On ion-frequency electron temperature gradient modes

Electron temperature gradient (ETG) modes are known to be capable of driving substantial electron heat transport in tokamaks. In pedestal plasmas specifically, they can exhibit complex structures and have recently shown unexpected behavior. Unlike the traditional picture in core plasmas, where they propagate in the electron diamagnetic direction, pedestal ETG modes can propagate in the ion diamagnetic direction. Using the GENE code and the ETG dispersion relation, we investigate ion-frequency ETG across relevant scales. The ion-frequency modes are linked to strong plasma shaping, high safety factor, and steep gradients, typical of pedestal conditions. Smooth transitions toward ion frequencies indicate they are not a separate branch but a natural feature of the ETG modes in edge-like conditions. Comparisons with core plasmas suggest these modes are restricted by the η threshold in such conditions. These results clarify the nature of ion-frequency ETG modes and advance our understanding of pedestal microturbulence at electron scales.