Quiet Solar Regions Research Articles

The O IV and S IV intercombination lines in solar and stellar ultraviolet spectra

New calculations of O IV electron density diagnostic emission-line ratios involving the 1399.8, 1401.2, 1404.8, and 14076.4 A transitions are presented. A comparison of these calculations with observational data from a quiet solar region, a sunspot, and an active region obtained with the High Resolution Telescope and Spectrograph (HRTS), two flares observed with the SO82B spectrograph on board Skylab, and Hubble Space Telescope (HST) observations by the Goddard High Resolution Spectrograph (GHRS) of Capella, gives good results using the ratio R(sub 1) = I(1407.4 A)/I(1401.2 A). However, the electron density obtained using the ratio R(sub 2) = I(1407.4 A)/I(1404.8 A) is often an order of magnitude smaller. The O IV 1404.8 A line is blended with the S IV 1404.8 A line, and we investigate whether this ratio may still be used as a density diagnostic if the S IV 1406.1 A line intensity is used to correct for the presence of S IV 1404.8 A, using previous S IV calculations by Dufton et al. We still find systematic differences compared to density determinations from line ratios that do not involve the O IV 1404.8 A line, which we suggest are due to errors in earlier theoretical calculations of the S IV atomic data, and also possibly to previously unconsidered fluorescent pumping of the upper level of the S IV 1404.8 A transition.

Modeling the Infrared Magnesium and Hydrogen Lines from Quiet and Active Solar Regions

The emission lines of Mg I at 7.4, 12.2, and 12.3 μm are now known to be formed in the upper photosphere; the line emission is due to collisional coupling of higher levels with the continuum together with radiative depopulation of lower levels. These combined effects cause the line source functions of high-lying transitions to exceed the corresponding Planck functions. However, there are uncertainties in a) the relevant atomic data, particularly the collisional rates and ultraviolet photoionization rates, and b) the sensitivity of the calculated results to changes in atmospheric temperature and density. These uncertainties are examined by comparing twelve calculated Mg I line profiles in the range 2.1-12.3 μm with ATMOS satellite observations. We show results based on different rates, and using different atmospheric models representing a range of dark and bright spatial features. The calculated Mg profiles are found to be relatively insensitive to atmospheric model changes, and to depend critically on the choice of collisional and photoionization rates. We find better agreement with the observations using collision rates from van Regemorter (1962) rather than from Seaton (1962). We also compare twelve calculated hydrogen profiles in the range 2.2-12.4 μm with ATMOS observations. The available rates and cross sections for hydrogen seem adequate to account for the observed profiles, while the calculated lines are highly sensitive to atmospheric model changes. These lines are perhaps the best available diagnostics of the temperature and density structure of the photosphere and low chromosphere. Further calculations based on these infrared hydrogen lines should lead to greatly improved models of the solar atmosphere.

Pervasive variability in the quiet solar transition region

Extreme UV spectroheliograms from an experimental time series are employed to investigate the nature of the quiet solar transition region with respect to temporal variability. A statistical treatment is developed to analyze the fraction of spatial elements that yield intensity variations significantly higher than the signal noise. The EUV intensity in every spatial resolution varies on a time scale of minutes by about 10-30 percent, and the fractional amplitude of the temporal variations is found to be nearly independent of mean intensity. The paper concludes that the quiet solar transition region is probably generated and modulated by small-scale magnetic activity, since the temporal variability in the transition region is spatially uniform and rather pervasive.

Solar SI II line ratios from the high-resolution telescope and spectrograph

Theoretical emission-line ratios involving multiplets near 1262, 1306, and 1530 A are derived on the basis of new calculations of electron-impact excitation rates for allowed transition in Si II. A comparison of these line ratios with observational data from a quiet solar region, a sunspot, and an active region, obtained with the HIRTS on board a sounding rocket flight reveals that the 1530-A multiplet is optically thick, which is consistent with a calculation of the optical depth of these lines through a model atmosphere. The 1262- and 1306-A multiplets appear to be effectively optically thin. The average discrepancy between the theoretical and observed ratios is about 40 percent, which may not be significant, since the estimated uncertainties in both the calculated and experimental data are approximately 30 percent.

Circular polarization of the CA II H and K lines in solar quiet and active regions

A representative set of profiles is presented for the Ca II H resonace line in Stokes V and I, for the quiet sun, plages, sunspot umbrae, and a flare, as well as one example of the Ca II K line in a sunspot penumbra. The degree of polarization is highest in the spots and zero in the quiet sun, within error limits. The V profile asymmetries are, however, highest in the flare. The spectra of the Ca II K line are used to obtain a linear relation between V(lambda) and -dI/d(lambda) and a value for B(parallel) of 820 + or - 40 G using the weak-field approximation. 25 refs.

A UV spectral atlas of a prominence

The ultraviolet spectrum of a large prominence has been observed with the High Resolution Telescope and Spectrograph (HRTS) on SPACELAB 2, 5 August, 1985. The spectrum covers the wavelength range λλ1335–1670 A and shows numerous emission lines from gas at chromospheric and transition region temperatures. An Atlas of the prominence spectrum has been made showing absolute calibrated intensities on an accurate wavelength scale. The Atlas includes for comparison the corresponding UV-spectrum from an average quiet solar region.

