described by a PDE that in addition includes a nonlinear term. In this case, the Fourier components of the solution would not be independent but would show some dependence that might be measurable. Supergranular dynamics are strongly nonlinear [according to]{} @Rincon2017 and divergence maps have a large nonzero skewness [@Duvall2000]. This should be reflected in the bispectrum.

Introductions to bispectral analysis are given by @Brillinger1965, @Kim1979, @Priestley1981 [Section 11.5.1], @Nikias1987, and [@Chandran1990]. Bispectral analysis has been used in a number of research domains. For example, it was used to measure nonlinear wave–wave coupling [e.g., @Kim1980] of ocean and surface water waves [e.g., @Elgar1989], to study [[non-Gaussianity]{}]{} in the cosmic microwave background [e.g., @Komatsu2001; @Ade2016], and in laboratory plasma physics [e.g., @Itoh2017]. The bispectrum is related to the spectral transfer of kinetic energy in turbulent flows [e.g., @Yeh1973]. In the context of stellar convective dynamos, measurements of the spectral energy transfer in magnetohydrodynamical simulations have been taken for example by [@Brandenburg2001], [@Moll2011], and [@Strugarek2013].

In this work, we apply bispectral analysis to near-surface horizontal divergence maps of supergranulation obtained by [@Langfellner2018] using local correlation tracking [LCT, @November1988] and time-distance helioseismology [TD, @Duvall1993] and data from the Helioseismic and Magnatic Imager [HMI, 