Please do NOT post ITAR-restricted content on this site.
Definitions of Units of Measure used in MMS data product files:
- Densities: [cm^-3]
- Speed, Velocities : [km/s]
- Angles, Phase shifts : [deg]
- Pressures (plasma - dynamic, thermal, magnetic): [nPa]
- Temperatures: [eV]
- HeatFlux: [mW/m^2]
- Entropy: [J/K]
- Electric Field: [mV/m]
- Probe to Spacecraft Potential: [V]
- Electric Field Power Spectral Density : [(V/m)^2/Hz]
- ExB velocity: [km/s]
- Poynting Flux : [mW/m^2]
- Magnetic Field: [nT]
- Magnetic field power spectral density: [nT^2/Hz]
- Current density: [nA/m^2]
- Differential Number Flux: [cm^-2 s^-1 sr^-1 eV^-1]
- Differential Energy Flux: eV [cm^-2 s^-1 sr^-1 eV^-1]
- Earth radius: 6371.2 km
- Distance: [km]
- Date/Time (PROPOSED): ISO8601 standard (e.g. YYYY-MM-DDTHH:MM:SS.SSS or YYYY-DDDTHH:MM:SS.SSS)
MMS Team members are welcome to ADD ADDITIONAL QUANTITIES as they are defined.
The CDF Format Guide requires that UNITS and SI_CONVERSION must be specified for each parameter.
- It is proposed that conventions should be defined, so that a standard nomenclature is used for these metadata.
- UNITS attribute
- units in Level 2 CDF files shall be taken from the list above
- The units shall be specified as a human-readable ASCII string, using the abbreviations listed above.
- guidelines for unit nomenclature are proposed, below.
- SI_CONVERSION attribute
- Gives the conversion from the MMS unit of measure to SI units
- This attribute allows for plotting/analysis tools to combine MMS data with data from other missions which use different units.
- Guidelines for SI unit nomenclature are proposed, below.
- Requires guidelines for syntax, which are also included below.
Proposed convention for unit nomenclature and SI conversion attribute:
- SI unit symbols to be used for SI conversion (rather than SI unit names) without SI prefixes: e.g. T, rather than nT or Tesla; sr, rather than steradians.
The SI_CONVERSION from nT would then be "1.0e-9>T". - SI prefixes are allowed in variable units, but not in the converted SI unit (units of [km] are required for distances, but the SI conversion must be to [m]).
- Note that the SI unit for angles is radians [rad].
The SI_CONVERSION for angles (in degrees as defined above) would be "pi/180>rad" (Alternative 1: uses division in unit conversion but preserves precision.)
The SI_CONVERSION for angles (in degrees as defined above) would be "0.0174532925>rad" (Alternative 2: gives number in unit conversion but loses precision.) - Dimensionless variables are required (by ISTP standard) to be a blank character. For consistency the conversion should also be a blank character.
The SI_CONVERSION for a dimensionless variable would be " > ". - Units that are already SI (e.g. Hz, V) will have a multiplicative factor of 1, so the SI_CONVERSION for spacecraft potential would be "1.0>V".
Proposed convention for compound units: [Note: this is proposed for the sake of consistency to facilitate future automation using this metadata]
- Compound units should be denoted by exponents only, with unit symbols separated by white space, as illustrated by the following examples:
Area - [m^2], not [m m]
Acceleration - [m s^-2] (or with LaTeX markup - see note - [m s^{-2}]), not [m/s/s] or [m/s^2] - Only dimensional units should be used. For example, a number density would have units of [cm^{-3}] not [#/cm^{3}]. Similarly an electron number flux would have units of [cm^-2 s^-1 sr^-1 eV^-1], not [#/(cm^-2 s^-1 sr^-1 eV^-1)] or [e/(cm^-2 s^-1 sr^-1 eV^-1)].
Additional note: Unless there's a reason not to, I suggest LaTeX math notation for the exponents, so the units for acceleration would be [m s^{-1}]
- This is usable through IDL with the graphics routines from "Coyote's guide" (David Fanning) or the TexToIDL package.
- This is usable through MATLAB, which directly supports TeX markup in graphics by specifying an interpreter to the text object.
- This is usable in Python through the graphics package matplotlib, which can use LaTeX to render the text in graphics output for several plotting backends.
- LaTeX is directly supported by GNUplot, in case anyone still uses that!
- Where this isn't currently supported is Autoplot, though it may not be difficult for Jeremy to support basic TeX-style markup for units.
- If LaTeX markup is allowed here then there is no reason people can't use it to give equations in other attributes, which could be handy when defining calculated quantities.
Overview
Content Tools
Activity
Tasks