PEM consists of four instruments: the Atmospheric X-ray Imaging Spectrometer (AXIS), the High Energy Particle Spectrometer (HEPS), the Medium Energy Particle Spectrometer (MEPS), and the Vector Magnetometer (VMAG). AXIS provides global images and energy spectra of bremsstrahlung x-rays produced by electron precipitation over the energy range from 3 to 100 keV. From these x-ray measurements, the intensities and energy spectra of the precipitating electrons are inferrred. HEPS and MEPS provide direct in situ measurements of precipitating electrons in the energy range from 1 eV to 5 MeV and protons in the energy range from 1 eV to 150 MeV. The altitude range to which the particles penetrate extends from several hundred km down to as low as ~30 km. VMAG references the particle measurements to the magnetic field direction and yields magnetic field perturbations from which field aligned and horizontal currents can be deduced. These combined measurements will be used to attain the primary PEM objective of determining the global energy input by charged-particles into the Earth's stratosphere, mesosphere, and thermosphere. Along with other measurements obtained from UARS, PEM data will be used to predict atmospheric responses to this energy source
Of the four PEM instruments, AXIS is the newest. In the descriptions that follow, we have therefore placed emphasis on that instrument. The particle instruments (HEPS and MEPS) and the magnetometer (VMAG) are similar to instruments that have flown before; with these we provide general descriptions citing references to the precursor instruments. We illustrate how they are used on the UARS spacecraft and place some emphasis on their calibration. In the last section we include examples of data and data presentation formats from observations made in the UARS mission.
Energetic particles precipitating into the atmosphere have a significant effect on the chemical composition of the stratosphere, the mesosphere, and the thermosphere by enhancing ionization and dissociation of atmospheric gases. Transport by winds and eddy mixing can extend these effects to regions far from those directly affected by particle precipitation, and may on occasion produce changes on a global scale.
Of particular interest are the changes caused by energetic solar protons and medium to high-energy precipitating electrons. Odd-nitrogen species resulting from dissociation of molecular nitrogen by these particles and odd-hydrogen species produced by ion-chemical reactions both lead to depletion of ozone in the stratosphere and mesosphere. The importance of this mechanism for ozone depletion relative to other natural and anthropogenic ozone sinks is still poorly understood. These precipitation events also provide a readily calculable input of odd nitrogen and odd hydrogen to the atmosphere and thus provide an opportunity to test models of atmospheric chemistry and transport.
In addition to the changes in the neutral composition of the middle atmosphere, energetic-particle precipitation causes substantial changes in the atmosphere's electrical properties through the creation of very large ion and electron concentrations in regions where the normal background levels are low. The changing conductivity affects the global electric circuit, and has a direct influence on high-latitude radio communication. Transport of the enhanced nitric oxide concentration from the polar regions to middle latitudes by planetary wave action extends its effect on a hemispheric scale when the nitric oxide reaches sunlit regions and becomes ionized by solar Lyman-alpha radiation.
The less energetic particles give rise to dramatic effects in the lower thermosphere. The extent to which these effects can couple downward into the middle atmosphere is still largely unknown, and the possibility of simultaneous measurements by the PEM instruments and the full complement of atmospheric remote-sensing instruments on board UARS should provide a unique opportunity to study these effects. In particular, the interaction between energetic particles and the polar mesospheric and noctilucent clouds that are a common feature of the high-latitude summer mesopause region will be of major scientific interest. These clouds are probably formed of ice particles and they should be readily detected by the UARS instruments. While their effect on the radiation budget of the polar regions is probably small, they provide important basic information on the water vapor content, the temperature structure, and the dynamics of the upper mesosphere.
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