The Solar Oscillations Investigation (SOI) uses the Michelson Doppler Imager (MDI) instrument to probe the interior of the Sun by measuring the photospheric manifestations of solar oscillations. Characteristics of the modes reveal the static and dynamic properties of the convection zone and core. Knowledge of these properties will improve our understanding of the solar cycle and of stellar evolution. Other photospheric observations will contribute to our knowledge of the solar magnetic field and surface motions. The investigation consists of coordinated efforts by several teams pursuing specific scientific objectives. The instrument images the Sun on a 10242 CCD camera through a series of increasingly narrow spectral filters. The final elements, a pair of tunable Michelson interferometers, enable MDI to record filtergrams with a FWHM bandwidth of 94 mA. Normally 20 images centered at 5 wavelengths near the Ni I 6768 spectral line are recorded each minute. MDI calculates velocity and continuum intensity from the filtergrams with a resolution of 4″ over the whole disk. An extensive calibration program has verified the end-to-end performance of the instrument. To provide continuous observations of the longest-lived modes that reveal the internal structure of the Sun, a carefully-selected set of spatial averages are computed and downlinked at all times. About half the time MDI will also be able to downlink complete velocity and intensity images each minute. This high rate telemetry (HRT) coverage is available for at least a continuous 60-day interval each year and for 8 hours each day during the rest of the year. During the 8-hour HRT intervals, 10 of the exposures each minute can be programmed for other observations, such as measurements in MDI's higher resolution (1.25″) field centered about 160″ north of the equator; meanwhile, the continuous structure program proceeds during the other half minute. Several times each day, polarizers will be inserted to measure the line-of-sight magnetic field. MDI operations will be scheduled well in advance and will vary only during the daily 8-hour campaigns. Quick-look and summary data, including magnetograms, will be processed immediately. Most high-rate data will be delivered only by mail to the SOI Science Support Center (SSSC) at Stanford, where a processing pipeline will produce 3 Terabytes of calibrated data products each year. These data products will be analyzed using the SSSC and the distributed resources of the co-investigators. The data will be available for collaborative investigations.

The Solar Oscillations Investigation - Michelson Doppler Imager

The Solar Oscillations Investigation (SOI) uses the Michelson Doppler Imager (MDI) instrument to probe the interior of the Sun by measuring the photospheric manifestations of solar oscillations. Characteristics of the modes reveal the static and dynamic properties of the convection zone and core. Knowledge of these properties will improve our understanding of the solar cycle and of stellar evolution. Other photospheric observations will contribute to our knowledge of the solar magnetic field and surface motions. The investigation consists of coordinated efforts by several teams pursuing specific scientific objectives.

The Solar Oscillations Investigation — Michelson Doppler Imager

Astrophysical magnetic fields and nonlinear dynamo theory

Spatially well-organized electron-transfer reactions in a series of membrane-bound redox proteins form the basis for energy conservation in both photosynthesis and respiration. The membrane-bound nature of the electron-transfer processes is critical, as the free energy made available in exergonic redox chemistry is used to generate transmembrane proton concentration and electrostatic potential gradients. These gradients are subsequently used to drive ATP formation, which provides the immediate energy source for constructive cellular processes. The terminal heme/copper oxidases in respiratory electron-transfer chains illustrate a number of the thermodynamic and structural principles that have driven the development of respiration. This class of enzyme reduces dioxygen to water, thus clearing the respiratory system of low-energy electrons so that sustained electron transfer and free-energy transduction can occur. By using dioxygen as the oxidizing substrate, free-energy production per electron through the chain is substantial, owing to the high reduction potential of O{sub 2} (0.815 V at pH 7). 122 refs.

Heme/Copper Terminal Oxidases

The splitting of the frequencies of the global resonant acoustic modes of the Sun by large-scale flows and rotation permits study of the variation of angular velocity Ω with both radius and latitude within the turbulent convection zone and the deeper radiative interior. The nearly uninterrupted Doppler imaging observations, provided by the Solar Oscillations Investigation (SOI) using the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) spacecraft positioned at the L1 Lagrangian point in continuous sunlight, yield oscillation power spectra with very high signal-to-noise ratios that allow frequency splittings to be determined with exceptional accuracy. This paper reports on joint helioseismic analyses of solar rotation in the convection zone and in the outer part of the radiative core. Inversions have been obtained for a medium-l mode set (involving modes of angular degree l extending to about 250) obtained from the first 144 day interval of SOI-MDI observations in 1996. Drawing inferences about the solar internal rotation from the splitting data is a subtle process. By applying more than one inversion technique to the data, we get some indication of what are the more robust and less robust features of our inversion solutions. Here we have used seven different inversion methods. To test the reliability and sensitivity of these methods, we have performed a set of controlled experiments utilizing artificial data. This gives us some confidence in the inferences we can draw from the real solar data. The inversions of SOI-MDI data have confirmed that the decrease of Ω with latitude seen at the surface extends with little radial variation through much of the convection zone, at the base of which is an adjustment layer, called the tachocline, leading to nearly uniform rotation deeper in the radiative interior. A prominent rotational shearing layer in which Ω increases just below the surface is discernible at low to mid latitudes. Using the new data, we have also been able to study the solar rotation closer to the poles than has been achieved in previous investigations. The data have revealed that the angular velocity is distinctly lower at high latitudes than the values previously extrapolated from measurements at lower latitudes based on surface Doppler observations and helioseismology. Furthermore, we have found some evidence near latitudes of 75° of a submerged polar jet which is rotating more rapidly than its immediate surroundings. Superposed on the relatively smooth latitudinal variation in Ω are alternating zonal bands of slightly faster and slower rotation, each extending some 10° to 15° in latitude. These relatively weak banded flows have been followed by inversion to a depth of about 5% of the solar radius and appear to coincide with the evolving pattern of torsional oscillations reported from earlier surface Doppler studies.

