An electromagnetic wave propagating through a partially ionized medium loses some of its energy to the plasma, and if the wave has sufficiently high power density, it perturbs the plasma properties. By observing the response of the ionospheric plasma to various to various kinds of HF excitation it is possible to learn a great deal about this plasma and the neutral upper atmosphere. The process is known as "HF heating" or more recently and generally as "RF interactions". The community interested in this process is broadly based, consisting of traditional aeronomers as well as plasma physicists from universities and national laboratories. This community meets every year at the RF Interactions Workshop to discuss current results and plan for the future, and it supports the building of the new Arecibo HF facility. The community has identified the specific problems that the new facility will address. Briefly, these are: 1. aeronomy studies with HF-accelerated electrons, 2. the electron thermal balance problem, 3. studies of resonant Langmuir/ion oscillations, 4. the electron acceleration process, 5. structuring of the ionospheric plasma at a range of scale lengths, and 6. the use of the facility as powerful HF radar for studying various aeronomy problems.

It is not possible to rebuild the facility on the government-owned land where it was previously located due to policy changes. Furthermore, for a variety of reasons, there is no available flat, dry land. The most practical solution is to construct a facility with new capabilities using the 300 meter main dish of the Observatory as a reflecting antenna. The feed antenna will be supported from the three main towers using a winch system allowing the feed to be lowered easily and removed when not in use.

The NAIC has proposed to design and construct the facility with appropriate assistance from outside engineers and contractors. NAIC will operate it as a visitor facility through its Space and Atmospheric Sciences Department as in the past. The design will bring to completion the efforts of several preliminary studies, assuring optimum performance as well as safe and convenient operation. The proposed facility consists of a high power HF transmitter connected through an HF transmission system to the feed antenna, the AC power transmission system, and a building to house the transmitter and auxiliary equipment. We will use coaxial line from the old facility and additional connecting components.

Full operation of the facility ideally requires three frequency bands, near 3.175, 5.1 and 8.175 MHz. NAIC proposes in this new facility a design that takes best advantage of the Arecibo reflector to meet the challenges of current scientific motivation. In contrast to the old facility, the new one provides its highest gain at the highest frequency and will allow reliable operation well above the threshold for the production of heater-induced airglow and significant fluxes of energetic electrons. The modern commercially available HF transmitter will allow continuous transmission of the necessary high average power, as well as modulated waveforms of much higher peak power than ever before. This will enhance the capability for producing accelerated electrons for aeronomy studies and will allow new studies of Strong Langmuir Turbulence. The proposed facility will transmit somewhat more average power than the old facility at 8.175 MHz, but the effects it will produce will be far greater because of the high peak power capability. Some experiments require a continuous wave (CW) transmission; for these a transmitter with a 500 KW carrier provides better performance at 8.175 MHz than the old facility. Other experiments use low- or high-duty cycle pulses; the transmitter provides about 2MW peak power for these. The modulator allows the transmitter to deliver about 622 KW of average power in CW applications, somewhat more than the nominal carrier level.

NAIC has proposed an antenna using a single feed combining the 5.1 and 8.175 MHz bands. Computer modeling makes it possible to account for the electrical interaction of the two feeds. Because of the large size and weight of the 3.175 MHz feed it requires separate implementation, to be proposed later. The primary motivation for building this facility at Arecibo is the availability of the powerful 430 MHz incoherent scatter radar in a quiet magnetic-mid-latitude environment as the essential, unique diagnostic. The site is ideal for studying the fundamental physics because it is usually free of the events more common at other latitudes causing "geophysical noise". The radar is the most sensitive in the world for these studies. The site also provides a suite of modern atmospheric optical instruments and the infrastructure to support visiting scientists with their additional instrumentation and other experimental requirements.

As in the past, the new facility will support PhD directed graduate and undergraduate research programs from many universities. More than twenty students have written doctoral theses associated with the old HF facility. The PhD graduates have gone on to serve the nation by strengthening its knowledge base, infrastructure, and international competitiveness in the ever increasingly technological marketplace. In general, heating projects are multidisciplinary endeavors that encompass virtually all aspects of upper atmospheric physics.

We will add a new exhibit to NAICŐs superb visitor facility, for promoting the understanding of aeronomy and the importance of ground-based observations for understanding our space and atmospheric environment. We will also introduce summertime teacherŐs workshops on geophysical topics to complement the current astronomy related workshops.

HF studies at Arecibo tie together aeronomy, space physics, plasma physics and education in a unique combination that allows for an effective implementation of NSFŐs 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.

The pending proposal was submitted to the NSF in April 2003.

For more information, please contact: Dr. Mike Sulzer, Sr. Research Associate
Arecibo Observatory/NAIC
Cornell University
787-878-2612 ext. 255 msulzer@naic.edu