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.

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