4. Pulse of the Sun


Pulsating Sun

Pulsating Sun

. Sound waves inside the Sun cause the visible solar disk to move in and out. This heaving motion can be described as the superposition of literally millions of oscillations, including the one shown here for regions pulsing in (red spots) and out (blue spots). The sound waves, represented here by black lines inside the cutaway section, resonate through the Sun. They are produced by hot gas churning in the convection zone, which lies above the radiative zone and the Sunís core. (Courtesy of John W. Harvey, National Optical Astronomy Observatories, except cross sections.)


Sound Paths

Sound Paths

. Sound rays are bent inside the Sun, like light within the lens of an eye, and circle the solar interior in spherical shells or resonant cavities. Each shell is bounded at the top by a large density drop near the photosphere and bounded at the bottom by an increase in sound speed with depth that refracts a downward propagating wave back toward the surface. The bottom turning points occur along the dotted circles shown here. How deep a wave penetrates and how far around the Sun it goes before it hits the surface depends on the harmonic degree, l. The white curve is for l = 0, the blue one for l = 2, green for l = 20, yellow for l = 25 and red for l = 75. (Courtesy of JÝrgen Christensen-Dalsgaard and Philip H. Scherrer.)


An l-nu Diagram

An <i>l</i>-nu Diagram

. The frequency, nu, of sound waves is plotted as a function of the spherical harmonic degree, l, for just eight hours of high-resolution data taken with the SOHO MDI instrument. A frequency of 3 milliHertz, or 0.003 cycles per second, corresponds to a wave period of five minutes. The degree, l, is the inverse of the spatial wavelength, or surface size; an l of 400 corresponds to waves on the order of 10 million meters in size. The oscillation power is contained within specific combinations of frequency and degree, demonstrating that the surface oscillations are due to standing waves confined within resonant cavities. (Courtesy of the SOHO MDI/SOI consortium. SOHO is a project of international cooperation between ESA and NASA.)


Radial Variations of Sound Speed

Radial Variations of  Sound Speed

. Red and blue correspond to faster and slower sound speeds, respectively, relative to a Standard Solar Model (yellow). When the sound travels faster than predicted by theory, the temperature is higher than expected; slow sound waves imply temperatures that are colder than expected. The conspicuous red layer, about a third of the way down, shows unexpected high temperatures at the boundary between the turbulent outer region (convective zone) and the more stable region inside it (radiative zone). Latitudinal variations in temperature are seen near the photosphere (center). These speed of sound measurements were made by the MDI/SOI and VIRGO instruments aboard the SOHO spacecraft. (Courtesy of Alexander G. Kosovichev, the SOHO MDI/SOI consortium, and the SOHO VIRGO consortium. SOHO is a project of international cooperation between ESA and NASA.)


Internal rotation of the Sun

Internal rotation of the Sun

. The rotation rate inside the Sun, determined from helioseismology. The outer parts of the Sun exhibit differential rotation, with high latitudes rotating more slowly than equatorial ones. This differential rotation persists to the bottom of the convective zone at 28.7 percent of the way down. The rotation period in days is given at the left axis, and the corresponding angular velocity scale is on the right axis in units of nanoHertz, abbreviated nHz, where 1 nHz = 10-9, or a billionth, of a cycle per second. A rotation rate of 320 nHz corresponds to a period of about 36 days (solar poles), and a rate of 460 nHz to a period of about 25 days (solar equator). The rotation in the outer parts of the Sun, at latitudes of zero (solar equator), 30, 45, 60 and 75 degrees, has been inferred from 144 days of data using the Michelson Doppler Imager, abbreviated MDI, aboard the SOlar and Heliospheric Observatory, or SOHO for short. Just below the convective zone, the rotational speed changes markedly, and shearing motions along this interface may be the dynamo source of the Sunís magnetism. By examining more than five years of low-order acoustic modes, obtained using the GOLF and MDI instruments aboard SOHO, the rotation rate has been inferred for the deep solar layers (error bars), mainly along the solar equator. There is uniform rotation in the radiative zone, from the base of the convective zone at 0.713 solar radii to about 0.25 solar radii. The acoustic modes (sound waves) do not reach the central part of the energy-generating core. (Courtesy of Alexander G. ďSashaĒ Kosovichev for the MDI data showing differential rotation in the convective zone, and Sebastien Couvidat, Rafael GarcŪa and Sylvaine Turck-ChiŤze for the GOLF/MDI data in the radiative zone. SOHO is a project of international cooperation between ESA and NA)


Interior Flows

Interior Flows

. In this picture, red corresponds to faster-than-average flows, yellow to slower than average, and blue to slower yet. On the left side, deeply rooted zones (yellow bands), analogous to the Earthís trade winds, travel slightly faster than their surroundings (blue regions). The right-hand cutaway reveals a slow movement poleward from the equator shown by the streamlines; the return flow below it is inferred. This image is the result of computations using one year of continuous observation, from May 1996 to May 1997, with the Michelson Doppler Imager (MDI) instrument aboard SOHO. (Courtesy of Philip H. Scherrer and the SOHO SOI/MDI consortium. SOHO is a project of international cooperation between ESA and NASA.)


What Lies Beneath A Sunspot

What Lies Beneath A Sunspot

. The temperature and flow structure under a sunspot have been probed using local helioseismology techniques with data obtained by the Michelson Doppler Imager instrument aboard the SOlar and Heliospheric Observatory, or SOHO for short. The intense magnetic fields in and below a sunspot act as a plug that prevents the up-flow of energy from the hot solar interior. As a result, the sunspot is cooler and darker than its surroundings (dark blue region in the bottom cross section). Heat builds up below the magnetic plug, so the material underneath the sunspotís magnetic fields becomes hotter (red area in cross section). The converging flows of surrounding cooler material, denoted by the arrows, help hold a sunspot together. (Courtesy of the SOHO MDI consortium. SOHO is a project of international cooperation between ESA and NASA.)


Looking through the Sun

Looking through the Sun

. The arcing trajectories of sound waves from the far side of the Sun are reflected internally before reaching the front side, where they are observed with the Michelson Doppler Imager, abbreviated MDI, aboard the SOlar and Heliospheric Observatory, or SOHO for short, or the Global Oscillation Network Group, abbreviated GONG. Here we show a two-skip correlation scheme (left) and a one-skip/three-skip correlation scheme (right) of seismic holography used to image active regions (focal point) on the otherwise unseen far side of the Sun. These methods permit complete seismic imaging of the entire far hemisphere of the Sun, provided daily at http://soi.stanford.edu/data/full_farside/ or http://gong.nso.edu/. This permits scientists to detect potentially threatening active regions on the far side of the Sun before the Sunís rotation brings them around to the front side that faces the Earth. [Adapted from Douglas C. Braun and Charles Lindsey, Astrophysical Journal (Letters) 560, L189 (2001).]


Gravity waves

Gravity waves

. A cross section of the solar interior showing the ray paths of sound waves (p modes) and gravity waves (g modes). Sound waves produce oscillations detected in the photosphere, and can be used to determine the internal properties of the Sun down to about 0.2 of the solar radius. Gravity waves never reach the photosphere and are instead turned around inside the Sun, probing its central depths and reaching the core.