At termination of an amplified and saturated (Bsat) Vlf pulse from Siple Station, Antarctica, a band-limited (50--500 Hz range) impulse (BLI) is generated which in turn triggers a narrowband emission, of about the same amplitude, that starts at about 50--150 Hz above the frequency of Bsat, called the positive frequency offset (PFO). The BLI creates the PFO by reducing the parallel velocities of existing phase-bunched cyclotron resonant electrons that form the source currents (J⊥) of the stimulated radiation. The PFO is proportional to the magnetic field intensity BBLI of the BLI. Since BBLI≈Bsat the PFO then becomes a measure of the saturated field Bsat. An example for L=4 and a pitch angle of 60¿, gives Bsat≅5 pT, in good agreement with previous estimates of Bsat. ¿ 2000 American Geophysical Union