  We present an energy-conserving discontinuous Galerkin scheme for the
full-$f$ electromagnetic gyrokinetic system in the long-wavelength limit. We
use the symplectic formulation and solve directly for $\partial
A_\parallel/\partial t$, the inductive component of the parallel electric
field, using a generalized Ohm's law derived directly from the gyrokinetic
equation. Linear benchmarks are performed to verify the implementation and show
that the scheme avoids the Amp\`ere cancellation problem. We perform a
nonlinear electromagnetic simulation in a helical open-field-line system as a
rough model of the tokamak scrape-off layer using parameters from the National
Spherical Torus Experiment (NSTX). This is the first published nonlinear
electromagnetic gyrokinetic simulation on open field lines. Comparisons are
made to a corresponding electrostatic simulation.
