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New Results on the Excitation of Solar Oscillations (1Jun95)
(from NSO, NOAO Newsletter No. 42, June 1995)

A group that includes P. Goode (NJIT), T. Stebbins (JILA), L. Strous (NJIT)
and T. Rimmele (NSO) is studying mechanisms that excite the solar global
oscillations. Simultaneous, high-resolution observations of solar granulation
and acoustic events in the photosphere were performed at the VTT/SP using a
narrow band (20mD) filter. The acoustic events observed in the photosphere
arise from an impulsive action that feeds mechanical energy into the
oscillations beneath, and thus are an observable by-product of the excitation of
solar oscillations (Goode, Gough and Kosovichev 1992, ApJ, 387 ). We find
that acoustic events occur preferentially in the dark intergranular lanes. This
indicates that "acoustic noise" is generated in narrow downdrafting plumes of
the granular convection as has been suggested by Rast (1995, ApJ, 443). At
the site of a typical acoustic event, the local granulation becomes darker over
several minutes leading up to the event, with a further abrupt darkening
immediately preceding the peak of the event. Thus, the excitation of solar
oscillations seems more closely associated with the rapid cooling occuring in
the upper convection layer, rather than with the overshooting of turbulent
convection, itself. We also find that the generation of acoustic noise seems to
be suppressed in magnetic regions.

T. Rimmele, P. Goode, T. Stebbins, L. Strous

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