  Injecting magnetized plasma with changes in magnetization or injection rate
necessitate a time variable magnetic field at the boundary of the simulation
box. Naive implementation will lead to electromagnetic precursor waves that can
affect the simulation results. The problem is well-known and is described e.g.
in Lemebege and Dawson 1987. Since then few papers have attempted to deal with
this problem and no standard solution exists.
  In this research note we suggest that the ad-hoc pulse shape that was used in
the paper by Lemebege and Dawson 1987 can be replaces by a standard digital low
pass filter. The cut off frequency of this filter should coincide with the gyro
frequency in the magnetized plasma, to make use of (Doppler-broadend) resonant
absorption caused by gyrating electrons. We illustrate this idea with a code
example and demonstrate the efficency of this method throug sample simulations
with our electromagnetic particle-in-cell code.
