Center-to-limb variation of Ca II line brightness oscillations in the solar chromosphere

We have investigated 15 time series of Ca II line spectrograms in quiet-Sun regions located at various distances from the disk center. Our goal is to reveal the center-to-limb variation of the brightness oscillations. The residual intensities at the centers of the Ca II K and 849.806-nm lines and the K index have been analyzed. We have considered separately two components of the chromospheric network. Our main result is that the power of the brightness oscillations in the chromosphere of the average quiet Sun decreases to the limb. This change for the boundaries of supergranulation cells (networks) is considerably larger than that for their inner parts (cells). It is mainly determined by the 5-min oscillations; the 3-min oscillations show virtually no center-to-limb variation. In addition to studying the dependence of the oscillation power on the heliocentric angle, we also consider other characteristics of the oscillatory regime of the chromosphere. For example, the low-frequency oscillations with periods longer than 700 s, which are inherent predominantly in the K line core in networks, have been separated into an isolated mode. No center-to-limb variation has been revealed for them. As a result of our discussion of the patterns found based on present-day publications on the chromosphere dynamics, we conclude that different mechanisms and sources of its heating can simultaneously make their contributions.     

On the center-to-limb variation of infrared photospheric carbon lines and the infrared continuum intensity around 1.75 μ M

The center-to-limb variation (CLV) of several infrared carbon lines and the infrared continuum intensity around 1.75 μm were measured. The results were compared with theoretical predictions using four different photospheric models. It was found that the model by Holweger and Müller (1974) describes the observations best.     

The anomalous intensities of helium lines in the quiet solar transition region

Previous studies using observations made at low spatial and spectral resolution showed that the resonance lines of He  i and He  ii are anomalously strong in the quiet Sun when compared with other transition region lines formed at similar temperatures. Here, the higher spatial and spectral resolution provided by the Coronal Diagnostic Spectrometer ( cds ) instrument on board the Solar and Heliospheric Observatory ( SOHO ) is used to re-examine the behaviour of the He  i and He  ii lines and other transition region lines, in quiet regions near Sun centre. Supergranulation cell boundaries and cell interiors are examined separately. Near-simultaneous observations with the sumer instrument provide information on the lower transition region and the electron pressure. While the lines of He  i and He  ii have a common behaviour, as do the other transition region lines, the behaviour of the helium lines relative to the other transition region lines is significantly different. The emission measure distributions that account for all transition region lines, except those of helium, fail to produce sufficient emission in the He  i and He  ii resonance lines by around an order of magnitude, in both supergranulation cell boundary and cell interior regions. The electron pressure appears to be higher in the cell interiors than in the average cell boundaries, although the uncertainties are large. While the VAL-D model gives a closer match to the He  i 584.3-Å line, it does not successfully reproduce other transition region lines.     

Variation in the Width of Transition Region Network Boundaries

The transition region network seen in solar extreme ultraviolet (EUV) lines is the extension of the chromospheric network. The network appears as an irregular web-like pattern over the solar surface outside active regions. The average width of transition region network boundaries is obtained from the two-dimensional autocorrelation function of SOlar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer (CDS) synoptic images of the Sun in two emission lines, He i 586 Å and O v 630 Å during 1996?–?2012. The width of the network boundaries is found to be roughly correlated with the solar cycle variation with a lag of about ten months. A comparison of the widths in the two emission lines shows that they are larger for the He i line. The SOHO/CDS data also show large asymmetry in boundary widths in the horizontal (x) and vertical (y) image directions, which is shown to be caused by image distortions that are due to instrumental effects. Since the network boundary widths are related to the magnetic flux concentration along the boundaries, the results are expected to have implications on the flux transport on the solar surface, solar cycle, and the mass and energy budget of network loops and jets.     

Microwave emission from a sunspot

A series of microwave observations of a sunspot in the active region NOAA 4741 was made with the Owens Valley Solar Array for the purpose of investigating the center-to-limb variation of both the spectral and spatial brightness distribution. In this investigation, several properties of the sunspot microwave radiation are found. First, sunspot microwave emission appears in two typical profiles depending on the heliocentric position of the spot: either the ring structure near disk center or single-peak structure near the limb. Second, the brightness temperature at high, optically thin frequencies (>6 GHz) increases slightly as the spot approaches the limb, which we interpret as being due to the increase of the gyroresonance opacity of the field lines near the spot center as they gain greater viewing angles. Third, the center-to-limb variation of the gyroresonance spectrum seems to be mostly characterized by a change of effective harmonic, which accompanies a discontinuous change of the degree of polarization. Fourth, a change of spectrum from gyroresonance to free-free emission is found in the passage of the spot over the solar limb, which gives a determination of the height of the gyroresonance layer to confirm its location low in the corona of the active region.     

