D-region research at Arecibo

Joumal of Atmospheric and Printed in Northern Ireland

Terrestrial F‘hysics, Vol.

43, No. 516. pp. 549-556, 1981.

0021-9169/81/050549-08$02.Do/0 @I 1981 Pergamon F’ressLtd.

D-region research at Arecibo J. D. MATHEWS Dept. of Electrical Engineering and Applied Physics, Case Institute of Technology, Case Western Reserve University, Cleveland, Ohio 44106, U.S.A. Ahstnuzt-The D-region research in progress at Arecibo Observatory is described. This research includes various meas~emen~ of the D-region proper and of the 85-100 km upper boundary of the D-region. Instruments utilized include the 430 h@Iz Thomson scatter radar, the airglow observatory, and the nearby meteor radar. The Thomson scatter radar is used to obtain electron and total negative ion density, ion-neutral collision frequency, and neutral atmosphere temperatures and winds. The airglow observatory is being used to obtain atmospheric temperatures near 85 km altitude using the OH@-3) emissions and near 97 km altitude using the 0,(0-l) emissions. Twilight resonant scattering from Fe1 and CaII is used to determine the height distributions of these metals in 60-150 km region. The meteor radar is employed as a continuous monitor of the S-105 km winds. The utiiity and implications of these meas~ements regarding knowledge of D-region processes is discussed.

1. EVIXODUCDON

The D-region is the least understood portion of the ionosphere. This situation is due both to the difficulties encountered in obtaining data regarding the region and to the complex chemistry which characterizes the region. Because of the difficulties in providing appropriate data, investigation of Dregion chemical processes has usually been via complicated theoretical model studies of the possible chemistry with experiment providing various constraints to the models (THOMAS, 1974: SECHRIST, 1977). These constraints generally take the form of laboratory-determined rate constants (see, for example, Chapter 10, BANKS and KOCKAR~, 1973; or ROWE et at., 1974), of relatively few rocket-borne, bass-spec~ometer negative and positive ion composition determinations (NAR~ISI et al., 1971; ARNOLD et al., 1971; ZBINDEN ef al.,

197S), and of numerous electron density measurements using a variety of techniques which are discussed and compared by SECHRIST (1974). Measurements of minor constituents such as NO {BAKER et al., 1977) and 0 (ZALPURI and OGAWA, 1979) are also vital to model studies of the D-region. Clearly, however, further experimental data concerning the D-region would prove useful particularly if from tropical latitudes. A program is described here, of D- and lower E-region measurements now in progress at Arecibo Observatory (18.3’N, 66.7” W). This program utilizes the 430 mz Thomson scatter radar, the airglow observatory facilities, and (to an increasing extent) the meteor radar (MAHEWS et at., 1981a) located about 40 km west northwest of the observatory. The measurements include Thomson scatter 549

power and spectral determinations in the 60100 km range, optical observations of various airglow emissions in the SO-130 km region, and operation of the meteor radar to obtain zonal winds between about 80 km and 105 km altitude. The intent of this observational program is to provide a data base which will contribute to the understanding of the chemistry and dynamics of the D-region as well as specifying atmospheric density, temperature, and metallic ion input at the upper boundary (85-100 km) of the tropical D-region. Other parameters obtained in this program of study include electron density, total negative ion density, ion-neutral collision frequency, and neutral atmosphere wind speeds. The uniqueness of these observations stems from the variety of techniques involved and from the tropical location. The lower D-region application of the Thomson scatter radar is a recent and perhaps controversial development and thus will receive considerable attention in Section 2 of this article. Section 3 includes airglow measurements of OH and O2 rotational temperatures (from about 85 and 97 km altitude, respectively) plus comments on Fe and Ca* altitude distributions obtained from twilight scattering experiments. Section 4 contains the summary and conclusions as well as some comments regarding future experiments and the meteor radar cont~butions to the D-region research program. 2. THOMSONSCATIRR RADAR MRASURRMR~ 2.1. Electron density The most basic and longest standing use of the Thomson scatter radar has been mesurement of ionosphere electron densities. The measurement

550

J. D. MATHEWS

procedure is reviewed by EVANS (1969) but basically received signal-plus-noise power vs height is accumulated, along with noise power alone, until suitable statistics are obtained. The noise power is then subtracted from the height profile of signalplus-noise power yielding a profile of signal power only. This ‘power profile’ of known statistical accuracy is then converted to electron density via the radar equation (EVANS, 1969). Equation (1) is a form of the radar equation P,(r) = KP,G(rMzMz)1z2

