Engineering topics

Thompson, W. T.

Publications and source records attributed to Thompson, W. T..

17 records

NASA NTRS2013

Alternating Twist Along an Erupting Prominence

Triangulation measurements using observations from the two Solar Terrestrial Relations Observatory (STEREO) spacecraft, combined with observations from the Solar Dynamics Observatory (SDO), are used to characterize the behavior of a prominence involved in two successive coronal mass ejections 6-7 December 2010. The STEREO separation at the time was 171.6 degrees, which was functionally equivalent to a separation of 8.4 degrees, and thus very favorable for feature co-identification above the limb. The first eruption at approximately 14:16 UT (Universal Time) on 6 December of the middle branch of the prominence starts off a series of magnetic reconfigurations in the right branch, which itself erupts at approximately 2:06 UT the next day, about 12 hours after the first eruption. The cool prominence material seen at 304 angstroms drains back down to the surface, but a flux-rope-like magnetic structure is seen to erupt in both 195 angstroms by the STEREO/Extreme Ultraviolet Imager (EUVI), and in white light by the STEREO/COR1 (Coronagraph 1) inner coronagraph. In between the two eruptions, two different signs of helicity are seen in the measured twist of the right branch. This is interpreted to be caused by the overall prominence channel being composed of different segments with alternating helicity signs. The erupting parts on 6 and 7 December both show positive twist, but negative twist is seen in between these positive sections. Negative twist is consistent with the dextral chirality signs seen in the He II line at 304 angstroms prior to both eruptions. However, during the period between the eruptions, a region of positive twist grows and replaces the region of negative twist, and finally erupts. We interpret these observations in the light of models that predict that helicity cancellation can be an important factor in the triggering of flares and coronal mass ejections.

Prominences↗

NASA NTRS2012

A Parametric Study of Erupting Flux Rope Rotation: Modeling the 'Cartwheel CME' on 9 April 2008

The rotation of erupting filaments in the solar corona is addressed through a parametric simulation study of unstable, rotating flux ropes in bipolar force-free initial equilibrium. The Lorentz force due to the external shear-field component and the relaxation of tension in the twisted field are the major contributors to the rotation in this model, while reconnection with the ambient field is of minor importance, due to the field's simple structure. In the low-beta corona, the rotation is not guided by the changing orientation of the vertical field component's polarity inversion line with height. The model yields strong initial rotations which saturate in the corona and differ qualitatively from the profile of rotation vs. height obtained in a recent simulation of an eruption without preexisting flux rope. Both major mechanisms writhe the flux rope axis, converting part of the initial twist helicity, and produce rotation profiles which, to a large part, are very similar within a range of shear-twist combinations. A difference lies in the tendency of twist-driven rotation to saturate at lower heights than shear-driven rotation. For parameters characteristic of the source regions of erupting filaments and coronal mass ejections, the shear field is found to be the dominant origin of rotations in the corona and to be required if the rotation reaches angles of order 90 degrees and higher; it dominates even if the twist exceeds the threshold of the helical kink instability. The contributions by shear and twist to the total rotation can be disentangled in the analysis of observations if the rotation and rise profiles are simultaneously compared with model calculations. The resulting twist estimate allows one to judge whether the helical kink instability occurred. This is demonstrated for the erupting prominence in the "Cartwheel CME" on 9 April 2008, which has shown a rotation of approximately 115 deg. up to a height of 1.5 Solar R above the photosphere. Out of a range of initial equilibria which include strongly kink-unstable (Phi = 5 pi), weakly kink-unstable (Phi = 3.5 pi), and kink-stable (Phi = 2.5 pi) configurations, only the evolution of the weakly kink-unstable flux rope matches the observations in their entirety.

initiation and propagation↗

NASA NTRS2012

The Height of a White-Light Flare and its Hard X-Ray Sources

We describe observations of a white-light (WL) flare (SOL2011-02-24T07:35:00, M3.5) close to the limb of the Sun, from which we obtain estimates of the heights of the optical continuum sources and those of the associated hard X-ray (HXR) sources. For this purpose, we use HXR images from the Reuven Ramaty High Energy Spectroscopic Imager and optical images at 6173 Ang. from the Solar Dynamics Observatory.We find that the centroids of the impulsive-phase emissions in WL and HXRs (30 -80 keV) match closely in central distance (angular displacement from Sun center), within uncertainties of order 0".2. This directly implies a common source height for these radiations, strengthening the connection between visible flare continuum formation and the accelerated electrons. We also estimate the absolute heights of these emissions as vertical distances from Sun center. Such a direct estimation has not been done previously, to our knowledge. Using a simultaneous 195 Ang. image from the Solar-Terrestrial RElations Observatory spacecraft to identify the heliographic coordinates of the flare footpoints, we determine mean heights above the photosphere (as normally defined; tau = 1 at 5000 Ang.) of 305 +/- 170 km and 195 +/- 70 km, respectively, for the centroids of the HXR and WL footpoint sources of the flare. These heights are unexpectedly low in the atmosphere, and are consistent with the expected locations of tau = 1 for the 6173 Ang and the approx 40 keV photons observed, respectively.

