62 research outputs found
Flow instabilities of magnetic flux tubes II. Longitudinal flow
Flow-induced instabilities are relevant for the storage and dynamics of
magnetic fields in stellar convection zones and possibly also in other
astrophysical contexts. We continue the study started in the first paper of
this series by considering the stability properties of longitudinal flows along
magnetic flux tubes. A linear stability analysis was carried out to determine
criteria for the onset of instability in the framework of the approximation of
thin magnetic flux tubes. In the non-dissipative case, we find Kelvin-Helmholtz
instability for flow velocities exceeding a critical speed that depends on the
Alfv{\'e}n speed and on the ratio of the internal and external densities.
Inclusion of a friction term proportional to the relative transverse velocity
leads to a friction-driven instability connected with backward (or negative
energy) waves. We discuss the physical nature of this instability. In the case
of a stratified external medium, the Kelvin-Helmholtz instability and the
friction-driven instability can set in for flow speeds significantly lower than
the Alfv{\'e}n speed. Dissipative effects can excite flow-driven instability
below the thresholds for the Kelvin-Helmholtz and the undulatory (Parker-type)
instabilities. This may be important for magnetic flux storage in stellar
convection zones and for the stability of astrophysical jets.Comment: accepted by Astronomy & Astrophysic
A weakly nonlinear Alfvénic pulse in a transversely inhomogeneous medium
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. (1997), 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. 2001). 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
Propagation of sausage soliton in the solar lower atmosphere observed by Hinode/SOT
Acoustic waves and pulses propagating from the solar photosphere upwards may
quickly develop into shocks due to the rapid decrease of atmospheric density.
However, if they propagate along a magnetic flux tube, then the nonlinear
steepening may be balanced by tube dispersion effects. This may result in the
formation of sausage soliton. The aim of this letter is to report an
observational evidence of sausage soliton in the solar chromosphere. Time
series of Ca II H line obtained at the solar limb with the Solar Optical
Telescope (SOT) on the board of Hinode is analysed. Observations show an
intensity blob, which propagates from 500 km to 1700 km above the solar surface
with the mean apparent speed of 35 km s. The speed is much higher than
expected local sound speed, therefore the blob can not be a simple pressure
pulse. The blob speed, length to width ratio and relative intensity correspond
to slow sausage soliton propagating along a magnetic tube. The blob width is
increased with height corresponding to the magnetic tube expansion in the
stratified atmosphere. Propagation of the intensity blob can be the first
observational evidence of slow sausage soliton in the solar atmosphere.Comment: 5 pages, 4 figures, accepted in MNRA
Swirling astrophysical flows - efficient amplifiers of Alfven waves
We show that a helical shear flow of a magnetized plasma may serve as an
efficient amplifier of Alfven waves. We find that even when the flow is purely
ejectional (i.e., when no rotation is present) Alfven waves are amplified
through the transient, shear-induced, algebraic amplification process. Series
of transient amplifications, taking place sequentially along the flow, may
result in a cascade amplification of these waves. However, when a flow is
swirling or helical (i.e., some rotation is imposed on the plasma motion),
Alfven waves become subject to new, much more powerful shear instabilities. In
this case, depending on the type of differential rotation, both usual and
parametric instabilities may appear. We claim that these phenomena may lead to
the generation of large amplitude Alfven waves and the mechanism may account
for the appearance of such waves in the solar atmosphere, in accretion-ejecion
flows and in accretion columns. These processes may also serve as an important
initial (linear and nonmodal) phase in the ultimate subcritical transition to
MHD Alfvenic turbulence in various kinds of astrophysical shear flows.Comment: 12 pages, 11 figures, accepted for publication (25-11-02) in
Astronomy and Astrophysic
Temporal evolution of the Evershed flow in sunspots. II. Physical properties and nature of Evershed clouds
Context: Evershed clouds (ECs) represent the most conspicuous variation of
the Evershed flow in sunspot penumbrae. Aims: We determine the physical
properties of ECs from high spatial and temporal resolution spectropolarimetric
measurements. Methods: The Stokes profiles of four visible and three infrared
spectral lines are subject to inversions based on simple one-component models
as well as more sophisticated realizations of penumbral flux tubes embedded in
a static ambient field (uncombed models). Results: According to the
one-component inversions, the EC phenomenon can be understood as a perturbation
of the magnetic and dynamic configuration of the penumbral filaments along
which these structures move. The uncombed inversions, on the other hand,
suggest that ECs are the result of enhancements in the visibility of penumbral
flux tubes. We conjecture that the enhancements are caused by a perturbation of
the thermodynamic properties of the tubes, rather than by changes in the vector
magnetic field. The feasibility of this mechanism is investigated performing
numerical experiments of thick penumbral tubes in mechanical equilibrium with a
background field. Conclusions: While the one-component inversions confirm many
of the properties indicated by a simple line parameter analysis (Paper I of
this series), we tend to give more credit to the results of the uncombed
inversions because they take into account, at least in an approximate manner,
the fine structure of the penumbra.Comment: Accepted for publication in A&
Phase diversity restoration of sunspot images I. Relations between penumbral and photospheric features
We investigate the dynamics of and the relations between small-scale
