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very dense streams of high energy particles. A theoretical model of the pulse phenomenon has been formulated, but the detailed physical processes involved are not clear.

Another discovery has been the detection of very intense and brief pulses of radiation from certain pulsars. The famous Crab Nebula pulsar emits every few minutes a pulse lasting less than 100 microseconds and of such intensity that an object would have to be heated to a temperature of 10 degrees Kelvin to duplicate the observed brightness. Even more remarkable is the discovery of pulses from the pulsar CP 0950 which last less than 7 microseconds. They are of such intensity that the pulsar is momentarily the strongest radio source in the sky and receivable on an ordinary television set. This time scale implies that the size of the emitting region is less than two miles, yet the power coming from it is so great that every square inch must radiate many times the total electrical power production of the earth's electrical generating plants. This is only possible if tens of billions of highly relativistic particles move in unison in the magnetosphere of the pulsar, a situation previously thought inconceivable.

Accurate timing of pulsar-pulse arrival times and the deduction of pulsar periods can also provide a test of the theory of general relativity. The theory predicts that when electromagnetic radiation passes near a massive object it will be effectively slowed by the curvature of space in the vicinity of the body. In the case of pulsar signals passing close to the Sun, the theory predicts delays in pulse arrival times of the order of 100 microseconds. Delays in pulse arrival time of the magnitude are detectable. Experiments to detect such delays in pulse arrival have been conducted whenever the motion of the earth in its orbit causes a suitable pulsar to pass close to the Sun. Preliminary results are promising, but final conclusions depend upon a better understanding of the effects of dispersion in the solar corona.

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(NAIC) FIGURE 2. Times of arrival and structures of pulses from pulsars.

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Additional radio astronomy programs have been conducted, including joint very long baseline (VLB) interferometry experiments with the Jodrell Bank telescope in Great Britain, synoptic observations of variable radio sources such as 3C120 at 611 MHz, and observations of nonthermal radiation from emission nebulae in our galaxy and extra-galactic objects.

In recent years the NAIC program to map the moon and inner planets at a frequency of 430 MHz has resulted in unambiguous maps of the radar scattering characteristics of the surfaces of these bodies (see Figures 3 a and b). NAIC astronomers have detected regions of the moon and Venus having great enhancements in radar reflectivity. In addition, Doppler ranging information on Mercury and Venus have led to more precise data on the orbits and rotation periods of these planets. Venus, in particular, continually shrouded in clouds, was found to be rotating "backwards". Accurate knowledge of the rotational periods of these inner planets enables astronomers to make estimates of their primordial densities. oblatenesses and rotation rates.

The ionspheric physics programs at NAIC are directed towards a better understanding of the important photochemical and dynamical processes governing the behavior of the ionized and neutral constituents of the upper atmosphere. The photochemical processes are concerned with the production and loss of charged particles and the associated energy transfers and optical emissions (airglow). The production of electrons is controlled by the incident solar flux intensity and spectrum, the structure of the neutral atmosphere, and the photoionization cross sections of the various constituents. The loss processes in the E and F regions of the ionosphere (Figure 4) involve primarily ion-atom interchange and dissociative recombination reactions, whose rates are determined by the neutral particle densities and temperature-dependent reaction rate coefficients. In the D region, metallic, hydrated, and negative ions are present whose reactions are complicated and poorly understood.

Dynamical processes include ambiopolar diffusion, electromagnetic drifts, neutral atmospheric winds (both local and global) and gravity waves, thermal expansion and contraction, and coupling between the ionosphere and protono

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(NAIC) FIGURE 4. Electron densities in the ionosphere as a function of altitude.

sphere at heights between 130 and 1400 km. The latter region acts as a reservoir of both plasma and thermal energy. Energetic photoelectrons escaping from the lower ionosphere can heat either the protonosphere or the magnetically conjugate ionosphere. The energy supplied to the protonosphere is slowly conducted back down to the lower ionosphere. Similarly, some plasma flows out into the protonosphere during the day, only to return during the night.

The Arecibo radar is one of the most powerful instruments devised for studying the ionosphere. The bulk of ionospheric data is obtained by collecting and analyzing 430 MHz radar signals that have been reflected, or scattered by layers of the ionosphere located overhead. From these data, atmospheric scientists can determine electron densities, electron and ion temperatures, ionic compositions, collision frequencies and ionospheric drift motions. Although the research effort at NAIC concentrates on ionized particles in the upper atmosphere, a large amount of information about the behavior and properties of neutral particles can also be deduced. Complementary ground-based observations are also obtained of the airglow emissions at optical wavelengths.

