  This paper develops a weighted $L^2$-method for the (half) Dirac equation.
For Dirac bundles over closed Riemann surfaces, we give a sufficient condition
for the solvability of the (half) Dirac equation in terms of a curvature
integral. Applying this to the Dolbeault-Dirac operator, we establish an
automatic transversality criteria for holomorphic curves in K\"ahler manifolds.
On compact Riemannian manifolds, we give a new perspective on some well-known
results about the first eigenvalue of the Dirac operator, and improve the
estimates when the Dirac bundle has a $Z_2$-grading. On Riemannian manifolds
with cylindrical ends, we obtain solvability in the $L^2$-space with suitable
exponential weights while allowing mild negativity of the curvature.
