Definitions of Units of Measure used in MMS data product files:
Quantity | Units (original notation) | Units (proposed notation - See below) | SI_CONVERSION (proposed) |
---|
Number densities | cm^-3 | cm^{-3} | 1e6>m^{-3} |
Speeds, velocities | km/s | km s^{-1} | 1.0e3>m |
Angles, phase shifts | deg | deg | 0.0174532925>rad |
Pressures (plasma - dynamic, thermal, magnetic) | nPa | nPa | 1.0e-9>Pa |
Temperatures | eV | eV | 11604.50520>K |
Heat Flux | mW/m^2 | mW m^{-2} | 1.0e-3>W m^{-2} |
Entropy | J/K | J K^{-1} | 1.0>J K^{-1} |
Electric Field | mV/m | mV m^{-1} | 1.0e-3>V m^{-1} |
Probe to Spacecraft Potential | V | V | 1.0>V |
Electric field power spectral density | (V/m)^2/Hz | V^2 m^{-2} Hz^{-1} | 1.0>V^2 m^{-2} Hz^{-1} |
ExB Velocity | km/s | km s^{-1} | 1.0e3>m s^{-1} |
Poynting Flux | mW/m^2 | mW m^{-2} | 1.0e-3>W m^{-2} |
Magnetic Field | nT | nT | 1.0e-9>T |
Magnetic Field Power Spectral Density | nT^2/Hz | nT^2 Hz^{-1} | |
Current Density | nA/m^2 | nA m^{-2} | 1.0e-9>A m^{-2} |
Differential Number Flux | 1/(cm^2 s sr eV) | cm^{-2} s^{-1} sr^{-1} eV^{-1} | |
Differential Energy Flux | eV/(cm^2 s sr eV) | eV cm^{-2} s^{-1} sr^{-1} eV^{-1} | |
Distance | km | km | 1.0e3>m |
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.