Twenty-one years of the ponderomotive force model of the first ionization potential effect

The ponderomotive force model of the first ionization potential (FIP) effect was first presented as a poster at the 2004 AAS meeting in Denver, CO (May 30th-June 3rd), and later as a talk at COSPAR in Paris (18th-25th July). The ponderomotive force appears to have been a completely new means of ion-neutral separation in solar physics or astrophysics, though it was not new to science. It represents an analogue in magnetohydrodynamics (MHD) of the optical component of a magneto-optical trap, now common in cold atom and related research. As it was, it was not until I read about Arthur Ashkin's Nobel Prize in 2018 in Physics Today that the penny dropped about the precise physics involved. Alfvén waves in the chromosphere refract from the ionized component of the plasma, and the ion recoil induces an ion-neutral separation. The coronal magnetic field geometry dictates the properties of Alfvén waves through resonance and interference, leading to the different FIP effects observed in different coronal structures, though ideas about the wave origins have evolved. I illustrate the varieties of FIP fractionation that can occur, and how such phenomena may be used to investigate other aspects of solar physics. This article is part of the Theo Murphy meeting issue 'Solar atmospheric abundances in space and time'.