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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6
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NASA NTRS1986

A study of solar filaments from high resolution microwave observations

The Very Large Array was used to observe several solar filaments at 1.5 and 5 GHz. The maximum temperature depressions appear to be associated with H-alpha filaments. Comparison with He 10,830 A spectroheliogram shows that 20 cm temperature depressions correspond to the regions of reduced intensity in the He 10,830 A around filaments, which correspond to coronal cavities. The temperature and density structure of the transition sheath between the filament and the surrounding corona was studied assuming that the energy radiated away is balanced by the energy conducted from the corona. It is found that the observations can be better explained by a model having a pressure gradient in the transition sheath around the filament.

Kundu, M. R.↗
NASA NTRS1987

Meterwave observations of a coronal hole

Meter-wave maps are presented showing a coronal hole at 30.9, 50.0, and 73.8 MHz using the Clark Lake Radioheliograph in October 1984. The coronal hole seen against the disk at all three frequencies shows interesting similarities to, and significant differences from its optical signatures in HeI lambda10830 spectroheliograms. The 73.8 MHz coronal hole, when seen near disk center, appears to coincide with the HeI footprint of the hole. At the lower frequencies, the emission comes from higher levels of the corona, and the hole appears to be displaced, probably due to the non-radial structure of the coronal hole. The contrast of the hole relative to the quiet Sun is much greater than reported previously for a coronal hole observed at 80 MHz. The higher contrast is certainly real, due to the superior dynamic range, sensitivity, and calibration of the Clark Lake instrument. Using a coronal hole model, the electron density is derived from radio observations of the brightness temperature. A very large discrepancy is found between the derived density and that determined from Skylab EUV observations of coronal holes. This discrepancy suggests that much of the physics of coronal holes has yet to be elucidated.

Wang, Z.↗
NASA NTRS1988

Coronal bright points at 6 and 20 centimeter wavelengths

The paper presents the results of near-simultaneous observations of bright points at 6 and 20 cm for a time interval of 10 hr using the VLA. It is shown that, at both 6 and 20 cm, the radio sources in the quiet sun have a very close association with bipolar features seen in magnetograms and with dark features in He I 10830 A spectroheliograms. The paper discusses the associations and the distributions of size, brightness, and temporal variation of the microwave bright points. It is found that the brightness temperatures of the 6 and 20 cm bright points are in the approximate ratio 0.09:1 expected from the emission mechanism being thermal bremsstrahlung.

Kundu, M. R.↗
NASA NTRS1988

The Lyman-alpha line in various solar features. I - Observations

Lyman-alpha line observations of the solar atmosphere obtained by the SMM ultraviolet spectrometer and polarimeter are presented. High spectral and spatial resolution data and broadband spectroheliograms show that, on the disk, the central reversal of the line is highly variable in depth and is frequently shifted, leading to asymmetric profiles. It is suggested that a dynamic layer overlying the limb may be responsible for distorting the background profile of the line and producing the observed asymmetric profiles. Narrow and extended prominences reaching velocities of several hundred km/sec are also noted.

Fontenla, J.↗
NASA NTRS1988

C 4 fluxes from the sun as a star and the correlation with magnetic flux

A total of 144 C 4 wavelength 1548 SMM-UVSP spectroheliograms of solar plages were analyzed, some of which are series of exposures of the same region on the same day. Also analyzed were C 4 wavelength 1551 rasters of plages and C 4 1548 rasters of the quiet sun. The sample contains data on 17 different plages, observed on 50 different days. The center-to-limb variations of the active regions show that the optical thickness effects in the C 4 wavelength 1548 line can be neglected in the conversion from intensity to flux density. As expected for the nearly optically thin situation, the C 4 1548 line is twice as bright as the C 4 wavelength 1551 line. The average C 4 wavelength 1548 flux density for a quiet is 2700 erg/cm/s and, with surprisingly little scatter, 18,000 erg/cm/s for plages. The intensity histograms of rasters obtained at disk centers can be separated into characteristic plage and quiet-sun contributions with variable relative filling factors. The disk-averaged flux density in the C 4 doublet and the disk-averaged magnitude of the magnetic flux density are related. The relationship between the C 4 and magnetic flux densities for spatially resolved data is inferred to be almost the same, with only an additional factor of order unity in the constant of proportionality.

