speeds. The speed grows
with the increase in plasma-β, which is consistent with analytical
theory. Slow magnetoacoustic pulse perturbs the magnetic field and
current density; this effect is stronger for higher amplitudes. Standing
waves spread across the field, forming a characteristic phase-mixing
pattern of anti-parallel flows. The two-dimensional effects are more
pronounced for higher β. Our results confirm the plausibility of the
interpretation of the observed evolution of two-ribbon flares in terms
of slow magnetoacoustic waves in magnetic arcades.
---------------------------------------------------------
Title: Entropy mode at a magnetic null point as a possible tool for
indirect observation of nanoflares in the solar corona
Authors: Murawski, K.; Zaqarashvili, T. V.; Nakariakov, V. M.
2011A&A...533A..18M Altcode:
<BR /> Aims: We aim to explore the dynamics of the entropy mode
perturbation excited by an energy release in the vicinity of a
magnetic null point that is embedded in a gravitationally stratified
solar corona. <BR /> Methods: We solve two-dimensional, time-dependent
magnetohydrodynamic equations numerically to find spatial and temporal
signatures of the entropy mode that is triggered impulsively by a
spatially localized pulse of the gas pressure. <BR /> Results: We
find that the properties of the entropy mode are determined by the
sign of the initial pressure pulse. The initial increase in the gas
pressure creates, together with the magnetoacoustic-gravity waves,
a stationary void of the rarefied plasma at the launching place,
associated with the entropy mode. In contrast, an initial decrease
in the gas pressure, which corresponds to a rapid (or catastrophic)
cooling, forms a blob of the dense plasma at the launching place. <BR
/> Conclusions: The cool, dense blobs at magnetic null points may
be observed in transition region and chromospheric spectral lines
at coronal heights off the solar limb and may be associated with the
places of nanoflare occurrence. Therefore, extensions of entropy mode
studies may produce a diagnostic tool for indirect observations of
nanoflares. The dense cool blobs may be affected by the gravity or
carried by downflows, hence may initiate a coronal rain.
---------------------------------------------------------
Title: Leakage of long-period oscillations from the chromosphere to
the corona
Authors: Yuan, D.; Nakariakov, V. M.; Chorley, N.; Foullon, C.
2011A&A...533A.116Y Altcode:
Long-period oscillations in a coronal diffuse structure are detected
with the Transition Region And Coronal Explorer (TRACE). The EUV
images of the NOAA active region 8253 are available in 171 Å and
195 Å bandpasses from 30 June to 4 July 1998. The average intensity
variation is found to be connected with the CCD temperature, which
varies with the orbital motion of the spacecraft. Hence, oscillations
with the orbital period and its higher harmonics appear as artifacts
in the light curves. After the exclusion of the orbital effects, we
identified several long-period oscillations in the diffuse fan-like
structure of the active region. Similar periodicities were detected
in the radio emission from the chromospheric part of that active
region, observed with the ground-based Nobeyama Radioheliograph (NoRH)
in the 17 GHz channel. It was found that 0.221, 0.312 and 0.573 mHz
oscillations were present in both EUV emission lines in the corona and
the radio signal from the sunspot in the chromosphere, just beneath
the active region. From the frequency values, the 1st and 3rd detected
oscillations could be associated with the l = 2, n = -3 or l = 3, n =
-5 and l = 1 gravity-driven solar interior modes, respectively. The
appearance of these oscillations in the coronal part of the active
region can be connected with the wave leakage or the evanescence of
chromospheric oscillations.
---------------------------------------------------------
Title: Magnetoacoustic shock formation near a magnetic null point
Authors: Gruszecki, M.; Vasheghani Farahani, S.; Nakariakov, V. M.;
Arber, T. D.
2011A&A...531A..63G Altcode:
<BR /> Aims: We investigate the interaction of nonlinear fast
magnetoacoustic waves with a magnetic null point in connection with the
triggering of solar flares. <BR /> Methods: We model the propagation of
fast, initially axisymmetric waves towards a two-dimensional isothermal
magnetic null point in terms of ideal magnetohydrodynamic equations. The
numerical simulations are carried out with the Lagrangian remap code
Lare2D. <BR /> Results: Dynamics of initially axisymmetric fast pulses
of small amplitude is found to be consistent with a linear analytical
solution proposed earlier. The increase in the amplitude leads to the
nonlinear acceleration of the compression pulse and deceleration of
the rarefaction pulse and hence the distortion of the wave front. The
pulse experiences nonlinear steepening in the radial direction either
on the leading or the back slopes for the compression and rarefaction
pulses, respectively. This effect is most pronounced in the directions
perpendicular to the field. Hence, the nonlinear evolution of the fast
pulse depends on the polar angle. The nonlinear steepening generates
the sharp spikes of the electric current density. As in the uniform
medium, the position of the shock formation also depends on the initial
width of the pulse. Only sufficiently smooth and low-amplitude initial
pulses can reach the vicinity of the null point, create there current
density spikes, and initiate magnetic reconnection by seeding anomalous
electrical resistivity. Steeper and higher amplitude initial pulses
overturn at larger distance from the null point, and cannot trigger
reconnection.
---------------------------------------------------------
Title: Damped large amplitude transverse oscillations in an EUV
solar prominence, triggered by large-scale transient coronal waves
Authors: Hershaw, J.; Foullon, C.; Nakariakov, V. M.; Verwichte, E.
2011A&A...531A..53H Altcode:
<BR /> Aims: We investigate two successive trains of large amplitude
transverse oscillations in an arched EUV prominence, observed
with SoHO/EIT on the north-east solar limb on 30 July 2005. The
oscillatory trains are triggered by two large scale coronal waves,
associated with an X-class and a C-class flare occurring in the same
remote active region. <BR /> Methods: The oscillations are tracked
within rectangular slits parallel to the solar limb at different
heights, which are taken to move with the apparent height profile
of the prominence to account for solar rotation. Time series for
the two prominence arch legs are extracted using Gaussian fitting
on the 195 Å absorption features, and fitted to a damped cosine
curve to determine the oscillatory parameters. <BR /> Results:
Differing energies of the two triggering flares and associated
waves are found to agree with the velocity amplitudes, of 50.6 ±
3.2 and 15.9 ± 8.0 km s<SUP>-1</SUP> at the apex, for the first
and second oscillatory trains respectively, as estimated in the
transverse direction. The period of oscillation is similar for both
trains, with an average of 99 ± 11 min, indicating a characteristic
frequency as predicted by magnetohydrodynamics. Increasing velocity
amplitude with height during the first oscillatory train, and in-phase
starting motions of the two legs regardless of height, for each train,
demonstrate that the prominence exhibits a global kink mode to a
first approximation. However, discrepancies between the oscillatory
characteristics of the two legs and an apparent dependence of period
upon height, suggest that the prominence actually oscillates as a
collection of separate but interacting threads. Damping times of around
two to three cycles are observed. Combining our results with those
of previously analysed loop oscillations, we find an approximately
linear dependence of damping time upon period for kink oscillations,
supporting resonant absorption as the damping mechanism despite
limitations in testing this theory.
---------------------------------------------------------
Title: Instrumental oscillations in RHESSI count rates during
solar flares
Authors: Inglis, A. R.; Zimovets, I. V.; Dennis, B. R.; Kontar, E. P.;
Nakariakov, V. M.; Struminsky, A. B.; Tolbert, A. K.
2011A&A...530A..47I Altcode: 2011arXiv1102.5349I
<BR /> Aims: We seek to illustrate the analysis problems posed
by RHESSI spacecraft motion by studying persistent instrumental
oscillations found in the lightcurves measured by RHESSI's X-ray
detectors in the 6-12 keV and 12-25 keV energy range during the
decay phase of the flares of 2004 November 4 and 6. <BR /> Methods:
The various motions of the RHESSI spacecraft which may contribute
to the manifestation of oscillations are studied. The response of
each detector in turn is also investigated. <BR /> Results: We find
that on 2004 November 6 the observed oscillations correspond to the
nutation period of the RHESSI instrument. These oscillations are of
greatest amplitude for detector 5, while in the lightcurves of many
other detectors the oscillations are small or undetectable. We also
find that the variation in detector pointing is much larger during this
flare than the counterexample of 2004 November 4. <BR /> Conclusions:
Sufficiently large nutation motions of the RHESSI spacecraft lead to
clearly observable oscillations in count rates, posing a significant
hazard for data analysis. This issue is particularly problematic for
detector 5 due to its design characteristics. Dynamic correction of
the RHESSI counts, accounting for the livetime, data gaps, and the
transmission of the bi-grid collimator of each detector, is required
to overcome this issue. These corrections should be applied to all
future oscillation studies.
---------------------------------------------------------
Title: Period persistence of long period oscillations in sunspots
Authors: Chorley, N.; Foullon, C.; Hnat, B.; Nakariakov, V. M.;
Shibasaki, K.
2011A&A...529A.123C Altcode:
Long period oscillations in the microwave radiation intensity
generated over the sunspot of NOAA AR 10330 are studied with the
Nobeyama Radioheliograph as the sunspot passes over the solar disk,
over the course of 9 days (06-15 April 2003). Periodogram, Fourier
and global wavelet analyses reveal the presence of a significant
oscillatory component in the range P ≈ 50-120 min over the course of
the observations. The spectral amplitudes of five significant Fourier
components in the range P = 50-150 min are also seen to be stable over
the observations, when the data are not affected by changes in magnetic
configuration in the region. The ground-based nature of the instrument
naturally introduces long data gaps in such long duration observations
and the presence of the gaps does not allow any conclusion as to the
stability of the phases of the oscillations. As a model to explain the
persistence of the dominant long periods, a simple oscillator with a
nonlinear driving term is proposed. The spectral difference between
distinct peaks within, e.g. the 3 min spectral band, is expected to
be able to resonate with the long period one hour oscillations.
---------------------------------------------------------
Title: Slow Magnetoacoustic Waves in Two-ribbon Flares
Authors: Nakariakov, V. M.; Zimovets, I. V.
2011ApJ...730L..27N Altcode:
We demonstrate that disturbances observed to propagate along the axis
of the arcade in two-ribbon solar flares at the speed of a few tens
of km s<SUP>-1</SUP>, well below the Alfvén and sound speeds, can
be interpreted in terms of slow magnetoacoustic waves. The waves can
propagate across the magnetic field, parallel to the magnetic neutral
line, because of the wave-guiding effect due to the reflection from
the footpoints. The perpendicular group speed of the perturbation
is found to be a fraction of the sound speed, which is consistent
with observations. The highest value of the group speed grows with the
increase in the ratio of the sound and Alfvén speeds. For a broad range
of parameters, the highest value of the group speed corresponds to the
propagation angle of 25°-28° to the magnetic field. This effect can
explain the temporal and spatial structure of quasi-periodic pulsations
observed in two-ribbon flares.
---------------------------------------------------------
Title: Magnetic Kelvin-Helmholtz Instability at the Sun
Authors: Foullon, Claire; Verwichte, Erwin; Nakariakov, Valery M.;
Nykyri, Katariina; Farrugia, Charles J.
2011ApJ...729L...8F Altcode:
Flows and instabilities play a major role in the dynamics of magnetized
plasmas including the solar corona, magnetospheric and heliospheric
boundaries, cometary tails, and astrophysical jets. The nonlinear
effects, multi-scale and microphysical interactions inherent to the
flow-driven instabilities, are believed to play a role, e.g., in plasma
entry across a discontinuity, generation of turbulence, and enhanced
drag. However, in order to clarify the efficiency of macroscopic
instabilities in these processes, we lack proper knowledge of their
overall morphological features. Here we show the first observations
of the temporally and spatially resolved evolution of the magnetic
Kelvin-Helmholtz instability in the solar corona. Unprecedented
high-resolution imaging observations of vortices developing at the
surface of a fast coronal mass ejecta are taken by the new Solar
Dynamics Observatory, validating theories of the nonlinear dynamics
involved. The new findings are a cornerstone for developing a unifying
theory on flow-driven instabilities in rarefied magnetized plasmas,
which is important for understanding the fundamental processes at work
in key regions of the Sun-Earth system.
---------------------------------------------------------
Title: Nonlinear long-wavelength torsional Alfvén waves
Authors: Vasheghani Farahani, S.; Nakariakov, V. M.; van Doorsselaere,
T.; Verwichte, E.
2011A&A...526A..80V Altcode:
<BR /> Aims: We investigate the nonlinear phenomena accompanying
long-wavelength torsional waves in solar and stellar coronae. <BR
/> Methods: The second order thin flux-tube approximation is used
to determine perturbations of a straight untwisted and non-rotating
magnetic flux-tube, nonlinearly induced by long-wavelength axisymmetric
magnetohydrodynamic waves of small, but finite amplitude. <BR />
Results: Propagating torsional waves induce compressible perturbations
oscillating with double the frequency of the torsional waves. In
contrast with plane shear Alfvén waves, the amplitude of compressible
perturbations is independent of the plasma-β and is proportional
to the torsional wave amplitude squared. Standing torsional waves
induce compressible perturbations of two kinds, that grow with
the characteristic time inversely proportional to the sound speed,
and that oscillate at double the frequency of the inducing torsional
wave. The growing density perturbation saturates at the level, inversely
proportional to the sound speed.
---------------------------------------------------------
Title: Chromospheric Resonances above Sunspot Umbrae
Authors: Botha, G. J. J.; Arber, T. D.; Nakariakov, V. M.; Zhugzhda,
Y. D.
2011ApJ...728...84B Altcode:
Three-minute oscillations are observed in the chromosphere above
sunspot umbrae. One of the models used to explain these oscillations
is that of a chromospheric acoustic resonator, where the cavity
between the photosphere and transition region partially reflects
slow magnetoacoustic waves to form resonances in the lower sunspot
atmosphere. We present a phenomenological study that compares simulation
results with observations. The ideal magnetohydrodynamic equations
are used with a uniform vertical magnetic field and a temperature
profile that models sunspot atmospheres above umbrae. The simulations
are initialized with a single broadband pulse in the vertical
velocity inside the convection zone underneath the photosphere. The
frequencies in the spectrum of the broadband pulse that lie below
the acoustic cutoff frequency are filtered out so that frequencies
equal and above the acoustic cutoff frequency resonate inside
the chromospheric cavity. The chromospheric cavity resonates with
approximately three-minute oscillations and is a leaky resonator, so
that these oscillations generate traveling waves that propagate upward
into the corona. Thus, there is no requirement that a narrowband
three-minute signal is present in the photosphere to explain the
narrowband three-minute oscillations in the chromosphere and corona. The
oscillations in the chromospheric cavity have larger relative amplitudes
(normalized to the local sound speed) than those in the corona and
reproduce the intensity fluctuations of observations. Different umbral
temperature profiles lead to different peaks in the spectrum of the
resonating chromospheric cavity, which can explain the frequency shift
in sunspot oscillations over the solar cycle.
---------------------------------------------------------
Title: The First Measurement of the Adiabatic Index in the Solar
Corona Using Time-dependent Spectroscopy of Hinode/EIS Observations
Authors: Van Doorsselaere, Tom; Wardle, Nick; Del Zanna, Giulio;
Jansari, Kishan; Verwichte, Erwin; Nakariakov, Valery M.
2011ApJ...727L..32V Altcode:
We use observations of a slow magnetohydrodynamic wave in the corona
to determine for the first time the value of the effective adiabatic
index, using data from the Extreme-ultraviolet Imaging Spectrometer
on board Hinode. We detect oscillations in the electron density,
using the CHIANTI atomic database to perform spectroscopy. From
the time-dependent wave signals from multiple spectral lines the
relationship between relative density and temperature perturbations is
determined, which allows in turn to measure the effective adiabatic
index to be γ<SUB>eff</SUB> = 1.10 ± 0.02. This confirms that the
thermal conduction along the magnetic field is very efficient in the
solar corona. The thermal conduction coefficient is measured from
the phase lag between the temperature and density, and is shown to be
compatible with Spitzer conductivity.
---------------------------------------------------------
Title: Height distribution of the power of 3-min oscillations over
sunspots
Authors: Kobanov, N. I.; Kolobov, D. Y.; Chupin, S. A.; Nakariakov,
V. M.
2011A&A...525A..41K Altcode: 2011arXiv1111.6676K
Context. The height structure of 3-min oscillations over sunspots is
studied in the context of the recently discovered effect of height
inversion: over the umbra, the spatial location of the maximum of
chromospheric 3-min oscillation power corresponds to the relative
decrease in the power of photospheric oscillations. <BR /> Aims:
We investigate whether the height inversion of the power of 3-min
oscillations is a common feature of the spatial structure of the
oscillations for the majority of sunspots. <BR /> Methods: Spectrogram
sequences of Hα 6563 Å and Fe i 6569 Å over sunspots, acquired with
very high cadency (about 2 s or better) are obtained. The distribution
of the oscillation power of the line-of-sight velocity signal is
studied by using methods of wavelet frequency filtration and Fourier
analysis. <BR /> Results: The effect of the height inversion is found
in 9 of 11 analyzed active regions. The interpretation of this effect
is possibly connected to both the decrease in the level of photosphere
in sunspot umbrae and the magnetic field topology.
---------------------------------------------------------
Title: Transient induced MHD oscillations: a tool to probe the solar
active regions
Authors: Srivastava, Abhishek K.; Nakariakov, V. M.; Dwivedi, B. N.;
Kumar, Pankaj
2011ASInC...2..271S Altcode: 2011arXiv1110.0705S
Solar transients and eruptive phenomena which are ubiquitous in
the solar atmosphere, can shed new light to the understanding of
the outstanding problems like coronal heating and the solar wind
acceleration. Observations in the entire electromagnetic spectrum of
such dynamical processes of large and small-scale transient/eruptive
events, with highly dynamic magnetic field configuration, and energetic
particles, provide crucial information about the plasma processes
at mega-Kelvin temperature embedded in a complex magnetic field,
and also energy build-up/energy-release processes, taking place in
such events. One of the most important phenomenological aspects of
solar eruptive phenomena is the induced magnetohydrodynamic (MHD)
waves generated during these energetic processes, which carry a
potential signature to probing the solar active regions. In this
paper, we briefly review the recent trends of the transient (e.g.,
flares) induced quasi-periodic oscillations in the solar atmosphere
and discuss their implications in diagnosing the solar active regions,
providing the clue to understanding local plasma dynamics and heating.
---------------------------------------------------------
Title: Oscillatory processes in solar flares
Authors: Nakariakov, V. M.; Inglis, A. R.; Zimovets, I. V.; Foullon,
C.; Verwichte, E.; Sych, R.; Myagkova, I. N.
2010PPCF...52l4009N Altcode: 2010arXiv1010.0063N
Electromagnetic (radio, visible-light, UV, EUV, x-ray and gamma-ray)
emission generated by solar and stellar flares often contains pronounced
quasi-periodic pulsations (QPPs). Physical mechanisms responsible
for the generation of long-period QPP (with periods longer than
1 s) are likely to be associated with MHD processes. The observed
modulation depths, periods and anharmonicity of QPP suggest that
they can be linked with some kind of MHD auto-oscillations, e.g. an
oscillatory regime of magnetic reconnection. Such regimes, of both
spontaneous and induced nature, have been observed in resistive-MHD
numerical simulations. The oscillations are essentially nonlinear and
non-stationary. We demonstrate that a promising novel method for their
analysis is the empirical mode decomposition technique.
---------------------------------------------------------
Title: Types of Microwave Quasi-Periodic Pulsations in Single
Flaring Loops
Authors: Kupriyanova, E. G.; Melnikov, V. F.; Nakariakov, V. M.;
Shibasaki, K.
2010SoPh..267..329K Altcode: 2010SoPh..tmp..179K
Quasi-periodic pulsations (QPP) of microwave emission generated
in single flaring loops observed with the Nobeyama Radioheliograph
(NoRH) and Nobeyama Radio Polarimeters (NoRP) are studied. Specific
features of the time profiles, i.e. the visible presence or absence
of QPPs, are not accounted for in the selection. The time evolution
of the periods of the QPPs is examined using wavelet and correlation
analyses. In ten out of twelve considered events, at least one or more
significant spectral components with periods from 5 - 60 s have been
found. The quality of the oscillations is rather low: Q=πN, where
N is the number of cycles, mostly varies in the range 12 to 40, with
an average of 25. We suggest that the detected QPPs can be classified
into four types: i) those with stable mean periods (e.g. of 15 - 20
s or 8 - 9 s, the prevailing type); ii) those with spectral drift to
shorter periods (mostly in the rise phase of the microwave emission);
iii) those with drift to longer periods (mostly in the decay phase);
iv) those with multiple periods showing an X-shaped drift (e.g. in
the range from 20 - 40 s in the rise phase).
---------------------------------------------------------
Title: Periodic Spectral Line Asymmetries in Solar Coronal Structures
from Slow Magnetoacoustic Waves
Authors: Verwichte, E.; Marsh, M.; Foullon, C.; Van Doorsselaere,
T.; De Moortel, I.; Hood, A. W.; Nakariakov, V. M.
