The Space Environment Monitor (SEM) that is regularly flown on the NOAA POES (formerly TIROS) series of low-altitude (850 km), polar-orbiting (98 degree inclination) spacecraft contains two sets of instruments that monitor the energetic charged-particle environment near Earth. An upgraded SEM, called SEM-2, began operations with the launch of NOAA-15 and is currently the prime source of observations.
The Total Energy Detector (TED) in SEM-2 provides the data used to determine the level of auroral activity and generate the statistical maps presented on SWPC's 'POES Auroral Activity' website. This instrument monitors the energy fluxes carried into the atmosphere by electrons and positive ions over the energy range between 50 and 20,000 electron Volts (eV). Particles of these energies are stopped by the atmosphere of the Earth at altitudes above 100 km, producing aurora. The instrument design utilizes cylindrical, curved-plate electrostatic analyzers to select (by impressing a variable voltage between the analyzer plates) the species and energy of those particles that are permitted to reach the detector. Those particles that pass through the analyzer are counted, one by one, by the detector (a Channeltron open-windowed electron multiplier). Eight such detector systems are included in the SEM, four to monitor positive ions and four to monitor electrons. They are mounted in groups of four, one group viewing radially outward from Earth and the other viewing at 30 degrees to the first. Whenever the satellite is poleward of a geographic latitude of about 30 degrees, all eight detectors view the charged particles that will be guided by the geomagnetic field into the atmosphere below the satellite. A data processing unit at the satellite converts the Channeltron responses to measures of integrated power flux; these are telemetered to the ground station along with crude information about the energy distribution of the electrons and positive ions. Data processing on the ground combines observations from the eight instruments to obtain the total power flux carried into the atmosphere by these particles.
The second instrument in the second-generation SEM-2 is the Medium Energy Proton and Electron Detector (MEPED) which provides the measurements used to create the plots on SWPC's 'POES Energetic Particles' website. This instrument includes four solid-state detector telescopes, two to measure the intensity of electrons between 30 and 1000 keV and two to measure the intensity of protons (positive ions) between 30 and 6900 keV, as well as solid-state "dome" detectors that measure the intensities of protons between 16 MeV and 275 MeV.
The solid-state detector telescopes are mounted in pairs. One pair views radially outward from Earth to monitor particles that will enter the atmosphere in the polar regions; the other pair is mounted at nearly 90 degrees to the first to view charged particles that will "magnetically mirror" near the satellite. The field of view of each detector system is only 30 degrees so that the angular distribution of the particles may be determined. These detectors are designed to monitor the intensities of energetic particles in Earth's radiation belts and during solar particle events.
In addition, the MEPED contains "dome" detectors that have very large fields of view (nearly 180 degrees) and are mounted on the side of the spacecraft facing away from Earth to monitor particles incident upon the atmosphere. These detectors are designed to detect and monitor energetic solar particles that cause severe ionospheric disturbances--blackouts--during solar particle events.