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Hudson, H. S.

Publications and source records attributed to Hudson, H. S..

At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

The solar-flare infrared continuum - Observational techniques and upper limits

Exploratory observations at 20 microns and 350 microns have determined detection thresholds for solar flares in these wavelengths. In the 20-micron range, solar atmospheric fluctuations (the 'temperature field') set the basic limits on flare detectability at about 5 K; at 350 microns, extinction in the earth's atmosphere provides the basic limitation of about 30 K. These thresholds are low enough for the successful detection of several infrared-emitting components of large flares. The upper limits obtained for subflares indicate that the thickness of the H-alpha flare region does not exceed approximately 10 km. This result confirms the conclusion of Suemoto and Hiei (1959) regarding the small effective thickness of the H-alpha-emitting regions in solar flares.

Hudson, H. S.↗
NASA NTRS1975

The solar-flare infrared continuum

Potential sources of infrared (1 micron to 1 mm) continuum in solar flares are considered. Several mechanisms should produce detectable fluxes: in the 350 micron window for ground-based observations, impulsive emission will arise in synchrotron radiation from 1-10 MeV electrons, and possibly thermal (free-free) continuum from the source of the white-light flare; the hot flare plasma responsible for soft X-ray emission will also emit detectable fluxes of free-free continuum in the largest flares. At shorter wavelengths the dominant infrared emission will come from the H alpha flare itself. Observations in the infrared wavelengths will help to complete the picture of flare structure in both the impulsive and gradual phases.

Ohki, K.↗
NASA NTRS1975

The peculiar object HD 44179 /'The red rectangle'/

A strong infrared source detected in the AFCRL sky survey is confirmed, and is identified with the binary star HD 44179, embedded in a peculiar nebula. UBVRI and broad-band photometry between 2.2 and 27 microns are combined with blue, red, and near-infrared spectra, polarimetry and spectrophotometry of the star, and a range of direct and image-tube photographs of the nebula, to suggest a composite model of the system. In this model, the infrared radiation derives from thermal emission by dust grains contained in a disklike geometry about the central object, which appears to be of spectral type B9-A0 III and which may be in pre-main-sequence evolution. Two infrared emission features are found, peaking at 8.7 and 11.3 microns, the latter corresponding to the feature seen in the spectrum of the planetary nebula NGC 7027. The complex nebular structure is discussed on the basis of photographs through narrow-band continuum and emission-line filters. The polarization data support the suggestion of a disk containing some large particles. No radio continuum emission is detected.

Cohen, M.↗
NASA NTRS1974

Submillimeter observations of planets

A new program of ground-based observations at submillimeter (about 400-micron) wavelengths has yielded observations of Mercury, Venus, Mars, Jupiter, and Saturn. We report here observations near planetary conjunctions, which have minimal corrections for atmospheric extinction: Mercury, 361 + or - 65 K; Venus, 231 + or - 35 K; and Saturn, 205 + or - 15 K (based upon the area of the planetary disk), using Jupiter (150 K) and Mars (220 K) as photometric standards. The Mercury observations show that the brightness temperature does not decrease at the submillimeter wavelengths, relative to observations at 3 mm; for Venus, however, the brightness temperature appears appreciably lower than at millimeter wavelengths. The results for Saturn indicate a strong and possibly optically thick contribution from the rings.

Hudson, H. S.↗
NASA NTRS1974

OSO-7 observations of solar X-rays in the energy range 10-100 keV

Data on 123 hard X-ray bursts observed by the satellite OSO-7 between Oct. 10, 1971 and June 6, 1972 are described and evaluated. Typical duration of a burst is 100 sec. Average spectral indices lie between 3.5 and 5.5 for two-thirds of the 123 bursts, with a median of 4.6. In some events a soft-hard-soft pattern is observed, but there are numerous examples in which the spectrum softens continuously throughout the burst. The mean shape of the hard X-ray time profile as measured by the full width at half maximum does not depend on burst amplitude; nor does the spectral hardness correlate with the flux. The distribution of burst peak fluxes and the observation of large soft X-ray bursts without accompanying hard X rays suggest the existence of a distinct class of solar flares which emit only soft X rays. No center-to-limb variation was found in the frequency of occurrence of bursts or in the fraction with a nonthermal component. Estimates of the energy in the form of nonthermal electrons and in the flare plasma derived from these data indicate that the total amounts in each are comparable.

