A statistical study of ion energization mechanisms in the auroral region


  • Patrik Norqvist,

  • Mats André,

  • Magnus Tyrland


More than one mechanism is causing ion energization perpendicular to the geomagnetic field in the auroral region. Observations by the Freja satellite obtained during 20 months at altitudes around 1700 km are used to study the most common O+ energization mechanisms. Ion energization can be due to broadband low-frequency waves, causing resonant energization by interaction at frequencies around the ion gyrofrequency. Energization can also be caused by resonant energization by electromagnetic ion cyclotron waves at about half the proton gyrofrequency, or by emissions near the lower hybrid frequency. About 90% of all O+ heating events and about 95% of the total O+ upflow are caused by ion energization associated with broadband low-frequency waves. The dependence of the different energization types on magnetic local time, magnetic latitude, magnetic activity, and season is studied. We find that the prenoon auroral region is a major source of O+ ions energized by broadband low-frequency waves.