PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM LABEL_REVISION_NOTE = " 2012-05-23 MESS:izenberg, GEO:ward Draft for peer review; 2013-01-09 GEO:ward, MESS:izenberg Updated for End Primary Mission (12085); 2013-06-18 GEO:ward Added surface DDR and atmospheric model DDR advanced products; 2013-12-19 MESS:vervack PDS Release 11 updates; 2014-01-09 GEO:ward, MESS:izenberg Updated for PDS Release 11; 2014-12-17 GEO:ward, MESS:izenberg Updated for PDS Release 13; 2016-03-07 GEO:ward, MESS:vervack Updated for PDS Release 15." OBJECT = DATA_SET DATA_SET_ID = "MESS-E/V/H-MASCS-4-UVVS-DDR-V1.0" OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = "MESSENGER E/V/H MASCS 4 UVVS DERIVED DATA V1.0" DATA_SET_COLLECTION_MEMBER_FLG = "N" DATA_OBJECT_TYPE = TABLE START_TIME = 2011-088T01:33:53 STOP_TIME = 2015-120T15:01:43 DATA_SET_RELEASE_DATE = 2016-05-06 ARCHIVE_STATUS = ARCHIVED PRODUCER_FULL_NAME = "NOAM IZENBERG" DETAILED_CATALOG_FLAG = "N" CITATION_DESC = " Izenberg, N., MESSENGER E/V/H MASCS 4 UVVS DERIVED DATA V1.0, MESS-E/V/H-MASCS-4-UVVS-DDR-V1.0, NASA Planetary Data System, 2013." DATA_SET_TERSE_DESC = "The MESSENGER MASCS UVVS derived data records consist of science and instrument data collected by the UVVS detector during orbital operations of Mercury." ABSTRACT_DESC = " Abstract ======== This data set consists of the MESSENGER MASCS UVVS derived data records, also known as DDRs. There are three types of UVVS DDRs: surface, atmosphere, and atmospheric model. There are two surface DDR data products associated with each UVVS observation set: a science header table and a science data table. There are nine atmosphere DDR data products, consisting of three different observation types for each of sodium (Na), calcium (Ca), and magnesium (Mg). There are 3 atmospheric model data products, one each for observations of Na, Ca, and Mg." DATA_SET_DESC = " Data Set Overview ================= This data set consists of the MESSENGER MASCS UVVS derived data records (DDRs). The UVVS experiment is equipped with three photomultiplier tubes, which are sensitive to three different wavelength ranges. Resulting spectra cover the wavelength ranges of the far ultraviolet (FUV)(115-190 nm), middle ultraviolet (MUV) (160-320 nm), and visible (VIS) (250-600 nm), with a resolution of 0.5 nm for the FUV channel and 1.0 nm for the MUV and VIS channels. There is a data overlap with VIRS in the VIS wavelength range. DDR products are generated from CDRs of orbital data. There are three types of UVVS DDRs: surface, atmosphere, and atmospheric model. For the UVVS surface DDR data products, one DDR will contain all the derived reflectance data from one MUV surface observation. Not all CDRs will be converted to DDRs; only targeted surface observations of the MUV PMT from Mercury orbit will be processed to a DDR. Thus, there will only be a few thousand total UVVS DDRs. CDRs are converted to DDRs by first filtering out non-planet-targeted observations, then converting to reflectance by dividing the solar irradiance spectrum out of the MASCS radiance data. A photometric correction is then applied to the reflectance data for corrected data. Bins of approximately 1 nm bandwidth (~5 UVVS steps) are created by coadding the signal in each step of the bin. In a 660 step scan of the standard UVVS surface observation, this results in a single UVVS spectrum of about 132 bins. One observation set is associated with two surface DDR data products: a science header table, showing the instrument command parameters for a given observation, and a science data table, showing counts, derived science data, and pointing information for each bin of 5 steps of an observation. The surface DDRs are in binary table format, and each is described by a detached PDS label. The label points to an associated format file that defines the fields of the binary table contained within the data file. The UVVS atmosphere DDRs consist of time-ordered sequences of radiance values that are measured along lines of sight perpendicular to Mercury radius vectors. The viewing direction for each measurement is determined by specifying the latitude, longitude, and altitude of the spacecraft and the latitude, longitude, and altitude of the minimum ray. There are three categories of observations in the DDRs: 1) dayside limb scans, which are acquired as sets of dedicated limb altitude profiles at specific local times, 2) dayside and nightside limb drift profiles, which are acquired in a more random fashion as time and observational circumstances allow, and 3) nightside tail sweeps, which are sets of back-and-forth sweeps of the UVVS line of sight across the nightside exosphere. Each of these products is generated for the three major exosphere species that are observed by the UVVS: sodium (Na), magnesium (Mg), and calcium (Ca). The Mg data in the DDRs are observations of the emission line at 285.3 nm, the Ca data are observations of the emission line at 422.8 nm, and the Na data are observations of the doublet emission lines at 589.2 nm (D2) and 589.8 nm (D1). All wavelengths are specified in vacuum. The total radiance calculated for Na pertains to the sum of the D1 and D2 lines, which overlap to some extent at the spectral resolution of the UVVS. A single atmosphere DDR contains all of the observations for a given species and observational category that were acquired during a single Mercury year. 