Sun: photosphere↗

NASA NTRS2011

3D Reconstruction of a Rotating Erupting Prominence

A bright prominence associated with a coronal mass ejection (CME) was seen erupting from the Sun on 9 April 2008. This prominence was tracked by both the Solar Terrestrial Relations Observatory (STEREO) EUVI and COR1 telescopes, and was seen to rotate about the line of sight a it erupted; therefore, the event has been nicknamed the "Cartwheel CME." The threads of the prominence in the core of the CME quite clearly indicate the structure of a weakly to moderately twisted flux rope throughout the field of view, up to heliocentric heights of 4 solar radii. Although the STEREO separation was 48 deg, it was possible to match some sharp features in the later part of the eruption as seen in the 304 A line in EUVI and in the H-alpha-sensitive bandpass of COR I by both STEREO Ahead and Behind. These features could then be traced out in three-dimensional space, and reprojected into a view in which the eruption is directed toward the observer. The reconstructed view shows that the alignment of the prominence to the vertical axis rotates as it rises up to a leading-edge height of approximately equal to 2.5 solar radii, and then remains approximately constant. The alignment at 2.5 solar radii differs by about 115 deg from the original filament orientation inferred from H-alpha and EUV data, and the height profile of the rotation, obtained here for the first time, shows that two thirds of the total rotation are reached within approximately equal to 0.5 solar radii above the photosphere. These features are well reproduced by numerical simulations of an unstable moderately twisted flux rope embedded in external flux with a relatively strong shear field component.

Thompson, W. T.↗

NASA NTRS2011

STEREO SECCHI COR1-A/B Intercalibration at 180 deg Separation

The twin Solar Terrestrial Relations Observatory (STEREO) spacecraft reached a separation angle of 180 deg on 6 February 2011. This provided a unique opportunity to test the intercalibration between the Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI) telescopes on both spacecraft for areas above the limb. So long as the corona is optically thin, at 180 deg separation each spacecraft sees the same corona from opposite directions. Thus, the data should appear as mirror images of each other. We report here on the results of the comparison of the images taken by the inner coronagraph (COR1) on the STEREO-Ahead and -Behind spacecraft in the hours when the separation was close to 180 deg. We find that the intensity values seen by the two telescopes agree with each other to a high degree of accuracy. This validates both the radiometric intercalibration between the COR1 telescopes, and the method used to remove instrumental background from the images. The relative error between COR1-A and COR1-B is found to be less than 10-9 B/B over most of the field-of-view, growing to a few 10-9 B/B for the brighter pixels near the edge of the occulter. The primary source of error is the background determination. We also report on the analysis of star observations which show that the absolute radiometric calibration of either COR1 telescope has not changed significantly since launch.

Thompson, W. T.↗

NASA NTRS2011

3D Reconstruction of a Rotating Erupting Prominence

A bright prominence associated with a coronal mass ejection (CME) was seen erupting from the Sun on 9 April 2008. This prominence was tracked by both the Solar Terrestrial Relations Observatory (STEREO) EUVI and COR1 telescopes, and was seen to rotate about the line of sight as it erupted; therefore, the event has been nicknamed the "Cartwheel CME." The threads of the prominence in the core of the CME quite clearly indicate the structure of a weakly to moderately twisted flux rope throughout the field of view, up to heliocentric heights of 4 solar radii. Although the STEREO separation was 48 deg, it was possible to match some sharp features in the later part of the eruption as seen in the 304 Angstrom line in EUVI and in the H alpha-sensitive bandpass of COR1 by both STEREO Ahead and Behind. These features could then be traced out in three dimensional space, and reprojected into a view in which the eruption is directed towards the observer. The reconstructed view shows that the alignment of the prominence to the vertical axis rotates as it rises up to a leading-edge height of approximately equals 2.5 solar radii, and then remains approximately constant. The alignment at 2.5 solar radii differs by about 115 deg. from the original filament orientation inferred from H alpha and EUV data, and the height profile of the rotation, obtained here for the first time, shows that two thirds of the total rotation is reached within approximately equals 0.5 solar radii above the photosphere. These features are well reproduced by numerical simulations of an unstable moderately twisted flux rope embedded in external flux with a relatively strong shear field component.

