  We construct a description of Ion Temperature Gradient (ITG) driven localised
linear modes which retains both wave-particle and magnetic drift resonant
effects while capturing the field-line dependence of the electrostatic
potential. We exploit the smallness of the magnetic drift and the strong
localisation of the mode to resolve the problem with a polynomial-gaussian
expansion in the field-following co-ordinate. A simple semi-analytical formula
for the spectrum of the mode is shown to capture long wavelength Landau
damping, ion-scale Larmor radius stabilization, weakening of Larmor radius
effects at short-wavelengths and magnetic-drift resonant stabilisation. These
elements lead to linear spectra with multiple maxima as observed in gyrokinetic
simulations in stellarators. Connections to the transition to extended
eigenfunctions and those localized by less unfavourable curvature regions
(hopping solutions) are also made. The model provides a clear qualitative
framework with which to interpret numerically simulated ITG modes linear
spectra with realistic geometries, despite its limitations for exact
quantitative predictions.