Schrijver, C. J.↗
NASA NTRS1988

C IV fluxes from the Sun as a star, and the correlation with magnetic flux

A total of 144 C IV wavelength 1548 Solar Maximum Mission (SMM)-UVSP spectroheliograms of solar plages were analyzed, some of which are series of exposures of the same region on the same day. Also analyzed were the C IV wavelength 1551 rasters of plages and C IV wavelength 1548 rasters of the quiet sun. The sample contained data on 17 different plages, observed on 50 different days. The center-to-limb variations of the active regions show that the optical thickness effects in the C IV wavelength 1548 line can be neglected in the conversion from intensity to flux density. As expected for the nearly optically thin situation, the C IV wavelength 1548 line is twice as bright as the C IV 1551 line. The average C IV wavelength 1548 flux density for a quiet region is 2700 ergs/cm/s and, with surprisingly little scatter, 18,000 erg/cm/s for plages. The intensity histograms of rasters obtained at disk center can be separated into characteristic plage and quiet sun contributions with variable relative filling factors. The relationship between the C IV and magnetic flux densities for spatially resolved data is inferred to be almost the same, with only an additional factor of order unity in the constant of proportionality.

Schrijver, C. J.↗
NASA NTRS1988

Mass and energy flow near sunspots. I - Observations of moat properties

New measurements of the velocities of moving magnetic features (MMFs) are made from spectroheliograms and filtergrams. The sample includes 200 MMFs found in the moats surrounding 7 different sunspots. Our data are compiled with the data from other authors to uncover common properties of moats. The moat radius is roughly twice the penumbral radius. No significant correlation between the average moat velocity or angular extent and any noted property of the spot or moat (size, age, stage of development) is found. Individual MMFs move radially outward with a constant speed, but nearby MMFs may have quite different speeds. The average moat speed is twice the supergranule flow speed. The speed of MMFs in moats is equal on average to the surface gas flow speed. A large bias is found in much of the existing observations of moat speeds from MMFs.

Brickhouse, Nancy S.↗
NASA NTRS1988

Coronal bright points in microwaves

An excellent map of the quiet sun showing coronal bright points at 20-cm wavelength was produced using the VLA on February 13, 1987. The locations of bright points (BPs) were studied relative to features on the photospheric magnetogram and Ca K spectroheliogram. Most bright points appearing in the full 5-hour synthesized map are associated with small bipolar structures on the photospheric magnetogram; and the brightest part of a BP tends to lie on the boundary of a supergranulation network. The bright points exhibit rapid variations in intensity superposed on an apparently slow variation.

Kundu, M. R.↗
NASA NTRS1989

Photometric measurements of solar irradiance variations due to sunspots

A photometric telescope constructed to obtain photometric sunspot areas and deficits on a daily basis is described. Data from this Cartesian full disk telescope (CFDT) are analyzed with attention given to the period between June 4 and June 17, 1985 because of the availability of overlapping sunspot area and irradiance deficit data from high-resolution digital spectroheliograms made with the San Fernando Observatory 28 cm vacuum solar telescope and spectroheliograph. The CFDT sunspot deficits suggest a substantial irradiance contribution from faculae and active region plage.

Chapman, G. A.↗
NASA NTRS1989

X-ray bright points and He I lambda 10830 dark points

Using near-simultaneous full disk Solar X-ray images and He I 10830 lambda, spectroheliograms from three recent rocket flights, dark points identified on the He I maps were compared with X-ray bright points identified on the X-ray images. It was found that for the largest and most obvious features there is a strong correlation: most He I dark points correspond to X-ray bright points. However, about 2/3 of the X-ray bright points were not identified on the basis of the helium data alone. Once an X-ray feature is identified it is almost always possible to find an underlying dark patch of enhanced He I absorption which, however, would not a priori have been selected as a dark point. Therefore, the He I dark points, using current selection criteria, cannot be used as a one-to-one proxy for the X-ray data. He I dark points do, however, identify the locations of the stronger X-ray bright points.

Golub, L.↗
NASA NTRS1989

Correspondence between solar fine-scale structures in the corona, transition region, and lower atmosphere from collaborative observations

The Soft X-Ray Imaging Payload and the High Resolution Telescope and Spectrograph (HRTS) instrument were launched from White Sands on 11 December 1987 in coordinated sounding rocket flights to investigate the correspondence of coronal and transition region structures, especially the relationship between X-ray bright points (XBPs) and transition region small spatial scale energetic events. The coaligned data from X-ray images are presented along with maps of sites of transition region energetic events observed in C IV (100,000 K), HRTS 1600 A spectroheliograms of the T sub min region and ground based magnetogram and He I 10830 A images.

Moses, J. Daniel↗
NASA NTRS1989

X-ray bright points and He I lambda 10830 dark points

Using near-simultaneous full disk Solar X-ray images and He I 10830 lambda, spectroheliograms from three recent rocket flights, dark points identified on the He I maps were compared with x-ray bright points identified on the X-ray images. It was found that for the largest and most obvious features there is a strong correlation: most He I dark points correspond to X-ray bright points. However, about 2/3 of the X-ray bright points were not identified on the basis of the helium data alone. Once an X-ray feature is identified it is almost always possible to find an underlying dark patch of enhanced He I absorption which, however, would not a priori have been selected as a dark point. Therefore, the He I dark points, using current selection criteria, cannot be used as a one-to-one proxy for the X-ray data. He I dark points do, however, identify the locations of the stronger X-ray bright points.

