  Prior to the EU Horizon 2020 and FP7 SOLARNET projects, metadata descriptions
of Solar observations were only standardized for space-based observations, but
the standards have been mostly within a single space mission at a time, at
times with significant differences between different mission standards. In the
context of ground-based Solar observations, data has typically not been made
freely available to the general research community, resulting in an even
greater lack of standards for metadata descriptions. This situation makes it
difficult to construct multi-instrument archives/virtual observatories with
anything more than the most basic metadata available for searching, as well as
making it difficult to write generic software for instrument-agnostic data
analysis. This document describes the metadata recommendations developed under
the SOLARNET EU project, which aims foster more collaboration and data sharing
between both ground-based and space-based Solar observatories. The
recommendations will be followed by data pipelines developed under the SOLARNET
project as well as e.g. the Solar Orbiter SPICE pipeline and the SST
CHROMIS/CRISP common pipeline. These recommendations are meant to function as a
common reference to which even existing diverse data sets may be related, for
ingestion into solar virtual observatories and for analysis by generic
software.
