4. Pulse of the Sun
- The Sunís unseen depths are explored by tracing the in-and-out vertical oscillations of the visible solar gases.
- The oscillations detected in the visible solar disk have a period of about five minutes. They are caused by sound waves generated in the convective zone and trapped inside the Sun. The sounds are too low-pitched to hear even if they could get out of the Sun.
- The five-minute oscillations are detected by the Doppler effect of a spectral line seen in the visible sunlight of the photosphere. When the pulsation is out toward us, the wavelength is shortened, and when the motion is into the Sun and away from us the wavelength becomes longer.
- Observations of the five-minute solar oscillations have been used to detect low-pitched sound waves that travel to different depths within the Sun, enabling determination of the inner structure and dynamics of the Sun by the techniques of helioseismology.
- Different sound waves follow different paths inside the Sun, penetrating to different depths.
- Only certain notes are played inside the Sun; they are the sounds that hit the photosphere at the same place, over and over again. These sound waves are said to be moving within a resonant cavity.
- Significant helioseismology results have been obtained with instruments aboard the SOlar and Heliospheric Observatory, abbreviated SOHO, which has had a continued, uninterrupted view of the Sun for more than ten years. The Sun has also been followed 24 hours a day for years by a worldwide network of ground-based observatories known as the Global Oscillations Network Group, or GONG for short.
- The Sunís visible oscillations are the combined effect of about 10 million separate notes. They can be used to look deep inside the Sun, in much the same way that an ultrasonic scanner can peer inside a motherís womb and map out the shape of an unborn infant.
- The velocity of a sound wave depends on the temperature and composition of the material it travels in.
- A small but definite change in sound speed has pinpointed the bottom of the
convective zone. It is located at a radius of 0.713 Ī 0.003 of the radius of the visible solar disk.
- The abundance of elements in the Sun is inferred by comparing solar models of internal opacity and sound wave propagation to the helioseismology results, with small but important differences from the observed solar abundances.
- By measuring the internal velocity of sound waves, helioseismologists have taken the temperature of the Sunís energy-generating core, showing that it agrees with model predictions, at 15.6 million K, and apparently ruling out any astrophysical solution to the solar neutrino problem.
- Motion splits the sound-wave velocity, as well as the oscillation periods, like the speed of an airplane traveling with or against the wind.
- The visible solar disk rotates at different speeds at different solar latitudes, with a faster rate at the solar equator than the solar poles and a smooth variation in between.
- Regions near the Sunís visible poles rotate with slow speeds and rotation periods of 35 days, while the visible equatorial regions spin rapidly with rotation periods of 25 days.
- Differential rotation persists through the convective zone to about a third of the way inside the Sun, where the rotation becomes uniform from pole to pole.
- Just below the convective zone, the Sun is a rigid rotator with rotation speeds independent of latitude, like any solid body including the Earth. The rotation of the core of the Sun is unknown.
- The Sunís magnetism is probably sustained by dynamo action at the tachocline interface between the deep interior, which rotates at one speed, and the overlying gas that spins faster in the equatorial middle.
- Helioseismic tomography, or time-distance helioseismology, has been used with SOHO Michelson Doppler Imager, abbreviated MDI, data to show that poleward motions, also called meridional flows, extend deep beneath the photosphere. They move with speeds of about 20 meters per second.
- Helioseismology techniques have been used to show that after average rotation is removed from both MDI and GONG data, zonal bands of slower- and faster-rotation, sometimes called torsional oscillations, are detected, which extend throughout the convective zone. The zonal rotation bands migrate in solar latitude together with belts of sunspots over the 11-year solar activity cycle.
- After subtracting the contributions of differential rotation, zonal rotation bands, and poleward meridional circulation, a swirling pattern of residual flows is found; this subsurface solar weather increases in complexity with the 11-year cycle of solar magnetic activity.
- The Sun is not strictly spherical, and its asphericity changes with solar activity, at least in the outer layers of the Sun.
- Helioseismic tomography has been used with SOHO MDI data to look beneath sunspots, and to discover that strong converging flows push and force the sunspot magnetic fields together, keeping sunspots from expanding at their edges and dispersing into the surrounding photosphere.
- A 10-year sequence of photosphere observations with SOHOís Global Oscillations at Low Frequencies, abbreviated GOLF, instrument exhibits a periodic structure that could be due to gravity waves, or g-modes, generated in the deeper parts of the Sun. If confirmed, this could also suggest that the solar core could be rotating three to five times faster than the radiative zone.
Copyright 2010, Professor Kenneth R. Lang, Tufts University