We can look inside the Sun by observing the slow, rhythmic, in-and-out motions of the photosphere. These widespread throbbing oscillations are caused by internal sound waves. When the sounds strike the photosphere and rebound back down, they disturb the gases there, causing them to move in and out with a period of about five minutes.
The sound waves are trapped inside the Sun and cannot travel through the near vacuum of space. Nevertheless, when these sounds propagate upward to the photosphere they disturb the gases there and cause them to rise and fall, producing widespread throbbing motions (Fig. 4.1). These vertical oscillations can be tens of thousands of meters high and travel a few hundred meters per second. Such movements are imperceptible to the eye, but sensitive instruments on the ground and in space routinely pick them out. They are detected as tiny, periodic changes in the wavelength of a well-defined spectral line, or as miniscule variations in the Sunís total light output.
When oscillations move part of the photosphere toward Earth, the wavelength of light emitted from that region becomes shorter, the wave fronts or crests appear closer together, and the light therefore becomes bluer. This shift occurs because each successive wave has a shorter distance to travel than the one before it did in order to reach Earth, so the distance between waves, the wavelength, becomes shorter. When the oscillations carry localized regions away from Earth, the wavelength becomes longer and the light redder. Each wave has farther to travel than the one before it did. The magnitude of the wavelength change, in either direction, establishes the velocity of motion along the line of sight, which is called the radial velocity.
Astronomers peel back the outer layers of the Sun, and glimpse inside it by examining sounds with different paths within the Sun. They use the term helioseismology to describe such investigations of the solar interior. It is a hybrid name combining the Greek word helios for Sun or light, the Greek word seismos meaning quake or tremor, and logos for reasoning or discourse. So literally translated helioseismology is the logical study of solar tremors. Geophysicists similarly unravel the internal structure of the Earth by recording earthquakes, or seismic waves, that travel to different depths; this type of investigation is called seismology. The techniques resemble the way that Computed Axial Tomography (CAT) scans derive views inside our bodies from numerous readings of X-rays that cross them from different directions.
The photosphereís oscillations are the combined effect of about 10 million separate notes. Each of these notes travels along a unique path inside the Sun, and sounds of different frequency or pitch descend to different depths (Fig. 4.2). Some of them stay within the convective zone, while others travel to the very center of the Sun. To trace our starís physical landscape all the way through Ė from its churning convective zone down into its radiative zone and core Ė we must determine the precise pitch (frequency) of all the notes.
The combined sound of all the notes reverberating inside the Sun has been compared to a resonating gong in a sandstorm, being repeatedly struck with tiny particles and randomly ringing with an incredible din. The Sun produces order out of this chaos by reinforcing certain notes that resonate within it, like the plucked strings of a guitar. They are called standing waves.
Scientists have examined the oscillation power in the various surface oscillations, or how often each and every note is played, confirming that the power is concentrated into such resonant standing waves. Instead of meaningless, random fluctuations, orderly motions are detected with specific combinations of size and period, or wavelength and frequency (Fig. 4.3). Destructive interference filters out all but the resonant waves that combine and reinforce each other. Yet, there are still millions of such notes resonating in the Sun, so prolonged observations with high spatial resolution and detailed computer analysis are required to sort them all out.
The dominant factor affecting each sound is its speed, which in turn depends on the temperature and composition of the solar regions through which it passes. The sound waves move faster through higher-temperature gas, and their speed increases in gases with lower than average molecular weight. So, by investigating many different waves we can build up a very detailed three-dimensional picture of the physical conditions inside the Sun, including the temperature and chemical composition, from just below the surface down to the very core of the Sun. The observed frequencies are integral measures of the speed along the path of the sound wave; helioseismologists have to use complex mathematical techniques and powerful computers to invert this measured data and get the sound speed.
