Observations of global Pi 2 pulsations in mid-latitudes during auroral substorms

Using geomagnetic pulsation observations obtained with induction magnetometers along the Norilsk meridian and from a network of mid-latitude observatories we investigate the excitation characteristics of Pi2 pulsations in the noon sector of the magnetosphere and in mid-latitudes. An additional (to nighttime) stable noon maximum was revealed in the diurnal distribution of the occurrence frequency of Pi2 pulsations at the midlatitude observatories, while the auroral stations recorded only a rare occurrence of Pi2 around noon. The meridional intensity distribution of the daytime Pi2 has its maximum at latitudes of magnetopause projection, whereas it occurs at auroral latitudes for nighttime Pi2. The development of the active phase of an auroral substorm is accompanied by a global excitation of Pi2 pulsations at mid-latitudes with identical dynamic spectra. At the same time the source size of Pi2 pulsations at high latitudes is limited longitudinally and appears to be determined by the zone of synchronous substorm development. The onset time of maximum amplitude in Pi2 pulsation intensifications (inferred from dynamic spectra) varied at different stations from a few fractions of a second to several tens of seconds, and these delays did not show any definite trend. Experimental results are discussed in terms of the existence of a global mode in the Earth's plasmasphere as a plausible source of mid-latitude Pi2 pulsations.