Mars Global Surveyor (MGS) ER Angular Flux Data Description ER - Electron Reflectometer PDS3 DATA_SET_ID = MGS-M-ER-4-MAP1/ANGULAR-FLUX-V1.0 ORIGINAL DATA_SET_NAME = MGS MARS/MOONS MAG/ER MAPPING ER ANGULAR FLUX V1.0 START_TIME = 1999-04-02T15:13:17.068 STOP_TIME = 2006-11-02T23:24:30.789 PDS3 DATA_SET_RELEASE_DATE = 2006-11-14 PRODUCER_FULL_NAME = David L. Mitchell ==================================================================== PDS4 Collections: ================= browse-full-calib (urn:nasa:pds:mgs-mager:browse-full-calib) data-angular-flux-map (urn:nasa:pds:mgs-mager:data-angular-flux-map) Collection Overview =================== The Mars Global Surveyor (MGS) Electron Reflectometer Angular Data (ERAD) consist of time-ordered series of 100-600-eV electron flux measurements for 16 look directions spanning a 360 x 14 degree field of view. These data are intended to supplement existing MGS ER data, which cover the entire instrumental energy range (10 eV - 20 keV), but are omni-directional. Separate data files are provided for each of four energy channels, 116, 191, 314, and 515 eV (dE/E = 25%). These energies were chosen because: 1) Electron trajectories are not significantly bent by the spacecraft floating potential at energies > 100 eV. 2) Count rates at energies < 600 eV are typically > 100 times greater than instrument background. 3) Sunlight contamination can be safely neglected in this energy range, except when sunlight directly enters the instrument aperture, which rarely occurs in the mapping configuration. Each record consists of a time tag (UTC, spacecraft event time) with 16 scalars representing measurements of electron energy flux in 16 look directions, 16 scalars indicating the uncertainties in those measurements, 1 scalar indicating instrument background, 2 scalars for the magnetic field azimuth and elevation angles in sensor coordinates, and 1 scalar indicating the size of the angular uncertainty cone around the magnetic field direction. Electron fluxes, uncertainties, and background are in units of electron differential energy flux, eV/(cm^2 sec ster eV). Angles are in degrees. The Electron Reflectometer field of view is a 360 x 14 degree fan that is divided into sixteen 22.5 x 14 degree sectors, numbered 0 through 15, each with its own counter. The ER is mounted on the nadir deck of the spacecraft. Its orientation is shown in the MGS ER mounting description currently located in the DOCUMENT directory of the PDS volume containing these data. ER azimuth is defined in the ER X-Y plane with zero at the boundary between sectors 0 and 15. Note that ER coordinates are left handed, so that azimuth increases with sector number. ER elevation is measured out of the ER X-Y plane. The field of view extends to +/- 7 degrees elevation. 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 (2-8 sec, depending on telemetry rate) and much longer than most timescales of interest in Mars' plasma environment. Thus, the field of view for each data record spans ~12% of the sky. However, since electrons are constrained to travel along magnetic field lines, it is more important to consider an electron's motion with respect to the magnetic field. In a uniform field, electrons gyrate around magnetic field lines on helical paths of constant radius (typically a few km) and pitch angle, which is the angle between an electron's velocity and the magnetic field. With knowledge of the ambient magnetic field direction measured by the MGS Magnetometer (MAG), the ER field of view can be mapped into pitch angle. In the ER X-Y plane, the relationship between azimuth (az) and pitch angle is: cos(Pitch Angle) = cos(az - Baz) * cos(Bel) where Baz and Bel are the azimuth and elevation of the magnetic field in ER sensor coordinates. When Bel = 0, the ER measures the entire pitch angle distribution (0-180 degrees) twice, once for each half of the field of view. When Bel = 90 degrees, the ER measures only pitch angles of 90 degrees. All values of Bel are possible, but the ER field of view is oriented in such a way that when the magnetic field has a large radial component (with respect to the planet), Bel is small, and most of the pitch angle distribution is observed. The magnetic field tends to have a large radial component on the night hemisphere and in the vicinity of crustal magnetic fields. ER Angular Data is intended to be used in conjunction with ER omni- directional data, MAG data, and spacecraft ephemeris data. Time tags are provided to synchronize the ERAD with all of these collections. The electron energy distribution (data-omni-flux-premap and data-omni-flux-map) data contain information about the plasma environment (i.e., whether the spacecraft is in the magnetosheath, magnetotail, or ionosphere). The MAG data provide the strength and direction of the local magnetic field (from crustal sources or induced by the Mars-solar wind interaction). The ERAD provide information about the large-scale configuration of the magnetic field -- for example, whether the spacecraft is on a closed crustal magnetic field loop, on an open crustal field line connected to the solar wind, or on a solar wind field line. Electron reflectometry can be used on open field lines to determine the magnetic field strength at altitudes near the exobase (~180 km) and to probe the atmospheric density between the spacecraft and the exobase. ==================================================================== Parameters ========== Each Mars Global Surveyor ER angle data file contains a time ordered series of electron differential energy flux measurements at a given energy (ranging from 100-600 eV). ER data are organized into 'packets', each of which contains 12, 24, or 48 seconds of data, for high, medium, and low spacecraft telemetry rates, respectively. Each packet is further subdivided into samples. There are 6 samples per packet for each of the relevant energy channels, as displayed in the table below, resulting in sample periods of 2, 4, or 8 seconds, depending on telemetry rate. The time listed for each record is the center of the sampling interval. The relevant energy channels and sampling intervals are as follows: Channel Number Energy Range Samples per Packet --------------------------------------------------------------- 7 415 - 639 eV 6 8 253 - 