Electron Telescope | Proton Telescope
Omnidirectional Proton Detectors
One-year Baseline Plots | Total Energy Detector (TED)

The NOAA POES (TIROS) Medium Energy Proton and Electron Detector

In addition to the Total Energy Detector (TED) that provides the data used to determine auroral activity, the second generation Space Environment Monitor (SEM-2) onboard NOAA POES contains a Medium Energy Proton and Electron Detector (MEPED) that monitors the intensities of charged particle radiation at higher energies extending up to cosmic rays. These higher energy particles can produce ionization deep within the Earth's atmosphere that can degrade radio communications (occasionally making short wave radio communication impossible in the polar regions), can disrupt the proper operation of satellite systems, and, when intensities are high, increase the radiation dose to astronauts in space.

In order to cover a wide range of particle energies for both protons and electrons, the MEPED includes a number of silicon solid-state detector systems.

The Electron Telescope (Channels 1 - 6)

The electron telescope is a 25 mm2 silicon solid state detector positioned behind a series of metal apertures that define a 15° (half-angle) cone where charged particles from space may reach the detector. The detector itself is covered with a thin nickel foil that stops protons of energies less than about 130 keV while allowing electrons of energies greater than 5 keV to pass through. Charged particles that enter the detector produce an electrical pulse whose amplitude is determined by the amount of energy the particle deposits in the detector. It requires about 1100 keV energy for an electron, and quite a bit more for a proton, to pass completely through the detector. The amplitudes of the electrical pulses produced by the energy loss of particles in the detector are analyzed electronically into three channels equivalent to energy losses (including the energy lost in the nickel foil) of

The electron telescope is sensitive to protons of energies between about 150 keV and 1200 keV, but the contribution to the total sensor response from these protons is ordinarily not significant except in the inner Van Allen radiation belt and the South Atlantic Anomaly.

0° and 90° Electron Telescopes

Two identical electron telescopes are included in the SEM-2 Space Environment Monitor. One, termed the 0° electron detector, is mounted on the 3-axis stabilized NOAA spacecraft to view outward along the Earth-center-to-satellite vector. Whenever the satellite is poleward of a geographic latitude of about 35°, this detector monitors electrons in the atmospheric loss cone that will enter the Earth's atmosphere below the satellite. At lower geographic latitudes, this detector measures electrons that are geomagnetically trapped; they will be reflected by the geomagnetic field at some point below the spacecraft and be returned up along the magnetic field into the magnetosphere.

The second electron telescope, called the 90° detector, is mounted to view in a direction approximately perpendicular to the 0° detector. It is important to note that the viewing azimuth of the 90° detector in the second generation SEM-2 differs from that in the earlier SEM-1 system. At higher geographic latitude locations along the satellite orbit the 90° detector in SEM-2 monitors electrons that are geomagnetically trapped since they will be reflected by the geomagnetic field at some point below the spacecraft and be returned up along the magnetic field into the magnetosphere. At lower geographic latitudes along the orbit, in contrast to the SEM-1 system, the 90° detector in SEM-2 views particles that are in the atmospheric loss cone and will not be magnetically reflected before reaching the atmosphere. For this reason the 90° detector responses at lower latitudes, especially in the region of the South Atlantic Magnetic Anomaly, are significantly lower than experienced by the SEM-1 system. The revised baseline patterns reflect this change.

The pair of electron telescopes provide 6 channels of energetic electron data.

The Proton Telescope (Channels 7 - 18)

The second generation SEM-2 proton telescope design is identical to the electron telescope, with two exceptions. The first is that a strong magnetic field is imposed across the aperture structure to prevent electrons from reaching the silicon solid-state detector. The second is that there is no nickel foil covering the detector, so very low energy protons are allowed to enter the detector. Electronic analysis of the pulses produced by the energy lost by protons in the solid state detector identify protons within six energy ranges (as compared with only five energy ranges for the SEM-1 system):

0° and 90° Proton Telescopes

Two identical proton telescopes are included in the SEM-2 Space Environment Monitor. They are mounted on the NOAA spacecraft in the same fashion as the pair of electron telescopes and respond to protons that are geomagnetically trapped and others that will be lost to the Earth's atmosphere in the same manner as described above.

The pair of proton telescopes provide 12 channels of energetic proton data.

The Omnidirectional Proton Detectors (Channels 19 - 22)

In order to monitor the intensities of still higher energy protons that arrive at the Earth because of solar energetic particle events, four additional silicon solid state detectors are included in the second generation Space Environment Monitor (compared with three detectors in the SEM-1 system.) Each of these four detectors contains a 50 mm2 area by 3 mm thick solid state detector mounted beneath a near-hemispherical (120° full angle) shell- shaped metal absorber. The material and thickness of the absorber determines the minimum energy that a proton needs to reach the detector and be counted.

The hemispheric shell metal absorbers are:

Detector channels 19, 20, and 21 will also respond to protons from all directions that have the energy sufficient to penetrate the satellite structure and reach the detector. This proton energy is estimated to be about 70 MeV. Detector 22 is surrounded by additional metal shielding and will respond to protons of greater than 140 MeV incident from all directions.

These solid state detectors will respond to protons up to a maximum energy of about 220 MeV for detectors 19 and 20, about 235 MeV for detector 21, and 275 MeV for detector 22. These upper bounds on the proton energies are set by the requirement that the proton must lose at least 2.5 MeV of energy in passing through or being stopped by the solid-state detector.