Small Scale Motions Over Concentrated Magnetic Field Regions of the Quiet Sun

Using a time sequence of filtergrams in the magnetically sensitive λ6103A CaI line (with circular polarization measurements), obtained with the SPO Vacuum Tower Telescope and the universal filter we mapped the line of sight velocity and the longitudinal magnetic field in three quiet solar regions. After elimination of the effect of the 5-minute photospheric oscillations we found in two regions of concentrated magnetic field no association with the line of sight velocity, while the third region was associated with small (<300 m/s) downflows.

Evolution of network magnetic fields of solar quiet regions

There are 4 types of evolution patterns of network magnetic fields: (1) flux cancellation, the mutual disappearance of encountering fluxes of opposite polarity, (2) flux increase by emergence of ephemeral regions, (3) flux decrease of one polarity and (4) flux increase of one polarity, without emergence of ephemeral regions. From a time sequence of magnetograms of a quiet region of 1983 October 14, the evolution of 300 network features was measured. The magnetograms have a spatial resolution of 2 to 3 arcsec and a time resolution of about 2 hr. The statistics show that the contribution to flux decrease by Type 3 is 1.28 times that by Type 1, and the contribution to flux increase by Type 4 is 7 times that by Type 2.

The Quiet Solar Transition Region

The results of recent studies (in the UV) of the structure and dynamics of the quiet solar transition region and its role in the mass and energy balance of the outer layers of the solar atmosphere are summarized. The region is one in which temperature increases from 25,000 K to about 1 million K and the matter density, fed by the outward flowing wind and the inward streaming plasma, increases by a factor of 40. The morphology of the region is described, along with the theoretical models being applied to the observed nonthermal line broadening, Doppler wavelength shifts, temporal fluctuations, and fine structure. Deficiencies in existing simple models of the region are discussed.

Emerging flux in active regions

We have compared the rates at which flux emerges in active and quiet solar regions within the sunspot belts. The emerging flux regions (EFRs) were identified by the appearance of arch filament structures in Hα. All EFRs in high-resolution films of active regions made at Big Bear in 1978 were counted. The comparable rate of flux emergence in quiet regions was obtained from SGD data and independently from EFRs detected outside the active region perimeter on the same films. The rate of flux emergence is 10 times higher in active regions than in quiet regions. A sample of all active regions in 31 days of 1983 gave a ratio of 7.5. We discuss possible mechanisms which might funnel new magnetic flux to regions of strong magnetic field.

X-Ray and EUV Emission from Optically Thin Plasma

The computation of the emission of an optically thin and low density plasma is performed at 104 < T < 108 K between 1 and 2000 Å. The computation is applied to explain the far EUV observations of Feldman et al. (1981) of interstellar medium emission towards the North galactic pole and the solar quiet region observations of Vernazza and Reeves (1978) in the spectral region from 300 to 1350 Å.

Fluid motions in the solar chromosphere-corona transition region. I - Line widths and Doppler shifts for C IV

Matrices of line profiles for C IV, lambda1548, observed with a 3'' aperture and sampled in 3'' intervals in active and quiet solar regions, show a pronounced tendency for line width to increase with increasing redshift. The correlation between incresing central intensity of the line and increasing redshift demonstrated by some authors is clearly present in these data but is weakened by the not infrequent occurrence of regions of strong redshift but with low intensity. Sunspots show both systematically narrower line profiles over umbral areas and strong redshifts on their Sun-center side. The latter result is consistent with a reverse Evershed effect.

A self-similar flow in Generalized Roche model with increasing energy

The self-similar flow of a gas, moving under the gravitational attraction of a central body of fixed mass behind a spherical shock wave driven out by a propelling contact surface into quiet solar wind region, is investigated. The total energy content between the inner expanding surface and the shock front increases with time. In the last section we briefly pose the self-similar isothermal flow of a gas behind a spherical shock wave.

Self-similar flows behind shock wave with increasing energy in magnetogasdynamics, I

Self-similar solutions for adiabatic and isothermal flows driven out by a propelling contact surface, moving into a quiet solar wind region, are investigated in the presence of magnetic field. The total energy of the flow between the shock and the contact surface is taken to be time-dependent obeying a power-law. The shock is assumed to be strong and propagating into a perfect gas at rest with non uniform density and magnetic field.

Enthalpy flux cooling of the solar corona

The differential emission measure profile for quiet and flaring solar regions is considered, using a model in which the principal downflow of heat is due to the enthalpy of downward-flowing material, rather than conduction. It is found that the emission measure profile for quiet solar regions is matched well by a downward particle number flux which decreases with temperature. This would be expected if this particle flux is due to heated spicular material falling back onto the chromosphere. In flaring regions, however, a particle flux which increases with temperature is required to explain the steep emission measure profile. This could be a result of mass motions downward out of the flaring loops.

