T H E N O R T H L I B E R T Y R A D I O O B S E R V A T O R Y

O F T H E U N I V E R S I T Y O F I O W A

(Report f r o m Solar Insti tute)

J. A. VAN ALLEN, C. D. WENDE, and S. R. MOSIER Dept. of Physics and Astronomy, The University of Iowa, Iowa City, Iowa, U.S.A.

(Received 26 November, 1968)

1. Location

The Nor th Liberty Radio Observatory (NLRO) of the University of Iowa is located 7.8 miles North-Northwest of Iowa City, Iowa, the site of the university campus, and approximately 2 miles Northeast of the small town of North Liberty, Iowa on 18 acres of land leased f rom the United States Army Corps of Engineers. The coordinates of the observatory are 41~ Lat., 91~ Long. at an elevation of 790 feet above mean sea level. The observatory is surrounded by federal government-controlled land, insuring freedom from local interference. Precipitation is moderate throughout the year.

2. Instrumentation

There are two instruments used primarily for solar research at NLRO: a total power monitor/polarimeter operating at 15375 MHz (1.95 cm) and a total power monitor operating at 40 MHz (7.5 m).

The 2-cm faci!ity was constructed jointly by the University of Iowa and the Collins Radio Company of Cedar Rapids, Iowa. The antenna is an equatorially mounted paraboloid 4 feet in diameter equipped with a dual polarization Cassegrain feed (see Figure 1). The receiver, operating at a center frequency of 15 375 MHz, is a dual mode instrument capable of operating either as a precision radiometer or as a polarimeter. The radiometer mode functions as a Ryle-Vonberg system in which the antenna noise is balanced against the noise from a gas discharge tube/precision attenuator system. In the polarimeter mode, the information necessary to determine the Stokes polariza- tion parameters is provided. The signals presented for storage are d.c. voltages of between 0 and 5 volts. These signals are stored both on strip chart recorders and on magnetic tape after being converted to a.c. signals by subcarrier oscillators. Time signals are provided by a time code generator using a crystal oscillator as a frequency standard.

In the radiometer mode, the receiver is capable of a flux resolution of 0.4 x 10-22 W m -2 Hz -1, but a stepping motor in the servo system sets the maximum resolution at about 1.6 x 10 -22 W m - 2 H z - 1 . The observed solar flux is about 480 x 10 -22 W

m - 2 Hz-1 . Because the operating wavelength is near the water vapor resonance at

Solar Physics 7 (1969) 159-163; �9 D. Reidel Publishing Company, Dordrecht- Holland

160 J .A. VAN ALLEN ET AL.

Fig. 1. The antenna and mount of the 2-cm solar patrol at NLRO. The receiver is contained in the box beneath the antenna and the controls and the data storage system are located in the building

in the background.

1.35 cm, da t a ob ta ined dur ing per iods of rain, fog, and snowfall are unusable .

The 7.5-m rad io telescope consists o f an a r ray o f two Yagi an tennas (see Figure 2)

fed in phase on a fixed a l t -az imuth m o u n t and a Dicke r ad iomete r receiver having a

pre-de tec t ion bandwid th o f 120 k H z and opera t ing with a pos t -de tec t ion in tegra t ion

THE NORTH LIBERTY RADIO OBSERVATORY 161

Fig. 2. The antenna array and fixed alt-azimuth mount of the 40 MHz radio telescope at NLRO. The antenna array is at a height of 35 feet above the ground.

t ime of 0.1 sec. The m i n i m u m detectable flux density from a point source under

normal quiet condit ions is of the order of 4 x 10-23 W m - 2 Hz-1 . Data are recorded

on a servo-controlled rectilinear strip chart recorder. Since the gain stability of the

receiver is extremely high, the receiver can be efficiently operated as a total power

162 J . A . V A N A L L E N E T A L .

receiver, resulting in an increase in sensitivity by a factor of 2.2. Time information

is provided by the time code generator. In addition to the two solar radio telescopes, a data acquisition telemetry facility

is in operation at NLRO using a 60-foot and a 28-foot parabolic dish antenna. These

~500 ~ I I I 1 I ' I , I I ' ' ' ' I ?""7 , ,

3 0 0 0

o: uJ ~: 2500

z

2 0 0 0

~5oo [ i I • r I i i i 1 8 0 0 1810 1815 1820

G . M . T .

Ii

p I 1 I I 1 8 0 5

Fig. 3. A 2-cm radio event during day 032, 1 February 1968 observed at NLRO. The conversion factor relating the antenna temperature, in degrees Kelvin, to the radio flux density is 0.4 • 10 22

W m 2 Hz-1 K-1. An atmospheric attenuation correction of the order of 10 percent has been incorporated in data reduction.

antennas are not presently being used for solar research but such use is contemplated.

The 60-foot antenna can be used at wavelengths down to 5 cm.

3. Research Programs

The 2-cm instrumentation has been operating daily as a solar patrol since 30 June 1967,

with the exception of periods when the equipment is shut down for maintenance. Data from this period (e.g., Figure 3) have been correlated with data gathered from

satellite experiments flown by the University of Iowa. In particular, the correlation between the 2-cm radio flux and the 2 to 12 • soft X-ray flux has been studied (WENDE,

1968). In normal use, the instrument has operated in the radiometer mode, although

modifications are being made to switch modes automatically during flares. It is anti- cipated that further studies will include the study of the polarization of flare emission at 2 cm and the correlation of flares with satellite-borne particle measurements.

The 7.5-m telescope has been in operation since October 1967. During the period from 27 October to 15 December 1967 data were taken from 1815 to 1945 UT each

THE NORTH LIBERTY RADIO OBSERVATORY 163

day, with the exception of maintenance periods. On 18 December 1967 this schedule

was changed to include only weekdays and so continued until 24 January 1968. Since

that date, the equipment has been operated only during periods of high solar activity

as indicated by the decametric sweep-frequency solar patrol operated by the University

Fig. 4. 40 MHz solar bursts of short duration observed on 13 November 1967 at NLRO. The con- version factor relating the antenna temperature, in degrees Kelvin, to the radio flux density is 2.8 • 10 -~5 W m -z Hz -1 K -1. The recorder gain was reduced on this day for increased dynamic range

resulting in a compressed intensity scale for these bursts.

of Colorado, under an arrangement with the Department of Astro-Geophysics there.

These data are being used to study the very-short duration bursts that make up periods of 'decametric continuum' emission and that seem to be associated with solar noise

storms. These bursts are being analyzed in comparison with the data obtained at

other radio observatories. Figure 4 shows an example of solar bursts of short duration

observed at NLRO.

References

WENDE, C. D. : 1968, The Correlation of Solar Microwave and Soft X-Ray Radiation, Ph.D. disserta- tion, The University of Iowa, Iowa City, Iowa.