Author name code: strous ADS astronomy entries on 2022-09-14 author:"Strous, Louis H." ------------------------------------------------------------------------ Title: The Formation of a Prominence in Active Region NOAA 8668. I. SOHO/MDI Observations of Magnetic Field Evolution Authors: Chae, Jongchul; Wang, Haimin; Qiu, Jiong; Goode, Philip R.; Strous, Louis; Yun, H. S. Bibcode: 2001ApJ...560..476C Altcode: We have studied the evolution of the photospheric magnetic field in active region NOAA 8668 for 3 days while the formation of a reverse S-shaped filament proceeded. From a set of full-disk line-of-sight magnetograms taken by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO), we have found a large canceling magnetic feature that was closely associated with the formation of the filament. The positive flux of the magnetic feature was initially 1.5×1021 Mx and exponentially decreased with an e-folding time of 28 hr throughout the period of observations. We also have determined the transverse velocities of the magnetic flux concentrations in the active region by applying local correlation tracking. As a result, a persistent pattern of shear motion was identified in the neighborhood of the filament. The shear motion had a speed of 0.2-0.5 km s-1 and fed negative magnetic helicity of -3×1042 Mx2 into the coronal volume during an observing run of 50 hr at an average rate of -6×1040 Mx2 hr-1. This rate is an order of magnitude higher than the rate of helicity change due to the solar differential rotation. The magnetic flux of the field lines created by magnetic reconnection and the magnetic helicity generated by the photospheric shear motion are much more than enough for the formation of the filament. Based on this result, we conjecture that the filament formation may be the visible manifestation of the creation of a much bigger magnetic structure that may consist of a flux rope and an overlying sheared arcade. Title: Bias in velocity determinations from full-disk solar observations Authors: Strous, L. H. Bibcode: 2000SoPh..195..219S Altcode: Estimates for the global solar surface velocity field can be obtained from time series of full-disk solar images. However, bias is introduced by mistakes in the assumptions about the geometry of the observations, and by imperfections in the optical system. I investigate many sources of bias and determine their first-order influence on the measured velocity field (both the transverse and longitudinal components). Results are presented in analytical and pictorial form. By comparing bias velocities of unknown origin with the results, one may obtain clues to the cause of the bias. Title: The Dynamics of the Excitation of Solar Oscillations Authors: Strous, Louis H.; Goode, Philip R.; Rimmele, Thomas R. Bibcode: 2000ApJ...535.1000S Altcode: We investigate seismic events, bursts of seismic waves that are generated locally just below the solar surface and that we detect traveling up through the photosphere. We identify a few thousand seismic events by their traveling wave character and find that they are associated with continuum darkening and downflow and have an extent of on average about 10-15 minutes and 1 Mm. Their birth rate is about 8×10-16 m-2 s -1. The observed upwardly traveling seismic flux in the average event (as derived from velocities in the p-mode region of k-ω space) is followed after about 3 minutes by some reflected downward flux. Only a small fraction of the energy generated in the hypocenter of the event below the surface travels straight up for us to see. The bulk of the generated energy is directed or reflected downward, and is eventually transformed into p-modes. The seismic events at the surface contain about 1.5×1019 J of seismic energy each, which corresponds to an average flux level of about 8.5 kW m-2 over the whole surface. The total energy flow is likely more than an order of magnitude greater, and is then in the same ballpark as the estimate of Libbrecht for the power required to sustain the p-mode spectrum. We find a roughly linear relation between the peak seismic flux and the peak downward convective velocity associated with each seismic event, which does not fit the highly nonlinear relations found theoretically by Lighthill and Goldreich & Kumar for stochastic excitation by turbulent convection, but does fit the monopole source deduced by Nigam & Kosovichev from a study of the p-mode spectrum. Title: Large-Scale Surface Flow Patterns on the Sun Authors: Strous, L. H. Bibcode: 