292 research outputs found
Seismology of curved coronal loops with vertically polarised transverse oscillations
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.
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.
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
Growing transverse oscillations of a multistranded loop observed by SDO/AIA
The first evidence of transverse oscillations of a multistranded loop with
growing amplitudes and internal coupling observed by the Atomspheric Imaging
Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) is presented. The
loop oscillation event occurred on 2011 March 8, triggered by a CME. The
multiwavelength analysis reveals the presence of multithermal strands in the
oscillating loop, whose dynamic behaviors are temperature-dependent, showing
differences in their oscillation amplitudes, phases and emission evolution. The
physical parameters of growing oscillations of two strands in 171 A are
measured and the 3-D loop geometry is determined using STEREO-A/EUVI data.
These strands have very similar frequencies, and between two 193 A strands a
quarter-period phase delay sets up. These features suggest the coupling between
kink oscillations of neighboring strands and the interpretation by the
collective kink mode as predicted by some models. However, the temperature
dependence of the multistarnded loop oscillations was not studied previously
and needs further investigation. The transverse loop oscillations are
associated with intensity and loop width variations. We suggest that the
amplitude-growing kink oscillations may be a result of continuous non-periodic
driving by magnetic deformation of the CME, which deposits energy into the loop
system at a rate faster than its loss.Comment: 6 pages, 4 color figures, 1 table, ApJ Letter, accepte
First direct observation of a torsional Alfvén oscillation at coronal heights
Context. Torsional Alfvén waves are promising candidates for the transport of energy across different layers of the solar atmosphere. They have been predicted theoretically for decades. Previous detections of Alfvén waves so far have however mostly relied on indirect signatures.
Aims. We present the first direct observational evidence of a fully resolved torsional Alfvén oscillation of a large-scale structure occurring at coronal heights.
Methods. We analysed IRIS imaging and spectral observation of a surge resulting from magnetic reconnection between active region prominence threads and surrounding magnetic field lines.
Results. The IRIS spectral data provide clear evidence of an oscillation in the line-of-sight velocity with a 180° phase difference between the oscillation signatures at opposite edges of the surge flux tube. This together with an alternating tilt in the Si IV and Mg II k spectra across the flux tube and the trajectories traced by the individual threads of the surge material provide clear evidence of torsional oscillation of the flux tube.
Conclusions. Our observation shows that magnetic reconnection leads to the generation of large-scale torsional Alfvén waves
Coronal loop seismology using multiple transverse loop oscillation harmonics
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.
Aims. The oscillation properties are studied to give seismological estimates of physical quantities, such as the density scale height.
Methods. A loop segment is traced during the oscillation, and the resulting time series is analysed for periodicities.
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
Coronal Alfvén speed determination : consistency between seismology using AIA/SDO transverse loop oscillations and magnetic extrapolation
Two transversely oscillating coronal loops are investigated in detail during a flare on the 6th September 2011 using data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory (SDO).We compare two independent methods to determine the Alfvén speed inside these loops. Through the period of oscillation and loop length information about the Alfvén speed inside each loop is deduced seismologically. This is compared with the Alfvén speed profiles deduced from magnetic extrapolation and spectral methods using AIA bandpass. We find that for both loops the two methods are consistent. Also, we find that the average Alfvén speed based on loop travel time is not necessarily a good measure to compare with the seismological result, which explains earlier reported discrepancies. Instead, the effect of density and magnetic stratification on the wave mode has to be taken into account. We discuss the implications of combining seismological, extrapolation and spectral methods in deducing the physical properties of coronal loops
Full orbit simulations of collisional impurity transport in spherical tokamak plasmas with strongly-sheared electric fields
The collisional dynamics of test impurity ions in spherical tokamak plasmas
with strongly-sheared radial electric fields is investigated by means of a test
particle full orbit simulation code. The strength of the shear is such that the
standard drift ordering can no longer be assumed and a full orbit approach is
required. The effect of radial electric field shear on neoclassical particle
transport is quantified for a range of test particle mass and charge numbers
and electric field parameters. It is shown that the effect of a sheared
electric field is to enhance the confinement of impurity species above the
level observed in the absence of such a field. The effect may be explained in
terms of a collisional drag force drift, which is proportional to particle
charge number but independent of particle mass. This drift acts inwards for
negative radial electric fields and outwards for positive fields, implying
strongly enhanced confinement of highly ionized impurity ions in the presence
of a negative radial electric field.Comment: 16 pages, 6 figures, submitted to Nuclear Fusio
Leakage of waves from coronal loops by wave tunneling
To better understand the decay of vertically polarised fast kink modes of coronal loops by the mechanism of wave tunneling, simulations are performed of fast kink modes in straight flux slabs which have Alfvén speed profiles which include a tunneling region. The decay rates are found to be determined by the mode number of the trapped mode and the thickness of the tunneling region. Two analytical models are suggested to explain the observed decay. The first is a extension of the work of Roberts (1981, Sol. Phys., 69, 39) to include a finite thickness tunneling region, and the second is a simpler model which yields an analytical solution for the relationship between decay rate, period and the thickness of the tunneling region. The decay rates for these straight slabs are found to be slower than in observations and those found in a previous paper on the subject by Brady & Arber (2005, A&A, 438, 733) using curved flux slabs. It is found that the difference between the straight slabs used here and the curved slabs used in Brady & Arber (2005, A&A, 438, 733) can be represented as a geometric correction to the decay rate
Recommended from our members
Aspects of Nonlinearity and Dissipation in Magnetohydrodynamics
We investigate two projects: (i) the weakly nonlinear evolution of two oppositely travelling waves and (ii) the dissipative instability of a tangential discontinuity.
We show that the ponderomotive force is a basic nonlinear force, which is, to leading-order, proportional to the product of a wave quantity and a gradient of a wave quantity. The ponderomotive force of Alfvén waves corresponds to a magnetic wave pressure force.
The motion of beads on a string and the fluid motions in an oscillating tube are shown to be good mechanical analogues for the weakly nonlinear evolution of bounded Alfvén waves, especially in plasmas with a low plasma β.
We examine, analytically and numerically, the weakly nonlinear evolution of bounded fast magneto-acoustic waves in the cold plasma limit and show that the ponderomotive force moves plasma along the equilibrium magnetic field. The maximum density enhancement is proportional to α2/β, with α the wave amplitude and β of the order of the plasma β, We obtain the wave amplitude and frequency modulation and discuss the problems with the cold-plasma assumption.
The weakly nonlinear interaction of Alfvén pulses is investigated in the cold-plasma limit and for finite-β plasmas. We find excellent agreement between analytical and numerical results. We describe a density enhancement, maximally of order α2β1/2 at the position of Alfvén pulse excitation, which splits into two slow pulses, We describe the shock-formation of the Alfvén and slow pulses.
The dissipative instability of the tangential discontinuity is examined and applied to coronal hole boundaries. We derive a dispersion relation, which includes weak viscosity and thermal conduction, and is solved for a specific model, using perturbation theory. The effect of viscosity and thermal conduction on stability are discussed, It is shown that dissipation lowers the threshold flow speed for instability
3D reconstruction of coronal loops by the principal component analysis
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
Coronal magnetic field measurement using loop oscillations observed by Hinode/EIS
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 kms−1 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
- …