Waves in the Solar Atmosphere

Abstract

The study of waves in the solar atmosphere started with an attempt to explain the observed Doppler shifts (wiggly line spectra) and broadening of photospheric spectral lines. Then Biermann (1946) and Schwarzschild (1948) suggested that acoustic waves, generated in the hydrogen convection zone, supplied the non­ radiative energy needed to heat the chromosphere and corona. In 1959 Leighton (1960) discovered that most of the solar surface was covered with regions that oscillated with periods near five minutes. Although it was assumed that these observed motions and theoretically postulated waves were related, development of these two aspects has been rather independent. The heating models have concen­ trated on shock wave propagation and dissipation, without much consideration of the observed motions, while the oscillation models have concentrated on explaining the oscillations without regard for their implications for energy and momentum transfer. The early observations have been summarized by Noyes (1967), and theories of wave generation, propagation, and heating have been reviewed by Lighthill (1967), Schatzman & Souffrin (1967), and Kuperus (1969). Michalitsanos (1974) has reviewed the recent literature on the five-minute oscillation. Research on solar motions has recently progressed in three areas. 1. Computers have made possible the numerical integration of the nonlinear equations of motion. This has shown that the upper chromosphere and corona can be heated by the five-minute oscillation (and so brought observations and heating models closer together), but short-period waves are needed to heat the low chromosphere. 2. A driving mechanism, which utilizes thermal overstability of trapped subphotospheric modes, has been suggested for the oscillations. This complements penetrative convection and turbulent motions in the convection zone, which generate short­ wavelength waves. 3. Observations of oscillations and transients in active regions