1. Introduction

[2]  The giant pulsations (Pg) used to be considered as one of unique wave phenomena observed at the Earth surface during weak geomagnetic activity ( Kp 0-2 ). This is due to both picturesque drop-like shape of the wave packet envelope and the monochromatic spectral composition of Pg, as well as to local, very seldom seen, observations of this type of pulsations [Chisham, 1996; Rostoker et al., 1979]. The excitation of Pg pulsations is limited by pronounced diurnal and seasonal variations and also by the phase of the solar activity cycle. It is widely known [Brekke et al., 1987; Chisham and Orr, 1991; Chisham et al., 1997; Green, 1979, 1985; Rostoker et al., 1979] that the giant pulsations are mainly registered at latitudes of the auroral zone at the early morning hours in the equinox seasons in the years of solar activity minimum. The characteristic carrier frequency of the Pg wave packets lies within the 7.0-22.0 mHz frequency band [Rostoker et al., 1979]. The duration of Pg pulsations is from 30 to 150 min and the amplitude varies within 10-30 nT. The most typical features of Pg making them different from geomagnetic pulsations of the same frequency range (of the Pc4 type) are the monochromatic character and dominant polarization in the D component [Chisham et al., 1997; Green, 1979].

[3]  One should note that the Pg pulsations may be observed in the form of separated wave packets [Chisham and Orr, 1991; Takahashi et al., 1992] and in the form of complicated regimes consisting of many wave packets, the examples presented by Pokhotelov et al. [2000], Thompson and Kivelson [2001], and Wright et al. [2001].

[4]  Earlier, Bondarenko et al. [1979], Guglielmi [1979], and Klain and Kurazhkovskaya [2000] showed that some types of geomagnetic pulsations having the form of separated wave packets demonstrate nonlinear properties. The shape of the amplitude envelope of these packets resembled a characteristic shape of the soliton envelope [Dodd et al., 1988; Kadomtsev, 1988]. The regularities in behavior of such signals were considered in the scope of the theory of magnetohydrodynamic (MHD) solitons (MHD solitons) [Mio, 1976a]. In particular, Bondarenko et al. [1979], Guglielmi [1979], and Klain and Kurazhkovskaya [2000] proved experimentally that the wave packets of the pulsations of Pc1 type ("pearl"), Pi2 oscillation trains, and ipcl pulsations bursts series satisfy the quantitative relation between the amplitude, duration and carrier frequency typical for solitons [Dodd et al., 1988; Kadomtsev, 1988].

[5]  The fact draws attention that the shape of the amplitude envelope of giant pulsations observed in the form of separate wave packets also visually similar to the shape of the solitons envelope. Taking this into account, one may assume that the Pg pulsations have properties of solitary waves or solitons.

[6]  This paper is dedicated to an experimental check of the assumption that the Pg pulsations observed in the form of separate wave packets are nonlinear structures where the relation between the amplitude, carrier frequency, and the width of the Pg wave packet corresponds to the theory of MHD solitons.