ION ENERGIZATION MECHANISMS AT 1700 KM IN THE AURORAL REGION

Observations obtained by the Freja satellite at altitudes around 1700 km in the high-latitude magnetosphere are used to study ion energizati on perpendicular to the geomagnetic field. Investigations of ions, ele ctrons, plasma densities, electric and magnetic wave fields, and field -aligned currents are used to study O+ heating mechanisms. Three ion h eating events are studied in detail, and 20 events are used in a detai led statistical study. More than 200 events are classified as belongin g to one of four major types of ion heating and are ordered as a funct ion of magnetic local time. The most common types of ion heating are a ssociated with broadband low-frequency electric wave fields occurring at all local times. These waves cover frequencies from below one up to several hundred hertz and correspond to the most intense O+ energizat ion. Heating by these waves at frequencies of the order of the O+ gyro frequency at 25 Hz seems to be the important energization mechanism, c ausing O+ ion mean energies up to hundreds of eV. The broadband waves are associated with Alfven waves with frequencies up to at least a few hertz and with field-aligned currents. Other types of O+ energization events are less common. During these events the ions are heated by wa ves near the lower hybrid frequency or near half the proton gyrofreque ncy. These waves are generated by auroral electrons or in a few cases by precipitating ions.