Plasmoid ejection and auroral brightenings

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Abstract
Geotail plasma and magnetic field observations of 24 plasmoids between 21 and 29 RE have been compared with Polar ultraviolet observations of auroral brightenings. Both single and multiple plasmoids almost always corresponded to brightenings, but the brightenings were sometimes weak and spatially limited and did not always grow to a global substorm. Even a case where a plasmoid event occurred with fast postplasmoid flow corresponded to a weak brightening but no substorm. Some brightenings did not correspond to plasmoids, but these brightenings were observed away from the longitude of Geotail, indicating that plasmoids have a small longitudinal extent in the near tail. The plasmoids were occasionally observed before the brightenings but more frequently were observed 0–2 min after the brightenings, with the delays probably due to the transit time to the observation point. It seems likely that 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.