Plasmoid ejection and auroral brightenings

Geotail plasma and magnetic field observations of 24 plasmoids between 21 a nd 29 R-E have been compared with Polar ultraviolet observations of auroral brightenings. Both single and multiple plasmoids almost always corresponde d to brightenings, but the brightenings were sometimes weak and spatially l imited and did not always grow to a global substorm. Even a case where a pl asmoid event occurred with fast postplasmoid flow corresponded to a weak br ightening but no substorm. Some brightenings did not correspond to plasmoid s, but these brightenings were observed away from the longitude of Geotail, indicating that plasmoids have a small longitudinal extent in the near tai l. The plasmoids were occasionally observed before the brightenings but mor e frequently were observed 0-2 min after the brightenings with the delays p robably due to the transit time to the observation point. It seems likely t hat; formation of a near-Earth neutral line causes each brightening in the polar ionosphere. but these formations do not always lead to a full-fledged substorm. What additional circumstance causes development of a full, large -scale substorm remains an open question.