CCSD1Z00000100000052CCSD1R00000300000032 DELIMITER=EOF; TYPE=CCSD1F000001; CCSD1C00000400000013 ADI=NURSPE00; CCSD1R00000300000032 DELIMITER=EOF; TYPE=CCSD1S000002; DataSetName PEM_Data DataSource UARS/PEM UARS = Upper Atmosphere Research Satellite PEM = Particle Environment Monitor ScientificContact Dr. J. David Winningham Instrumentation and Space Research Division Southwest Research Institute P.O. Drawer 28510 6220 Culebra Road San Antonio, Texas 78228-0510, USA e-mail: david@dews1.space.swri.edu (Internet) PEM::DAVID (UARSNET) FAX: 210-647-4325 phone: 210-522-3075 SourceCharacteristics The UARS satellite is in a 57 degree orbit which precesses 5 degrees per day relative to the sun. Thus, for one day's data, local solar time around each orbit is an approximately constant function of latitude (differing by 12 hours for ascending and descending nodes) but over a period of 72 days, the entire diurnal cycle is sampled. In order to maintain thermal stability during this orbital precession, the satellite is yawed through 180 degrees approximately every 36 days. This is to ensure that one side (+Y) of the satellite is never illuminated by the sun. PEM is comprised of four types of instruments. These instruments are the Atmospheric X-ray Imaging Spectrometer (AXIS), the High Energy Particle Spectrometer (HEPS), the Medium Energy Particle Spectrometer (MEPS), and the Vector Magnetometer (VMAG). PEM is distributed over the spacecraft with the HEPS and MEPS sensors located on the PEM zenith and nadir booms. They determine the particle distribution at the spacecraft. VMAG is also located on the zenith boom and determines the vector magnetic field locally. AXIS measures Bremsstrahlung x-rays from the atmosphere in order to determine the remote electron energy deposition into the atmosphere. AXIS is mounted on the nadir side of the spacecraft body at the opposite end from the multimission modular spacecraft (MMS) subsystem and performs its measurements over the global atmosphere. InvestigationObjectives PEM will determine the type, amount, energy, and distribution of charged particles injected into the Earth's thermosphere, mesosphere, and stratosphere. It will utilize three separate boom-mounted sensors to measure electrons with energies from 1 eV to 5 MeV, protons with energies from 1 eV to 150 MeV, and the strength of the Earth's magnetic field--all in the vicinity of the spacecraft. In addition to the in situ particle measurements, PEM will include a 16-element array of X-ray detectors to provide wide spatial coverage of the energy injected into the upper atmosphere by high-energy electrons. As these electrons are slowed in their passage through the atmosphere, X-rays are emitted and scattered in all directions. PEM will provide X-ray images in the energy range from 3 to 100 keV, leading to the reconstruction of the global, three-dimensional energy input spectrum of electrons up to 1 MeV in energy. InstrumentAttributes PEM is comprised of four types of instruments: 1) AXIS AXIS measures the x-ray energy spectrum produced by energetic electrons incident upon the atmosphere. It includes 16 detector modules (pixels) that view the atmosphere from limb-to-limb. As the spacecraft orbits the earth, AXIS provides a "strip chart" image of the x-ray intensity on either side of the UARS ground track. AXIS measures x-rays in the energy range from 3 to 100 keV. Each pixel contains a solid-state silicon detector surrounded by tungsten and tantalum collimator shields. The pixels are passively cooled by a two-stage thermal radiator system to 160 degrees K to reduce detector leakage current and noise. The geometric factor for each of the 16 pixels is 0.07 cm^2 sr, with an angular resolution of 8 degrees Full Width at Half Maximum (FWHM). AXIS looks forward of nadir (towards the +X spacecraft axis) by 22.5 degrees. The eight pixels of AXIS 1 (AXIS 2) view from near the spacecraft ground track to the limb on the -Y (+Y) side of the spacecraft. In the AXIS instrument, the x-ray spectrum of each detector is divided into 128 linear energy channels. They are compressed under microprocessor control for the down-link telemetry to 32 logarithmic