A Standard for Units in Heliophysics

A Standard for Units in Heliophysics Bob Weigel, Baptiste Cecconi, Brian Thomas, Doug Lindholm, Andriy Koval, Jeremy Faden, Lee Bargatze, Bobby Candey, and Shing Fung We describe an effort by representatives from the metadata standards groups of HAPI, SPASE, and ISTP Metadata Guidelines to develop a recommendation for the representation of scientific units in metadata. The recommendation is “Some metadata models in Heliophysics do not have a constraint on unit strings (that is, a standard has not been chosen). Our recommendation is that if a standard is used, it should be VOUnits.” 10.5281/zenodo.17353237 ISTP Metadata Guidelines - Part of the International Solar-Terrestrial Program that began in 1980s, guidelines developed in 90s in parallel with CDF file format. SPASE - Space Physics Search and Extract. Began ~2004. More formal metadata model than ISTP and community developed. HAPI - Heliophysics API. Began in 2016. An API and metadata standard for serving time series data via an API. Metadata model is not domain specific - allows linking to domain-specific metadata. Background In the metadata standard for the HAPI API, we have units and unitsSchema attributes; the optional unitsSchema can be used by units-aware software to interpret units. Two Heliophysics metadata standards, ISTP and SPASE, do not have constraints on unit strings (UNITS and Units). As a result, units aware software cannot use the unit string values. Motivation Example usage Regular Unit UNITS = 0.01nT UNITS = 1.0e-15Tesla/Hz**0.5 UNITS = 1/(SQcm-ster-s) Mix of unit and coordinate information UNITS = Deg N Motivation Motivation Example usage cont. Mix of quantity and unit UNITS = % of H+ E-Flux" Mix of quantity and coordinate UNITS = (0=A East & B West) Use to inject warning on plot label where unit string shown UNITS = [DO NOT USE-NO QFLAG FILTER] Motivation ISTP has SI_CONVERSION and SPASE has UnitsConversion, which allow (in principle) automatic unit conversion. Examples UNITS = Re SI_CONVERSION = 6371200 > m UNITS = eV SI_CONVERSION = 1.602e-19 > J UNITS = eV SI_CONVERSION = 1.60217646E-19 > J UNITS = deg SI_CONVERSION = 0.01745 > rad UNITS = deg SI_CONVERSION = 0.0174532925 > rad UNITS = deg SI_CONVERSION = 1.0 > degree UNITS = deg SI_CONVERSION = 1.0 > degrees UNITS = deg SI_CONVERSION = 1.0 > (degree) Primary Questions 1. Is SI_CONVERSION/UnitsConversion enough? 2. If not, what standard should we use for unit strings? Advantages: ●Used in Heliophysics community for many years ●Avoids requirement that data providers use a standard style for unit strings (providers have strong opinions about this) Question: 1. Is SI_CONVERSION/UnitsConversion enough? Disadvantages: ●Does not seemed to be used outside of Heliophysics ●Lack of software that uses ●Uneven use by (especially older) missions ●Some inconsistency in implementation across missions, e.g., “degree”, “degrees”, and “(degree)” ●Additional standards needed (e.g., precision on numbers, what terms are valid, e.g., degree or degrees or deg). ISTP and SPASE documentation is “... the factor that the variable must be multiplied by in order to turn it to generic SI units” Question: 1. Is SI_CONVERSION/UnitsConversion enough? Plan HAPI - already has units and unitsSchema and will add VOUnits as an option for unitsSchema. Some work needed to allow unitsSchema version to change without updating HAPI version. ISTP - already has UNITS. Will add UNITS_VO or equivalent. Will also update SI_CONVERSION documentation. SPASE - already has Units. Will add a way to express the schema of units, possibly with an XML attribute on Units.