Transverse Magnetohydrodynamic Research Articles

The role of linearly polarized transverse MHD waves in heating the solar coronal plasma

ABSTRACT We derive a general dispersion relation for linearly polarized transverse magnetohydrodynamic (MHD) waves in an inhomogeneous, viscous, and resistive coronal plasma. We assume density structuring along the direction of polarization of the wave. In the linear regime and for an incompressible and homogeneous plasma, where the criterion of ignorable coordinates is satisfied, our solution corresponds to the customary Alfvén wave, which is basically dissipated by shear viscosity or resistivity. However, the assumption of density stratification along the direction of polarization of waves breaks down the criterion of ignorable coordinates, due to which transversal wave perturbations become compressible and, consequently, compressive viscosity turns out to be an important process. We find that for a typical coronal plasma with a magnetic field of 5 to 22 G, transverse body waves with a period of less than 10 s can dissipate energy on a time scale comparable to the coronal radiative time in their antisymmetric mode. Thus, it is possible that linearly polarized transverse wave perturbations dissipated by ion compressive viscosity maintain a hot coronal temperature.

Magnetohydrodynamic waves in braided magnetic fields

Aims. We investigate the propagation of transverse magnetohydrodynamic (MHD) wave fronts through a coronal plasma containing a braided magnetic field. Methods. We performed a series of three dimensional MHD simulations in which a small amplitude, transverse velocity perturbation is introduced into a complex magnetic field. We analysed the deformation of the wave fronts as the perturbation propagates through the braided magnetic structures and explore the nature of Alfvénic wave phase mixing in this regime. We considered the effects of viscous dissipation in a weakly non-ideal plasma and evaluate the effects of field complexity on wave energy dissipation. Results. Spatial gradients in the local Alfvén speed and variations in the length of magnetic field lines ensure that small scales form throughout the propagating wave front due to phase mixing. Additionally, the presence of complex, intricate current sheets associated with the background field locally modifies the polarisation of the wave front. The combination of these two effects enhances the rate of viscous dissipation, particularly in more complex field configurations. Unlike in classical phase mixing configurations, the greater spatial extent of Alfvén speed gradients ensures that wave energy is deposited over a larger cross-section of the magnetic structure. Further, the complexity of the background magnetic field ensures that small gradients in a wave driver can map to large gradients within the coronal plasma. Conclusions. The phase mixing of transverse MHD waves in a complex magnetic field will progress throughout the braided volume. As a result, in a non-ideal regime wave energy will be dissipated over a greater cross-section than in classical phase mixing models. The formation rate of small spatial scales in a propagating wave front is a function of the complexity of the background magnetic field. As such, if the coronal field is sufficiently complex it remains plausible that phase mixing induced wave heating can contribute to maintaining the observed temperatures. Furthermore, the weak compressibility of the transverse wave and the observed phase mixing pattern may provide seismological information about the nature of the background plasma.

RESONANT ABSORPTION OF TRANSVERSE OSCILLATIONS AND ASSOCIATED HEATING IN A SOLAR PROMINENCE. II. NUMERICAL ASPECTS

Transverse magnetohydrodynamic (MHD) waves are ubiquitous in the solar atmosphere and may be responsible for generating the Sun's million-degree outer atmosphere. However, direct evidence of the dissipation process and heating from these waves remains elusive. Through advanced numerical simulations combined with appropriate forward modeling of a prominence flux tube, we provide the observational signatures of transverse MHD waves in prominence plasmas. We show that these signatures are characterized by thread-like substructure, strong transverse dynamical coherence, an out-of-phase difference between plane-of-the-sky motions and LOS velocities, and enhanced line broadening and heating around most of the flux tube. A complex combination between resonant absorption and Kelvin-Helmholtz instabilities (KHI) takes place in which the KHI extracts the energy from the resonant layer and dissipates it through vortices and current sheets, which rapidly degenerate into turbulence. An inward enlargement of the boundary is produced in which the turbulent flows conserve the characteristic dynamics from the resonance, therefore guaranteeing detectability of the resonance imprints. We show that the features described in the accompanying paper (Okamoto et al. 2015) through coordinated Hinode and IRIS observations match well the numerical results.

Observations from Hinode/EIS of intensity oscillations above a bright point: signature of the leakage of acoustic oscillations in the inner corona

We study the intensity oscillations in the upper chromosphere/transition region (TR) and corona, above a bright point (BP) in the solar atmosphere. We analyse the time series of He ii 256 A, Fe xii 195 A and Fe xv 284 A, observed in a 40-arcsec slot close to the centre of the Sun above the BP by the extreme ultraviolet (EUV) imaging spectrometer (EIS) on board Hinode. Using standard wavelet and periodogram tools, we produce power spectra of intensity oscillations. In the He ii 256.32 A and Fe xii 195.12 A EUV light curves, we detect intensity oscillations of the periods ∼ 263 ± 80 s and ∼ 241 ± 60 s, respectively, with a probability >95 per cent in wavelets, which are also consistent with their periodograms. This provides the most likely signature of the propagation of acoustic oscillations around the ∼5.0-min period from the photosphere to the inner corona. The radiative cooling and thus the finite radiative relaxation time are found to be the most likely mechanisms for the reduced cut-off frequency environment above the observed BP. This may allow the transfer of ∼5.0-min acoustic oscillations from the upper chromosphere/TR into the corona. We find that intensity oscillations in He ii 256.32 A show temporal damping during the total span of the observation. This may be the first most likely observational signature of acoustic wave damping in the upper chromosphere caused by the radiative cooling effect. The intensity oscillations in Fe xii 195.12 A show an amplification, which may be a most likely signature of the mode-coupling (two-wave interaction) and then resonant energy conversion, probably from transverse magnetohydrodynamic (MHD) waves of the double period (e.g. Alfven waves) to the observed acoustic waves in the lower solar atmosphere where the plasma beta tends to unity. However, we find no evidence of real oscillations around the ∼5.0-min period with its amplification in the higher corona where the Fe xv 284.16 A line is formed, which rules out this type of wave activity there. Almost 1.6 per cent of the solar surface is covered with small BPs, probably associated with the small-scale closed-loop system, which may be a subset of expanding flux tubes. Hence, the leakage of ∼5.0-min oscillations above such BPs, which is associated with the highest powers of strong convective motions, and probably resonantly amplified by transverse MHD waves (e.g. Alfven waves), may be significant for heating the solar atmosphere locally.