Thompson, W. T.↗

NASA NTRS2011

STEREO SECCHI COR1-A/B Intercalibration at 180 deg Separation

The twin Solar Terrestrial Relations Observatory (STEREO) spacecraft reached a separation angle of 180 degrees on 6 February 2011. This provided a unique opportunity to test the intercalibration between the Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI) telescopes on both spacecraft for areas above the limb. So long as the corona is optically thin, at 180 degree separation each spacecraft sees the same corona from opposite directions. Thus, the data should appear as mirror images of each other. We report here on the results of the comparison of the images taken by the inner coronagraph (COR1) on the STEREO Ahead and Behind spacecraft in the hours when the separation was close to 180 degrees. We find that the intensity values seen by the two telescopes agree with each other to a high degree of accuracy. This validates both the radiometric intercalibration between the COR1 telescopes, and the method used to remove instrumental background from the images. The relative error between COR1-A and COR1-B is found to be less than 10(exp -9) B/B solar over most of the field-of-view, growing to a few x 10(exp -9) B/B solar for the brighter pixels near the edge of the occulter. The primary source of error is the background determination. We also report on the analysis of star observations which show that the absolute radiometric calibration of either COR1 telescope has not changed significantly since launch.

Thompson, W. T.↗

NASA NTRS2010

Stereoscopic Analysis of STEREO/SECCHI Data for CME Trajectory Determination

The Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) coronagraphs on the twin Solar TErrestrial RElations Observatory (STEREO) spacecraft provide simultaneous views of the corona and coronal mass ejections from two view points. Here, we analyze simultaneous image pairs using the technique of tie-pointing and triangulation (T&T) to determine the three-dimensional trajectory of seven coronal mass ejections (CMEs). The bright leading edge of a CME seen in coronagraph images results from line-of-sight integration through the CME front; the two STEREO coronagraphs see different apparent leading edges, leading to a systematic error in its three-dimensional reconstruction. We analyze this systematic error using a simple geometric model of a CME front. We validate the technique and analysis by comparing T&T trajectory determinations for seven CMEs with trajectories determined by Thernisien et al. (2009) using a forward modeling technique not susceptible to this systematic effect.

CORONAGRAPHS↗

NASA NTRS2010

Precision Effects for Solar Image Coordinates Within the FITS World Coordinate System

The FITS world coordinate system (WCS) provides a number of tools for precisely specifying the spatial coordinates of an image. Many of the finer details that the WCS addresses have not historically been taken into account in solar image processing. This paper examines various effects which can affect the expression of coordinates in FITS headers, to determine under what conditions such effects need to be taken into account in data analysis, and under what conditions they can be safely ignored. Effects which are examined include perspective, parallax, spherical projection, optical axis determination, speed-of-light effects, stellar aberration, gravitational deflection, and scattering and refraction at radio wavelengths. Purely instrumental effects, such as misalignment or untreated optical aberrations, are not considered. Since the value of the solar radius is an experimental quantity, the effect of adopting a specific radius value is also examined. These effects are examined in the context of a previous paper outlining a WCS standard for encoding solar coordinates in FITS files. Aspects of that previous paper are clarified and extended in the present work.

Thompson, W. T.↗

NASA NTRS1994

Evidence of mass outflow in the low corona over a large sunspot

An extreme ultraviolet (EUV) imaging spectrograph designed for sounding rocket flight has been used to search for velocity fields in the low solar corona. During a flight in May, 1989, we obtained emission line profile measurements along a chord through an active region on the Sun. Relative Doppler velocities were measured in emission lines of Mg IX, Fe XV, and Fe XVI with a sensitivity of 2-3 km/s at 350 A. The only Doppler shift appreciably greater than this level was observed in the line of Mg IX at 368.1 A over the umbra of the large sunspot. The maximum shift measured at that location corresponded to a velocity toward the observer of 14 plus or minus 3 km/s relative to the mean of measurements in that emission line made elsewhere over the active region. The magnetic field in the low corona was aligned to within 10 deg of the line of sight at the location of maximum Doppler shift. Depending on the magnetic field geometry, this mass outflow could either re-appear as a downflow of material in distant footprints of closed coronal loops or, if along open field lines, could contribute to the solar wind. The site of the sunspot was near a major photospheric magnetic field boundary. Such boundaries have been associated with low-speed solar winds as observed in interplanetary plasmas.