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Helioseismic Studies of Differential Rotation in the Solar Envelope by the Solar Oscillations Investigation Using the Michelson Doppler Imager

THE rotation of the Sun is not that of a rigid body; at its surface, the gas near the poles has a lower angular velocity than that near the equator(1). This latitudinal variation persists to the base of the convection zone, below which the angular velocity becomes approximately uniform(2,3). Any variations of angular velocity at much greater depths are, however, poorly constrained(4-10). Observations of solar oscillation modes have been used to probe density variations in the Sun; rotational splitting of degenerate modes, although difficult to resolve, provides important constraints on the dynamical structure(11). Here we report observations of rotationally split: modes made over a three-year period with the Birmingham Solar Oscillations Network. Our results indicate that there is a substantial region inside the Sun that is rotating more slowly than the surface. This situation seems likely to be transient-the minimum-energy state would have all the deeper regions rotating with the same angular velocity-and is at variance with our current ideas about the rotational evolution of main-sequence stars(12). We have no solution to the dynamical problem this poses.

Slow rotation of the Sun's interior

GLOBAL acoustic oscillation modes of the Sun were discovered eleven years ago1. The possibility of temporal variations in the oscillation frequencies was suggested by fluctuations in the flux of solar neutrinos2, and would also be implied by changes in the size of the solar cavity or in the speed of sound within the Sun. Our group has studied solar oscillations for many years1,3; over the past decade, a network of three stations deployed at sites that can permit 24-h data collection has provided p-mode (acoustic) spectra of very high quality. Here we present evidence that the frequencies of the lowest-order (l≤2) modes have varied over the period of observation (1977–88) in a manner that is correlated with solar activity (as measured by sunspot number). The frequency variation has a peak-to-peak amplitude of 0.46±0.06μHz, and could reflect variations in the solar dimensions or in the sound speed in the Sun, which might in turn be due to changes in solar temperature and/or magnetic field.

Variation of low-order acoustic solar oscillations over the solar cycle

High sensitivity Doppier spectroscopy of integral sunlight in the 769.9 nm line of neutral potassium reveals several equally spaced lines of Q > 1,000 centred on the well known period of 5 min. These lines are interpreted as low l, high n overtones of the entire Sun and, if viewed in terms of current solar models, appear to imply a low heavy element abundance and a consequent low solar neutrino flux. Previous workers1–3 have used spatial resolution to study the 5-min oscillation of the solar photosphere and have shown that power is concentrated along narrow ridges in the k, w diagram. These oscillations are mainly low n, high l value acoustic modes of the entire Sun. In contrast to this, the present work (for a summary see ref. 4) is aimed at an overall view of the solar surface and consequently is biased to a study of low l value oscillations. These modes penetrate deeply5 and thus probe the structure as well as the interior angular velocity of the Sun6.

Solar structure from global studies of the 5-minute oscillation

The observation of line-of-sight velocities of the solar surface by means of Doppler shift measurements of the Fraunhofer absorption lines, using optical resonance scattering, has proved a useful technique in solar physics enabling the discovery of long period solar oscillation1, the discrete, regular structure in the 5-min oscillation2 and, as we report here the splitting of this structure by rotational effects. A measurement of the splitting of the discrete lines in the 5-min oscillations of the solar surface produced experimental evidence for the rapid internal rotation of the Sun. These data demonstrate that the Sun does not rotate uniformly but that the ‘core’ of the Sun rotates 2–9 times as rapidly as the observed surface rotation. The number of components into which the lines are split also allows unambiguous identification of 11 examples of each of the l = 0, l = 1 and l = 2 modes in the frequency range 2.40–3.85 mHz.

Rapid rotation of the solar interior

We present here high-accuracy determinations of the frequencies of low-l solar p-modes and their solar-cycle dependence. The data were obtained using the Birmingham network of solar spectrometers (BISON). The precision of the measurements is discussed. Our previously published results of a significant frequency shift between solar minimum and solar maximum, apparently independent of l and similar to that found by other workers for intermediate-l modes, is confirmed and extended. This suggests that at most only a small fraction of the variation is due to the solar core. Sets of frequencies at high and low solar activity, and an average corrected for solar-activity effects, are presented. There is now evidence that the solar-activity dependence of the frequencies varies across the 5 minute spectrum.

C. P. McLeod

Papers

Slow rotation of the Sun's interior

Variation of low-order acoustic solar oscillations over the solar cycle

Solar structure from global studies of the 5-minute oscillation

Rapid rotation of the solar interior

Solar p-mode frequencies and their dependence on solar-activity - recent results from the BISON network