Observations in the transverse ion energization region

We present here the first direct observations of thermal (0 ≤ E/Q ≤ 25 Volts) and suprathermal (E/Q > 50 Volts) ion distributions from the Suprathermal Ion Mass Spectrometer (SMS) on the EXOS‐D spacecraft in and near the Transverse Ion Energization (TIE) region. It is shown that the TIE region exists on field lines closely related to the auroral zone and at altitudes which vary between 3000 to 6000 km on the day side. This process also occurs on the night side at lower altitudes, but no clear examples when the spacecraft was in the TIE region at these local times have been found to date. The altitude range over which the energization occurs is narrow, less than 100 km. In the region, all ions (major and minor species) are energized to approximately the same energy (temperature) perpendicular to the magnetic field ( ) and ejected into the magnetosphere by the gradient force. Above the region, the energized plasma expands outward along magnetic field lines forming “conic” distributions. However, distribution function observations above the TIE region indicate that significant variations from that expected from conservation of the first invariant occur when the ions have travelled only a few thousand kilometers upwards from the source.