Definitions of Units of Measure used in MMS data product files:
| Quantity | Units (original) | Proposal #1 | Proposal #2 | Notation in CDF | SI_CONVERSIONS (proposed) | |||
|---|---|---|---|---|---|---|---|---|
| markup in CDF | as rendered by plot routines | markup in CDF | as rendered | |||||
| Number densities | cm^-3 | cm^{-3} | cm-3 | cm^{-3} | cm-3 | 1e6>m^{-3} | ||
| Speeds, velocities | km/s | km/s | km/s | km s^{-1} | km s-1 | 1.0e3>m/s | ||
| Angles, phase shifts | deg | degdeg | deg | deg | 0.0174532925>rad | |||
| Pressures (plasma - dynamic, thermal, magnetic) | nPa | nPAnPa | nPa | nPa | 1.0e-9>Pa | |||
| Temperatures | eV | eV | eV | eV | eV | 11604.50520>K | ||
| Heat Flux | mW/m^2 | mW/m^{2} | mW/m2 | mW m^{-2} | mW m-2 | 1.0e-3>W/m^{-2} | ||
| Entropy | J/K | J/K | J/K | J K^{-1} | J K-1 | 1.0>J K^{-1}/K | ||
| Electric Field | mV/m | mV/m | mV/m | mV m^{-1} | mV m-1 | 1.0e-3>V m^{-1}/m | ||
| Probe to Spacecraft Potential | V | V | V | V | V | 1.0>V | ||
| Electric field power spectral density | (V/m)^2/Hz | (V/m)^{2}/Hz | (V/m)2/Hz | V^{2} m^{-2} Hz^{-1} | V2 m-2 Hz-1 | 1.0>V^2 m^{-2} Hz^{-1}1.0>(V/m)^{2}/Hz | ||
| ExB Velocity | km/s | km/s | km/s | km s^{-1} | km s-1 | 1.0e3>m s^{-1}/s | ||
| Poynting Flux | mW/m^2 | mW/m^{2} | mW/m2 | mW m^{-2} | mW m-2 | 1.0e-3>W/m^{-2} | ||
| Magnetic FieldnT | nT | nT | nT | nT | 1.0e-9>T | |||
| Magnetic Field Power Spectral Density | nT^2/Hz | nT^{2}/Hz | nT2/Hz | nT^{2} Hz^{-1} | nT2 Hz-1 | |||
| Current Density | nA/m^2 | nA/m^{2} | nA/m2 | nA m^{-2} | nA m-2 | 1.0e-9>A/m^{-2} | ||
| Differential Number Flux | 1/(cm^2 s sr eV) | 1/(cm^{2} s sr eV) | 1/(cm2 s sr eV) | cm^{-2} s^{-1} sr^{-1} eV^{-1}cm-2 s-1 sr-1 eV-1 | ||||
| Differential Energy Flux | eV/(cm^2 s sr eV) | eV/(cm^{2} s sr eV) | eV/(cm2 s sr eV) | eV cm^{-2} s^{-1} sr^{-1} eV^{-1} | eV cm-2 s-1 sr-1 eV-1 | |||
| Distance | km | km | km | km | km | 1.0e3>m | ||
- Earth radius: 6371.2 km
ASCII Date/Time: ISO8601 standard (e.g. YYYY-MM-DDTHH:MM:SS.SSS or YYYY-DDDTHH:MM:SS.SSS)
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- 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 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 "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]
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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}]. 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)].
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- LaTeX math notation is to be used exponents, so the units for acceleration would be [m/s^{
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- 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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