2010ApJ...724L.194V Altcode:
Recent spectral observations of upward moving quasi-periodic intensity
perturbations in solar coronal structures have shown evidence of
periodic line asymmetries near their footpoints. These observations
challenge the established interpretation of the intensity perturbations
in terms of propagating slow magnetoacoustic waves. We show that slow
waves inherently have a bias toward enhancement of emission in the
blue wing of the emission line due to in-phase behavior of velocity
and density perturbations. We demonstrate that slow waves cause line
asymmetries when the emission line is averaged over an oscillation
period or when a quasi-static plasma component in the line of sight
is included. Therefore, we conclude that slow magnetoacoustic waves
remain a valid explanation for the observed quasi-periodic intensity
perturbations.
---------------------------------------------------------
Title: Web-Based Data Processing System for Automated Detection of
Oscillations with Applications to the Solar Atmosphere
Authors: Sych, R. A.; Nakariakov, V. M.; Anfinogentov, S. A.; Ofman, L.
2010SoPh..266..349S Altcode: 2010SoPh..tmp..154S; 2010arXiv1005.3591S
A web-based, interactive system for the remote processing of imaging
data sets (i.e., EUV, X-ray, and microwave) and the automated
interactive detection of wave and oscillatory phenomena in the solar
atmosphere is presented. The system targets localized, but spatially
resolved, phenomena such as kink, sausage, and longitudinal propagating
and standing waves. The system implements the methods of Periodmapping
for pre-analysis, and Pixelized Wavelet Filtering for detailed analysis
of the imaging data cubes. The system is implemented on the dedicated
data-processing server http://pwf.iszf.irk.ru, which is situated at
the Institute of Solar-Terrestrial Physics, Irkutsk, Russia. Input
data in the .sav, .fits, or .txt formats can be submitted via the
local and/or global network (the Internet). The output data can be in
the png, jpeg, and binary formats, on the user's request. The output
data are periodmaps; narrowband amplitude, power, phase and correlation
maps of the wave's sources at significant harmonics and in the chosen
spectral intervals, and mpeg movies of their evolution. The system
was tested by the analysis of the EUV and microwave emission from the
active region NOAA 10756 on 4 May 2005 observed with TRACE and the
Nobeyama Radioheliograph. The similarity of the spatial localization
of three-minute propagating waves, near the footpoint of locally open
magnetic-field lines determined by the potential-field extrapolation,
in both the transition region and the corona was established. In the
transition region the growth of the three-minute amplitude was found
to be accompanied by the decrease in the line-of-sight angle to the
wave-propagation direction.
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Title: From Large-scale Loops to the Sites of Dense Flaring Loops:
Preferential Conditions for Long-period Pulsations in Solar Flares
Authors: Foullon, C.; Fletcher, L.; Hannah, I. G.; Verwichte, E.;
Cecconi, B.; Nakariakov, V. M.; Phillips, K. J. H.; Tan, B. L.
2010ApJ...719..151F Altcode:
Long-period quasi-periodic pulsations (QPPs) of solar flares are
a class apart from shorter period events. By involving an external
resonator, the mechanism they call upon differs from traditional QPP
models, but has wider applications. We present a multi-wavelength
analysis of spatially resolved QPPs, with periods around 10 minutes,
observed in the X-ray spectrum primarily at energies between 3 and 25
keV. Complementary observations obtained in Hα and radio emission in
the kHz to GHz frequency range, together with an analysis of the X-ray
plasma properties provide a comprehensive picture that is consistent
with a dense flaring loop subject to periodic energization and
thermalization. The QPPs obtained in Hα and type III radio bursts,
with similar periods as the QPPs in soft X-rays, have the longest
periods ever reported for those types of data sets. We also report 1-2
GHz radio emission, concurrent with but unrestricted to the QPP time
intervals, which is multi-structured at regularly separated narrowband
frequencies and modulated with ~18 minute periods. This radio emission
can be attributed to the presence of multiple "quiet" large-scale loops
in the background corona. Large scale but shorter inner loops below
may act as preferential resonators for the QPPs. The observations
support interpretations consistent with both inner and outer loops
subject to fast kink magnetohydrodynamic waves. Finally, X-ray imaging
indicates the presence of double coronal sources in the flaring sites,
which could be the particular signatures of the magnetically linked
inner loops. We discuss the preferential conditions and the driving
mechanisms causing the repeated flaring.
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Title: Phenomenon of Alfvénic Vortex Shedding
Authors: Gruszecki, M.; Nakariakov, V. M.; van Doorsselaere, T.;
Arber, T. D.
2010PhRvL.105e5004G Altcode:
Generation of Alfvénic (magnetohydrodynamic) vortices by the
interaction of compressible plasma flows with magnetic-field-aligned
blunt obstacles is modeled in terms of magnetohydrodynamics. It is
found that periodic shedding of vortices with opposite vorticity
is a robust feature of the interaction in a broad range of plasma
parameters: for plasma beta from 0.025 to 0.5, and for the flow speeds
from 0.1 to 0.99 of the fast magnetoacoustic speed. The Strouhal
number is the dimensionless ratio of the blunt body diameter to the
product of the period of vortex shedding and the inflow speed. It is
found to be consistently in the range 0.15-0.25 in the whole range
of parameters. The induced Alfvénic vortices are compressible and
contain spiral-armed perturbations of the magnetic field strength
and plasma mass density up to 50%-60% of the background values. The
generated electric current also has the spiral-armed structuring.
---------------------------------------------------------
Title: Long-wavelength torsional modes of solar coronal plasma
structures
Authors: Vasheghani Farahani, S.; Nakariakov, V. M.; van Doorsselaere,
T.
2010A&A...517A..29V Altcode:
<BR /> Aims: We consider the effects of the magnetic twist and
plasma rotation on the propagation of torsional m = 0 perturbations
of cylindrical plasma structures (straight magnetic flux tubes)
in the case when the wavelength is much longer than the cylinder
diameter. <BR /> Methods: The second order thin flux tube approximation
is used to derive dispersion relations and phase relations in linear
long-wavelength axisymmetric magnetohydrodynamic waves in uniformly
twisted and rotating plasma structures. <BR /> Results: Asymptotic
dispersion relations linking phase speeds with the plasma parameters
are derived. When twist and rotation are both present, the phase speed
of torsional waves depends upon the direction of the wave propagation,
and also the waves are compressible. The phase relations show that in
a torsional wave the density and azimuthal magnetic field perturbations
are in phase with the axial magnetic field perturbations and anti-phase
with tube cross-section perturbations. In a zero-β non-rotating plasma
cylinder confined by the equilibrium twist, the density perturbation is
found to be about 66 percent of the amplitude of the twist perturbation
in torsional waves.
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Title: Independent Signals from the Influence of Internal Magnetic
Layers on the Frequencies of Solar p-modes
Authors: Foullon, C.; Nakariakov, V. M.
2010ApJ...714L..68F Altcode:
The discovery that p-mode frequencies of low degree do not follow
changes of solar surface activity during the recent solar minimum
offers the possibility of a new diagnostic signature of the responsible
pressure perturbation in the wave guiding medium, potentially rich
of information regarding the structure of the Sun and the cause
of the unusually long solar minimum. Magnetic fields, as well as
temperature changes, introduce equilibrium pressure deviations that
modify the resonant frequencies of p-mode oscillations. Assuming the
perturbation to be caused by a horizontal layer of magnetic field
located in a plane-stratified model of the Sun, we compile analytical
frequency shifts and process them to allow direct comparison with
observations. The effect of magnetism itself on the central p-mode
frequencies can be neglected in comparison with the thermal effect of
a perturbative layer buried in the solar interior. A parametric study
shows that a layer as thin as 2100 km at subsurface depths is able to
reproduce reported mean anomalous frequency shifts (not correlated with
the surface activity), while a layer of size around 4200 km increasing
by a small amount at depths near 0.08 R <SUB>sun</SUB> can explain
individual low-degree shifts. It is also possible to obtain the mean
shifts via the upward motion through depths near 0.03 R <SUB>sun</SUB>
of a rising perturbative layer of thickness around 7000 km. Hence,
the anomalous frequency shifts are best explained by thermal effects
in the upper regions of the convection zone. The effects of latitudinal
distribution are not treated here.
---------------------------------------------------------
Title: Quasi-Periodic Pulsations in the Gamma-Ray Emission of a
Solar Flare
Authors: Nakariakov, V. M.; Foullon, C.; Myagkova, I. N.; Inglis, A. R.
2010EGUGA..12.8615N Altcode:
Quasi-periodic pulsations (QPPs) of gamma-ray emission with a period
of about 40 s are found in a single loop X-class solar flare on 2005
January 1 at photon energies up to 2-6 MeV with the SOlar Neutrons
and Gamma-rays (SONG) experiment aboard the CORONAS-F mission. The
oscillations are also found to be present in the microwave
emission detected with the Nobeyama Radioheliograph, and in the
hard X-ray and low energy gamma-ray channels of RHESSI. Periodogram
and correlation analysis shows that the 40 s QPPs of microwave,
hard X-ray, and gamma-ray emission are almost synchronous in
all observation bands. Analysis of the spatial structure of hard
X-ray and low energy (80-225 keV) gamma-ray QPP with RHESSI reveals
synchronous while asymmetric QPP at both footpoints of the flaring
loop. The difference between the averaged hard X-ray fluxes coming
from the two footpoint sources is found to oscillate with a period
of about 13 s for five cycles in the highest emission stage of the
flare. The proposed mechanism generating the 40 s QPP is a triggering
of magnetic reconnection by a kink oscillation in a nearby loop. The
13 s periodicity could be produced by the second harmonics of the
sausage mode of the flaring loop.
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Title: Long period oscillations in sunspots
Authors: Chorley, N.; Hnat, B.; Nakariakov, V. M.; Inglis, A. R.;
Bakunina, I. A.
2010A&A...513A..27C Altcode:
Long period oscillations of the gyroresonant emission from sunspot
atmospheres are studied. Time series data generated from the sequences
of images obtained by the Nobeyama Radioheliograph operating at a
frequency of 17 GHz for three sunspots have been analysed and are
found to contain significant periods in the range of several tens
of minutes. Wavelet analysis shows that these periods are persistent
throughout the observation periods. The presence of the oscillations
is confirmed by several methods (periodogram, wavelets, Fisher
randomisation and empirical mode decomposition). Spatial analysis
using the techniques of period, power, correlation and time lag
mapping reveals regions of enhanced oscillatory power in the umbral
regions. Also seen are two regions of coherent oscillation of about 25
pixels in size, that oscillate in anti-phase with each other. Possible
interpretation of the observed periodicities is discussed, in terms
of the shallow sunspot model and the leakage of the solar g-modes.
---------------------------------------------------------
Title: Spatially-resolved X-ray observations of the global sausage
mode oscillations in a coronal loop
Authors: Zimovets, Ivan; Nakariakov, Valery
2010cosp...38.1813Z Altcode: 2010cosp.meet.1813Z
We present the first identification of the global (or fundamental) fast
magnetoacoustic sausage mode of a coronal loop in the X-ray band, based
upon the detailed analysis of multi-wavelength spatially-resolving
observations of the M9.3 solar flare on 6 November 2004 by
RHESSI. High quality harmonic oscillations with the time period of
about 78 s and Q=240 have been detected in the thermal X-ray (with
energies lower than 25 keV) emission flux in the decay phase of the
flare. Similar quasi-periodic pulsations were also observed in the
decimetric-centimetric radio emission flux (as pulsations of type
IV radio burst). In the non-thermal hard X-ray emission (higher than
25 keV) the oscillations were less pronounced. It is found that the
area of the quasi-stationary and cooling soft X-ray source (lower
than 15 keV), which was situated near the top of the flaring loop,
varied in anti-phase with the oscillating flux of its radiation. The
oscillation period remained constant during all the oscillations. The
observed properties of the oscillations coincide with the theoretically
predicted properties of standing sausage modes. Cooling of heated plasma
during the oscillations is explained mainly by the conductive losses.
---------------------------------------------------------
Title: Quasi-Periodic Pulsations in the Gamma-Ray Emission of a
Solar Flare
Authors: Nakariakov, V. M.; Foullon, C.; Myagkova, I. N.; Inglis, A. R.
2010ApJ...708L..47N Altcode:
Quasi-periodic pulsations (QPPs) of gamma-ray emission with a period
of about 40 s are found in a single loop X-class solar flare on 2005
January 1 at photon energies up to 2-6 MeV with the SOlar Neutrons
and Gamma-rays (SONG) experiment aboard the CORONAS-F mission. The
oscillations are also found to be present in the microwave
emission detected with the Nobeyama Radioheliograph, and in the
hard X-ray and low energy gamma-ray channels of RHESSI. Periodogram
and correlation analysis shows that the 40 s QPPs of microwave,
hard X-ray, and gamma-ray emission are almost synchronous in
all observation bands. Analysis of the spatial structure of hard
X-ray and low energy (80-225 keV) gamma-ray QPP with RHESSI reveals
synchronous while asymmetric QPP at both footpoints of the flaring
loop. The difference between the averaged hard X-ray fluxes coming
from the two footpoint sources is found to oscillate with a period
of about 13 s for five cycles in the highest emission stage of the
flare. The proposed mechanism generating the 40 s QPP is a triggering
of magnetic reconnection by a kink oscillation in a nearby loop. The
13 s periodicity could be produced by the second harmonics of the
sausage mode of the flaring loop.
---------------------------------------------------------
Title: Foreword
Authors: Nakariakov, V. M.; Erdélyi, R.
2009SSRv..149....1N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Quasi-Periodic Pulsations in Solar Flares
Authors: Nakariakov, V. M.; Melnikov, V. F.
2009SSRv..149..119N Altcode:
Quasi-periodic pulsations (QPP) are a common feature of flaring
energy releases in the solar atmosphere, observed in all bands, from
radio to hard X-ray. In this review we concentrate on QPP with the
periods longer than one second. Physical mechanisms responsible for
the generation of long QPP split into two groups: “load/unload”
mechanisms and MHD oscillations. Load/unload mechanisms are repetitive
regimes of flaring energy releases by magnetic reconnection or by
other means. MHD oscillations can affect all elements of the flaring
emission generation: triggering of reconnection and modulation of its
rate, acceleration and dynamics of non-thermal electrons, and physical
conditions in the emitting plasmas. In the case of MHD oscillations,
the periodicity of QPP is determined either by the presence of some
resonances, e.g. standing modes of plasma structures, or by wave
dispersion. Periods and other parameters of QPP are linked with
properties of flaring plasmas and their morphology. Observational
investigation of the QPP generation mechanisms based upon the use of
spatial information, broadband spectral coverage and multi-periodicity
is discussed.
---------------------------------------------------------
Title: Relationship between wave processes in sunspots and
quasi-periodic pulsations in active region flares
Authors: Sych, R.; Nakariakov, V. M.; Karlicky, M.; Anfinogentov, S.
2009A&A...505..791S Altcode: 2010arXiv1005.3594S
A phenomenological relationship between oscillations in a sunspot
and quasi-periodic pulsations (QPP) in flaring energy releases at an
active region (AR) above the sunspot is established. The analysis of
the microwave emission recorded by the Nobeyama Radioheliograph at 17
GHz shows a gradual increase in the power of the 3-min oscillation
train in the sunspot associated with AR 10756 before flares in this
AR. The flaring light curves are found to be bursty with a period of 3
min. Our analysis of the spatial distribution of the 3-min oscillation
power implies that the oscillations follow from sunspots along coronal
loops towards the flaring site. It is proposed that QPP in the flaring
energy releases can be triggered by 3-min slow magnetoacoustic waves
leaking from sunspots.
---------------------------------------------------------
Title: Ultra-long-period Oscillations in EUV Filaments Near to
Eruption: Two-wavelength Correlation and Seismology
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.
2009ApJ...700.1658F Altcode:
We investigate whether or not ultra-long-period oscillations in EUV
filaments can be related to their eruption. We report new observations
of long-period (~10-30 hr) oscillatory motions in an apparently
quiescent filament, as it crosses the solar disk in a 12 minute cadence
SOHO/Extreme-Ultraviolet Imaging Telescope (EIT) 195 Å uninterrupted
data set. This data set is chosen to explore characteristics of the
filament oscillations depending on its eruptive behavior, which is
observed while the filament is still on the disk. The periods are
found to increase in a near-stable regime prior to eruption. For the
two sequences reported so far, we compare and link the EUV filament
oscillations with pulsations in full-disk solar EUV irradiance from
SOHO/CELIAS/SEM 304 Å flux measurements. In intervals with stationary
periods, we find that the 304 Å pulsations and the 195 Å filament
oscillations have similar periodicities, but are phase-shifted by
about a quarter of period. The two-wavelength correlation serves to
show that, when the filament is the dominant dynamical feature but
can no longer be tracked on the disk, the full-disk irradiance may
provide a mean to identify the period increase prior to the filament
eruption. We use the periods thus obtained to estimate the height
increase of filaments' suspending coronal magnetic field lines,
based on a magnetohydrodynamic (MHD) wave interpretation of the
oscillations. The results are consistent with changes in prominence
heights detected off-limb and thus support the seismological tool
employed. Other interpretations connected with thermal overstability or
MHD piston effect are possible. These theoretical predictions however
do not explain the quarter-period shift between the two EUV-wavelength
signals. In any case, the detected variations may provide a powerful
diagnostic tool for the forecasting of prominence eruptions.
---------------------------------------------------------
Title: Characteristics of magnetoacoustic sausage modes
Authors: Inglis, A. R.; van Doorsselaere, T.; Brady, C. S.; Nakariakov,
V. M.
2009A&A...503..569I Altcode: 2013arXiv1303.6301I
Aims: We perform an advanced study of the fast magnetoacoustic
sausage oscillations of coronal loops in the context of MHD coronal
seismology to establish the dependence of the sausage mode period
and cut-off wavenumber on the plasma-β of the loop-filling plasma. A
parametric study of the ratios for different harmonics of the mode is
also carried out. <BR />Methods: Full magnetohydrodynamic numerical
simulations were performed using Lare2d, simulating hot, dense loops in
a magnetic slab environment. The symmetric Epstein profile and a simple
step-function profile were both used to model the density structure of
the simulated loops. Analytical expressions for the cut-off wavenumber
and the harmonic ratio between the second longitudinal harmonic and the
fundamental were also examined. <BR />Results: It was established that
the period of the global sausage mode is only very weakly dependent on
the value of the plasma-β inside a coronal loop, which justifies the
application of this model to hot flaring loops. The cut-off wavenumber
k<SUB>c</SUB> for the global mode was found to be dependent on both
internal and external values of the plasma-β, again only weakly. By
far the most important factor in this case was the value of the density
contrast ratio between the loop and the surroundings. Finally, the
deviation of the harmonic ratio P_1/2P<SUB>2</SUB> from the ideal
non-dispersive case was shown to be considerable at low k, again
strongly dependent on plasma density. Quantifying the behaviour of the
cut-off wavenumber and the harmonic ratio has significant applications
to the field of coronal seismology.
---------------------------------------------------------
Title: The possible role of vortex shedding in the excitation of
kink-mode oscillations in the solar corona
Authors: Nakariakov, V. M.; Aschwanden, M. J.; van Doorsselaere, T.
2009A&A...502..661N Altcode:
We propose a model for the excitation of horizontally polarised
transverse (kink) magnetoacoustic oscillations of solar coronal loops
by upflows associated with coronal mass ejections. If the magnetic
field in the plasma that is dragged in the vertical direction by the
flow is parallel to the loop, the phenomenon of vortex shedding causes
the appearance of a quasi-periodic horizontal force that is applied to
alternating sides of the loop. The period of the force is determined
by the flow speed and the loop's minor radius. The oscillations are
excited the most effectively when the force is in resonance with the
natural frequency of the kink oscillations. This model can explain
the selectivity of the excitation of the oscillations and the initial
growth of the oscillation amplitude.
---------------------------------------------------------
Title: Seismology of a Large Solar Coronal Loop from EUVI/STEREO
Observations of its Transverse Oscillation
Authors: Verwichte, E.; Aschwanden, M. J.; Van Doorsselaere, T.;
Foullon, C.; Nakariakov, V. M.
2009ApJ...698..397V Altcode:
The first analysis of a transverse loop oscillation observed by both
Solar TErrestrial RElations Observatories (STEREO) spacecraft is
presented, for an event on the 2007 June 27 as seen by the Extreme
Ultraviolet Imager (EUVI). The three-dimensional loop geometry is
determined using a three-dimensional reconstruction with a semicircular
loop model, which allows for an accurate measurement of the loop
length. The plane of wave polarization is found from comparison with
a simulated loop model and shows that the oscillation is a fundamental
horizontally polarized fast magnetoacoustic kink mode. The oscillation
is characterized using an automated method and the results from
both spacecraft are found to match closely. The oscillation period
is 630 ± 30 s and the damping time is 1000 ± 300 s. Also, clear
intensity variations associated with the transverse loop oscillations
are reported for the first time. They are shown to be caused by the
effect of line-of-sight integration. The Alfvén speed and coronal
magnetic field derived using coronal seismology are discussed. This
study shows that EUVI/STEREO observations achieve an adequate accuracy
for studying long-period, large-amplitude transverse loop oscillations.
---------------------------------------------------------
Title: Propagating transverse waves in soft X-ray coronal jets
Authors: Vasheghani Farahani, S.; Van Doorsselaere, T.; Verwichte,
E.; Nakariakov, V. M.
