Coupling between ion Bernstein waves and electron temperature gradient modes in magnetically confined plasmas under steep temperature gradients

Steep density and temperature gradients in collisionless magnetized plasmas alter high-frequency ion dynamics beyond the scope of conventional gyrokinetic ordering. Herein, the linear stability of electron-gyroradius-scale modes is analyzed in a shearless slab geometry using fully kinetic ion and gyrokinetic electron models. Ion Bernstein waves (IBWs) remain stable under moderate gradients; however, we reveal that sufficiently steep electron temperature gradients destabilize them through coupling between ion cyclotron and electron Landau resonances. These IBWs possess real frequencies near the ion cyclotron harmonics and become unstable at perpendicular wavenumbers below those at which slab electron-temperature-gradient (ETG) modes peak. In parameter regions where the IBW and ETG branches approach one another, an exceptional point characteristic of non-Hermitian eigenvalue problems arises, at which both eigenvalues and eigenfunctions coalesce. In the vicinity of this exceptional point, branch exchange produces a hybrid IBW–ETG mode characterized by a nonzero eigenvalue winding number. These results reveal a previously unrecognized kinetic pathway for IBW destabilization in steep-gradient plasmas and clarify the spectral structure arising from IBW–ETG coupling.