  Recent gyrokinetic stability calculations have revealed that the spherical
tokamak is susceptible to tearing parity instabilities with length scales of a
few ion Larmor radii perpendicular to the magnetic field lines. Here we
investigate this 'micro-tearing' mode in greater detail to uncover its key
characteristics, and compare it with existing theoretical models of the
phenomenon. This has been accomplished using a full numerical solution of the
linear gyrokinetic-Maxwell equations. Importantly, the instability is found to
be driven by the free energy in the electron temperature gradient as described
in the literature. However, our calculations suggest it is not substantially
affected by either of the destabilising mechanisms proposed in previous
theoretical models. Instead the instability is destabilised by interactions
with magnetic drifts, and the electrostatic potential. Further calculations
reveal that the mode is not significantly destabilised by the flux surface
shaping or the large trapped particle fraction present in the spherical
tokamak. Its prevalence in spherical tokamak plasmas is primarily due to the
higher value of plasma {\beta}, and the enhanced magnetic drifts due to the
smaller radius of curvature.