2009A&A...498L..29V Altcode:
Aims: The theoretical model for magnetohydrodynamic (MHD) modes guided
by a field-aligned plasma cylinder with a steady flow is adapted
to interpret transverse waves observed in solar coronal hot jets,
discovered with Hinode/XRT in terms of fast magnetoacoustic kink
modes. <BR />Methods: Dispersion relations for linear magnetoacoustic
perturbations of a plasma jet of constant cross-section surrounded by
static magnetised plasma are used to determine the phase and group
speeds of guided transverse waves and their relationship with the
physical parameters of the jet and the background plasma. The structure
of the perturbations in the macroscopic parameters of the plasma inside
and outside the jet, and the phase relations between them are also
established. <BR />Results: We obtained a convenient expansion for
the long wave-length limit of the phase and group speeds and have
shown that transverse waves observed in soft-X-ray solar coronal
jets are adequately described in terms of fast magnetoacoustic kink
modes by a magnetic cylinder model, which includes the effect of a
steady flow. In the observationally determined range of parameters,
the waves are not found to be subject to either the Kelvin-Helmholtz
instability or the negative energy wave instability, and hence they
are likely to be excited at the source of the jet.
---------------------------------------------------------
Title: Sausage oscillations in loops with a non-uniform cross-section
Authors: Pascoe, D. J.; Nakariakov, V. M.; Arber, T. D.; Murawski, K.
2009A&A...494.1119P Altcode:
Context: Flaring coronal loops are often observed in microwave
and sometimes in soft X-rays, as extended structures of variable
cross-section or width. The quasi-periodic pulsations associated with
flaring coronal loops have been interpreted in terms of standing,
fast sausage magnetoacoustic modes and suggested for the diagnostics
of the magnetic field outside the oscillating loop. <BR />Aims: We
investigate the effects of a non-uniform cross-section on a coronal loop
with respect to the standing sausage modes it supports. <BR />Methods:
Numerical simulations of standing sausage modes are performed for a
straight, zero-β coronal loop with a varying cross-section. The global
sausage mode and higher harmonics are considered, and simulations were
performed for a range of density contrast ratios and loop divergence
parameter. We consider modifications of the period of oscillation
and the spatial profile of the standing modes. <BR />Results: As the
loop divergence parameter increases, the period of the standing modes
decreases. The fractional change period is independent of the density
contrast ratio. The spatial profiles of the standing modes are modified
by the cross-section inhomogeneity, but this effect is too weak to be
observable by modern instruments. For the global sausage mode, the
cross-section radius divergence by a factor of 2 is found to cause
the decrease in the period of about 5%.
---------------------------------------------------------
Title: A multi-periodic oscillatory event in a solar flare
Authors: Inglis, A. R.; Nakariakov, V. M.
2009A&A...493..259I Altcode:
Aims: Ratios of different significant periods found in the light curves
of a solar flare exhibiting quasi-periodic pulsations (QPP) are used
to distinguish between the possible physical mechanisms responsible
for such periodic behaviour. <BR />Methods: Time series data of the
flaring event of 2002 July 3, observed via the Nobeyama Radioheliograph,
Nobeyama Radiopolarimeters, and the RHESSI satellite, are investigated
with the use of the Lomb-Scargle periodogram technique. Images of
the event are also recovered via the RHESSI, SOHO, and Nobeyama
Radioheliograph instruments. <BR />Results: Statistical analysis
of the Lomb-Scargle periodogram results indicates three distinct
periods above the 99% confidence level in Nobeyama Radioheliograph
and Radiopolarimeter data, at 28 s, 18 s, and 12 s, respectively. The
two longest of these periods were also observed in the RHESSI data at
the same confidence level. Wavelet analysis demonstrated that multiple
periods occurred simultaneously without any significant frequency shift
over time. Reconstructed images of the event reveal a very compact flare
structure unsuitable for spatially resolved analysis. Consideration
of the period ratios leads to the conclusion that the cause of this
multi-periodic event is likely to be a kink mode periodically triggering
magnetic reconnection.
---------------------------------------------------------
Title: Seismological demonstration of perpendicular density
structuring in the solar corona
Authors: Van Doorsselaere, T.; Brady, C. S.; Verwichte, E.; Nakariakov,
V. M.
2008A&A...491L...9V Altcode:
The peculiarities of the propagating transverse waves observed in the
solar corona with the Coronal Multi-channel Polarimeter (CoMP) indicate
the existence of fine field structuring in the coronal density. We
present results of numerical simulations studying the evolution of a
localised transverse magneto-hydrodynamic wave in a uniform magnetic
field. We consider two initial low plasma-beta equilibria: one with
a homogeneous density, and one with a field-aligned dense structure
(such as a loop or a plume). The perpendicular localisation of the
wave strongly determines the angular distribution of the energy
propagation. If the perpendicular scale of the wave is significantly
smaller than the parallel scale (e.g. wavelength), as established by
CoMP, the wave develops as an oblique fast magneto-acoustic wave. In
an unstructured medium, the energy of such a wave is transferred
mainly across the magnetic field. However, it is possible to channel
the energy of the transverse wave along the magnetic field in the
presence of a field-aligned density enhancement. We conclude that the
CoMP results provide an independent seismological proof that the corona
is structured in density in the perpendicular direction.
---------------------------------------------------------
Title: Magnetohydrodynamic seismology of solar and stellar coronae
Authors: Nakariakov, V. M.
2008JPhCS.118a2038N Altcode:
The review presents the study of long period (longer than a few seconds)
wave and oscillation phenomena in the corona of the Sun in radio,
VL, EUV and X-ray bands, interpreted in terms of MHD theory. This
study provides us with the observational foundation for the remote
diagnostics of solar and stellar coronae with MHD waves. Kink and
sausage magnetoacoustic modes and longitudinal (or acoustic) modes of
coronal loops have already been confidently identified in the data. At
present, the main research emphasis is put on the determination
of parametric relations in the wave and oscillatory phenomena, the
study of multi-modal events, and on the identification of MHD modes
in quasi-periodic pulsations of solar and stellar flares.
---------------------------------------------------------
Title: Forward Modeling of EIS Spectroscopic Data Emitted by a
Vertically Oscillating Loop
Authors: Doorsselaere, T. V.; Nakariakov, V. M.
2008ASPC..397...58D Altcode:
This paper focuses on forward modeling of spectroscopic data, as would
be emitted by a coronal loop oscillating in the vertical fast kink
mode. The Doppler shift, broadening and deformation of the observed
spectral lines are investigated. It is concluded that the vertical
kink mode causes strong line broadening, either inside the loop or
at the edges of the loop, depending on the time integration and the
oscillatory phase observed.
---------------------------------------------------------
Title: Detection of Waves in the Solar Corona: Kink or Alfvén?
Authors: van Doorsselaere, T.; Nakariakov, V. M.; Verwichte, E.;
Young, P. R.
2008ESPM...12.2.81V Altcode:
Last year, Tomczyk et al. (2007) have conclusively proven that low
amplitude (1km/s) waves are ubiquitously present in the corona. A few
months later, this was followed up with the discovery that chromospheric
spicules carry tremendous wave power (De Pontieu et al., 2007). These
new developments form a basis for coronal seismological magnetic field
mapping of the corona and chromosphere. However, this must be based
upon a confident identification of the observed wave mode. <P />Using
basic MHD wave theory, we demonstrate that the only way to interpret
the observed wave motions is in terms of fast magnetoacoustic kink
waves. We prove that the interpretation in terms of Alfven waves,
as was put forward in the original articles, does not explain the
observed phenomenology, while the fast magnetoacoustic kink waves
reproduce the required observational constraints. <P />To underline the
potential of magnetic field mapping, we discuss a recent observation
of coronal loop kink oscillations observed with Hinode/EIS. Because
of its spectroscopic and imaging capabilities, we are able to measure
the loop density, simultaneously with the loop length. This allows us
to determine the magnetic field with unprecedented accuracy.
---------------------------------------------------------
Title: Multi-wavelength spatially resolved analysis of quasi-periodic
pulsations in a solar flare
Authors: Inglis, A. R.; Nakariakov, V. M.; Melnikov, V. F.
2008A&A...487.1147I Altcode:
Aims: We aim to perform a spatially resolved analysis of a
quasi-periodic pulsation event from 8th May 1998 using microwave data
from the Nobeyama Radioheliograph and Radiopolarimeter, and X-ray
data from the Yohkoh satellite. <BR />Methods: Time spectra of the
signals integrated over the emission source are constructed with the
use of the Lomb-Scargle periodogram method, revealing the presence of
a pronounced 16 s periodicity. The Pixon image reconstruction algorithm
and Hanaoka algorithm are used to reconstruct images from the hard X-ray
data from Yohkoh/HXT and Nobeyama Radioheliograph respectively. The
phase relationship of the microwave emission was analysed with the
use of cross-correlation techniques. <BR />Results: The flaring loop
was resolved in the microwave band. The hard X-ray sources are found
to be located near the footpoint and at the loop apex determined by
the soft X-ray image. The apex source is much fainter than footpoint
one. In microwave, all parts of the loop are seen to oscillate with the
same period and almost in phase. It was not possible to determine the
spatial structure of the oscillation in the hard X-ray band. The period
and the coherent spatial structure of the oscillation are indicative
of the presence of either an MHD sausage mode or a periodic regime of
magnetic reconnection.
---------------------------------------------------------
Title: Coronal magnetic field measurement using loop oscillations
observed by Hinode/EIS
Authors: Van Doorsselaere, T.; Nakariakov, V. M.; Young, P. R.;
Verwichte, E.
2008A&A...487L..17V Altcode:
We report the first spectroscopic detection of a kink MHD oscillation
of a solar coronal structure by the Extreme-Ultraviolet Imaging
Spectrometer (EIS) on the Japanese Hinode satellite. The detected
oscillation has an amplitude of 1 km s<SUP>-1</SUP> in the Doppler
shift of the FeXII 195 Å spectral line (1.3~MK), and a period of
296~s. The unique combination of EIS's spectroscopic and imaging
abilities enables us to measure simultaneously the mass density and
length of the oscillating loop. This enables us to measure directly
the magnitude of the local magnetic field, the fundamental coronal
plasma parameter, as 39 ± 8~G, with unprecedented accuracy. This
proof of concept makes EIS an exclusive instrument for the full scale
implementation of the MHD coronal seismological technique.
---------------------------------------------------------
Title: Coronal loop seismology using multiple transverse loop
oscillation harmonics
Authors: Van Doorsselaere, T.; Nakariakov, V. M.; Verwichte, E.
2008IAUS..247..140V Altcode: 2007IAUS..247..140V
TRACE observations (23/11/1998 06:35:57-06:48:43UT) in the 171 Å
bandpass of an active region are studied. Coronal loop oscillations are
observed after a violent disruption of the equilibrium. The oscillation
properties are studied to give seismological estimates of physical
quantities, such as the density scale height. A loop segment is traced
during the oscillation, and the resulting time series is analysed
for periodicities. In the loop segment displacement, two periods are
found: 435.6±4.5 s and 242.7±6.4 s, consistent with the periods
of the fundamental and 2nd harmonic fast kink oscillation. The small
uncertainties allow us to estimate the density scale height in the loop
to be 109 Mm, which is about double the estimated hydrostatical value
of 50 Mm. The eigenfunction is used to do spatial coronal seismology,
but that method does not give any conclusive results.
---------------------------------------------------------
Title: The Pixelised Wavelet Filtering Method to Study Waves and
Oscillations in Time Sequences of Solar Atmospheric Images
Authors: Sych, R. A.; Nakariakov, V. M.
2008SoPh..248..395S Altcode:
Pixelised wavelet filtering (PWF) for the determination of the spatial,
temporal, and phase structure of oscillation sources in temporal
sequences of 2D images, based upon the continuous wavelet transform,
has been designed and tested. The PWF method allows us to obtain
information about the presence of propagating and nonpropagating waves
in the data and localise them precisely in time and in space. The
method is tested on the data sets obtained in microwaves with the
Nobeyama Radioheliograph and in the EUV with TRACE. The method reveals
fine spatial structuring of the sources of 3-, 5-, and 15-minute
periodicities in the microwave and EUV emission generated in sunspot
atmospheres. In addition, the PWF method provides us with unique
information about the temporal variability of the power, amplitude,
and phase narrowband maps of the observed oscillations and waves. The
applicability of the method to the analysis of coronal wave phenomena
is discussed.
---------------------------------------------------------
Title: Detection of Waves in the Solar Corona: Kink or Alfvén?
Authors: Van Doorsselaere, T.; Nakariakov, V. M.; Verwichte, E.
2008ApJ...676L..73V Altcode:
Recently, the omnipresence of waves has been discovered in the corona
using the CoMP instrument. We demonstrate that the observational
findings can be explained in terms of guided kink magnetoacoustic
modes. The interpretation of the observations in terms of Alfvén waves
is shown to be inconsistent with MHD wave theory. The implications of
the interpretation in terms of kink waves are discussed.
---------------------------------------------------------
Title: Interactive remote data processing using Pixelize Wavelet
Filtration (PWF-method) and PeriodMap analysis
Authors: Sych, Robert; Nakariakov, Valery; Anfinogentov, Sergey
2008cosp...37.3106S Altcode: 2008cosp.meet.3106S
Wavelet analysis is suitable for investigating waves and oscillating in
solar atmosphere, which are limited in both time and frequency. We have
developed an algorithms to detect this waves by use the Pixelize Wavelet
Filtration (PWF-method). This method allows to obtain information
about the presence of propagating and non-propagating waves in the
data observation (cube images), and localize them precisely in time as
well in space. We tested the algorithm and found that the results of
coronal waves detection are consistent with those obtained by visual
inspection. For fast exploration of the data cube, in addition,
we applied early-developed Period- Map analysis. This method based
on the Fast Fourier Transform and allows on initial stage quickly
to look for "hot" regions with the peak harmonic oscillations and
determine spatial distribution at the significant harmonics. We
propose the detection procedure of coronal waves separate on two
parts: at the first part, we apply the PeriodMap analysis (fast
preparation) and than, at the second part, use information about
spatial distribution of oscillation sources to apply the PWF-method
(slow preparation). There are two possible algorithms working with
the data: in automatic and hands-on operation mode. Firstly we use
multiply PWF analysis as a preparation narrowband maps at frequency
subbands multiply two and/or harmonic PWF analysis for separate
harmonics in a spectrum. Secondly we manually select necessary spectral
subband and temporal interval and than construct narrowband maps. For
practical implementation of the proposed methods, we have developed
the remote data processing system at Institute of Solar-Terrestrial
Physics, Irkutsk. The system based on the data processing server -
http://pwf.iszf.irk.ru. The main aim of this resource is calculation
in remote access through the local and/or global network (Internet)
narrowband maps of wave's sources both in whole spectral band and at
significant harmonics. In addition, we can obtain temporal dynamics
(mpeg- files) of the main oscillation characteristics: amplitude,
power and phase as a spatial-temporal coordinates. For periodogram
mapping of data cubes as a method for the pre-analysis, we developed
preparation of the color maps where the pixel's colour corresponds
to the frequency of the power spectrum maximum. The computer system
based on applications ION-scripts, algorithmic languages IDL and PHP,
and Apache WEB server. The IDL ION-scripts use for preparation and
configuration of network requests at the central data server with
subsequent connection to IDL run-unit software and graphic output on
FTP-server and screen. Web page is constructed using PHP language.
---------------------------------------------------------
Title: Sausage Oscillations in Multishell Coronal Structures
Authors: Pascoe, D. J.; Nakariakov, V. M.; Arber, T. D.
2007SoPh..246..165P Altcode:
The effect of fine multilayered structuring on the resonant periods of
global sausage (m=0), fast magnetoacoustic oscillations of coronal loops
is studied for the straight magnetic slab model filled with a zero-β
plasma. The slab is considered to have a one-dimensional inhomogeneity
of the Alfvén speed in the direction perpendicular to the axis of
the slab, which coincides with the equilibrium, straight, magnetic
field. This magnetic configuration models an oscillating coronal
loop that is formed by a bundle of field-aligned, smooth, concentric,
circular shells of variable density and width. The number of the layers
(or shells), along with their density, width, and radial steepness,
are randomly distributed. It is shown that the resonant properties of
long-wavelength sausage standing modes of the slab are not sensitive
to the details of fine structuring. It is concluded that the coronal
seismological technique for the determination of the magnetic field
strength by the period of global sausage oscillations is not sensitive
to the fine multilayered or multishell structuring.
---------------------------------------------------------
Title: Preface: A Topical Issue in Honor of Professor Bernard Roberts
Authors: Ballester, J. L.; Erdélyi, R.; Hood, A. W.; Leibacher,
J. W.; Nakariakov, V. M.
2007SoPh..246....1B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal loop seismology using multiple transverse loop
oscillation harmonics
Authors: Van Doorsselaere, T.; Nakariakov, V. M.; Verwichte, E.
2007A&A...473..959V Altcode:
Context: TRACE observations (23/11/1998 06:35:57-06:48:43 UT) in the 171
Å bandpass of an active region are studied. Coronal loop oscillations
are observed after a violent disruption of the equilibrium. <BR />Aims:
The oscillation properties are studied to give seismological estimates
of physical quantities, such as the density scale height. <BR />Methods:
A loop segment is traced during the oscillation, and the resulting
time series is analysed for periodicities. <BR />Results: In the
loop segment displacement, two periods are found: 435.6 ± 4.5~s and
242.7 ± 6.4~s, consistent with the periods of the fundamental and
2nd harmonic fast kink oscillation. The small uncertainties allow us
to estimate the density scale height in the loop to be 109~Mm, which
is about double the estimated hydrostatical value of 50~Mm. Because
a loop segment is traced, the amplitude dependence along the loop is
found for each of these oscillations. The obtained spatial information
is used as a seismological tool to give details about the geometry of
the observed loop.
---------------------------------------------------------
Title: Solar-B as a Tool for Coronal Wave Studies
Authors: Nakariakov, V. M.
2007ASPC..369..221N Altcode:
Direct observational evidence of wave and oscillatory activity of the
solar corona is abundant in all observational bands and includes recent
discoveries of propagating compressible waves in polar plumes and near
loop footpoints, flare-generated transverse oscillations of loops,
and longitudinal and sausage standing oscillations within loops. These
phenomena are successfully interpreted in terms of magnetohydrodynamic
(MHD) waves. This observational breakthrough gave rise to the rapid
development of a new method for the remote diagnostics of the coronal
plasma, MHD coronal seismology, allowing for estimation of the absolute
value of the magnetic field in coronal loops, Alfvén speeds, transport
coefficients, fine structuring, heating function and other important
coronal parameters. EIS and XRT instruments are excellent tools for
the further development of coronal wave studies and especially flare
generated oscillations and waves. We present the recent findings,
theoretical estimations of the observability of different MHD modes
with Solar-B instruments, theory-based observational and data analysis
strategies and some ideas about the utilisation of the results for
coronal seismology. The expected results are of particular interest
for the revealing of coronal heating mechanisms.
---------------------------------------------------------
Title: Coronal dynamics
Authors: Nakariakov, V. M.
2007AIPC..919..214N Altcode:
The lectures present the foundation of solar coronal physics with
the main emphasis on the MHD theory and on wave and oscillatory
phenomena. We discuss major challenges of the modern coronal physics;
the main plasma structures observed in the corona and the conditions for
their equilibrium; phenomenology of large scale long period oscillatory
coronal phenomena and their theoretical modelling as MHD waves. The
possibility of the remote diagnostics of coronal plasmas with the use
of MHD oscillations is demonstrated.
---------------------------------------------------------
Title: Quasi-periodic oscillations associated with solar flares
Authors: Nakariakov, V. M.
2007AGUSMSH22A..06N Altcode:
Mechanisms for the generation of quasi-periodic pulsations (QPP) in
microwave, X-ray and visible light emission generated by solar and
stellar flares, based upon magnetohydrodynamic (MHD) oscillations, are
discussed. QPP can be produced by internal MHD oscillations of flaring
magnetic structures, in particular by kink, sausage and longitudinal
modes. Different modes have different observational signatures in
different emission bands, allowing for the correct identification
of the mode. Another mechanism is based upon periodic triggering of
flaring energy releases by oscillations in a non-flaring loop situated
nearby the flaring region. The use of observable parameters of QPP for
remote diagnostics of coronal plasmas, in particular of the magnetic
field in the flaring regions is demonstrated.
---------------------------------------------------------
Title: Coronal Periodmaps
Authors: Nakariakov, V. M.; King, D. B.
2007SoPh..241..397N Altcode:
The search for signatures of wave and oscillatory processes in the solar
corona in the data obtained with imaging instruments can be automated by
using the periodmap method. The method reduces a three-dimensional data
cube to a two-dimensional map of the analysed field of view. The map
reveals the presence and distribution of the most pronounced frequencies
in the power spectrum of the time signal recorded at spatial pixels. We
demonstrate the applicability of this method as a pre-analysis tool
with the use of TRACE EUV coronal data, which contain examples of
transverse and longitudinal oscillations of coronal loops. The main
advantage of using periodmaps over other possible (more sophisticated)
pre-analysis tools, such as wavelet analysis, is their robustness and
efficiency (both in speed and computational power). The method can be
implemented in the Hinode/XRT and SDO/AIA data pre-analysis.
