  The propagation of a streamer near an insulating surface under the influence
of a transverse magnetic field is theoretically investigated. In the weak
magnetic field limit it is shown that the trajectory of the streamer has a
circular form with a radius that is much larger than the cyclotron radius of an
electron. The charge distribution within the streamer head is strongly
polarized by the Lorentz force exerted perpendicualr to the streamer velocity.
A critical magnetic field for the branching of a streamer is estimated. Our
results are in good agreement with available experimental data.
