SAMPEX Re-Entry into the Atmosphere
The End of a Twenty-Year Mission
After 20 years of very successful operation, the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) re-entered the atmosphere on 13 November 2012. One of its instruments, the Heavy Ion Large Telescope (HILT) – developed at the Max Planck Institute for Extraterrestrial Physics – was operated to the very end of the mission, at an altitude of about 220 km, to provide correlative measurements between SAMPEX and the new Van Allen Probes mission.
SAMPEX was the very first of a new series of low-cost small explorer missions, which impressively outlived its planned three years mission time. It was launched on 3 July 1992 to a high-inclination, low altitude (520-670 km) orbit to systematically study the Earth’s radiation belts. Other research objectives included in particular high-sensitivity measurements of electrons and ions of solar and interplanetary origin during the polar passages. To infer the ionic charge states of these solar and interplanetary particles at high energies where an in-situ determination of charge states is technically not feasible, SAMPEX used the Earth’s magnetic field as a mass per charge spectrometer.
The scientific payload consisted of four instruments designed to measure electrons and ions in the mass range from protons to iron ions, over a wide range in energy. The Heavy Ion Large Telescope (HILT) was developed at MPE, in cooperation with The Aerospace Corporation, USA, to precisely measure the energy and mass of ions from helium to iron. Its sensitivity window covered in particular medium-energy solar ions, galactic cosmic rays, and the maximum intensity range of the anomalous cosmic ray component. Moreover, one of the operational modes of HILT also allows the measurement of precipitating relativistic electrons at the unprecedented high time resolution of up to 20 ms – and this mode was now used for its last measurements ever.
Over its lifetime, SAMPEX provided for the first time a continuous record of high sensitivity measurements of energetic ions and relativistic electrons over almost two solar cycles. These data resulted in new insights into the acceleration, transport and loss processes in the Earth’s magnetosphere, driven by high speed streams or coronal mass ejections in the solar wind. SAMPEX also discovered doubly charged anomalous cosmic rays of interstellar origin (N, O, Ne), thus limiting the time scale for acceleration of these ions in the outer heliosphere to a few years. The measurements also confirmed the existence of the trapped component of these anomalous cosmic rays and showed that this is located in a narrow belt within the inner region of the Van Allen radiation belts. Furthermore, the SAMPEX mission also successfully addressed important scientific objectives concerning solar energetic particles - when SAMPEX launched, the sun was just finishing the peak of its 11-year solar cycle and beginning to move toward solar minimum.