Partial polarizer filter

A birefringent filter module comprises, in seriatum. (1) an entrance polarizer, (2) a first birefringent crystal responsive to optical energy exiting the entrance polarizer, (3) a partial polarizer responsive to optical energy exiting the first polarizer, (4) a second birefringent crystal responsive to optical energy exiting the partial polarizer, and (5) an exit polarizer. The first and second birefringent crystals have fast axes disposed + or -45 deg from the high transmitivity direction of the partial polarizer. Preferably, the second crystal has a length 1/2 that of the first crystal and the high transmitivity direction of the partial polarizer is nine times as great as the low transmitivity direction. To provide tuning, the polarizations of the energy entering the first crystal and leaving the second crystal are varied by either rotating the entrance and exit polarizers, or by sandwiching the entrance and exit polarizers between pairs of half wave plates that are rotated relative to the polarizers. A plurality of the filter modules may be cascaded

Waller Creek Tunnel Project

Waller Creek Working Grou

Optimization of a field-widened Michelson interferometer

This paper considers the optical design of a wide-angle fixed-path Michelson interferometer consisting of two arm glasses and an air gap. It is shown that this configuration can be optimized to give (a) extra large fringes (over 50° in diameter) over a range of wavelength, (b) a path difference nearly independent of wavelength, or (c) a path difference specified differently at two different wavelengths for observing a pair of doublets. Specific examples refer to the airglow wavelengths of 557.7, 630.0, 732.0 nm and others, and to a path difference of 4.5 cm. The properties of different glass combinations are discussed

The quiet Sun's magnetic flux estimated from CaIIH bright inter-granular G-band structures

We determine the number density and area contribution of small-scale inter-granular calcium-II bright G-band structures in images of the quiet Sun as tracers of kilo-Gauss magnetic flux-concentrations. In a 149" x 117" G-band image of the disk center at the activity minimum, 7593 small inter-granular structures ['IGS']were segmented with the `multiple-level tracking' pattern recognition algorithm ['MLT_4']. The scatter-plot of the continuum versus the G-band brightness shows the known magnetic and non-magnetic branches. These branches are largely disentangled by applying an intrinsic Ca-II excess criterion. The thus obtained 2995 structures contain 1152 G-band bright points ['BP'] and 1843 G-band faint points ['FP']. They show a tendency of increasing size with decreasing G-band excess, as expected from the `hot wall' picture. Their Ca-H and G-band brightness are slightly related, resembling the known relation of Ca-II and magnetic field strength. The magnetic flux density of each individual BP and FP is estimated from their G-band brightness according to MHD-model calculations. The entity of BP and FP covers the total field-of-view ['FOV'] with a number density of 0.32/Mm^2 and a total area contribution of 2.0%. Their individual calibrations yield a mean flux density of 20 Mx/cm^2 in the entire FOV and 13 Mx/cm^2 for inter-network regions

Balltracking: an highly efficient method for tracking flow fields

We present a method for tracking solar photospheric flows that is highly efficient, and demonstrate it using high resolution MDI continuum images. The method involves making a surface from the photospheric granulation data, and allowing many small floating tracers or balls to be moved around by the evolving granulation pattern. The results are tested against synthesised granulation with known flow fields and compared to the results produced by Local Correlation tracking (LCT). The results from this new method have similar accuracy to those produced by LCT. We also investigate the maximum spatial and temporal resolution of the velocity field that it is possible to extract, based on the statistical properties of the granulation data. We conclude that both methods produce results that are close to the maximum resolution possible from granulation data. The code runs very significantly faster than our similarly optimised LCT code, making real time applications on large data sets possible. The tracking method is not limited to photospheric flows, and will also work on any velocity field where there are visible moving features of known scale length

Can the Solar Wind be Driven by Magnetic Reconnection in the Sun's Magnetic Carpet?

The physical processes that heat the solar corona and accelerate the solar wind remain unknown after many years of study. Some have suggested that the wind is driven by waves and turbulence in open magnetic flux tubes, and others have suggested that plasma is injected into the open tubes by magnetic reconnection with closed loops. In order to test the latter idea, we developed Monte Carlo simulations of the photospheric "magnetic carpet" and extrapolated the time-varying coronal field. These models were constructed for a range of different magnetic flux imbalance ratios. Completely balanced models represent quiet regions on the Sun and source regions of slow solar wind streams. Highly imbalanced models represent coronal holes and source regions of fast wind streams. The models agree with observed emergence rates, surface flux densities, and number distributions of magnetic elements. Despite having no imposed supergranular motions, a realistic network of magnetic "funnels" appeared spontaneously. We computed the rate at which closed field lines open up (i.e., recycling times for open flux), and we estimated the energy flux released in reconnection events involving the opening up of closed flux tubes. For quiet regions and mixed-polarity coronal holes, these energy fluxes were found to be much lower than required to accelerate the solar wind. For the most imbalanced coronal holes, the energy fluxes may be large enough to power the solar wind, but the recycling times are far longer than the time it takes the solar wind to accelerate into the low corona. Thus, it is unlikely that either the slow or fast solar wind is driven by reconnection and loop-opening processes in the magnetic carpet.Comment: 25 pages (emulateapj style), 13 figures, ApJ, in pres