---------------------------------------------------------
Title: Sausage oscillations of coronal loops
Authors: Pascoe, D. J.; Nakariakov, V. M.; Arber, T. D.
2007A&A...461.1149P Altcode:
Aims:Analytical theory predicts the existence of trapped global (or
fundamental) sausage fast magnetoacoustic modes in thick and dense
coronal loops only, with the periods estimated as the ratio of double
the loop length and the Alfvén speed outside the loop. We extend
this study to the leaking regime, considering global sausage modes of
long loops with small density contrasts. <BR />Methods: Anti-symmetric
fast magnetoacoustic perturbations (sausage, or m=0 modes) of a low β
plasma slab with the symmetric Epstein profile of plasma density are
modelled numerically. <BR />Results: It was found that long loops with
sufficiently small density contrast can support global sausage leaky
modes of detectable quality. The periods of the leaky modes are found to
be approximately determined by the loop length and the external Alfvén
speed. If the loop length can be estimated from imaging observations,
the observed period of this mode provides us with the information about
the Alfvén speed outside the loop. For typical flaring coronal loops,
the estimated periods of the global sausage modes are about 5-60 s.
---------------------------------------------------------
Title: A Magnetometer For The Solar Orbiter Mission
Authors: Carr, C. M.; Horbury, T. S.; Balogh, A.; Baumjohann, W.;
Bavassano, B.; Breen, A.; Burgess, D.; Cargill, P. J.; Brooker, N.;
Erdõs, G.; Fletcher, L.; Forsyth, R. J.; Giacalone, J.; Glassmeier,
K. -H.; Hoeksema, J. T.; Goldstein, M. L.; Lockwood, M.; Magnes, W.;
Masimovic, M.; Marsch, G.; Matthaeus, W. H.; Murphy, N.; Nakariakov,
V. M.; Pacheco, J. R.; Pincon, J. -L.; Riley, P.; Russell, C. T.;
Schwartz, S. J.; Szabo, A.; Thompson, M.; Vainio, R.; Velli, M.;
Vennerstrom, S.; Walsh, R.; Wimmer-Schweingruber, R.; Zank, G.
2007ESASP.641E..41C Altcode:
The magnetometer is a key instrument to the Solar Orbiter mission. The
magnetic field is a fundamental parameter in any plasma: a precise
and accurate measurement of the field is essential for understanding
almost all aspects of plasma dynamics such as shocks and stream-stream
interactions. Many of Solar Orbiter's mission goals are focussed
around the links between the Sun and space. A combination of in situ
measurements by the magnetometer, remote measurements of solar magnetic
fields and global modelling is required to determine this link and
hence how the Sun affects interplanetary space. The magnetic field
is typically one of the most precisely measured plasma parameters and
is therefore the most commonly used measurement for studies of waves,
turbulence and other small scale phenomena. It is also related to the
coronal magnetic field which cannot be measured directly. Accurate
knowledge of the magnetic field is essential for the calculation of
fundamental plasma parameters such as the plasma beta, Alfvén speed
and gyroperiod. We describe here the objectives and context of magnetic
field measurements on Solar Orbiter and an instrument that fulfils those
objectives as defined by the scientific requirements for the mission.
---------------------------------------------------------
Title: Quasi-periodic Pulsations as a Diagnostic Tool for Coronal
Plasma Parameters
Authors: Nakariakov, V. M.; Stepanov, A. V.
2007LNP...725..221N Altcode:
Quasi-periodic pulsations of radio emission generated in solar and
stellar flares are interpreted in terms of MHD oscillations of coronal
loops and in the frame of equivalent electric circuit approach,
and compared with coronal wave and oscillatory phenomena recently
discovered in the EUV, X-ray and visible light bands. Various methods
of remote diagnostics of the coronal plasma, based upon the use of
observationally detectable properties of quasi-periodic pulsations -
their period, amplitude and quality - are discussed. The applicability
of these methods to the diagnostics of stellar coronae is demonstrated.
---------------------------------------------------------
Title: MHD oscillations in solar and stellar coronae: Current results
and perspectives
Authors: Nakariakov, Valery M.
2007AdSpR..39.1804N Altcode:
Wave and oscillatory activity is observed with modern imaging and
spectral instruments in the visible light, EUV, X-ray and radio
bands in all parts of the solar corona. Magnetohydrodynamic (MHD)
wave theory gives satisfactory interpretation of these phenomena
in terms of MHD modes of coronal structures. The paper reviews the
current trends in the observational study of coronal oscillations,
recent development of theoretical modelling of MHD wave interaction with
plasma structures, and implementation of the theoretical results for the
mode identification. Also the use of MHD waves for remote diagnostics of
coronal plasmas is discussed. In particular, the applicability of this
method to the estimation of the coronal magnetic field is demonstrated.
---------------------------------------------------------
Title: MHD-Oscillation Modes of a Flaring Loop Using Microwave
Observations With High Spatial Resolution
Authors: Reznikova, Veronika E.; Melnikov, Victor F.; Nakariakov,
Valery M.; Shibasaki, Kiyoto
2006AIPC..848..133R Altcode:
Study of an oscillating loop observed with high spatial resolution
(NoRH) shows the presence of two Fourier spectral peaks: P1 = 14-17
s, which is more pronounced at the loop apex and P2 = 8-11 s, that is
stronger at the loop legs. We found the phase shift, P2/4, between the
pulsations at the northern leg and at the loop top for the P2 spectral
component and a lack of the phase shift between different parts of the
flare loop for the P1 component. The computed phase speeds and periods
of MHD modes of a coronal loop confirm identification of the first
periodicity P1 as the global (fundamental) sausage mode. The second
periodicity can be associated with several modes: the second and the
third longitudinal harmonics of the kink mode, the third harmonics of
the sausage mode and the second harmonics of ballooning mode.
---------------------------------------------------------
Title: Quasi-periodic modulation of solar and stellar flaring emission
by magnetohydrodynamic oscillations in a nearby loop
Authors: Nakariakov, V. M.; Foullon, C.; Verwichte, E.; Young, N. P.
2006A&A...452..343N Altcode:
We propose a new model for quasi-periodic modulation of solar
and stellar flaring emission. Fast magnetoacoustic oscillations
of a non-flaring loop can interact with a nearby flaring active
region. This interaction occurs when part of the oscillation situated
outside the loop reaches the regions of steep gradients in magnetic
field within an active region and produces periodic variations of
electric current density. The modulation depth of these variations is
a few orders of magnitude greater than the amplitude of the driving
oscillation. The variations of the current can induce current-driven
plasma micro-instabilities and thus anomalous resistivity. This can
periodically trigger magnetic reconnection, and hence acceleration
of charged particles, producing quasi-periodic pulsations of X-ray,
optical and radio emission at the arcade footpoints.
---------------------------------------------------------
Title: Seismology of curved coronal loops with vertically polarised
transverse oscillations
Authors: Verwichte, E.; Foullon, C.; Nakariakov, V. M.
2006A&A...452..615V Altcode:
Aims.Using a model of vertically polarised fast magnetoacoustic waves
in curved coronal loops, the method of coronal seismology is applied
to observations of transverse loop oscillations. <BR /> Methods: . A
coronal loop is modeled as a curved magnetic slab in the zero plasma-β
limit. For an arbitrary piece-wise continuous power law equilibrium
density profile, the dispersion relation governing linear vertically
polarised fast magnetoacoustic kink waves is derived. The ways in which
this model can be used for coronal seismology are explored and applied
to two observational examples. <BR /> Results: . The Alfvén speed and
equilibrium density profile are determined from observations. It is
shown that the mechanism of lateral leakage of fast magnetoacoustic
kink oscillations described in this model is efficient. In fact, the
damping is so efficient that in order to match predicted values with
observational ones, either the loop needs to be highly contrasted or the
transverse Alfvén speed profile needs to be close to linear. Possible
improvements to make the modeling of lateral wave leakage in loops more
realistic, allowing a lower damping efficiency, are discussed. <BR />
---------------------------------------------------------
Title: Fast magnetoacoustic waves in curved coronal
loops. II. Tunneling modes
Authors: Verwichte, E.; Foullon, C.; Nakariakov, V. M.
2006A&A...449..769V Altcode:
Aims. Fast magnetoacoustic waves in curved coronal loops are
investigated and the role of lateral leakage in wave damping, which
includes the mechanism of wave tunneling, is explored. Methods. A
coronal loop is modeled as a curved, magnetic slab in the zero
plasma-β limit. In this model and for an arbitrary piece-wise
continuous power law equilibrium density profile, the wave equation
governing linear vertically polarised fast magnetoacoustic waves is
solved analytically. An associated dispersion relation is derived
and the frequencies and eigenfunctions of the wave modes are
characterised. Results. For some equilibria, the waves are shown
to be all damped due to lateral leakage. It is demonstrated that
waves either leak straight out into the external medium or have to
overcome an evanescent barrier, which is linked to wave tunneling. The
wave solutions consist of alternating vertically polarised kink
and sausage branches. Fast kink oscillations may have a non-zero
density perturbation when averaged across the loop. The calculated
damping rate of fast magnetoacoustic kink oscillations is shown to be
consistent with related numerical simulations and show that lateral
leakage may explain the observed damping of (vertically polarised)
fast magnetoacoustic kink oscillations.
---------------------------------------------------------
Title: Modulation of gyrosynchrotron emission in solar and stellar
flares by slow magnetoacoustic oscillations
Authors: Nakariakov, V. M.; Melnikov, V. F.
2006A&A...446.1151N Altcode:
Gyrosynchrotron emission generated by non-thermal electrons in solar
and stellar coronal flares can be efficiently modulated by slow
magnetoacoustic oscillations in the flaring loops. The modulation
mechanism is based upon perturbation of the efficiency in the Razin
suppression of optically thin gyrosynchrotron emission. Modulation
of the emission is in anti-phase with the density perturbation in the
slow wave. The observed emission modulation depth can be up to an order
of magnitude higher than the slow wave amplitude. This effect is more
pronounced at lower frequencies. Observations with spatial resolution,
together with analysis of the modulation frequency, are shown to be
sufficient for providing the information needed to identify the mode.
---------------------------------------------------------
Title: Magnetohydrodynamic waves in coronal polar plumes
Authors: Nakariakov, Valery M.
2006RSPTA.364..473N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Hydromagnetic surface waves on a tangential discontinuity
Authors: Joarder, P. S.; Nakariakov, V. M.
2006GApFD.100...59J Altcode:
Propagation of hydromagnetic surface waves on an MHD tangential
discontinuity, separating two uniform magnetic plasmas, across which
the magnetic field changes both its direction and field-strength,
is examined. The study is arbitrarily confined to discontinuities one
side of which has a plasma-ß value larger than unity. A classification
scheme to identify the ‘fast’ and the ‘slow’ surface waves is
employed thus enabling us to find new features in the modal structure
of these waves. In the case of sufficiently small values for the
density ratio between the higher and the lower Alfvén speed regions
on opposite sides of the interface, this study reveals the presence
of a ‘forbidden region’ for propagation of fast surface waves with
values of their propagation angle being smaller than a certain critical
value. The study also reveals the existence of two distinct slow surface
waves with angles of propagation larger than a certain critical value
but for two different ranges of values for the angle between magnetic
field vectors across the interface. While revealing such new structures
of the surface modes for intermediate parameter values, the present
investigation also confirms earlier results regarding the asymptotic
behaviour of the modes in the incompressible limit.
---------------------------------------------------------
Title: Fast magnetoacoustic waves in curved coronal loops
Authors: Verwichte, E.; Foullon, C.; Nakariakov, V. M.
2006A&A...446.1139V Altcode:
A study of vertically polarised fast magnetoacoustic waves in a curved
coronal loop is presented. The loop is modeled as a semi-circular
magnetic slab in the zero plasma-β limit. The governing equations for
linear waves are derived. We show that the wave mode behaviour depends
on the slope of the equilibrium density profile, which is modeled as
a piece-wise continuous power law curve of index α. For all profiles,
except for α=-4, wave modes are not trapped in the loop and leak out
into the external medium through wave tunneling. The particular case of
α=-4, which corresponds to a linearly increasing Alfvén speed profile,
is examined in more detail as this is the only model that can support
trapped wave modes. We compare the results with a straight slab model
and find similar behaviour. Coupling between sausage and kink wave
modes has not been found in the model.
---------------------------------------------------------
Title: A parametric study of the effect of pressure anisotropy on
slow nonlinear acoustic waves in high-beta plasmas
Authors: Simon, B.; Nakariakov, V.; Rowlands, G.; Chapman, S.;
Dendy, R.
2006cosp...36.1041S Altcode: 2006cosp.meet.1041S
Solitary wave solutions arise naturally from one dimensional
Hall-Magnetohydrodynamic MHD equations These solutions have been
recently identified as models for slow-mode solitary structures observed
in the magnetosheath and other magnetospheric boundary layers and as a
model for magnetic holes MH which are localized depressions of magnetic
field observed in the solar wind We extend the analysis of the model of
Stasiewicz which is based on one dimesional Hall-MHD with an explicit
pressure anisotropy Applying the methods of the Sagdeev potential and
direct numerical modelling we performed parametric studies of highly
oblique magnetoacoustic nonlinear waves in a high- beta ratio of kinetic
to magnetic pressures plasmas We found that anisotropy is crucial for
the existence of solitary structures in this model Furthermore if the
perpendicular pressure is greater than the parallel pressure there
can be both field enhancing bright and field depleting dark solitary
structures whereas increased parallel pressure compared to perpendicular
pressure only allows for dark solitary structures Combinations of the
plasma parameters corresponding to the different regimes are determined
---------------------------------------------------------
Title: Coronal waves and oscillations
Authors: Nakariakov, Valery M.
2006IAUS..233..464N Altcode:
Wave and oscillatory activity of the solar corona is confidently
observed with modern imaging and spectral instruments in the
visible light, EUV, X-ray and radio bands, and interpreted in
terms of magnetohydrodynamic (MHD) wave theory. The review reflects
the current trends in the observational study of coronal waves and
oscillations, theoretical modelling of interaction of MHD waves with
plasma structures, and implementation of the theoretical results for the
mode identification. Also the use of MHD waves for remote diagnostics
of coronal plasma - MHD coronal seismology - is discussed.
---------------------------------------------------------
Title: Current results and perspectives of coronal wave investigations
Authors: Nakariakov, V. M.
2006cosp...36..794N Altcode: 2006cosp.meet..794N
Wave and oscillatory activity is observed with modern imaging and
spectral instruments in the visible light EUV X-ray and radio bands
in all parts of the solar corona in particular in active regions and
polar plumes Magnetohydrodynamic MHD wave theory gives satisfactory
interpretation of these phenomena in terms of MHD modes of coronal
structures The talk reviews the current trends in the observational
study of coronal waves and oscillations with spaceborne and ground-based
facilities with the main emphasis on the standing kink and sausage modes
and propagating fast wave recent development of theoretical modelling
of interaction of MHD waves with plasma structures and implementation
of the theoretical results for the mode identification The use of MHD
waves for remote diagnostics of coronal plasma is discussed and the
applicability of this method for the estimation of coronal magnetic
field and fine structuring is demonstrated
---------------------------------------------------------
Title: Diagnostics of Mhd-Oscillation Modes of a Flaring Loop Using
Microwave Observations with High Spatial Resolution
Authors: Reznikova, V. E.; Nakariakov, V. M.; Melnikov, V. F.;
Shibasaki, K.
2005ESASP.600E.140R Altcode: 2005ESPM...11..140R; 2005dysu.confE.140R
No abstract at ADS
---------------------------------------------------------
Title: Transverse Waves in a Post-Flare Supra-Arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Cooper, F. C.
2005ESASP.600E.101V Altcode: 2005dysu.confE.101V; 2005ESPM...11..101V
No abstract at ADS
---------------------------------------------------------
Title: X-Ray Quasi-Periodic Pulsations in Solar Flares as
Magnetohydrodynamic Oscillations
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.; Fletcher, L.
2005ESASP.600E..33F Altcode: 2005ESPM...11...33F; 2005dysu.confE..33F
No abstract at ADS
---------------------------------------------------------
Title: Magnetoacoustic Solitons in High-β Plasmas
Authors: Simon, B.; Nakariakov, V. M.; Chapman, S. C.; Dendy, R. O.
2005AGUFMSH53A1238S Altcode:
Solitary wave solutions arise naturally from one dimensional
Hall-Magnetohydrodynamic equations. These solutions have been recently
identified as models for slow-mode solitary structures observed in the
magnetosheath and other magnetospheric boundary layers, and as a model
for magnetic holes (MH), which are localized depressions of magnetic
field observed in the solar wind. We perform a parametric study of
highly oblique magnetoacoustic solitons in frames of Stasiewicz's
analytical model [1], taking plasma pressure anisotropy into account. We
study the stability of these structures using a 1.5D hybrid code. In
particular, we focus on magnetic structures in a high-β (ratio of
kinetic to magnetic pressures) plasmas. These plasma conditions are
typical in the region around MHs, and the study of stability is vital in
verifying the theory that MHs are manifestations of such solitons. Our
simulations are aimed at studying linear magnetic holes, where there
is almost no variation in direction of the ambient magnetic field
direction. The majority of MHs discovered in space plasmas fall into
this category. [1] Stasiewicz,K., Phys. Rev. Lett., 93, 125004, 2004
---------------------------------------------------------
Title: Transverse Waves in a Post-Flare Supra-Arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Cooper, F. C.
2005ESASP.596E..38V Altcode: 2005ccmf.confE..38V
No abstract at ADS
---------------------------------------------------------
Title: Short Quasi-Periodic MHD Waves in Coronal Structures
Authors: Nakariakov, V. M.; Pascoe, D. J.; Arber, T. D.
2005SSRv..121..115N Altcode:
The possibility of remote diagnostics of coronal structures with
impulsively-generated short-period fast magnetoacoustic wave trains is
demonstrated. An initially broad-band, aperiodic fast magnetoacoustic
perturbation guided by a 1D plasma inhomogeneity develops into
a quasi-periodic wave train with a well-pronounced frequency and
amplitude modulation. The quasi-periodicity results from the geometrical
dispersion of the modes, determined by the transverse profile of the
loop, and hence contains information about the profile. Wavelet images
of the wave train demonstrate that their typical spectral signature
is of a “crazy tadpole’’ shape: a narrow spectrum tail precedes
a broad-band head. The instantaneous period of the oscillations in
the wave train decreases gradually with time, with a mean value of
several seconds for typical coronal values. The period and the spectral
amplitude evolution are determined by the steepness of the transverse
density profile and the density contrast ratio in the loop, which offers
a tool for estimation of the sub-resolution structuring of the corona.
---------------------------------------------------------
Title: X-Ray Quasi-Periodic Pulsations in Solar Flares as
Magnetohydrodynamic Oscillations
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.; Fletcher, L.
2005ESASP.596E..46F Altcode: 2005ccmf.confE..46F
No abstract at ADS
---------------------------------------------------------
Title: X-ray quasi-periodic pulsations in solar flares as
magnetohydrodynamic oscillations
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.; Fletcher, L.
2005A&A...440L..59F Altcode:
We report the first observation at high spatial resolution of
long-period quasi-periodic pulsations (QPP) of X-ray radiation during
solar flares, made possible with the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI), supported by complementary data at other
wavelengths from space-based and ground-based telescopes. Evidence
for the presence of a transequatorial loop possibly responsible for
the detected periodicity connected with its kink mode is found. Our
findings suggest that QPP can be interpreted as a periodic pumping
of electrons in a compact flaring loop, modulated by oscillations
in a magnetically linked and larger loop acting as a long-period
magnetohydrodynamic resonator.
---------------------------------------------------------
Title: High-frequency Alfvén waves in multi-ion coronal plasma:
Observational implications
Authors: Ofman, L.; Davila, J. M.; Nakariakov, V. M.; ViñAs, A. -F.
2005JGRA..110.9102O Altcode: 2005JGRA..11009102O
We investigate the effects of high-frequency (of order ion
gyrofrequency) Alfvén and ion-cyclotron waves on ion emission lines
by studying the dispersion of these waves in a multi-ion coronal
plasma. For this purpose we solve the dispersion relation of the
linearized multifluid and Vlasov equations in a magnetized multi-ion
plasma with coronal abundances of heavy ions. We also calculate the
dispersion relation using nonlinear one-dimensional hybrid kinetic
simulations of the multi-ion plasma. When heavy ions are present the
dispersion relation of parallel propagating Alfvén cyclotron waves
exhibits the following branches (in the positive Ω - k quadrant):
right-hand polarized nonresonant and left-hand polarized resonant
branch for protons and each ion. We calculate the ratio of ion to
proton velocities perpendicular to the direction of the magnetic field
for each wave modes for typical coronal parameters and find strong
enhancement of the heavy ion perpendicular fluid velocity compared
with proton perpendicular fluid velocity. The linear multifluid cold
plasma results agree with linear warm plasma Vlasov results and with
the nonlinear hybrid simulation model results. In view of our findings
we discuss how the observed nonthermal line broadening of minor ions
in coronal holes may relate to the high-frequency wave motions.
