Coordinated tidal observations at Arecibo

Jownal of Ahasphcrie and TerrestrialPhysics, Vol. 40, pp. 887-890. Q Pergmon FTC.MLtd.,1978.Printed in Northern Ireland.

OOZl-9169/78/08014887$02.00/0

Coordinated tidal observations

at Are&o

R. M. HARPER National Astronomy and Ionosphere Center, Arecibo Observatory, Arecibo, PR 00612, U.S.A. and

R. H. WAND MIT.

Lincoln Laboratory, Lexington, MA 02134, U.S.A. (Received 22 November 1977)

Abstract-The

Arecibo Observatory recently began a regular program of coordinated tidal observations. Arecibo directly measures the ion velocity vector, the electron and ion temperatures, and the electron density over the 100-500 km region for this program. In addition, the ion neutral collision frequency can be inferred below 120km. From these measurements neutral temperatures, the southward component of the neutral wind, and the eastward wind component below 130 km can be inferred. The height resolution is 2.3 km below 200 km, and 35 km above, while the time resolution is about 14 h for velocities and 30 min for temperatures. ‘Ibis paper describes the Arecibo measurements, and the method of analysis of the inferred neutral winds and temperatures for tidal components. Results for the initial observing period in August 1974 are presented.

The installation of a new line feed and reflector surface resulted in a 6 dB increase in the sensitivity of the Arecibo radar, substantially improving the observatory’s low altitude measurement capability by permitting good height and time resolution studies of E-region temperatures and collision frequensies (ZULU-I-I-I and FARIBY, 1975), determination of E-region neutral winds (HARPERet al., 1976), and detailed measurements of night-time E-region electron density profiles (ROWE, 1973). The measurements presented here determine these E-region parameters as well as F-region temperatures and velocities, with an appropriate height resolution in each altitude range, and with a time resolution sufficient to follow large scale (tidal) changes in thermospheric structure. The measurements are made by transmitting three different pulse schemes: (1) E- and Flregion complex autocorrelation functions are measured by a 5 pulse scheme with a 16 ps basic pulse length, giving a height resolution of 2.3 km. The technique is essentially that described by ZAML.U?TI and FAFUEY (1975). 60% of the observing time is spent on these measurements. (2) F-region complex autocorrelation functions are measured by a single 296 ps pulse. The technique is essentially that used by BEHNKE and HARPER (1974). 20% of the observing time is spent on this pulse scheme. (3) A

single 24 p.s pulse

is used to measure

a

power profile with a 3.6 km height resolution. 20% of the observing time is used. The Arecibo antenna must be pointed in at least three directions to measure the ion velocity vector. The antenna is offset 15” from the vertical and alternately pointed at 180, 270 and 360” azimuth. At a given position the 5 pulse scheme is transmitted for 15 min, then the 296 ws pulse for 5 min. The antenna is then rotated to the next position, which takes about 5 min. during which time the 24 ps pulse is transmitted. The line-of-sight velocities are determined by a least squares ‘fit to a linear change in the phase of the complex autocorrelation function with time delay. The line-of-sight velocities at the three positions are interpolated in time and combined to give the ion velocity vector. We use a coordinate system with the x-axis positive southward, the y-axis positive eastward, and the z-axis positive upward. The amplitude of the autocorrelation function is used to determine other ionospheric parameters. Below 120 km the ion temperature, x, and the ion-neutral collision frequency, Vi”,are obtained by matching the measured autocorrelation function to theoretical functions given by DOUGHERTV and FARBY (1963) for an ion mass of 31. From 120 to 130 km the effect of ion-neutral collisions on the incoherent scatter spectrum is negligibly small, and only the ion temperature (which equals the electron temperature below 130 km) is derived from the

887

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R. M. HARPERand R. H. WAND

measurements. The electron and ion temperatures are derived above 130 km by assuming a fixed composition model in which the molecular to atomic ion ratio decreases to 50% near 160 km and the ionization is essentially all atomic oxygen above 215 km. Electron densities are determined from the measured power protile, corrected for the measured electron-ion temperature ratio, and normalized to the F-region peak electron density as determined by an on-site ionosonde. Velocities and temperatures are calculated at a total of 26 heights, 10 spaced at 3.6 km intervals over the 100-135 km region, 5 spaced at 7.2 km intervals over the 135-170 km region, and 11 spaced at 30 km intervals over the 200-500km interval. The uncertainties in the inferred velocity components have been derived from comparisons of the r.m.s. deviations of independent estimates of V, (HARPER et al., 1976), and are of the order of 5 m s-’ for V,, 20 m s-l for V,, and 25 m s-’ for V,. There is a systematic bias in the line-of-sight velocities of approximately 15 m s-l due to a transmitter frequency offset. All line-of-sight velocities are corrected by this amount. As the bias is not measured directly, it introduces a systematic uncertainty of several meters/second in the measurement of V, and V,, but does not affect the measurements of v.. No attempt is made to gather E-region autocorrelation information at night. Rather, the 296 ps pulse is transmitted for 10 minutes to obtain Fregion temperatures and velocities. The antenna is then rotated in the daytime azimuth sequence. During the swing, a 24 ps pulse is transmitted to determine the electron density profile. The determination of neutral winds from the measured ion velocity vector has been discussed in HARPER et al. (1976). The uncertainties in the southward wind component are about 20 ms-‘, while those in the eastward component are near 30 ms-‘. It is not possible to infer the eastward wind above about 130 km from the Arecibo measurements. ANALYSIS

FOR TIDAL.COMPO-

Below 200 km winds and temperatures are only obtained during the daylight hours at Arecibo due to the low electron densities in the night-time Eand Fl -regions. About 12 hours of temperature data and 10 hours of velocity data will be available on any given day. It has proved impossible to accurately determine both the diurnal and semidiurnal components of the wind and temperature oscillations from these short data records.

