362 research outputs found

### Observation of coronal loop torsional oscillation

We suggest that the global torsional oscillation of solar coronal loop may be
observed by the periodical variation of a spectral line width. The amplitude of
the variation must be maximal at the velocity antinodes and minimal at the
nodes of the torsional oscillation. Then the spectroscopic observation as a
time series at different heights above the active region at the solar limb may
allow to determine the period and wavelength of global torsional oscillation
and consequently the Alfv{\'e}n speed in corona. From the analysis of early
observation (Egan & Schneeberger \cite{egan}) we suggest the value of coronal
Alfv{\'e}n speed as $\sim 500 {\rm km}{\cdot}{\rm s}^{-1}$.Comment: Accepted in Astronomy and Astrophysic

### Kelvin-Helmholtz instability of kink waves in photospheric twisted flux tubes

We investigate conditions under which kink magnetohydrodynamic waves
propagating along photospheric uniformly twisted flux tubes with axial mass
flows become unstable as a consequence of the Kelvin-Helmholtz instability. We
employed the dispersion relations of kink waves derived from the linearised
magnetohydrodynamic equations. We assumed real wave numbers and complex angular
wave frequencies, namely complex wave phase velocities. The dispersion
relations were solved numerically at fixed input parameters and several mass
flow velocities. We show that the stability of the waves depends upon four
parameters, the density contrast between the flux tube and its environment, the
ratio of the background magnetic fields in the two media, the twist of the
magnetic field lines inside the tube, and the value of the Alfven-Mach number
(the ratio of the jet velocity to Alfv\'en speed inside the flux tube). At
certain densities and magnetic field twists, an instability of the
Kelvin-Helmholtz type of kink (m = 1) mode can arise if the Alfven-Mach number
exceeds a critical value. The observed mass flows may trigger the
Kelvin-Helmholtz instability of the kink (m = 1) mode in weakly twisted
photospheric magnetic flux tubes at critical Alfven-Mach numbers lower that
those in untwisted tubes if the magnetic field twist lies in the range
0.36--0.4 and the flow speed exceeds a critical value. A weak external magnetic
field (with a ratio to the magnetic field inside the tube in the range
0.1--0.5) slightly increases that critical value.Comment: 11 pages, 10 figures. arXiv admin note: text overlap with
arXiv:1105.112

### Doppler shift oscillations in solar spicules

Consecutive height series of Ha spectra in solar limb spicules taken on the
53 cm coronagraph of Abastumani Astrophysical Observatory at the heights of
3800-8700 km above the photosphere have been analyzed. The aim is to observe
oscillatory phenomena in spicules and consequently to trace wave propagations
through the chromosphere. The Discrete Fourier Transform analysis of Ha Doppler
shift time series constructed from the observed spectra at each height is used.
Doppler velocities of solar limb spicules show oscillations with periods of
20-55 and 75-110 s. There is also the clear evidence of 3-min oscillations at
the observed heights. The oscillations can be caused by wave propagations in
thin magnetic flux tubes anchored in the photosphere. We suggest the
granulation as a possible source for the wave excitation. Observed waves can be
used as a tool for spicule seismology; the magnetic field strength in spicules
at the height of about 6000 km above the photosphere is estimated as 12-15 G.Comment: 7 pages, 8 figures, accepted in A&

### Torsional Alfv\'en waves in solar partially ionized plasma: effects of neutral helium and stratification

Ion-neutral collisions may lead to the damping of Alfven waves in
chromospheric and prominence plasmas. Neutral helium atoms enhance the damping
in certain temperature interval, where the ratio of neutral helium and neutral
hydrogen atoms is increased. Therefore, the height-dependence of ionization
degrees of hydrogen and helium may influence the damping rate of Alfven waves.
We aim to study the effect of neutral helium in the damping of Alfven waves in
stratified partially ionized plasma of the solar chromosphere. We consider a
magnetic flux tube, which is expanded up to 1000 km height and then becomes
vertical due to merging with neighboring tubes, and study the dynamics of
linear torsional Alfven waves in the presence of neutral hydrogen and neutral
helium atoms. We start with three-fluid description of plasma and consequently
derive single-fluid magnetohydrodynamic (MHD) equations for torsional Alfven
waves. Thin flux tube approximation allows to obtain the dispersion relation of
the waves in the lower part of tubes, while the spatial dependence of
steady-state Alfven waves is governed by Bessel type equation in the upper part
of tubes. Consecutive derivation of single-fluid MHD equations results in a new
Cowling diffusion coefficient in the presence of neutral helium which is
different from previously used one. We found that shorter-period (< 5 s)
torsional Alfven waves damp quickly in the chromospheric network due to
ion-neutral collision. On the other hand, longer-period (> 5 s) waves do not
reach the transition region as they become evanescent at lower heights in the
network cores. Propagation of torsional Alfven waves through the chromosphere
into the solar corona should be considered with caution: low-frequency waves
are evanescent due to the stratification, while high-frequency waves are damped
due to ion neutral collisions.Comment: 9 pages, 7 figures (accepted in A&A

### Kelvin-Helmholtz instability of twisted magnetic flux tubes in the solar wind

Solar wind plasma is supposed to be structured in magnetic flux tubes carried
from the solar surface. Tangential velocity discontinuity near the boundaries
of individual tubes may result in Kelvin-Helmholtz instability, which may
contribute into the solar wind turbulence. While the axial magnetic field may
stabilize the instability, a small twist in the magnetic field may allow to
sub-Alfvenic motions to be unstable. We aim to study the Kelvin-Helmholtz
instability of twisted magnetic flux tube in the solar wind with different
configurations of external magnetic field. We use magnetohydrodynamic equations
in the cylindrical geometry and derive the dispersion equations governing the
dynamics of twisted magnetic flux tube moving along its axis in the cases of
untwisted and twisted external fields. Then we solve the dispersion equations
analytically and numerically and found thresholds for Kelvin-Helmholtz
instability in both cases of external field. Both analytical and numerical
solutions show that the Kelvin-Helmholtz instability is suppressed in the
twisted tube by external axial magnetic field for sub-Alfvenic motions.
However, even small twist in the external magnetic field allows the
Kelvin-Helmholtz instability to be developed for any sub-Alfvenic motions. The
unstable harmonics correspond to vortices with high azimuthal mode numbers,
which are carried by the flow. Twisted magnetic flux tubes can be unstable to
Kelvin-Helmholtz instability when they move with small speed relative to main
solar wind stream, then the Kelvin-Helmholtz vortices may significantly
contribute into the solar wind turbulence.Comment: 8 pages, 3 figures, accepted in A&

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