Ultraviolet continuum absorption /less than about 1000 A/ above the quiet sun transition region

view Abstract Citations (20) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Ultraviolet continuum absorption /less than about 1000 A/ above the quiet sun transition region Doschek, G. A. ; Feldman, U. Abstract Lyman continuum absorption shortward of 912 A in the quiet sun solar transition region is investigated by combining spectra obtained from the Apollo Telescope Mount experiments on Skylab. The most recent atomic data are used to compute line intensities for lines that fall on both sides of the Lyman limit. Lines of O III, O IV, O V, and S IV are considered. The computed intensity ratios of most lines from O IV, O V, and S IV agree with the experimental ratios to within a factor of 2. However, the discrepancies show no apparent wavelength dependence. From this fact, it is concluded that at least part of the discrepancy between theory and observation for lines of these ions can be accounted for by uncertainties in instrumental calibration and atomic data. However, difficulties remain in reconciling observation and theory, particularly for lines of O III, and one line of S IV. The other recent results of Schmahl and Orrall (1979) are also discussed in terms of newer atomic data. Publication: The Astrophysical Journal Pub Date: March 1982 DOI: 10.1086/159741 Bibcode: 1982ApJ...254..371D Keywords: Emission Spectra; Lyman Spectra; Oxygen Spectra; Solar Atmosphere; Solar Spectra; Ultraviolet Absorption; Calibrating; Opacity; Resonance Lines; Solar Physics full text sources ADS |

Steady flows in the solar transition region observed with SMM

Steady flows in the quiet solar transition region have been observed with the Ultraviolet Spectrometer and Polarimeter experiment on the Solar Maximum Mission (SMM) satellite. The persistent vertical motions seen at disk center have spatial rms amplitudes of 1.4 km/s in the C II line, 3.9 km/s in Si IV, and 4.2 km/s in C IV. The amplitudes of the more horizontal flows seen toward the limb tend to be somewhat higher. Plots of steady vertical velocity versus intensity seen at disk center in Si IV and C IV show two distinct branches.

Observations of magnetic fields on two late-type dwarf stars

view Abstract Citations (145) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of magnetic fields on two late-type dwarf stars. Robinson, R. D. ; Worden, S. P. ; Harvey, J. W. Abstract The detection of magnetic fields on the G8 V star Xi Boo A and on the K0 B star 70 Oph A is reported. A method has been developed and applied which provides estimates of the total field strength and fraction of the visible stellar surface covered by fields. The essence of this method is to perform the Fourier deconvolution of a nonmagnetically sensitive line profile from a magnetically sensitive one. This method is insensitive to magnetic field geometry and does not depend on polarization measurements. Results indicate fields of 2550 + or 390 gauss covering 20-45% Xi Boo A, and that fields of 1880 + or - 350 gauss may cover 10% of 70 Oph A. Comparison observations of solar active regions show fields of 1800 + or - 550 gauss covering 10% of the solar active regions, in line with previous work. Tests of solar quiet regions and sunspots also support the validity of the stellar measurements. Publication: The Astrophysical Journal Pub Date: March 1980 DOI: 10.1086/183217 Bibcode: 1980ApJ...236L.155R Keywords: Dwarf Stars; Late Stars; Magnetic Stars; Stellar Magnetic Fields; Field Strength; Line Spectra; Solar Activity; Zeeman Effect; Astrophysics full text sources ADS | data products SIMBAD (6)

The emission-line spectrum above the limb of a solar coronal hole - 1175-1940 A

Emission-line spectra of a coronal hole that coincided with the north pole of the sun are discussed which were obtained with a slit spectrograph aboard Skylab at positions within and above the solar white-light limb in the wavelength range from 1175 to 1940 A. Relative line intensities, line profiles, and full widths at half-maximum are presented for selected chromospheric and transition-zone lines observed above the present polar coronal hole. Average mass motions in the transition zone are determined as a function of electron temperature from the widths of the optically thin lines by assuming ionization equilibrium. The line intensities and profiles are compared with corresponding results deduced from spectra obtained above a quiet solar region. The coronal-hole spectra are found to imply an angular dependence for the source function as well as a radial dependence such that the source function is the smallest at the south pole and increases with decreasing solar latitude.

The emission-line spectrum above the limb of the quiet sun - 1175-1940 A

Spectra of a quiet solar region obtained at positions within and above the solar white-light limb in the wavelength region from 1175 to 1940 A are discussed. The spectra were obtained by the slit spectrograph (SO82-B) on Skylab. The spectral resolution is 0.06 A, and the projected slit area on the sun was 2 x 60 arcsec (1450 x 43500 km). Relative line intensities are presented for lines formed in the temperature region of the solar atmosphere from about 8000 to 220,000 K. Representative line profiles of both optically thin and optically thick lines are shown as a function of height above the limb. Random mass-motion velocities are deduced from the optically thin lines, and the relative intensities and profiles of the lines are discussed in terms of current theoretical models. A wavelength list with identifications is given for the spectrum obtained at +4 arcsec above the white-light limb.