(1)

where z is height and P, is the scattered power at the receiver, K the system constant, P, the transmitted power, G(z) the near field correction, P&(z) the electron density, and u,(z) the total scattering cross-section per electron. The most crucial parameter in Equation (1) is K, the system constant, which is obtained by comparing power profile and ionosonde or plasma line results. The system constant is currently known to about 20% which represents a systematic error in the electron density. Other error sources are the near field correction discussed by SHEN and BRUCE (1973) and the Debye effect correction to the total scattering cross-section. These last two terms are known to about 95% accuracy each and also represent systematic error in the D-region electron densities.

The total systematic error in converting the power profile to electron density could then be about 30%. A discussion of these and other error sources is given by TROST (1979) and MATHEWS er al. (1981b).

Figure 1 is a plot, on a logarithmic scale, of the diurnal electron density variations at fixed heights from Arecibo for August 14, 1977. The original data was in the form of one-minute averaged power profiles obtained with the (13 baud, 4 PS baud-‘) Barker coder/decoder system (GRAY and F-WY, 1973). Profiles containing obvious aircraft returns (large amplitude, single gates), a problem below 80 km altitude, were removed from the data base before further averaging. Smoothing functions used to produce final electron densities are discussed in the figure caption. Figure 1 is most important in that electrons, as indicated by the diurnal variation, are clearly being detected even at the lowest height of 64 km. Combined statistical and systematic errors at the lowest height indicate that 100 e cc-’ in Fig. 1 is actually between W-200 e cc-l. Current experiments should yield significantly better error levels. Also note that the electron density variations shown in Fig. 1 clearly exhibit sunrise, sunset, and the basic symmetry about noon. Low altitude presunrise data displays a large variance which is not

August 14, -1977 m-

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Local Time (hrs) Fig. 1. Thomson scatter-derived electron densities plotted versus time for 14 altitudes ranging from 64 to 103 km in steps of 3 km. These results are derived from 1 mm time averaged, 600 m height resolution, Thomson scatter power profiles with 20 min, full width-half rn~~, Gaussian weighted smoothing in time. The lower seven heights were ‘Gaussian smoothed’ over 4.8km altitude (futl width-half maximum) while the upper seven were ‘box’ smoothed 2.4 km altitude only. The upper seven heights are then independent of each other and the cross-hatched area was interpolated. Notice the wave-like feature in the box on the left center of the figure. (Figure courtesy of J. K. Breakall and G. Karawas.)

D-region research at Arc&o understood at this point although troposcatter selfclutter in the antenna sidelobes is suspected. Waves which are apparently not low-level interference are sometimes observed. The boxed-in area (dashed lines) in Fig. 1 is an example of a wave-like structure which appears to show a phase shift with altitude. The variations near sunset at the lower heights are almost certainly due to interference but have been left in the plot. Correlated waves at higher altitudes and in the morning hours are due to sporadic-E. The current D-region research program should provide diurnal electron densities on a quarterly basis. 2.2. Spectral information Individual Thomson scatter spectra and the altitude variation of these spectra contain considerable information. TEPLEY and MATHEWS (1978) determined the altitude variation of the ion-neutral collision frequency (for momentum transfer) and temperature in the 80-100 km region using the pulse-to-pulse correlation scheme described by MATHEWS (1976). Direct spectral measurements are now possible (HARPER, 1978) due to various computing advances. Figure 2 shows a representative altitude sequence of spectra along with the corresponding electron density profile. These spectra are described as collision-dominated because Yi> 1 where ‘I’i is normalized ion-neutral

551

collision frequency

and

Yi = Vi”/(Jz ~vi)

where vin is the ion-neutral collision frequency of momentum transfer, k, the scattering wave number, and Vi the ion thermal speed (MATHEWS, 1978). The central ion component of collisiondominated Thomson scatter spectra become progressively narrower as Yin and Yi become larger. This effect is obvious in the data displayed in Fig. 2 where the ‘ion-line’ spectra progressively narrow with decreasing height and thus increasing atmospheric density or ion-neutral collision frequency. Figure 3 shows the effective Yi plotted versus altitude for several adjacent times. The Yi profile is determined by use of a non-linear, least squares fitting procedure which compares a library of theoretical spectra to the experimental spectra. The power profile, or apparent electron density is a parameter in the fitting procedure. The Yi profile may be interpreted in terms of neutral number density and temperature as discussed in TEPLJZY and MATHEWS (1978). This interpretation of the Yi data has been done as shown in Fig. 4 where daytime variations of vin or total neutral number density (N) are given at seven heights. As described by TEPLEY and MATHEWS (1978) the local slope of the Yi profile is used to determine temperature (or local scale height) and this temperature value is

