Engineering topics

Bogart, R. S.

Publications and source records attributed to Bogart, R. S..

14 records

NASA NTRS2012

Systematic Center-To-Limb Variation in Measured Helioseismic Travel Times and Its Effect on Inferences of Solar Interior Meridional Flows

We report on a systematic center-to-limb variation in measured helioseismic travel times, which must be taken into account for an accurate determination of solar interior meridional flows. The systematic variation, found in time-distance helioseismology analysis using SDO/HMI and SDO/AIA observations, is different in both travel-time magnitude and variation trend for different observables. It is not clear what causes this systematic effect. Subtracting the longitude-dependent east-west travel times, obtained along the equatorial area, from the latitude-dependent north-south travel times, obtained along the central meridian area, gives remarkably similar results for different observables. We suggest this as an effective procedure for removing the systematic center-to-limb variation. The subsurface meridional flows obtained from inversion of the corrected travel times are approximately 10 m s−1 slower than those obtained without removing the systematic effect. The detected center-to-limb variation may have important implications in the derivation of meridional flows in the deep interior and needs to be better understood.

Sun: oscillations↗

NASA NTRS2012

Design and Ground Calibration of the Helioseismic and Magnetic Imager (HMI) Instrument on the Solar Dynamics Observatory (SDO)

The Helioseismic and Magnetic Imager (HMI) investigation will study the solar interior using helioseismic techniques as well as the magnetic field near the solar surface. The HMI instrument is part of the Solar Dynamics Observatory (SDO) that was launched on 11 February 2010. The instrument is designed to measure the Doppler shift, intensity, and vector magnetic field at the solar photosphere using the 6173 Fe I absorption line. The instrument consists of a front-window filter, a telescope, a set of wave plates for polarimetry, an image-stabilization system, a blocking filter, a five-stage Lyot filter with one tunable element, two wide-field tunable Michelson interferometers, a pair of 4096(exo 2) pixel cameras with independent shutters, and associated electronics. Each camera takes a full-disk image roughly every 3.75 seconds giving an overall cadence of 45 seconds for the Doppler, intensity, and line-of-sight magnetic-field measurements and a slower cadence for the full vector magnetic field. This article describes the design of the HMI instrument and provides an overview of the pre-launch calibration efforts. Overviews of the investigation, details of the calibrations, data handling, and the science analysis are provided in accompanying articles.

magnetic field↗

NASA NTRS2011

The Helioseismic and Magnetic Imager (HMI) Investigation for the Solar Dynamics Observatory (SDO)

The Helioseismic and Magnetic Imager (HMI) instrument and investigation as a part of the NASA Solar Dynamics Observatory (SDO) is designed to study convection-zone dynamics and the solar dynamo, the origin and evolution of sunspots, active regions, and complexes of activity, the sources and drivers of solar magnetic activity and disturbances, links between the internal processes and dynamics of the corona and heliosphere, and precursors of solar disturbances for space-weather forecasts. A brief overview of the instrument, investigation objectives, and standard data products is presented.

Helioseismology↗

NASA NTRS2011

Time-Distance Helioseismology Data-Analysis Pipeline for Helioseismic and Magnetic Imager Onboard Solar Dynamics Observatory (SDO-HMI) and Its Initial Results

The Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO/HMI) provides continuous full-disk observations of solar oscillations. We develop a data-analysis pipeline based on the time-distance helioseismology method to measure acoustic travel times using HMI Doppler-shift observations, and infer solar interior properties by inverting these measurements. The pipeline is used for routine production of near-real-time full-disk maps of subsurface wave-speed perturbations and horizontal flow velocities for depths ranging from 0 to 20 Mm, every eight hours. In addition, Carrington synoptic maps for the subsurface properties are made from these full-disk maps. The pipeline can also be used for selected target areas and time periods. We explain details of the pipeline organization and procedures, including processing of the HMI Doppler observations, measurements of the travel times, inversions, and constructions of the full-disk and synoptic maps. Some initial results from the pipeline, including full-disk flow maps, sunspot subsurface flow fields, and the interior rotation and meridional flow speeds, are presented.

