A sub-class of pi 1 micropulsations associated with the diurnal transit of the neutral sheet

A composite pi 1-hydromagnetic noise class of micropulsations is found to have pronounced local time occurrence maxima at 0930 and 2130. It is shown that the night-time events tend to occur within a geomagnetic local time ‘slot’, the extent of which correlates quasi-linearly with the width of the neutral sheet recombination slot. No such correlation exists for night-time pi 1 bursts. The suggested energy source for these night-time pi 1-hydromagnetic noise events is geotail field-line merging and the concomitant injection and acceleration of the geotail plasma.