A case study of a periodic structure in the atmosphere near the 90 km level

Journal of Atmospheric and Terrcstrial Physics, 19{}8, Vel. 80, pp. 885-89{}° P e r g a m o n P r e ~ . Printed in Northern Ireland

A case study of a periodic structure in the atmosphere near the 90 km level EArL E. GOSSARD AND M. R. PAULSON Radio Physics Division, Naval Electronics Laboratory Center, Command Control and Communications, San Diego, California 92152 (Received 11 November 1967)

Abstract--Radio waves at three frequencies in the VLF/LF range are reflected from the l o w e r ionosphere and received at spaced receiver sites on the ground. This paper presents a case study of a periodic structure near 90 kin observed on the night of 13-14 June 1966. It is concluded that the records are most reasonably interpreted as being due to 1st mode gravity waves whose kinetic energy resides mainly in the upper sound channel near 90 kin. DESCRIPTION OF EXPERIMENT THIS paper describes the results of an experiment which measures the fluctuation characteristics of radio waves reflected from the lower ionosphere over very short radio paths. Spaced receivers are used to determine the space and time variable characteristics of the ionosphere. Unlike most experiments of this sort, the present experim e n t employs very low radio frequencies which at night are reflected from about the 90 km level. Three frequencies are t r a n s m i t t e d simultaneously--21"6 kHz, 33"5 kHz, and 44'3 kHz. The t r a n s m i t t e r is pulsed and the receivers are gated to ensure t h a t there is no contamination from t h e ground wave or from multiple reflected skywaves. The geometry of the experiment is shown schematically in Fig. 1. The receiver sites were chosen so t h a t the mid-path reflection point would occur over the U.S. A r m y ' s Y u m a Test Station in Arizona. During the time period t h a t will be described wind profile measurements were obtained at the Y u m a Test Station b y the U.S. A r m y Ballistics Research Laboratories under the direction of Dr. Charles M u r p h y in a program jointly sponsored with the Space Research I n s t i t u t e of McGill University. The wind trails were obtained b y firing high altitude ballistic probes from a 16 in. gun and releasing t r i m e t h y l aluminum at altitudes from about 88 k m to about 125 kin. The chemiluminescent glow allowed the trails to be photographed and the photographs were analyzed to provide wind information b y Space I n s t r u m e n t s Research Incorporated. Some of the results of these wind profile measurements will be used in a t t e m p t i n g to interpret the drift measurements obtained from the triangle of radio receivers. DISCUSSION OF RECORDS We have pointed out previously (Goss~LRD, 1967) t h a t one should be very cautious in interpreting periodic fluctuations in the radio records as an indication of a periodic structure in the ionosphere. M a n y such records which appear wavelike in character have in the past been shown to be due to m u l t i p a t h interference 885

886

EA_~, E. GOSSARD and M. R. PA~LSO~

from off-path reflectors. This is shown b y the fact t h a t the phase and amplitude records tend to be 90 ° out of phase and b y the fact t h a t the fluctuation period depends on the radio frequency being employed. An example of this t y p e of record is shown in Fig. 2. Figure 2 shows records at all 3 frequencies for the same time interval recorded at one of the receiver sites. I t is evident even to the eye t h a t the period of the

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fluctuations depends on the radio frequency and this is borne out b y t h e power spectra shown in Fig. 3. Note t h a t the spectral band is shifted up in frequency as the radio frequency is increased. This paper concerns a different t y p e of r e c o r d - - o n e t h a t indicates a truly periodic structure in the ionosphere and possibly provides evidence of internal w a v e motions near the 9 0 k m level. The records are shown in Fig. 4. These were recorded at a b o u t local midnight on 13 J u n e 1966. The wave-like structure lasted for an hour and a half and the apparent period of the waves was a b o u t 6 min. The power spectra of the amplitude records of three radio

Periodic structure in the atmosphere near the 90 k m level

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frequencies are shown for a single station in Fig. 5. Note t h a t there is no shift in frequency of the spectral line with increasing radio frequency, and the fluctuations must be due to a periodic structure of the ionosphere itself. ~KNALYSIS OF ~:~ECORDS

Carrying out a cross spectrum analysis between the records obtained for tlfis period a very precise speed and direction of movement of the fading p a t t e r n across the receiver triangle can be obtained. This p a t t e r n of movement is shown in Fig. 6. The radial lines directed outward from the vector triangle indicate the orientation of the 3 legs of the receiver triangle. E a c h point located on the vector triangle represents the velocity component of m o v e m e n t along t h a t leg of the receiver triangle. The crosses are the points obtained from the three phase records at 44"3 kHz and the circles are points obtained from the corresponding three amplitu de

Periodic structure in the atmosphere near the 90 k m level

889

13-14 J U N E 6 6

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EARL E. GOSSIP snd M. R. PAELSO~

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records• I f the three component velocities which make up each leg of the vector triangle lie nearly on a straight line, it is an indication t h a t the magnitude and direction of m o v e m e n t is reliable since this is a measure of the accuracy of the statistical matching of one record against another to get the appropriate time lag. The indicated movements are not only reliable b y this criterion b u t the agreement between the phase records and the amplitude records provides an independent assurance t h a t the indicated movements are reliable. The three vectors I, I I and I I I represent inherent ambiguities in determining the drift pattern. For example, in matching t w o sinusoidal records, there is no w a y of knowing whether a record should be slipped forward or b a c k w a r d in time to match another record. This ambiguity in deciding whether we are dealing with a time lead or a time lag between records leads to the three possible solutions I, I I

