MESSENGER MASCS UVVS Derived Data Record (UVVDDR)

UVVDDR – Derived Data Record

Instrument: MESSENGER Mercury Atmospheric and Surface Composition Spectrometer

PDS Data Set ID: MESS-E/V/H-MASCS-4-UVVS-DDR-V1.0

The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) Ultraviolet and Visible Spectrometer (UVVS) Derived Data Record (UVVDDR) data products consist of UVVS derived data. The UVVS experiment is a scanning grating monochromator equipped with three photomultiplier tubes (PMTs). There are three UVVEDR data products, one for each detector, which 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. UVVDDR products are generated from Calibrated Data Records (UVVCDRs) of orbital data.

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 IAU2000 reference system for cartographic coordinates and rotational elements was used for computing latitude and longitude coordinates of planets.

There are three types of UVVDDRs: surface, atmosphere, and atmospheric model.

Surface UVVDDRs:

One surface UVVDDR will contain all the derived reflectance data from one MUV surface observation. Not all UVVCDRs will be converted to UVVDDRs; 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 UVVDDRs.

UVVCDRs are converted to UVVDDRs 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 co-adding 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.

There are two surface UVVDDR 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 of 5 steps of an observation. The surface UVVDDRs 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 UVVDDR 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).

MASCS surface UVVDDR products are named according to the following file naming convention:

UMD_mmm_xx_yyddd_hhmmss_zzz.DAT (with detached PDS label)

where:

mmm = mission phase

ORB = orbit insertion until end of nominal orbit mission

OB2 = second year of orbital operations

OB3 = third year of orbital operations

OB4 = fourth year of orbital operations

xx = 2-digit macro id; 00 for data created prior to the existence of UVVS macros or when PIPE (software) cannot determine the macro id used; value 48 or 49 for surface observations

yy = 2-digit year in UTC converted from the first Mission Elapsed Time (MET) in the EDR

ddd = 3-digit day of year in UTC converted from the first MET in the EDR

hhmmss = 6-digit hour, minute, second in UTC converted from the first MET in the EDR

zzz = data type

HDR = per observation (science header) data

SCI = per step (science DDR) data

In ODE, surface UVVDDR products appear under the following product IDs:

UMD_MMM_XX_YYDD_HHMMSS_ZZZ_DAT

Atmosphere UVVDDRs:

The atmosphere UVVDDR consists 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 UVVDDRs: 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 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 in the spectral resolution of the UVVS.

Atmosphere UVVDDRs for each species and observation category consist of individual files spanning one Mercury year (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) 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].

MASCS atmosphere UVVDDR products are named according to the following file naming convention:

UD_mm_xx_ss.DAT (with detached PDS label)

where:

mm = mercury year

01-04 = Primary mission

05-09 = Extended mission (orbit year 2)

10-13 = Extended mission (orbit year 3)

14-17 = Extended mission (orbit year 4)

18 = Extended mission (orbit year 5)

xx = record category

LS = Dayside Limb Scan

LD = Dayside and Nightside Limb Drift

NS = Nightside Sweep

ss = species

NA = sodium

MG = magnesium

CA = calcium

In ODE, atmosphere UVVDDR products appear under the following product IDs:

UD_MM_XX_SS_DAT

Atmospheric Model UVVDDRs:

The atmospheric model UVVDDRs 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 2 atmospheric model UVVDDRs for the sodium (UD_NA_MOD.TAB, with its corresponding label file) and calcium (UD_CA_MOD.TAB, with its corresponding label file) models.

In ODE, atmospheric model UVVDDR products appear under the following product IDs:

UD_NA_MOD_TAB

UD_CA_MOD_TAB