Golub, L.↗
NASA NTRS1990

Relations between the photospheric magnetic field and the emission from the outer atmospheres of cool stars. II - The C IV 1550 A doublet

Solar and stellar data are used to determine the relationship between C IV 1548 + 1551 A and magnetic flux densities. C IV SMM-UVSP spectroheliograms of solar quiet regions and plages are analyzed. Typical distribution functions of C IV flux densities over both quiet and active solar regions are presented. The C IV emission is shown to be dependent on magnetic flux density.

Schrijver, C. J.↗
NASA NTRS1990

Hallmarks of the magnetic field in the solar atmosphere - Structure, evolution, heating, and flaring

Recent observations of the solar magnetic field and its effects on the solar atmosphere are discussed, with an emphasis on large-scale active regions and their implications for the fine-scale magnetic structure and for activity in the so-called quiet regions. Sample magnetograms, sunlight images, H-alpha images, X-ray images, and spectroheliograms are presented and characterized in detail, and the form and action of the magnetic field in flares are considered. It is pointed out that simultaneous observations of all levels (from the photosphere to the corona) at 100-km (about 100-marcsec) resolution are needed to see the extent of fields looping into the corona and understand their structure and activity; large space-based observatories would be required.

Moore, Ronald L.↗
NASA NTRS1991

The magnetic network location of explosive events observed in the solar transition region

Compact short-lived explosive events have been observed in solar transition region lines with the High-Resolution Telescope and Spectrograph (HRTS) flown by the Naval Research Laboratory on a series of rockets and on Spacelab 2. Data from Spacelab 2 are coaligned with a simultaneous magnetogram and near-simultaneous He I 10,380 -A spectroheliogram obtained at the National Solar Observatory at Kitt Peak. The comparison shows that the explosive events occur in the solar magnetic network lanes at the boundaries of supergranular convective cells. However, the events occur away from the larger concentrations of magnetic flux in the network, in contradiction to the observed tendency of the more energetic solar phenomena to be associated with the stronger magnetic fields.

Porter, J. G.↗
NASA NTRS1991

Weak magnetic fields and solar irradiance variations

NOAA active region 5643 was observed from August 17 to 21, 1989. Sets of video spectra-spectroheliograms including the Fe I line at 6302.5 A were made at least daily with the San Fernando Observatory 28 cm vacuum telescope and vacuum spectroheliograph. These give simultaneous, co-registered digital images representing monochromatic continuum intensity, line core intensity and line-of-sight magnetic field. Three different criteria are used to define the pixels representing the quiet sun and the facular portions of the images. These criteria are the magnetic field strength, the line core intensity, and the distribution of continuum intensities. Each of these definition schemes is used to estimate the irradiance change due to facular emission. The magnetic field and the continuum intensity distribution definitions give estimates which agree closely. The line core intensity definition leads to larger estimates of the facular irradiance contribution. Some model-dependent investigations of the contrasts and sizes of individual facular elements also are presented.

Lawrence, J. K.↗
NASA NTRS1991

Large-scale features of the sun at 20 centimeter wavelength

Results are reported from an experimental study of the characteristics of large-scale coronal structures such as active regions, plages, filaments, and coronal holes using data obtained with the VLA at 1.5 GHz during the period September 11-17, 1988. The radio data were supplemented with He 10830- A, H-alpha, and Calcium-K spectroheliograms. A statistical analysis of some of the characteristics of the active regions is performed. Most of the active region sources were found to be about 100 arcsec in size, with bridges between regions common; lower brightness temperature regions showed a higher degree of polarization in general. The maximum polarization was found at the edge of active regions but well within the associated plages. The degree of polarization from bright active regions was small (not more than 20 percent), in agreement with previous results. Evidence was found for compression of preexisting flux by the emerging flux from a new region, which took place in the apparent absence of magnetic reconnection.

Gopalswamy, N.↗
NASA NTRS1992

An EUV imaging spectrograph for high-resolution observations of the solar corona

An extreme ultraviolet (EUV) imaging spectrograph for the wavelength range from 235 to 450 A has been developed and used for resolution observations of the sun. The instrument incorporates a glancing incidence Wolter Type II Telescope and a near-normal incidence toroidal grating spectrograph to achieve near-stigmatic performance over this spectral range. The design of the spectrograph entrance aperture enables both stigmatic spectra with spectral resolution adequate to observe emission line profiles and spectroheliograms of restricted portions of the sun to be obtained concurrently. The design and performance of the instrument are described, and an overview of results obtained during a sounding rocket flight on May 5, 1989 is provided.

Neupert, Werner M.↗