Some comments on the limb shift of solar lines

It has been suggested that pressure shifts are the sole cause for the systematic blue shift of solar fraunhofer lines (Hart, 1974). To check this we evaluate the significance of pressure shifts for Fe i lines. The observed wavelength shifts of a large number of lines are compared with the shifts calculated for the Lennard-Jones potential following Hindmarsh et al. (1967). The Lennard-Jones potential for the interatomic forces yield pressure shifts caused by neutral hydrogen, which explain only a small fraction of the observed blue shift. It is also shown that the quadratic Stark effect contributes insignificantly to the position of Fe i lines. Table I summarizes the average line shifts for iron lines at the center of the solar disk after correction for pressure shifts.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 74-04129 with the National Science Foundation.     

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.     

Some comments on the problem of solar cycle prediction

The paper provides a number of regression equations that can be used to calculate the height of the odd Wolf number cycle. The feasibility of the rule of Gnevyshev-Ohl is analyzed as applied to the geomagneticaa-index. A modified rule of Gnevyshev-Ohl has been formulated to describe the behaviour ofaa-indices. A new method is suggested for early prediction of the next solar cycle. In this method, the angular coefficient (straightline slope) of linear dependence ofaa-indices on the Wolf number at the descending branch of the cycle has been used as a prediction index. It is shown to a high degree of certainty that the new prediction index is related to the height of the forthcoming cycle. While the methods based on the ratio of the even-odd cycles in a pair give very high values of cycle 23 maximum (203.2 ± 10.7), our new index, on the contrary, gives very low values (74.7 ± 6.9). There are some contradictory symptoms indicating that the forthcoming cycle 23 is likely to violate the regularities established for the past 125 years.     

Detection of small scale structure in metal lines at the extreme solar limb

We describe a tangential limb spectrum at 5870 Å which geometrically probes the high photosphere through the low chromosphere. Velocity and brightness structures with sizes ranging from 500 to 1500 km are present in the stronger emission lines. Such structure is consistent between the Fe i and Ba ii lines, and emission knots in these lines coincide with continuum bright streaks. But no correlation is evident between structure in the He i D₃ line, emission in the Na i D₂ line, and emission in the Fe i and Ba ii lines as a group. Two classes of near-horizontal velocity structure are seen in the height range from 0 to 500 km above the limb: υ ? 1 km s^?1 for the weaker metals and υ ～ 7–10 km s^?1 for the Na i line. Differences in line opacity are suggested as the cause of the low correlation between the fine structure in the various lines.

     

Some comments on the Oort cloud

The arguments used by Lyttleton to prove the nonexistence of the Oort cloud are reviewed, and it is shown that Oort's hypothesis remains consistent with observation. The 1950 model of the cloud cannot be correct and, by use of the results from a number of more recent papers, an improved model is described and compared with observations. It is emphasized that comparison of the predictions of theory with observations should concentrate on thea-distribution, as the 1/a-distribution masks much of the detailed structure of the theory. An order of magnitude argument is given which shows that 20% of so-called new comets have passed through the planetary system before, and the implications of this to the statistics of near-parabolic comet orbits are briefly investigated.     

Absolute wavelengths of Fraunhofer lines: Convective motions in the solar photosphere and the gravitational red shift

Absolute wavelengths for Fraunhofer lines are compared with laboratory measurements for several atomic and molecular spectra. The wavelength differences are shown to be consistent with the proposal that the deeper layers of the photosphere are in convective motion: _e -3 km/sec for log ₀> -1.0. Convective motions in the outer layers (log₀< - 1.0) are shown to be very small. Wavelength shifts of Fraunhofer lines formed in these outer layers are in good quantitative agreement with the predictions of the General Theory of Relativity.     

Some further comments on approximations of the radial dependence of the electron temperature in two-fluid models of the solar wind

In a previous paper (Cosmic Electrodyn.3, 116) we had suggested the use of the radialr ^?2/5 dependence rather than the often usedr ^?2/7 one in approximations of the electronic temperature profile in some two-fluid models. This note is intended to clarify and restate some points on this subject.     

A first order analysis of variations of the limb darkening and the shapes for solar Fraunhofer lines

New center-to-limb measurements in FeI lines show changes in both the line profiles and the limb darkening curves that appear to be characteristic of many other solar lines. Here we seek the constraints placed on the atmospheric model by these effects. We find that in addition to a depth varying source function we must also allow the ratio of the continuous absorption coefficient to the total absorption coefficient to pass through a minimum in the mid-photosphere. Such an effect is consistent with inward increases of the Doppler width and damping constant in the upper photosphere and an inward increase of the ionization for both iron and hydrogen in the low photosphere.The National Center for Atmospheric Research and Kitt Peak National Observatory are sponsored by the National Science Foundation.     

Microwave solar limb brightening

Previous models of microwave limb brightening have omitted the alignment of spicules along supergranule boundaries, have neglected the high temperature sheath around spicules, and have assumed an interspicular medium which was averaged over chromospheric network and non-network regions. We present a model which includes these factors. By constraining the model to conform to results from earlier UV and optical studies we are effectively left with two free parameters: the temperature at the core of the spicules, T _core, and (at solar minimum), the interspicular chromospheric network density model of the lower transition zone. The absence of limb brightening at the short millimeter wavelengths implies T _core 6000 k. Differences between the model and certain deconvolved observations near 9 mm are expected as a consequence of an extension of emission beyond the optical limb, predicted by the model, which affects the accuracy of the deconvolution technique. Unlike models which assume homogeneous spicules in a random distribution, ours does not require an abnormally high spicule area.     

