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Definitions of Units of Measure used in MMS data product files:

markup in CDFas rendered
by plot routines
Number densitiescm^-3cm^{-3}cm-31e6>m^{-3}
Speeds, velocitieskm/skm/skm/s1.0e3>m/s
Angles, phase shiftsdegdeg


Pressures (plasma - dynamic, thermal, magnetic)nPanPAnPa1.0e-9>Pa
Heat FluxmW/m^2mW/m^{2}mW/m21.0e-3>W/m^{2}
Electric FieldmV/mmV/mmV/m1.0e-3>V/m
Probe to Spacecraft PotentialVVV1.0>V
Electric field power spectral density(V/m)^2/Hz(V/m)^{2}/Hz(V/m)2/Hz1.0>(V/m)^{2}/Hz
ExB Velocitykm/skm/skm/s1.0e3>m/s
Poynting FluxmW/m^2mW/m^{2}mW/m21.0e-3>W/m^{2}
Magnetic FieldnTnTnT1.0e-9>T
Magnetic Field Power Spectral DensitynT^2/HznT^{2}/HznT2/Hz1.0e-18>T^{2}/Hz
Current DensitynA/m^2nA/m^{2}nA/m21.0e-9>A/m^{2}
Differential Number Flux1/(cm^2 s sr eV)1/(cm^{2} s sr eV)1/(cm2 s sr eV) 
Differential Energy FluxeV/(cm^2 s sr eV)eV/(cm^{2} s sr eV)eV/(cm2 s sr eV) 
  • Earth radius: 6371.2 km
  • ASCII Date/Time: 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.

Convention for unit nomenclature and SI conversion attribute (see table above):

  • 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 "0.0174532925>rad"
  • 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".


Convention for compound units (see table above): 

  • Only dimensional units should be used. For example, a number density would have units of [cm^{-3}] not [#/cm^{3}]. 
  • LaTeX math notation is to be used exponents, so the units for acceleration would be [m/s^{2}]
    • 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.


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