channels per pixel every 8 seconds and also to four channels per pixel every 1 second. In addition, there are integral spectra for 10 pixels every 0.5 seconds. The microprocessor tables that control the accumulation over energy are stored in the flight read-only memory, but they may be changed by ground command. The capability exists as well to patch the microprocessor flight software in either AXIS 1 or AXIS 2, if needed. Housekeeping functions like digitizing analog instrument monitors and details of timing between the spacecraft telemetry interface and AXIS are performed by hardware, independent of the flight computers. AXIS communicates directly with the observatory telemetry system. 2) HEPS HEPS observes the local precipitating (particles coming down the magnetic field line) and trapped (the mirroring particle distribution returning from the earth up the magnetic field line) electron and proton distributions at the spacecraft. Electrons are measured over 32 logarithmic energy steps from 30 keV to 5000 keV at angles of -15, +15, +45, and +90 degrees to the zenith. Positive angle is in the +X direction measured from the -Z axis of the spacecraft. These data are accumulated continuously and are read every 4 seconds. They are also observed from 30 keV to 1500 keV at angles of -165 and +165 degrees and are read-out every 16 seconds. Protons are detected over 32 logarithmic energy steps from 0.10 MeV to 150 MeV at angles of -15 and +15 degrees to zenith and in 24 energy channels from 0.50 MeV to 150 MeV at +45 and +90 degrees. They are read every 16 seconds. Additional integral counting rates are collected in each sensor telescope for both electrons and protons. The field of view for each of the eight particle telescopes is a cone 30 degrees wide. The multiple view directions resolve the angular distribution of electrons and protons to distinguish the precipitating particles from the trapped populations. Energy deposition in the atmosphere is produced by the precipitating electrons and protons. The geometric factor for the low-energy (0.10 to 0.50 MeV) proton sensors at -15 and +15 degrees is 0.07 cm^2 sr. The geometric factor for the other HEPS 1 and HEPS 2 sensors is 0.54 cm^2 sr. The geometric factor for both HEPS 3 telescopes is 1.53 cm^2 sr. For angular coverage and redundancy there are three HEPS packages at two locations: on the PEM zenith boom are HEPS 1 with telescopes at +15 and +45 and HEPS 2 with telescopes at -15 and +90 degrees, while HEPS 3 is on the nadir boom with electron sensors at -165 and +165 degrees. Each HEPS package is controlled by its own microprocessor, which accumulates simultaneously the electron and proton data from each telescope, compresses the energy channels and transfers data to the CEP (Central Electronics Package). If needed, the microprocessor flight software itself or the energy processing tables can be modified using ground supplied patches to the RAM memory. The body mounted CEP handles the boom instrument telemetry formatting for transfer to the observatory telemetry system. Housekeeping functions such as digitizing analog instrument monitors are performed by hardware, independent of the flight computers. 3) MEPS MEPS measures the local particle population in the range from 1 eV to 32000 eV. It generates a 31-point logarithmic energy spectra every two seconds. Telemetry restrictions force every other point of data from most of the ion sensors to be lost, resulting in 15-point energy spectra. Five MEPS analyzer heads are mounted on the zenith boom, each determining the electron and ion population simultaneously. Their positions are defined in spacecraft coordinates at +6.3, +21.3, -23.7, +36.3, and +66.3 degrees relative to the -Z spacecraft axis in the direction of +Y spacecraft axis (positive angle is in the +Y direction measured from the -Z axis). Three MEPS analyzers are mounted on the nadir boom and they only measure the electron population. Their positions are defined in spacecraft coordinates at +126.3, +156.3, and -158.7 degrees. Each MEPS analyzer has a 5 x 20 degree full-field-of-view producing a geometric factor of 1.79 x 10**-4 cm^2 sr. Samplings of