Sun: helioseismology↗

NASA NTRS2006

The Virtual Solar Observatory and the Heliophysics Meta-Virtual Observatory

The Virtual Solar Observatory (VSO) has survived its infancy and provides metadata search and data identification for measurements from 45 instrument data sets held at 12 online archives, as well as flare and coronal mass ejection (CME) event lists. Like any toddler, the VSO is good at getting into anything and everything, and is now extending its grasp to more data sets, new missions, and new access methods using its application programming interface (API). We discuss and demonstrate recent changes, including developments for STEREO and SDO, and an IDL-callable interface for the VSO API. We urge the heliophysics community to help civilize this obstreperous youngster by providing input on ways to make the VSO even more useful for system science research in its role as part of the growing cluster of Heliophysics Virtual Observatories.

Gurman, J. B.↗

NASA NTRS2005

The Virtual Solar Observatory: Still a Small Box

Two and a half years after a design study began, and a year and a half after development commenced, version 1.0 of the Virtual Solar Observatory (VSO) was released at the 2004 Fall AGU meeting. Although internal elements of the VSO have changed, the basic design has remained the same, reflecting the team's belief in the importance of a simple, robust mechanism for registering data provider holdings, initiating queries at the appropriate provider sites, aggregating the responses, allowing the user to iterate before making a final selection, and enabling the delivery of data directly from the providers. In order to make the VSO transparent, lightweight, and portable, the developers employed XML for the registry, SOAP for communication between a VSO instance and data services, and HTML for the graphic user interface (GUI's). We discuss the internal data model, the API, and user responses to various trial GUI's as typical design issues for any virtual observatory. We also discuss the role of the "small box" of data search, identification, and delivery services provided by the VSO in the larger, Sun-Solar System Connection virtual observatory (VxO) scheme.

Gurman, J. B.↗

NASA NTRS1999

The Photospheric Convection Spectrum

Spectra of the cellular photospheric flows are determined from observations acquired by the MDI instrument on the SOHO spacecraft. Spherical harmonic spectra are obtained from the full-disk observations. Fourier spectra are obtained from the high-resolution observations. The p-mode oscillation signals and instrumental artifacts are reduced by temporal filtering of the Doppler data. The resulting spectra give power (kinetic energy) per wavenumber for effective spherical harmonic degrees from 1 to over 3000. The spectra show distinct peaks representing granules and supergranules but no distinct features at wavenumbers representative of mesogranu;es or giant cells. The observed cellular patterns and spectra are well represented by a model that only includes granules and supergranules.

Hathaway, D. H.↗

NASA NTRS1997

Time-Distance Helioseismology with the MDI Instrument: Initial Results

In time-distance helioseismology, the travel time of acoustic waves is measured between various points on the solar surface. To some approximation, the waves can be considered to follow ray paths that depend only on a mean solar model, with the curvature of the ray paths being caused by the increasing sound speed with depth below the surface. The travel time is effected by various inhomogeneities along the ray path, including flows, temperature inhomogeneities, and magnetic fields. By measuring a large number of times between different locations and using an inversion method, it is possible to construct 3-dimensional maps of the subsurface inhomogeneities. The SOI/MDI experiment on SOHO has several unique capabilities for time-distance helioseismology. The great stability of the images observed without benefit of an intervening atmosphere is quite striking. It his made it possible for us to detect the travel time fo separations of points as small as 2.4 Mm in the high-resolution mode of MDI (0.6 arc sec 1/pixel). This has enabled the detection of the supergranulation flow. Coupled with the inversion technique, we can now study the 3-dimensional evolution of the flows near the solar surface.

Duvall, T. L., Jr.↗

NASA NTRS1997

Structure and Rotation of the Solar Interior: Initial Results from the MDI Medium-L Program