'Mercury year' is defined as the time to cover the full 360 degrees of Mercury's orbit around the Sun, with the starting point being at 0 degrees true anomaly. Because the orbital phase of the MESSENGER mission did not start at 0 degrees true anomaly, the first 'Mercury year' will be partial, running from 73 to 360 degrees. The UVVS Atmospheric Model DDRs consist of a series of model fits to data averaged over specific Mercury true anomalies and local times. These model fits, and the parameters provided in the DDRs, are described below in 'Derived Product Overview.' The description is given for Na, but Ca and Mg are similarly fitted. Instrument Overview =================== The MASCS instrument consists of a small Cassegrain telescope that simultaneously feeds the UVVS and VIRS experiments. The MASCS UVVS experiment is a scanning grating monochromator equipped with three photomultiplier tubes, providing spectral information in the far ultraviolet (115-190 nm), middle ultraviolet (160-320 nm), and visible (250-600 nm) wavelength ranges. The UVVS detector will determine the composition of Mercury's exosphere by measuring the spatial and vertical distribution of known species as well as search for new ones. See the INST.CAT file for more information and [MCCLINTOCK&LANK2007] for full details. Derived Product Overview ======================== This data set consists of data derived from MASCS UVVS Calibrated Data Records (CDR). The Science CDRs are the processed data records used to derive emission or reflectance data used for scientific analysis. The UVVS Science CDRs contain emission data of the UVVS photomultiplier tubes (PMT) at the commanded step of the UVVS grating, which corresponds to a specific wavelength of light. Wavelength range and sensitivity of each PMT at each grating step vary, as documented in the MASCS Calibration Report, MASCS_CAL_RPT.PDF, provided in the DOCUMENT directory. Before the science data can be used for scientific analysis, the count rates in the EDRs must be converted to physical units and the data must be transformed into meaningful physical reference systems. This conversion yields calibrated data which are stored in Calibrated Data Records (CDRs). Surface DDRs: The CDR to surface DDR processing steps include: 1. Filter out non-surface observations. 2. Divide by solar spectrum for reflectance, bin steps to 1 nm bandwidths. 3. Apply photometric correction. Atmosphere DDR: The CDR data for Na, Ca, and Mg is filtered by observational category and combined according to Mercury year to create the atmosphere DDR file. The processing steps from the CDR to the atmosphere DDR level include: For each of: a) Dayside limb altitude profiles b) Dayside and nightside limb drift profiles c) Night side tail sweep profiles carry out the following: 1. Filter CDRs for a specified species and observational category. 2. Order as a time series, which keeps individual altitude profiles together. Atmospheric Model DDR: The model employed is adapted from the model developed by Chamberlain [1963]. The temperature and density of the dayside sodium exosphere were found by fitting the estimated column densities from individual limb scans. The fit applies only to the lower 700-1000 km of the exosphere, which is relatively cold and dense compared to the exosphere at higher altitudes. The DDRs give the averages of these fits as a function of true anomaly in 5 degree increments. The column density N (cm^-2) is derived from radiance I (kR) using N=I/(g 10^6) (1) The g-value is the rate (s^-1) at which an atom scatters sodium D1 and D2 solar photons. It depends on distance from, and radial velocity relative to, the sun. The sodium atoms have a distribution of speeds, but because this distribution is relatively narrow for the low-altitude, low-temperature portion of the exosphere described in this data product, we use Mercury's radial velocity and distance from the sun to calculate g. The exospheric column density is related to the density via N=KHn (2) where n is the density of the exosphere at the line-of-sight tangent point, H is the scale height of the exosphere, and K is the ratio between the line-of-sight column density and the vertical column density (~Hn) and given, approximately, by [pi*r/(2H)]^(1/2). These formulas come from Chamberlain [1963]. The density is approximated by n=n0e^-(U/kT) (3) where U is the gravitational potential energy due to gravity and photon acceleration, T the temperature, and n0 the surface density (see Feynman [1963] for a general discussion of this