Neupert, W. M.↗

NASA NTRS1993

Correlation of He II Lyman alpha with He I 10830 A, and with chromospheric and EUV coronal emission

This paper describes the results of comparing Solar EUV Rocket Telescope and Spectrograph (SERTS-3) images obtained in the transition region line of He II 304 A with chromospheric He I 10830 A absorption; with strong coronal lines of Mg IX 368 A, Fe XV 284 A and 417 A, and Fe XVI 335 A and 31 A; with H-alpha; with Ca II 8542 A; and with magnetograms in Fe I 8688H-alpha. All of the images are illustrated, and the image reconstruction techniques used are described and evaluated. The high correlation of the He II 304 A and He I 10830 A images, originally found by Harvey and Sheeley (1977), is confirmed and is put on a quantitative basis. We find that the supergranulation network has greater contrast, and that filaments appear darker, in 10830 A than in 304 A. In active regions, the 304 A line follows more closely the behavior of H-alpha and Ca II 8542 A than the 10830 A line.

Thompson, W. T.↗

NASA NTRS1993

Solar coronal observations and formation of the He II 304 A line

Although a large body of recent work supports the formation of the He II 304 A resonance line by collisional excitation in the quiet sun, the formation mechanism is less clear in strong coronal active regions and flares. The 1989 May 5 flight of the Goddard Solar Extreme Ultraviolet Rocket Telescope and Spectrograph (SERTS-3) provided a data set that is well suited to addressing this question. This paper develops a method of assessment of the line formation mechanism that is based on simple non-LTE theory and is applied to these data. The results support the conclusion of other authors that the 304 A line is formed by collisional excitation in the quiet sun, but that photoionization-recombination (p-r) may play a significant role in coronal active regions, and that p-r is important, and may even be predominant, in many flares.

Jordan, S. D.↗

NASA NTRS1992

Observations of the solar corona with a high resolution EUV imaging spectrograph

An extreme ultraviolet imaging spectrograph was used to obtain coronal observations with high spectral and spatial resolution. The imaging properties of the instrument enable measurements of spectral line shapes and positions in discrete spatial elements of a region, so that the dynamic characteristics of the coronal plasma, as well as the distribution of emission measure with temperature and well known density diagnostics, can be studied for specific features. The instrumentation is described and several results of a sounding rocket flight on 5 May 1989, when the corona over NOAA Region 5464 including emission over the umbra of the region's largest sunspot and the pre-impulsive phase emission of a small flare was recorded, are summarized.

Neupert, Werner M.↗

NASA NTRS1992

A rotating tomographic imager for solar extreme-ultraviolet/soft X-ray emission

A concept is presented for a high-resolution EUV/soft-X-ray imager that has much in common with the medical imaging procedure of tomography. The resulting instrument is compatible with a simpler, less costly spin-axis-stabilized spacecraft. To demonstrate the fidelity of the reconstruction procedure, the observation and reconstruction is simulated to compare the results with the original image.

Davila, Joseph M.↗

NASA NTRS1991

Coordinated observations with high resolution spectrographs (HRTS, SERTS, MSDP)

Some of the advantages of coordinated observations with ground-based and space instrumentation in research on prominences are examined. The dynamical behavior of a prominence in transition zone lines (HRTS) and the coronal environment of a filament (SERTS) are discussed. Fe XIV and Si X line ratios were found to be important for NE diagnostic.

Schmieder, B.↗

NASA NTRS1988

Hydrogen and helium excitation by EUV radiation for the production of white-light flares

Non-LTE radiative transfer calculations for hydrogen and helium in a simple model atmosphere are used to demonstrate that EUV radiation cannot be the main energy source for white-light flares. The opacities in the Lyman continuum and the helium I and II continua are found to be much larger than the enhanced opacity in the visible hydrogen continuum. It is shown that the EUV radiation is absorbed before it can have a significant effect on the visible light continuum.

Poland, A. I.↗

NASA NTRS1981

Saturn's rings - Azimuthal variations, phase curves, and radial profiles in four colors

Four-color photographic photometry, at effective wavelengths of 5900, 5350, 4150, and 3560 A, obtained during the 1977, 1978, and 1979 Saturn apparitions, were analyzed to determine the ring brightness as a function of wavelength, solar phase angle, ring particle orbital phase (the azimuthal effect), declination of the earth relative to the ring plane (tilt effect), and radial distance from Saturn. Data were obtained using the International Planetary Patrol network when the ring tilt angle B was approximately 16.5, 11.5, and 6 deg, respectively. From the approximately 40,000 original images, 364 were scanned and digitized, along with corresponding calibration exposures. The smearing correction technique used achieved better signal-to-noise by utilizing such constraints as the known width of Cassini's Division on the major axis. The most important observational results were summarized, and included the following: the azimuthal brightness variations for the brighter portion of ring A increased as the ring tilt decreased from B = 26 deg to less than 16 deg, reaching the order of + or 20% with respect to the ansae; they are not detectable for ring B or the outer portion of ring A.

Thompson, W. T.↗