Data Set Information
DATA_SET_NAME MGS MARS/MOONS MAG/ER MAPPING ER OMNIDIRECTIONAL FLUX V1.0
DATA_SET_ID MGS-M-ER-3-MAP1/OMNIDIR-FLUX-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION ==================================================================== Data Set Overview ================= The Electron Reflectometer Data Record (ERDR) is a time ordered series of electron measurements from the Mars Global Surveyor (MGS) Mission. Each record consists of a time tag with 19 scalar data points representing measurements of the electron flux in 19 different energy channels, ranging from 10 eV to 20 keV, with an energy resolution of 25%. Each data point is a measure of the electron flux (cm-2 sec-1 ster-1 eV-1) averaged over a 360 x 14 degree disk-shaped field of view (FOV). During the Mapping Phase, as the spacecraft orbits the planet the ER field of view sweeps out the entire sky (4-pi ster) every 58.5 minutes, which is much longer than the integration time per record (2 to 48 sec, depending on energy and telemetry rate) and much longer than most timescales of interest in Mars' plasma environment. The ERDR is intended to be used in conjunction with MGS Magnetometer (MAG) data records, which provide the magnetic field vector and spacecraft ephemeris data as a function of time. Electrons travel along the magnetic field lines in tight helices (few km radius) at high speed (roughly one Mars diameter per second). Thus the electron data contain information about the plasma environment as well as the large-scale configuration of the magnetic field, which is sampled locally by the MAG. ==================================================================== Parameters ========== The Mars Global Surveyor ER data set consists of a time ordered series of electron flux measurements in 19 energy channels, ranging from 10 eV to 20 keV. The ER data are organized into ''packets,'' each of which contains 12, 24, or 48 seconds of data, respectively, for high, medium, and low spacecraft telemetry rates. Each packet is further subdivided into samples. There are from 1 to 6 samples per packet, depending on the energy channel, as given in the table below. The ER data set is generated at the rate of 6 samples per packet, regardless of energy. When there are fewer than 6 samples per packet at a particular energy, data values are repeated in order to maintain a uniform table. The time listed for each record is the center of the sampling interval. When records are repeated, taking an average of the times for all repeated records provides the center time for that sample. The energy channels and sampling intervals are as follows: Channel Number Energy Range Samples per Packet --------------------------------------------------------------- 0 13 - 20 keV 1 1 8.0 - 12 keV 1 2 4.9 - 7.5 keV 1 3 2.9 - 4.6 keV 3 4 1.8 - 2.8 keV 3 5 1.1 - 1.7 keV 3 6 680 - 1046 eV 6 7 415 - 639 eV 6 8 253 - 390 eV 6 9 153 - 237 eV 6 10 92 - 144 eV 6 11 72 - 87 eV 2 12 56 - 67 eV 2 13 43 - 52 eV 1 14 33 - 40 eV 2 15 25 - 30 eV 1 16 18 - 23 eV 2 17 14 - 17 eV 1 18 10 - 13 eV 2 --------------------------------------------------------------- The ER has a 360 x 14 degree disk-shaped field of view. The 360 degrees are divided into 16 angular sectors, each with a separate counter that is read out in telemetry. The sizes and look directions of these sectors are programmable to within an accuracy of 1.4 degrees. For these data, the FOV is divided into 16 equally sized 22.5 x 14 degree sectors that remained fixed in spacecraft coordinates. Throughout pre-mapping, some of these sectors were masked because of obstructions in the FOV, most notably the stowed high gain antenna (HGA), which blocked 3 sectors. Three additional sectors are partially obstructed by the -Y solar array gimbal/yoke assembly and corners of the spacecraft bus. Apart from the HGA, these obstructions are minimal, so data from these sectors are still considered to be of good quality. After HGA deployment just prior to mapping, the entire FOV became useable. An in-flight calibration was then performed to determine the relative instrumental sensitivity around the FOV to an accuracy of about 10%. For this mapping data set, we average data from all 16 sectors to form a scalar ''omnidirectional'' value. ==================================================================== Processing ========== Processing is carried out at the Space Sciences Laboratory (SSL) of the University of California, Berkeley, (UCB) to convert the raw data to measurements of the omnidirectional electron flux (cm-2 s-1 ster-1 eV-1). Because of the instrument's high dynamic range (six decades), the onboard digital processing unit (DPU) compresses the raw counts in a logarithmic scale. The first step is to decompress the raw counts and construct a three-dimensional data array, where the first dimension is time (6 elements per telemetry packet), the second dimension is direction around the FOV (16 elements), and the third dimension is energy (19 elements). The next step is to sum over the 16 angular sectors to produce a two-dimensional time/energy array. Raw count rate (R) is then obtained by