the particle population are taken at various angles relative to the magnetic field for the purpose of reconstructing the three-dimensional particle distribution. MEPS measures particles within the loss cone (measuring precipitating particles), outside of the loss cone (the trapped particles), and electrons escaping the earth. These spectra enable determination of the energy being input into the upper atmosphere within the MEPS energy range. Data from all MEPS sensors are not available at all times. When the spacecraft is flying with its velocity vector in the +X direction, data from electron sensors mounted at +21.3, -158.7, and the ion sensor mounted at +21.3 degrees are not available when the spacecraft is in the northern hemisphere. Data from electron and ion sensors mounted at -23.7 and +6.3 degrees are not available when the spacecraft is in the southern hemisphere. This reverses when the spacecraft is flying with its velocity vector in the -X direction. Here, data from electron and ion sensors mounted at -23.7 and +6.3 degrees are not available when the spacecraft is in the northern hemisphere, and data from electron sensors mounted at +21.3, -158.7, and ion sensor mounted at +21.3 degrees are not available when the spacecraft is in the southern hemisphere. Because of telemetry restrictions, data from ion sensors mounted at -23.7, +6.3, +36.3, and +66.3 degrees are multiplexed. As a result, all 31 samples of data are not placed into the telemetry stream. Only every other data sample is telemetered, producing 15-point logarithmic energy spectra. Detection energies for ion sensors mounted at -23.7 and +36.3 degrees are staggered from detection energies for ion sensors mounted at +6.3 and +66.3 degrees. This type of multiplexing causes gaps in the energy spectra at every other energy. These gaps result because the analyzers are designed such that full energy coverage occurs within the MEPS energy range in a 31-point spectrum and, since every other data point is missing, full energy coverage is not included within the energy range. 4) VMAG VMAG provides vector measurements of the local magnetic field using a 3-axis fluxgate magnetometer. The fluxgate sensor detects the ambient field in the following way. The sensor head consists of a pair of coils, a driver, and a sensor wrapped on a core of high-permeability material. An alternating current in the driver coil is used to force the core to saturation with alternating polarity. The sensor coil records the rate of change of flux that passes through the windings. In the absence of a background field, the polarity reversals of core magnetization occur equally spaced in time. For nonzero background fields, the core spends slightly more time in the sense of magnetization parallel to the background field and less in the opposite sense. This asymmetry in the magnetization-versus-time waveform is sensed by Fourier decomposition of the sensor coil signal. The amplitude of the second harmonic, relative to the frequency of the current in the driver coil, is proportional to the ambient field in the direction of the coil axes. Vector measurements are provided by using three sets of core/coil assemblies. For VMAG, the sampling rate is five samples/second, and the range is from +65500 nT to -65500 nT with a resolution of 8 nT (commensurate with 13-bit resolution). The VMAG sensor is on the zenith boom, removed as far as possible from magnetic contamination generated by the spacecraft. VMAG serves two purposes, the first of which is its primary function: (1) to provide a reference for the plasma measurements which determine energy input from particle precipitation, and (2) to determine the energy deposited in the ionosphere by field aligned currents. The direction of particle precipitation relative to the local field determines the altitude in the atmosphere at which particles deposit their energy. Hence, knowledge of the local magnetic field is required to evaluate energy deposition due to precipitating particles from the energetic particle data acquired by PEM. In addition, deviations in the magnetic field result from current at or near the spacecraft, primarily flowing