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Kink oscillations of coronal loops, i.e., standing kink waves, is one of the most studied dynamic phenomena in the solar corona. The oscillations are excited by impulsive energy releases, such as low coronal eruptions. Typical periods of the oscillations are from a few to several minutes, and are found to increase linearly with the increase in the major radius of the oscillating loops. It clearly demonstrates that kink oscillations are natural modes of the loops, and can be described as standing fast magnetoacoustic waves with the wavelength determined by the length of the loop. Kink oscillations are observed in two different regimes. In the rapidly decaying regime, the apparent displacement amplitude reaches several minor radii of the loop. The damping time which is about several oscillation periods decreases with the increase in the oscillation amplitude, suggesting a nonlinear nature of the damping. In the decayless regime, the amplitudes are smaller than a minor radius, and the driver is still debated. The review summarises major findings obtained during the last decade, and covers both observational and theoretical results. Observational results include creation and analysis of comprehensive catalogues of the oscillation events, and detection of kink oscillations with imaging and spectral instruments in the EUV and microwave bands. Theoretical results include various approaches to modelling in terms of the magnetohydrodynamic wave theory. Properties of kink oscillations are found to depend on parameters of the oscillating loop, such as the magnetic twist, stratification, steady flows, temperature variations and so on, which make kink oscillations a natural probe of these parameters by the method of magnetohydrodynamic seismology.

Characterized by cyclic axisymmetric perturbations to both the magnetic and fluid parameters, magnetohydrodynamic fast sausage modes (FSMs) have proven useful for solar coronal seismology given their strong dispersion. This review starts by summarizing the dispersive properties of the FSMs in the canonical configuration where the equilibrium quantities are transversely structured in a step fashion. With this preparation we then review the recent theoretical studies on coronal FSMs, showing that the canonical dispersion features have been better understood physically, and further exploited seismologically. In addition, we show that departures from the canonical equilibrium configuration have led to qualitatively different dispersion features, thereby substantially broadening the range of observations that FSMs can be invoked to account for. We also summarize the advances in forward modeling studies, emphasizing the intricacies in interpreting observed oscillatory signals in terms of FSMs. All these advances notwithstanding, we offer a list of aspects that remain to be better addressed, with the physical connection of coronal FSMs to the quasi-periodic pulsations in solar flares particularly noteworthy., Comment: Accepted for publication in Space Science Reviews

We present a model for the intensity of optically thin EUV emission for a plasma atmosphere. We apply our model to the solar corona as observed using the six optically thin EUV channels of the SDO/AIA instrument. The emissivity of the plasma is calculated from the density and temperature using CHIANTI tables and the intensity is then determined by integration along the line of sight. We consider several different profiles for the radial density and temperature profiles, each of which are constrained by the observational data alone with no further physical assumptions. We demonstrate the method first by applying it to a quiet region of the corona, and then use it as the background component of a model including coronal holes, allowing the plasma densities and temperatures inside and outside the hole to be estimated. We compare our results with differential emission measure (DEM) inversions. More accurate estimates for the coronal density and temperature profiles have the potential to help constrain plasma properties such as the magnetic field strength when used in combination with methods such as seismology.

Recent observations with the Atmospheric Imaging Assembly (AIA) instrument on the SDO spacecraft have revealed the existence of decayless coronal kink oscillations. These transverse oscillations are not connected to any external phenomena like flares or coronal mass ejections, and show significantly lower amplitudes than the externally excited decaying oscillations. Numerical studies have managed to reproduce such decayless oscillations in the form of footpoint driven standing waves in coronal loops, and to treat them as a possible mechanism for wave heating of the solar corona. Our aim is to investigate the correlation between the observed amplitudes of the oscillations and input the energy flux from different drivers. We perform 3D MHD simulations in single, straight, density-enhanced coronal flux tubes for different drivers, in the presence of gravity. Synthetic images at different spectral lines are constructed with the use of the FoMo code. The development of the Kelvin-Helmholtz instability leads to mixing of plasma between the flux tube and the hot corona. Once the KHI is fully developed, the amplitudes of the decayless oscillations show only a weak correlation with the driver strength. We find that low amplitude decayless kink oscillations may correspond to significant energy fluxes of the order of the radiative losses for the Quiet Sun. A clear correlation between the input energy flux and the observed amplitudes from our synthetic imaging data cannot be established. Stronger drivers lead to higher vales of the line width estimated energy fluxes. Finally, estimations of the energy fluxes by spectroscopic data are affected by the LOS angle, favoring combined analysis of imaging and spectroscopic data for single oscillating loops., Comment: 22 pages, 9 figures

