Duvall, Harvey and Pomerantz (1986) reported the existence of a ‘structural asymmetry’ inside the Sun. We show that this asymmetry is not a consequence of the Sun's rotation. We attribute the asymmetry, rather, to a toroidal magnetic field inside the Sun. Consistency requires a field of about one megagauss located in the lower part of the convection zone. Accounting for such a field would wreak havoc on our understanding of the solar dynamo and convection.

The component of the frequency splitting of solar five-minute oscillations observed by Duvall, Harvey and Pomerantz that is even in azimuthal degree measures latitudinal and depth variations in the structure of the sun. We indicate how the data hint that there is a shallow perturbation, possibly associated with a magnetic field, that is concentrated at low latitudes.

The 5 minute oscillations in a sunspot umbra are the response of the sunspot to forcing by the 5 minute p-modes in the surrounding convection zone (Thomas 1981). This interaction of solar p-modes with a sunspot can be used to probe the structure of a sunspot beneath the visible surface of the Sun (Thomas, Cram, and Nye 1982). Here we report briefly the results of both an observational study and a simple theoretical analysis of this interaction. A full account of these results will be published elsewhere (Abdelatif, Lites, and Thomas 1986; Abdelatif and Thomas 1987).

Doppler shifts of the nonsplit line Fe I 7090Å have been measured simultaneously in the sunspot umbra, penumbra and the photosphere using a 100 × 100 diode array. The power in the range 2.5–4.5 mHz is found to decrease from the photosphere towards the umbra. Besides the minimum at the center of the umbra, further power minima occur at the transition region between umbra and penumbra. No significant power is found in the range 5.0–6.0 mHz.

In this paper, The author has discussed the coupling processes between the magnetic field and the unsteady plasma motion, and analysed the features of the energy storage and conversions in the active region.

In an expansion scheme in velocity space, the first order perturbations of a stellar system bear close resemblance to those of a fluid. This feature is exploited to study the structure of the Hilbert space of the linear perturbations of a stellar system, to provide a classification for the modes, and to construct ansatz for variational calculations. The first order non-radial modes appear to be trispectral in that they are predominantly derived from a scalar potential, a toroidal vector potential, or a poloidal vector potential. The eigenfrequencies and the eigenfunctions of radial (ℓ = 0) and non-radial (ℓ, = 1) modes of polytropes and of truncated isothermal distributions are calculated. The density waves associated with these modes are also reported.

A comparison of three separate years (1980, 1984, 1985) of SMM/ACRIM solar total irradiance data reveals small but significant changes in the frequencies of low-degree solar p-modes. Specifically, a decrease in the mean frequency of the nine strongest ℓ = 0 and 1 oscillation peaks was observed between 1980 and 1984 such that

Data obtained at Izana (Tenerife) and Haleakala (Maui), using optical resonant scattering with a potassium vapour cell over the years 1980–84, are used to determine the frequencies of the low ℓ p modes. Possible variation in these frequencies with the solar cycle are investigated.

During the summer seasons of the years 1977 to 1985 daily velocity measurements of solar global oscillations have been obtained using a resonant scattering spectrometer. After calculating the power spectra of the daily residuals, the mean for each season is found. Several discrete frequency intervals are defined in the spectrum which yield information on the characteristics of the oscillations and noise levels. The mean power in these intervals and the cut-off frequency of the p-mode spectrum, determined for each year, are correlated with the solar activity cycle. Furthermore, several series of 13 contiguous days for each year are analyzed. The frequency and amplitude of the p modes of 1 ≤ 3 and 11 ≤ n ≤ 33 are determined and their average for these years is found. The frequency differences between modes of the same degree 1 and between neighbouring modes are found. Their variation over the years is compared with the solar activity cycle.

From the analysis of radial velocity measurements of the Sun, obtained at Izaña during long observing seasons covering ten years, from 1976 to 1985, stable periods longer than 1 day, have been found in the observed signal. In particular the appearance of an oscillation with a 13 day period, discovered by Claverie et al (1982), has been confirmed. The comparison, for 1981 - 82 - 83 and 84, of the observations with a calibrated numerical model of the passage of inhomogeneities (spots and plages) on the solar surface, shows that the signal is not only due to this effect. The obtained signal from the model has a phase lag of almost two days relative to the observed one. So, it is believed that another velocity field, probably related to the surface inhomogeneities, contributes to the observed signal.

Measurements of the line of sight velocity of the sun with respect to earth have been obtained at Izaña (Tenerife) during the years 1976 to 1985. The mean values found for each year show a trend of ~30 m/s from minimum to maximum. Their mean value is of 583.1 ± .2 m/s which is 92% of the gravitational red shift predicted by theory and their variation seems to be related to the solar cycle with the clear exception of 1985. The most likely interpretation is that the velocity limb shift effect, averaged over the whole sun, is the cause of the slight disagreement with theory and this effect changes with time.