Some comments on the low intensity Hα emission observed by J.-L. Leroy in the solar corona

Some comments are presented on the important observations of faint prominences made recently by Dr Jean-Louis Leroy at the Pic du Midi Observatory. The writer draws attention to the very probable connection with faintly luminous H obscuring prominences which appear sometimes as dark lanes and markings in ordinary prominences.     

Precise measurement of the solar gravitational red shift

We present the concept and the status of a multi-year project based on a new method to measure the Gravitational Red Shift of the Solar Spectrum with high precision. This project is aimed to conduct experimental verifications of the effect that the Einstein theory of General Relativity predicts for the frequencies of the Fraunhofer lines, that is, the light spectrum emitted by the Sun in its strong gravitational field. Previous determinations of such effect is limited to a precision of 2%. In order to discriminate between classical and relativistic explanations, we need to be sensitive to one part per million of the predicted effect. We have developed a new powerful technique, the Magneto-Optical Filter, that is able to provide far better precision and, for the future, possible space instrumentations able to extend our test to the second-order effect of the relativistic equivalence principle, never done before. The present paper is intended to describe the instrumentation, the procedure and the first encouraging results.     

On the broadening and shift of spectral lines

The Van der Waals interaction between a radiator and a light neutral perturber explains the broadening and shift of spectral lines only partly satisfactorily. Comparison with solar observations suggests that these theoretical damping constants are systematically too low. Moreover, the theoretically predicted shifts of the lines agree only incidentally with those obtained in laboratory experiments. We used a combination of a Smirnov-Roueff (S-R) interaction potential and a Van der Waals (VdW) potential. The damping constants, thus obtained, are always somewhat higher than those obtained by a VdW potential alone. The agreement between theory and experiment for the shift of the line is - although not completely satisfactory - better than the very poor agreement resulting from a VdW potential. However, the S-R potential may not be used in the case that heavy perturbers are involved. In that case the VdW potential yields the better results. Fairly good agreement is found between theoretically and empirically determined values ofγ for the solar centre.     

Magneto-sensitive lines in the solar spectrum

In this paper a numerical solution of the equations of transfer of the Stokes parameters in the presence of magnetic fields has been carried out. This yields the profiles of the Stokes parameters for three magneto-sensitive spectral lines (FEI λλ 6302.499, 6173.341, 5250.216). The magnetic broadening, temperature sensitivity and some other properties of these lines have also been investigated.In Unno's pioneering work, an algebraic solution of the equations of transfer of the Stokes parameters were obtained under a series of assumptions. Two of these are: 1) The source function B, varies linearly with the optical depth τ_λ; 2) The atmosphere follows the Milne-Eddington model. In this paper we have abandoned both these unrealistic assumptions. In consequence, our numerical solution agrees with observations better than the algebraic solution of Unno.     

UV observations of macrospicules at the solar limb

During the Spacelab 2 mission, the NRL High Resolution Telescope and Spectrograph (HRTS) obtained a time-series of broad-band ultraviolet images of macrospicules at the solar limb inside a polar coronal hole with a temporal resolution of 20 and 60 s. The properties of the macrospicules observed in the Spacelab data are measured and compared with the properties reported for EUV macrospicules observed during Skylab (Bohlin et al., 1975; Withbroe et al., 1976). There is a general agreement between the data sets but several differences. Because of the higher temporal resolution of the Spacelab data, it is possible to see macrospicules with shorter lifetimes than seen during Skylab, as well as variations on faster timescales. The largest (30–60) and fastest (150 km s ^-1) macrospicules seen during Skylab were not found in the Spacelab observations. The Spacelab data support the conclusion that many macrospicules decay by simply fading away.     

The contrast of faculae near the solar limb

We have measured the contrast of solar faculae near the limb on direct digital video images made with the 65 cm vacuum reflector at the Big Bear Solar Observatory. We used six broad band filters with different wavelengths from red to violet. The range of heliocentric angle covered in our measurements is 0.05 < = cos < 0.4 ( = 87°–66°). About 300 images were measured from observations made during the summers of 1983 and 1985. Over 20000 faculae were measured.By averaging the contrasts of faculae and plotting them vs heliocentric angle, we found that contrast increases monotonically towards the limb for the shorter wavelengths; for longer wavelengths, contrast has a tendency to peak around = 0.15, and then decrease towards the extreme limb. The contrast increases as wavelength decreases.     

Center-to-limb analysis of the solar oxygen lines

Several lines of neutral oxygen observed at various positions on the solar disk were used to study the influence of (1) the temperature distribution, (2) the velocity field, and (3) the damping on the line profiles and the abundance of oxygen in the photosphere. Theoretical profiles were calculated on the basis of four different model atmospheres in LTE. It was found that the model proposed by Holweger (1967) best reproduced the center-to-limb observations of the lines studied. The weighted mean of the oxygen abundance turned out to be log _O = 8.83 on the basis of log _H = 12.00.     

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.     

Some remarks on solar neutrinos

In 1970 Fred Hoyle encouraged a study of solar neutrino production which led to along-term investigation of the influence of what have become known as `non-standard' processes (i.e. processes that are not accounted for in the relatively naively constructed so-called `standard' theoretical solar models). The outcome is a very much sounder understanding of the structure and dynamics of the Sun, which has yielded a knowledge of conditions in the energy-generating core so precise that one can set quite tight reliable constraints on neutrino-producing nuclear reactions, and thereby provide an important contribution to the study of neutrino transitions. This revised version was published online in August 2006 with corrections to the Cover Date.