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

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

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

Analysis of coronal rain observed by IRIS, HINODE/SOT and SDO/AIA : transverse oscillations, kinematics and thermal evolution

Coronal rain composed of cool plasma condensations falling from coronal heights along magnetic field lines is a phenomenon occurring mainly in active region coronal loops. Recent high resolution observations have shown that coronal rain is much more common than previously thought, suggesting its important role in the chromosphere-corona mass cycle. We present the analysis of MHD oscillations and kinematics of the coronal rain observed in chromospheric and transition region lines by IRIS, Hinode/SOT and SDO/AIA. Two different regimes of transverse oscillations traced by the rain are detected: small-scale persistent oscillations driven by a continuously operating process and localised large-scale oscillations excited by a transient mechanism. The plasma condensations are found to move with speeds ranging from few km s−1 up to 180 km s−1 and with accelerations largely below the free fall rate, with the likely reasons being pressure effects and the ponderomotive force resulting from the loop oscillations. The observed evolution of the emission in individual SDO/AIA bandpasses is found to exhibit clear signatures of a gradual cooling of the plasma at the loop top. We determine the temperature evolution of the coronal loop plasma using regularised inversion to recover the differential emission measure (DEM) and by forward modelling the emission intensities in the SDO/AIA bandpasses using a two-component synthetic DEM model. The inferred evolution of the temperature and density of the plasma near the apex is consistent with the limit cycle model and suggests the loop is going through a sequence of periodically repeating heating-condensation cycles

Excitation of vertical coronal loop oscillations by impulsively driven flows

Context Flows of plasma along a coronal loop caused by the pressure difference between loop footpoints are common in the solar corona. Aims We aim to investigate the possibility of excitation of loop oscillations by an impulsively driven flow triggered by an enhanced pressure in one of the loop footpoints. Methods We carry out 2.5D magnetohydrodynamic (MHD) simulations of a coronal loop with an impulsively driven flow and investigate the properties and evolution of the resulting oscillatory motion of the loop. Results The action of the centrifugal force associated with plasma moving at high speeds along the curved axis of the loop is found to excite the fundamental harmonic of a vertically polarised kink mode. We analyse the dependence of the resulting oscillations on the speed and kinetic energy of the flow. Conclusions We find that flows with realistic speeds of less than 100 km s−1 are sufficient to excite oscillations with observable amplitudes. We therefore propose plasma flows as a possible excitation mechanism for observed transverse loop oscillations

Excitation and evolution of vertically polarised transverse loop oscillations by coronal rain

Context. Coronal rain is composed of cool dense blobs that form in solar coronal loops and are a manifestation of catastrophic cooling linked to thermal instability. The nature and excitation of oscillations associated with coronal rain is not well understood. Aims. We consider observations of coronal rain in a bid to elucidate the excitation mechanism and evolution of wave characteristics. Methods. We analyse IRIS and Hinode/SOT observations of an oscillating coronal rain event on 17 Aug 2014 and determine the wave characteristics as a function of time using tried and tested time-space analysis techniques. Results. We exploit the seismological capability of the oscillation to deduce the relative rain mass from the oscillation amplitude. This is consistent with the evolution of the oscillation period showing the loop loosing a third of its mass due to falling coronal rain in a 10-15 minute time period. Conclusions. We present first evidence of the excitation of vertically polarised transverse loop oscillations triggered by a catastrophic cooling at the loop top and consistent with two thirds of the loop mass comprising of cool rain mass

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

Ultra-long-period oscillations in EUV filaments near to eruption: Two-wavelength correlation and seismology

Copyright © 2009 American Astronomical Society / IOP PublishingWe 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

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