Experimental and theoretical methods for studying gyroresonant wave-particle interactions (WPI) in the magnetosphere commonly assume that the magnetic field is static over the time scales of interest. In order to investigate the effect of nonequilibrium or dynamic magnetospheric processes on WPI, the response of the interaction mechanism to time-dependent perturbations in the magnetic field was examined using the global field compressions that occur during sudden commencements (sc). By measuring modifications of the wave spectrum and electron distribution during sc, the response of the WPI mechanism to slow variations in the magnetic field and plasma environment can be determined. This technique was used to examine the applicability of both nonlinear and quasi-linear descriptions of gyroresonant WPI to dynamic environments. Nonlinear interactions were investigated by calculating the nonlinear parameters (interaction length, wave trapping threshold, and bunching efficiency) as a function of time for a given field line during a simulated ac compression. The results showed that the occurrence probability for discrete emissions generation increases during the compression, which is consistent with the observed triggering of discrete emissions during sc.

Quasi-linear interactions were investigated using a modified formulation of quasi-linear theory in which a small time-dependent perturbation is introduced in the magnetic field and the resulting change in the wave growth rate is determined. The observed growth rate of 0.3--2.7 dB/s, total growth of 12--29 dB, and growth duration or 10--20 s were found to be comparable to the values of 1.5 dB/s, 30 dB, and 20 s predicted by the theory. In addition, the modified theory predicts the occurrence of 3--4 cycles of damped oscillations with period 35--40 s in response to the sc, similar to the observed 3--4 cycles of 60--90 s period damped oscillations which were observed. The results indicate that current theories of gyroresonant WPI, when properly modified to include a time-dependent magnetic field, can be used to predict properties of WPI in a dynamic magnetospheric environment. ¿ American Geophysical Union 1990