A spherical-harmonic-fourier analysis of the maximum areas of sunspot groups listed in Ledgers I and II of Greenwich photoheliographic results for 1933–1954 yield significant peaks at the 11y periodicity for some spherical harmonic modes: especially the mode (ℓ = 6, m = 0). A similar analysis of the daily areas of the spotgroups during 1944–1954 yields 11y periodicity peaks only for some non-axisymmetric modes. These results suggest that the sunspot activity may be physically related to long period global oscillations of the sun.

Long periods have been detected in solar diameter measurements obtained during the past 10 years. The same periodicities are also present in other global Sun parameters such as the 10 cm radio flux, the total irradiance, and possibly the neutrino flux. Special attention is paid to periods corresponding to the solar cycle (“C”), 1000 days (“P”) and 320 days (“W”). Studying the respective phases of the signals give some insight to the possible physical origins of these variations.

The Paris observatory is in possession of a 53 y record of solar diameters and sunspot positions during the Maunder minimum (1666–1719). For the period (1666 to 1684), the solar diameter was 32′9″ and slowly decreased down to 32′6″ when sunspot activity had resumed. Nowadays, the solar diameter is 32′2″ when taking into account of the solar irradiance. During the same period, the sunspot rotation was smaller than the present one (3% less near the equator), and the differential rotation was greater.

The two phenomena (a larger Sun and a slower rotation) suggest that the Sun undergoes a cyclic expansion and contraction, on timescale of several centuries.

Asteroseismology is interpreted as an extension of the field of variable stars, and not just as the stellar analogue of helioseismology. The main effects of stellar mass and evolution on oscillation frequencies are discussed with the help of simplified wave-propagation diagrams. Frequency separations resulting from asymptotic expressions are compared with the corresponding results from numerical computations. The validity of asymptotic theory can be gauged in this way. The seismological issues of solar-like stars and Ap stars are discussed in some detail, and a progress report on the equation of state for stellar interiors is given. The review ends with a summary of properties and important physical problems for selected classes of variable stars.

Helioseismology bas proved to be a powerful tool to probe the internal structure of the sun. With a new adapted optical resonance spectrophotometer, an extension has been attempted to two bright stars, namely α Centauri Å and Procyon. Results obtained from two observing runs on α Centauri Å in May 1983 and May 1984 at La Silla, and one observing run on Procyon in February 1984 at Hawaii are presented. In both cases, solar-like pressure oscillations have been detected. The complete analysis is presented, which leads to determine essentially five parameters: the frequency range, the amplitudes, the mean equidistance Δν, the departure around this equidistance (curvature of the echelle-diagram) and the fine frequency spacing between modes of degree 0 and 2. For Procyon, all results are consistent with theoretical predictions, including excitation of oscillations, mass, radius, and age of the star. For α Centauri, all the results are consistent together and are confirmed by the 1984 observations. They suggest that α Cen might be younger than estimated, consistent with a zero age main sequence star.

This is the abstract of a paper recently published in Astronomy and Astrophysics. We have selected here under some parts of this paper. For more detail, it is recommended to refer to this journal.

The discovery of global solar 5-min oscillations has prompted the search for such oscillations in other stars. The main problem in detecting these variations is their expected small amplitude (in the sun it is less than 1 m/s in velocity and 10−6 in integrated light). Two recent studies by Fossat et al. (1984) and Noyes et al. (1984) have suggested that the 5-min oscillations in α Cen and ε Eri may have been detected.

We attempted to detect these oscillations by means of conventional broad-band photometry in the components of three double star systems (α Cen, HR4677 and HR5568). We selected these stars because they cover a wide range of spectral types (F2V to M2V). It was hoped that there may be a region where the pulsation amplitudes are particularly large. By using one component of a double star system to correct for transparency variations in the other, we also expected to obtain a larger signal-to-noise ratio.

A double magneto optical filter attached to the Cassegrain focus of the 1.9 m reflector of the South African Astronomical Observatory was used to observe Procyon in the 769.9 nm line of potassium during all clear hours of six nights. Velocity calibration was provided by the rotational and orbital velocity changes of the observer as well as by a continuous magnetic modulation.

Twelve cool Ap stars are known at present to undergo rapid light oscillations at low amplitude (ΔB < 0m.012) and with periods of minutes. These rapidly oscillating Ap (roAp) stars also exhibit amplitude modulation with timescales of days, which correspond to the magnetic/rotation periods (when those have been measured). In at least one case (HR 3831), 180° phase shifts in the dominant oscillation are seen twice per modulation cycle.

Photometric observations of some Ap stars with a 50 cm telescope have been carried out at Izaña (Tenerife) in February 1986. The stars observed were, generally, magnetic - cool Ap stars. A search for global oscillations in the range 3 to 15 minutes has been undertaken and they seem to be present in some of the stars observed.