penumbral and photospheric features near the outer penumbral boundary:
penumbral grains (PGs), dark penumbral fibrils, granules, and photospheric
G-band bright points. The analysis is based on a 2 h time sequence of a sunspot
close to disc center, taken simultaneously in the G-band and in the blue
continuum at 450.7 nm. Observations were performed at the Swedish Vacuum Solar
Telescope (La Palma) in July 1999. A total of 2564 images (46 arcsec x 75
arcsec) were corrected for telescope aberrations and turbulence perturbations
by applying the inversion method of phase diversity. Our findings can by
summarized as follows: (a) One third of the outward-moving PGs pass through the
outer penumbral boundary and then either continue moving as small bright
features or expand and develop into granules. (b) Former PGs and G-band bright
points next to the spot reveal a different nature. The latter have not been
identified as a continuation of PGs escaping from the penumbra. The G-band
bright points are mostly born close to dark penumbral fibrils where the
magnetic field is strong, whereas PGs stem from the less-magnetized penumbral
component and evolve presumably to non-magnetic granules or small bright
features.Comment: Accepted by A&A, 9 pages and 5 figure
Models and Observations of Sunspot Penumbrae
The mysteries of sunspot penumbrae have been under an intense scrutiny for
the past 10 years. During this time, some models have been proposed and
refuted, while the surviving ones had to be modified, adapted and evolved to
explain the ever-increasing array of observational constraints. In this
contribution I will review two of the present models, emphasizing their
contributions to this field, but also pinpointing some of their inadequacies to
explain a number of recent observations at very high spatial resolution. To
help explaining these new observations I propose some modifications to each of
them. These modifications bring those two seemingly opposite models closer
together into a general picture that agrees well with recent 3D
magneto-hydrodynamic simulations.Comment: 9 pages, 1 color figure. Review talk to appear in the proceedings of
the International Workshop of 2008 Solar Total Eclipse: Solar Magnetism,
Corona and Space Weather--Chinese Space Solar Telescope Scienc
Resonantly damped surface and body MHD waves in a solar coronal slab with oblique propagation
The theory of magnetohydrodynamic (MHD) waves in solar coronal slabs in a
zero- configuration and for parallel propagation of waves does not allow
the existence of surface waves. When oblique propagation of perturbations is
considered both surface and body waves are able to propagate. When the
perpendicular wave number is larger than a certain value, the body kink mode
becomes a surface wave. In addition, a sausage surface mode is found below the
internal cut-off frequency. When non-uniformity in the equilibrium is included,
surface and body modes are damped due to resonant absorption. In this paper,
first, a normal-mode analysis is performed and the period, the damping rate,
and the spatial structure of eigenfunctions are obtained. Then, the
time-dependent problem is solved, and the conditions under which one or the
other type of mode is excited are investigated.Comment: 19 pages, 9 figures, accepted for publication in Solar Physic
Effects of Steady Flow on Magnetoacoustic-Gravity Surface Waves: I. The Weak Field Case
Magnetoacoustic gravity (MAG) waves have been studied for some time. In this article, we investigate the effect that a shear flow at a tangential discontinuity embedded in a gravitationally stratified and magnetised plasma has on MAG surface waves. The dispersion relation found is algebraically analogous to the relation of the non-flow cases obtained by Miles and Roberts (Solar Phys.141, 205, 1992), except for the introduction of a Doppler-shifted frequency for the eigenvalue. This feature, however, introduces rather interesting physics, including the asymmetric presence of forward- and backward-propagating surface waves. We find that increasing the equilibrium flow speed leads to a shift in the permitted regions of propagation for surface waves. For most wave number combinations this leads to the fast mode being completely removed, as well as more limited phase speed regimes for slow-mode propagation. We also find that upon increasing the flow, the phase speeds of the backward propagating waves are increased. Eventually, at high enough flow speeds, the waveâs direction of propagation is reversed and is in the positive direction. However, the phase speed of the forward-propagating wave remains mainly the same. For strong enough flows we find that the KelvinâHelmholtz instability can also occur when the forward- and backward-propagating modes couple
Observing Kelvin-Helmholtz instability in solar blowout jet
KelvinâHelmholtz instability (KHI) is a basic physical process in fluids and magnetized plasmas, with applications successfully modelling e.g. exponentially growing instabilities observed at magnetospheric and heliospheric boundaries, in the solar or Earthâs atmosphere and within astrophysical jets. Here, we report the discovery of the KHI in solar blowout jets and analyse the detailed evolution by employing high-resolution data from the Interface Region Imaging Spectrograph (IRIS) satellite launched in 2013. The particular jet we focus on is rooted in the surrounding penumbra of the main negative polarity sunspot of Active Region 12365, where the main body of the jet is a super-penumbral structure. At its maximum, the jet has a length of 90âMm, a width of 19.7âMm, and its density is about 40 times higher than its surroundings. During the evolution of the jet, a cavity appears near the base of the jet, and bi-directional flows originated from the top and bottom of the cavity start to develop, indicating that magnetic reconnection takes place around the cavity. Two upward flows pass along the left boundary of the jet successively. Next, KHI develops due to a strong velocity shear (âŒ204âkmâsâ1) between these two flows, and subsequently the smooth left boundary exhibits a sawtooth pattern, evidencing the onset of the instability
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