Asymmetries of the Stokes V profiles observed by HINODE SOT/SP in the quiet Sun

We present the first classification of SOT/SP circular polarization measurements with the aim of highlighting exhaustively the whole variety of Stokes V shapes emerging from the quiet Sun. k-means is used to classify HINODE SOT/SP Stokes V profiles observed in the quiet Sun network and internetwork (IN). We analyze a 302 x 162 square arcsec field-of-view (FOV) which can be considered a complete sample of quiet Sun measurements performed at at the disk center with 0.32 arcsec angular resolution and 0.001 polarimetric sensitivity. Such a classification allows us to divide the whole dataset in classes, with each class represented by a cluster profile, i.e., the average of the profiles in the class. The set of 35 cluster profiles derived from the analysis completely characterizes SOT/SP quiet Sun measurements. The separation between network and IN profile shapes is evident - classes in the network are not present in the IN, and vice versa. Asymmetric profiles are approximatively 93 % of the total number of profiles. Among these, approximatively 34 % of the profiles are strongly asymmetric profiles, and they can be divided in three families: blue-lobe, red-lobe, and Q-like profiles. The blue-lobe profiles tend to be associated with upflows (granules), whereas the red-lobe and Q-like ones appear in downflows (intergranular lanes). Such profiles need to be interpreted considering model atmospheres different from a uniformly magnetized Milne-Eddington (ME) atmosphere, i.e., characterized by gradients and/or discontinuities in the magnetic field and velocity along the line-of-sight (LOS).Comment: 11 pages, 4 figures, accepted for publication in Astronomy and Astrophysic

Spectroscopic analysis of interaction between an EIT wave and a coronal upflow region

We report a spectroscopic analysis of an EIT wave event that occurred in active region 11081 on 2010 June 12 and was associated with an M2.0 class flare. The wave propagated near circularly. The south-eastern part of the wave front passed over an upflow region nearby a magnetic bipole. Using EIS raster observations for this region, we studied the properties of plasma dynamics in the wave front, as well as the interaction between the wave and the upflow region. We found a weak blueshift for the Fe XII {\lambda}195.12 and Fe XIII {\lambda}202.04 lines in the wave front. The local velocity along the solar surface, which is deduced from the line of sight velocity in the wave front and the projection effect, is much lower than the typical propagation speed of the wave. A more interesting finding is that the upflow and non-thermal velocities in the upflow region are suddenly diminished after the transit of the wave front. This implies a significant change of magnetic field orientation when the wave passed. As the lines in the upflow region are redirected, the velocity along the line of sight is diminished as a result. We suggest that this scenario is more in accordance with what was proposed in the field-line stretching model of EIT waves.Comment: 13 pages, 7 figures, accepted for publication in Ap

Modified p-modes in penumbral filaments?

Aims: The primary objective of this study is to search for and identify wave modes within a sunspot penumbra. Methods: Infrared spectropolarimetric time series data are inverted using a model comprising two atmospheric components in each spatial pixel. Fourier phase difference analysis is performed on the line-of-sight velocities retrieved from both components to determine time delays between the velocity signals. In addition, the vertical separation between the signals in the two components is calculated from the Stokes velocity response functions. Results: The inversion yields two atmospheric components, one permeated by a nearly horizontal magnetic field, the other with a less-inclined magnetic field. Time delays between the oscillations in the two components in the frequency range 2.5-4.5 mHz are combined with speeds of atmospheric wave modes to determine wave travel distances. These are compared to expected path lengths obtained from response functions of the observed spectral lines in the different atmospheric components. Fast-mode (i.e., modified p-mode) waves exhibit the best agreement with the observations when propagating toward the sunspot at an angle ~50 degrees to the vertical.Comment: 8 pages, 12 figures, accepted for publication in Astronomy & Astrophysic

Kinematics and Fine Structure of An Unwinding Polar Jet Observed by SDO/AIA

We present an observational study of the kinematics and fine structure of an unwinding polar jet, with high temporal and spatial observations taken by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) and the Solar Magnetic Activity Research Telescope (SMART). During the rising period, the shape of the jet resembled a cylinder with helical structures on the surface, while the mass of the jet was mainly distributed on the cylinder's shell. In the radial direction, the jet expanded successively at its western side. The radial expansion presented three distinct phases: the gradually expanding phase, the fast expanding phase, and the steady phase. Each phase lasted for about 12 minutes. The angular speed of the unwinding jet and the twist transferred into the outer corona during the eruption are estimated to be 11.1 \times 10{-3} rad/s (period = 564 s) and 1.17 to 2.55 turns (or 2.34 to 5.1{\pi}) respectively. On the other hand, by calculating the azimuthal component of the magnetic field in the jet and comparing the free energy stored in the non-potential magnetic field with the jet's total energy, we find that the non-potential magnetic field in the jet is enough to supply the energy for the ejection. These new observational results strongly support the scenario that the jets are driven by the magnetic twist, which is stored in the twisted closed field of a bipole, and released through magnetic reconnection between the bipole and its ambient open field