Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “CHROMOSPHERE”

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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS2023

Modeling the Chromosphere and Transition Region of Planet-hosting Star GJ 436

Ahead of upcoming space missions intending to conduct observations of low-mass stars in the ultraviolet (UV) spectral region it becomes imperative to simultaneously conduct atmospheric modeling from the UV to the visible (VIS) and near-infrared (NIR). Investigations on extended spectral regions will help to improve the overall understanding of the diversity of spectral lines arising from very different atmospheric temperature regions. Here we investigate atmosphere models with a chromosphere and transition region for the M2.5V star GJ 436, which hosts a close-in Hot Neptune. The atmosphere models are guided by observed spectral features from the UV to the VIS/NIR originating in the chromosphere and transition region of GJ 436. High-resolution observations from the Hubble Space Telescope and Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs (CARMENES) are used to obtain an appropriate model spectrum for the investigated M dwarf. We use a large set of atomic species considered in nonlocal thermodynamic equilibrium conditions within our PHOENIX model computations to approximate the physics within the low-density atmospheric regions. In order to obtain an overall match for the nonsimultaneous observations, it is necessary to apply a linear combination of two model spectra, where one of them better reproduces the UV lines while the other better represents the lines from the VIS/NIR range. This is needed to adequately handle different activity states across the observations.

stars↗
DOE OSTI2014

The impact of chromospheric activity on observed initial mass functions

Using recently established empirical calibrations for the impact of chromospheric activity on the radii, effective temperatures, and estimated masses of active low-mass stars and brown dwarfs, we reassess the shape of the initial mass function (IMF) across the stellar/substellar boundary in the Upper Sco star-forming region (age ∼ 5-10 Myr). We adjust the observed effective temperatures to warmer values using the observed strength of the chromospheric Hα emission, and redetermine the estimated masses of objects using pre-main-sequence evolutionary tracks in the H-R diagram. The effect of the activity-adjusted temperatures is to shift the objects to higher masses by 3%-100%. While the slope of the resulting IMF at substellar masses is not strongly changed, the peak of the IMF does shift from ≈0.06 to ≈0.11 M {sub ☉}. Moreover, for objects with masses ≲ 0.2 M {sub ☉}, the ratio of brown dwarfs to stars changes from ∼80% to ∼33%. These results suggest that activity corrections are essential for studies of the substellar mass function, if the masses are estimated from spectral types or from effective temperatures.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

General Model for Light Curves of Chromospherically Active Binary Stars

The starspots on the surface of many chromospherically active binary stars concentrate on long-lived active longitudes separated by 180°. Shifts in activity between these two longitudes, the “flip-flop” events, have been observed in single stars like FK Comae and binary stars like σ Geminorum. Recently, interferometry has revealed that ellipticity may at least partly explain the flip-flop events in σ Geminorum. This idea was supported by the double-peaked shape of the long-term mean light curve of this star. Here we show that the long-term mean light curves of 14 chromospherically active binaries follow a general model that explains the connection between orbital motion, changes in starspot distribution, ellipticity, and flip-flop events. Surface differential rotation is probably weak in these stars, because the interference of two constant period waves may explain the observed light curve changes. These two constant periods are the active longitude period (P{sub act}) and the orbital period (P{sub orb}). We also show how to apply the same model to single stars, where only the value of P {sub act} is known. Finally, we present a tentative interference hypothesis about the origin of magnetic fields in all spectral types of stars.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

Supergranulation-driven Alfven waves in the solar chromosphere and related phenomena.

It has recently been recognized that Alfven waves frequently dominate the microstructure of the solar wind at the orbit of the earth. We seek a solar source for these waves, and consider here their excitation by the supergranular motions. The wave equation is solved in a horizontally stratified, bi-exponential solar atmosphere. The interaction of Alfven wave motions associated with adjacent supergranules is discussed qualitatively. The Alfven wave effectively conveys the supergranular motions to great heights in the chromosphere. These motions are oppositely directed above intersupergranule boundaries, and compress the magnetic field there. A naive calculation of the compression, based on balancing dynamic and magnetic pressures, leads to adequate agreement with observations of the chromospheric network. We find that the magnetic field is appreciably compressed only below about 1500 km, and on this basis we reject theories of spicule formation which require large vertical magnetic fields at the heights reached by spicules. We advance a theory for spicule formation, in which spicules form as a result of matter being squeezed upward, out of the compression region between adjacent supergranules.

