Triggering of whistler mode emissions by the band‐limited impulse (BLI) Associated with amplified Vlf signals from Siple Station, Antarctica

At termination of an amplified and saturated ( B sat ) 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 B sat , 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 B BLI of the BLI. Since B BLI ≈ B sat the PFO then becomes a measure of the saturated field B sat . An example for L = 4 and a pitch angle of 60°, gives B sat ≅ 5pT, in good agreement with previous estimates of B sat .