July

5, 1978

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Fig. 2. Electron density and associated Thomson scatter power spectra plotted versus altitude. These spectra are formed by FFT techniques as described by %UWER (1978). The spectral results shown here represent 20 min time averages and 3 km height resolution. Both spectral and electron density (power profile) information is used to determine parameters such as ion-neutral collision frequency (see text). Note that progressive narrowing of the spectra with decreasing height indicates increasing ion-neutral collision frequency (or qI). (Figure courtesy of C. A. Tepley and R. B. Webster.)

552

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Fig. 3. Normalized collision frequency plotted versus altitude for four different times. The solid line is the Pi variation calculated from the CIRA 1972 (mean) model atmosphere for a 31 amu ion. The tick marks on the solid lines indicate Vi = 1, 10,100 respectively from top to bottom and the full scale on the abscissa is for the lirst curve only. All experimental points are independent and determined from 600 m height resolution Thomson scatter power spectra via a numerical fitting procedure. The large deviations from the model curves at low heights are apparently a combination of increased ion mass and negative ions. (Figure courtesy of C. A. Tepley and R. B. Webster.) 100

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Fig. 4. Daytime variations at seven heights of vin and neutral number density (N) from 5 June 1978. The CIRA 1972 (mean) model atmosphere equivalent results are also presented for comparison purposes (indicated by ‘h%‘).Experimental and model values of vi, (or N) compare very well at middle heights. The progressive underestimate of vi, at lower heights is apparently due to negative ion effects as discussed in the text. The variations with time of the results are considered real since all data are independent. (Figure courtesy of C. A. Tepley.)

D-region research at Arecibo used to rescale the effective vi to its true value from which vi, and thus total atmospheric number density are readily found. In Fig. 4, the CIRA (1972) mean model (M) neutral number densities are indicated for all altitudes. The agreement between experimental and model values is good at the middle heights. The progressive underestimate of N at lower heights is the same effect seen in Fig. 3. That is, the deviation of qi from linear behavior at low heights (all data points are independent) in Fig. 3 is due not only to temperature or Vi” variations but has been interpreted in terms of a progressive ion mass increase and the presence of negative ions by GANGULY et al. (1979). The GANGULY et al. (1979) results are based on the theoretical effect of negative ions on Thomson scattering as given by MATHEWS (1978). Aliasing of the experimental spectra causes the problems with the 100 km results. It should be pointed out that the work of both GANGULY et al. (1979) and MATHEWS (1978) assume a single representative species of negative ion with a mass typically between 30 and 60 amu. Also below 80 km altitude the average positive ion mass is assumed to be close to 60 amu. Clearly in order to most effectively employ these results iteration between the photo-chemical models of the region and analysis of the Thomson scatter data must occur. That is, theory and experiment must provide mutually consistent constraints which when considered as a function of height and solar zenith angle will, hopefully, lead to a more complete understanding of the region. 3. OPTICAL MEAS3.1. Atmospheric temperatures As stated in the introduction, one objective of the Arecibo D-region research program is to monitor the upper boundary of the region. To this end the mesopause (-85 km) and -97 km temperatures are measured optically employing, respectively, the OH(&3) and Oz(O-1) airglow emission bands. These observations are similar to those reported by NOXON (1978) and complement the radar measurements described in the previous section. Observations of the hydroxyl emission for rotational temperature determinations are widely reported (e.g. SIVJEE et al., 1972; MJZRWETHER, 1975; TAICAHASHIet al., 1974). The emission region has been determined to have a centroid near 85 km altitude and a Gaussian-like shape with an approximate 10 km half-maximum width (PACKER,

