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Withbroe, G. L.

Publications and source records attributed to Withbroe, G. L..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Analysis of EUV limb brightening observations from ATM. II - Influence of spicules

A comparison of limb brightening measurements with theoretical limb brightening curves calculated for simple atmospheric models indicates that a significant fraction of the EUV emission from lines formed in the transition layer comes from spicules. We find that spicules contribute about 20% of the total quiet sun intensity for lines of the C II, C III, N III, and O VI ions. The uncertainty in the magnitude of the percentage is about a factor of 2. The EUV-emission scale heights above the limb are consistent with the assumption that the variation with height of the number of EUV-emitting spicules is the same as for H-alpha-emitting spicules. Optical depths derived for the C II 1335 A and C III 977 A lines yield estimates for the pressure in the transition layer that are in good agreement with pressures derived under the assumption that the transition layer is in pressure equilibrium with the chromosphere and corona.

Withbroe, G. L.↗
NASA NTRS1976

Extreme-ultraviolet transients observed at the solar pole

Extreme-ultraviolet (EUV) observations of two polar transient features ('macrospicules') are described. These features appear to be caused by jets of chromospheric material that shoot upward to a height of 35,000 km above the limb and then fall back into the chromosphere, reaching terminal velocities of about 140 km/s. On the basis of a model developed from the EUV measurements, it is found that the energy required to produce each event is about 3 by 10 to the 26th power ergs, about two orders of magnitude more than that required to produce an ordinary spicule. This indicates that macrospicules may be an important factor in the energy balance of the chromosphere and corona.

Withbroe, G. L.↗
NASA NTRS1976

Initial results from the EUV spectroheliometer on ATM

The Harvard College Observatory photoelectric spectroheliometer on the Apollo Telescope Mount operated correctly in orbit from May 29, 1973 to February 7, 1974. During this period, many thousands of spatial and spectral scans at extreme ultraviolet wavelengths were recorded during observations of a variety of solar features. The construction and modes of operation of the instrument are outlined, and the principal scientific results from a preliminary analysis of the data are described.

Reeves, E. M.↗
NASA NTRS1975

The analysis of XUV emission lines

A technique for analyzing measurements of XUV spectral-line intensities is described. Application of the technique to OSO-4 and OSO-6 spectra indicates that the mean coronal temperature is 2.1 million K in typical active solar regions and that the mean coronal temperature in typical quiet regions ranges from 1.5 to 2.1 million K. One active-region spectrum shows evidence for substantial quantities of coronal material with temperatures between 2 and 3.5 million K. Measurements from limb spectra show evidence that (1) coronal abundances of N and O are low relative to heavier elements; or (2) that the ionization-equilibrium calculations used may contain systematic errors; or (3) that the XUV intensity measurements may be incorrectly calibrated.

Withbroe, G. L.↗
NASA NTRS1975

Time variations in extreme-ultraviolet emission lines and the problem of coronal heating

We have analyzed the time structure of the intensity of solar chromospheric and coronal extreme-ultraviolet lines, obtained by the Harvard College Observatory spectrometer aboard Skylab. We find changes in the intensity of up to 50 percent in times as short as 1 minute, but not periodic oscillations. Some evidence is found for the presence of shock waves in the chromosphere and the transition region. It is suggested that the solar chromosphere and corona are heated by nonperiodic waves.

Vernazza, J. E.↗
NASA NTRS1975

EUV analysis of an active region

A sequence of extreme-ultraviolet (EUV) spectroheliograms of McMath region No. 10283 were obtained by OSO-6. The lines O VI (1032 A) Mg X (625 A), Si XII (499 A), and Fe XVI (335 A) were used to determine coronal temperatures and densities above the active region. A comparison of theoretical and observed line ratios yielded coronal temperatures of 2.2 to 2.3 million K above the active region and 2.0 to 2.1 million K in the surrounding area. The temperatures derived from ratios involving the O VI intensities are systematically higher than the others. This is attributed to an error in the theoretical O VI intensities. The intensities observed above the limb are compared with intensities predicted by a simple model based on cylindrical geometry. The overall agreement shows that the assumption of an isothermal corona in hydrostatic equilibrium above the active region is a resonable working hypothesis and that the adopted geometrical model for the electron density distribution is adequate.

Raghavan, N.↗
NASA NTRS1975

EUV observations of the active sun from the Harvard experiment on ATM

Some extreme UV observations of solar active regions made with a scanning spectroheliometer are described. Spectroheliograms constructed from digital data using a computer-driven cathode-ray tube display show clearly how the appearance of an active region changes as a function of temperature. Flare studies indicate that the impulsive rise in EUV emission occurs essentially simultaneously at all levels from the transition zone to the corona. Observations of sunspots reveal a very intense emission in transition zone lines. A matrix of Mg x rasters covering the entire sun reveals several hundred bright points having dimensions of 30 arc seconds or less. Other observations include coronal holes and prominences.

Noyes, R. W.↗
NASA NTRS1974

Extreme-ultraviolet observations of coronal holes - Initial results from Skylab

We compare the appearance and physical parameters of the solar chromosphere, transition zone, and corona in areas of coronal holes with that of quiet areas outside the hole. Measurements of the height of emission of various ions in a coronal hole appearing at the polar limb give a quantitative indication of the increased thickness of the transition zone underlying coronal holes.

