Search NASA⌕ Search

SEARCH · Search NASA

Results for “CORONA”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

'Coronae' of rotating interstellar clouds

This letter considers differential rotation of cool interstellar clouds in the presence of internal magnetic fields, and shows that because of the relative ineffectiveness of field dissipation within the clouds, magnetized gas experiences buoyant forces. The resulting field loops emerge from the cloud and dissipate their energy by field reconnection. The consequent heating is sufficient to produce relatively hot (T approximately 10,000 K) 'coronae' about the clouds.

Rosner, R.↗
NASA NTRS1980

The minimum flux corona; theory or concept

The reply to the criticisms of the minimum flux theory is discussed. These criticisms are correct in substance, as well as in detail. Counter arguments that the minimum flux corona theory is untenable, because of errors in its formulation, are presented.

Underwood, J. H.↗
NASA NTRS1979

Chromospheres and coronae of late-type stars

IUE short wavelength (1150-2000 A) spectra of late-type dwarfs, giant, and supergiant stars show a variety of emission features varying in excitation from about 10,000 K to about 3 x 10 to the 5th K. High excitation species are found most strongly in main sequence stars. Dwarf stars with active chromospheres (e.g., Xi Boo) and flare stars (EQ Peg) show enhancement of surface flux as compared to the quiet sun; binary systems of W UMa and RS CVn types show even larger surface fluxes. The enhancement increases with temperature of formation much like a solar active region. The presence of high excitation species in the giant and supergiant stars suggests that parameters other than effective temperature and luminosity are important in establishing a transition region and corona.

Dupree, A. K.↗
NASA NTRS1980

The oxygen polar corona

At high-latitude regions, energetic oxygen atoms are produced by a nonthermal mechanism associated with precipitated energetic atomic oxygen ions during disturbed periods. The upward moving nonthermal atoms can redistribute themselves at higher altitudes. For the storm of December 17, 1971, assuming the energetic O(+) ions are uniformly precipitated between 50 deg and 70 deg magnetic latitudes, the nonthermal oxygen atoms become dominant over the ambient at about 700 km on the dayside and at about 600 km on the nightside at 60 deg magnetic latitude. Globally, a high-number-density atomic oxygen corona appears between 50 deg and 70 deg magnetic latitudes above the exobase, and the maximum is located at a few degrees poleward from the center of the band (63 deg magnetic latitude).

Yee, J. H.↗
NASA NTRS1980

Evidence for hot gaseous coronae around the Magellanic Clouds

High-dispersion IUE far-UV spectra were obtained for five stars in the LMC and two stars in the SMC. At Magellanic Cloud velocities, all the spectra exhibit very strong interstellar lines of Al III, Si IV, and C IV, in addition to the usual lower ionization stage lines associated with H I region absorption. In two stars interstellar N V is also detected at the 50 mA level. From a consideration of the strength, width, and radial velocity of these high ionization stage lines, it is argued that regions of hot halo gas have probably been detected in front of the Magellanic Clouds. The regions detected exhibit absorption characteristics very similar to that of the Milky Way corona described previously by Savage and de Boer (1979). A cloud detected in the line of sight to HD 5980 in the SMC at a radial velocity of 300 km/s is of unknown origin.

De Boer, K. S.↗
NASA NTRS1980

Is the galactic corona produced by galactic flares

The effect of the differential rotation of the disk of the galaxy on magnetic field which penetrates the disk is considered. The magnetic field will be progressively distorted from a potential (current-free) form and will at some stage become unstable. We expect, from knowledge of solar flares, that an MHD instability, a resistive instability, or a combination of the two, will result in the release of the excess magnetic energy, and that part of the released energy will be converted into heat. By estimating the energy release and the rate at which this process will occur and by assuming that this energy input is balanced by radiation, we obtain estimates of the parameters of the resulting plasma. It appears that this process alone can heat a galactic corona to temperatures of order one-million K.

Sturrock, P. A.↗
NASA NTRS1980

Detection of a corona of fast oxygen atoms during solar maximum

A series of twilight interferometric observations of the near infrared O(+)(2P) doublets at 7320 and 7330 A between April 1979 and October 1979 detected excessive amounts of emission at shadow heights above 550 km. The scale height deduced from the vertical brightness profile determined from data taken on September 26, 1979, when the 10.7-cm flux was 231, showed a marked increase above 550 km. The equivalent temperature was estimated to be 4000 K or higher. Observations of the emission line shape with the high-resolution capability of the interferometer substantiated these results by a very apparent progressive broadening of the emission profile between times of low and high shadow height. It is concluded from these results that there exists an atomic oxygen corona overlying the thermosphere during the solar maximum period.

