Temperature Density Structure Research Articles

FTS atlas of the Sun’s spectrally resolved center-to-limb variation

The Sun's spectrum varies with center-to-limb distance. This variation is governed by the underlying temperature-density structure of the solar atmosphere. To explore the spectrally resolved center-to-limb variation (CLV) we make use of two spectral atlases recorded with the Fourier transform spectrometer (FTS) at the McMath-Pierce facility at Kitt Peak. One spectral atlas obtained 10 arcsec inside the solar limb was recorded in 1978-79 as part of the first survey of the Second Solar Spectrum, while the other atlas is the well used reference NSO/Kitt Peak FTS atlas for the disk center. Both atlases represent fully resolved spectra without any spectral stray light. We then construct an atlas of the limb/disk-center ratio between the two spectra over the wavelength range 4084-9950 \AA. This ratio spectrum, which expresses the CLV amplitude relative to the continuum, is as richly structured as the intensity spectrum itself, but the line profiles differ greatly in both shape and amplitude. It is as if we are dealing with a new, unfamiliar spectrum of the Sun, distinctly different from both the intensity spectrum (which we here refer to with the acronym SS1) and the linear polarization of the Second Solar Spectrum (for which we use acronym SS2). In analogy we refer to the new ratio spectrum as SS3. While there is hardly any resemblance between SS3 and SS2, we are able to identify a non-linear mapping that can translate SS1 to SS3 in the case of weak to medium-strong spectral lines that are mainly formed in LTE (being directly coupled to the local temperature-density structure). This non-linear mapping is successfully modeled in terms of two free parameters that are found to vary approximately linearly over the entire wavelength range covered. These parameters and the various SS3 line profiles provide a novel, rich set of observational constraints on model atmospheres.

The Atmosphere of Mira Variables: A View with theHubble Space Telescope

Ultraviolet spectra obtained with Hubble Space Telescope (HST) of two Mira-type variable stars, R Leo and R Hya, are presented, along with analysis providing information on their outer atmospheres. These high-dispersion spectra were taken with the Goddard High Resolution Spectrograph (HRS) in two spectral regions: 2320-2368 Å to record the C II] (UV0.01) multiplet and 2785-2835 Å to obtain the Mg II h and k lines. The R Hya spectrum was obtained at visual light phase 0.26 and shows a Mg II spectrum that is very clean, showing clear evidence for the overlying circumstellar absorption from Fe I (UV3) and Mn I (UV1) over the k line. The fluoresced Fe I (UV44) feature at 2824 Å is plainly visible in this spectrum, whereas past International Ultraviolet Explorer (IUE) observations of Mira variables at high dispersion were unable to record this feature. Remarkably, the newly identified fluoresced Fe I (UV45) feature near 2807 Å is seen in this spectrum. Until now, this line has been seen only in cool carbon stars with HST/HRS. This line is pumped by the thin C II] (UV0.01) emission line at 2325.5 Å. Two of the strongest C II] (UV0.01) lines near 2325 Å are plainly seen in this spectrum. This region of the spectrum, however, is dominated by the Si II] (UV0.01) line near 2335 Å, in contrast to that observed in the carbon stars and the non-Mira oxygen-rich red giant stars. Very weak Mg II lines are seen in the R Leo spectrum at phase 0.12. At this phase, these lines are typically absent in IUE spectra. Velocity shifts of emission features in the UV spectra of Mira variables are consistent with previously published hydrodynamic models of these stars. These velocities indicate, however, that the C II] (UV0.01) emission lines are not formed in the same atmospheric layers as the Mg II emission. The electron density deduced from the C II] (UV0.01) multiplet is ~109 cm-3. Finally, the temperature-density structure of the semiregular variable carbon stars is similar to the oxygen-rich Mira variables—both are hydrodynamic in nature; however, the carbon stars macroscopic velocity fields are not identical to the Mira stars in the atmosphere layers between the Mg II emission region and the circumstellar shell.

