Transversely accelerated ions - An ionospheric source of hot magnetospheric ions

A source of hot magnetospheric ions has been discovered operating in the auroral topside ionosphere. A part of the cold ionospheric ion distribution is being transversely accelerated within a source region as low as 1000 km. Subsequent to their transverse acceleration the ions are driven upward into the magnetosphere by the gradient B mirror force. These ions are observed by the soft particle spectrometers on the Isis 1 and 2 satellites, with sharply defined pitch angle distributions centered between 90° and ∼120° and having energy distributions typically up to several hundred electron volts. Observations at 1400 km indicate the transversely accelerated ion (TAI) events are predominantly a winter nightside auroral zone phenomenon. However, in the summertime at altitudes above 2750 km, TAI events are observed on the dayside within the cleft. Probabilities of occurrence at some invariant latitude-magnetic local time locations within the nightside auroral zone at 1400 km exceed 50%. Typical intensities of >108 ions/cm² s sr during TAI events suggest that they may be a significant source of hot ions for the magnetosphere.