Stephen H. Ferguson

Statistical Properties of Solar Granulation Derived from the Soup Instrument on Spacelab 2

Abstract : The Solar Optical Universal Polarimeter (SOUP) on Spacelab 2 collected movies of solar granulation completely free from the distortion and blurring introduced by the Earths atmosphere. Individual images in the movies are diffraction-limited (30 cm aperture) and are not degraded by pointing jitter. The movies illustrate that the solar 5 minute oscillation has a major role in the appearance of solar granulation and that exploding granules are a common feature of the granule evolution. Using three-dimensional Fourier filtering techniques, we have been able to remove the oscillations and demonstrate that they dominate the temporal autocorrelation functions of the granulation pattern. When the oscillations are removed, the autocorrelation lifetime of granulation is a factor of 2 greater in magnetic field regions than in field-free quiet Sun. Using a technique called local correlation tracking, we have been able to measure horizontal velocities and observe flow patterns on the scale of meso- and supergranulation. In quiet regions the mean flow velocity is 370 m/s, while in magnetic regions it is about 125 m/s. We have also found that the root mean square fluctuating horizontal velocity field in quiet Sun increases from 0.45 to 1.4 km/s, and in strong magnetic field regions it increases from 0.3 to 0.75 km/s as the measuring aperture decreases.

On the Relation between Photospheric Flow Fields and the Magnetic Field Distribution on the Solar Surface

Using the technique of local correlation tracking on a 28 minute time sequence of white-light images of solar granulation, the horizontal flow field on the solar surface is measured. The time series was obtained by the Solar Optical Universal Polarimeter (SOUP) on Spacelab 2 (Space Shuttle flight 51-F) and is free from atmospheric blurring and distortion. The SOUP flow fields have been compared with carefully aligned magnetograms taken over a nine hour period at the Big Bear Solar Observatory before, during, and after the SOUP images. The flow field and the magnetic field agree in considerable detail: vectors which define the flow of the white-light intensity pattern (granulation) point toward magnetic field regions, magnetic fields surround flow cells, and magnetic features move along the flow arrows. The projected locations of free particles (corks) in the measured flow field congregate at the same locations where the magnetic field is observed. 31 references.

White-light movies of the solar photosphere from the soup instrument on spacelab 2

Abstract We present initial results on solar granulation, pores and sunspots from the white-light films obtained by the Solar Optical Universal Polarimeter (SOUP) instrument on Spacelab 2. SOUP contains a 30-cm Cassegrain telescope, an active secondary mirror for image stabilization, and a white-light optical system with 35-mm film and video cameras. Outputs from the fine guidance servo provided engineering data on the performance of the ESA Instrument Pointing System (IPS). Several hours of movies were taken at various disk and limb positions in quiet and active regions. The images are diffraction-limited at 0.5 arc second resolution and are, of course, free of atmospheric seeing and distortion. Properties of the granulation in magnetic and non-magnetic regions are compared and are found to differ significantly in size, rate of intensity variation, and lifetime. In quiet sun on the order of fifty percent of the area has at least one “exploding granule” occurring in it during a 25 minute period. Local correlation tracking has detected several types of transverse flows, including systematic outflow from the penumbral boundary of a spot, motion of penumbral filaments, and cellular flow patterns of supergranular and mesogranular size. Feature tracking has shown that in quiet sun the average granule fragment has a velocity of about one kilometer per second.

The relation between convection flows and magnetic structure at the solar surface

Abstract We describe recent results from the comparison of data from the Solar Optical Universal Polarimeter instrument on Spacelab 2 and magnetograms from Big Bear Solar Observatory. We show that the Suns surface velocity field governs the structure of the observed magnetic field over the entire solar surface outside sunspots and pores. We attempt to describe the observed flows by a simple axisymmetric plume model. Finally, we suggest that these observations may have important implications for the prediction of solar flares, mass ejections, and coronal heating.

Variability of solar mesogranulation

Abstract From white-light photographs of solar granulation obtained with the SOUP instrument on Space Shuttle Flight STS-19 we have measured the motions of granules using local correlation tracking techniques. The granules are organized into larger-scale structures (mesogranular and supergranular) which exhibit outflow from upwellings, convergence into sinks, as well as significant vorticity. Magnetic fields follow these same flow patterns. We describe these velocity structures, and suggest that their effect on magnetic field structures may be important to the solar flare buildup process.