The Future of Helio- and Asteroseismology (L.Gizon) Millions of modes of vibration, excited by turbulent convection, permeate the solar interior. Surface observations of the motions caused by these waves enable solar physicists to see inside the Sun, just as geophysicists can probe the internal structure of the Earth using records of seismic activity. Over the past twenty years, helioseismology has produced a considerable number of discoveries in solar, stellar, and fundamental physics. The best is still to come, however: three dimensional helioseismic techniques offer unique prospects for probing turbulent magnetoconvection and uncovering the mechanism of the solar cycle, while the extension of seismic investigations to distant stars will open a new era of observational stellar research. A cutaway drawing of the Sun showing a particular mode of global acoustic oscillation (right). The blowup reveals horizontal flows 1000 km below a sunspot deduced from local helioseismology (the surface magnetic field is shown in red and the Earth gives the scale). The Sun s hot and extended corona is visible in the background.
The study of solar oscillation requires long, nearly continuous observations from space or from ground-based networks of telescopes. The most precious dataset so far is provided by the MDI instrument aboard the SOHO spacecraft, an international collaboration between the European Space Agency (ESA) and NASA. Since 1996, MDI has been returning one image per minute of the Sun s surface velocity field. A spectral analysis reveals that oscillatory power peaks near 3 mhz and is localized along ridges in wavenumber-frequency space. These ridges correspond to different overtones of the acoustic modes of oscillation (i.e. pressure is the restoring force). Modes with similar horizontal phase speed (the ratio frequency/wavenumber) propagate down to similar depths inside the Sun. Methods of helioseismology can be divided into two classes: global and local. The more traditional technique of global helioseismology consists of measuring the millions of frequencies of oscillation and searching for a seismic solar model whose frequencies of oscillation match the observed ones, within measurement uncertainties. The solution to this problem tells us about the Sun s large-scale structure and rotation as a function of depth and (unsigned) latitude. Restricting the analysis of solar oscillations to mode frequencies, however, does not bring about the full potential of helioseismology. Recent methods of local helioseismology produce three dimensional maps of the solar interior. The basic idea is to measure the time it takes for solar waves to propagate through the interior between any two locations on the surface. Wave travel times contain the seismic signature of buried inhomogeneities and flows situated along the propagation paths. Global helioseismology has provided by far the most precise tests for the theory of stellar structure and evolution, probably the most complete theoretical structure in astrophysics. In particular, modeling the equilibrium structure of the Sun requires a good understanding of how energy flows from the nuclear burning core to the surface. One of the first triumphs of helioseismology was the precise measurement of the depth of Sun s outer convective envelope. This enabled us to identify the correct model of the convection zone among a wide variety of predictions. In addition, the values of opacities, which determine the transport of radiation through matter, had to be revised to accommodate the seismological constraint on the internal temperature profile. We can now measure the temperature throughout the solar interior to a precision better than 0.05%. This example shows that the Sun can be used as a laboratory to study microphysical phenomena at densities and temperatures so extreme that they cannot be reached by experiments on Earth. Another great contribution of helioseismology concerns the solar neutrino problem. As neutrino detectors became accurate enough, it was found that the number of solar neutrinos measured on Earth was surprisingly low. Early attempts to explain the discrepancy proposed that solar nuclear reaction rates, controlled by temperature and pressure in the solar core, were slightly different from what solar models predicted. However, this hypothesis became untenable when conditions in the solar core became known with sufficient accuracy -- thanks to global helioseismology. It is clear today that it is the standard model of particle physics which is at fault: neutrinos are not massless