The medium-l program of the Michelson Doppler Imager instrument on board SOHO provides continuous observations of oscillation modes of angular degree, l, from 0 to approximately 300. The data for the program are partly processed on board because only about 3% of MDI observations can be transmitted continuously to the ground. The on-board data processing, the main component of which is Gaussian-weighted binning, has been optimized to reduce the negative influence of spatial aliasing of the high-degree oscillation modes. The data processing is completed in a data analysis pipeline at the SOI Stanford Support Center to determine the mean multiplet frequencies and splitting coefficients. The initial results show that the noise in the medium-l oscillation power spectrum is substantially lower than in ground-based measurements. This enables us to detect lower amplitude modes and, thus, to extend the range of measured mode frequencies. This is important for inferring the Sun's internal structure and rotation. The MDI observations also reveal the asymmetry of oscillation spectral lines. The line asymmetries agree with the theory of mode excitation by acoustic sources localized in the upper convective boundary layer. The sound-speed profile inferred from the mean frequencies gives evidence for a sharp variation at the edge of the energy-generating core. The results also confirm the previous finding by the GONG (Gough et al., 1996) that, in a thin layer just beneath the convection zone, helium appears to be less abundant than predicted by theory. Inverting the multiplet frequency splittings from MDI, we detect significant rotational shear in this thin layer. This layer is likely to be the place where the solar dynamo operates. In order to understand how the Sun works, it is extremely important to observe the evolution of this transition layer throughout the 11-year activity cycle.

Kosovichev, A. G.↗

NASA NTRS1988

On the feasibility of correlation tracking at moderate resolution

The measurement of photospheric surface flows via the correlation tracking of white-light intensity features at low resolution has been demonstrated using Solar Optical Universal Polarimeter data obtained from Spacelab-2. The data were artificially degraded, but still free of seeing distortion. It is suggested that apertures as small as 10-14 cm on a space-based platform can be used to measure large-scale horizontal motions.

Bogart, R. S.↗

NASA NTRS1985

Confirmation of a 152 day periodicity in the occurrence of solar flares inferred from microwave data

Evidence for a periodicity of about 155 + or - 5 days in the production of energetic solar flares was reported in 1984 by Rieger et al. and Kiplinger et al. To see whether this periodicity is a persistent phenomenon, the occurrences of flares inferred from microwave data, which are available for most of the present and previous solar cycles, were examined. Strong confirmation of a 152 day periodicity in the time interval previously studied is found, demonstrating that these flares are a useful indicator for the observed periodicity. Evidence is found for persistence of the periodicity in the previous cycle (cycle 20). In cycle 20 the periodic modulation of the flare occurrence rate was weaker than in cycle 21, but the phase has apparently remained coherent through both cycles.

Bogart, R. S.↗

NASA NTRS1982

On the distribution of orbits among long-period comets

The distribution of the axes defining the planes and orientations of the orbits of 542 long-period comets are analyzed. The directions of the perihelia and of the oriented plane normals show significant nonuniformity in their distributions. The preferred direction of perihelia near the apex of solar motion is refined to an (1 sigma) error circle of 20.6 deg, and possible concentrations of the plane normals and the directions of perihelion velocity along roughly orthogonal directions are noted, although the error circles extend to nearly full hemispheres. Planes of preference (and avoidance) are found from the distribution ellipsoids of the three orbital axes, with the perihelion directions lying preferentially along the galactic plane. The distribution ellipsoids of all three sets of orbital axes exhibit anisotropies roughly twice those expected for random distributions.

Bogart, R. S.↗

NASA NTRS1982

Recurrence of solar activity - Evidence for active longitudes

It is pointed out that the autocorrelation coefficients of the daily Wolf sunspot numbers over a period of 128 years reveal a number of interesting features of the variability of solar activity. Besides establishing periodicities for the solar rotation, solar activity cycle, and, perhaps, the 'Gleissberg Cycle', they suggest that active longitudes do exist, but with much greater strength and persistence in some solar cycles than in others. Evidence is adduced for a variation in the solar rotation period, as measured by sunspot number, of as much as two days between different solar cycles.

Bogart, R. S.↗

NASA NTRS1980

Linear modes of convection in the solar envelope

The structure of the solar convective envelope calculated according to mixing-length theory is not consistent with the transport of the convective heat flux by any superposition of the fundamental linear convective modes. If the amplitudes of these modes can be estimated by consideration of the most important nonlinear terms in the equations of motion, it becomes possible to find solar models which are consistent with mode transport of the heat through the convective zone. These models may be characterized by a mixing length which varies from point to point through the envelope. The linear modes in such models show peaks in velocity at certain preferred length scales, but these scales do not necessarily correspond to those of the solar motions, such as the supergranulation.

Bogart, R. S.↗