Geomagnetic storm and substorm aurora observed from Spitsbergen

The present study confirms that the auroras of the nightside oval population consist of two main types, namely storm and substorm aurora. Storm-type aurora, which is relatively infrequent, results from bombardment of the upper ionosphere by fast particle streams generated in the solar wind by cataclysmic solar events related to coronal mass ejections. The associated turbulent plasma that is injected into the magnetosphere produces great magnetic storms of world-wide dimensions that may last for days. In contrast, substorm aurora is a frequent (almost daily) occurrence that lasts for an hour or two around geomagnetic midnight. It is generated by the impulsive release of stored magnetospheric energy from the substorm onset region in the Earth's magnetotail and is associated with localised negative magnetic bays in the H (horizontal)-component of the Earth's magnetic field in the vicinity of the auroral oval.