We have analyzed 9 yr of non-imaged Doppler velocity observations of the visible disc of the Sun in an effort to search for pseudo-mede-like structure in the data above the acoustic cut-off frequency of the solar atmosphere (approximate to 5400 muHz). These data were collected by the ground-based Birmingham Solar-Oscillations Network (BiSON) over the period 1992 January through 2000 December. Our analysis uncovers the presence of a pseudo-mode-like structure above the acoustic cut-off frequency that persists up to approximate to 8500 muHz, with a spacing between adjacent peaks (or troughs) of similar to 68 muHz. The signature - which disappears at higher frequencies - has a slightly different repeat period (i.e., frequency separation between successive peaks or troughs) to that found by Garcia et al. (1998) in full-disc GOLF data.

The predominant contribution to the frequency splitting of low-l solar p modes arises from the rotation of the solar interior and this lifts the frequency degeneracy in to give a symmetric pattern where the observed (synodic) separation between adjacent m (i.e., for \Deltam\ = 1) is similar to400 nHz. Magnetic fields can also contribute to the splitting, but they do so in such a way as to introduce asymmetries in the arrangement of the components within each multiplet. In full-disc data this effect may become apparent when l greater than or equal to 2. Here, we extract estimates of the frequency asymmetry for l = 2, investigate variations over the activity cycle and compare tile results with the near-surface predictions of Dziembowski et al. (2000) which are based upon observations from higher l.

We make use of 9 yr of full-disc helioseismic data - as collected by the ground-based Birmingham Solar-Oscillations Network (BiSON) - to search for low-frequency, low-angular-degree (low-l) acoustic modes. A range of tests are applied to the power spectrum of the observations that search for prominent mode-like structure: strong spikes, structure spanning several bins signifying the presence of width (from damping), and the occurrence of prominent multiplet structure at l greater than or equal to 1 arising principally from the solar rotation and made from several spikes separated suitably in frequency. For each test we present analytical expressions that allow the probability that the uncovered structure is part of the broad-band noise background to be assessed. These make use of the cumulative binomial (Bernoulli) distribution and serve to provide an objective measure of the significance of the detections. This work has to date uncovered nine significant detections of non-broad-band origin that we have identified as low-l modes with radial overtone numbers n less than or equal to 9.

We present data taken from BiSON to illuminate solar-activity related changes in the oscillation parameters. We also use the full dataset to hunt for low order p-modes.

We have studied variations in the frequencies of low-l solar p modes through the analysis of nine years of helioseismic data collected by the Birmingham Solar Oscillations Network (BiSON)dagger. This is the first time that such a long data set has been explored with the extra accuracy afforded by fitting the modes to asymmetric profiles. The epoch covered (1991-99) spans the declining activity phase of solar cycle 22, and a substantial portion of the initial activity increase during cycle 23. The complete time series has been split into contiguous segments of length 27, 54, 108 and 216 d in order to facilitate the study of changes occurring on different time-scales. Further, we have characterized the observed shifts as a function of six well-known indicators of solar activity. These indices reflect changes taking place in the photosphere, chromosphere and corona, but only over the visible hemisphere of the Sun. Since the low-l eigenfrequencies respond to global variations in activity, we discuss the implications of this mismatch for the analyses performed.We demonstrate that, as expected, the low-l modes adjust to changes in the activity measures on time-scales as short as a few months. Our analysis indicates that all six proxies correlate equally well (at the level of precision of the data) with the measured shifts. Further, the sensitivity of the shifts to changes in five of the activity indices is the same, to within similar to 15 per cent (1 sigma) or so, on the falling and rising phases considered. There is, however, a slight suggestion that the sensitivity to changes in the disc-averaged line-of-sight magnetic field component (as determined from daily Kitt Peak magnetograms) may be higher on the rising phase of the cycle.

We have analyzed 60 months of non-imaged Doppler velocity observations of the visible disc of the Sun in an effort to search for pseudo-mode-like structure in the data above the acoustic cut-off frequency of the solar atmosphere (approximate to 5400 mu Hz). These data were collected by the ground-based Birmingham Solar-Oscillations Network (BiSON). Our analysis uncovers marginally significant evidence for the presence of a pseudo-mode-like structure in the range 5900 less than or equal to nu less than or equal to 6600 mu Hz, with a spacing between adjacent peaks (or troughs) of similar to 70 mu Hz. The signature which, owing to the low signal to noise, disappears at higher frequencies - has a similar repeat period and phase to that found by Garcia et al. (1998) in full-disc GOLF data. However, the zero-to-peak power is a factor of similar to 10 weaker in strength.

