Figure 5.

Linear analysis close to the onset pattern formation. Panel (a) represents the Fourier space for $\text{Re}\lambda (k)$, where the blue plane indicates $\lambda =0$. The condition for pattern formation is depicted in (b) as a projection; as $\eta$ increases towards a critical value ${\eta }_c$, the curve surpasses the $\{k_x,k_y\}$ plane, allowing a band of unstable modes to appear. This region is shown in (c), where the discrete unstable modes, represented by dots, are within the solid line, while those outside satisfies $\text{Re}\lambda (k)<0$. For $\eta =-0.219$, there are three unstable modes, as shown in (d), two obliques $O_l$ and $O_r$, and the horizontal one H.