Of Mikes and butterflies

More often than not, after having worked for some time on a problem related to dynamos or magnetic instabilities, I said to myself: "I should have read Michael Proctor before!" Actually, not many scientists have influenced magnetohydrodynamics so profoundly as he did. In a personally biased selection, I discuss some experimental and theoretical MHD topics which were strongly influenced by Michael Proctor's ideas. Those include: 1) The distinction between convective and absolute instabilities, which turned out to be essential for the experimental demonstration of the dynamo effect in Riga and the helical magnetorotational instability (MRI) in Dresden, 2) the Malkus-Proctor effect, as nicely illustrated by the saturation mechanism of the Riga dynamo, 3) double-diffusive magnetic instabilities, such as buoyancy instabilities, but also helical and azimuthal MRI and Super-AMRI, 3) spectral degeneracies of dynamo operators in diabolic or exceptional points, and their (putative) role for reversals of the geodynamo, 4) highly nonlinear dynamo mechanisms, such as MRI dynamos and Tayler-Spruit dynamos.