390 eV 6 9 153 - 237 eV 6 10 92 - 144 eV 6 --------------------------------------------------------------- In-flight calibrations are available for each of the pre-mapping mission phases (AB1, SPO1, SPO2, and AB2), and are performed about once per month in the Mapping and Extended phases. This calibration is used to determine the relative instrumental sensitivity around the FOV to an accuracy of 5-10%. The sensitivity varies slowly with time primarily because of aging of the microchannel plate (MCP), which is used to amplify the signal from a single electron into an electrical pulse that can be detected on the anode. This information is taken into account when converting count rates to calibrated electron differential energy fluxes. The local magnetic field measured by the Magnetometer is rotated into ER sensor coordinates (see the MGS ER mounting description mentioned above) to determine the pitch angle range spanned by each angular sector of the instrument. The magnetic field direction changes continuously, so the pitch angle map must be determined separately for every data record. ==================================================================== 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 electron energy flux eV/(cm^2 s ster eV). 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 (4 elements). The data array is converted from raw counts to differential energy flux as follows. Raw count rate (R) is 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 0.5 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 instrumental background, which is caused primarily by high energy particles that penetrate the ~2-mm-thick instrument casing and impact the MCP. Electrons > few MeV and protons > 20 MeV have sufficient energy to do this. (Lower energy particles are also detected if any of the secondaries they produce reach the detector.) Since these particles bypass the electostatic analyzer section, they produce a count rate that is independent of the instrument's energy sweep, typically dominating the signal in the highest 2-3 energy channels. During quiet times, the background count rate over the entire anode is 7-10 counts/sec, which translates to 350-500 eV/(cm^2 s ster eV) in the ERAD. 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 or interplanetary shock crossings (when significant fluxes of >10 keV electrons are present) or during energetic solar events, when bursts of solar energetic particles can increase the flux of penetrating particles by several orders of magnitude. Assuming that the highest energy channel contains 100% background, the background level for the lower energy channels used in this collection can be estimated as B(E) = B(20 keV), where B(E) is the background level (in units of eV cm-2 s-1 ster-1 eV-1) at energy E. The background is typically negligible at energies below about 1 keV. Finally, we divide by the geometric factor (0.02 cm2 ster) to obtain the differential energy flux eV/(cm^2 s ster eV). In order for the user to calculate pitch angles for each sector and observation, the orientation of the magnetic field with respect to the ER instrument must be supplied. To accomplish this, the magnetic field vectors recorded by MGS MAG, expressed in a payload coordinate system, are first resampled to the time resolution of the electron observations. Then, the orientation of the local field vector in azimuth and elevation are recorded, along with an uncertainty. The ER sectors are numbered 0-15 in a clockwise fashion as viewed from above the ER instrument. Azimuth is defined from 0-360 degrees, also in a clockwise fashion as viewed from above ER, with 0 degrees located at the boundary between instrument sectors 0 and 15. Elevation is defined between -90 and 90 degrees, with 90 degrees indicating the local magnetic field is orthogonal to the instrument aperture and pointed 'up' away from the instrument (in the +z direction for the left-handed coordinate system defined by the azimuth angles), 90 degrees pointing 'down' toward the instrument, and 0 degrees indicating that the local magnetic field is in the plane of the instrument aperture. The uncertainty in the local magnetic field vector is supplied as a single angle defining a cone around the nominal vector. Therefore, the uncertainty for a given observation could be in the azimuth direction or the elevation direction or (most likely) both. Finally, data for sectors 9 and 10 are supplied as for all other sectors in this collection. However, in-flight intercalibration indicates that sectors 9 and 10 have significantly higher uncertainties than the other sectors. Therefore we recommend that sectors 9 and 10 be disregarded in any science analysis using these data. After processing, data are written to ASCII tables, described below. ==================================================================== Data ==== The ERDR collection consists of four time-ordered tables containing electron fluxes in each of four different instrument energy ranges as a function of instrument sector. Each table contains 43 columns, as follows: COLUMN CONTAINS ------ ---------------------------------------------------- 1-6 time stamp (yr, day of year, hr, min, sec, msec) 7-22 electron energy fluxes for 16 ER sectors 23-38 one-sigma error estimates for 16 ER sectors 39 estimate of instrument background (energy flux units) 40-41 azimuth and elevation of local B-field wrt ER 42 angular uncertainty in local B-field direction (Note: The column numbering given here differs from the column numbering in the PDS labels for these data. This is not a real discrepancy; it reflects the fact that the PDS labels treat the time stamp as a single multi-part column.) File names in this collection currently follow the format MyyDddd_PAD_eee.STS where yy = 2-digit year, ddd = 3-digit day of year, and eee = 3-digit energy channel in eV. ==================================================================== Ancillary Data ============== No additional ancillary data is required for use of these data. However, the user may wish to use MAG data and MGS spacecraft ephemeris information in conjunction with this collection, also available from the PDS. ==================================================================== Coordinate System ================= The data are presented in