Kinetic treatments of drift-tearing modes that match an inner resonant layer solution to an external MHD region solution, characterised by $Delta^{prime}$, fail to properly match the ideal MHD boundary condition on the parallel electric field, $E_{parallel}.$ In this paper we demonstrate how consideration of ion sound and ion Landau damping effects achieves this and place the theory on a firm footing. As a consequence, these effects contribute quite significantly to the critical value of $Delta^{prime}$ for instability of drift-tearing modes and play a key role in determining the minimum value for this threshold.
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