2000SPD....31.0104S Altcode: 2000BAAS...32..801S I derive the large-scale flow field at the solar surface from tracking features at supergranular scale (10 Mm) through SOHO/MDI full-disk Dopplergrams recorded during 62 days in 1996 and reduced at Stanford University to a dataset with a pixel scale of 1.39 Mm and a cadence of 15 minutes. Both the dataset and the reduction algorithms are improved compared to earlier investigations. I present measurements, derived from supergranular motion, of differential rotation and meridional flow, and synoptic maps of the solar surface flow and its variation in time. This work is supported by the SOI/MDI project at Stanford University and Lockheed Martin Solar & Astrophysics Laboratory (grant NAG5-3077). Title: De veranderlijke zon. Authors: Strous, L. Bibcode: 2000Zenit..27..148S Altcode: No abstract at ADS Title: Phenomena in an Emerging Active Region. II. Properties of the Dynamic Small-Scale Structure Authors: Strous, Louis H.; Zwaan, Cornelis Bibcode: 1999ApJ...527..435S Altcode: The magnetic flux emergence in growing active region NOAA 5617, when it is about 8 hr old, shows an intricate fine structure. The small-scale emergence events are characterized by a coincident upflow and transient darkening (of about 2 Mm and 10 minutes) in the continuum and line-center intensity followed by the appearance of one, or in some cases two, new bright grains flanking the line-center darkening. The bright grains (faculae) coincide with magnetic flux concentrations and downflows. The footpoints move apart at on average 1.4 km s-1. Flux emergence happens recurrently in a number of locations widely distributed over the active region, which appear to form a pattern with a wavelength of about 8 Mm. A preferred orientation that fits Hale's polarity law is displayed by the spatial pattern in the emergence locations, the emergence events themselves, subsequent footpoint motion, and the Hα arch filament system. We find long (~15 Mm) alignments of unipolar faculae of each magnetic polarity that also follow the preferred orientation. We adapt the model for flux emergence to accommodate the observed dynamic fine structure. Essential new features are (1) the emerging bundle of flux tubes is frayed in two systems, in vertical stacks, arranged in slightly curved, nearly parallel sheets; and (2) many flux tubes emerge in multiple locations. Title: Solar Differential Rotation Derived from H-alpha Full Disk Images by Means of Local Correlation Tracking Authors: Woodard, M. F.; Denker, C.; Strous, L. H.; BBSO Collaboration; LMSAL Collaboration Bibcode: 1999AAS...19410007W Altcode: 1999BAAS...31..997W We present the application of Local Correlation Tracking (LCT) techniques to time series of contrast-enhanced H-alpha full disk images taken as part of the synoptic observing program at Big Bear Solar Observatory (BBS0) during the summer of 1998. A typical set of daily H-alpha full disk images consists of 600 to 800 individual frames, taken 30 to 60 s apart, with a 2k x 2k pixel Kodak 4.2 MegaPlus CCD camera at BBSO's Singer telescope. For each pair of successive images, we compute displacement vectors over a 64 x 64 element cartesian grid covering the solar disk. The resulting daily-averaged flow maps show predominantly solar differential rotation and proper motions in active regions. We remap the flow maps to heliographic coordinates and determine a Legendre polynomial expansion of the daily differential rotation profile. We present preliminary findings regarding differential rotation based on different types of features seen in H-alpha, such as quiet-sun fibrils, plages, and dark filaments. We discuss the relation of our differential rotation profiles to profiles derived by other methods and address the question of time variability. The work at BBSO is supported by ONR under grant N00014-97-1-1037, by NSF under grant ATM 97-14796, and by NASA under grant NAG 5-4919. Louis Strous is supported by NASA NAG5-3077 to Stanford University. Title: Average Properties of Flux Emergence in Young Active Region 5617 Authors: Strous, L. Bibcode: 1999ASPC..183..551S Altcode: 1999hrsp.conf..551S No abstract at ADS Title: Properties of Small-Scale Flux Emergence in a Young Active Region Authors: Strous, Louis; Zwaan, Cornelis Bibcode: 1999soho....9E..82S Altcode: The magnetic flux emergence in growing active region NOAA 5617, when it is about 8 hours old, shows an intricate fine structure. The small-scale emergence events are characterized by