parallel to the magnetic field direction. VMAG data thus provides information on the current densities flowing into and out of the ionosphere and can be used to determine the energy deposited by the currents. MeasuredParameters X-Rays Energy Range 3 to 100 keV Energy Resolution 1.5 keV FWHM Number of Energy Channels 128 compressed to 32 per pixel every 8 s 4 per pixel ever 1 s Geometrical factor 0.07 cm^2 sr for each of 16 pixels Field of View Full earth coverage from limb to limb Angular Resolution 8 degrees FWHM per pixel Minimum Spatial Resolution 100 km or less, depending on brightness Accumulation Interval 0.5, 1, and 8 seconds Temporal Coverage Continuous day and night Low Energy Particles Available Samples Zenith Species Available Velocity Per Angle Hemisphere Direction Spectra +6.3 electron north +x 31 south -x 31 ion north +x 15 south -x 15 +21.3 electron south +x 31 north -x 31 ion south +x 31 north -x 31 -23.7 electron north +x 31 south -x 31 ion north +x 15 south -x 15 +36.3 electron both both 31 ion both both 15 +66.3 electron both both 31 ion both both 15 +126.3 electron both both 31 +156.3 electron both both 31 -158.7 electron south +x 31 north -x 31 energy range l eV to 32 keV Accumulation Interval 2 seconds per spectrum Field-of-view 5 degrees x 20 degrees Geometric factor 1.79 x 10**-4 cm^2 sr High Energy Particles Angle Minimum Geometric Particle Zenith FOV Energy Energy Energy Factor Instr Measured (Degrees) (Degrees) Range (MeV) Steps Resolution (cm^2 sr) HEPS1 Electrons +15 +/-15 0.029-0.30 16 6 keV 0.54 0.30-5.1 16 90 keV 0.54 Protons +15 +/-15 0.068-0.52 8 10 keV 0.07 0.52-5.0 8 175 keV 0.54 5.0-45 8 .8 MeV 0.54 45-150 8 .8 MeV 0.54 Electrons +45 +/-15 0.029-0.30 16 6 keV 0.54 0.30-5.1 16 90 keV 0.54 Protons +45 +/-15 0.52-5.0 8 175 keV 0.54 5.0-45 8 .8 MeV 0.54 45-150 8 4.7 MeV 0.54 HEPS2 Electron -15 +/-15 0.029-0.30 16 6 keV 0.54 0.30-5.1 16 90 keV 0.54 Protons -15 +/-15 0.068-0.52 8 10 keV 0.07 0.52-5.0 8 175 keV 0.54 5.0-45 8 .8 MeV 0.54 45-150 8 4.7 MeV 0.54 Electrons +90 +/-15 0.029-0.30 16 6 keV 0.54 0.30-5.1 16 90 keV 0.54 Protons +90 +/-15 0.52-5.0 8 175 keV 0.54 5.0-45 8 .8 MeV 0.54 45-150 8 4.7 MeV 0.54 HEPS3 Electrons +165 +/-15 0.029-0.30 16 6 keV 1.53 0.30-1.68 16 50 keV 1.53 Electrons -165 +/-15 0.029-0.30 16 6 keV 1.53 0.30-1.68 16 50 keV 1.53 Magnetic Field Components Three orthogonal axis Magnitude Range +65500nT to -65500nT Magnitude Resolution 8nT Time Resolution 5 samples per second DataSetQuality During inflight operation all data will be tested for quality with an automatic data test program. PEM is an energy input instrument and the Level 3AT data products are altitude profiles of energy deposition rate determined for (a) incident electrons (HEPS and MEPS), (b) incident protons (HEPS and MEPS), and (c) incident electrons via the bremsstrahlung x-rays (AXIS). In the case of HEPS and MEPS data, the program will test the measured spectral form that is most directly related to energy deposition rate [erg/(cm^2 s)] vs. height. This spectrum will be an energy moment of the differential energy flux [erg/(cm^2 s eV)] as a function of energy. Upper and lower acceptance windows will be established, and each spectral value will be tested for fit within these limits. A flag based on location of data value within or outside the acceptance window bounds will be assigned. Confidence limits will be set using count rate statistics, the data compression error associated with each spectral value, and the range of expected values (based on similar previous satellite measurements). All data will be examined with this program. In addition, to the data-test program, a representative subset of the PEM data will be examined on a regular basis by the PI or a designee. In the first months of operation this subset will be essentially all of the data, displayed in a quick-look format. DataProcessingOverview The level 2 PEM data is created directly from the level 0 PEM data. The level 2 Instrument Data File Sets (IDFS - described in the DataOrganization and FileClassRelationships sections) provide both physical parameters (level 1) and geophysical measures (level 2), thus allowing the transition