Helioseismology

The structure of a star is uniquely determined by its mass and chemical composition. Unique models are constructed by varying the assumed composition with the known mass until the observed radius, luminosity, and surface temperature are matched. The process also requires assumptions about the convective zone. Such models can now be tested by the new science known as helioseismology.

Helioseismology is analogous to geoseismology; frequencies and wavelengths of various waves at the Sun's surface are measured to map the internal structure. On Earth the waves are observed only after earthquakes, while on the Sun they are continuously excited, probably by the currents in the convec-tive zone. While a wide range of frequencies are observed, the intensity of the oscillation patterns, or modes, peaks strongly at a mode having a period of five minutes. The surface amplitudes range from a few centimetres per second to several metres per second. The modes where the entire Sun expands and contracts or where sound waves travel deeply through the Sun, only touching the surface in a few nodes (i.e., points of no vibration), make it possible to map the deep Sun. Modes with many nodes are, by contrast, limited to the outer regions. Every mode has a definite frequency determined by the structure of the Sun. From a compilation of thousands of mode frequencies, one can develop an independent solar model, which reproduces the observed oscillations quite well. The frequencies of the modes vary slightly with the sunspot cycle.

As the Sun rotates, one half is moving toward us, and the other away. This produces a splitting in the frequencies of the modes (owing to the Doppler shift from the two halves of the Sun). Because the different modes reach different depths in the Sun, the rotation at different depths can be mapped. The interior below the convective zone rotates as a solid body. At the surface rotation is fastest at the equator and slowest at the poles. This differential rotation is easily visible as sunspots rotate across the solar surface, and it has been known since the first telescopic studies. At the equator the sunspots rotate at a 25-day rate, and at high latitudes at a 28- or 29-day rate. The differential rotation, apparently generated by the convective zone, is thought to play an important role in the generation of the magnetic field of the Sun. Much is not understood, however, for many solar features show less differential rotation.

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