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.
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.
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
K. The large
sunspot the oscillations of which we are looking for forms the centre of the
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.
- 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)
- Horn, T., Staude, J., Landgraf, V.:
Observations of sunspot umbral oscillations.
Solar Phys. 172, 69 (1997)
- 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)
- Staude, J., Dzhalilov, N.S., Zhugzhda, Y.D.:
Radiation-hydrodynamic waves and global solar oscillations.
Solar Phys. 152, 227 (1994)
- Zhugzhda, Y.D., Staude, J., Bartling, G.:
Spectral darkening functions of solar p-modes - an effective tool
A&A 305, L33 (1996)
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