---------------------------------------------------------
Title: Spatially resolved microwave pulsations of a flare loop
Authors: Melnikov, V. F.; Reznikova, V. E.; Shibasaki, K.; Nakariakov,
V. M.
2005A&A...439..727M Altcode:
A microwave burst with quasi-periodic pulsations was studied with high
spatial resolution using observations with the Nobeyama Radioheliograph
(NoRH). We found that the time profiles of the microwave emission at
17 and 34 GHz exhibit quasi-periodic (with two well defined periods
P_1= 14-17 s and P_2= 8-11 s) variations of the intensity at different
parts of an observed flaring loop. Detailed Fourier analysis shows the
P<SUB>1</SUB> spectral component to be dominant at the top, while the
P<SUB>2</SUB> one near the feet of the loop. The 14-17 s pulsations
are synchronous at the top and in both legs of the loop. The 8-11 s
pulsations at the legs are well correlated with each other but the
correlation is not so obvious with the pulsations at the loop top. For
this P<SUB>2</SUB> spectral component, a definite phase shift, P_2/4≈
2.2~s, between pulsations in the northern leg and loop top parts of
the loop have been found. The length of the flaring loop is estimated
as L = 25 Mm (≈ 34 arcsec) and its average width at half intensity
at 34 GHz as about 6 Mm (≈ 8 arcsec). Microwave diagnostics shows
the loop to be filled with a dense plasma with the number density
n<SUB>0</SUB> ≈ 10<SUP>11</SUP> cm<SUP>-3</SUP>, penetrated by the
magnetic field changing from B<SUB>0</SUB> ≈ 100 G near the loop top
up to B<SUB>0</SUB> ≈ 200 G near the north footpoint. A comparative
analysis of different MHD modes of the loop demonstrates the possibility
of the simultaneous existence of two modes of oscillations in the loop:
the global sausage mode, with the period P_1= 14-17 s and the nodes at
the footpoints, and a higher harmonics mode (possibly with the radial
wave number l>1), with P_2= 8-11 s.
---------------------------------------------------------
Title: Acoustic Oscillations in Solar and Stellar Flaring Loops
Authors: Kelly, A.; Nakariakov, V. M.; Arber, T. D.
2005PADEU..15..125K Altcode:
Observations of flaring loops in radio, visible and x-ray bands
show quasi-periodic pulsations with periods from a few seconds
to several minutes. Recent numerical studies have shown that
some of these oscillations can be interpreted as standing slow
magnetoacoustic waves. Energy deposition from the flare excites the
second standing harmonic, with a period determined by the temperature
and loop length. The excited longitudinal oscillations can be
practically dissipationless and can, possibly, be considered MHD
autowaves. Numerical simulations with a wide range of flare durations
and choices of heat deposition location show that the second harmonic
is a common feature of flaring loops.
---------------------------------------------------------
Title: Coronal Waves and Oscillations
Authors: Nakariakov, Valery M.; Verwichte, Erwin
2005LRSP....2....3N Altcode:
Wave and oscillatory activity of the solar corona is confidently
observed with modern imaging and spectral instruments in the
visible light, EUV, X-ray and radio bands, and interpreted in terms
of magnetohydrodynamic (MHD) wave theory. The review reflects
the current trends in the observational study of coronal waves
and oscillations (standing kink, sausage and longitudinal modes,
propagating slow waves and fast wave trains, the search for torsional
waves), theoretical modelling of interaction of MHD waves with plasma
structures, and implementation of the theoretical results for the mode
identification. Also the use of MHD waves for remote diagnostics of
coronal plasma — MHD coronal seismology — is discussed and the
applicability of this method for the estimation of coronal magnetic
field, transport coefficients, fine structuring and heating function
is demonstrated.
---------------------------------------------------------
Title: Transverse waves in a post-flare supra-arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Cooper, F. C.
2005A&A...430L..65V Altcode:
Observations of propagating transverse waves in an open magnetic field
structure with the Transition Region And Coronal Explorer (TRACE)
are presented. Waves associated with dark tadpole-like sunward
moving structures in the post-flare supra-arcade of NOAA active
region 9906 on the 21st of April 2002 are analysed. They are seen as
quasi-periodic transverse displacements of the dark tadpole tails, with
periods in the range of 90-220 s. Their phase speeds and displacement
amplitudes decrease as they propagate sunwards. At heights of 90 and
60 Mm above the post-flare loop footpoints the phase speeds are in
the ranges 200-700 km s<SUP>-1</SUP> and 90-200 km s<SUP>-1</SUP>
respectively. Furthermore, for consecutive tadpoles the phase speeds
decrease and periods increase as a function of time. The waves are
interpreted as propagating fast magnetoacoustic kink waves guided by
a vertical, evolving, open structure.
---------------------------------------------------------
Title: Detection of Ultra-Long Oscillations in an EUV Filament
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.
2004ESASP.575..394F Altcode: 2004soho...15..394F
No abstract at ADS
---------------------------------------------------------
Title: Transverse Oscillations in a Coronal Loop Arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Ofman, L.; Deluca, E. E.
2004ESASP.575..460V Altcode: 2004soho...15..460V
No abstract at ADS
---------------------------------------------------------
Title: Coronal Waves: Coronal Heating and Coronal Seismology
Authors: Nakariakov, V. M.
2004ASPC..325..253N Altcode:
SOHO and TRACE missions have demonstrated that wave phenomena are
abundant in the corona and provided us with an observational basis for
MHD coronal seismology. This review covers the recent observational
finding of coronal wave phenomena and their theoretical models. The
relevance of these observational and theoretical efforts to the coronal
heating is discussed. Possible observability of these phenomena with
EIS and XRT instruments is discussed.
---------------------------------------------------------
Title: Transverse Waves in a Post-Flare Supra-Arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Cooper, F. C.
2004ESASP.575..126V Altcode: 2004soho...15..126V
No abstract at ADS
---------------------------------------------------------
Title: The Nature and Excitation Mechanisms of Acoustic Oscillations
in Solar and Stellar Coronal Loops
Authors: Tsiklauri, D.; Nakariakov, V. M.; Arber, T. D.; Aschwanden,
M. J.
2004ESASP.575..114T Altcode: 2004soho...15..114T; 2004astro.ph..9556T
In the recent work of Nakariakov et al. (2004), it has been shown
that the time dependences of density and velocity in a flaring loop
contain pronounced quasi-harmonic oscillations associated with the
2nd harmonic of a standing slow magnetoacoustic wave. That model
used a symmetric heating function (heat deposition was strictly at
the apex). This left outstanding questions: A) is the generation of
the 2nd harmonic a consequence of the fact that the heating function
was symmetric? B) Would the generation of these oscillations occur if
we break symmetry? C) What is the spectrum of these oscillations? Is
it consistent with a 2nd spatial harmonic? The present work (and
partly Tsiklauri et al. (2004b)) attempts to answer these important
outstanding questions. Namely, we investigate the physical nature
of these oscillations in greater detail: we study their spectrum
(using periodogram technique) and how heat positioning affects the
mode excitation. We found that excitation of such oscillations is
practically independent of location of the heat deposition in the
loop. Because of the change of the background temperature and density,
the phase shift between the density and velocity perturbations is
not exactly a quarter of the period, it varies along the loop and is
time dependent, especially in the case of one footpoint (asymmetric)
heating. We also were able to model successfully SUMER oscillations
observed in hot coronal loops.
---------------------------------------------------------
Title: Detection of ultra-long-period oscillations in an EUV filament
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.
2004A&A...427L...5F Altcode:
We report the first detection of long-period (8-27 h) oscillatory
intensity variations in a coronal filament. The filament is observed
continuously as it crosses the solar disk in a 12-min-cadence SoHO/EIT
195 Å, uninterrupted data set. Cyclic intensity variations are found to
be correlated along the filament, while the most pronounced oscillations
are detected at its southern end for nearly 6 days. The dominant period
of these oscillations is 12.1 h and the amplitude of the intensity
variations reaches approximately 10% of the background intensity. The
ultra-long-period oscillations may be interpreted in terms of slow
string MHD modes or may be connected with thermal over-stability
associated with peculiarities of the cooling/heating function and with
the effect of neutrals. These theoretical predictions however do not
explain the spatial structure of the oscillations along the filament.
---------------------------------------------------------
Title: Meeting report: Waves in solar and magnetospheric plasmas
Authors: Nakariakov, V. M.; Roberts, B.; Wright, A. N.
2004A&G....45e..32N Altcode:
This RAS discussion meeting was arranged with the idea of bringing
together solar physicists and magnetospheric physicists, so as
to understand better how the two disciplines approach waves, and
ultimately to learn from each other. V M Nakariakov, B Roberts and A
N Wright report.
---------------------------------------------------------
Title: CDS wide slit time-series of EUV coronal bright points
Authors: Ugarte-Urra, I.; Doyle, J. G.; Nakariakov, V. M.; Foley, C. R.
2004A&A...425.1083U Altcode:
Wide slit (90<SUP>”</SUP>×240<SUP>”</SUP>) movies of four Extreme
Ultraviolet coronal bright points (BPs) obtained with the Coronal
Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric
Observatory (SoHO) have been inspected. The wavelet analysis of the He
I 584.34 Å, O V 629.73 Å and Mg VII/IX 368 Å time-series confirms
the oscillating nature of the BPs, with periods ranging between
600 and 1100 s. In one case we detect periods as short as 236 s. We
suggest that these oscillations are the same as those seen in the
chromospheric network and that a fraction of the network bright points
are most likely the cool footpoints of the loops comprising coronal
bright points. These oscillations are interpreted in terms of global
acoustic modes of the closed magnetic structures associated with BPs.
---------------------------------------------------------
Title: Characteristics of transverse oscillations in a coronal
loop arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Ofman, L.; Deluca, E. E.
2004SoPh..223...77V Altcode:
TRACE observations from 15 April 2001 of transverse oscillations in
coronal loops of a post-flare loop arcade are investigated. They
are considered to be standing fast kink oscillations. Oscillation
signatures such as displacement amplitude, period, phase and damping
time are deduced from 9 loops as a function of distance along the loop
length. Multiple oscillation modes are found with different amplitude
profile along the loop length, suggesting the presence of a second
harmonic. The damping times are consistent with the hypothesis of
phase mixing and resonant absorption, although there is a clear bias
towards longer damping times compared with previous studies. The
coronal magnetic field strength and coronal shear viscosity in the
loop arcade are derived.
---------------------------------------------------------
Title: Coronal seismology: Seismology of the corona of the Sun
Authors: Nakariakov, V. M.; Verwichte, E.
2004A&G....45d..26N Altcode:
Seismology now includes study of the Sun's corona, a promising research
target both in its own right and for its role in the relationship
between the Sun and the Earth and its links to the solar magnetic
field. In addition, the corona, as a natural plasma, is itself an
objective for fundamental physics. Observations using spacecraft such
as SOHO and TRACE highlight the interrelationship between the magnetic
field and corona.
---------------------------------------------------------
Title: Astrophysical Implications of Magnetoacoustic Autowaves in
Thermally Unstable Plasmas
Authors: Kelly, A.; Nakariakov, V. M.
2004plph.confE...1K Altcode:
The formation and evolution of magnetoacoustic autowaves in thermally
unstable plasmas is discussed, with particular reference to solar
and stellar flares. The competition of thermal overstability, thermal
conductivity and radiative losses leads to self-organisation of weakly
nonlinear magnetoacoustic perturbations into dissipative structures with
the parameters independent of the initial perturbation. This evolution
is modelled in terms of the extended Burgers equation. The properties
of the autowaves are expressed through the plasma parameters such
as the magnetic field, plasma temperature and density. The autowaves
are saw-tooth shaped waves, with an eventual amplitude that depends
upon the balance between the amplification due to radiative losses and
decay due to thermal conductivity. Increased dissipation due to thermal
conductivity or stronger non-linearity leads to a lower amplitude, while
stronger thermal instability leads to a higher final amplitude. X-ray,
white light and radio band observations and numerical simulations of
flaring loops show quasi-periodic pulsations during the flares. These
pulsations are standing acoustic waves (primarily the second harmonic)
and show little decay over a number of periods despite the large
theoretically predicted damping due to thermal conductivity. It is
suggested that these waves are magnetoacoustic autowaves.
---------------------------------------------------------
Title: Flare-generated acoustic oscillations in solar and stellar
coronal loops
Authors: Tsiklauri, D.; Nakariakov, V. M.; Arber, T. D.; Aschwanden,
M. J.
2004A&A...422..351T Altcode: 2004astro.ph..2261T
Long period longitudinal oscillations of a flaring coronal loop are
studied numerically. In the recent work of Nakariakov et al. (2004)
it has been shown that the time dependence of density and velocity in a
flaring loop contain pronounced quasi-harmonic oscillations associated
with the 2nd harmonic of a standing slow magnetoacoustic wave. In
this work we investigate the physical nature of these oscillations
in greater detail, namely, their spectrum (using the periodogram
technique) and how heat positioning affects mode excitation. We found
that excitation of such oscillations is practically independent of the
location of the heat deposition in the loop. Because of the change of
the background temperature and density, the phase shift between the
density and velocity perturbations is not exactly a quarter of the
period; it varies along the loop and is time dependent, especially in
the case of one footpoint (asymmetric) heating.
---------------------------------------------------------
Title: Radiative hydrodynamic modeling of the Bastille-Day flare
(14 July, 2000). I. Numerical simulations
Authors: Tsiklauri, D.; Aschwanden, M. J.; Nakariakov, V. M.; Arber,
T. D.
2004A&A...419.1149T Altcode: 2004astro.ph..2260T
A 1D loop radiative hydrodynamic model that incorporates the effects
of gravitational stratification, heat conduction, radiative losses,
external heat input, presence of helium, and Braginskii viscosity is
used to simulate elementary flare loops. The physical parameters for the
input are taken from observations of the Bastille-Day flare of 2000 July
14. The present analysis shows that: a) the obtained maximum values of
the electron density can be considerably higher (4.2 × 10<SUP>11</SUP>
cm<SUP>-3</SUP> or more) in the case of footpoint heating than in
the case of apex heating (2.5 × 10<SUP>11</SUP> cm<SUP>-3</SUP>);
b) the average cooling time after the flare peak takes less time in
the case of footpoint heating than in the case of apex heating; c)
the peak apex temperatures are significantly lower (by about 10 MK)
for the case of footpoint heating than for apex heating (for the
same average loop temperature of about 30 MK). This characteristic
would allow to discriminate between different heating positioning; d)
in both cases (of apex and footpoint heating), the maximum obtained
apex temperature T<SUP>max</SUP> is practically independent of the
heating duration σ<SUB>t</SUB>, but scales directly with the heating
rate E<SUB>H0</SUB>; e) the maximum obtained densities at the loop
apex, n<SUB>e</SUB><SUP>max</SUP>, increase with the heating rate
E<SUB>H0</SUB> and heating duration σ<SUB>t</SUB> for both footpoint
and apex heating. In Paper II we will use the outputs of these
hydrodynamic simulations, which cover a wide range of the parameter
space of heating rates and durations, as an input for forward-fitting
of the multi-loop arcade of the Bastille-day flare.
---------------------------------------------------------
Title: Time signatures of impulsively generated coronal fast wave
trains
Authors: Nakariakov, V. M.; Arber, T. D.; Ault, C. E.; Katsiyannis,
A. C.; Williams, D. R.; Keenan, F. P.
2004MNRAS.349..705N Altcode:
Impulsively generated short-period fast magneto-acoustic wave
trains, guided by solar and stellar coronal loops, are numerically
modelled. In the developed stage of the evolution, the wave trains
have a characteristic quasi-periodic signature. The quasi-periodicity
results from the geometrical dispersion of the guided fast modes,
determined by the transverse profile of the loop. A typical feature
of the signature is a tadpole wavelet spectrum: a narrow-spectrum
tail precedes a broad-band head. The instantaneous period of the
oscillations in the wave train decreases gradually with time. The period
and the spectral amplitude evolution are shown to be determined by the
steepness of the transverse density profile and the density contrast
ratio in the loop. The propagating wave trains recently discovered
with the Solar Eclipse Coronal Imaging System (SECIS) instrument are
noted to have similar wavelet spectral features, which strengthens
the interpretation of SECIS results as guided fast wave trains.
---------------------------------------------------------
Title: Theoretical Aspects of MHD Coronal Seismology
Authors: Nakariakov, V. M.
2004ESASP.547..407N Altcode: 2004soho...13..407N
The revolutionary breakthrough in observational detection of propagating
and standing quasi-periodic disturbances of coronal plasma parameters,
made in the EUV band with SOHO and TRACE mission instruments, toin
terms of magnetohydrodynamic (MHD) waves, has given rise to MHD coronal
seismology. This review presents the theoretical background of these
studies and reflects current trends in the development of the theory.
---------------------------------------------------------
Title: Global Sausage Magnetoacoustic Modes of Coronal Loops
Authors: Nakariakov, V. M.; Melnikov, V. F.; Reznikova, V. E.
2004IAUS..223..653N Altcode: 2005IAUS..223..653N
A sufficiently thick and dense flaring magnetic loop is shown to be
able to support global sausage magnetoacoustic modes. The oscillation
period of this mode, calculated in the straight cylinder approximation,
is determined by the length of the loop, not by its diameter, as
it has previously been assumed. For the existence of this mode, the
ratio of the loop length to its diameter is to be less than about a
half of the square root of the density contrast ratio. This mode has a
maximum of the magnetic field perturbation at the loop apex and nodes
at the footpoints. We demonstrate that the 14-17 sec quasi-periodical
pulsations of microwave emission, oscillating in phase at a loop apex
and at its legs, as observed with the Nobeyama Radioheliograph, are
interpreted in terms of the global sausage mode.
---------------------------------------------------------
Title: Coronal Seismology by MHD Autowaves
Authors: Kelly, A.; Nakariakov, V. M.
2004ESASP.547..483K Altcode: 2004soho...13..483K
Slow magnetoacoustic waves are routinely observed in solar coronal
structures. These waves can be strongly affected by non-adiabatic
effects leading to self-organising compressible propagating disturbances
-- magnetoacoustic autowaves. Autowaves are propagating disturbances
which have parameters independent of the excitation that are determined
by the parameters of the medium only and, consequently, are potentially
an ideal tool for coronal seismology. The influence of non-adiabatic
effects is studied and magnetoacoustic autowaves are modelled with
the extended Burgers equation. A numerical code is developed to
study the evolution of such waves and a parametric study is carried
out. Observable parameters of coronal magnetoacoustic autowaves could
provide a tool for the determination of heat deposition mechanisms in
the corona.
---------------------------------------------------------
Title: Acoustic Oscillations in Solar and Stellar Flaring Loops
Authors: Tsiklauri, D.; Nakariakov, V. M.; Kelly, A.; Aschwanden,
M. J.; Arber, T. D.
2004ESASP.547..473T Altcode: 2004soho...13..473T
No abstract at ADS
---------------------------------------------------------
Title: Acoustic oscillations in solar and stellar flaring loops
Authors: Nakariakov, V. M.; Tsiklauri, D.; Kelly, A.; Arber, T. D.;
Aschwanden, M. J.
2004A&A...414L..25N Altcode: 2004astro.ph..2223N
Evolution of a coronal loop in response to an impulsive energy release
is numerically modelled. It is shown that the loop density evolution
curves exhibit quasi-periodic perturbations with the periods given
approximately by the ratio of the loop length to the average sound
speed, associated with the second standing harmonics of an acoustic
wave. The density perturbations have a maximum near the loop apex. The
corresponding field-aligned flows have a node near the apex. We
suggest that the quasi-periodic pulsations with periods in the range
10-300 s, frequently observed in flaring coronal loops in the radio,
visible light and X-ray bands, may be produced by the second standing
harmonic of the acoustic mode.
---------------------------------------------------------
Title: Magnetohydrodynamic Sausage-Mode Oscillations in Coronal Loops
Authors: Aschwanden, Markus J.; Nakariakov, Valery M.; Melnikov,
Victor F.
2004ApJ...600..458A Altcode: 2003astro.ph..9493A
A recent study by Nakariakov and coworkers pointed out that the
dispersion relation of MHD sausage-mode oscillations has been
incorrectly applied to coronal loops, neglecting the highly dispersive
nature of the phase speed and the long-wavelength cutoff of the
wavenumber. In the light of these new insights, we revisit previous
observations that have been interpreted in terms of MHD sausage-mode
oscillations in coronal loops and come to the following conclusions:
(1) fast sausage MHD-mode oscillations require such a high electron
density imposed by the wavenumber cutoff that they can only occur in
flare loops, and (2) in the previously reported radio observations
(ν~100 MHz to 1 GHz) with periods of P~0.5-5 s, the fast sausage
MHD-mode oscillation is likely to be confined to a small segment
(corresponding to a high harmonic node) near the apex of the loop,
rather than involving a global oscillation over the entire loop
length. The recent microwave and soft X-ray observations of fast periods
(P~6-17 s) by Asai and coworkers and Melnikov and coworkers, however,
are consistent with fast sausage MHD oscillations at the fundamental
harmonic.
---------------------------------------------------------
Title: 3D Numerical Simulations of Impulsively Generated MHD Waves
in Solar Coronal Loops
Authors: Selwa, M.; Murawski, K.; Kowal, G.; Nakariakov, V.;
Aschwanden, M.; Oliver, R.; Ballester, J. L.
