Pi 2 micropulsations as indicators of substorm onsets and intensifications.

Pi2 micropulsations are impulsive wave trains which accompany the onset of magnetospheric substorm disturbances. At middle and low latitudes Pi2's are observed as relatively monochromatic damped wave trains, while at auroral observatories onsets of substorms are marked by impulsive increases in noise across the Pi1 and Pi2 bands. Because they are often clearly apparent on normal low latitude magnetograms their presence or absence has been used as an indicator of the presence or absence of substorm activity. In this paper we shall demonstrate the relationship between Pi2 bursts and substorm onsets and Intensifications. We shall show that the degree of spatial localization of the Pi2 disturbance often makes it unidentifiable at middle and lower latitudes using conventional recording techniques. Thus while the presence of a Pi2 pulsation may be taken as an indication of substorm occurrence, its absence at a particular station may be due to the location of the station relative to the substorm region and may not necessarily imply the absence of substorm activity. Sonograms from an east-west array of observatories in the auroral oval will be employed to demonstrate the use of Pi signals at auroral zone latitudes to define the times of substorm onsets and intensifications. It is recommended that the present subjective means of identifying Pi2's be replaced by more objective criteria, based upon careful analysis of Pi2 signals.