a coincident upflow and transient darkening (of about 2 Mm and 10 minutes) in the continuum and line-center intensity, followed by the appearance of one, or in some cases two, new bright grains flanking the line-center darkening. The bright grains (faculae) coincide with magnetic flux concentrations and downflows. The footpoints move apart at on average 1.4 km/s. Flux emergence happens recurrently in a number of locations widely distributed over the active region, which appear to form a pattern with a wavelength of about 8 Mm. A preferred orientation which fits Hale's polarity law is displayed by the spatial pattern in the emergence locations, the emergence events themselves, subsequent footpoint motion, and the H-alpha arch filament system. We find long (~ 15 Mm) alignments of unipolar faculae of each magnetic polarity that also follow the preferred orientation. We adapt the model for flux emergence to accommodate the observed dynamic fine structure. Essential new features are: (1) the emerging bundle of flux tubes is frayed in two systems; in vertical stacks, arranged in slightly curved, nearly parallel sheets, and (2) many flux tubes emerge in multiple locations. Title: The Excitation of Solar Oscillations -- Observations and Simulations Authors: Goode, P.; Strous, L.; Rimmele, T.; Stein, R.; Nordlund, Å. Bibcode: 1999ASPC..183..456G Altcode: 1999hrsp.conf..456G No abstract at ADS Title: The Excitation of Solar Oscillations Authors: Strous, Louis H.; Goode, Philip R.; Rimmele, Thomas R. Bibcode: 1999soho....9E..81S Altcode: We investigate seismic events, bursts of seismic waves that are generated locally just below the solar surface and that we detect travelling up through the photosphere. We identify 646 seismic events, which are associated with intergranular lanes and have an extent of on average about 10 minutes and 3 Mm. Their birth rate is about 10-16 m-2 s-1. The observed upwardly travelling seismic flux in the average event (as derived from velocities in the p-mode region of k-omega space) is followed after about 5 minutes by some reflected downward flux. Only some of the energy generated in the hypocenter of the event below the surface travels up for us to see. We propose that this energy is converted into surface (f-mode-like) waves, while the unseen, initially downward going energy is eventually transformed into p-modes. The seismic events at the surface contain about 5 * 1019 J of seismic energy each, which corresponds to an average flux level of about 4 kW/m2 over the whole surface. The initially downward directed energy flow is likely substantially greater, and is then in the same ballpark as the estimate of Libbrecht (1988) for the power required to sustain the p-mode spectrum. We find a roughly linear relation between the peak seismic flux and the peak downward convective velocity associated with each seismic event, which is not equal to the v8 relation found theoretically by Lighthill (1952) for stochastic excitation by turbulent convection. Title: On the Origin of Solar Oscillations Authors: Goode, Philip R.; Strous, Louis H.; Rimmele, Thomas R.; Stebbins, Robin T. Bibcode: 1998ApJ...495L..27G Altcode: 1998astro.ph..1008G We have made high-resolution observations of the Sun in which we identify individual sunquakes and see power from these seismic events being pumped into the resonant modes of vibration of the Sun. A typical event lasts about 5 minutes. We report the physical properties of the events and relate them to theories of the excitation of solar oscillations. We also discuss the local seismic potential of these events. Title: 62 Days Around the Sun: A Search for Supergranular Evolution and Giant Cells Authors: Strous, Louis H.; Simon, George W. Bibcode: 1998ASPC..140..161S Altcode: 1998ssp..conf..161S No abstract at ADS Title: Supergranular Evolution, Solar Rotation, and a Search for Giant Cells, using Full-disk SOHO/SOI/MDI Dopplergrams Authors: Simon, George W.; Strous, Louis H. Bibcode: 1997AAS...19112002S Altcode: 1997BAAS...29.1402S SOHO/SOI/MDI obtained full-disk dopplergrams of the Sun once per minute for 62 days from 1996 May 23 to 1996 July 23. From hourly averages of these dopplergrams we have studied the evolution of supergranules, measured solar rotation up to high latitudes, and searched for giant cells. Title: Photospheric Differential Rotation from Full-Disk SOI/MDI Dopplergrams Authors: Bogart, R. S.; Bai, T.; Scherrer, P. H.; Strous, L. H.; Simon, G. W.; Tarbell, T. D. Bibcode: 1997SPD....28.0258B