of level 0 data to level 2 data directly (i.e., level 1 processing is bypassed). The transition from level 0 to level 2 occurs in three basic steps. The first step does general quality checking and generates a level 0 PEM temporary file to be used in the second step. The second step then splits the level 0 PEM temporary file into four level 0 temporary files corresponding to the four physical instruments of PEM (AXIS, HEPS, MEPS, and VMAG) while doing specific instrument housekeeping and quality checks. The third step is a four-process step where each of the four processes reads the corresponding temporary physical instrument file and performs the necessary algorithms to generate the level 2 logical instrument data file sets. The level 0 data is not transformed per se, but is re-arranged and stored in the level 2 data files accordingly. DataUsage DataOrganization The PEM instrument is considered to be several "logical" instruments. The table below relates the logical instrument name to each PEM instrument subsystem. ---------------------------------------------------------------------------- Physical Instrument Logical Name Definition ---------------------------------------------------------------------------- MEPS MA 31 energy levels, 2-sec spectrum MEPS MB 30 energy levels, 2-sec spectrum HEPS HA 24 or 32 energy levels, 16-sec spectrum HEPS HB 32 energy levels, 4-sec spectrum AXIS AA 4 energy levels, 1-sec spectrum AXIS AB 32 energy levels, 8-sec spectrum AXIS AC 16-sec singles data VMAG VM magnetometer data VMAG VA magnetometer AC filter data ---------------------------------------------------------------------------- The PEM data at levels 1 and 2 will be stored and cataloged utilizing the Instrument Data File Set (IDFS) paradigm. These files are cataloged as level 2 data. The data for each logical instrument will be contained in a set of four files: (1) the Science Unit File (SUF), (2) the Instrument Description File (IDF), (3) the Header File (HF), and (4) the Data File (DF). There is one SUF and one IDF per logical instrument. These files are cataloged as calibration files since the information they contain will not change often throughout the mission. The SUF contains science unit look-up tables for the logical instrument (e.g., differential number flux, differential energy flux, etc.) as described in the IDF. The IDF contains information about the logical instrument in an attempt to describe the logical instrument. A one-to-one correspondence exists between the header and data files. Ideally, one set of these files is created daily from the level 0 PEM data by the level 0 to level 2 processing. FileClassRelationships When using the level 2 IDFS data, the IDF should be opened and read first because it contains information about the logical instrument that is pertinent for opening and reading the rest of the data set. It gives such information as how many and what types of science units are stored in the SUF. It also contains the geometric factors, center energies, sample resolutions, etc. as appropriate for that logical instrument. Virtually everything that is known about the logical instrument is stored in the IDF for public use. The SUFs are used only if table look-up for the given precomputed science units is desired. The data files contain the compressed counts taken directly from level 0 which are used to index into the SUF look-up tables. There is a header record offset in each data file record which indicates the corresponding header record. Thus, there are several data records for each header record. Also, the header record offsets are not necessarily sequential in the data records. For example, data record number 1 may point to header record number 2, and data record number 10 may point to header record number 1. LitReferences "Instrument Systems Description Document for the UARS Particle Environment Monitor (PEM)," Southwest Research Institute Document 7845-IS-1, San Antonio, Texas, (October 1990). "Particle Environment Monitor Software Specifications, Data Descriptions, and Algorithms," Southwest Research Institute Document 7845-SDD-1, San Antonio, Texas. Referred to as the SDD document.