Evolution of the wavelength of the fastest-growing mode, λ_eMRI, of the extended MRI. All coloured points meet the generalized instability criteria. Focusing on the region close to the equatorial plane – the one that becomes progressively more suited for the instability to develop – we see that λ_eMRI becomes progressively smaller and smaller, of the order of 1 − 10 m at late times. This means that in the central region the characteristic wavelength of the instability is small, thus making the instability more difficult to resolve.