Altcode: 1997BAAS...29..903B We report on measurements of the solar surface differential rotation made from SOI/MDI full-disk Dopplergrams obtained once per minute during the 2-month Dynamics Program from 23 May through 26 July 1996. We infer the rotation profile both from the direct photospheric Doppler signal and also by tracking Doppler features (supergranules) across the solar disk. We study the rotation curve as a function of latitude, feature size, and tracking method, and look for global scale flows. This work was supported by NASA Grant NAG5-3077 at Stanford and Lockheed Martin, and by AFOSR and the Fellows Program of AF Phillips Lab at NSO/SP. Title: Comparison of Granulation Correlation Tracking (CT) and Feature Tracking (FT) Results from SOHO/MDI and the Swedish Vacuum Solar Telescope on La Palma Authors: Shine, R.; Strous, L.; Simon, G.; Berger, T.; Hurlburt, N.; Tarbell, T.; Title, A.; Scharmer, G. Bibcode: 1997SPD....28.0262S Altcode: 1997BAAS...29Q.904S We have computed photospheric velocity flow maps from simultaneous observations taken with MDI and at the Swedish Vacuum Solar Tower (SVST) on La Palma on August 15, 1996. Both sets consist of a series of photospheric images, and flow maps are computed by following the local motions of granules. The MDI data have the important advantages of very stable images and longer continuous coverage of the same area of the solar surface. This longer coverage is necessary to study the evolution of mesogranules, supergranules, and to detect possible low amplitude motions on scales larger than supergranules. However, the high resolution mode of MDI is limited by the small telescope size to about 1.2 arc seconds angular resolution and uses a 0.6 arc second pixel size. This is adequate to show granulation but has the rms constrast significantly reduced to about 2%. Early efforts adapting techniques that were successful with higher resolution ground based images gave poor results and although new methods have now been developed, there are still some problems with accuracy. On the other hand, the SVST images have much higher angular resolution (as good as 0.2 arc second) but suffer from variable atmospheric distortion. They also have a much smaller field of view. By detailed comparison of the two data sets and by using CT and FT techniques to track the motions, we hope to understand the sources of any differences between them and to develop credible correction parameters to the MDI data sets if necessary. This work was supported by NASA Grant NAG5-3077 at Stanford and Lockheed Martin, by AFOSR and the Fellows Program of AF Phillips Lab at NSO/SP, and by the Swedish Royal Academy of Sciences. Title: Preliminary SoHO/MDI Observations of Supergranular Evolution Authors: Simon, G. W.; Strous, L. H.; Matt, S.; Title, A. M.; Schrijver, C. J. Bibcode: 1997SPD....28.0264S Altcode: 1997BAAS...29R.904S We present preliminary results of a study into the evolution of supergranules, using data from SoHO/MDI. We discuss the supergranular size spectrum, lifetimes, and topological evolution. We compare structures of supergranular size visible in high-resolution SoHO/MDI dopplergrams and in divergence maps derived from tracking of features in dopplergrams. This work was supported by NASA Grant NAG5-3077 at Stanford and Lockheed Martin, and by AFOSR and the Fellows Program of AF Phillips Lab at NSO/SP. Title: Horizontal Velocity Structure of Supergranules near Disk Center from High-Resolution SoHO/MDI Observations Authors: Strous, L. H.; Simon, G. W.; Shine, R. A.; Hurlburt, N. Bibcode: 1997SPD....28.0265S Altcode: 1997BAAS...29S.904S We determine the average surface flows in supergranules from high-resolution SoHO/MDI observations near disk center, using local correlation and feature tracking methods. We present results as a function of distance to the supergranule center and of supergranule size, and as a function of normalized distance to the supergranule center. This work was supported by NASA Grant NAG5-3077 at Stanford and Lockheed Martin, and by AFOSR and the Fellows Program of AF Phillips Lab at NSO/SP. Title: Comparison of supergranular flows from doppler and local correlation tracking velocities Authors: Frank, Z.; Hurlburt, N.; Shine, R.; Tarbell, T.; Simon, G.; Strous, L.; Matt, S. Bibcode: 1997SPD....28.0259F Altcode: 1997BAAS...29..903F Measurements of the flows in the solar photosphere rely upon two techniques: doppler measurements of the line-of-sight velocity or tracking of features or