formula). The temperature and surface density are the free parameters of our fits to the limb scans and are provided in the DDR. Photon acceleration (also called photon pressure or radiation pressure) is an antisunward acceleration due to the resonant scattering of sunlight. It is directly proportional to the g-value. At Mercury, for sodium, it can be nearly half as large as surface gravity (e.g., Wang and Ip [2011]). For that reason we included it along with the gravitational potential term in Eq. (1), so that the potential energy is written as U=GMm/r + mbrcos(theta) (4) as described in Bishop [1985], where b is the photon acceleration, theta is the angle between the local radial vector and the Mercury-sun axis, G is the gravitational constant, M is the mass of Mercury, m is the mass of a sodium atom, and r is the distance from Mercury's center. For each limbscan, because they all have line-of-sight tangent points near the equator, theta is derived, approximately, by the angular distance from noon as measured by the local time provided in the DDR. The scale height, used in the formulas above and provided in the data product, is defined by H = n/(dn/dr) = kT/( GMm/r^2 + mbcos(theta) ). (5) Note that in the absence of photon pressure and ignoring the radial variation in gravity, this definition reduces to the classic kT/ma, where a is gravitational acceleration. Alternately, we can write H as kT/m(a + bcos(theta) ) (6) where the parenthetical term in the denominator can be seen as a sum of two terms, the gravitational term and the radial component of the photon pressure. This scale height (measured in km) is also provided in the DDR. Coordinate Systems ================== MASCS UVVS data are represented in the following coordinate systems: * Planetocentric body fixed: The MBF coordinate system is defined by the planetocentric position, Cartesian X, Y, Z coordinates related to the planetocentric distance, latitude measured positive northward from the equator, and longitude measured positive eastward from the prime meridian. * Cartographic: Surface observations use IAU planetocentric system with East longitudes being positive for planetary surfaces. The MESSENGER-derived reference system (pck00010_msgr_v21.tpc) for cartographic coordinates and rotational elements was used for computing latitude and longitude coordinates of planets. Data ==== Surface DDRs: There are two surface DDR data products associated with each UVVS observation set: a science header table, showing the instrument command parameters for a given observation, and a science data table, showing counts, derived science data, and pointing information for each bin of 5 steps of an observation. The surface DDRs are in binary table format, and each is described by a detached PDS label. The label points to an associated format file that defines the fields of the binary table contained within the data file. There are four types of spectra stored in each surface DDR science data file: IOF_BIN_DATA (table column 14), PHOTOM_IOF_BIN_DATA (table column 15), IOF_ BIN_NOISE_DATA (table column 16), and PHOTOM_IOF_ BIN_NOISE_DATA (table column 17). Atmosphere DDR: There are nine UVVS atmosphere DDR data products per Mercury year (as defined above), one for each species (Na, Ca, Mg) and for each observation category (dayside limb altitude profile, dayside and nightside limb drift profile, and night side tail sweep profile). Data from the MASCS housekeeping EDR product generated by the MASCS instrument, which is the same for both the UVVS and VIRS components of the MASCS experiment, are incorporated into the CDRs, and data from both EDR and CDR are passed through to the DDR. Atmosphere DDRs for each species and observation category consist of individual files spanning one Mercury year from true anomaly 0 to 360 degrees. Fractional years (including the beginning of the orbital phase and the end of mission) will not contain data for all true anomaly values. There are two types of data stored in each science file: RADIANCE versus WAVELENGTH (spectra) and TOTAL_RADIANCE in each observed emission line [total radiance integrated over the emission line(s) as measured at each tangent altitude]. Atmospheric Model DDR: The UVVS Atmospheric Model DDRs consist of average fits to observations at a series of Mercury true anomaly and local times. The model parameters near-surface density, temperature, and scale height are provided. The data are provided in an ASCII table format. There are 3 atmospheric model DDRs for the sodium, magnesium, and calcium models, respectively. " CONFIDENCE_LEVEL_NOTE = " Confidence Level Overview ========================= The MASCS UVVS Derived Data Records (DDRs) consist of derived data converted to physical units and represented in physical coordinate systems. Data presented here are an accurate representation of the UVVS data as received from the spacecraft, and reflect the processing steps from the MASCS UVVS Calibrated Data Record (CDR) to the Derived Data Record (DDR) level as detailed in the document UVVS_CDR2DDR located in the DOCUMENT directory. The UTC and MET time tags have been corrected for timing latencies in the instrument so that the UTC and MET correspond to the physical time of the observation. It should be noted that the atmospheric models are intended only as a first-order approximation of the average atmospheric state on the dayside using a well-known and accepted model (the Chamberlain model). However, such models lack some of the relevant physics that are pertinent to the Mercury exosphere. As such, they are excellent starting points for more detailed models but should not be interpreted as a true representation of the actual exosphere. More detailed models are too complicated to provide in any meaningful way. Review ====== The UVVS DDR was reviewed internally by the MASCS team prior to release to the PDS. PDS also performed an external review of the MASCS UVVS DDRs. Data Coverage and Quality ========================= Data reported are the derived data received from the spacecraft during the orbital mission phases: Mercury Orbit, Mercury Orbit Year 2, Mercury Orbit Year 3, Mercury Orbit Year 4, and Mercury Orbit Year 5. These mission phases are defined as: Start time End time Phase Name Date (DOY) Date (DOY) Mercury Years ------------------------- ----------- ----------- ------------- ORB: Mercury Orbit 04 Mar 2011 17 Mar 2012 1 (partial), (063) (077) 2,3,4, 5 (partial) OB2: Mercury Orbit Year 2 18 Mar 2012 17 Mar 2013 5 (partial), (078) (076) 6,7,8, 9(partial) OB3: Mercury Orbit Year 3 18 Mar 2013 17 Mar 2014 9 (partial), (077) (076) 10,11,12, 13 (partial) OB4: Mercury Orbit Year 4 18 Mar 2014 17 Mar 2015 13 (partial), (077) (076) 14,15,16, 17 (partial) OB5: Mercury Orbit Year 5 18 Mar 2015 30 Apr 2015 17 (partial), (077) (112) 18 (partial) To validate the initial DDR dataset, the MASCS team did the following: * Examined one Mercury year of data for each DDR product type. - Checked all data columns for format and sanity numbers. - Compared calibration numbers for earlier independent analysis before DDR development. - Compared pointing information (vectors, latitudes, longitudes, distances, angles) to previous independent solutions. * Spot-checked additional data sets from other Mercury years. The orbital DDR dataset is evaluated on an ongoing basis, checking for completeness and data integrity. MASCS UVVS data were collected during all phases except Venus 1 Flyby. DDR data are derived from orbit only. Data quality information for DDR observations are available in the DQI of matching CDRs. Removal of Low-Altitude Magnesium Data A systematic instrument effect on the UVVS data has been found in some of the dayside observations. This effect, likely due to a scattering problem of unknown origin, mimics a constant background that shows a dependence on local time. While the UVVS team tracks down the cause and develops a correction, all dayside magnesium DDR data (LS and LD) have been excluded from the archive to prevent analysis of bad data. This effect has minimal to no impact on the nightside magnesium tail sweeps (NS) nor on any of the calcium or sodium observations, so these data have not been excluded from the DDRs. Limitations =========== This data set is derived data. The data are received from the spacecraft telemetry and ingested into the MESSENGER Science Operations Center (SOC), then run through the MASCS pipeline. No data uncharacterized gaps have been identified for any of the MASCS operational periods. The data have been calibrated to the best possible level; however, there are temperature-dependent effects in the background removal and wavelength calibration that are still being evaluated on an ongoing basis. The user is warned that these effects may sometimes result in false positives in the calibrated radiance (i.e., false detections). Care should be exercised in using the data, particularly when results are anomalous in appearance. It is noted that the UVVS DDRs represent the best calibrated set of exosphere observations for Na, Ca, and Mg. Because of issues that remain in the CDRs (as stated in the confidence notes), users are STRONGLY ENCOURAGED to use the DDRs for scientific analysis. " END_OBJECT = DATA_SET_INFORMATION OBJECT = DATA_SET_MISSION MISSION_NAME = "MESSENGER" END_OBJECT = DATA_SET_MISSION OBJECT = DATA_SET_TARGET TARGET_NAME = "MERCURY" END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_HOST INSTRUMENT_HOST_ID = MESS INSTRUMENT_ID = "MASCS" END_OBJECT = DATA_SET_HOST OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "BISHOP1985" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "CHAMBERLAIN1963" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "FEYNMAN1963" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "MCCLINTOCK&LANK2007" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "WANG&IP2011" END_OBJECT = DATA_SET_REFERENCE_INFORMATION END_OBJECT = DATA_SET END