Influence of ionospheric conductivity on mid-latitude Pc 3-4 pulsations

Diurnal variations of the parameters of the magnetospheric Alfven resonator at different latitudes have been calculated using a semi-empirical model of the ionosphere-magnetosphere plasma distribution. The ionospheric plasma density is taken from the IRI model, the electron density at the magnetospheric equator is based on the ISEE/whistler model, and the field-aligned magnetospheric plasma distribution is calculated under the assumption of diffusive equilibrium. It is shown that for the mid-latitude ionosphere the Hall conductivity has no effect on the parameters of the magnetospheric Alfven resonator. The calculated values of damping rates of Alfven oscillations at middle latitudes during the dark period are too high for the "free-end" and "quarter-wave" oscillation regimes to be realized. At low latitudes quality factors are relatively high both at daytime and nighttime conditions. An expected change of a field-aligned structure of Alfven oscillations during the transition from dayside to nightside ionospheric conditions does not occur. The analysis of the experimental data recorded at middle and low latitude stations of the "210 degrees Magnetic Meridian" magnetometer network and station 1'Aquila gives the results, compatible with the predictions of the numerical model: (a) the pulsation amplitude in a frequency band near the fundamental harmonic of the Alfven field line resonance has the strongest dependence on the ionospheric conductivity; (b) the influence of day/night ionospheric conditions on the Pc 3 amplitudes is less at low (L less than or equal to 2) geomagnetic latitudes than at middle latitudes; (c) the ionospheric conductivity control of the Pc 3 amplitude at middle latitudes weakens with increasing harmonic number.