Line asymmetries and shifts in the presence of granulation and oscillations: The CLV of the Ki 7699 resonance line

We study the effects of both the solar granulation and short-period oscillations on the solar profile of the Ki 7699 resonance line and its center-to-limb variations.The granular model we used is that of Nelson (1978). The wave field is obtained by adding two 30 s acoustic waves with opposite horizontal wavenumbers to allow for the center-to-limb calculations.We find that the broadening and strengthening produced by our dynamical models do not fill the gap existing between the observations and the values computed in a static atmosphere. Then, either the granular model has to be revised (in particular its r.m.s. velocities are too low) or other types of motions, like gravity waves, have to be accounted for.The upper part of the mean bisectors is affected by granulation and the lower part by waves. With some qualification concerning the data used for comparison, a satisfactory fit of the observed center-to-limb variations of the mean bisectors is obtained when both dynamical models are included and the granular velocities are increased by a factor 1.5.Currently NAS/NRC Research Associate, on leave from the Universita' di Napoli.Operated by the Association of Universities for Research in Astronomy Inc., under contract with the National Science Foundation. Partial support for the NSO is provided by the USAF under a Memorandum of understanding with the NSF.     

Umbral intensities of large sunspots

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.     

Influence of the Atmospheric Model on Hanle Diagnostics

We clarify the uncertainty in the inferred magnetic field vector via the Hanle diagnostics of the hydrogen Lyman-\(\upalpha\) line when the stratification of the underlying atmosphere is unknown. We calculate the anisotropy of the radiation field with plane-parallel semi-empirical models under the nonlocal thermal equilibrium condition and derive linear polarization signals for all possible parameters of magnetic field vectors based on an analytical solution of the atomic polarization and Hanle effect. We find that the semi-empirical models of the inter-network region (FAL-A) and network region (FAL-F) show similar degrees of anisotropy in the radiation field, and this similarity results in an acceptable inversion error (e.g., \({\sim}\, 40~\mbox{G}\) instead of 50 G in field strength and \({\sim}\,100^{\circ}\) instead of \(90^{\circ}\) in inclination) when FAL-A and FAL-F are swapped. However, the semi-empirical models of FAL-C (averaged quiet-Sun model including both inter-network and network regions) and FAL-P (plage regions) yield an atomic polarization that deviates from all other models, which makes it difficult to precisely determine the magnetic field vector if the correct atmospheric model is not known (e.g., the inversion error is much larger than 40% of the field strength; \({>}\,70~\mbox{G}\) instead of \(50~\mbox{G}\)). These results clearly demonstrate that the choice of model atmosphere is important for Hanle diagnostics. As is well known, one way to constrain the average atmospheric stratification is to measure the center-to-limb variation of the linear polarization signals. The dependence of the center-to-limb variations on the atmospheric model is also presented in this paper.     

Center-to-limb variations in the two-dimensional contrast of photospheric faculae

A semi-empirical model of a facular flux tube has been used to determine its center-to-limb appearance. The model is that of Osherovich et al. (1983). This model is in MHSE and includes tension forces. Results are presented in the form of contour maps, cross-sections of the contrast on axis, and integrated contrast as a function of viewing angle. Results are generally consistent with those of Caccin and Severino (1979), but not with observations of Chapman and Klabunde (1982).     

Closed magnetic structures in the chromosphere and in the transition region

Using simultaneous observations of the same solar regions in the lines H and Civ 1548 Å, we have derived schematic models of closed magnetic lines from dynamical constraints. We conclude that the magnetic loops are closed at higher levels above facular than above non-facular regions. This result remains valid whatever are the assumed density models and even if we take into account the 3 min oscillations. The center-to-limb behaviour is well predicted by taking into account the relative opacity in chromosphere and transition region.     

On the spatial directivity of solar radio bursts

In this paper, a new method of estimating the spatial directivity (in the form of center-to-limb variation) of microwave burst emission is proposed and derived. Estimations of radioemission directivity values vs observation frequency are obtained. Results are compared to the radio source model using an inhomogeneous magnetic field, source size and particle density, and show a high degree of agreement. Values of model parameters from earlier estimations are confirmed.     

