All evidence of the solar origin of 160-min period oscillations is collected, and the present state of observations of this oscillation in optical and radio-ranges is considered. The main results are summarized: (a) the 160-min oscillation was observed in 1981 as well as before, (b) an attempt to find a nonradial component with l = 2 has failed, (c) the intensity and circular polarization of radioemission show with statistical significance the presence of this 160-min periodicity.

The method developed and the instrument designed for detecting variations of the solar limb darkening at the atmospheric transparency window of the solar opacity minimum region of λ 1.65 µ are described. This differential technique proved to be successful in rejecting undesirable low frequency noises due to the atmosphere and to the instrument. Analysis of observations made in 1977, 1978, and 1981 indicates the persistance of global fluctuations of the IR differential, center-to-limb intensity at the wellknown 160 min period with an average amplitude of about ± 2 x 10-4 in units of the ‘average Sun’ intensity near 1.65 µm.

The solar oscillation with period near 160 min is found to be unique in a spectrum computed over the range of periods from about 71 to 278 min. Our best estimate of the period is 160.0095 ± 0.001 min, which is different from 160 min (1/9 of a day) by a highly significant amount. The width of the peak is approximately equal to the limiting resolution that can be obtained from an observation lasting 6 years, which suggests that the damping time of the oscillations is considerably longer than 6 years. A suggestion that this peak might be the result of a beating phenomenon between the five minute data averages and a solar oscillation with period near five minutes is shown to be incorrect by recomputing a portion of the spectrum using 15 s data averages.

A program to measure long-period brightness oscillations at the solar limb has been pursued at Sacramento Peak Observatory for several years. Past improvements in observing technique and data analysis are reviewed. The encouraging results aid in the verification of the reality and the origin of oscillatory signals. However, the main stumbling block to this and other observational programs is the length of observing sequences imposed by the day/night cycle. The South Pole has received considerable attention as a site where extended observations might be possible. Currently, the Sacramento Peak program is developing a South Pole telescope designed for the observing technique and data analysis proven in Sunspot. A review of pertinent South Pole site parameters is given here for other workers who may be considering South Pole observations. Observing sequences longer than 150 hr are possible, though rare. Data sets of this duration are very attractive for solar oscillation studies.

This paper presents the latest results obtained from the analysis of the full-disk Doppler shift observations obtained at the geographic South Pole in 1981. About 80 normal modes of oscillation (l = 0–3) have now been identified. Their frequencies range from 1886 μHz (l = 1, n = 12) to 5074.5 μHz (l = 2, n = 35), and their amplitudes are as low as 2.5 cm s–1. Amplitude modulation occurs with periods of 1–2 days, and the individual oscillations appear to be excited randomly and independently. In cases where other groups have observed some of the modes identified by us, the agreement in frequency is good.

The total solar irradiance measurements obtained by the active-cavity radiometer on board the Solar Maximum Mission have been analyzed for evidence of global oscillations. We find that the most energetic low-degree p-mode oscillations in the five-minute band have amplitudes of a few parts per million of the total irradiance, and we positively detect modes with l = 0, 1, and 2. The distribution in l differs from that of the velocity spectrum, with relatively more power at lower l values. The individual modes have narrow line widths, corresponding to values of Q greater than a few thousand, or lifetimes of at least a week. We do not detect the 160-min oscillation in the power spectrum, and place an upper limit of 5 parts per million (99.9% confidence) on its amplitude.

Solar five-minute oscillations of degree l = 3, 4, and 5 have been observed at Stanford, in the Doppler shift of the Fe 5124 line. The frequencies and amplitudes are in broad agreement with previous observations of modes with l ≤ 3, though we note that there are some systematic discrepancies between the results of different observers.

New observations of solar oscillations are reported. Power density spectra derived from these observations reveal narrow-band oscillations that are spatially global, have spatial symmetry properties that are either symmetric or antisymmetric for reflection about the center of the solar disk and also about the solar equator, and have coherence times ≿41 days. Large-scale differential refraction effects have been reduced by a factor of 105 over that found in previous solar diameter studies by the design of the experiment; thus, these effects are eliminated as a possible source of the oscillations. A discussion is presented of this reduction as well as other features of the observing and analysis program. It is concluded that the probability is very high that individual normal modes of oscillation of the Sun have been detected in the period range from 2 hr to 10 min.

The value of p-modes of high degree l as a diagnostic for the structure and dynamics of the solar envelope is reviewed.

A brief summary is given of a program which is currently being carried out with the McMath telescope of the Kitt Peak National Observatory in order to study high-degree (l ≳ 150) solar p-mode oscillations. This program uses a 244 x 248 pixel CID camera and the main spectrograph of the McMath telescope to obtain velocity-time maps of the oscillations which can be converted into two-dimensional (kh – ω) power spectra of the oscillations. Several different regions of the solar spectrum have been used in order to study the oscillations at different elevations in the solar atmosphere. The program concentrates on eastward- and westward-propagating sectoral harmonic waves so that measurements can be made of the absolute rotational velocities of the solar photospheric and shallow sub-photospheric layers. Some preliminary results from this program are now available. First, we have been unable to confirm the existence of a radial gradient in the equatorial rotational velocity as was previously suggested. Second, we have indeed been able to confirm the presence of p-mode waves in the solar chromosphere as was first suggested by Rhodes et al. (1977). Third, we have been able to demonstrate differences in photospheric and chromospheric power spectra.