Kink oscillations of a coronal loop are observed and studied in detail because they provide a unique probe into the structure of coronal loops through MHD seismology and a potential test of coronal heating through the phase-mixing of Alfv\'en waves. In particular, recent observations show that standing oscillations of loops often involve also higher harmonics, beside the fundamental mode. The damping of these kink oscillations is explained by mode coupling with Alfv\'en waves. We investigate the consequences for wave-based coronal heating of higher harmonics and what coronal heating observational signatures we may use to infer the presence of higher harmonic kink oscillations. We perform a set of non-ideal MHD simulations where the damping of the kink oscillation of a flux tube via mode coupling is modelled. Our MHD simulation parameters are based on the seismological inversion of an observation for which the first three harmonics are detected. We study the phase-mixing of Alfv\'en waves that leads to the deposition of heat in the system, and we apply the seismological inversion techniques to the MHD simulation output. We find that the heating due to phase-mixing of the Alfv\'en waves triggered by the damping of the kink oscillation is relatively small, however we can illustrate i) how the heating location drifts due to the subsequent damping of lower order harmonics. We also address the role of the higher order harmonics and the width of the boundary shell in the energy deposition. We conclude that the coronal heating due to phase-mixing seems not to provide enough energy to maintain the thermal structure of the solar corona even when multi-harmonics oscillations are included, and these oscillations play an inhibiting role in the development of smaller scale structures.

We perform numerical simulations of impulsively generated magnetic swirls in an isolated flux tube which is rooted in the solar photosphere. These swirls are triggered by an initial pulse in a horizontal component of the velocity. The initial pulse is launched either: (a) centrally, within the localized magnetic flux tube; or (b) off-central, in the ambient medium. The evolution and dynamics of the flux tube is described by three-dimensional, ideal magnetohydrodynamic equations. These equations are numerically solved to reveal that in case (a) dipole-like swirls associated with the fast magnetoacoustic kink and $m=1$ Alfv\'en waves are generated. In case (b), the fast magnetoacoustic kink and $m=0$ Alfv\'en modes are excited. In both these cases, the excited fast magnetoacoustic kink and Alfv\'en waves consist of similar flow pattern and magnetic shells are also generated with clockwise and counter-clockwise rotating plasma within them, which can be the proxy of dipole-shaped chromospheric swirls. The complex dynamics of vortices and wave perturbations reveals the channelling of sufficient amount of energy to fulfill energy losses in the chromosphere ($\sim$ 10$^{4}$ W m$^{-1}$) and in the corona ($\sim$ 10$^{2}$ W m$^{-1}$). Some of these numerical findings are reminiscent of signatures in recent observational data.

This short review paper gives an overview of recently observed transverse, propagating velocity perturbations in coronal loops. These ubiquitous perturbations are observed to undergo strong damping as they propagate. Using 3D numerical simulations of footpoint-driven transverse waves propagating in a coronal plasma with a cylindrical density structure, in combination with analytical modelling, it is demonstrated that the observed velocity perturbations can be understood in terms of coupling of different wave modes in the inhomogeneous boundaries of the loops. Mode coupling in the inhomogeneous boundary layers of the loops leads to the coupling of the transversal (kink) mode to the azimuthal (Alfven) mode, observed as the decay of the transverse kink oscillations. Both the numerical and analytical results show the spatial profile of the damped wave has a Gaussian shape to begin with, before switching to exponential decay at large heights. In addition, recent analysis of CoMP (Coronal Multi-channel Polarimeter) Doppler shift observations of large, off- limb, trans-equatorial loops shows that Fourier power at the apex appears to be higher in the high-frequency part of the spectrum than expected from theoretical models. This excess high-frequency FFT power could be tentative evidence for the onset of a cascade of the low-to-mid frequency waves into (Alfvenic) turbulence.