dividing the raw counts by the integration time (0.0625 sec per energy step). The data are next corrected for deadtime. During the time it takes the instrument to process a single electron (known as the ''deadtime'', which is about 0.4 microsec for the ER), it ignores any other electrons. The raw count rate is multiplied by the factor 1/(1 - RT), where T is the deadtime, to obtain corrected count rate. Data values are masked (set to -9.999e-9) when the deadtime correction factor exceeds 1.25. These data are NOT CORRECTED for a background count rate due to cosmic rays and noise in the electronics (about 10 counts/sec). Most of the time, the signal in the highest energy channel (13-20 keV) is dominated by background. Exceptions to this sometimes occur during bowshock crossings or during energetic solar events. Assuming that the highest energy channel contains 100% background, the background level for the lower energy channels can be estimated as follows: Channels 0-10 (92 eV - 20 keV): B(E) = B(20 keV) * (20 keV/E) Channels 11-18 (10 eV - 87 eV): B(E) = B(20 keV) * (20 keV/E) * 43.5 where B(E) is the background level (in units of cm-2 s-1 ster-1 eV-1) at energy E. The background is typically negligible at energies below about 1 keV. Background correction is essential at higher energies. Data are collected through two separate apertures that differ in their transmission by a factor of 43.5. At low energies (10 eV to 87 eV), the smaller aperture is used to attenuate high fluxes, and at high energies (92 eV to 20 keV), the larger aperture is used to maximize the sensitivity to low fluxes. The corrected count rates in energy channels 11-18 (10-87 eV) are multiplied by the factor 43.5 to compensate for the smaller aperture size. Finally, we divide by the geometric factor (0.02 cm2 ster) and the center energy (eV) to obtain the differential flux (cm-2 s-1 ster-1 eV-1). The data are organized into a table with a uniform time step for all energy channels. Since the sampling interval is different for different energy channels, data values are repeated within each packet, as necessary, to enforce a uniform time step. The data are sent via FTP to the Principal Investigator (Mario Acuna) at Goddard Space Flight Center (GSFC), where they are incorporated with the magnetometer data. ==================================================================== Data ==== The ERDR data set consists of a single time-ordered table. Each record contains a time stamp and 19 data values, representing the omnidirectional electron flux in 19 different energy channels ranging from 10 eV to 20 keV. ==================================================================== Ancillary Data ============== No additional ancillary data is required beyond that described for the MAG. ==================================================================== Coordinate System ================= The data are presented in omnidirectional format. The time tags contained in the ER data set should be used to obtain the corresponding spacecraft ephemeris information from the MAG data set. ==================================================================== Software ======== Data reduction software for the ER is written in IDL. ==================================================================== Media/Format ============ The ER data are provided in the form of 20-column ascii tables. Storage media are described in the MAG documentation.
DATA_SET_RELEASE_DATE 2009-05-14T00:00:00.000Z
START_TIME 1999-03-08T12:00:00.000Z
STOP_TIME 2006-11-02T11:24:30.793Z
MISSION_NAME MARS GLOBAL SURVEYOR
MISSION_START_DATE 1994-10-12T12:00:00.000Z
MISSION_STOP_DATE 2007-09-30T12:00:00.000Z
TARGET_NAME MARS
TARGET_TYPE PLANET
INSTRUMENT_HOST_ID MGS
INSTRUMENT_NAME ANALYZER OF SPACE PLASMA AND ENERGETIC ATOMS (3RD VERSION)
ELECTRON REFLECTOMETER
INSTRUMENT_ID ER
INSTRUMENT_TYPE ION MASS SPECTROMETER
ELECTRON SPECTROMETER
NEUTRAL PARTICLE IMAGER
NEUTRAL PARTICLE DETECTOR
Plasma Analyzer
ELECTRON REFLECTOMETER
NODE_NAME planetary plasma interactions
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE ==================================================================== Confidence Level Overview ========================= The ER is mounted on the spacecraft body, where measurements are susceptible to spacecraft charging and FOV blockage. This ER instrument design is typically used on a rapidly spinning spacecraft (few seconds period), on which the disk-shaped FOV would sweep out the entire sky in a time that is short compared with most timescales of interest. However, since MGS spins slowly (once per orbit), each data record covers only a small region of the sky. Despite this limitation, the scalar flux provided in the ERDR is suitable for identification of plasma boundaries (bow shock, magnetic pile-up boundary, ionopause) and following the evolution of the electron energy distribution, which is useful for evaluation of the plasma environment and interpretation of the magnetometer data. Any application of these data that requires an unbiased average over all look directions (4-pi ster) is NOT RECOMMENDED. ==================================================================== Review ====== The ERDR will be reviewed internally by the MGS MAG/ER team prior to release to the planetary community. The ERDR will also be reviewed by PDS. ==================================================================== Data Coverage and Quality ========================= ER data are recorded continuously. Data coverage depends almost entirely on the fraction of the spacecraft telemetry that can be received by the DSN. The mapping orbit lies close to the ionopause altitude. Because of spatial and temporal variations in the ionopause, the ER can sample several different plasma environments, including the ionosphere, the magnetosheath, the magnetotail, and closed magnetic field lines anchored to remanent crustal sources. Data quality is best when the spacecraft is within the planet's shadow. In sunlight, data quality is a function of spacecraft rotation phase, since photoelectron contamination depends on the illumination pattern. ==================================================================== Limitations =========== The ER is mounted on the spacecraft instrument deck and has a disk-shaped FOV that is orthogonal to the spacecraft XY plane and nearly orthogonal to the spacecraft Y axis. (There is a 10-degree rotation about the Z axis to minimize spacecraft obstructions in the FOV.) This 360-degree FOV is divided into 16 angular sectors, each 22.5 degrees wide. Throughout mapping, the ER is in ''fixed-sector'' mode, meaning that these 16 angular sectors remained constant in the spacecraft reference frame, sweeping out the entire sky every 1/2 of an orbit. Parts of the spacecraft are within the instrument's FOV. The high gain antenna (HGA), which blocked ~70 degrees of the FOV during aerobraking is not in the FOV during mapping. Smaller amounts of blockage are caused by attitude control thrusters and the -Y solar array gimbal and yoke assembly. One effect this has on the measurements is to block ambient electrons from the directions of the obstacles. This is most clearly seen at high energies (> 100 eV), which are only slightly deflected by the spacecraft floating potential. In addition, when these obstacles are illuminated by the sun, they emit photoelectrons up to ~50 eV, which can enter the ER aperture and elevate the counting rate at low energies. The detailed signature of this effect depends on the illumination pattern as the spacecraft rotates, which is a function of the angles between Earth, Mars, and the Sun. These angles varied throughout the mapping phase. Photoelectron contamination has not been removed from the data; however, the presence of contamination is readily identified in the low energy channels (< 50 eV) by a sharp (nearly discontinuous) increase in counting rate which appears at regular 100-minute intervals. The contamination disappears as abruptly as it appears. For a duration of ~4 minutes every 1/2 spacecraft spin (when the spacecraft illuminated) sunlight can directly enter the ER aperture and scatter inside the instrument, creating secondary electrons. (Note: the spacecraft spins once per orbit to keep the nadir deck pointed at the planet.) A tiny fraction of these photons and secondary electrons can scatter down to the anode and create a ''pulse'' of spurious counts. This sunlight pulse appears at all energies, but is most noticeable from 10 to 80 eV and above 1 keV. Sunlight pulses have not been removed from the data. The instrument's energy scale is referenced to spacecraft ground. In sunlight, spacecraft ground floats a few volts positive relative to the plasma in which the spacecraft is immersed. Electrons are accelerated by the spacecraft potential before they can enter the ER aperture, thus all energies are shifted upward by a few eV. In addition to shifting the electron energy, the trajectories of low energy electrons can be significantly bent by electric fields around the spacecraft. Thus, the energy scale and imaging characteristics are relatively poor at the lowest energies (10-30 eV), becoming much more accurate at higher energies.
CITATION_DESCRIPTION Mitchell, D. L., MGS-M-ER-3-MAP1/OMNIDIR-FLUX-V1.0, MGS Mars/Moons MAG/ER Mapping ER Omnidirectional Flux V1.0, NASA Planetary Data System, 2009.
ABSTRACT_TEXT The Electron Reflectometer DataRecord (ERDR) is a time ordered series of electron measurements from theMars Global Surveyor (MGS) Mission. Each record consists of a time tagwith 19 scalar data points representing measurements of the electron fluxin 19 different energy channels, ranging from 10 eV to 20 keV, with anenergy resolution of 25%. Each data point is a measure of the electronflux (cm-2 sec-1 ster-1 eV-1) averaged over a 360 x 14 degree disk-shapedfield of view (FOV). During the Mapping Phase, as the spacecraft orbits theplanet the ER field of view sweeps out the entire sky (4-pi ster)every 58.5 minutes, which is much longer than the integration time perrecord (2 to 48 sec, depending on energy and telemetry rate) and much longer than most timescales of interest in Mars' plasma environment.
PRODUCER_FULL_NAME DAVID L. MITCHELL
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