Oscillations and waves related to sunspots

In order to study umbral oscillations, running penumbral waves and the relationship between them, we analyzed CCD, high-resolution, sunspot observations obtained at the center and the wings of the Hα line and the Fe I 5576 ˚A line using a UBF filter. We produced “space/time slice images” which show that there is not a clear relationship between umbral oscillations and running penumbral waves as they observed in upper chromospheric layers. We found that the running penumbral waves are observable at least up to the formation height of the Hα±0.5 ˚A line, but not in the Hα±0.75 ˚A or the Fe I±0.12 ˚A. The correlation between umbral oscillations at various atmospheric heights and running penumbral waves strongly indicates that the latter are excited by photospheric umbral oscillations and not the chromospheric ones

Evaluation of a method for the resolution improvement of near limb solar images

We present a methodology, based onthe correctionfor the limb darkening and the use of a directionally sensitive operator the “MadMax”, for the image processing of observations obtained near the solar limb. Our image processing method substantially enhances near-limb observations and permits an insight into the studies of the very fine chromospheric structures, over higher-resolution images. Space/time images produced from filtergrams processed with our method indicate that polar surges and spicules are probably related to different physical mechanisms

The Evershed flow from simultaneous chromospheric and photospheric observations

We study the Evershed flow in the photosphere and the reverse Evershed flow inthe chromosphere giving emphasis to the temporal evolution of the phenomenon. Our results verify that the velocity of the Evershed flow has a maximum above the penumbra in the photosphere and well outside the penumbra in the chromosphere. We found a quasi-periodic behavior of the reverse Evershed flow in the chromosphere with period between10–15 min. We were not able to identify an obvious repetitive behavior in the photosphere, except from the propagation of the slow photospheric waves

Increasing the Fine Structure Visibility of the Hinode SOT Ca II H Filtergrams

We present the improved so-called Madmax (OMC) operator selecting maxima of convexities computed in multiple directions around each pixel rewritten in MatLab and shown to be very efficient for pattern recognition. The aim of the algorithm is to trace the bright hair-like features (for ex. chromospheric thin jets or spicules) of solar ultimate observations polluted by a noise of different origins. This popular spatial operator uses the second derivative in the optimally selected direction for which its absolute value has a maximum value. Accordingly, it uses the positivity of the resulting intensity signal affected by a superposed noise. The results are illustrated using a test artificially generated image and real SOT (Hinode) images are also used, to make your own choice of the sensitive parameters to use in improving the visibility of images.Comment: 12 pages, 3 figurs, submitted in Solar Physic

Non-linear numerical simulations of magneto-acoustic wave propagation in small-scale flux tubes

We present results of non-linear, 2D, numerical simulations of magneto-acoustic wave propagation in the photosphere and chromosphere of small-scale flux tubes with internal structure. Waves with realistic periods of three to five minutes are studied, after applying horizontal and vertical oscillatory perturbations to the equilibrium model. Spurious reflections of shock waves from the upper boundary are minimized thanks to a special boundary condition. This has allowed us to increase the duration of the simulations and to make it long enough to perform a statistical analysis of oscillations. The simulations show that deep horizontal motions of the flux tube generate a slow (magnetic) mode and a surface mode. These modes are efficiently transformed into a slow (acoustic) mode in the vA < cS atmosphere. The slow (acoustic) mode propagates vertically along the field lines, forms shocks and remains always within the flux tube. It might deposit effectively the energy of the driver into the chromosphere. When the driver oscillates with a high frequency, above the cut-off, non-linear wave propagation occurs with the same dominant driver period at all heights. At low frequencies, below the cut-off, the dominant period of oscillations changes with height from that of the driver in the photosphere to its first harmonic (half period) in the chromosphere. Depending on the period and on the type of the driver, different shock patterns are observed.Comment: 22 pages 6 color figures, submitted to Solar Physics, proceeding of SOHO 19/ GONG 2007 meeting, Melbourne, Australi

Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results

The chromosphere is a thin layer of the solar atmosphere that bridges the relatively cool photosphere and the intensely heated transition region and corona. Compressible and incompressible waves propagating through the chromosphere can supply significant amounts of energy to the interface region and corona. In recent years an abundance of high-resolution observations from state-of-the-art facilities have provided new and exciting ways of disentangling the characteristics of oscillatory phenomena propagating through the dynamic chromosphere. Coupled with rapid advancements in magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly investigate the role waves play in supplying energy to sustain chromospheric and coronal heating. Here, we review the recent progress made in characterising, categorising and interpreting oscillations manifesting in the solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review