Huber, M. C. E.↗
NASA NTRS1974

Solar prominences in the extreme ultraviolet as observed from the Apollo Telescope Mount

Observations of quiescent solar prominences with the Harvard College Observatory spectrometer aboard Skylab show that prominence material is optically thick in the Lyman alpha line and the Lyman continuum. The color temperature of the Lyman continuum has a mean of 6600 K and an upward gradient toward the top of the prominence. The departure coefficient of the ground state of hydrogen is found to be of the order of unity, as expected from theory. The optical depth of the C III sheath region is determined directly from the observation of the limb through the prominence and is used to infer the mean electron density and the temperature gradient of the sheath. The result implies that the sheath density is about 0.4, and the temperature gradient about 1.4 times the respective value in the C III transition zone of the quiet sun. The C III triplet-singlet ratio for the prominence is found to give a density compatible, within the uncertainty of the atomic parameters, with the density obtained from the optical depth.

Schmahl, E. J.↗
NASA NTRS1974

Initial results from the EUV spectroheliometer on ATM

The Harvard College Observatory photoelectric spectroheliometer on the Apollo Telescope Mount operated correctly in orbit from May 29, 1973 to Feb. 7, 1974. During this period many thousands of spatial and spectral scans at extreme ultraviolet wavelengths were recorded during observations of a variety of solar features. The construction and modes of operation of the instrument are outlined and the principal scientific results from a preliminary analysis of the data are described.

Reeves, E. M.↗
NASA NTRS1974

Observations of the chromospheric network - Initial results from the Apollo Telescope Mount

A preliminary analysis of early data taken by the HCO spectrometer on Skylab shows that the solar chromospheric network can be clearly seen with varying contrast in the extreme-ultraviolet emission characteristic of temperatures between 10,000 K (the Lyman continuum) and 300,000 K (O VI). In the emission of Mg X, a coronal line formed at about 1,500,000 K, the network is generally unrecognizable. This is interpreted as being due to a spreading of the magnetic field lines of the network boundary in the height interval corresponding to the temperature difference between 300,000 and 1,500,000 K. We note that in certain anomalous cases, bright points of the network are seen to extend with high contrast and essentially unchanged in their cross-section through the full range of temperatures characteristic of the chromosphere, transition region, and low corona.

Reeves, E. M.↗
NASA NTRS1974

A comparison of EUV spectroheliograms and photospheric magnetograms

A comparison of EUV data from an experiment on the OSO-6 satellite with photospheric magnetograms from Kitt Peak National Observatory indicates relationships between the strength of the photospheric magnetic field and density in the overlying corona in quiet and in active regions, distinctly differing from previous notions. In addition, the photospheric field beneath a coronal hole is found to be virtually identical to that beneath normal quiet regions.

Gurman, J. B.↗
NASA NTRS1973

The Harvard experiment on OSO-6 - Instrumentation, calibration, operation, and description of observations.

The Harvard experiment carried by OSO-6 was an extreme-ultraviolet (EUV) spectrometer-spectroheliometer with a wavelength range of 285 to 1385 A, a spatial and spectral bandwidth of 35 x 35(arc sec) squared and 3 A, respectively. The instrument acquired data that have been deposited with the National Space Science Data Center and World Data Center A at the Goddard Space Flight Center in Greenbelt, Maryland, and are now available in their entirety to the scientific community. Aspects of the experiment that are relevant to potential users of the data are described - namely, instrument configuration and parameters, laboratory and inflight calibrations, as well as operational capabilities and procedures. The observations obtained are reported, and the nature, number, and dates of observation, where relevant, are listed.

Huber, M. C. E.↗
NASA NTRS1972

Extreme-ultraviolet emission from solar prominences.

Spectra and spectroheliograms of prominences have been obtained at wavelengths of 300 to 1400 A from instruments aboard the OSO 4 and OSO 6 spacecraft. Quiescent prominences appear in emission above the limb for all strong lines formed at temperatures below 300,000 K, but not at higher temperatures. The gas pressure in the 100,000-K transition zone around prominences is approximately equal to that in the cooler (6500 K) central regions. The temperature and the hydrogen ground-state departure coefficient in the central regions are determined from the Lyman-continuum spectrum. Prominences on the disk (filaments) are visible in absorption in many lines, especially those at wavelengths below the hydrogen Lyman limit at 912 A.

Noyes, R. W.↗
NASA NTRS1972

Observing programs in solar physics during the 1973 ATM Skylab program.

Description of the Apollo Telescope Mount (ATM), a set of spaceborne X-ray, ultraviolet, and visible-light telescopes intended to implement the extensive series of solar observations representing one of the major tasks assigned to the three-man crews who will perform three (respectively 28-, 56-, and 56-day long) manned orbital flights in mid-1973, known collectively as the Skylab mission. The ATM capabilities, its principal scientific goals, and planned observing program are reviewed, along with the methods to be used for coordinating the observations with ground-based and other kinds of solar observations.

Reeves, E. M.↗