Characteristics of the Ca ii K-line profiles in the quiet Sun

Feature-to-feature identification is made on simultaneous Ca ii K-line spectrograms (SG) and K_2v spectroheliograms (SHG). The line profiles in plages and in the network boundary nearly always have double-peaked reversal in the core, while those inside the cells present all possibilities: double-peaked, single-peaked on violet side, single-peaked on red side, and unreversed absorption. Statistics of the profiles in the quiet chromosphere show that 50% are K₂₃₂ double-peaked, 20% are K_2v single-peaked, 10% are K_2r single-peaked, and 20% show only incipient reversal or even totally lack any reversal. We call attention to the nontrivial contribution of these absorption profiles which are formed in dark regions shown on SHG's.The physical conditions inferred from different kinds of profiles are briefly discussed.In part to be included in a dissertation to be submitted to the Graduate School, University of Maryland, by S. Y. Liu in partial fulfillment of the requirements for the Ph. D. degree in Astronomy.Visiting Graduate Student, Solar Division, Kitt Peak National Observatory.Visiting Astronomer, Solar Division, Kitt Peak National Observatory.     

Forbidden lines of Ca II in the photospheric spectrum

On scans obtained with the Oxford photoelectric spectrometer, the [Ca ii] transition 4s ²S_1/2-3d ²D_3/2; is identified with a weak Fraunhofer line. It is argued that the strength of the line is consistent with the abundance of calcium which is derived from the permitted lines of Ca i and Ca ii.     

Asymmetries of the solar Ca ii lines

A theoretical study of the influence of propagating acoustic pulses in the solar chromosphere upon the line profiles of the Ca ii resonance and infrared triplet lines has been made. The major objective has been to explain the observed asymmetries seen in the cores of the H and K lines and to predict the temporal behavior of the infrared lines caused by passing acoustic or shock pulses. The velocities in the pulses, calculated from weak shock theory, have been included consistently in the non-LTE calculations. The results of the calculations show that these lines are very sensitive to perturbations in the background atmosphere caused by the pulses. Only minor changes in the line shapes result from including the velocities consistently in the line source function calculations. The qualitative changes in the line profiles vary markedly with the strength of the shock pulses. The observed differences in the K line profiles seen on the quiet Sun can be explained in terms of a spectrum of pulses with different wave-lengths and initial amplitudes in the photosphere.     

Fine structure in Ca ii on the solar disc

High-dispersion spectra of the core of the K line of Ca ii as seen at the center of the solar disc have been reduced. Resolution on the spectra approach 1 arc sec. Line profiles of individual elements are very asymmetric and often are peaked on only one side of the line center. Variations of the line profiles and the emission peaks are discussed. The doubly reversed mean profile of the K line is explained as a spatial average of individual profiles, and it is suggested that single peaks may be caused by Doppler-shifted discrete elements in the chromosphere.Donald H. Menzel Research Fellow in Astronomy.     

The Balmer 9 and Balmer 11 lines of He ii in the Sun

We identify the Balmer 9 and 11 lines of He ii at 959 Å and 942 Å in solar spectra. These lines are produced mainly by recombination following photoionization of He ii by coronal XUV radiation. From analysis of the line intensities, we confirm the theoretical model of Avrett et al. (1976), who found that an appreciable amount of He⁺⁺ is present at temperatures of 1–2 × 10⁴ K and that the anomalously strong He ii 304 line is produced primarily by collisional excitation. We also confirm the suggestion of Kohl (1977) that the photoionization-recombination process is more important in active regions than in the quiet Sun, and we find that the 304 line is produced largely by recombination in solar flares.     

The Ca ii emission lines in quiescent prominences

Observations of the Ca ii H, K, and infrared triplet lines are compared with theoretical predictions from the slab models of Heasley and Milkey (1976). While the theoretical models describe the hydrogen and helium emission spectra of quiescent prominences satisfactorily the predicted Ca ii lines are systematically too bright. The most likely reason for the discrepancy is the inapplicability of the symmetric slab prominence model for lines which become even moderately optically thick in prominences.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Visiting scientist at Kitt Peak National Observatory.     

Width of emission cores of the line K Ca ii in sunspots

Emission core widths of K Ca ii line in the umbra and penumbra of 9 sunspots and in their vicinity are measured. All sunspots are located near the solar disc center. Data on variation of widths W _K along the mean sunspot radius are obtained. Values W _K in the umbra and penumbra centers are equal or somewhat less than on stars of the same luminosity.     