and change flavor as they travel through space, some escaping detection on Earth by traditional neutrino detectors. Perhaps one of the most exciting aspect of helioseismology, however, is the possibility of discovering new phenomena beyond the standard solar model, which by definition is spherically symmetric and does not take into account the effects of mass motions or the magnetic field. Of central importance is the search for clues regarding the origin and variability of the Sun s magnetic field, which lies at the heart of all solar active phenomena such as flares, coronal mass ejections, x- and γ-ray emission, etc. The origin of the Sun s magnetic field is not understood yet; it is perhaps the most important unsolved problem in solar physics today. The general belief is that a dynamo process is responsible for the solar magnetic cycle. In this scenario, magnetic field lines are stretched and twisted by internal shearing motions. Variations in the internal rotation with radius and latitude play a prominent role in the theory of the solar dynamo. Motions in the north-south direction are also believed to be important to explain why the global magnetic field reverses polarity every eleven year. One of the main goals of helioseismology is to map these internal mass motions, structural asphericities, and their temporal variations, in order to help solve the mystery of the solar cycle. Time-varying component of solar rotation, showing the migration in latitude of bands of faster (red) and slower (blue) angular velocity. Credit: J. Schou and MDI-SOHO/ESA/NASA. Global helioseismology has revealed regions of rotational shear in the Sun s interior. In the convection zone, rotation varies mostly with latitude: faster at the equator (25 day period) than at high latitudes (35 day period). The radiative zone as a whole appears to rotate like a rigid body with a period of about 27 days. Thus, at low or high latitudes, a layer of sharp radial rotational shear exists near the bottom of the convection zone (the tachocline), a preferred location for the seat of the solar dynamo. Furthermore, there is a strong radial gradient in the angular velocity immediately below the surface; this needs to
be studied in more detail with local helioseismology. The first direct evidence of a connection between internal mass motions and the large-scale characteristics of the solar cycle came from the detection of latitudinal bands of faster and slower rotation which exhibit an eleven year periodicity and persist deep into the convection zone. One theory suggests that it could be part of a migrating dynamo wave. Mysterious quasi-periodic changes in the rotational velocity with a period of about 1.3 year, which were discovered near the bottom of the convection zone, are an indicator that our understanding of the solar internal dynamics is still very fragmentary. Local helioseismology, although still a young science, has pinpointed flows between the equator and the poles, which could serve as a mechanism for the latitudinal transport of the magnetic flux and determine the period of the solar cycle. In addition, local helioseismology provides detailed 3D maps of local flows in the upper 5% of the convection zone, which have been named solar subsurface weather. Motions appear to be highly organized in the vicinity of magnetic active regions. Away from active regions are complex flows like meanders, jets, and vortices, possibly related to the largest scales of deep convection. Smaller scale phenomena can also be studied, such as supergranulation, a convective pattern, and sunspots. Wave-speed perturbations, caused by a combination of thermal and magnetic anomalies, have been detected below sunspots. Cellular flows around sunspots may explain why they can remain stable structures for many weeks. In yet another application, local helioseismology can be used to construct maps of active regions on the far side of the Sun. These few examples illustrate the richness of the science possible with local helioseismology. In all these cases a taste of the possibilities has been provided, but better data and in particular further developments in the technique are required to realize the full potential. Solar subsurface weather revealed by local helioseismology. Horizontal flows 1000 km below the surface (arrows) show a connection between mass motions and regions of magnetic activity (red/green shades). Credit: L. Gizon and MDI-SOHO/ESA/NASA.
Wave speed perturbations below a sunspot. Red is faster than average. The maximum depth is 24000 km. Credit: A. Kosovichev and MDI-SOHO/ESA/NASA. The next big technological step for helioseismology will come with HMI on the Solar Dynamics Observatory of NASA to be launched in 2008. With a high spatial resolution over the entire visible solar hemisphere, HMI is the first instrument specifically designed for local helioseismology. In a low Earth orbit, it will make available a flood of data (50 Mbit/s over at least 5 years) to the whole scientific community. Among the most important science goals will be the study of the fine structure and the temporal evolution of magnetic regions in the upper convection zone. In the 2013 timeframe, ESA is planning to send Solar Orbiter on an interplanetary journey that will lead it out of the plane of the ecliptic. Solar Orbiter should give access, for the first time, to the subsurface structure and dynamics of the Sun s polar regions. Also, using Solar Orbiter s views of the far side of the Sun in combination with other Earth-side observations, stereoscopic local helioseismology will enable us to probe deeper into the convection zone with improved resolution. Future missions for helioseismology: the Solar Dynamics Observatory of NASA (left) and Solar Orbiter of ESA (right).
