Solar and Stellar Oscillations and their Interaction with Radiation

by Jürgen Staude
in cooperation with Y.D. Zhugzhda, N.S. Dzhalilov (IZMIRAN)
and T. Horn, G. Bartling (AIP)

(You can download the full paper here!)
The rapidly increasing interest in investigations of oscillations and waves in the atmospheres and interiors of the Sun and other stars is inspired by two different aspects: waves are likely candidates of energy transport and heating in higher atmospheric layers, in active regions in particular, and helioseismological sounding provides a useful tool of diagnostics of the interior by observing the surface oscillations. Observations hint at a non-adiabatic behavior of the waves, very probably by interaction with radiation and turbulence. The influence of oscillations on the observed radiation is another important aspect of diagnostics.

So far existing models are limited to strongly simplifying assumptions, making any comparison with observations very difficult. We have been trying to approach the problems from various sides by doing both modeling and observations; below follow some short descriptions of such projects.

Radiation-hydrodynamic waves

The theory of radiation-hydrodynamic waves has been generalized to consider waves in a radiative, thermally conducting, homogeneous stellar atmosphere in LTE with a finite mean free path of photons (Dzhalilov et al., 1994; Staude et al., 1994), resulting in a self-consistent treatment for velocities of order (v/c). The results of our model calculations point out the shortcomings in the appoximations of existing theories and in their applications to the interpretation of observations.

The consideration of the influence of hydrodynamic waves on radiative transfer in a stratified atmosphere leads us to the conception of spectral darkening functions, that is a new effective tool for helio- and astero-seismology (Staude et al., 1994; Zhugzhda et al., 1996): The relative intensity fluctuations due to periodic disturbances in waves depend in a characteristic way on the observed position on the stellar disk and on the wavelength of the light, sometimes changing even the sign of the fluctuations when passing from the centre of the disk to the limb; the effect depends critically on the assumed non-adiabatic processes. This effect gives us the possibility to observe global acoustic p-modes with an angular degree l>3 even on stars. Moreover, it provides a new tool for checking the numerical models of p-modes and for imposing constraints on their outer boundary conditions. Supported by the German Space Agency DARA a special satellite experiment `DIFOS-M' has been developed to measure that effect on the Sun; it is planned to be launched on the Russian-Ukrainian solar satellite KORONAS-F in 1998.

Figure 1: Power spectra of velocity and magnetic field fluctuations versus frequency (in mHz) and the related time series (time in min) in a selected position on the umbra (Horn et al., 1997). The dashed horizontal lines are the 99% confidence limits corresponding to 2.33 sigma standard deviation according to Groth (1975).
Figure 2: Example from the time series of filtergrams obtained by EIT in the line combination Fe IX/X at 171A, formed in the upper transition region at a temperature of 1.3 x tex2html_wrap_inline65 K. The large sunspot the oscillations of which we are looking for forms the centre of the active region.

Sunspot oscillations

Sunspots are a unique laboratory to investigate the dynamics of a magnetized atmosphere and of magneto-atmospheric (MAG) waves in particular. Earlier modeling efforts (Staude, 1994) resulted in a scenario of coupled resonators or cavities acting at different heights in the sunspot atmosphere and subphotosphere, where various modes of MAG waves are partly trapped and partly transmitted. The predicted properties agree with those which are observed in different bands of oscillatory power peaks at periods around 3 min, 5 min, and perhaps 20 min. Some observations, however, are not yet understood; they seem to require non-adiabatic model calculations with a more general approach of radiation-magneto-hydrodynamic theory. Such calculations are now being prepared.

New observational techniques are now available which provide time-dependent, two-dimensional, spectro-polarimetric measurements with unprecedented high spatial resolution from ground-based observatories and from space as well. By means of a polarimeter in front of a Fabry-Perot interferometer at the German Vacuum Tower Telescope at Tenerife we collected a time series of two hours duration in a sunspot. The data reduction showed some unexpected results (Horn et al., 1997): The well known velocity oscillations are accompanied by significant oscillations of the magnetic field at the same resonant periods (see Figure 1). So far the possible existence of such magnetic oscillations has been controversially discussed. Maximum power is measured in those parts of the sunspot where we are looking along the lines of force of the magnetic field, thus demonstrating the longitudinal character of the oscillations with respect to the magnetic field direction. The oscillations are characterized by a marked spatial fine structure and a non-stationary behavior.

The SUMER and EIT instruments aboard the SOHO satellite have been used to get two-dimensional EUV observations of oscillations in the chromosphere and transition region to the corona above a large sunspot (see Figure 2). These unique data are now being treated; we expect crucial information on the non-adiabatic behavior of magneto-atmospheric waves in the higher sunspot atmosphere.

Selected Publications

  1. Dzhalilov, N.S., Zhugzhda, Y.D., Staude, J.: Radiation-hydrodynamic waves in an optically grey atmosphere. II. Analysis of wave propagation and effects of thermal conductivity in a homogeneous model. A&A 291, 1001 (1994)
  2. Horn, T., Staude, J., Landgraf, V.: Observations of sunspot umbral oscillations. Solar Phys. 172, 69 (1997)
  3. Staude, J.: Interpretation of sunspot oscillations. In: R.J. Rutten and C.J. Schijver (eds.), `Solar Surface Magnetism'. NATO ASI Series C 433, p. 189 (1994)
  4. Staude, J., Dzhalilov, N.S., Zhugzhda, Y.D.: Radiation-hydrodynamic waves and global solar oscillations. Solar Phys. 152, 227 (1994)
  5. Zhugzhda, Y.D., Staude, J., Bartling, G.: Spectral darkening functions of solar p-modes - an effective tool for helioseismology. A&A 305, L33 (1996)

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