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Evidence for stellar chromospheres presented by ultraviolet observations of the sun and stars
Observations of emission lines in stellar sources in the ultraviolet region of the spectrum show P Cygni-like evidence of mass outflow as well as many ultraviolet emission lines which cannot be interpreted wholly in terms of geometric effects. These are considered powerful chromospheric indicators.
Mechanical heating in stellar chromospheres using the sun as a test case
The shock dissipation hypothesis for solar atmospheric heating is extended to stellar chromospheres of late type stars which have significant convective envelopes. Principle wave modes are studied for their generation, propagation, and heating properties in the presence of a magnetic field, and various approximations on when a sound wave becomes strong enough to produce heating in the atmosphere are presented.
Chromospheric activity and stellar evolution
A study of stellar chromospheres based on the internal structure of particular stars is presented. Used are complex flow diagrams of the linkage paths between mass loss, angular momentum loss, magnetic field from the turbulent dynamo and its relations to differential rotations and the convection zone, and stellar evolution.
The absence of flares in 3835 A and the heating of the chromosphere.
Simultaneous observations of flares in H-alpha and a band 15 A wide centered on 3835 A show no change whatever in 3835 A at the time of several flares, although the chromospheric network is easily visible. Flares are therefore transparent in this wavelength. Since the flare represents a test heat input from above, and no brightening of the network appears, we conclude it is impossible for the corona to produce the bright network by downward convection.
Further aspects of weak shock theory applied to the solar chromosphere.
The low chromosphere now seems definitely to require mechanical heating, and dissipation of initially acoustic waves by shocking is one of the most promising possibilities. Results of recent calculations indicate that the weak shock theory may be applicable in this case, but discrepancies exist among various applications of this theory, and the explanations offered to date are not completely satisfactory. It is shown that the different approximations used by different authors to evaluate the mechanical flux integral play an important role in producing these discrepancies, in addition to the already well-known effects of the density scale heights and the wave periods. Arguments are presented favoring Ulmschneider's (1970, 1971) method for evaluation of this flux integral.
A short-lived chromospheric flare-point with a lifetime of 20 seconds and rise and fall times of 5 seconds
We have observed a chromospheric brightening in the H alpha and Ca II K lines with a diameter of about 1 arc second. The time structure of this event, obtained with a relative resolution of 1 second, shows the rise time to be 4 seconds, the lifetime (FWHM) to be 20 seconds, and the decay time to be 5 seconds. This imposes new constraints on flare-point models. These restrictions can be accommodated easily by either an infall-impact flare model or a model invoking the precipitation of high-energy particles from the corona.
Line profiles and turbulence generated by acoustic waves in the solar chromosphere. II - Contours of the Ca II and Mg II K lines
Making use of the time-averaged absorption profiles derived by Oster and Ulmschneider, non-LTE line formation in the context of a two-level atom is investigated for an isothermal atmosphere and for the Ca II and Mg II K lines in the solar chromosphere as represented by the Harvard-Smithsonian Reference Atmosphere. Source functions and emergent line profiles are computed for a variety of assumptions concerning the acoustically broadened profiles and the solar velocity fields.
A three-component concept of the chromosphere and transition region
It is proposed that present observations of the chromosphere and transition region in EUV, optical and mm wavelengths are best described by a three-component concept. The three components are taken to be the interiors of supergranular cells, the hot plagettes overlying faculae, and the cooler, transient mottles which surround them in the network boundaries. The enhanced emission of the hot plagettes in transition ions is interpreted as a direct result of the increased pressure scale height over faculae relative to the cell interiors.
Height of helium emission in the chromosphere
Filtergrams of the limb show the He I D3 chromosphere as a shell which is separated from the limb by a gap. The height of maximum D3 contribution occurs at about 1350 km above the limb and is independent of the intensity of the D3 emission. We interpret this effect as the height to which coronal EUV radiation is capable of penetrating the atmosphere.
The calculation of theoretical chromospheric models and predicted OSO 1 spectra
Theoretical solar chromospheric and photospheric models are computed for use in analyzing OSO 8 spectra. The Vernazza, Avrett, and Loeser (1976) solar model is updated and self-consistent non-LTE number densities for H I, He I, He II, C I, Mg I, Al I, Si I, and H(-) are produced. These number densities are used in the calculation of a theoretical solar spectrum from 90 to 250 nm, including approximately 7000 lines in non-LTE. More than 60,000 lines of other elements are treated with approximate source functions.
Stellar model chromospheres. IV - The formation of the H-epsilon feature in the sun /G2 V/ and Arcturus /K2 III/
The formation of the Balmer-series member H-epsilon in the near-red wing of the Ca II H line is discussed for two cases: the sun (H-epsilon absorption profile) and Arcturus (H-epsilon emission profile). It is shown that although the H-epsilon source functions in both stars are dominated by the Balmer-continuum radiation field through photoionizations, the line-formation problems in the two stars are quantitatively different, owing to a substantial difference in the relative importance of the stellar chromosphere temperature inversion as compared with the stellar photosphere.