Magnetohydrodynamic waves interact with structured plasmas and reveal the internal magnetic and thermal structures therein, thereby having seismological applications in the solar atmosphere. We investigate the evolution of fast magnetoacoustic pulses in randomly structured plasmas, in the context of large-scale propagating waves in the solar atmosphere. We perform one dimensional numerical simulations of fast wave pulses propagating perpendicular to a constant magnetic field in a low-$\beta$ plasma with a random density profile across the field. Both linear and nonlinear regimes are considered. We study how the evolution of the pulse amplitude and width depends on their initial values and the parameters of the random structuring. A randomly structured plasma acts as a dispersive medium for a fast magnetoacoustic pulse, causing amplitude attenuation and broadening of the pulse width. After the passage of the main pulse, secondary propagating and standing fast waves appear in the plasma. Width evolution of both linear and nonlinear pulses can be well approximated by linear functions; however, narrow pulses may have zero or negative broadening. This arises because a narrow pulse is prone to splitting, while a broad pulse usually deviates less from their initial Gaussian shape and form ripple structures on top of the main pulse. A linear pulse decays at almost a constant rate, while a nonlinear pulse decays exponentially. A pulse interacts most efficiently with a random medium which has a correlation length of about half of its initial pulse width. The development of a detailed model of a fast MHD pulse propagating in highly structured medium substantiates the interpretation of EIT waves as fast magnetoacoustic waves. Evolution of a fast pulse provides us with a novel method to diagnose the sub-resolution filamentation of the solar atmosphere., Comment: 25 pages, 8 figures

We present a Bayesian seismology inversion technique for propagating magnetohydrodynamic (MHD) transverse waves observed in coronal waveguides. The technique uses theoretical predictions for the spatial damping of propagating kink waves in transversely inhomogeneous coronal waveguides. It combines wave amplitude damping length scales along the waveguide with theoretical results for resonantly damped propagating kink waves to infer the plasma density variation across the oscillating structures. Provided the spatial dependence of the velocity amplitude along the propagation direction is measured and the existence of two different damping regimes is identified, the technique would enable us to fully constrain the transverse density structuring, providing estimates for the density contrast and its transverse inhomogeneity length scale.

Erythrocytes and vesicles shed by erythrocytes from human and rat blood were collected and analyzed after temperature was elevated by physical exercise or by exposure to external heat. The images of erythrocytes and vesicles were analyzed by the light microscopy system with spatial resolution of better than 90 nm. The samples were observed in an aqueous environment and required no freezing, dehydration, staining, shadowing, marking or any other manipulation. Temperature elevation, whether passive or through exercise, resulted in significant concentration increase of structurally transformed erythrocytes (echinocytes) and vesicles in blood. At temperature of 37 oC, mean vesicle concentrations and diameters in human and rat blood were (1.50+-0.35)x10^6 and (1.4+-0.2)x10^6 vesicles/{\mu}L, and 0.365+-0.065 and 0.436+-0.03 {\mu}m, respectively. It was estimated that 80% of all vesicles found in human blood are smaller than 0.4 {\mu}m. Thermodynamic analysis of experimental and literature data showed that erythrocyte transformation, vesicle release and other associated processes are driven by entropy with enthalpy-entropy compensation. It is suggested that physical state of hydrated cell membrane is responsible for the compensation. The increase of vesicle number related to elevated temperatures may be indicative of the heat stress level and serve as diagnostic of erythrocyte stability and human performance., Comment: 18 pages, 7 figures, Submitted to the Journal of Thermal Biology on January 25, 2013

We use images of high spatial and temporal resolution, obtained with the Rapid Oscillations in the Solar Atmosphere instrument at the Dunn Solar Telescope, to reveal how the generation of transverse waves in Type I spicules is a direct result of longitudinal oscillations occurring in the photosphere. Here we show how pressure oscillations, with periodicities in the range 130 - 440 s, manifest in small-scale photospheric magnetic bright points, and generate kink waves in the Sun's outer atmosphere with transverse velocities approaching the local sound speed. Through comparison of our observations with advanced two-dimensional magneto-hydrodynamic simulations, we provide evidence for how magnetoacoustic oscillations, generated at the solar surface, funnel upwards along Type I spicule structures, before undergoing longitudinal-to-transverse mode conversion into waves at twice the initial driving frequency. The resulting kink modes are visible in chromospheric plasma, with periodicities of 65 -220 s, and amplitudes often exceeding 400 km. A sausage mode oscillation also arises as a consequence of the photospheric driver, which is visible in both simulated and observational time series. We conclude that the mode conversion and period modification is a direct consequence of the 90 degree phase shift encompassing opposite sides of the photospheric driver. The chromospheric energy flux of these waves are estimated to be approximately 300,000 W/m^2, which indicates that they are sufficiently energetic to accelerate the solar wind and heat the localized corona to its multi-million degree temperatures., Comment: Accepted as an ApJ Letter, 4 figures