This is an award to rebuild the Arecibo Observatory High Frequency (HF) facility using an innovative antenna system over the 300m telescope dish. The Arecibo Observatory has obtained surplus transmitters from a decommissioned Over the Horizon (OTH) radar facility in Maine, and has obtained funding from both the Office of Naval Research and the Air Force Office of Scientific Research Defense University Research Instrumentation Program (DURIP). This award supports the balance of the effort needed to construct the facility at the Observatory. The HF Facility is essential for continuing progress toward the understanding of the interaction between a powerful HF radio wave and the ionospheric plasma and for making new discoveries in several aspects of aeronomy. The facility will consist of an HF antenna system to feed the 300 m dish, HF transmitters with a building to house them, and a power transmission system. None of the existing HF facilities in the world has Arecibo's combination of diagnostic capabilities and geophysical environment. This includes the extraordinarily sensitive incoherent scatter radar at Arecibo, as well as a suite of supporting optical instrumentation, and a magnetic mid-latitude location with a quiet stable background ionosphere. This unique combination is ideal to study the underlying physical processes of the interaction in full detail. Until its destruction in September 1998 by Hurricane Georges, the Arecibo HF facility was located to the northeast of the Observatory near the coast of Puerto Rico. Since it is not possible to rebuild in that area due to policy changes in land use by the Puerto Rican Government, the most practical and least expensive solution is to construct a facility with new capabilities using the 300 meter main dish of the Observatory as a reflecting antenna. The surplus transmitters from the OTH facility are in perfect condition and easy to operate. They will be connected to feeds located near the center of the surface of the dish and transmit to a wire mesh sub-reflector (Cassegrain) located well below the platform. This reflector then illuminates the main dish to make a high gain HF signal. The combination will provide the power needed for a variety of experiments, including the production of artificial airglow and accelerated electrons at the higher frequency. As in the past, the new facility will support PhD directed graduate and undergraduate research programs from many universities. The investigators will add a new exhibit to the observatory's superb visitor facility, for promoting the understanding of aeronomy and the importance of ground-based observations for understanding our space and atmospheric environment. HF studies at Arecibo tie together aeronomy, space physics, plasma physics and education in a unique combination that allows for an effective implementation of NSF's strategic plan for enhancing diversity, making state of the art infrastructure available to the community, and providing opportunities to advance discovery and understanding of science and engineering.
Radio waves interact with the solid objects, liquids, or gas media in which they travel in different ways. In the case of the ionosphere, the gas is made of particles that have an electrical charge ( http://en.wikipedia.org/wiki/Ionosphere ). These charged particles (electrons and positive ions) are set in motion by the oscillating electric field of the radio signal; it turns out the since the mass of the electrons is much lower than any of the other particles, they are more effective at absorbing energy from the radiowave. As they vibrate they may collide with other particles, resulting in energy deposition in the electron gas and a consequent rise in temperature. If the number density of electrons in the ionosphere is such that the plasma frequency ( http://en.wikipedia.org/wiki/Plasma_frequency) is equal to the frequency of the trasmitted radio wave, the interaction is resonant, enhancing the energy transfer and resulting in the growth of several interesting plasma instabilities. The Arecibo High Frequency facility uses the 305 m diameter radio telescope, together 6 100 kW HF transmitters connected to 6 crossed dipoles and an associated subreflector mesh to briefly energize a small region in the ionosphere 100 to 350 km or in altitude over the observatory. The very sensitive incoherent scatter radar at Arecibo ( http://en.wikipedia.org/wiki/Arecibo_Observatory ) can then be used to probe the interaction region and measure the resulting effects. When this facility is commisioned in late summer 2013 it will allow researchers, scientists and university students from across the nation to visit Arecibo not only to perform plasma physics experiments but also to better understand the ionosphere.