The formation of the Ca ii K line in a spinning spicule

The emission of the Ca ii K line from a spinning cylindrical spicule model is considered. In order to reproduce the observed spicule K line profiles, the model must have both radial and axial gradients in N _e and T _e. The rotating spicule model is optically thin at all heights and is hotter and more dense than a stationary model computed in a previous paper. It is proposed that the so-called Type I and Type II spicules may be structurally the same features, with different rotational velocities.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A facular model based on the wings of the Ca ii lines

We develop a relatively simple procedure for deriving models of upper photospheric regions based on the damping wings of the Ca ii resonance and infrared triplet lines. The procedure is used to derive a facular model but can also be applied to late-type stars. We compare our model to that of Chapman.Visiting Astronomer at Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Staff Member, Laboratory Astrophysics Division, National Bureau of Standards.     

Formation of the Ca ii K-line core with arbitrary temperature minima

The formation of the solar Caii K-line core is studied under the assumption of a two component chromosphere for many atmosphere models, which are tested by comparing observed and calculated average intensity profiles at several places on the solar disk. Non-LTE-line profile calculations are made using model atoms of two and three bound levels while the value and depth of the temperature minimum and turbulent velocity gradient are varied. A critique of several broad types of models is provided.Limb darkening of the entire core, a K1 radiation temperature of 4300 K and increasing limbward separation of the K2 peaks are predicted by a combination of a cool component, covering 90% of the solar disk, and a hot component, representing the bright calcium network. The optimum cool component is characterized by a 4200 K temperature minimum and low ( 1 km/sec) microturbulent velocities. The hot component begins its outward temperature rise in the photosphere at 4300 K and is never more than 500 K hotter than the cool component.Publications of the Goethe Link Observatory, Indiana University, No. 116.Presently at New Mexico State University, Las Cruces, New Mexico.     

The [Ca ii] λ7323.90 line in the solar spectrum

The possibility of detecting abundance inhomogeneities in the photosphere of the Sun is discussed. These inhomogeneities are postulated to be a result of nuclear reactions caused by high energy protons from solar flares.From our present knowledge of flares and of nuclear reaction cross sections it is found that temporary inhomogeneities in Li-abundance of spectroscopically detectable amounts should not be excluded.Preliminary observations have given negative results.     

Magnetic states of the ionosphere of Venus observed by Venus Express

Strong ultraviolet radiation from the Sun ionizes the upper atmosphere of Venus, creating a dense ionosphere on the dayside of the planet. In contrast to Earth, the ionosphere of Venus is not protected against the solar wind by a magnetic field. However, the interaction between charged ionospheric particles and the solar wind dynamic and magnetic pressure creates a pseudo-magnetosphere which deflects the solar wind flow around the planet (Schunk and Nagy, 1980). The combination of changing solar radiation and solar wind intensities leads to a highly variable structure and plasma composition of the ionosphere. The instrumentation of the Venus Express spacecraft allows to measure the magnetic field (MAG experiment) as well as the electron energy spectrum and the ion composition (ASPERA-4 experiment) of the upper ionosphere and ionopause. In contrast to the earlier Pioneer Venus Orbiter (PVO) measurements which were conducted during solar maximum, the solar activity was very low in the period 2006-2009. A comparison with PVO allows for an investigation of ionospheric properties under different solar wind and EUV radiation conditions. Observations of MAG and ASPERA have been analyzed to determine the positions of the photoelectron boundary (PEB) and the “magnetopause” and their dependence on the solar zenith angle (SZA). The PEB was determined using the ELS observations of ionospheric photoelectrons, which can be identified by their specific energy range. It is of particular interest to explore the different magnetic states of the ionosphere, since these influence the local plasma conductivity, currents and probably the escape of electrons and ions. The penetration of magnetic fields into the ionosphere depends on the external conditions as well as on the ionospheric properties. By analyzing a large number of orbits, using a combination of two different methods, we define criteria to distinguish between the so-called magnetized and unmagnetized ionospheric states. Furthermore, we confirm that the average magnetic field inside the ionosphere shows a linear dependence on the magnetic field in the region directly above the PEB.     

