Is there an Anisotropy of p-Modes ?

Abstract

The question of the isotropy of the p-modes with respect to orientation on th solar disk was recently addressed by Kuhn (1984). He found significantly greater power at low wavenumbers (1≲30) in his measurements with the slit oriented in E-W direction as compared to the N-S orientation.

A similar experiment was conducted at the Capri station of the Kiepenheuer Institute, including the intermediate and the high wave-number range from 1 = 12 to 1 = 3000. In view of the question of isotropy the purpose of this experiment was both to provide an update of the older (Deubner, 1975) measurements which suffered from rather low statistical stability, and to have a second look at the lower wave-numbers, if feasible.

The one-dimensional scanning technique, using a long slit to average the data in the other dimension, was applied. For this reason, the N-S and the E-W recordings had to be obtained on two different days. (This certainly poses some problems with respect to changing seeing quality and instrument stability).

After correcting the E-W data for solar rotation two-dimensional FFT transforms were performed in order to compare the power in the two sets of data as a function of V and l. The preliminary results shown at this workshop unfortunately were largely mutilated by a trivial error in the data reduction code. The trends in the diagrams which seemed to support Kuhn’s results do not exist.

In the correct results, any excess power in the E-W scans is certainly less than 30% in amplitude, it is not peaked at low 1- values, and it is not restricted to frequencies with high p-mode power. It is expected that taking instrumental and seeing effects fully into account the upper limit of 30% can be lowered further substantially. This work is still in progress.

A full account of this study is in preparation for Astronomy and Astrophysics.