Although the method of superposed epochs (Chree analysis) has been utilized for seven decades, a procedure to determine the statistical significance of the results has not been available heretofore. Consequently, various subjective methods have been utilized in the interpretation of Chree analysis results in several fields. The major problem in the statistical treatment of Chree analysis results arises from the fact that in most studies of natural phenomena, data are neither random nor sequentially independent. In this paper, a statistical procedure which takes this factor into account is developed.

The accuracy of frequency determination by a least squares technique for an autoregressive spectral estimator is studied and compared with the Fourier method. Using numerical tests the probability distribution function of the peak location is calculated. The autoregressive filter order is optimized in the sense of minimum variance of the peak location. Simple sinusoidal signals with additive Gaussian noise are considered and the effect of other components is only indicated. Generally, a filter order between 1/3 and 1/2 of the total data number and a not very dense data sampling, gives the most stable spectrum. The results are numerical.

Interpretations of current and past results from ground-based solar diameter measurements, as well as the planning of scientific programs for the 1980’s, are strongly dependent on the perceived level of the degrading effects of the Earth’s atmosphere. One of the more effective approaches has been to design the observing program and the subsequent data analysis such that the solar diameter measurements themselves could provide an evaluation of atmospheric effects. Many important results have been obtained in studies of this type and these results are collected here to help in appraising the current situation. This evidence all points in one direction: the Earth’s atmosphere, while complicating the design of observational programs, is not the source of the oscillations observed in solar diameter measurements. Further, this same evidence indicates that the Earth’s atmosphere will not pose any serious limitations in ground-based solar diameter studies during the 1980’s.

Because of the renewed attention now paid to the solar diameter, its variations from equator to pole, or its secular or long-period changes, the question: what is a solar diameter? is not meaningless. Two kinds of definitions may be given: either astrophysical, each one relating to a specific physical parameter, or observational, relating to a given quantity to be measured. Only the second kind is directly accessible, and astrophysical definitions should be linked to these quantities, once they are determined with the highest possible accuracy. In practice, all the programs under way refer to the point of the limb where the brightness gradient is maximum, or to a higher order approximation of the shape of the profile. Two of them are compared: the Pic-du-Midi experiment, using fast scans of the limb to define the inflection point after a correction for the blurring effect of the atmosphere, and the SCLERA experiment, using the algorithm called FFTD to eliminate this correction. The advantage of a fast scan is emphasized, and the remark is formulated that, once the signal is digitized and stored, FFTD or any processing of it can be performed. In collecting day-long one-limb scans to calibrate the blurring correction, the authors have found fluctuations of the maximum brightness gradient which provide a new entry to the field of solar oscillations.

Short period oscillations (with periods less than 150 s) are shown to be non-uniformly distributed on the solar surface, and in time. Rather, they appear concentrated in short bursts which preferentially occur in regions with strong instantaneous downflow motion throughout the observed extent of the solar atmosphere.

A spectral-spatial analysis is made in the region of temperature minimum of the Sun's atmosphere. Filtergrams in the Ba II 4554 + 0.05 Å line have been used. The wavelengths corresponding to the sequence of peaks on the spectrum are in close agreement with those theoretically calculated by Ulrich and Rhodes (1977) for five-minute oscillations. We thus have good cause to surmize that the observed oscillations in the region of temperature minimum pertain to the p-mode class of general oscillations of the Sun, the main fraction of their energy falling on the period of 300 s.

Vertically propagating traveling waves have been observed in the solar photosphere. These waves have a period of 278 ± 41 seconds and a vertical phase velocity of about 2 km s-1. It is noted that these waves also have approximately the same period as the well-studied five-minute-period acoustic mode, which is evanescent in the photosphere. The only consistent interpretation of the traveling waves implies that they are gravity waves. About half the time the gravity waves are outgoing, while the remainder of the time they are ingoing.

The data were collected by Stebbins et al. (1980) to study the vertical structure of the photosphere. They examined velocity pertubations at nine altitudes in the photosphere using a Doppler shift technique. The current work represents a reanalysis of that data which uncovered the five-minute-period traveling waves.

The mean velocity amplitude of disturbances at a given altitude, as registered in the observed Doppler shift, was found to be directly proportional to the mean velocity amplitude at the base of the photosphere. This was the expected relationship between the velocity amplitudes. It was not expected that the standard deviation of the velocity amplitude at a particular altitude would be independent of the velocity amplitude at the base of the photosphere. In addition, the phase difference between velocities at different altitudes is, unexpectedly, inversely proportional to the velocity amplitude at any altitude. These two traits of the data are consistent with a five-minute-period nonacoustic traveling wave being superimposed on the five-minute-period acoustic mode. This supposition is borne out by a detailed examination of the data in the complex plane of amplitude and phase and by a calculation of the power spectrum of the traveling waves as a function of their vertical wavenumber (see Hill et al., 1982).

We consider the atmospheric behaviour of solar oscillations in a model including a detailed, semi-empirical atmosphere. Equations of radial and non-radial oscillation, with consistent treatment of the radiation field, are derived. These equations are solved in the radial, grey case; the results show that departure from the Eddington approximation has little effect on the properties of the oscillations. Preliminary results are presented for the non-radial case, indicating substantial deviations from the Eddington approximation when the optical thickness across a horizontal wavelength is of the order of or less than unity.

Recent observations of a cut-off frequency in the acoustic modes of the Sun (Claverie et al., 1981b) should help determine the mean molecular weight and, thereby, the helium abundance in the visible layers of the solar atmosphere. A first preliminary result of Y = 0.42 ± 0.04 is obtained for an assumed minimum photospheric temperature of 4400 K ± 200 K.