Yee, J. H.↗
NASA NTRS1980

Solar corona electron density distribution

The paper discusses the three and one-half months of single-frequency time delay data which were acquired from the Helios 2 spacecraft around the time of its solar occultation. The excess time delay due to integrated effect of free electrons along the signal's ray path could be separated and modeled following the determination of the spacecraft trajectory. An average solar corona and equatorial electron density profile during solar minimum were deduced from the time delay measurements acquired within 5-60 solar radii of the sun. As a point of reference at 10 solar radii from the sun, an average electron density was 4500 el/cu cm. However, an asymmetry was found in the electron density as the ray path moved from the west to east solar limb. This may be related to the fact that during entry into occultation the heliographic latitude of the ray path was about 6 deg, while during exit it was 7 deg. The Helios density model is compared with similar models deduced from different experimental techniques.

Esposito, P. B.↗
NASA NTRS1980

The coronae of 40 Eridani

High-resolution X-ray and radio images of the 40 Eridani triple star system have been obtained. The X-ray image, obtained with the Einstein Observatory, shows a corona of approximately solar intensity at the position of the K1 V star; the dM4e flare star is a strong X-ray source estimated at 2 x 10 to the 28th ergs/s. The hot white dwarf 40 Eridani B shows no X-ray signal down to a level of 8 x 10 to the 26th ergs/s. A VLA 4885 MHz radio image shows no evidence of signal from any of the three stars, down to the 0.95 mJy level.

Cash, W.↗
NASA NTRS1981

The C I opacity and physical structure of cool, very dense plasma in the solar corona

The physical structure of the cool material in the volume of the solar corona is investigated. The observational evidence is summarized in a table giving the brightness contrast in optical and EUV radiations. The state of ionization and the opacity sources are discussed, and a physical model is described that is consistent with the EUV and optical data. It is noted that a comparison of the EUV raster pictures with H-alpha and Ca K photoheliograms indicates that the material is commonly injected from below into relatively low-lying magnetic loops (fibrils) seen near sunspots and plages.

Foukal, P.↗
NASA NTRS1981

On the relationship between coronae and mass loss in late-type stars

High-dispersion spectra taken with the IUE are used to examine the temperature stratification of the wind of the hybrid atmosphere star Alpha TrA (K4 II). C IV emission is shown to have a line width of 150-200 km/s, with Si III and C III exhibiting 100 km/s widths. It is suggested that the line widths reflect wind expansion, with C IV formed in an extended region flowing near the wind terminal velocity of 85 km/s. It is shown that the observed line widths can be accounted for by the stellar wind theory of Hartmann and MacGregor, in which the schematic dissipation of Alfven waves drives the outflow in addition to heating the wind. The observations of the warm expanding corona in Alpha TrA suggest a continuous progression between the high temperature low-mass-flux solar wind and the winds of luminous cool stars.

Hartmann, L.↗
NASA NTRS1979

Heating of chromospheres and coronae in cool stars

Recent evidence is summarized that suggests that neither of two traditional views of chromospheric heating (that the flux of energy required to heat the solar chromosphere) is only a small fraction of the total radiative energy emerging from the solar interior and that chromospheres are heated by acoustic waves generated in the convection zone (which lies close to the surface of the star) is valid. Particular attention is given to cool stars, both dwarfs and giants. The directions in which research is currently heading in attempting to understand heating of chromospheres and coronae in these stars are indicated. Hot stars are excluded from consideration because radiation pressure plays an important role in their atmospheric heating. It is concluded that the role of magnetic fields in giants is different from the role in dwarfs.

Mullan, D. J.↗
NASA NTRS1981

Kinematical analysis of flare spray ejecta observed in the corona

Observations in H-alpha, soft X-ray, white light and radio wavelengths are used to track cool and hot material from limb de-occultation to the extent of six solar radii. The kinematics and thermodynamics of the internal material are determined, along with the overall mass and energy budget of the event. It is found that the majority of the mass and energy is linked with the coronal material, but at least 20% of the ejected mass originated as near-surface prominence material, leading to the conclusions that the upper part of the prominence was being heated to coronal temperatures as it rose through the corona. Evidence was found that a moving type IV burst, indicative of strong magnetic fields, was associated with the upper part of the prominence, and the observations are judged to be most consistent with magnetic propulsion models of coronal transients.