patterns moving perpendicular to the line-of-sight. These methods have differing characteristics. Doppler measurements can easily measure surface flows near the limb which are not seriously contaminated by p-modes or other solar sources. However, they require excellent instrument calibration over the full field of view. Even then vertical flows within supergranules are barely detectable. Correlation and feature tracking have proven useful for estimating transverse velocity using granules and other tracers. Nevertheless, they can be degraded by the intensity variations of p-modes and possibly other oscillatory motions, as well as by effects of limb darkening and foreshortening. The two methods would both be strengthened through detailed comparisons. Data collected by MDI/SOHO is ideal for this purpose. The data is co-spatial and co-temporal, and is all obtained through the same instrument. We compare Doppler velocities with those obtained through correlation tracking using high-resolution MDI/SOHO images. We focus on motions at positions exceeding 30 degrees from disk center. After taking projection effects into account, we combine the two measurements to form a three-dimensional picture of the flows in the average supergranule. This work was supported by NASA Grant NAG5-3077 at Stanford and Lockheed Martin, and by AFOSR and the Fellows Program of AF Phillips Lab at NSO/SP. Title: Dynamics of the Chromospheric Network: Mobility, Dispersal, and Diffusion Coefficients Authors: Schrijver, Carolus J.; Shine, Richard A.; Hagenaar, Hermance J.; Hurlburt, Neal E.; Title, Alan M.; Strous, Louis H.; Jefferies, Stuart M.; Jones, Andrew R.; Harvey, John W.; Duvall, Thomas L., Jr. Bibcode: 1996ApJ...468..921S Altcode: Understanding the physics behind the dispersal of photo spheric magnetic flux is crucial to studies of magnetoconvection, dynamos, and stellar atmospheric activity. The rate of flux dispersal is often quantified by a diffusion coefficient, D. Published values of D differ by more than a factor of 2, which is more than the uncertainties allow. We propose that the discrepancies between the published values for D are the result of a correlation between the mobility and flux content of concentrations of magnetic flux. This conclusion is based on measurements of displacement velocities of Ca II K mottles using an uninterrupted 2 day sequence of filtergrams obtained at the South Pole near cycle minimum. We transform the Ca II K intensity to an equivalent magnetic flux density through a power-law relationship defined by a comparison with a nearly simultaneously observed magnetogram. One result is that, wherever the network is clearly defined in the filtergrams, the displacement vectors of the mottles are preferentially aligned with the network, suggesting that network-aligned motions are more important to field dispersal than deformation of the network pattern by cell evolution. The rms value of the inferred velocities, R = <|v|2>½, decreases with increasing flux, Φ, contained in the mottles, from R ≍ 240 m s-1 down to 140 s-1. The value of R(Φ) appears to be independent of the flux surrounding the concentration, to the extreme that it does not matter whether the concentration is in a plage or in the network. The determination of a proper effective diffusion coefficient requires that the function R(Φ) be weighted by the number density n(Φ) of mottles that contain a total flux. We find that n(Φ) decreases exponentially with Φ and propose a model of continual random splitting and merging of concentrations of flux to explain this dependence. Traditional methods used to measure D tend to be biased toward the larger, more sluggish flux concentrations. Such methods neglect or underestimate the significant effects of the relatively large number of the more mobile, smaller concentrations. We argue that the effective diffusion coefficient for the dispersal of photo spheric magnetic flux is ∼600 km2 s-1. Title: Observation of the excitation of solar oscillations Authors: Goode, Philip R.; Strous, Louis H. Bibcode: 1996BASI...24..223G Altcode: No abstract at ADS Title: Preliminary SOI/MDI Observations of Surface Flows by Correlation Tracking in the Quiet Solar Photosphere and an Emerging Active Region Authors: Tarbell, T.; Frank, Z.; Hurlburt, N.; Saba, J.; Schrijver, C.; Shine, R.; Title, A.; Simon, G.; Strous, L. Bibcode: 1996AAS...188.6914T Altcode: 1996BAAS...28..937T The extended observation of the solar surface with frequent sampling provided by MDI