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Ion-gyroradius-scale microinstabilities typically have a frequency comparableto the ion transit frequency. Due to the small electron-to-ion mass ratio andthe large electron transit frequency, it is conventionally assumed that passingelectrons respond adiabatically in ion-gyroradius-scale modes. However, ingyrokinetic simulations of ion-gyroradius-scale modes in axisymmetric toroidalmagnetic fields, the nonadiabatic response of passing electrons can drive themode, and generate fluctuations with narrow radial layers, which may haveconsequences for turbulent transport in a variety of circumstances. In fluxtube simulations, in the ballooning representation, these instabilities revealthemselves as modes with extended tails. The small electron-to-ion mass ratiolimit of linear gyrokinetics for electrostatic instabilities is presented, inaxisymmetric toroidal magnetic geometry, including the nonadiabatic response ofpassing electrons and associated narrow radial layers. This theory reveals theexistence of ion-gyroradius-scale modes driven solely by the nonadiabaticpassing electron response, and recovers the usual ion-gyroradius-scale modesdriven by the response of ions and trapped electrons, where the nonadiabaticresponse of passing electrons is small. The collisionless and collisionallimits of the theory are considered, demonstrating parallels in structure andphysical processes to neoclassical transport theory. By examining initial-valuesimulations of fastest-growing eigenmodes, the predictions for mass-ratioscaling are tested and verified numerically for a range of collisionfrequencies. Insights from the small electron-to-ion mass ratio theory may leadto a computationally efficient treatment of extended modes.

Extended electron tails in electrostatic microinstabilities and the nonadiabatic response of passing electrons