2004ESASP.547..495S Altcode: 2004soho...13..495S
Impulsively generated magnetohydrodynamic waves in a typical EUV solar
coronal loop are studied numerically with a use of the three-dimensional
FLASH code. Our results reveal several 3D effects such as distinctive
time signatures which are collected at a detection point inside the
loop. A slow magnetosonic wave generates a significant variation in a
mass density profile with a time-scale of the order of s. A fast kink
wave affects a mass density too but its magnitude is much lower than
in the case of a slow wave. Time-scales which are associated with the
fast kink wave are generally lower than in the case of a slow wave;
they are in the range of a dozen or so seconds. Temporal signatures
of a fast sausage wave reveal s oscillations in the quasi-periodic
phase. Impulses which are launched outside the loop excite few seconds
oscillations in the mass density. Time-signatures depend on a position
of the detection point; they are usually more complex further out from
the exciter.
---------------------------------------------------------
Title: Observations of sausage mode oscillations in a flaring loop
Authors: Melnikov, V. F.; Reznikova, V. E.; Shibasaki, K.; Nakariakov,
V. M.
2004IAUS..223..647M Altcode: 2005IAUS..223..647M
We provide an observational proof of the existence of the global sausage
mode oscillations in a flaring loop studying a microwave burst with
quasi-periodical pulsations observed with the Nobeyama Radioheliograph
(NoRH).
---------------------------------------------------------
Title: The possible back-rotation of sunspots
Authors: Williams, D. R.; van Driel-Gesztelyi, L.; Nakariakov, V. M.
2004cosp...35.4391W Altcode: 2004cosp.meet.4391W
The curious case of sunspot rotation in two senses, presented by
Kucera (1982), provided an interesting challenge to the idea of sunspot
rotation being due simply to the differential rotation of the Sun above
the tachocline. As the rotation rate and direction, in that case, are
seen to vary as a damped sinusoid, this presents the possibility that
such motion is the result of torsional waves trapped in the sunspot's
sub-photospheric flux tube. We therefore analyse the rotation of a
number of sunspots using SoHO/MDI and track their rotation, presenting
evidence of 'back-rotation' -- rotational motion which reverses --
in a several of these spots. Intuitively, this raises the possibility
of extending the field of 'Coronal Seismology' back down to beneath
the photosphere (where helioseismology already exists).
---------------------------------------------------------
Title: Alfvén Waves in Multi-ion Coronal Plasma: Observational
Implications
Authors: Ofman, L.; Nakariakov, V. M.; Davila, J. M.
2003AGUFMSH11D1129O Altcode:
We investigate low-frequency Alfvén waves in a multi-ion coronal plasma
by deriving the dispersion relation of the linearized three-fluid
equations in magnetized plasma with coronal parameters. We study
the effect of collisions on the velocity amplitude of minor ions due
to the Alfvén waves compared to the proton wave amplitude. We find
that in the collisionless plasma the minor ion velocity is reduced
by the factor Z/A compared to protons, where Z is the ion charge,
and A is mass in units of proton mass. When the collision frequency
is much larger then the Alfvén wave frequency the velocity amplitude
of the minor ions is equal to the proton velocity amplitude. We show
the effects of minor ions and collisions on the dispersion relation
of Alfvén waves in the three fluid plasma. In view of our results we
show how the observed nonthermal line broadening of minor ions relates
to the wave motions of protons as a function of heliocentric distance,
and the plasma physical parameters.
---------------------------------------------------------
Title: Global sausage modes of coronal loops
Authors: Nakariakov, V. M.; Melnikov, V. F.; Reznikova, V. E.
2003A&A...412L...7N Altcode:
Sufficiently thick and dense coronal loops can support global sausage
magnetoacoustic modes. We demonstrate that the oscillation period
of this mode, calculated in the straight cylinder approximation, is
determined by the length of the loop, not by its diameter, as it was
previously assumed. The existence condition for this mode is the ratio
of the loop length to its diameter to be less than about a half of
the square root of the density contrast ratio. This mode has a maximum
of the magnetic field perturbation at the loop apex and nodes at the
footpoints. We demonstrate that the 14-17 s quasi-periodic pulsations,
oscillating in phase at a loop apex and at its legs, observed with
the Nobeyama Radioheliograph, are interpreted in terms of the global
sausage mode.
---------------------------------------------------------
Title: Statistical characterisation of full-disk EUV/XUV solar
irradiance and correlation with solar activity
Authors: Greenhough, J.; Chapman, S. C.; Dendy, R. O.; Nakariakov,
V. M.; Rowlands, G.
2003A&A...409L..17G Altcode: 2003astro.ph..2277G
We investigate the distribution of fluctuations in solar irradiance when
integrated over the full disk, obtained using extreme ultraviolet/soft
X-ray observations from the SOHO CELIAS/SEM instrument. This time
series sums over both the contributions of single distinguishable
flares, and of many other processes. By detrending we select events
with timescales of less than a few hours such as waves, slow flows,
and CMEs. The statistics generated by this range of phenomena can be
characterised by power-law-tailed distributions. We show that (i)
during the high activity period 2000 Jan.-June the tail exponent
a<SUB>T</SUB>=1.5+/-0.1; (ii) during the low activity period 1996
Jan.-June a<SUB>T</SUB>=3.0+/-0.2; and (iii) in general a<SUB>T</SUB>
decreases with increasing activity.
---------------------------------------------------------
Title: Short period fast waves in solar coronal loops
Authors: Cooper, F. C.; Nakariakov, V. M.; Williams, D. R.
2003A&A...409..325C Altcode:
Short period fast magnetoacoustic waves propagating along solar
coronal loops, perturbing the loop boundary along the line of sight
(LOS), may be observed by imaging telescopes. The relationship between
the difference in emission intensity, the angle between the LOS and
the direction of propagation and the wave amplitude and wavelength,
is explored for kink and sausage fast waves. It is shown that the
compressibility of the plasma in the loop significantly affects the
observability of the waves. For both wave types there is an optimal
observation angle which is determined by the ratio of the wave length
and the loop radius. The change of the observational conditions
because of the loop curvature predicts a significant, up to an order
of magnitude, change in the observed wave amplitude. This prediction
is confirmed by the analysis of the evolution of the fast wave train
amplitude, observed with the SECIS instrument. The wave train amplitude
experiences a sharp increase and then a decrease along the loop. The
observational results are in a good agreement with the theory.
---------------------------------------------------------
Title: A Three Dimensional Magnetohydrodynamic Pulse in a Transversely
Inhomogeneous Medium
Authors: Tsiklauri, D.; Nakariakov, V. M.
2003AIPC..679..570T Altcode:
Interaction of impulsively generated MHD waves with a one-dimensional
plasma inhomogeneity, transverse to the magnetic field, is considered in
the three-dimensional regime. Because of the transverse inhomogeneity,
MHD fluctuations, even if they do not include initially any
density perturbation, evolve toward states where the compressible
components tend to become predominant. The propagating MHD pulse
asymptotically reaches a quasi-steady state with the final levels of
density perturbation weakly depending on the degree of non-planeness
of the pulse in the homogeneous transverse direction and somewhat
stronger depending on plasma β. Our study demonstrates the necessity
of incorporation of compressible and 3D effects in theory of Alfvén
wave phase mixing. However, as far as the dynamics of weakly non-plane
Alfvén waves is concerned it can still be qualitatively understood
in terms of the previous 2.5D models.
---------------------------------------------------------
Title: Propagating EUV disturbances in the Solar corona:
Two-wavelength observations
Authors: King, D. B.; Nakariakov, V. M.; Deluca, E. E.; Golub, L.;
McClements, K. G.
2003A&A...404L...1K Altcode:
Quasi-periodic EUV disturbances simultaneously observed in 171 Å and
195 Å TRACE bandpasses propagating outwardly in a fan-like magnetic
structure of a coronal active region are analysed. Time series of
disturbances observed in the different bandpasses have a relatively
high correlation coefficient (up to about 0.7). The correlation has
a tendency to decrease with distance along the structure: this is
consistent with an interpretation of the disturbances in terms of
parallel-propagating slow magnetoacoustic waves. The wavelet analysis
does not show a significant difference between waves observed in
different bandpasses. Periodic patterns of two distinct periods: 2-3 min
and 5-8 min are detected in both bandpasses, existing simultaneously
and at the same distance along the loop, suggesting the nonlinear
generation of the second harmonics.
---------------------------------------------------------
Title: Coronal `Tadpoles' Observed by TRACE
Authors: Seaton, D. B.; Deluca, E. E.; Cooper, F. C.; Nakariakov, V. M.
2003SPD....34.1618S Altcode: 2003BAAS...35..836S
The X1.5 flare event observed by TRACE in 195 Å on 21 April 2002,
showed dark, downward-propagating features over the flare arcade which
we call `coronal tadpoles.' We analyzed these features and determined
their distinguishing characteristics. Estimating plasma densities
inside the features, we determined that they are of very low density
and possibly >10 million K. We propose that the coronal tadpoles
my be interpreted in terms of kink modes of the flaring current sheet
filled by hot, rarefied plasma.
---------------------------------------------------------
Title: Nonlinear Multi-Scale Processes in the Heliosphere in the IHY
Authors: Chapman, S. C.; Nakariakov, V. M.; Arber, T. D.
2003EAEJA....10486C Altcode:
University of Warwick's Space and Astrophysics Group have undertaken
an intensive research programme in magnetospheric, heliospheric and
solar physics, including analytical theory, large scale numerical
simulation, advanced visualization techniques and theory led data
analysis. The unifying theme is that these are nonlinear multiscale
processes, which control the energy release and conversion processes,
turbulence and self-organisation. The coordinated studies of solar,
heliospheric and magnetospheric processes proposed in frames of the
IHY provide a unique opportunity to unify knowledge that is presently
fragmented. For example, remote sensing of physical processes in the
solar corona with the use of ground-based and space-borne facilities
(potentially, including the multiple vantage point observations with
STEREO), remote (e.g. IPS) and in-situ (e.g. WIND, CLUSTER) studies of
the development of these processes in the solar wind and the coupling to
the dynamic Earth magnetosphere (as indicated by scaling in geomagnetic
indices and POLAR UVI images of dynamic auroras) and ionosphere, will
allow us to understand the chain of solar-terrestrial connections and,
in particular, the nonlinear processes governing these connections.
---------------------------------------------------------
Title: Phase mixing of a three dimensional magnetohydrodynamic pulse
Authors: Tsiklauri, D.; Nakariakov, V. M.; Rowlands, G.
2003A&A...400.1051T Altcode: 2002astro.ph.10117T
Phase mixing of a three dimensional magnetohydrodynamic (MHD) pulse is
studied in the compressive, three-dimensional (without an ignorable
coordinate) regime. It is shown that the efficiency of decay of an
Alfvénic part of a compressible MHD pulse is related linearly to
the degree of localization of the pulse in the homogeneous transverse
direction. In the developed stage of phase mixing (for large times),
coupling to its compressive part does not alter the power-law decay
of an Alfvénic part of a compressible MHD pulse. The same applies
to the dependence upon the resistivity of the Alfvénic part of the
pulse. All this implies that the dynamics of Alfvén waves can still be
qualitatively understood in terms of the previous 2.5D models. Thus,
the phase mixing remains a relevant paradigm for the coronal heating
applications in the realistic 3D geometry and compressive plasma.
---------------------------------------------------------
Title: Quasi-periodic behaviour of a flare ribbon system
Authors: Ludmány, A.; Major, B.; Nakariakov, V. M.
2003PADEU..13..139L Altcode:
A series of quasi-periodic pulsations have been detected in the
flare of 19th July 1999. The total area occupied by the flare ribbons
exhibited temporary increases in the declining phase of the flare at
certain moments, the period of the pulses was about 20-30 minutes. The
phenomenon has been recognized in the center, as well as at 0.5 and
1.0 Å wings on both sides of the H-alpha line.
---------------------------------------------------------
Title: Coronal oscillations
Authors: Nakariakov, V. M.
2003dysu.book..314N Altcode:
The detection of coronal waves provides us with a new tool for the
determination of the unknown parameters of the corona - MHD seismology
of the corona. The method is similar to helioseismology. But MHD coronal
seismology is much richer as it is based upon three different wave modes
- Alfvén, slow and fast magnetoacoustic modes. These MHD modes have
quite different dispersive, polarization and propagation properties,
which make this approach even more powerful. The delicate interplay of
MHD wave theory and the observations of coronal waves and oscillations
are presented, illustrating it with several examples.
---------------------------------------------------------
Title: Line-of-sight effects on observability of kink and sausage
modes in coronal structures with imaging telescopes
Authors: Cooper, F. C.; Nakariakov, V. M.; Tsiklauri, D.
2003A&A...397..765C Altcode: 2002astro.ph..7167C
Kink modes of solar coronal structures, perturbing the loop in the
direction along the line-of-sight (LOS), can be observed as emission
intensity disturbances propagating along the loop provided the angle
between the LOS and the structure is not ninety degrees. The effect
is based upon the change of the column depth of the loop (along the
LOS) by the wave. The observed amplitude of the emission intensity
variations can be larger than the actual amplitude of the wave by a
factor of two and there is an optimal angle maximizing the observed
amplitude. For other angles this effect can also attenuate the observed
wave amplitude. The observed amplitude depends upon the ratio of the
wave length of kink perturbations to the width of the structure and on
the angle between the LOS and the axis of the structure. Sausage modes
are always affected negatively from the observational point of view,
as the observed amplitude is always less than the actual one. This
effect should be taken into account in the interpretation of wave
phenomena observed in the corona with space-borne and ground-based
imaging telescopes.
---------------------------------------------------------
Title: Book Review: Current theoretical models and high resolution
solar observations / Astronomical Society of the Pacific, 2003
Authors: Nakariakov, Valery
2003Obs...123..391N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Phase mixing of a three dimensional magnetohydrodynamic pulse
Authors: Tsiklauri, D.; Nakariakov, V. M.
2002ESASP.506...67T Altcode: 2002ESPM...10...67T; 2002svco.conf...67T
Phase mixing of a three dimensional magnetohydrodynamic (MHD) pulse
is studied in the compressive three-dimensional (without an ignorable
coordinate) regime. It is shown that the coupling to the compressive
modes dramatically enhances the efficiency of decay of an Alfvénic part
of a compressible MHD pulse. However, the dynamics of weakly non-plane
Alfvén waves can still be qualitatively understood in terms of the
previous 2.5D models.
---------------------------------------------------------
Title: Line-of-sight effects on observability of kink modes in
coronal structures with imaging telescopes
Authors: Cooper, F. C.; Nakariakov, V. M.; Tsiklauri, D.
2002ESASP.506..577C Altcode: 2002svco.conf..577C; 2002ESPM...10..577C
Kink modes of solar coronal structures, perturbing the loop in the
direction along the line-of-sight (LOS), can be observed as emission
intensity disturbances propagating along the loop provided the angle
between the LOS and the structure is not ninety degrees. The phenomenon
is based upon the change of the column depth of the loop (along the LOS)
by the wave. The observed amplitude of the emission intensity variations
can be larger than the actual amplitude of the wave by a factor of two
and there is an optimal angle maximizing the observed amplitude. The
observed amplitude depends upon the ratio of the wavelength of kink
perturbations to the width of the structure and on the angle between
the LOS and the axis of the structure. This phenomenon should be taken
into account in the interpretation of wave phenomena observed in the
corona with space-borne and ground-based imaging telescopes.
---------------------------------------------------------
Title: Slow magnetoacoustic waves in coronal loops
Authors: Nakariakov, V. M.; King, D. B.; Tsiklauri, D.
2002ESASP.506..705N Altcode: 2002ESPM...10..705N; 2002svco.conf..705N
Propagating disturbances of EUV emission, recently discovered in coronal
loops with SOHO/EIT and TRACE, are interpreted as slow magnetoacoustic
waves. This interpretation is based upon several observational findings:
(a) the disturbances seem to propagate along the magnetic field
(b) at a constant speed which is below the estimated sound speed,
(c) the perturbations are compressible, (d) the periods are shorter
than the acoustic cut-off period in the corona. A theoretical model
describing the wave evolution and incorporating effects of dissipation,
stratification and weak nonlinearity is created. In particular, the
model explains why only the upwardly propagating waves are observed. The
interpretation suggested is in a good agreement with obervational
findings. It is shown that wide spectrum slow magnetoacoustic waves,
consistent with currently available observations in the low frequency
part of the spectrum, can provide the rate of heat deposition sufficient
to heat the loop. In this scenario, the heat would be deposited near
the loop footpoints which also agrees with the current observational
data. The analysis of simultaneous observations of propagating EUV
disturbances in 171 Å and 195 Å bandpasses of TRACE telescope shows
high correlation which systematically decreases with the distance
along the loop.
---------------------------------------------------------
Title: Kink oscillations of coronal loops as a tool for the
determination of coronal magnetic field
Authors: Nakariakov, Valery; Ofman, Leon
2002ESASP.506..461N Altcode: 2002svco.conf..461N; 2002ESPM...10..461N
We develop a new method for the determination of the absolute value
of the magnetic field strength in coronal closed magnetic structures,
based on the analysis of flare-generated oscillations of coronal
loops. Interpretation of the quasi-harmonic oscillations observed in
terms of global standing kink waves allows to connect the period of
the oscillations and the loops length with the magnetic field strength
in the loops. Improved diagnostic of the loop length, the oscillation
period, and the plasma density in the loop will significantly improve
the method's precision.
---------------------------------------------------------
Title: A strongly nonlinear Alfvénic pulse in a transversely
inhomogeneous medium
Authors: Tsiklauri, D.; Nakariakov, V. M.; Arber, T. D.
2002A&A...395..285T Altcode: 2002astro.ph.10625T
We investigate the interaction of a plane, linearly polarized, Alfvénic
pulse with a one-dimensional, perpendicular to the magnetic field,
plasma density inhomogeneity in the strongly nonlinear regime. Our
numerical study of the full MHD equations shows that: (i) Plasma
density inhomogeneity substantially enhances (by about a factor of
2) the generation of longitudinal compressive waves. (ii) Attained
maximal values of the generated transverse compressive perturbations
are insensitive to the strength of the plasma density inhomogeneity,
plasma beta and the initial amplitude of the Alfvén wave. Typically,
they reach about 40% of the initial Alfvén wave amplitude. (iii)
Attained maximal values of the generated relative density perturbations
are within 20-40% for 0.5 <= beta <= 2.0. They depend upon
plasma beta strongly; and scale almost linearly with the initial
Alfvén wave amplitude.
---------------------------------------------------------
Title: A three dimensional magnetohydrodynamic pulse in a transversely
inhomogeneous medium
Authors: Tsiklauri, D.; Nakariakov, V. M.
2002A&A...393..321T Altcode: 2002astro.ph..2489T
Interaction of impulsively generated MHD waves with a one-dimensional
plasma inhomogeneity, transverse to the magnetic field, is considered in
the three-dimensional regime. Because of the transverse inhomogeneity,
MHD fluctuations, even if they do not include initially any density
perturbation, evolve toward states where the compressible components
tend to become predominant. The propagating MHD pulse asymptotically
reaches a quasi-steady state with the final levels of density
perturbation weakly depending on the degree of non-planeness of the
pulse in the homogeneous transverse direction and somewhat stronger
depending on plasma beta . Our study demonstrates the necessity of
incorporation of compressible and 3D effects in theory of Alfvén wave
phase mixing. However, as far as the dynamics of weakly non-plane
Alfvén waves is concerned it can still be qualitatively understood
in terms of the previous 2.5D models.
---------------------------------------------------------
Title: Nonlinear MHD waves in the solar wind plasma structures
Authors: Tsiklauri, D.; Nakariakov, V. M.; Arber, T. D.
2002ESASP.477..381T Altcode: 2002scsw.conf..381T; 2001astro.ph.10494T
We investigate the interaction of magnetohydrodnamic waves with
plasma density inhomogeneities. Our numerical study of the full MHD
equations shows that: (A) Plasma density inhomogeneities are a source
of non-linear generation of transverse compressive waves by a plane
Alfvén wave, and substantially enhance (by about a factor of 2) the
generation of the longitudinal compressive waves. (B) Attained maximal
values of the generated transverse compressive waves are insensitive
to the strength of the plasma density inhomogeneity and the initial
amplitude of the Alfvén wave. (C) Efficiency of the generation depends
weakly upon the plasma β parameter. The maximum generated amplitude
of transverse compressive wave, up to 32% of the initial Alfvén
wave amplitude, is reached for about β = 0.5. The results obtained
demonstrate that plasma inhomogeneities enhance the efficiency of the
non-linear wave coupling.
---------------------------------------------------------
Title: Wide-spectrum slow magnetoacoustic waves in coronal loops
Authors: Tsiklauri, D.; Nakariakov, V. M.