Origin of the weakening of EUV emission lines formed in the chromosphere-corona transition zone

EUV spectroheliograms of the quiet Sun obtained with the Harvard experiment on Skylab are analyzed to identify the structure causing the weakening of the EUV line emission due to Lyman continuum absorption. The weakening at the network boundaries can be explained by overlapping of several spicules each of which being wrapped in an EUV emitting sheath. Part of the cell interiors show moderate weakening, this has the shape of a belt surrounding the network boundaries. There are a number of patches showing intense weakening near network boundaries and in cell interiors; the weakening at the points cannot be explained by overlapping of chromospheric structures with an EUV emitting sheath. A possible explanation is that the intense weakening is caused by cool chromospheric clouds or moving blobs over the EUV emitting sources in the cell interiors. Some of the points showing intense weakening are associated with an enhancement of the EUV emission. These points have lifetime shorter than the time interval of 5.5 min between successive observations, which stresses again that the chromosphere-corona transition zone is in a dynamic state.     

Statistical Feature Recognition for Multidimensional Solar Imagery

A maximum a posteriori (MAP) technique is developed to identify solar features in cotemporal and cospatial images of line-of-sight magnetic flux, continuum intensity, and equivalent width observed with the NASA/National Solar Observatory (NSO) Spectromagnetograph (SPM). The technique facilitates human understanding of patterns in large data sets and enables systematic studies of feature characteristics for comparison with models and observations of long-term solar activity and variability. The method uses Bayes’ rule to compute the posterior probability of any feature segmentation of a trio of observed images from per-pixel, class-conditional probabilities derived from independently-segmented training images. Simulated annealing is used to find the most likely segmentation. New algorithms for computing class-conditional probabilities from three-dimensional Gaussian mixture models and interpolated histogram densities are described and compared. A new extension to the spatial smoothing in the Bayesian prior model is introduced, which can incorporate a spatial dependence such as center-to-limb variation. How the spatial scale of training segmentations affects the results is discussed, and a new method for statistical separation of quiet Sun and quiet network is presented.     

Formation of spectral lines in planetary atmosphere IV. Theoretical evidence for structure of the jovian clouds from spectroscopic observations of methane and hydrogen quadrupole lines

The theory of formation of pressure-broadened methane lines and collision-narrowed hydrogen quadrupole lines in a Jovian atmosphere is studied in detail for a physically realistic model of the planet's lower atmosphere. Only observations of the center-to-limb (CTL) variations of the equivalent width of absorption lines for both of these molecules can identify the structure of the visible cloud layers. Observations of the CTL variation of methane and hydrogen quadrupole lines are the most suitable for studying the Jovian atmosphere. The CTL variations for hydrogen are much greater and more sensitive to variations of the properties of the thin upper tropospheric cloud layer than the corresponding observations of methane lines. A detailed comparison of hydrogen quadrupole with methane lines is made for the same continuum conditions, enabling us to develop a detailed understanding of the formation of the collision-narrowed hydrogen quadrupole lines in a Jovian atmosphere.     

Motions in solar magnetic tubes

Three independent observations by rocket, Skylab, and OSO-8 have all indicated the presence of steady downflows of the order of a few kilometers per second in the solar transition region overlying the chromospheric network. Using density estimates at these heights from traditional transition region models, we find that the downward mass fluxes associated with these velocities are comparable with the estimated upward mass flux in spicules, originating in the same regions. Since both observations and theoretical calculations show that the solar wind can accept only a small fraction of the upward spicule flux, we suggest that the downflow represents spicular material returning to the chromosphere after being heated to coronal temperatures. In this context, the differential velocity measurement of Cushman and Rense is interpreted as indicating a difference in downflow speeds rather than a difference in expansion speeds.Moreover, the enthalpy flux associated with the downflow of coronal material into these regions is shown by various estimates to exceed the inward heat flow expected by thermal conduction and it may constitute the dominant energy source for the transition region. Simplified analytical models are used to explore the nature of the transition region overlying the supergranulation boundaries, under the assumption that the thermal structure results from a balance of the downward convection of enthalpy and radiative losses. Models based upon these considerations are shown to be consistent with the observed emission measures.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Propagation and confinement of energetic electrons in solar flares

The propagation, cofinement and total energy of energetic (>25 keV) electrons in solar flares are examined through a brief review of the following hard X-ray measurements: (1) spatially resolved observations obtained by imaging instruments; (2) stereoscopic observations of partially occulted sources providing radial (vertical) spatial resolution; and (3) directivity of the emission measured through stereoscopic observations and the center-to-limb variation of the occurrence frequency of hard X-ray flares. The characteristics of the energetic electrons are found to be quite distinct in impulsive and gradual hard X-ray flares. In impulsive flares the non-thermal electron spectrum seems to extend down to 2 keV indicating that the total energy of non-thermal electrons is much larger than that assumed in the past.     

