E. Verwichte

Detection of waves in the solar corona : kink or Alfven?

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 Alfven waves is shown to be inconsistent with MHD wave theory. The implications of the interpretation in terms of kink waves are discussed.

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 A 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. 5sa nd 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.

Quasi-periodic modulation of solar and stellar flaring emission by magnetohydrodynamic oscillations in a nearby loop

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.

Magnetic Kelvin-Helmholtz Instability at the Sun

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.

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 A 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.

Decaying and decayless transverse oscillations of a coronal loop

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. 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. 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.

Transverse coronal loop oscillations seen in unprecedented detail by AIA/SDO

Aims. Detailed analysis of 11 transverse coronal loop oscillations in three events observed with the Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory (SDO) spacecraft. Detailed analysis includes analysis of the displacement time series, intensity variations and comparing EUVI and AIA data to estimate the 3D loop geometry. Methods. Time distance images extracted from cuts made perpendicular to the oscillations are obtained. A Gaussian plus background fitting technique is used to extract the time series which is then fitted with a damped cosine curve. Intensity variations are extracted along the time series points. EUVI/STEREO data is compared to AIA/SDO data to obtain three-dimensional models of the loop geometry. Results. Timeseries analysis revealed periods between 1.7 and 10 min and damping timesbetween 2.9 and 13 min. Intensity variations are reliably observed for six of the loops and a comparison between EUVI/STEREO and AIA/SDO data is performed to simulate the polarisation of the kink mode. We conclude that the intensity variations are due to variations in the line of sight column depth of a horizontally polarised transverse loop oscillation. Coronal seismology of the kink mode was applied to determine the range of the internal Alfven speed and the magnetic field strength for each loop.

Seismology of a large solar coronal loop from EUVI/STEREO observations of its transverse oscillation

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 Alfven 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.

Seismological demonstration of perpendicular density structuring in the solar corona

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.

The first measurement of the adiabatic index in the solar corona using time-dependent spectroscopy of hinode/eis observations

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 γeff = 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.