Interruption of exponential growth of MTI modes with whistler suppression of thermal diffusivity. Left column: Compensated amplitude of the MTI eigenmode as a function of time (nondimensionalized by the growth rate s) for perturbations of different wavenumbers: k = 2πm/L, with m = 1, 2, 12, 36. The green vertical lines show when the second term in Eq. (B.3) becomes unity, which roughly corresponds to the point where the thermal diffusivity is suppressed. Right column: Real part of MTI dispersion relation and growth rates for our runs (top panel), and amplitude of the numerical prefactor A as a function of the wavenumber (bottom panel). The gray shaded area indicates the wavenumbers for which the growing MTI eigenmodes are in the regime of slow conduction (i.e., k²χ₀/s < 1).