Datlowe, D. W.↗
NASA NTRS1974

Submillimeter observations of the Orion Nebula and NGC 2024

New observations at submillimeter wavelengths of the Orion Nebula and NGC 2024 are shown to provide a rather detailed map of the Orion Nebula at these wavelengths. The Orion Nebula emission is attributed to optically thin thermal radiation from dust grains in the molecular cloud with peak emission centered in the Kleinmann-Low nebula source.

Soifer, B. T.↗
NASA NTRS1974

Observations of solar X-ray bursts in the energy range 5-15 keV

Bursts of solar X-rays in the energy range 5-15 keV are associated with flares and are due to thermal emission from a hot coronal plasma. The results of the first study of a large sample of separate bursts, 197 events associated with subflares, and of a few events of importance 1 are presented. The observations were made by a proportional counter on the satellite OSO-7 from October, 1971 to June, 1972. In most cases, the temperature characterizing the X-ray spectrum rises impulsively at the onset of the burst and then declines slowly throughout the remainder of the burst. The emission measure rises exponentially with a time scale of 30-100 sec and then declines slowly on a time scale of the order of 1,000 sec. It is shown that the growth of the thermal energy in the flare plasma throughout the burst can be due to the heating of new cool material.

Datlowe, D. W.↗
NASA NTRS1974

Direct observation of temperature amplitude of solar 300-second oscillations

Intensity variations in the Rayleigh-Jeans portion of the solar continuum spectrum, longward of 1 micron, permit direct observations of the temperature structure of the solar atmosphere. A table indicates the source heights in the HSRA model atmosphere (Gingerich et al. 1971) for several of the infrared windows available for ground-based observations. The existence of 300-sec oscillations in the infrared continuum provides a powerful new tool for the exploration of this phenomenon: variation with altitude, spatial structure, relative phase of temperatures and velocity variations, etc. The initial observations at 20 micron are reported.

Hudson, H. S.↗
NASA NTRS1973

Nonthermal X-rays and related processes

The hard X-rays associated with solar flares represent the bremsstrahlung of nonthermal electron streams. In the initial phase of the flare, the X-rays probably come from a dense region in the chromosphere, as a consequence of precipitation from the lower corona. A gradual nonthermal X-ray burst may occur in a later phase, probably caused by the electrons responsible for the type 4 burst. The bombardment of the chromosphere during the early phase of the flare releases a large amount of energy there. This energy can support many of the observed flash-phase phenomena. We therefore consider this picture of flare energetics: The energy initially goes into fast particles. These interact with the chromosphere, which expands to form the hot region observed in the corona. This hot region then provides energy for many of the main-phase flare phenomena, such as H-alpha emission. The bulk of the energy in the bombardment must reside in the electron component. A clear test of the whole picture lies in its prediction of a strong infrared excess during the flash phase.

Hudson, H. S.↗
NASA NTRS1972

Long-term temporal variations of the hard X-ray flux from the Centaurus region.

An X-ray telescope aboard OSO 3 observed the Centaurus region daily from October 1967 to February 1968, and also for five days in June 1968. For this period, we derive a stable minimum flux of 0.33 plus or minus 0.33 photons per sq cm per sec between 7.7 and 38 keV from a persistent hard X-ray source around 305 deg longitude. Several single days show enhanced fluxes; and two extensive flaring episodes, one with a soft and the other with a very hard spectrum, last at least 10 days.

Schwartz, D. A.↗
NASA NTRS1972

Upper limits to the X-ray luminosities of five supernovae.

Examination of data from the OSO-III X-ray telescope for evidence of X-ray emission from five optically detected extragalactic supernovae during the period from March 1967 to June 1968. Upper limits to the X-ray emission in the range from 7.7 to 113 keV near optical maximum (within 30 days) fall in the range from 10 to the minus 8th to 10 to the minus 10th ergs/sq cm-sec. Reasonable estimates of the distances to these supernovae lead to upper limits on the total energies of from 10 to the 50th to 10 to the 51st ergs.

Ulmer, M.↗
NASA NTRS1971

Long term temporal variations of the hard X-ray flux from the Centaurus region

The X-ray telescope aboard the third Orbiting Solar Observatory (OSO-3) observed the Centaurus region daily from 1967 October to 1968 February, and also for five days in 1968 June. A stable minimum flux of 0.33 + or - 0.03 photons (sq cm sec)/1 between 7.7 and 38 keV from a source around l = 305 deg is derived. Several single days show enhanced fluxes, and two extensive flaring episodes, one with a soft and the other a very hard spectrum, lasting at least ten days.

Schwartz, D. A.↗