In FY 71 a significant project was carried out by a visiting scientific team from Rice University. Collaborating with the NAIC staff scientists, this team developed a new experimental technique involving artificial heating of the ionosphere that promises to yield a wealth of new information on the ionosphere. In this technique, a high power transmitter is tuned to a frequency at which the radio power will be strongly absorbed by material at a certain height of interest in the ionosphere. The transmitter is then activated, causing intense heating and an increase in pressure in the ionosphere (Figure 5). The effects of this heating are measured by a simultaneously activated transmitter receiver system which is used to study the density, temperature, and motions in the ionosphere by the incoherent backscatter method.

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Using the existing transmitters and a special large tunable antenna system to produce the heating, it has been possible to increase the temperature of a few tens of cubic miles of the ionosphere by hundreds of degrees within a few seconds. The resultant local increase in pressure causes this volume to expand with the ionospheric plasma contained within the volume interacting continuously with the earth's magnetic field. Through these artificial heating experiments a new approach to the study of plasma dynamics has become possible. Further, carefully controlled experiments are now possible in plasmas of very low density.

In response to recommendations from the scientific community to convert the 1000-foot diameter antenna into an instrument capable of operating at wavelengths as low as 5 cm (its present operational capability is at wavelengths greater than 50 cm), proposals were solicited from the industrial community to fabricate and install a new surface for the spherical dish. After several months of detailed proposal evaluations by Cornell, the design concept of LTV-Electrosystems, Incorporated was selected by Cornell and approved by the NSF in November 1971. Installation of the new aluminum surface will begin in 1972 after completion of the final design and necessary civil engineering, and will take 20 months to complete. The telescope will be able to continue its research operations during the installation of the new surface. Funds for the new surface were provided in FY 1971 and FY 1972.

In conjunction with the resurfacing, NASA is sponsoring a program at NAIC to upgrade the radar planetary mapping capability of the NAIC telescope. Costing $3 million over a three-year period (FY 71–73), modifications to the overhead portion of the antenna and its support structures and improved electrical services will enable astronomers to range-Doppler map the inner planets at S-band (12 cm) wavelengths with a resolution equal to that achieved on the moon with an optical telescope. These modifications will be carried on at the same time as the resurfacing project and both are scheduled for completion in late FY 1974.

In FY 72, a new high efficiency 430 MHz line feed is being installed. This will increase by an order of magnitude the present radar signal sensitivity for planetary and ionosphere research. In FY 73 the budget includes funds for a low noise receiver development program. The new receivers resulting from this development effort will operate in the new short wavelength regime that becomes available when the upgrading of the antenna is completed. Over the next few years, new receivers will become operational, making it possible for users to make detailed observations of faint sources of 21 cm neutral hydrogen emission and other interstellar molecules. In addition, in FY 73 a specially designed multichannel signal recorder and data processor, called an autocorrelator, will be designed and built to handle the multitude of scientific data received during the course of telescope observing runs.

The FY 73 budget request includes funds for the construction of a portable interferometer antenna capable of operating at distances up to 100 miles away from the 1000-foot spherical dish. In conjunction with the upgraded surface and S-band transmitter, this satellite antenna will be used extensively for conventional radio source interferometry and high resolution planetary radar observations.

The long-range plans for NAIC envision the establishment of a headquarters administrative and engineering support complex in San Juan. The FY 73 budget request includes funds for the design of this facility.

In FY 73 the manpower level at the NAIC will remain the same as the FY 72 level of 160 personnel.

KITT PEAK NATIONAL OBSERVATORY

The Kitt Peak National Observatory (KPNO) is operated under a National Science Foundation contract with the Association of Universities for Research in Astronomy, Inc. (AURA), a nonprofit corporation representing a consortium of nine U.S. universities. The observatory headquarters provides office, research, and engineering support facilities for its staff and visitors at its location in Tucson, Arizona, adjacent to the campus of the University of Arizona. The observing facilities are located atop Kitt Peak, a 6800 foot mountain located 40 miles west of Tucson in the Papago Indian reservation. The mission of the observatory is to provide research facilities in astronomy for the U.S.

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