Structure of the solar chromosphere. II - The underlying photosphere and temperature-minimum region
The paper presents a non-LTE empirical model of the quiet solar photosphere and the temperature-minimum region. The continuous spectrum computed from this model is in good overall agreement with available disk-center observations throughout the wavelength range from 0.125 to 500 microns. It is found that (1) absolute-intensity measurements are needed in the range between 1 and 2 microns to establish the structure of the deepest observable layers; (2) absolute-intensity or flux measurements are needed in the range between 20 and 200 microns to determine whether the minimum solar temperature occurring between the photosphere and the chromosphere is as low as indicated by present observations or much higher, as recent theoretical predictions indicate; (3) studies of the far-ultraviolet spectrum based on the assumption of LTE can be substantially in error; and (4) line opacity seems to account for the 'missing opacity' in the ultraviolet.
Heating of the solar chromosphere and corona. I - Generalized inhomogeneous wave equation for magnetoacoustic motions
The generalized inhomogeneous wave equation that governs magnetoacoustic, vortical, and thermal motions in compressible fluids and that is applicable to the problem of heating of the solar chromosphere and corona is obtained. The effects of kinematic and bulk viscosity, heat conduction, Joule dissipation, and magnetic diffusivity are included. Under the usual assumptions, the generalized wave equation reduces to the well-known equations of Lighthill, Kulsrud, Phillips, and others. The major problems encountered in applying Lighthill's (1952) mechanism to sound generation in turbulent media are reviewed for both the subsonic and supersonic cases.
Properties of the chromosphere-corona transition region in Capella
Analysis of recent ultraviolet observations of the Capella binary system (Alpha Aur) indicates a dense geometrically narrow chromosphere-corona transition region in the Capella system primary (G5 III) similar in many respects to a solar active region. An examination of the coronal energy balance, together with the coronal base pressure derived from the line fluxes, predicts a corona with a mean temperature of 1.2 million K and a large stellar wind consistent with observations.
Association of X-ray arches with chromospheric neutral lines
Daily maps of magnetic neutral lines derived from H-alpha observations have been superimposed on solar X-ray images for the period from June 15 to 30, 1973. Nearly all X-ray-emitting structures consist of systems of arches covering chromospheric neutral lines. Areas of low emissivity, coronal holes, appear as the areas between arcades of arches. The presence of a coronal hole, therefore, is determined by the spacing between neutral lines and the scale of the arches over those neutral lines. X-ray emissivity on the solar disk extends from neutral lines in proportion to the vertical and horizontal scale of the arches over those neutral lines. Increasing scale of arches corresponds with increasing age of magnetic fields associated with the neutral line. All X-ray filament cavities coincided with neutral lines, but filaments appeared under cavities for only part of their length and for only a fraction of the disk passage.
The prominence-corona interface compared with the chromosphere-corona transition region
The intensities of 52 optically thin EUV emission lines formed at temperatures of 350,000 to 2.2 million K in nine hedgerow prominences observed at the limb are compared with the intensities of the same lines formed within network cells at the center of the solar disk in order to compare the prominence-corona interface (PC) with the chromosphere-corona transition region (CC). It is found for all nine prominences that the ratio of the intensity of a line measured in a cell to that in a prominence decreases with increasing temperature approximately as the -0.6 power of temperature. This ratio is used as the basis for comparing the PC with the CC in the framework of two different geometries wherein the prominence consists of one or more identical fully resolved slabs or threads in the line of sight or contains one or more identical unresolved cylindrical threads. It is concluded that three effects may contribute to the systematic difference between the PC and the CC: (1) the pressure within the PC might increase outward; (2) the temperature gradient within the PC might increase more slowly with temperature than in the CC; and (3) the unresolved internal geometry of a prominence can directly explain some, but not all, of the systematic difference.
Chromospheric limb spectra from Skylab - 2000 to 3200 A
Chromospheric limb spectra of a quiet-sun region between 2000 and 3200 A recorded by the normal-incidence spectrograph on Skylab are discussed. The spectral resolution is 0.12 A, and the projected slit area on the sun is 2 by 60 arcsec. A list of lines with wavelengths, identifications, and absolute intensities is given for the spectrum recorded at +4 arcsec outside the white-light limb. The intensity behavior outside the limb is shown for lines of the ions C II, Si II, Cr II, Mn II, Fe II, Fe III, Co II, and Ni II. The widths of the intersystem lines of Si II and C II increase monotonically with height above the limb. The full width at half-maximum of the Si II lines increases from 0.034 A at the limb to 0.27 A at +12 arcsec above the limb. The widths of the C II lines increase from 0.17 A at +2 arcsec to 0.31 A at +12 arcsec.