Effects of geomagnetic impulses in midlatitude VLF emissions

The peculiarities of the manifestations of geomagnetic impulses in prolonge d bursts of midlatitude VLF emissions (hiss) with a weakly varying backgrou nd amplitude are analyzed. It is shown that drops in the VLF hiss amplitude s at the time of the impulses may be observed both during abrupt variations in the dynamical pressure of the solar-wind plasma and in the absence of s uch variations. The sensitivity of prolonged VLF hiss events to various typ es of external factors (negative and positive geomagnetic impulses, Pi2 geo magnetic pulsations) indicates that, in these cases, the regime of cyclotro n excitation of VLF emissions is close to that of weak pitch-angle diffusio n. Estimates show that the quasi-stationary excitation of prolonged VLF his s events can be due to the high coefficient of reflection of VLF waves from the ionosphere.