553

1960; VALLANCE-JONES, 1973). The 02(0-1) night-time airglow emission has received relatively little attention, except for the work of NOXON (1978), at middle and low latitudes probably because of the relative weakness (-5 Rayleighs) of the emission. The vertical width of the O&l-l) emission regions is similar to that of the OH(8-3) region but centered near 97 km altitude (PACKER, 1960; NOXON, 1975). Selected OH(8-3) and O*(O-1) temperature determinations are given in Fig. 5. These temperatures display distinct variations with time which are probably associated with the dynamics of the region (see, for example, NOXON, 1978) and will be discussed in the conclusions. Of major importance are the mean temperatures of 175 K near 85 km altitude and 224 K near 97 km altitude. The hydroxyl temperatures are based on the ratio of the P,(3) line to P,(5) line intensities (see, for example, SrvJEE et al., 1972) which are determined using separate computer-controlled tilting filter photometers in a technique similar to that described by MJZFUWKWR (1975). The 0,(0-l) rotational temperature is found by a non-linear, least-squares fitting of a library of theoretical 0,(0-l) spectra to the experimental spectra. The experimental spectra were obtained using the computer-controlled 1m Ebert-Fastie spectrophotometer described by MERWETHER (1979) while the theoretical spectra were calculated as described by SUE and BAKER (1976) and convolved with the instrumental function to form the correct spectra for the library. An experimental 0,(0-l) spectrum and the corresponding best-fit theoretical spectrum is given in Fig. 6. It should be noted that this approach to determining the 0*(0-l) temperatures is exactly parallel in procedure to the analysis of Thomson scatter spectra and makes maximal use of the available spectral information. 3.2. Metals That many metals and alkali metals play an important role in the E-region is clear from rocketborne, mass spectrometer observations of sporadicE layers (e.g. AIIUN and GOLDBERG, 1973). Figure 6 displays a time series of radar-derived electron density profiles from Arecibo where the various layers appear to be controlled by the diurnal and semidiurnal tides via the u XB or windshear mechanism (CHIMONAS and AFFORD, 1968; MATHEWS and BEKENY, 1979). Of particular importance to this topic is the ‘dumping’ of the metal ions into the 85-95 km region (MATHEWS and BEKENY, 1979) where, because of efficient three-body

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Local Time - AST Fig. 5. Atmospheric temperatures derived from the 0,(0-l) band (open circles) and the OH@-3) band (open squares) airglow emissions. The 0, emission is centered near 97 km altitude while OH emission is centered near 85 km altitude. The ratio of the P,(3) to P,(5) of OH@-3) band is used to determine the OH rotational temperature while a numerical fitting procedure, described in the text, is used to find the O&-l) rotational temperature+ Note that two different days in January are given and the dashed lines are the mean temperatures for the respective data sets. (Figure courtesy of C. A. Tepley and R. G. Bumside.)

.

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the ions are neutralized and/or become further involved in the chemistry of the region (BROWN, 1973; MURAD, 1978). SEC(1977) gives a brief overview of the possible importance of metal ions in D-region chemistry. At Arecibo optical observations of neutral iron (Fe) and calcium II (Ca’) in twilight resonant scattering address some aspects of D-region metal chemistry. The iron was observed at 3860 A while Ca’ was observed at the H and I( line wavelengths (3948 and 3934 A, respectively). Complete resufts of these observations will be given in TEPLEY et al. (1981a,b), however some details are appropriate here. Neutral iron was detected in about 20% of the twilight observations and when detected was largely restricted to the 60-W km altitude range. Upper limits for column density appear to be 1 x 109cm-* with less than 5% contribution from above 100 km altitude. That is, the maximum observed Fe1 density was an average 300 cmV3 in the 60-90 km range but exhibited considerable day-today variations. Calcium II was found to be distributed between 90 and 140 km altitude and to be associated with layers of ionization, similar to those in Fig. 7, observed with the Thomson scatter radar. In particular Ca’ was found in low-lying (-115 km) intermediate layers. The height dis~butions of both Fe and Ca” conform with current understanding of metal reactions,

15 January 1980 0O:SO - 04:55 AST

8600

Wavelength (i) Fig.6. A first-order spectrum (-) of the O&l-l) atmospheric band measured in the nightglow with the 1 m Ebert-Fastie spectrometer. Eight spectra from the time period shown were averaged. The spectral resolution is 3.5 A. A fitted synthetic spectrum (-----) for a temperature 205 *6 K is overlaid. The fit ranged from about 8694 to 8607 a to avoid contamination of nearby features such as the P(7) lines of the OH(g2) band shown on the figure. (Figure courtesy of C. A. Tepiey.)