Hollweg, J. V.↗
NASA NTRS1972

EUV observations of the chromospheric network.

Extreme ultraviolet observations of a quiet region of the sun on Aug. 18, 1969, with the Harvard spectroheliometer on OSO 6 indicate that the chromospheric network can be observed in lines of the chromosphere and transition region (T = 840,000 K) with almost identical structure. At coronal heights, the network changes but some residual structure can still be discerned in Mg X and perhaps Si XII, although there is little or no evidence remaining in Fe XVI.

Reeves, E. M.↗
NASA NTRS1973

Theoretical understanding of chromospheric inhomogeneities

Detailed theoretical studies of chromospheric inhomogeneities consider dynamics as well as radiative transfer of mass flow as a consequence of energy deposition. It is shown that pressure is exerted by the heating waves, especially in inhomogeneous structures, where they can be defracted. A dynamical model is formulated that depicts the inhomogeneous structure of the chromosphere-corona transition region through mass flow regimes.

Delache, P.↗
NASA NTRS1973

New observations of the solar ultraviolet chromosphere.

We present some of the results of a rocket flight which obtained a stigmatic spectrum of the sun in the region 1190 to 1320A. The experiment achieved a spectral resolution of 0.01A throughout this range, and the effective angular resolution was about 20 sec. Lines which are formed in the chromosphere and transition zone show strong fluctuations with position on the disk. The correspondence between the H Lyman-alpha profile and chromospheric details seen in the Ca K-line is demonstrated.

Bruner, E. C., Jr.↗
NASA NTRS1973

Solar rotation in the chromosphere and corona

Extreme ultraviolet spectroheliograms in Mg X (625 A) and the Lyman continuum (897 A) obtained from OSO-6 are used to determine the differential rotation rate in the solar chromosphere and corona. The equatorial rotation rate agrees with spectroscopic measurements of the photospheric plasma velocity; the variation of rate with latitude is less pronounced than in most other determinations. We cannot discern a variation in the rotation rate between the chromosphere and corona.

Henze, W., Jr.↗
NASA NTRS1974

The galloping chromosphere

Oscillating velocity fields can be observed on H-alpha filtergrams as a shifting pattern of intensity fluctuations known as 'the galloping chromosphere'. The characteristics of this activity are those of horizontal running waves of typical period of about 300 sec and long wavelength (about 20,000 km) that can be interpreted as acoustic-gravity waves propagating in the acoustic domain. Periods are longer in dark, structured regions, and in fibrils, and the change is quantitatively consistent with the reduction of resonance frequency in a magnetic field of 1 to 10 gauss. These easily observed fluctuations thus offer a means of estimating magnetic-field strength at specific locations in the chromosphere. Phase velocities are high, ranging upward from typical values between 50 and 100 km per sec, and tending to be lower in active regions and toward the limb.

Sawyer, C.↗
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

On spatial variations in the intensity of chromospheric H-alpha

Investigation of the formation of patterns in H-alpha spectroheliograms and filtergrams. Introducing a source-sink-control diagram, it is concluded that the H-alpha line source function in the quiet solar chromosphere is indirectly controlled by the photospheric radiation fields in the Balmer and Paschen continua. It is demonstrated that in producing the observed patterns, horizontal spatial variations in the shape of the absorption profile are extremely effective compared to changes in the source and sink terms. Applying this mechanism, asymptotic values are computed for the contrasts and visibilities in chromospheric H-alpha.

Gebbie, K. B.↗
NASA NTRS1975

Slow X-ray bursts and chromospheric flares with filament disruption

The data from OGO-5 and OSO-7 X-ray experiments have been analyzed to study six chromospheric flares with filament disruption associated with slow thermal X-ray bursts. Filament activation accompanied by a slight X-ray enhancement precedes the first evidence of H alpha flare by a few minutes. Rapid increase of the soft X-ray flux is accompanied by a sudden brightening of the filament when viewed on-band H alpha. Thereafter the bright chromospheric strands reach their maximum brightness with maximum X-ray flux. Any plateau or slow decay phase in the X-ray flux is accompanied by a quieting in filament activity and even by filament re-appearance. The height of the disrupted prominence is proportional to the soft X-ray flux for the August 3, 1970 limb occulted event.

