Search for Travelling Waves

For a pair of wavelengths, λ₁ and λ₂, at a frequency ν, the power curves, P_λ1(ν) and P_λ2(ν) were created. The cross correlation coefficient, C(Δt, ν), of these power curves as a function of timeshift, Δt, is given by

This is used to evaluate the shift in P_λ2(ν) which results in maximum correlation with P_λ1(ν). This shift, Δt_{max_corr}, then corresponds to the timelag of P_λ2(ν) onto P_λ1(ν) at frequency ν, and C(Δt_{max_corr}, ν) is an indication of how good the correlation is. This procedure was then repeated for all frequencies, and between all pairs of wavelengths. From this timelag information, and from a knowledge of the approximate heights of formation (Table 5.1), the speed of possible travelling waves at specific frequencies were calculated (Bocchialini & Baudin 1995; Baudin, Bocchialini, & Koutchmy 1996).