Properties of solar pores

We present the results of an extensive investigation of the properties of solar pores. Spectra of all 4 Stokes parameters of several magnetic sensitive absorption lines as well as Stokes I only spectra of lines with low or vanishing Landéfactor have been observed. An inversion code based on the Levenberg-Marquardt algorithm was written, incorporating the full radiative transfer equations. This code adapts a set of parameters that are used to represent the magnetic and thermodynamic state of the atmosphere to best-fit a set of observed lineprofiles. The results show that the value of all relevant parameters are intimately related to the size of the pores, therewith confirming the role of solar pores as a link between (bright) magnetic flux tubes and sunspots. In particular the inclination of themagnetic field lines at the outer rim of a pore, which is affectedby the vertical field gradient, seems to put an upper limit onthe diameter of pores where the inclination reaches a value thatmight trigger the formation of a penumbra

Physics of Complex Plasmas.

Physics of complex plasmas is a wide and varied field. In the context of this PhD thesis I present the major results from my research on fundamental properties of the plasma sheath, the plasma dust interaction, non-Hamiltonian dynamics, and on non-equilibrium phase transitions, using complex plasmas as a model system. The first chapter provides a short overview of the development of physics of Complex Plasmas. From fundamental plasma physics, properties of dust in plasmas, to the exceptional and unique features of complex plasmas. A summary of twenty years of research topics is also presented. This is followed by three chapters that illustrate publications based on experiments I did during my PhD. These publications, in my opinion, reflect nicely the large diversity of complex plasma research. • The investigation of nonlinear vertical oscillations of a particle in a sheath of an rf discharge was a simultaneous test of (pre-)sheath models and parameters. The nonlinear oscillations were shown to derive from a (strong) nonlinearity of the local sheath potential. They could be described quantitatively applying the theory of anharmonic oscillations, and the first two anharmonic terms in an expansion of the sheath potential were measured. On top of that we provided a simple experimentally, theoretically and mathematically based method that allows for in situ measurement of these coefficients for other experimental conditions. • The vertical pairing of identical particles suspended in the plasma sheath demonstrated some of the unique features that complex plasmas have as an open (non-Hamiltonian) system. Particle interaction becomes non-reciprocal in the presence of streaming ions. The symmetry breaking allows for mode-coupling of in plane and out of plane motion of particles. • Lane formation is a non-equilibrium phase transition. I summarize the main result of my papers on the dynamics of lane formation, i.e., the temporal evolution of lanes. This is followed by an outlook on my future research on non-equilibrium phase transitions, how they relate to our research of systems at the critical point, and how they allow us to test fundamental theories of charging of particles and the shielding of the resulting surface potential. Finally there is an appendix on the scaling index method. A versatile mathematical tool to quantify structural differences / peculiarities in data, that I used to define a suitable order parameter for lane formation

The size of penumbral fine structure

I present power spectra of penumbral and granular intensity variations from a speckle-restored G-band image sequence of sunspot NOAA 9407 taken on April 1, 2001 with the Dutch Open Telescope on La Palma. I compare spatial power spectra of the sunspot penumbra with spatial power spectra from granulation with and without filigree. Relative to the granular power distribution, the penumbral power spectrum is enhanced over a wide range in spatial frequency peaking at 0."35. For smaller scales, the penumbral power distribution closely resembles that of the granular intensity variations. In contrast, the power spectrum of granulation with filigree exhibits increased power down to the resolution limit of 0."22, indicating the presence of unresolved magnetic elements

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Temperature mapping of sunspots and pores from speckle reconstructed three colour photometry