The solar UV continuum 1440–1680 Å and its center-to-limb variation

The solar UV continuum has been derived from intensity-calibrated observations with the High-Resolution Telescope and Spectrograph - HRTS - on its second rocket flight in 1978. A database has been constructed using the spatially-resolved solar spectrum 1180–1700 Å along a slit extending from near disk center to the solar limb and crossing a sunspot and two active regions. The angular resolution is approximately 1.8″. The data consist of 1772 spectral scans in the full center-to-limb range, including both quiet and active solar regions. The distribution of solar UV intensities has been derived and the center-to-limb variations of the continuum intensities in the quiet Sun are studied. Both quantities show spectral variations, particularly across the Sii continuum edge at 1521 Å. The spectra have been fitted to curves of constant color temperature above and below the Sii edge. The derived center-to-limb variations have been compared to the values of Samain (1979) which are frequently referred to in the literature. A relatively large discrepancy may be explained by the higher spectral and angular resolution of the HRTS as compared to the rocket instrument used by Samain. Comparisons with the VAL III model calculations by Vernazza, Avrett, and Loeser (1981) show discrepancies between the observations and the model predictions, particularly with regard to the sign and amount of the intensity change across the Sii continuum edge. It is noted that some of the results presented, i.e., absolute intensities and brightness temperatures may change, pending confirmation of the SUSIM Spacelab 2 irradiance results (VanHoosieret al., 1988).     

Probing the Quiet Solar Atmosphere from the Photosphere to the Corona

We investigate the morphology and temporal variability of a quiet-Sun network region in different solar layers. The emission in several extreme ultraviolet (EUV) spectral lines through both raster and slot time-series, recorded by the EUV Imaging Spectrometer (EIS) on board the Hinode spacecraft is studied along with \(\mbox{H}\upalpha\) observations and high-resolution spectropolarimetric observations of the photospheric magnetic field. The photospheric magnetic field is extrapolated up to the corona, showing a multitude of large- and small-scale structures. We show for the first time that the smallest magnetic structures at both the network and internetwork contribute significantly to the emission in EUV lines, with temperatures ranging from \(8\times 10^{4}~\mbox{K}\) to \(6\times 10^{5}~\mbox{K}\). Two components of transition region emission are present, one associated with small-scale loops that do not reach coronal temperatures, and another component that acts as an interface between coronal and chromospheric plasma. Both components are associated with persistent chromospheric structures. The temporal variability of the EUV intensity at the network region is also associated with chromospheric motions, pointing to a connection between transition region and chromospheric features. Intensity enhancements in the EUV transition region lines are preferentially produced by \(\mbox{H}\upalpha\) upflows. Examination of two individual chromospheric jets shows that their evolution is associated with intensity variations in transition region and coronal temperatures.     

Some comments on the limb shift of solar lines

The well-known correlation between granulation intensity and velocity fluctuations causes a shift of the average line position called the convective blue shift. It is argued that this convective blue shift is most likely reponsible for the limb effect of solar Fraunhofer lines. To explain the center-to-limb variation of this limb effect it is essential that both horizontal and vertical motions in the granulation are considered. The effects of a variation in the granulation properties across the Sun on large scale velocity pattern observations are discussed. Abnormal granulation patterns observed inactive regions and at the boundaries of supergranules could be responsible for part or all of the downflow observed there.On leave from Astronomy Department, University of Washington, Seattle, Wash., U.S.A.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 74-04129 with the National Science Foundation.     

Structure of the chromosphere-corona transition region

The structure and energy balance of the chromosphere-corona transition region is investigated by means of a static, planar model which is compared with the results of XUV-resonance-line observations. In this model, the transition region is heated by thermal conduction from the corona and cooled by radiative losses. Comparison of the model with observational results implies that this is the dominant process in the energy balance of the transition region, and that the base of the transition region is inherently non-static and/or non-planar. The model explains the observational finding of Noyes et al. (1970) that the number density and the downward heat flux both increase by the same factor from quiet regions to active regions. The implications of these results are discussed with regard to spicules.