The forbidden line [Ca ii] λ7323 in the fraunhofer spectrum

New observations of the [Caii] 7323 Fraunhofer line are reported. The blending H₂O line was weak at the time of observation. Accurate estimates of the centre-limb variation of the equivalent width of the [Caii] transition are obtained and shown to be consistent with the calcium abundance log N(Ca) = 6.33.     

Increasing the Fine Structure Visibility of the Hinode SOT Ca ii H Filtergrams

We present an image processing technique, the improved OMC filter (also called Madmax), which selects maxima of convexities of intensity modulations of an image. The filter computes second derivatives of the image in multiple directions around each pixel. It is shown to be efficient for pattern recognition, and bright hair-like or small anisotropic features can be enhanced. The filter is tested on artificially generated images, and the effect of a different number of directions in which the second derivatives are calculated is evaluated. Ca ii H line images of spicules and prominences obtained with Hinode/SOT are also used to illustrate its efficiency on real data. It is shown how to make the choice of the sensitive parameters to be used in improving the image visibility.     

Ca II K line profile of the truly quiet Sun

While evaluating the chromospheric variability (solar cycle related or any other) using the Ca II K line (3933.684 Å) as an indicator, an essential prerequisite is the knowledge of the profile of a truly quiet Sun in the integrated light. Such a profile can serve as a bench mark over which enhancements can be measured, particularly when modelling variability. This paper describes how such a K-line profile has been derived for the quiet Sun using disc-integrated light.     

On peculiarities of the H and K Ca ii lines of quiescent prominences

A study of the metallic lines in bright quiescent prominences indicates that the optical thickness in the K line of Ca ii may reach values as high as 10³. This is about 10 times larger than the optical thickness in the H line and may explain some peculiarities of the H and K lines in solar prominences.     

Empirical calibration of the near-infrared Ca ii triplet – III. Fitting functions

Using a near-infrared stellar library of 706 stars with a wide coverage of atmospheric parameters, we study the behaviour of the Ca  ii triplet strength in terms of effective temperature, surface gravity and metallicity. Empirical fitting functions for recently defined line-strength indices, namely CaT*, CaT and PaT, are provided. These functions can be easily implemented into stellar population models to provide accurate predictions for integrated Ca  ii strengths. We also present a thorough study of the various error sources and their relation to the residuals of the derived fitting functions. Finally, the derived functional forms and the behaviour of the predicted Ca  ii are compared with those of previous works in the field.     

Sun as a star: Rotation rates from the Ca K-index

Daily measurements for 18 months made at Tucson of the Ca K-index in integrated sunlight have been used to derive solar rotation rates. Power spectral analysis shows that one can obtain a well defined value of solar rotation rate in a period of six months, provided the data are fairly continuous. One might suppose it is possible to study the variation of solar (or stellar) rotation rate with time, a variation arising from a combination of differential rotation and shifts in the active latitude. A comparison between our observed rates and the prevailing dominant activity zones does not support this supposition, at least for the interval studied. Rather, our rates seem to depend on the circumstances of sampling and active region birth and decay.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Scientific goals for the observation of Venus by VIRTIS on ESA/Venus express mission

The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board the ESA/Venus Express mission has technical specifications well suited for many science objectives of Venus exploration. VIRTIS will both comprehensively explore a plethora of atmospheric properties and processes and map optical properties of the surface through its three channels, VIRTIS-M-vis (imaging spectrometer in the 0.3–1 μm range), VIRTIS-M-IR (imaging spectrometer in the 1–5 μm range) and VIRTIS-H (aperture high-resolution spectrometer in the 2–5 μm range). The atmospheric composition below the clouds will be repeatedly measured in the night side infrared windows over a wide range of latitudes and longitudes, thereby providing information on Venus's chemical cycles. In particular, CO, H₂O, OCS and SO₂ can be studied. The cloud structure will be repeatedly mapped from the brightness contrasts in the near-infrared night side windows, providing new insights into Venusian meteorology. The global circulation and local dynamics of Venus will be extensively studied from infrared and visible spectral images. The thermal structure above the clouds will be retrieved in the night side using the 4.3 μm fundamental band of CO₂. The surface of Venus is detectable in the short-wave infrared windows on the night side at 1.01, 1.10 and 1.18 μm, providing constraints on surface properties and the extent of active volcanism. Many more tentative studies are also possible, such as lightning detection, the composition of volcanic emissions, and mesospheric wave propagation.