on SoHO offers the chance to observe the evolution of supergranules and to measure surface flows associated with active regions and perhaps larger scale zonal and meridonal flows. We have used local correlation tracking of the granulation pattern for measuring surface flows from MDI high resolution continuum images. The datasets consist of 1024 x 1024 pixel images collected with a cadence of one minute and extending many hours each. The images are typically centered upon the central meridian of the sun and offset to the north of sun center, spanning roughly 40 degrees of solar longitude and from approximately -10 to +30 degrees of solar latitude. The latitude dependence of the differential rotation is evident. We present preliminary results of our search for signatures of mesogranules, supergranules and giant cells. On 23 Feb. 1996, we obtained a 12-hour continuous sequence including quiet sun near disk center and NOAA region 7946 at about N08 E30. The active region grew rapidly over this interval, forming several sunpots. We show preliminary comparisons of the measured flow fields with coaligned SOI/MDI magnetograms taken at 15-minute intervals. The SOI/MDI program is supported by NASA grant NAG5-3077. Title: Phenomena in an emerging active region. I. Horizontal dynamics. Authors: Strous, L. H.; Scharmer, G.; Tarbell, T. D.; Title, A. M.; Zwaan, C. Bibcode: 1996A&A...306..947S Altcode: Horizontal dynamics in observations of NOAA AR 5617 are studied by tracking individual elements through the field of view. Small magnetic elements of both magnetic polarities occur everywhere in the active region, and define unipolar thread-like concentrations of magnetic field of up to 15Mm length. The horizontal granular flow field in the active region is divergent (e-time scale 2.1hours) and clockwise (time scale 32hours). Facular elements are tracers of (clumps of) fluxtubes. A hierarchy of movement of magnetic elements appears: Facular elements everywhere in the active region move obliquely toward the edges of the active region of the same polarity as their own, faster than those edges (as defined by strings of pores) move apart. The pores move along the edges toward the major sunspots of their own polarity, and the major sunspots of either polarity move apart. The separation velocity of both polarities of facular elements is about 0.84km/s, of pores about 0.73km/s, and that of the major sunspots is about 0.50km/s. Title: Dark Lanes in Granulation and the Excitation of Solar Oscillations Authors: Rimmele, T. R.; Goode, P. R.; Strous, L. H.; Stebbins, R. T. Bibcode: 1995ESASP.376b.329R Altcode: 1995help.confP.329R; 1995soho....2..329R No abstract at ADS Title: Warning: Local Correlation Tracking may BE Dangerous to your (scientific) Health Authors: Simon, G. W.; Brandt, P. N.; November, L. J.; Shine, R. A.; Strous, L. H. Bibcode: 1995ESASP.376b.223S Altcode: 1995soho....2..223S; 1995help.confP.223S No abstract at ADS Title: Feature Tracking: Deriving Horizontal Motion and More Authors: Strous, L. H. Bibcode: 1995ESASP.376b.213S Altcode: 1995help.confP.213S; 1995soho....2..213S No abstract at ADS Title: Comparison of Flow Maps Derived by Various Techniques Authors: Strous, L. H. Bibcode: 1995ESASP.376b.219S Altcode: 1995help.confP.219S; 1995soho....2..219S No abstract at ADS Title: Dynamics in Solar Active Regions: Patterns in Magnetic-Flux Emergence Authors: Strous, L. H. Bibcode: 1994PhDT.......347S Altcode: No abstract at ADS Title: Dynamics of small magnetic elements in a growing active region Authors: Strous, L. H. Bibcode: 1994ASIC..433...73S Altcode: No abstract at ADS Title: Predictions of infrared recombination lines in stellar winds of hot stars Authors: Strous, L. H.; Lamers, H. J. G. L. M. Bibcode: 1989ESASP.290..385S Altcode: 1989isa..book..385S Infrared recombination lines can provide accurate estimates for the velocity and density structure of stellar winds and thus of mass loss. The authors use a simple stellar wind model, hydrogenic approximations and scaling laws to derive information about recombination lines in the ISO range (1 - 100 μm). They estimate that, for a 40000K star, the boundary-level from above which recombination lines originate is in the range n = 2 - 10 for the most abundant elements. The authors give a simple formula to estimate the population of levels above the boundary-level and use this to produce a table of predictions of fluxes and equivalent widths of recombination lines for H, He, C III-IV, N III-V and O III-V.