However, if it is known a priorithat the semidiurnal component is dominant, or even of comparable magnitude to the diurnal component, it has been shown that a 3 parameter fit to the data of a prevailing component and a semidiurnal wave produces an estimate of the semidiurnal amplitude that is generally accurate to about 25%, while the phase is generally estimated to within 1 hour (BERNARD, 1974). The consistent descent of a layer of enhanced electron density through the night-time Fl -valley about 12 hours after the directly measured daytime descent of a convergent null in the vertical ion velocity V, is strong evidence that the winds in the 115-170 km region have a predominant semidiurnal component (HARPER, 1976). Thus we have analyzed the winds above 115 km for the semidiurnal component using the 3 parameter fits. The temperatures have similarly been analyzed over the 115-135 km region. The interpretation of the incoherent scatter data for temperatures is more difficult above 135 km due to the uncertainty in the ion composition, and the analysis for tidal temperature oscillations is beyond the scope of the current paper. A diurnal wind oscillation, identified as the S1,l mode, is frequently dominant below 110-115 km (HARPER, 1976). However, the penetration height of this tide appears to vary considerably from day to day and with season. This has complicated the routine processing of the data, as in general it is not possible to determine both the diurnal and semidiurnal components simultaneously. We have chosen to select days with clearly dominant diurnal oscillations over the 100-110 km region, and analyze these for amplitude and phase. Thus the diurnal amplitudes we present are upper bound amplitudes. Results for August 1974

Figure 1 shows the average semidiurnal amplitude and phase resulting from a 3 parameter fit to the southward wind data above 115 km for 10, 12 and 13 August 1974. The estimated amplitude and phase of the diurnal wind for 13 August are shown below 115 km. The raw August wind data is presented in HARPER (1976). The error bars on the semidiurnal results indicate the range of variation on the 3 days that went into the average. These wind results are discussed in HARPER (1976). Basically, they indicate a dominant & component of the semidiurnal tide near 15Okm, with an amplitude about twice that predicted by HONG and LMDZEN (1976). Higher order

889

Coordinated tidal observations at Are&o

TIDAL

AMPLITUDE a PHASE - SOUTHWARD - AUGUST 10,12,13 1974 ARECIBO

B +

SEMIDIURNAL

a-

+

DIURNAL-

02 PREVAILING

(M/SEC)

HOUR

Al;L/l;;;y

WIND

OF

MAXIMUM WIND

06

IO

SOUTHWARD

F&l. Tidal amplitude and phases from the southward wind component for the August 1974 measurements. Dashed curve is downward extrapolated S,,, amplitude assuming an undamped S,,, oscillation at 1.50 km. Dot-dash curve are the predictions of Hong and Lindzen for summer conditions at WN.

semidiumal modes appear to contribute to the semidiumal oscillation chiefly below 125 km for these summer observations. Figure 2 shows the average results of a 3 parameter fit to the temperature data over the 115-135 km region for lo,12 and 13 August 1974. The error bars again indicate the range of variation on the 3 days that went into the average, A very TIDAL

small temperature oscillation was observed at Arecibo on 10 August (10-25 K) while the other days showed substantial (60 K) amplitudes. The estimated diurnal temperature amplitude and phase for the 13 August data are shown below 115 km, though it was not possible to obtain a reliable estimate at 100 km. The vertical wavelength of the diurnal tide that is inferred from

AMPLITUDE AND ARECIBO - AUGUST

PHASE 10.12.13,

- TEMPERATURE 1974

130 i 120 + z g

y

II0

IO0

1 0

---?O w 200

l2

16

20

24

HOUR

OF

MAXIMUM

04

08

I24

400

BACKQROUND TEMPERATUREPK

I

AMPLITUDE

(OK)

TEMPERATURE

Fig. 2. Tidal amplitude and phases for the temperature oscillations for the August 1974 measurements.

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R. M. HARPERand R. H. WAND

the temperatures, about 22 km, agrees quite well with that inferred from the wind measurements. The maximum temperatures occur approximately in phase with the maximum southward wind for the 13 August observation. The diurnal temperature amplitudes reach a maximum of about 25% of the background temperature.

Acknowledgements-The

National Astronomy and Ionosphere Center, Are&o Observatory, ia operated by Cornell University under contract to the National Science Foundation . The work by one of us (RHW) was supported by the National Science Foundation under grant GA42230 and the paper was prepared under grant ATM7522193.

REFERENCES

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C. A.

geophys. Res. 78, 8222. atmos. tcrr. Phys. 36, 1105. geophys. Res. 68, 5473. geophys. Res. 81, 25.

1973 1974 1963 1976

J. J. J. J.

1976 1976 1973 1975

J. geophys, Res. to be published. J. atmos. Sci. 33, 135. J. geophys. Res. 78, 6811. Radio Sci. 10, 573.