Small sound-speed discrepancies between measurements and theory are significant (Fig. 4.4). Just below the convection zone, there is an increase in the observed sound speed compared to the model, suggesting that turbulent material is mixing in and out within this base layer. Since the speed of sound depends on temperature as well as composition, the temperature might also increase in this place. Without additional information, scientists cannot distinguish between temperature and compositional mixing as causes of discrepancies from standard models. There is a sharp decrease of the observed speed relative to theoretical expectations at the boundary of the energy-generating core, hinting that either cooler material or turbulent churning motions might occur there. Scientists speculate that they could be due to unstable nuclear burning processes. If substantiated by further studies, this could be very important for studies of stellar evolution; they usually assume that nuclear reactions proceed without any mixing of fresh material into the core.
Helioseismology instruments have shown how the Sun rotates inside, using the Doppler effect in which motion changes the pitch of sound waves. Regions near the Sunís poles rotate with exceptionally slow speeds, while the equatorial regions spin rapidly. This differential rotation persists to about a third of the way inside the Sun, where the rotation becomes uniform from pole to pole. The Sunís magnetism is probably generated at the interface between the deep interior, that rotates at one speed, and the overlying gas that spins faster in the equatorial middle. Internal flows have also been discovered by helioseismologists. White-hot currents of gas move beneath the Sun we see with our eyes, streaming at a leisurely pace when compared to the rotation. They circulate near the equator, and between the equator and poles, describing a sort of solar meteorology. Internal tremors, or sunquakes, have also been detected, generated by flaring explosions in the solar atmosphere.
For at least a century, astronomers have known from watching sunspots that the photosphere rotates faster at the equator than it does at higher latitudes, decreasing in speed evenly toward each pole. The photosphere spins about the Sunís rotation axis with a sidereal rotation period, from east to west against the stars, of 25.7 days; its rotation period reaches 33.4 days at 75 degrees latitude north or south. These rotation periods can be converted into velocities - just divide the circumference at the latitude by its period.
Armed with this sensitive technique, scientists have found that the latitude-dependent rates exhibited by the photosphere persist throughout the convection zone (Figure 4.5). However, the rotation speed becomes uniform from pole to pole nearly one-third of the way to the core, 220 million meters beneath the photosphere. Lower down the rotation rate remains independent of latitude, acting as if the Sun were a solid body.
Thus, the rotation velocity changes sharply at the top of the radiative zone. There the outer parts of the radiative interior, which rotates at one speed, meet the overlying convection zone, which spins faster in its equatorial middle. Scientists suspect that the forces generated by the two zones moving against each other may create the Sunís magnetic field.
The roughly 20-million-meter-wide layer where these very different zones meet and shear against one another is the likely site of the solar dynamo, the source of the Sunís magnetism. It amplifies and regenerates the Sunís magnetic field within the solar interior. The hot circulating gases, which are good conductors of electricity, generate electrical currents that create magnetic fields; these fields in turn sustain the generation of electricity, just as in a power-plant dynamo. The same thing also happens in the fluid interiors of many planets, including the Earth.
Scientists have recently used helioseismology observations from both the ground and space to examine rotation rates at the bottom edge of the convection zone, providing important clues to how the solar dynamo works. The Sun does not rotate at a fixed rate down there. The rotation speed varies periodically, spinning fast and slow and then fast again (Fig. 4.5). These alterations in rotation speed have a period of 1.3 years, or 16 months, in equatorial regions. There is a more complicated variation with a dominant 1.0-year period at higher latitudes.
Our starís interior flows in ways other than rotation. Ponderous, slow rivers of gas circulate beneath the visible surface of the Sun. They have been detected by measuring travel times and distances for sound waves that probe regions just below the photosphere.
The method is quite straightforward: SOHOís MDI instrument records small periodic changes in the wavelength of light emitted from a million points across the Sun every minute. By keeping track of them, it is possible to determine how long it takes for sound waves to travel from one point on the solar photosphere, through the interior, to another point. The time taken to skim through the Sunís outer layers tells of both the temperature and gas flows along the internal path connecting the two points. If the local temperature is high, sound waves move more quickly - as they do if they travel with the flow of gas. This data is then inverted in a computer to chart the three-dimensional internal structure and dynamics of the Sun, including the sound speed, flow speed, and direction of motion.
