are reduced in amplitude because of the increase in power from the noise; see Eqs.  and . It is therefore not possible to discriminate between a dataset with small noise and a small coupling coefficient, and another one with larger-amplitude noise and a larger coupling coefficient.

In other words, for a given power spectrum, which is due to the signal plus noise, stronger noise results in a smaller bispectrum. For the case of possible hexagonal convection cells, it might therefore be that their signal in the bispectrum is reduced by noise.

Results {#secresults}
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In this section, we show results for data averaged over the azimuthal direction of $\bmm$. We do not find any direction-dependence of the bispectrum.

The bispectrum of supergranulation {#secresultsbispec}
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Figure \[figbicoh\] shows the estimated bicoherence $\hat b$ from LCT. The bicoherence is significantly nonzero for a large number of combinations of wave vectors. The bicoherence is largest at wave vectors of similar amplitude, $|\bk|\approx|\bl|$. In general, it is strongest if the coupling vectors are close to the supergranular peak in the power spectrum, in our case at $m R_\sun = 155.5$. At these wave vectors, the amount of power due to a nonlinear component in 