2001A&A...379.1106T Altcode: 2001astro.ph..7579T
A model interpreting variations of EUV brightness upward propagating
in solar coronal loops as slow magnetoacoustic waves is developed. A
loop is considered to have a non-zero plane inclination angle and
offset of the circular loop centre from the baseline. The model also
incorporates effects of dissipation and gravitational stratification. A
linear evolutionary equation is derived and applied to investigations of
slow wave dynamics. Both the non-zero plane inclination and the offset
decrease the growth of the wave amplitude due to stratification. It
is shown that wide-spectrum slow magnetoacoustic waves, consistent
with currently available observations in the low frequency part of
the spectrum, can provide a rate of heat deposition sufficient to heat
the loop. In this scenario, the heat would be deposited near the loop
footpoints, which agrees with the current observational data.
---------------------------------------------------------
Title: A weakly nonlinear Alfvénic pulse in a transversely
inhomogeneous medium
Authors: Tsiklauri, D.; Arber, T. D.; Nakariakov, V. M.
2001A&A...379.1098T Altcode: 2001astro.ph..7580T
The interaction of a weakly nonlinear Alfvénic pulse with an Alfvén
speed inhomogeneity in the direction perpendicular to the magnetic
field is investigated. Identical to the phase mixing experienced by
a harmonic Alfvén wave, sharp transverse gradients are generated in
the pulse by the inhomogeneity. In the initial stage of the evolution
of an initially plane Alfvénic pulse, the transverse gradients
efficiently generate transversely propagating fast magnetoacoustic
waves. However, high resolution full MHD numerical simulations of
the developed stage of the pulse evolution show that the generation
saturates due to destructive wave interference. It is shown that the
weakly non-linear description of the generated fast magnetoacoustic
wave is well described by the driven wave equation proposed in
Nakariakov et al. (\cite{Nakariakov97}), and a simple numerical code
(2D MacCromack), which solves it with minimal CPU resources, produces
identical results to those obtained from the full MHD code (Lare2d,
Arber et al. \cite{Arber01}). A parametric study of the phenomenon
is undertaken, showing that, contrary to one's expectations, steeper
inhomogeneities of the Alfvén speed do not produce higher saturation
levels of the fast wave generation. There is a certain optimal gradient
of the inhomogeneity that ensures the maximal efficiency of the fast
wave generation.
---------------------------------------------------------
Title: Non-resonant nonlinear coupling of magnetohydrodynamic waves
in inhomogeneous media
Authors: Nakariakov, V. M.; Tsiklauri, D.; Arber, T. D.
2001astro.ph.10493N Altcode:
A new mechanism for the enhanced generation of compressible fluctuations
by Alfven waves is presented. A strongly nonlinear regime of Alfven
wave phase-mixing is numerically simulated in a one-dimensionally
inhomogeneous plasma of finite temperature. It is found that the
inhomogeneity of the medium determines the efficiency of nonlinear
excitation of magnetoacoustic waves. The level of the compressible
fluctuations is found to be higher (up to the factor of two) in
inhomogeneous regions. The amplitude of the generated magnetoacoustic
wave can reach up to 30% of the source Alfven wave amplitude, and
this value is practically independent of the Alfven wave amplitude
and the steepness of Alfven speed profile. The highest amplitudes of
compressible disturbances are reached in plasmas with beta of about
0.5. The further growth of the amplitude of compressible fluctuations
is depressed by saturation.
---------------------------------------------------------
Title: Nonlinear magneto-acoustic waves in the solar atmosphere
Authors: Mendoza-Briceño, César A.; Ibáñez, Miguel H.; Nakariakov,
Valery M.
2001DyAtO..34..399M Altcode:
The weakly nonlinear evolution of quasi-isentropic magnetoacoustic
waves in the solar atmosphere is analyzed. The plasma is assumed to
be initially homogeneous, in thermal equilibrium and with a straight
and homogeneous magnetic field frozen in. Additionally, the plasma is
assumed to be cooled by the standard cooling function in the range of
10 <SUP>4</SUP> K<T<10 <SUP>7</SUP> K and heated by a mechanism
which is proportional to the density and temperature. The range of
temperature where the plasma is isentropically unstable, the e-folding
time for thermal instability and the corresponding time and length-scale
for wave breaking are found.
---------------------------------------------------------
Title: Slow magneto-acoustic waves in coronal loops
Authors: Verwichte, E.; Nakariakov, V. M.; Berghmans, D.; Hochedez,
J. -F.
2001ESASP.493..395V Altcode: 2001sefs.work..395V
No abstract at ADS
---------------------------------------------------------
Title: Determination of the coronal magnetic field by coronal loop
oscillations
Authors: Nakariakov, V. M.; Ofman, L.
2001A&A...372L..53N Altcode:
We develop a new method for the determination of the absolute value
of the magnetic field strength in coronal closed magnetic structures,
based on the analysis of flare-generated oscillations of coronal
loops. Interpretation of the oscillations observed in terms of global
standing kink waves allows to connect the period of the oscillations and
the loops length with the magnetic field strength in the loops. For loop
oscillations observed with TRACE on 14th July 1998 and 4th July 1999,
we estimate the magnetic field strength as 4-30 G. Using TRACE 171 Å
and 195 Å images of the loop, taken on 4th July 1999 to determine the
plasma density, we estimate the magnetic field in the loop as 13+/-
9 G. Improved diagnostic of the loop length, the oscillation period,
and the plasma density in the loop will significantly improve the
method's precision.
---------------------------------------------------------
Title: Slow magnetoacoustic waves in coronal loops: EIT and TRACE
Authors: Robbrecht, E.; Verwichte, E.; Berghmans, D.; Hochedez, J. F.;
Poedts, S.; Nakariakov, V. M.
2001A&A...370..591R Altcode:
On May 13, 1998 the EIT (Extreme ultraviolet Imaging Telescope) on board
of SoHO (Solar and Heliospheric Observatory) and TRACE (Transition
Region And Coronal Explorer) instruments produced simultaneous high
cadence image sequences of the same active region (AR 8218). TRACE
achieved a 25 s cadence in the Fe Ix (171 Å) bandpass while EIT
achieved a 15 s cadence (operating in “shutterless mode”, SoHO JOP
80) in the Fe Xii (195 Å) bandpass. These high cadence observations
in two complementary wavelengths have revealed the existence of weak
transient disturbances in an extended coronal loop system. These
propagating disturbances (PDs) seem to be a common phenomenon in
this part of the active region. The disturbances originate from small
scale brightenings at the footpoints of the loops and propagate along
the loops. The projected propagation speeds roughly vary between 65
and 150 km s<SUP>-1</SUP> for both instruments which is close to and
below the expected sound speed in the coronal loops. The measured slow
magnetoacoustic propagation speeds seem to suggest that the transients
are sound (or slow) wave disturbances. This work differs from previous
studies in the sense that it is based on a multi-wavelength observation
of an entire loop bundle at high cadence by two EUV imagers. The
observation of sound waves along the same path shows that they propagate
along the same loop, suggesting that loops contain sharp temperature
gradients and consist of either concentric shells or thin loop threads,
at different temperatures.
---------------------------------------------------------
Title: Fast magnetoacoustic waves in a randomly structured solar
corona
Authors: Murawski, K.; Nakariakov, V. M.; Pelinovsky, E. N.
2001A&A...366..306M Altcode:
The propagation of fast magnetoacoustic waves in a randomly structured
solar corona is considered in the linear and cold plasma limits. The
random field is assumed to be static and associated with plasma density
inhomogeneities only. A transcendental dispersion relation for the
fast magnetoacoustic waves which propagate perpendicularly to the
magnetic field is derived in the weak random field approximation. It
is shown analytically that the fast magnetosonic waves experience
acceleration, attenuation, and dispersion in comparison to the
homogeneous case. These analytical findings are essentially confirmed
by numerical simulations for a wide-spectrum pulse, except that the
waves were found decelerated. It is concluded that the coronal Moreton
waves can be applied to MHD seismology of the solar corona.
---------------------------------------------------------
Title: MHD Seismology of the Solar Cornoa with SOHO and TRACE
Authors: Nakariakov, V. M.
2001IAUS..203..353N Altcode:
Recent discoveries of MHD wave motions in the solar corona (slow
magnetoacoustic waves in polar plumes and long loops, oscillations of
coronal loops, coronal Moreton waves) done with EUV imaging telescopes
onboard SOHO and TRACE provide an observational basis for the MHD
seismology of the corona. Measuring the properties of MHD waves and
oscillations (periods, wavelengths, amplitudes, temporal and spatial
signatures), combined with theoretical modelling of the wave phenomena,
allow us to determine values of the mean parameters of the corona
(the magnetic field strength, transport coefficients, etc.). As
an example, we consider post-flare decaying oscillations of loops,
observed with TRACE (14th July 1998 at 12:55 UT). An analysis of the
oscillations shows that they are quasi-harmonic, with a period of
about 265 s, and quickly decaying with the decay time of about 14.5
min. We interpret these oscillations as a standing kink global modes
of the loops. The period of oscillations allows us to determine the
Alfvén speed in the oscillating loop about 770 km/s. This value can
be used for deduction of the value of the magnetic field in the loop
(giving 15-25 G). The decay time, together with the assumption that
the decay is caused by viscous (or resistive) dissipation, gives us
the Reynolds number of 10<SUP>5.3-6.1</SUP> (or the Lundquist number
of 10<SUP>5.0-5.8</SUP>). Other possible means for the oscillation
decay, such as wave leakage and linear and nonlinear wave coupling
are estimated.
---------------------------------------------------------
Title: A developed stage of Alfvén wave phase mixing
Authors: Botha, G. J. J.; Arber, T. D.; Nakariakov, V. M.; Keenan,
F. P.
2000A&A...363.1186B Altcode:
Alfvén wave phase mixing is an extensively studied mechanism for
dissipating wave energy in an inhomogeneous medium. It is common in the
vast majority of phase mixing papers to assume that even though short
scale lengths and steep gradients develop as a result of phase mixing,
nonlinear wave coupling does not occur. However, weakly nonlinear
studies have shown that phase mixing generates magnetoacoustic
modes. Numerical results are presented which show the nonlinear
generation of magnetosonic waves by Alfvén wave phase mixing. The
efficiency of the effect is determined by the wave amplitude, the
frequency of the Alfvén waves and the gradient in the background
Alfvén speed. Weakly nonlinear theory has shown that the amplitude
of the fast magnetosonic wave grows linearly in time. The simulations
presented in this paper extend this result to later times and show
saturation of the fast magnetosonic component at amplitudes much lower
than that of the Alfvén wave. For the case where Alfvén waves are
driven at the boundary, simulating photospheric footpoint motion,
a clear modulation of the saturated amplitude is observed. All the
results in this paper are for a low amplitude (<= 0.1), single
frequency Alfvén wave and a uniform background magnetic field in
a two dimensional domain. For this simplified geometry, and with a
monochromatic driver, we concluded that the nonlinear generation of
fast modes has little effect on classical phase mixing.
---------------------------------------------------------
Title: Slow magnetoacoustic waves in coronal loops
Authors: Nakariakov, V. M.; Verwichte, E.; Berghmans, D.; Robbrecht, E.
2000A&A...362.1151N Altcode:
A theoretical model interpreting propagating disturbances of EUV
emission intensity, recently observed in coronal loops, is constructed
in terms of slow magnetoacoustic waves. The model is one-dimensional
and incorporates effects of nonlinearity, dissipation due to finite
viscosity and thermal conduction, and gravitational stratification of
plasma in the loop. It has been found that, for the observationally
detected parameters of the waves, the main factors influencing the
wave evolution are dissipation and stratification. The upwardly
propagating waves of observed periods (5-20 min) are found to decay
significantly in the vicinity of the loop apex, explaining the rarity
of observational detection of downwardly propagating waves. The model
provides a theoretical basis for development of MHD seismology of the
coronal loops.
---------------------------------------------------------
Title: Flare-generated coronal loop oscillations: A tool for MHD
coronal seismology
Authors: Nakariakov, Valery M.
2000AIPC..537..264N Altcode: 2000wdss.conf..264N
Quasi-periodical oscillations of positions of coronal loops were
observed in the extreme-ultraviolet band (171A, FeIX) with the imaging
telescope onboard the TRACE spacecraft. Oscillating transversal
displacements (swinging) of a long (~130 Mm) thin (diameter ~2 Mm)
bright loop, were detected. The oscillations were excited by a flare
(14th July 1998 at about 12:55 UT) at distance of about 60 Mm from the
loop. The frequency of the oscillations was about 4 mHz (the period
about 265 s) and was determined by the wavelet analysis. Neighboring
perpendicular slits show synphase temporal behavior, suggesting that the
oscillations are produced by a kink global fast magnetoacoustic mode
of the loop. With this interpretation, we determine the kink speed in
the loop as 1040 km/s, which gives the Alfvén speed of about 770+/-40
km/s. The detection and analysis of post-flare oscillations of coronal
loops provides us with an efficient tool for indirect determination of
coronal parameters, e.g. the magnetic field found to be of about 20 G. .
---------------------------------------------------------
Title: Propagational Aspects of Sunquake Waves
Authors: Medrek, M.; Murawski, K.; Nakariakov, V.
2000AcA....50..405M Altcode:
We present the results of numerical simulations of impulsively generated
seismic waves excited by a spatially localized impulse source which is
connected with a nearby solar flare. The solar atmosphere is modeled
as a two layer medium with constant temperature over the photosphere
and linearly growing temperature below the photosphere. Effects
of magnetic fields are neglected. Only two dimensional effects are
considered. The source is localized slightly below the photosphere. The
numerical results show that the initial pulse of enhanced pressure,
which can be connected with the thermal energy release by interaction
of flare-generated particles with the sub-photospheric medium in the
flare-loop footpoint, generates an acoustic (seismic) wave. Interaction
of the wave with the solar surface produces perturbations registered as
sunquakes. Typical observationally registered features of the sunquakes,
such as characteristic wave signatures and acceleration of the wave
with the distance from the epicenter, are well reproduced with the
model developed. It is found that the seismic waves are essentially
dispersive and non-linear. The proposed model provides us with a
theoretical basis for sunquake seismology of the solar interior.
---------------------------------------------------------
Title: Dissipation of Slow Magnetosonic Waves in Coronal Plumes
Authors: Ofman, L.; Nakariakov, V. M.; Sehgal, N.
2000ApJ...533.1071O Altcode:
Recently, slow magnetosonic waves were identified in polar plumes,
at heights up to about 1.2 R<SUB>solar</SUB> using the Extreme
Ultraviolet Imaging Telescope (EIT) observations of quasi-periodic EUV
intensity fluctuations, and higher in the corona using the Ultraviolet
Coronagraph Spectrometer (UVCS) white-light channel. First, we derive
the linear dispersion relation for the slow waves in the viscous
plasma. Next, we derive and solve an evolutionary equation of the
Burgers type for the slow waves, incorporating the effects of radial
stratification, quadratic nonlinearity, and viscosity. Finally, we
model the propagation and dissipation of slow magnetosonic waves in
polar plumes using one-dimensional and two-dimensional MHD codes in
spherical geometry. The waves are launched at the base of the corona
with a monochromatic source. We find that the slow waves nonlinearly
steepen as they propagate away from the Sun into the solar wind. The
nonlinear steepening of the waves leads to enhanced dissipation owing to
compressive viscosity at the wave fronts. The efficient dissipation of
the slow wave by compressive viscosity leads to damping of the waves
within the first solar radii above the surface. We investigate the
parametric dependence of the wave properties.
---------------------------------------------------------
Title: Nonlinear dissipative spherical Alfvén waves in solar
coronal holes
Authors: Nakariakov, V. M.; Ofman, L.; Arber, T. D.
2000A&A...353..741N Altcode:
The weakly nonlinear dynamics of linearly polarized, spherical Alfvén
waves in coronal holes is investigated. An evolutionary equation,
combining the effects of spherical stratification, nonlinear steepening
and dissipation due to shear viscosity is derived. The equation is a
spherical analog of the scalar Cohen-Kulsrud-Burgers equation. Three
main stages of the wave evolution are distinguished: geometrical
amplification, wave breaking and enhanced dissipation. The wave
dissipation is dramatically increased by the nonlinear transfer of
energy to smaller scales. The scenario of the nonlinear dissipation is
practically independent of viscosity. The dissipation rate is stronger
for highest amplitudes, and depends weakly on the wave period and the
temperature of the atmosphere. Waves with periods less than 300 s and
initial amplitudes about 2-3% of the Alfvén speed at the base of the
corona are subject to the nonlinear steepening and dissipation in less
than 10 solar radii. For the Alfvén waves with amplitudes less than
25 km s<SUP>-1</SUP> at the base of the corona, the maximum amplitude
of up to 200 km s<SUP>-1</SUP> is reached at several solar radii. The
nonlinear distortion of the wave shape is accompanied by the generation
of longitudinal motions and density perturbations.
---------------------------------------------------------
Title: Magnetoacoustic Waves of Small Amplitude in Optically Thin
Quasi-isentropic Plasmas
Authors: Nakariakov, Valery M.; Mendoza-Briceño, César A.; Ibáñez
S., Miguel H.
2000ApJ...528..767N Altcode: 1999astro.ph..9318N
The evolution of quasi-isentropic magnetohydrodynamic waves of small
but finite amplitude in an optically thin plasma is analyzed. The
plasma is assumed to be initially homogeneous, in thermal equilibrium
and with a straight and homogeneous magnetic field frozen in. Depending
on the particular form of the heating/cooling function, the plasma may
act as a dissipative or active medium for magnetoacoustic waves, while
Alfvén waves are not affected directly. An evolutionary equation for
fast and slow magnetoacoustic waves in the single wave limit has been
derived and solved that allows us to analyze the wave modification
by competition of weakly nonlinear and quasi-isentropic effects. It
was shown that the sign of the quasi-isentropic term determines
the scenario of the evolution, either dissipative or active. In the
dissipative case, when the plasma is first-order isentropically stable,
the magnetoacoustic waves are damped and the time for shock wave
formation is delayed. However, in the active case when the plasma is
isentropically overstable, the wave amplitude grows, the strength of
the shock increases, and the breaking time decreases. The magnitude
of the above effects depends on the angle between the wavevector and
the magnetic field. For hot (T>10<SUP>4</SUP> K) atomic plasmas
with solar abundances either in the interstellar medium or in the
solar atmosphere, as well as for the cold (T<10<SUP>3</SUP> K)
ISM molecular gas, the range of temperatures in which the plasma is
isentropically unstable and the corresponding timescale and length
scale for wave breaking have been found.
---------------------------------------------------------
Title: Slow Magnetoacoustic Waves in Coronal Loops?
Authors: Robbrecht, E.; Berghmans, D.; Nakariakov, V.; Poedts, S.
1999ESASP.446..575R Altcode: 1999soho....8..575R
On May 13, 1998 the EIT and TRACE instruments produced simultaneous
high cadence image sequences of the same active region (AR 8218). TRACE
achieved a 25 sec cadence in the 171 deg passband while EIT achieved
a 15 sec cadence (operating in 'shutterless mode', SOHO JOP 80) in the
195 deg passband. These high cadence observations in two complementary
wavelengths have revealed the existence of weak disturbances in an
extended coronal loop system. The disturbances originate from small
scale brightenings at the footpoints of the loops and propagate along
the loops at an apparant speed of the order of 150 km/s which is close
to the expected sound speed. To conclude whether these propagating
disturbances should be interpreted as slow magnetoacoustic waves or as
mass motions ('microflows'), we compare our observational findings with
theoretical models. Our results suggest that the recent discovery of
DeForest and Gurman (1998) of slow MHD waves in polar plumes, are in
fact not typical of polar plumes but occur also in extended coronal
structures elsewhere.
---------------------------------------------------------
Title: MHD Waves in Open Magnetic Structures
Authors: Nakariakov, V. M.; Roberts, B.; Murawski, K.
1999ASPC..184..243N Altcode:
The dynamics of Alfvén and magnetoacoustic waves of finite amplitude in
open magnetic structures is considered. A transversal inhomogeneity in
the magnetic structure in the density and/or in a field-aligned steady
flow gives rise to Alfvén wave phase mixing, which, in the almost
collisionless and dissipationless plasma of the corona, nonlinearly
generates obliquely propagating fast magnetoacoustic waves. The
generated fast waves are more effectively damped than the Alfvén
waves and so the process of heating by phase mixing is enhanced. This
"indirect" heating leads to the spreading of the heated plasma across
the magnetic field. We point out also that an inhomogeneous flow gives
rise to the secular generation of longitudinal motions through Alfvén
wave phase mixing, accelerating the plasma along the field.
---------------------------------------------------------
Title: TRACE observation of damped coronal loop oscillations:
Implications for coronal heating
Authors: Nakariakov, V. M.; Ofman, L.; Deluca, E. E.; Roberts, B.;
Davila, J. M.
1999Sci...285..862N Altcode:
The imaging telescope on board the Transition Region and Coronal
Explorer (TRACE) spacecraft observed the decaying transversal
oscillations of a long [(130 ± 6) × 10<SUP>6</SUP> meters], thin
[diameter (2.0 ± 0.36) × 10<SUP>6</SUP> meters], bright coronal
loop in the 171 angstrom FeIX emission line. The oscillations were
excited by a solar flare in the adjacent active region. The decay
time of the oscillations is 14.5 ± 2.7 minutes for an oscillation
with a frequency 3.90 ± 0.13 millihertz. The coronal dissipation
coefficient is estimated to be eight to nine orders of magnitude
larger than the theoretically predicted classical value. The larger
dissipation coefficient may solve existing difficulties with wave
heating and reconnection theories.
