ORFEES - a radio spectrograph for the study of solar radio bursts and space weather applications

Radio bursts are sensitive tracers of non-thermal electron populations in the solar corona. They are produced by electron beams and shock waves propagating through the corona and the Heliosphere, and by trapped electron populations in coronal mass ejections (CMEs) and in quiescent active regions. Combining space borne and ground-based radio spectrographs allows one to track disturbances all the way between the low corona, near or at the sites of particle acceleration, and the spacecraft. Radio observations are therefore a significant tool in probing the solar origin of heliospheric disturbances, which is a central research topic as witnessed by the Parker Solar Probe and Solar Orbiter missions. The full scientific return of these projects needs vigorous ground-based support, which at radio wavelengths covers altitudes up to about a solar radius above the photosphere. Besides research in solar and heliospheric physics, monitoring solar radio bursts also supports space weather services. On occasion radio bursts can themselves be a space weather hazard. The Nan\c{c}ay radio astronomy station in central France has a long tradition of monitoring radio emission at decimetre-to-metre wavelengths. This article describes the radio spectrograph ORFEES ({\it Observations Radiospectrographiques pour FEDOME et l'Etude des Eruptions Solaires}). It observes the whole-Sun flux density between 144 and 1004 MHz, which pertains to regions between the low corona and about half a solar radius above the photosphere. ORFEES is the result of a partnership between Observatoire de Paris and the French Air Force, which operates the experimental space weather service FEDOME. The primary use of the instrument at Paris Observatory is the astrophysical observation. Low-resolution data with rapid availability are presently produced for the French Air Force. Similar information can be made available to a broader range of space-weather service providers. This article gives an overview of the instrument design and the access to the data, and shows a few illustrative observations.