D-region research at Arecibo

I.b+A

\hS

Altitude

204i

(bn)

Fig. 7. Electron density profiles plotted for a sequence of times. The arrows indicate lo4 el cc-l and the logarithmic scale is indicated. Of particular interest are the welldefined ‘sporadic’ layers which are composed of metallic ions and formed and transported vertically by the diurnal

and semi-diurnal tides. Note that the lowest layer occurs between 90 and 95 km altitude and that metal ions are ‘dumped’ at the top of the D-region. At later times an intermediate layer (-130 km altitude) is seen descending into the E-region. (Figure courtesy of J. K. Breakall.) chemistry

(BROWN, 1973). However,

the apparent known.

time variations

the origin of of Fe remains un-

4. SIJMMARY AND CONCLUSIONS The

Arecibo 430 MHz Thomson scatter radar and airglow observatory are involved in a program of ionosphere and neutral atmosphere measurements encompassing the 60-140 km altitude region but especially directed toward D-region problems.

555

These observations are unique both in that they occur at tropical latitudes and in the array of instruments involved. As an example of the long-term utility of these measurements, note that the noontime solar zenith angle (SZA) varies from 0” in the summer to 43” in the winter. The winter SZA corresponds to a 66” latitude summer noon-time and the range of noon-time solar zenith angles coupled with measurements of the dynamics of the region may well separate seasonal effects into local and transport components. The measurements of the dynamics of the region involve the Thomson scatter radar which measures winds between 65 and 95 km altitude during daylight hours and the meteor radar which measures winds in the 85 to 105 km region continuously. As mentioned in the introduction, the meteor radar is located about 40 km from Arecibo and the two measurement techniques are compared by MATHEWS et al. (1981a). The importance of the meteor radar to the study of the D-region and its upper-boundary is that the measurements will be continuous allowing separation of tidal and longerperiod phenomena (such as prevailing winds) so that the seasonal behavior of each may be incorporated into the overall study. Another consideration which involves both temperature and wind measurements is the variation of tidal energy input to the region (MATHEWS, 1976) and the variation of eddy diffusion coefficient. Eddy diffusion plays a major role in the distribution and availability of minor constituents such as NO and 0 in the D-region and is certainly dependent on tidal amplitudes (ROPER, 1977). The optical temperature measurements may yield tidal temperature variations which would provide more information on the tides themselves as well as give further indication of the overall stability of the atmosphere. The diurnal tide at Arecibo apparently reaches maximum amplitude in March with wind speeds in excess of 100 m s-’ observed at 80 km height. In short, wind measurements, particularly from the meteor radar, will play a major role in the Arecibo D-region research program. In conclusion, the Arecibo D-region research program should provide, over the next few years, an extensive data base for studying numerous Dregion problems. The amount and quality of the data will improve as the data-gathering systems become more sophisticated and the number of parameters measured will certainly increase. Acknowledgements-Contributions to the research program herein described from J. K. BREAKALL,S.GANGULY,

556

J. D. h4~mws

J. W. v, C. A. TEPLEY, R. G. ROPER, J. C. G. Ww, and R. B. WEBSTEX are gratefully acknowledged. J. K. Em, C. A. TFPLEY, and R. B. WEBSTER did much of the data reduction for the figures presented here. The Are&o Observatory is part of the Nationat Astronomy and Ionosphere Center, which is

operated by Cornell University under contract with the National Science Foundation. Much of the material presented here and the preparation of this paper was supported by the National Science Foundation under grants ATM77-19645 and A~9-18379 to Case Western Reserve University.

~RJIXXY AKIN A. C. and GOLDBERGR. A. ARNOLDF., KISSEL J., KRANKOWSKY D., WEIDERH. and ZABRXNGER J. BAKERK. D., NAGY A. F., OLSEN R. O., ORAN E. S., RANDHAWA J., STROBELD. F.

and TOHMATSUT BANKSP. M. and KOCKARTSG. BROWNT. L. CHIMONASG. and Axporn W. I.

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~&MEWS M~mws

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1971

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197.5 1978 1960 1977

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Atmosphere,

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1974

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1974 1973

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1972 1974 1978 1981a

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Reference is made to the foZ~owi~g unpublished material: SUE S. and BAKER D. J. 1976

J. geophys. Res. 86.

Computer-add estimates of the rotational temperatures of 0, in the mesosphere, Air Force Geophysics Laboratory report AFGL-TR-76-0212.