---------------------------------------------------------
Title: Determination of the Reynolds number from TRACE Observation
of Damped Coronal Loop Oscillations Induced by a Flare
Authors: Ofman, L.; Nakariakov, V. M.; Deluca, E.; Roberts, B.;
Davila, J. M.
1999AAS...194.7909O Altcode: 1999BAAS...31..964O
The Transition Region and Coronal Expolorer (TRACE) observes the solar
corona with unprecedented spatial and temporal resolution. We analyzed
active region loop observation in the 171 Angstroms Fe IX emission line,
and report the direct observations of damped transverse oscillations
of a long (130+/-6 Mm) thin (diameter 2+/-0.36 Mm) bright active region
loop. The oscillations were detected following a flare in the adjacent
active region. We determined the oscillation frequency and the decay
time by the least-square fit of an exponentially decaying sinusoidal
function. Using the dispersion relation for the transverse oscillations,
and the observed loop geometry we estimated the Alfven crossing time
in the loop. The Alfven time can be used to determine the magnetic
field strength in the loop if the density is known. All parts of the
loop were observed to oscillate transversly in-phase, implying that the
ocillation is a global mode of the loop. Using dissipative MHD model for
resonant absorption of global mode oscillations for the coronal loop
we determined the Reynolds number that produces the observed damping
rate of the observed global mode. The value of the Reynolds number is
in the 10(5-10^6) range, which is eight to nine orders of magnitude
smaller than the classical coronal value. We discuss the important
implication of the small Reynolds number on coronal heating theories.
---------------------------------------------------------
Title: Slow Magnetosonic Waves in Coronal Plumes
Authors: Ofman, L.; Nakariakov, V. M.; DeForest, C. E.
1999ApJ...514..441O Altcode:
Recent observations of polar plumes in the southern solar coronal
hole by the Extreme-Ultraviolet Imaging Telescope (EIT) on board
the SOHO spacecraft show signatures of quasi-periodic compressional
waves with periods of 10-15 minutes. The relative wave amplitude
was found to increase with height in the plumes up to about 1.2
R<SUB>solar</SUB>. Using a one-dimensional linear wave equation for
the magnetosonic wave, we show that the waves are propagating and
that their amplitude increases with height. The observed propagation
velocity agrees well with the expected sound velocity inside the
plumes. We present the results of the first nonlinear, two-dimensional,
magnetohydrodynamic (MHD) simulation of the magnetosonic waves in
plumes for typical coronal conditions consistent with observations
and gravitationally stratified solar corona. We find numerically
that outward-propagating slow magnetosonic waves are trapped, and
nonlinearly steepen in the polar plumes. The nonlinear steepening of
the magnetosonic waves may contribute significantly to the heating of
the lower corona by compressive dissipation.
---------------------------------------------------------
Title: Excitation of magnetospheric waveguide modes by magnetosheath
flows
Authors: Mann, Ian R.; Wright, Andrew N.; Mills, Katharine J.;
Nakariakov, Valery M.
1999JGR...104..333M Altcode:
Standard models of the Earth's outer magnetospheric waveguide assume
that a perfectly reflecting magnetopause can trap energy inside the
waveguide. In contrast, we show that the near-noon magnetopause often
acts as a leaky boundary, wave trapping only being possible for large
magnetosheath flow speeds. Moreover, for sufficiently fast flow speeds,
we show how waveguide modes may be energized by magnetosheath flows via
the overreflection mechanism. Unbounded simulations of the growth of
surface waves via the development of a Kelvin-Helmholtz instability
(KHI) vortex sheet show growth rates which increase without limit
proportional to wavenumber (k<SUB>y</SUB>), until the assumption of a
thin boundary is no longer valid. For a bounded magnetosphere, however,
overreflected body type waveguide modes can introduce wavenumber
selection, that is, generate modes with maximum linear growth rates
at finite k<SUB>y</SUB>. A necessary condition is that the wave is
propagating in the magnetosphere, that is, the wave's turning point
lies inside the magnetosphere. By developing a new description of
both KHI and waveguide mode growth in terms of overreflection and
the propagation of negative energy waves, we show how the maximum
growth rate can be understood in terms of the reflection coefficient
of waves incident upon the magnetopause. Our model can also explain
the observed local time dependence of Pc5 field line resonance wave
power, and can explain the observed correlation between high solar wind
speeds and Pc5 wave power. Finally, we show how a waveguide with a free
magnetopause boundary supports quarter-wavelength modes. These modes
have lower frequencies than the standard (magnetopause velocity node)
half-wavelength modes, perhaps generating the millihertz waveguide
mode eigenfrequencies which appear to drive field line resonances in
HF radar data.
---------------------------------------------------------
Title: Alfvén wave phase mixing in two-dimensional open magnetic
configurations.
Authors: Ruderman, M. S.; Nakariakov, V. M.; Roberts, B.
1999joso.proc..143R Altcode:
The phase mixing of Alfvén waves in planar two-dimensional open
magnetic configurations is considered. It is assumed that the
characteristic vertical spatial scale of the configuration is much
larger than the horizontal scale, and that the latter is of the order
of a wavelength. The WKB method is used to derive the governing
equation for the wave amplitude, which in appropriate coordinates
is the diffusion equation with the diffusion coefficient being
spatially dependent. The dependency of the energy flux on the vertical
coordinate is obtained for monochromatic waves, and illustrated for
three particular cases. The theory is applied to Alfvén wave damping
in coronal holes.
---------------------------------------------------------
Title: Alfven wave phase mixing in two-dimensional open magnetic
configurations.
Authors: Ruderman, Michael S.; Nakariakov, Valery M.; Roberts, Bernard
1998A&A...338.1118R Altcode:
The phase mixing of Alfven waves in planar two-dimensional open
magnetic plasma configurations is considered. It is assumed that
the characteristic vertical spatial scale of the configuration is
much larger than the horizontal scale, and that the latter is of
the order of a wavelength. The WKB method is used to derive the
governing equation for the wave amplitude, which in appropriate
coordinates is the diffusion equation with the diffusion coefficient
being spatially dependent. The dependency of the energy flux
on the vertical coordinate is obtained for monochromatic waves,
and illustrated for three particular cases. In all three cases,
at low heights phase-mixed Alfven waves damp at the same rate as
in a one-dimensional configuration. However, in the first and third
cases phase mixing operates only at low and intermediate heights and
practically stops at heights larger than a few characteristic vertical
length scales. Only a part of the energy flux is damped due to phase
mixing. In the second case the situation is reversed: the damping of
the energy flux with height is much faster that in one-dimensional
configurations. The rate of damping of the energy flux with height
due to phase mixing in two-dimensional configurations thus depends
strongly on the particular form of the configuration. The theory is
applied to Alfven wave damping in coronal holes.
---------------------------------------------------------
Title: Nonlinear coupling of MHD waves in inhomogeneous steady flows
Authors: Nakariakov, V. M.; Roberts, B.; Murawski, K.
1998A&A...332..795N Altcode:
The nonlinear coupling of MHD waves in a cold (beta =3D 0) compressible
plasma with a smoothly inhomogeneous low-speed steady flow directed
along the magnetic field is considered. The effect is similar to Alfven
wave phase mixing in a static, inhomogeneous medium and leads to the
production of steep transversal gradients in the plasma parameters,
which increases dissipation. Transversal gradients in the total
pressure, produced by phase mixing, lead to the secular generation of
obliquely propagating fast magnetosonic waves, at double the frequency
and the wavenumber of the source Alfven waves. The efficiency of the
generation is defined by the Alfven wave amplitude and the transversal
spatial scale of the flow inhomogeneity. The secular growth of density
perturbations, connected with fast waves, takes place for flow speeds
that are considerably below the thresholds of the Kelvin - Helmholtz and
negative energy wave instabilities. The initial stage of the nonlinear
generation of the fast waves is considered analytically and illustrated
by numerical simulations.
---------------------------------------------------------
Title: Nonlinear Waves in Force-Free Fibrils
Authors: Zhugzhda, Y. D.; Nakariakov, V. M.
1998ASPC..150..151Z Altcode: 1998npsp.conf..151Z; 1998IAUCo.167..151Z
No abstract at ADS
---------------------------------------------------------
Title: A Manifestation of Negative Energy Waves in the Solar
Atmosphere
Authors: Joarder, P. S.; Nakariakov, V. M.; Roberts, B.
1997SoPh..176..285J Altcode:
Magnetosonic modes of magnetic structures of the solar atmosphere in
the presence of inhomogeneous steady flows are considered. It is shown
that, when the speed of the steady flow exceeds the phase speed of
one of the modes, the mode has negative energy, and can be subject to
an over-stability due to the negative energy wave instabilities. It is
shown that registered steady flows in the solar atmosphere, with speeds
below the threshold of the Kelvin-Helmholtz instability, can provide
the existence of the magnetosonic negative energy wave phenomena. In
particular, in isolated photospheric magnetic flux tubes, there are kink
surface modes with negative energy, produced by the external granulation
downflows. Dissipative instability of these modes due to finite thermal
conductivity and explosive instability due to nonlinear coupling of
these modes with Alfvén waves are discussed. For coronal loops, it is
found that only very high-speed flows (>300 km s<SUP>-1</SUP>) can
produce negative energy slow body modes. In solar wind flow structures,
both slow and fast body modes have negative energy and are unstable.
---------------------------------------------------------
Title: Latent Heating of Coronal Loops
Authors: Zhugzhda, Y. D.; Nakariakov, V. M.
1997SoPh..175..107Z Altcode:
The theory of weakly nonlinear slow magnetosonic waves in thin flux
tubes is developed. A Korteweg-de Vries equation is derived for
slow body waves. The analysis of solitary and periodic solutions
shows that the nonlinear waves produce running narrowings of the
tube. The similarity of the shallow-water theory and the thin-flux-tube
approximation helps to outline the peaking and breaking of the nonlinear
slow body waves. It appears that the running narrowing of the tube
works as the de Laval nozzle in generating a cool supersonic jet
and a subsequent shock that heats the plasma. The presented theory of
nonlinear slow body waves suggests a mechanism of coronal loop heating,
which meets many of the observational constraints.
---------------------------------------------------------
Title: Alfvén Wave Phase Mixing as a Source of Fast Magnetosonic
Waves
Authors: Nakariakov, V. M.; Roberts, B.; Murawski, K.
1997SoPh..175...93N Altcode:
The nonlinear excitation of fast magnetosonic waves by phase mixing
Alfvén waves in a cold plasma with a smooth inhomogeneity of
density across a uniform magnetic field is considered. If initially
fast waves are absent from the system, then nonlinearity leads to
their excitation by transversal gradients in the Alfvén wave. The
efficiency of the nonlinear Alfvén-fast magnetosonic wave coupling is
strongly increased by the inhomogeneity of the medium. The fast waves,
permanently generated by Alfvén wave phase mixing, are refracted
from the region with transversal gradients of the Alfvén speed. This
nonlinear process suggests a mechanism of indirect plasma heating by
phase mixing through the excitation of obliquely propagating fast waves.
---------------------------------------------------------
Title: Oscillations in Prominence Fine-Structures
Authors: Joarder, P. S.; Nakariakov, V. M.; Roberts, B.
1997SoPh..173...81J Altcode:
Oscillatory spectra of solar quiescent prominences highlight the
importance of incorporating the effect of prominence fine-structure
in the theory of prominence oscillations. We determine the
magnetohydrodynamic modes of oscillation of an elementary, zero-β
model of a prominence fibril, arguing that the fast body kink modes,
namely, the string and the internal magnetic Love modes, produce the
observed short periodicities in prominence fine-structures. Estimates
for the periods of these modes are presented: the modes are subject
to testing in future high-resolution observations.
---------------------------------------------------------
Title: On nonlinear dynamics of twisted magnetic flux tubes.
Authors: Zhugzhda, Yu. D.; Nakariakov, V. M.
1997joso.proc...95Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Negative energy wave instabilities in magnetic flux tubes.
Authors: Nakariakov, V. M.; Roberts, B.; Joarder, P. S.
1997joso.proc...64N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Nonlinearly Selected Frequencies in Coronal Loops
Authors: Nakariakov, V. M.; Roberts, B.
1996SoPh..168..273N Altcode:
A nonlinear process for the resonant generation of low-frequency fast
magnetosonic kink waves in coronal loops is discussed. The efficiency of
the process is strongly enhanced due to the existence of a nonlinearly
selected frequency produced by a constant frequency difference in the
dispersion curves in the short wavelength limit. The kink wave with
the selected frequency interacts with high-frequency kink and sausage
waves. The efficiency of such interaction does not require coherence in
the interactive waves. In a loop of width 2 × 10<SUP>3</SUP> km, field
strength 50 G and number density 5 × 10<SUP>15</SUP> m<SUP>−3</SUP>,
the nonlinearly selected frequency is of order 46 mHz (period 21.8 s),
but this may range through 11 mHz to 184 mHz (periods 86.5 s to 5.4 s)
for typical coronal conditions.
---------------------------------------------------------
Title: On the interaction of longitudinal and transversal waves in
thin magnetic flux tubes.
Authors: Nakariakov, V. M.; Zhugzhda, Y. D.; Ulmschneider, P.
1996A&A...312..691N Altcode:
An analytical investigation of the nonlinear interaction of longitudinal
and transversal waves in thin magnetic flux tubes is presented and
the nonlinear terms which give rise to wave generation of other modes
and to shock formation are isolated. The nonlinear resonant three-wave
interaction of longitudinal and transversal waves is studied together
with the growth and decay behaviour of these waves. This analytical
study clarifies our previous numerical computations of nonlinear wave
generation and of the steepening of longitudinal as well as transversal
wave profiles.
---------------------------------------------------------
Title: MHD modes of solar wind flow tubes.
Authors: Nakariakov, V. M.; Roberts, B.; Mann, G.
1996A&A...311..311N Altcode:
The properties of linear magnetosonic waves trapped within solar wind
flow tubes are considered. The trapping is due to reflection of the
waves from the tube boundary, which may correspond to either a jump
in plasma density or magnetic field, or a jump in the steady flow
velocity. The structure of the waves transverse to the magnetic field
is defined by the boundary conditions. The phase and group speeds of
the waves depend upon the wave frequency and wavelength. For realistic
conditions in a flow tube, corresponding to a "spaghetti structure",
there are two types of trapped waves propagating away from the Sun, fast
and slow waves. These waves are "backward", in the sense that they would
propagate towards the Sun in the absence of any difference in the steady
flow velocity inside and outside the tube. The "forward" magnetosonic
waves are not trapped under solar wind conditions. Both slow and fast
backward waves have phase velocities that are sub-Alfvenic. There are
no MHD waves propagating towards the Sun.
---------------------------------------------------------
Title: Resonant interactions of modes in coronal magnetic flux tubes
Authors: Nakariakov, V. M.; Oraevsky, V. N.
1995SoPh..160..289N Altcode:
Nonlinear resonant interactions of different kinds of fast magnetosonic
(FMS) waves trapped in the inhomogeneity of a low-β plasma density,
stretched along a magnetic field (as, for example, in coronal loops)
are investigated. A set of equations describing the amplitudes of
interactive modes is derived for an arbitrary density profile. The
quantitative characteristics of such interactions are found. The
decay instability of the wave with highest frequency is possible in
the system. If amplitudes of interactive modes have close values,
the long-period temporal and spatial oscillations are in the system.
---------------------------------------------------------
Title: Effects of plasma temperature anisotropy on the
Kelvin-Helmholtz instability of the heliopause
Authors: Kruznetsov, V. D.; Nakaryakov, V. M.; Tsyganov, P. V.
1995AstL...21..710K Altcode: 1995PAZh...21..793K
No abstract at ADS
---------------------------------------------------------
Title: Magnetosonic Waves in Structured Atmospheres with Steady
Flows, I
Authors: Nakariakov, V. M.; Roberts, B.
1995SoPh..159..213N Altcode:
The magnetosonic modes of magnetic plasma structures in the solar
atmosphere are considered taking into account steady flows of plasma
in the internal and external media and using a slab geometry. The
investigation brings nearer the theory of magnetosonic waveguides,
in such structures as coronal loops and photospheric flux tubes, to
realistic conditions of the solar atmosphere. The general dispersion
relation for the magnetosonic modes of a magnetic slab in magnetic
surroundings is derived, allowing for field-aligned steady flows in
either region. It is shown that flows change both qualitatively and
quantitatively the characteristics of magnetosonic modes. The flow may
lead to the appearance of a new type of trapped mode, namelybackward
waves. These waves are the usual slab modes propagating in the
direction opposite to the internal flow, but advected with the flow. The
disappearance of some modes due to the flow is also demonstrated.
---------------------------------------------------------
Title: On Fast Magnetosonic Coronal Pulsations
Authors: Nakariakov, V. M.; Roberts, B.
1995SoPh..159..399N Altcode:
The linear properties of the fast magnetosonic modes of a coronal loop
modelled as a smooth density inhomogeneity in a uniform magnetic field
are compared with the case of a step function slab. It is shown that
the group velocityC<SUB>g</SUB>of the modes, important in determining
the structure of impulsively excited wave packets, possesses a minimum
for a wide class of profile including the slab, with the exception of
the Epstein profile for which the minimum inC<SUB>g</SUB>moves out
to infinity. Results for the simple step profile are thus of wider
validity, and likely to be applicable to coronal loops.
---------------------------------------------------------
Title: Waves in Cosmic Magnetic Structures Taking into Account the
Anisotropic Plasma Pressure
Authors: Nakariakov, V. M.; Oraevsky, V. N.
1995SoPh..158...29N Altcode:
This paper provides an analysis of magneto-sonic eigenwaves travelling
in magnetic plasma structures based on the Chew-Goldberger-Low
approximation, for which the plasma kinetic pressure is different
along and across the magnetic field. The anisotropy does not lead to
the emergence of new modes. The dependence of phase velocities of the
waves, trapped by a single magnetic surface, on the pressure anisotropy
is investigated. For a magnetic slab with field-free surroundings,
the dispersion relations for the eigenwaves are obtained. The
pressure anisotropy may change dispersion relations of such modes
significantly. In particular, backward waves are possible in the
case of strong anisotropy. The dependences of the thresholds for the
mirror and hose instabilities on the system parameters are obtained. In
particular, hose and mirror instabilities of such waves are absent for
some wave number regions. The results are used to obtain the eigenwave
characteristics in coronal loops and chromospheric flux tubes.
---------------------------------------------------------
Title: Superreflection of fast magnetosonic waves from a tangential
discontinuity of velocity
Authors: Nakaryakov, V. M.; Stepanyants, Yu. A.
1994AstL...20..763N Altcode: 1994PAZh...20..873N
No abstract at ADS
---------------------------------------------------------
Title: Parametric modulation of magnetoacoustic waves in a magnetic
field tube by an external wave
Authors: Nakaryakov, V. M.; Petrukhin, N. S.
1994AstL...20..365N Altcode: 1994PAZh...20..431N
No abstract at ADS
---------------------------------------------------------
Title: Effect of pressure anisotropy on the magnetosonic wave
propagation in a plasma sheet
Authors: Gromov, E. M.; Nakariakov, V. M.; Oraevskii, V. N.
1992SvAL...18..341G Altcode: 1992PAZh...18..841G
The propagation of magnetosonic waves localized within a sheet of
magnetized plasma is studied, with the anisotropy of plasma kinetic
pressure in the magnetic field taken into account. Dispersion
dependences of wave phase velocities on the ratio of the sheet size
to the wavelength are derived for various plasma parameters within and
outside the sheet. The thresholds of the appearance of mirror and hose
instabilities are investigated. The ranges of the parameters of the
system under consideration for which anisotropy must be taken into
account are indicated. Changes due to anisotropy are estimated for
various magnetic structures in the solar atmosphere.
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Title: Generation of Low-Frequency Pulsations in Magnetic Waveguides
in the Sun's Atmosphere
Authors: Nakaryakov, V. M.; Petrukhin, N. S.; Fainshtein, S. M.
1991SvAL...17..423N Altcode:
No abstract at ADS
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Title: Generation of low-frequency pulsations in magnetic waveguides
in the solar atmosphere
Authors: Nakariakov, V. M.; Petrukhin, N. S.; Feinstein, S. M.
1991PAZh...17.1008N Altcode:
An analytical conclusion is presented of Schroedinger's nonlinear
equation describing the nonlinear dynamics of the envelope of the
eigenmodes of the magnetic flux tube in the solar atmosphere within the
framework of the ideal magnetic layer approximation. The generation of
LF pulsations arising as a result of the instability of the packets
of captured modes is investigated, and estimates of the period of LF
pulsations for photospheric magnetic tubes are made.
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Title: Generation of the Alfven-wave second harmonic in a plane
plasma waveguide
Authors: Nakariakov, V. N.; Fainshtein, S. M.
1991RaF....34..211N Altcode: 1991R&QE...34..211N
The interaction and self-defocusing of different types of waves in
unbounded media are studied. A plane waveguide filled with 'cold'
plasma with a constant magnetic field guided along the waveguide walls
is considered. It is shown that efficient excitation of the Alfven
wave harmonic is possible in this plasma system.
