  Using multi-dimensional PIC simulations we show that the magnetic
undulator-type field of the plasma magnetostatic mode is indeed produced by the
interaction of a laser pulse with a relativistic ionization front, as predicted
by linear theory for a cold plasma. When the front with this magnetostatic mode
is followed by a relativistic electron beam, the interaction of the beam with
this magnetic field, produces FEL-type synchrotron radiation, providing a
direct signature of the magnetostatic mode. The possibility of generating
readily detectable ultrashort wavelength radiation using this mode, by
employing state-of-the-art laser systems, is demonstrated, thus opening the way
towards experimental observation of the hitherto unseen magnetostatic mode and
the use of this plasma FEL mechanism to provide a source of high-brilliance
ultrashort wavelength radiation.
