"The goal of the NASA heliophysics digital observatory (HDO) is to provide a convenient portal for searching, discovering, and accessing digital resources to support heliophysics and space weather research. The purpose of the HDO is to augment and enhance the functionality and operations of the existing NASA heliophysics data and model services provided by the Solar Data Analysis Center (SDAC), Space Physics Data Facility (SPDF), and the Community Coordinated Modeling Center (CCMC). To that end, the HDO focuses on providing higher-level search capabilities for meeting digital resource requirements stemming from heliophysics cross-disciplinary, system-science research tasks. Following the paradigm established by the discipline-oriented heliophysics virtual observatories (VxOs), the HDO is a middleware developed for searching, locating, and retrieving digital heliophysics resources that may be stored at distributed locations. Taking full advantages of the standard ways in which the Space Physics Archive and Extract (SPASE) metadata model describes and locate digital resources, the HDO can thus allow effective search and delivery of the desired resources to the users. In developing the HDO, we have taken steps to consolidate VxOs developed at the NASA Goddard Space Flight Center, namely the virtual Heliospheric (VHO), Magnetospheric (VMO), and Wave (VWO) observatories, into a single virtual observatory. In doing so, we are preserving all the advantages and rationale of the virtual observatory paradigm, but with a more streamlined middleware system."