With these new observations, will come the need for improvements in solar modeling. Much is expected from powerful numerical simulations to identify some of the mechanisms that control the interplay between rotation, convection, and magnetism. Numerical simulations will also be required to understand wave propagation in strongly magnetized fluids, a necessary condition for the application of local helioseismology to active regions. Traditionally, travel times have been computed in the geometrical acoustics approximation. Improved computations that take wave effects into account will provide more reliable inversions, as is done in geophysics. Local helioseismology is very much under development today and promises many more discoveries. Among the most ambitious goals is to directly image the magnetic field in the solar interior. If measurable, this information will be retrieved from local anisotropies in wave propagation, wave speeds being different along and across magnetic field lines. An important discovery would certainly be the detection of longitudinal variations in the structure of the tachocline. Asteroseismology is the study of distant stars using various modes of oscillations. Many stars, covering a wide range of masses and evolutionary states, are known to exhibit global oscillations. The challenge for astronomers is to measure spectra of oscillations that are rich enough to provide stringent constraints on internal structure. The seismology of white-dwarf stars (gravity-mode pulsators) has been particularly successful. Only in the last few years, however, has it been possible to detect oscillations on solar-like stars using large ground-based telescopes. Such discoveries followed spectacular advances in technology, driven by the search for exoplanets. Since the surface of a star is not resolved, only modes associated with the smoothest spatial patterns (radial, dipole and quadrupole) can be detected, so that asteroseismology is at the stage of early global helioseismology. The high-overtone frequencies of solarlike oscillations form a nearly regularly spaced pattern, from which can be extracted two basic parameters: the large and small frequency separations. The large frequency separation is the inverse of the sound travel time across a stellar diameter, a global property intimately connected with the mean stellar density. The small frequency separation, on the other hand, is strongly weighted toward the central parts of the star and decreases with stellar age as helium content increases in the core. Assuming that other parameters of a star, such as its composition, are known, then its mass and age can be determined with unprecedented precision from the small and large frequency separations. It is expected that such knowledge for a sufficient sample of stars will revolutionize stellar evolution and galactic evolution studies. Accurately determined frequencies of stellar oscillations contain much more information, well beyond the large and small frequency separations. They have the potential to lead to the detection of sharp features in stellar interiors, such as the borders of convection zones or changes in chemical ionization. Locating convection zones would enable to calibrate the theory of energy transport by convection, which remains very approximate. An estimate of internal rotation can also be determined with asteroseismology. Such
information would help understand dynamo-generated stellar activity cycles, as well as the solar-stellar connection. Furthermore, there is the possibility to determine the inclination of the rotation axis of the star, which is particularly interesting in the case of binary stars and central stars of planetary systems. These exciting possibilities for the study of stellar structure, evolution, and activity will be fully realized only once highquality oscillation spectra become available for a large sample of stars. Power spectrum of oscillations on α Cen A, a solar-type star, using one week of observations at Europe s Very Large Telescope in Chile. Inset shows the Sun s spectrum of oscillations (no spatial resolution). Credit: R. Butler et al., Astrophysical Journal, vol. 600, L75 (2004). Asteroseismology is entering a very exciting period of discoveries with opportunities for high-precision velocity measurements using sophisticated spectrographs on large telescopes. Currently, the best measurements are made at the European Southern Observatory in Chile, with the HARPS spectrograph on the 3.6-m telescope at La Silla and the UVES spectrograph at the 8-m class Very Large Telescope at Paranal. The precision on the frequencies of the global modes of stellar oscillations, however, is very much limited by available telescope time, which is why dedicated space telescopes are an attractive solution to obtain nearly uninterrupted long-term coverage of many types of pulsating stars. The satellites WIRE and MOST have already detected stellar oscillations in intensity light curves. High precision photometry from space is expected in 2006 with the launch of COROT of the ESA. The field of asteroseismology will make much progress in the following decades with more ambitious missions like NASA s Kepler and possibly ESA s Eddington. Perhaps one day it will even be possible to spatially resolve many hundreds of modes of oscillation on a single star with optical interferometry from space, as is proposed by the Stellar Imager project of NASA. As in the case of helioseismology, there is a strong need to improve our understanding of the oscillations and how they interact with the magnetohydrodynamical processes in stars.
The Very Large Telescope of ESO (left) and the satellite COROT of CNES/ESA (right) will deliver data of unprecedented quality for asteroseismology.