Webb, D. F.↗
NASA NTRS1982

Collisional damping of surface waves in the solar corona

The damping of surface waves by viscosity and heat conduction is evaluated. For the solar corona, it is found that surface waves dissipate efficiently only if their periods are shorter than a few tens of seconds and only if the background magnetic field is less than about 10 Gauss. Heating of quiet coronal regions is possible if the coronal waves have short periods, but they cannot heat regions of strong magnetic field, such as coronal active region loops.

Gordon, B. E.↗
NASA NTRS1982

Chromospheres and coronae in the T Tauri stars

The Tauri stars exhibit strong far ultraviolet emission lines of C II, III, IV; OI; Si II, III; IV; and sometimes N V and He II. Surface fluxes of the lines for several T Tauri stars were computed, drawing upon both International Ultraviolet Explorer observations and published spectra. The surface fluxes are quite high. The FUV lines together account for 0.1% of the stellar luminosity. These results indicate the presence of active relatively dense chromospheres. The Tauri stars showing very strong visual emission spectra exhibit weakened high temperature FUV lines of C IV and N V, as well as He II which may be produced by X-rays. In the same stars, no X-ray emission was detected with Einstein. Among all the T Tauri stars the X-ray luminosities are deficient by factors of 100 to 1000 compared to the FUV lines. It is argued that the X-ray flux has not been heavily absorbed by circumstellar gas, as has been previously suggested, but that the X-rays are truly underluminous, perhaps due to the stellar wind. It is suggested that the birth and development of the chromosphere and corona may be occurring during the T Tauri stage of protostellar evolution.

Imhoff, C. L.↗
NASA NTRS1981

Possible detection of far-ultraviolet line emission from a hot galactic corona

The presence of an emission-line component to the radiation field is suggested by rocket observations, at low resolution, of the spectrum of the diffuse, far-ultraviolet background near the north galactic pole. Removal of the line emission leaves a residual uniform cosmic ultraviolet background radiation of only 150 + or - 50 photons/sq cm s sr A, or about half that previously reported. The lines, which are at the wavelengths of the collisionally-excited atom emissions that have been predicted to arise from a hot galactic corona, suggest that there is no need to explain observed high-ionization states at high galactic latitudes as being due to photoionization caused by neutrino decay.

Feldman, P. D.↗
NASA NTRS1981

The smothered coronae of T Tauri stars

Einstein IPC observations of 10 fields containing pre-main-sequence stars are presented. The detection of significant soft X-ray flux from eight cataloged T Tauri stars plus the optical identification of two new T Tauri-like stars associated with X-ray sources are reported. It is demonstrated than an inverse correlation between the H-alpha equivalent width and the detection of X-ray flux exists. This is interpreted in terms of a model wherein the X-ray emission arises in a corona similar to that of active late-type stars, which is small compared to the extent of the circumstellar envelope. It is postulated that the lack of correlation of X-ray flux (or H-alpha) with A(v) is due to a lack of grains in the envelopes of the more active T Tauri stars.

Walter, F. M.↗
NASA NTRS1981

First detection of nonflare microwave emissions from the coronae of single late-type dwarf stars

Results are presented of a search for nonflare microwave radiation from the coronae of nearby late-type dwarf stars comparable to the sun: single stars without evidence for either a large wind or circumstellar envelope. The observing program consisted of flux measurements of six stars over a 24-h period with the VLA in the C configuration at a wavelength of 6 cm with 50 MHz bandwidth. Positive detections at 6 cm were made for Chi 1 Ori (0.6 mJy) and the flare star UV Cet (1.55 mJy), and upper limits were obtained for the stars Pi 1 UMa, Xi Boo A, 70 Oph A and Epsilon Eri. It is suggested that Chi 1 Ori, and possibly UV Cet, represent the first detected members of a new class of radio sources which are driven by gyroresonance emission, i.e. cyclotron emission from nonrelativistic Maxwellian electrons.

Gary, D. E.↗