The temperature-density structure of coronal loops in hydrostatic equilibrium

The temperature and density structure are computed for a comprehensive set of coronal loops that are in hydrostatic and thermal equilibrium. The effect of gravity is to produce significant deviations from the usual uniform-pressure scaling law (T∼(pL) 1/3) when the loops are taller than a scale height. For thermally isolated loops it lowers the pressure throughout the loop, which in turn lowers the density significantly and also the temperature slightly; this modifies the above scaling law considerably. For more general loops, where the base conductive flux does not vanish, gravity lowers the summit pressure and so makes the radiation decrease by more than the heating. This in turn raises the temperature above its uniform pressure value for loops of moderate length but lowers it for longer loops. A divergence in loop cross-section increases the summit temperature by typically a factor of 2, and decreases the density, while an increase in loop height (for constant loop length) changes the temperature very little but can halve the density. One feature of the results is a lack of equilibrium when the loop pressure becomes too large. This may explain the presence of cool cores in loops which originally had temperatures below 2 × 106 K. Loops hotter than 2 × 106 K are not expected to develop cool cores because the pressure necessary to produce non-equilibrium is larger than observed.

The solar wind and the temperature-density structure of the solar corona

The influence of the solar wind on large-scale temperature and density distributions in the lower corona is studied. This influence is most profoundly felt through its effect upon the geometry of coronal magnetic fields since the presence of expansion divides the corona into magnetically ‘open’ and ‘closed’ regions. Each of these regions is governed by entirely different energy transport processes. This results in significant temperature differences since only the open field regions suffer outward conductive heat losses. Because the temperature influences the density in an exponential manner, large density inhomogeneities are to be expected.

A model for the polar transition layer and corona for November 1967

A model for the chromospheric-coronal transition layer and lower corona has been constructed for the south polar region. EUV observations acquired by the Harvard OSO-4 experiment in the fall of 1967 were used in the analysis. The observations can be explained with a simple model consisting of two types of regions. One region has a temperature-density structure similar to that in models developed for typical equatorial quiet areas. The other region has a corona in which the temperature and density are a factor of about 2 lower and the chromospheric-coronal temperature gradient is less steep by a factor of 4.

Temperature-Density Structure in Coronal Helmets: the Quiescent Prominence and Coronal Cavity

view Abstract Citations (39) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Temperature-Density Structure in Coronal Helmets: the Quiescent Prominence and Coronal Cavity Pneuman, G. W. Abstract Physical processes which are most important in determining temperature and density distributions in closed coronal loop systems are studied within the context of how this type of magnetic geometry influences the local balance of mechanical energy flux, conductive flux, and radiative losses. For regions above the temperature maximum, there can be no net conductive loss since closed field lines prevent outward beat conduction and the vanishing temperature gradient at the level of the temperature maximum prohibits downward heat conduction there. Hence, the mechanical energy input at the base must ultimately be radiated away. This requirement, in addition to a condition on the lateral distribution of gas pressure at the level of T , is used in a computation which yields the density and temperature distribution throughout the helmet as a function of the mechanical energy input and magnetic field geometry. It is demonstrated that the quiescent prominences normally observed at the base of coronal helmets as well as the low-density cavity surrounding these structures are the natural consequences of the adjustment of density and temperature structui e in the helmet required to satisfy energy and mass balance conditions as modulated by the magnetic field. Publication: The Astrophysical Journal Pub Date: November 1972 DOI: 10.1086/151756 Bibcode: 1972ApJ...177..793P full text sources ADS |

The Lower Solar Corona: Interpretation of the Ultraviolet Spectrum.

An analysis of the resonance lines of nine elements (27 ions) formed in the chromosphere and corona of the sun yields the following results: the chemical composition of this region of the sun can be determined, without any knowledge of the detailed temperature-density structure in this region; a further clue concerning the detailed structure of this region may be obtained. A prediction of the expected emission of the sun in radio frequencies can be used both as a check on the correctness of the analysis and as a means of obtaining the ratio of the observed elements to hydrogen. (auth)