Enhancement of the power of dayside geomagnetic pulsations in periods of magnetospheric substorm onset

Spectral and morphological characteristics of the dayside geomagnetic pulsations within a period range of 3 to 100 s during the development of ten magnetospheric substorms were investigated on the basis of optical and magnetic observations at the high-latitude Barentsburg observatory (Phi' = 74.5 degrees, MLT = UT + 4.7). A beginning of the expansion phase of a substorm (T-0) in the nightside of the Earth was determined from data of the meridian scanning photometers and magnetometers of the Canadian network of stations (CANOPUS). It was found that the onset of the substorm is accompanied by an enhancement of the power of geomagnetic pulsations with a duration of similar to 10-15 min and within the period ranges of 3-8 s and 30-80 s. Before the beginning of the expansion phase of the substorm, approximately within 5-10 min prior to T-0, a relatively low activity of geomagnetic pulsations with periods of 30-80 s is frequently observed. Possible mechanisms, which may be responsible for the enhancement of the power of geomagnetic pulsations in the initial period of the substorm growth phase, are discussed.