HELIO Virtual Observatory
HELIO defined a hierarchical data model specified by a couple of XML Schema Files (XSD).
http://sourceforge.net/p/helio-vo/code/HEAD/tree/trunk/helio-datamodel/src/main/resources/ - The XML Schema with auto-generated bindings to Java classes.
http://www.helio-vo.eu/documents/public/HELIO_UNIMAN_R1_004_TN_DataModel_v0.3.pdf - Introduction to the HELIO data model and auto-generated documentation from the XML Schema.
Could be interesting for FLARECAST, but not very much used in the community so far.
Virtual Solar Observatory
http://docs.virtualsolar.org/wiki/DataModel18
Relatively small data model that focuses on the search parameters of the VSO.
Is there anything else?
SPASE Data model
http://www.spase-group.org/data/dictionary/spase-2_2_3.pdf
Hierarchical data model related to Space Physics
Integrated Medium for Planetary Exploration
http://impex-fp7.oeaw.ac.at/xsd/doc/IMPEx-Sim-DM-1_0_3.pdf
Extension of the SPASE model for observations
References
http://solarsystem-interoperability.eu/wp-content/uploads/2014/01/CASSIS_D2_001_d3_v2.3_140108.pdf - Report on Data models heliophysics domain (2012)