The MDI has provided travel times for sounds crossing thousands of paths, linking myriad surface points. Researchers have fed one year of nearly continuous observations into a supercomputer, using it to work out temperatures and flows along these intersecting paths. After weeks of number crunching, the SOHO scientists have identified vast new currents coursing through the Sun and clarified the form of previously discovered ones (Fig. 4.6). These flows are not the dominant, global rotational motions, but rather the ones found when rotation is removed from the data. Their speeds only reach tens of meters per second.
Looking Beneath Sunspots
For centuries people have wondered about those strange dark spots on the Sun.What holds them together, so they last for weeks without breaking apart?The outward pressure of their strong magnetic fields ought to make sunspots expand at their edges and disperse into the surrounding photosphere, just as magnets with like polarity repel each other.And how far do the sunspots extend below the photosphere?Are they magnetic islands floating on the top of the convective zone, or are they anchored deep within it?These questions were finally answered when helioseismic tomography, or time-distance helioseismology, was used to trace out the motions of hot flowing gas in, around, and below a sunspot (Fig. 4.7).The helioseismologits detected strong converging flows around the sunspot and downward directed flows in it.The adjacent streams of gas strengthen and converge towards the sunspots, pushing and concentrating the magnetic fields into them.Cool down-flowing material beneath the sunspots may also draw the surrounding gas and magnetic fields inward.Swirling currents of gas beneath solar active regions may account for powerful outbursts known as solar flares.
Detecting Active Regions on the Far Side of the Sun
Scientists are also now using sound waves to see right through the Sun to its hidden, normally invisible, backside, describing active regions on the far side of the Sun days before they rotate onto the side facing Earth.They use the technique of helioseismic holography with observations of the Sunís oscillations to create a sort of mathematical lens that focuses to different depths.A wide ring of sound waves is examined, which emanates from a region on the side of the Sun facing away from the Earth, the far side, and reaches the near side that faces the Earth (Fig. 4.8).
When a large solar active region is present on the backside of the Sun, its intense magnetic fields cool the gas there, thus lowering the level at which sound waves are reflected.A sound wave that would ordinarily take about 6 hours to travel from the near side to the far side of the Sun and back again takes approximately 12 seconds less when it bounces off an active region on the far side.When nearside photosphere oscillations are examined, the quick return of these sound waves can be detected.
Images and movies of active regions on the far side of the Sun are available at and .
Solar astronomers are using this technique to monitor the structure and evolution of large regions of magnetic activity as they cross the back side of the Sun, thereby revealing the regions that are growing in magnetic complexity or strength and seem primed for explosive outbursts.Since the solar equator rotates with a period of 27 days, when viewed from the Earth, this can give more than a weekís extra warning of potential solar flares or coronal mass ejections before the active region swings into view, threatening the Earth with the possible intense radiation, energetic particles or mass ejections from these outbursts.
Waves in the Sunís Core
In 2007 scientist working with SOHO data reported their detection of long-period resonant oscillations that are generated in the deeper parts of the Sun and have their largest amplitudes there (Fig. 4.9).And they are known as gravity waves, or g-modes, because it is the force of gravity that determines how quickly they rise and fall.They are produced when a parcel of gas oscillates above and below an equilibrium position, like waves in the deep sea.When a high-density parcel moves up into a lower-density region, it is pulled back into place by gravity, and then moves back due to the restoring force of buoyancy.In contrast, sound waves are restored by pressure, and are therefore designated p-modes with p for pressure.
Gravity waves become evanescent, or non-propagating, in regions where the gas is not stably stratified, such as the turbulent convective zone.As a result, they are largely confined to the Sunís deep interior where they are the strongest.
Since the g-mode detection has not been confirmed by any other measurements, there is still a possibility that it may not survive the test of time.If confirmed, however, these exciting reports of gravity waves and hints of a rapidly spinning core will become one of the key discoveries of helioseismology.