a coordinate system tied to the ER instrument. Each of the 16 energy fluxes and error bars are associated with a single angular sector of the instrument. Azimuth and elevation angles necessary to compute pitch angles for each sector are supplied with respect to the instrument, as described in the sections above. ==================================================================== Software ======== No software is provided with this PDS4 collection. Data reduction software for the ERAD is written in IDL. Users are advised that the following line of pseudocode can be used to compute the pitch angle range spanned by each of the ER's 16 angular sectors, given the azimuth (Baz) and elevation (Bel) of the ambient magnetic field in sensor coordinates: cos(pitch angle) = cos(az - Baz) * cos(Bel) where az represents the azimuth range spanned by one of the sectors. For example, to compute the pitch angle range spanned by Sector 0, one would use a range of 0-22.5 degrees for az, together with values of Baz and Bel in sensor coordinates provided for each data record. Note that Baz and Bel vary continuously, so a separate pitch angle map must be computed for each data record. Also note that the maximum and minimum pitch angles sampled by a given sector need not be at the edges of the sector. One way to present these data on a plot of energy flux (Y axis) vs. pitch angle (X axis), is to plot horizontal 'error bars' representing the pitch angles spanned by each sector and vertical error bars for the one-sigma flux uncertainties. ==================================================================== Confidence Level Overview ========================= The 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. Any application of these data that requires an unbiased average over all look directions (4-pi ster) is NOT RECOMMENDED. Instead, use the formula above for calculating a separate pitch angle map for each data record. Complete (0-180 degrees) pitch angle coverage is rarely achieved, but the pitch angle coverage is often sufficient to determine magnetic field topology and to detect loss cones caused by atmospheric absorption of electrons. The ER is mounted on the spacecraft body, where measurements are susceptible to the following effects: 1. Spacecraft charging: The spacecraft typically charges to several volts positive in sunlight and several tens of volts negative in shadow. Since electrons have to cross this potential before reaching the ER aperture, the energy scale of the ER is effectively shifted by the amount of the spacecraft potential. In addition, it is possible for different parts of the spacecraft to charge to different potentials, because of differences in material properties. Differential charging sets up small-scale electric fields that can bend the trajectories of low energy electrons. At the >100 eV energies used in this collection, these effects are relatively small and can be safely neglected. For increased confidence, the user may compare pitch angle distributions at 100 and 500 eV. 2. Spacecraft photoelectrons: Parts of the spacecraft that are illuminated by the sun will emit photoelectrons. Most of these spacecraft photoelectrons have energies below ~10 eV, but a small part of the distribution extends up to ~60 eV. Since parts of the spacecraft are close to the ER field of view, some of these spacecraft photoelectrons can enter the ER aperture and contaminate the measurements. The flux and angular distribution of spacecraft photoelectrons as seen by the ER depends on the illumination pattern of the spacecraft. At the >100 eV energies of this collection, spacecraft photoelectron contamination can be safely neglected. 3. Field-of-view blockage: Parts of the field of view are partially obstructed by the spacecraft. In the mapping configuration (with the high gain antenna deployed) these obstructions include the -Y solar array gimbal and corners of the spacecraft bus. To first order, these fixed obstructions are accounted for in the calibration of the ER field of view. (Sectors that contain an obstruction are assigned a lower effective sensitivity.) 4. Solar energetic particle (SEP) events: The instrument background is dominated by penetrating particles due to galactic cosmic rays (GCRs) and solar energetic particles (SEPs), which are produced during solar flares and associated coronal mass ejections (CMEs). The GCR background is negligible in the 100-500 eV energy range -- the fluxes of ambient electrons dominate by several orders of magnitude. However, during large solar events ('space weather'), SEPs can increase the background level enough to be significant even in the 100-500 eV range. The user is cautioned to use the background data provided in this collection to identify SEP events. Large events may prevent the reliable use of these data. ==================================================================== 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. Additionally, pitch angle coverage is controlled by a combination of ER look direction and the local magnetic field direction. In some instances it is possible that the local magnetic field is orthogonal to the ER field of view, so that the pitch angle coverage is limited to a small range around 90 deg. However, a much more typical situation is partial pitch angle coverage, which is often sufficient to establish the topology of the magnetic field and to identify loss cones caused by atmospheric absorption of electrons. The ER field of view is oriented in such a way that the pitch angle coverage is best whenever there is a large radial component (with respect to the planet) of the ambient magnetic field. This situation often occurs on the night hemisphere (because of the draped magnetotail) and over strong crustal magnetic fields. ==================================================================== 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, observations that were likely to have been contaminated have not been included in this collection. 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 is 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, outside the range of energies included with this collection. Sunlight pulses, therefore, 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 the higher energies used in this collection.