Excitation and Damping of Solar P-Modes

Citation

Kumar, Pawan (1988) Excitation and Damping of Solar P-Modes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/q6y2-3263. https://resolver.caltech.edu/CaltechETD:etd-09092008-090628

Abstract

I have carried out detailed analysis of the interaction of acoustic radiation with homogeneous turbulence in order to understand the excitation of solar p-modes by turbulent convection. The most significant outcome of this investigation is the finding that, for certain types of forced turbulences, the absorption of acoustic waves is no greater than a free turbulence, whereas the emission is always enhanced by a factor M⁻², where M is the Mach number of the turbulence. Turbulent convection in the sun is an example of this kind of turbulence. This leads to the conclusion that energies in solar p-modes, due to their interaction with the convection, should be approximately equal to the thermal energy in a resonant eddy. This is found to be in good agreement with the observations. The ideas developed in the above work have been applied to explain the recently observed absorption of acoustic waves by sunspots as well. Work has also been carried out to determine the probability distribution function for the time averaged energy of stochastically excited modes. We hope to learn about the nature of the excitation and damping processes for the solar modes by comparing this theoretically determined distribution with the observations.

In an effort towards resolving the overstability question of solar p-modes, I have investigated the effectiveness of 3-mode couplings, the most plausible process for limiting the amplitudes of overstable modes. The 3-mode coupling mechanism is also a good candidate for exciting fundamental modes which are found to be linearly stable, but are observed to have energies comparable to p-modes of similar frequencies. The issue of mode stability remains inconclusive due to the unknown energies of modes with period ~3.5 minutes. However, we find the fundamental modes to be damped as a result of mode couplings and hence they require excitation by a mechanism other than the overstability.

Thesis Files