The two-dimensional temperature distribution in a highly structured sunspot and in two small umbrae is determined from a three-colour photometry in narrow spectral continua. Disturbing influences from the earths atmosphere are removed by speckle masking techniques, yielding a spatial resolution limited by the telescopes aperture. The corresponding colour temperatures are consistent over a range of more than 2000 K, although the numerical correction introduced by the reconstruction differs largely for the three colours. Part of the scatter in the temperature relation disappears when convoluting the final images with artificial PSFs that compensate for the different, colour dependent spatial resolution. The remaining spread in the scatter plots does not reflect noise, but is related to local variations of the temperature difference between the continuum emitting layers. This is most obvious for a small umbra which yields branches in the scatter plots the bluer of which corresponding to the limb-side umbral border. Here, the hot rim of a Wilson depressed umbra becomes visible. The temperature map of the large spot shows that the bright umbral dots do not reach the temperature of the non-spot surroundings. Instead, they exceed the 2000K cooler umbral temperature minimum by 900-1300 K. The filamentary structure of the surrounding penumbra has spatial temperature fluctuations of typically 700 K, a value which fits earlier observed ontrasts. However, the mean temperatures of 5650K in the dark and 6250K in the bright penumbral fine structures exceed former findings. Exceptionally bright penumbral grains are 250K than the mean solar surface and thus exceed even brightest granules

Applying speckle-masking to spectra

We have applied the technique of speckle masking to spectra. The obser- vation of elongated solar structures avoids the problem of missing information in one-dimensional spectra. Image motion perpendicular to the slit was diminished by a one-dimensional image stabilization system. The remaining influence of the earth's atmosphere was removed by a modified speckle-masking algorithm, adapted to the single spatial dimension occurring in the spectra. The reconstructed spectra achieve the diffraction limit of the telescope and the spectrograph. The first application of this technique to observations of spicules and penumbral filaments reveals more details and also yield line profiles which differ from those before reconstruction. The Ha emission in spicules shows line-of-sight velocities two times larger than in the unprocessed spectra. The non-magnetic line Fe 709.03 nm shows penumbral line widths, reflecting mostly the line asymmetry from the Ever- shed effect, which are tightly correlated to the continuum intensity fluctuations. Our reconstruction increases the coherence between both from 0.6 to 0.8

A jigsaw puzzle framework for homogenization of high porosity foams

An approach to homogenization of high porosity metallic foams is explored. The emphasis is on the \Alporas{} foam and its representation by means of two-dimensional wire-frame models. The guaranteed upper and lower bounds on the effective properties are derived by the first-order homogenization with the uniform and minimal kinematic boundary conditions at heart. This is combined with the method of Wang tilings to generate sufficiently large material samples along with their finite element discretization. The obtained results are compared to experimental and numerical data available in literature and the suitability of the two-dimensional setting itself is discussed.Comment: 11 pages, 7 figures, 3 table

Opposite polarity field with convective downflow and its relation to magnetic spines in a sunspot penumbra

We discuss NICOLE inversions of Fe I 630.15 nm and 630.25 nm Stokes spectra from a sunspot penumbra recorded with the CRISP imaging spectropolarimeter on the Swedish 1-m Solar Telescope at a spatial resolution close to 0.15". We report on narrow radially extended lanes of opposite polarity field, located at the boundaries between areas of relatively horizontal magnetic field (the intra-spines) and much more vertical field (the spines). These lanes harbor convective downflows of about 1 km/s. The locations of these downflows close to the spines agree with predictions from the convective gap model (the "gappy penumbra") proposed six years ago, and more recent 3D MHD simulations. We also confirm the existence of strong convective flows throughout the entire penumbra, showing the expected correlation between temperature and vertical velocity, and having vertical RMS velocities of about 1.2 km/s.Comment: Accepted for publication in A&A (06-March-2013). Minor corrections made in this version

Oscillatory behavior in the quiet Sun observed with the New Solar Telescope

Surface photometry of the quiet Sun has achieved an angular resolution of $0".1$ with the New Solar Telescope at Big Bear Solar Observatory revealing that a disproportionate fraction of the oscillatory events appear above observed bright point-like structures. During the tracking of these structures, we noted that the more powerful oscillatory events are cospatial with them, indicating that observed flux tubes may be the source of many observed oscillatory events.Comment: 5 pages 4 figure