Roy, J. R.↗
NASA NTRS1975

Magnetic fields and dense chromospheres in dMe stars

The hypothesis is investigated that dense chromospheres of dMe stars are heated by dissipation of hydromagnetic waves which may be generated in active regions where the nonspot magnetic field strength can be as large as 5 to 10 kG. It is proposed that dMe stars are a set of magnetic stars on the lower main sequence which have strong fields generated by dynamo action in deep convective envelopes, while dM stars are nonmagnetic or weakly magnetic stars having no starspots on their surfaces. The combination of magnetic fields and dense chromospheres in dMe stars is shown to provide consistent evidence for several conclusions, including: (1) the dMe stars which are most likely to be flare stars are those with hydrogen emission lines and (2) propagation of flare-initiated coronal waves can trigger sympathetic stellar flares. It is suggested that grain formation occurs in starspots of dMe stars and that such grains in a circumstellar shell are responsible for the systematic IR excesses of dMe stars relative to dM stars.

Mullan, D. J.↗
NASA NTRS1975

Flare model chromospheres and photospheres

Homogeneous plane-parallel model atmospheres for solar flares have been constructed to approximately simulate observations of flares. The wings of the Ca II lines have been used to derive flare upper photosphere models, which indicate temperature increases of about 100 K over the temperature distribution in the pre-existing facula at a height of 300 km. In the case of flares covering sunspots the temperature rise seems to occur much higher in the atmosphere. We find that with increasing flare importance the heights of the upper chromosphere and transition region occur lower in the solar atmosphere, accounting for the factor of 60-600 increase in pressure in these regions relative to the quiet sun. The Ca II line profiles agree with observations only by assuming a macrovelocity distribution that increases with height. Also, the chromospheric parts of flares appear to be highly inhomogeneous. We show that shock and particle heated flare models do not agree with the observations and propose a thermal response model for flares. In particular, it appears that heating in the photosphere is an essential aspect of flares.

Machado, M. E.↗
NASA NTRS1976

The EUV chromospheric network in the quiet sun

Investigations on the structure and intensity of the chromospheric network from quiet solar regions have been carried out with EUV data obtained with a spectroheliometer on the Apollo Telescope Mount of Skylab. The distribution of intensities within supergranulation cell interiors follows a near-normal function, where the standard deviation exceeds the value expected from the counting rate, which indicates fine-scale structure below the 5 arcsec resolution of the data. The intensities from the centers of supergranulation cells appear to be the same in both quiet regions and coronal holes, although the network is significantly different in the two types of regions. The average halfwidth of the network elements was measured as 10 arcsec and was independent of the temperature of formation of the observing line for effective-temperature logarithms (log Te) from 3.8 to 5.8. The contrast between the network and the centers of cells is greatest for lines with log Te about equal to 5.2, where the network contributes approximately 75% of the intensity of quiet solar regions. The contrast and fractional-intensity contributions decrease to higher and lower temperatures characteristic of the corona and chromosphere.

Reeves, E. M.↗
NASA NTRS1976

Stellar model chromospheres. V - Alpha Centauri A /G2 V/ and Alpha Centauri B /K1 V/

Models for the upper photospheres and lower chromospheres of Alpha Centauri A and B are derived from high-dispersion spectrograms of the Ca II K-line emission cores and damping wings. Effective temperatures, surface gravities, and ages consistent with the measured broadband colors, metallicities, and absolute magnitudes are estimated for the two stars. The spectrograms are calibrated by fitting the far-wing K profiles with synthetic fluxes based on radiative-equilibrium models, and the model atmospheres are obtained through a partial-redistribution analysis of the line cores and damping wings. These models are compared with those previously proposed for Procyon, Arcturus, and the sun. Many features in all the models are found to be quite similar, and some evidence suggests that Alpha Cen A may be significantly older and more evolved than the sun, even though both stars exhibit similar chromospheric properties.

Ayres, T. R.↗