The results of the global sound-speed inversion obtained with solar p-mode frequencies provided by the recent high-quality observational data (BiSON, SOI/MDI) are presented and discussed. The iterative nonlinear inversion technique used here is a generalization (for the case of exact solution of the adiabatic oscillation equations) of the Born quasiasymptotic approximate inversion developed by Marchenkov et al. (2000).

We present low-l rotational p-mode splittings from the analysis of 8 yr of observations made by the Birmingham Solar-Oscillations Network (BiSON) of the full solar disc. These data are presented in the light of a thorough investigation of the fitting techniques used to extract them. Particular attention is paid to both the origin and magnitude of bias present in these estimates. An extensive Monte Carlo strategy has been adopted to facilitate this study-in all, several thousand complete, artificial proxies of the 96-month data set have been generated to test the analysis of real 'full-disc' data. These simulations allow for an assessment of any complications in the analysis which might arise from variations in the properties of the p modes over the 11-yr solar activity cycle. The use of such an extended data set affords greater precision in the splittings, and by implication the rotation rate inferred from these data, and reduces bias inherent in the analysis, thereby giving a more accurate determination of the rotation. The grand, weighted sidereal average of the BiSON set is 434 +/-2 nHz, a value consistent with that expected were the deep radiative interior (r/R<0.5) to rotate at the same frequency, and in the same 'rigid' manner, as the more precisely and accurately studied outer part of the radiative zone.

We present the results of a non-linear inverse analysis for the hydrostatic, spherically symmetric component of the solar internal structure using the observed p-mode frequencies. The iterative non-linear inversion technique used here is based on the succesive Born approximation description of solar p-modes developed by Roxburgh & Vorontsov. This description can give a high resolution of regions of rapid variation of seismic parameters with depth (e.g., the base of the convection zone), and accounts accurately for the strong influence of gravity perturbations on low-degree modes which penetrate deep into the solar core. The inversion procedure is non-linear; the eigenfrequency equation obtained from the Born approximation is solved by iteration. The particular target of our inverse analysis is to achieve the highest possible resolution of the region near the base of the solar convection zone, searching for possible signatures of penetrative convection, element diffusion and/or strong magnetic fields. The results of the global inversion obtained with solar p-mode frequencies provided by the recent high-quality observational data (GONG, SOI/MDI, GOLF) are presented and discussed.

The inverse scattering problem for reconstruction of the structure of reflecting potential from the observed frequency dependence of the phase shift of reflected acoustic waves is considered. The linearized formulation of the ill-posed inverse problem is used, which is solved using a perturbation technique. The potential perturbation of the standard model as a combination of five B-splines leads to a constructive solution of the discrepancy problem between the observational and theoretical frequencies of the 5-min oscillations. The discrepancy is reduced by an order of magnitude. The corresponding change of the shape of the reflecting potential is interpreted as a requirement of a general increase of convection efficiency in the standard solar model. In this way, the agreement of the oscillation frequencies of high degree is also improved.

The inverse scattering problem for recovery of the structure of the reflecting potential is investigated on the basis of the observed phases of acoustic-wave reflection. The linearized formulation of the ill-posed inverse problem, which is solvable with perturbation theory, is used. The perturbation of the potential of the standard model as a combination of five B-splines leads to a constructive solution of the problem of the discrepancies among the frequencies of the 5-min oscillations. The mismatch is reduced by an order of magnitude. The corresponding change in the reflecting potential profile is interpreted as a general increase in the convection efficiency. Agreement in the frequencies of oscillations with large values of l is also achieved.

The accurately measured frequencies of intermediate-degree acoustic oscillations permit to infer the frequency dependence of the phase shift, corresponding to the reflection of the trapped acoustic waves from the solar surface. This function is sensitive to the structure of the outermost solar layers down to to the depth of second helium ionization zone. It deviates significantly from the predictions of standard solar models, representing the main source of discrepancies of oscillation frequencies in all the degree range. We report the results of studying this problem in the framework of adiabatic approximation, using both linearized inversions for the reflecting acoustic potential and direct tests of a variety of solar envelope models.

Normal mode spectra and neutrino counting rates are calculated for a set of chemically-inhomogeneous solar models. Each model has a core with a high concentration of heavy elements; high opacity makes the core convective. The structure of the envelope is that of the standard model. It is shown that (1) the spectrum of g modes becomes less densely separated than that of the standard model, which simplifies the problem of interpreting 160-min oscillations; (2) low neutrino counting rates may be achieved for a low initial helium concentration in the core; (3) the models do not contradict the frequency spacing of global 5-min oscillations.