Periodic structure in the atmosphere near the 90 k m level

891

and I I I shown in the figure. There are of course a n y n u m b e r of higher order ambiguities if we assume t h a t the structure size of the fading p a t t e r n is smaller t h a n t h a t of the receiver triangle itself. I n our case the size of the triangle was so chosen with respect to the radio wavelengths employed in the experiment t h a t

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EARL E. GOSSA2D and M. R. PAULSON

892

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Fig. 7. The wind sounding between 92 and 102 kilometers altitude obtained 1½ hr prior to the radio records shown in Fig. 4. t i m e of t h e w i n d s o u n d i n g s h o w s o m e e v i d e n c e of w a v e m o t i o n b u t i n d i c a t e t h a t it w a s p r o b a b l y n o t n e a r l y as e v i d e n t as in t h e r e c o r d s s h o w n here. T h e r e is s o m e e v i d e n c e of a v e r t i c a l s t r u c t u r e size of p e r h a p s 2 k m s on t h e w i n d profiles a n d t h e q u e s t i o n i m m e d i a t e l y arises as t o w h e t h e r t h e p e r i o d i c s t r u c t u r e in t h e r a d i o r e c o r d s is r e l a t e d t o t h e p e r i o d i c s t r u c t u r e in t h e w i n d profile. I t is possible to c a l c u l a t e t h e v e r t i c a l s t r u c t u r e size of i n t e r n a l w a v e s n e a r 90 k m f r o m t h e c h a r a c t e r i s t i c e q u a t i o n if t h e p h a s e v e l o c i t y a n d p e r i o d a r e k n o w n . F u r t h e r m o r e , it is possible t o c a l c u l a t e t h e r e l a t i o n s h i p b e t w e e n t h e a m p l i t u d e of t h e w a v e Table I. Observed wind and wave parameters F r o m wind profile (92.5 km)

Average wind

Speed Direct. (mps) (°cwn) 35

165

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Speed

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Period (rain)

Height fluctuation (M) *

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• Radio phase perturbation indicates amplitude of height fluctuation is 130 m.

Periodic structure in the atmosphere near the 90 k m level

893

perturbation and the perturbation in wind velocity due to the wave motion. T h e equations required for these calculations are as follows:

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c, p(z) = sound velocity and unperturbed density w, v ---- vertical and horizontal velocity perturbations p, A = pressure and height perturbations C, ~ ---- phase velocity and frequency noted b y observer moving with mean wind. 2~r n -= the vertical component of the wave number Table 1 shows the pertinent features of the wind sounding and the radio fading pattern. Table 2 shows calculated values of wave parameters for comparison. The mean wind at the 92"5 km level has been used to correct the apparent movement of the fading p a t t e r n across the receiver triangle to obtain wave phase velocities corresponding to the three vectors indicated earlier b y roman numerals I, I I and I I I . Vector I can u n d o u b t e d l y be rejected immediately since it leads to a horizontal wavelength for the irregularities t h a t is less t h a n the radio wavelengths (about equal to the radio wavelength at 44"3 kHz.) I t is well known from diffraction t h e o r y t h a t perturbations in the radio wave field due to such small features in the diffracting screen are evanescent so we shall confine our attention to vectors I I and I I I . The amplitude of the perturbation is assumed to be t h a t calculated from the radio phase perturbation and is about 130 mr. The vertical structure size and the wind perturbations have been calculated according to the equations shown above. The results are shown in the last two columns of Table 2. These values should be compared with the observed vertical size and the observed wind speed perturbation shown in Table 1 under wind fluctuation data. I t is seen t h a t the calculated vertical size from vector I I agrees reasonably well with the observed vertical

EARL E. GOSSARD and M. 1~. PAULSOI~

894

Table 2. Calculated wave parameters

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Direct. (°own)

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Period (sec)

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316 212 356

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600 900 207

Frequency Vert. size (rad/sec) (kin) 0"0105 0.0070 0.030

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Wind fluctuation (lnps) 3"0 3.3 --

size. Ho.wever, t h e w i n d f l u c t u a t i o n of 3"3 m p s c a l c u l a t e d f r o m t h e 130 m a m p l i t u d e f l u c t u a t i o n is a t o d d s w i t h t h e 24 raps w i n d s p e e d p e r t u r b a t i o n o b s e r v e d in t h e w i n d profile a t t h e 92"5 k m level. I n f a c t , it t u r n s o u t t h a t a n a m p l i t u d e p e r t u r b a t i o n of a p p r o x i m a t e l y 800 m w o u l d b e r e q u i r e d in o r d e r t o a c c o u n t f o r t h e 24 raps w i n d f l u c t u a t i o n b y i n t e r n a l w a v e s . I t s h o u l d be p o i n t e d o u t h e r e t h a t perturbation theory and the linearized equations have been used throughout. 300 KM MODEL

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e.xtending to 300 kin elevation and having a stratospheric and a mesospheric sound channel. The bottom frame shows the kinetic energy as a function of elevation for the first gravity wave mode for several different fluctuation periods.

Periodic structure in the atmosphere near the 90 km level

896

However, for the wavelengths and amplitudes considered, the linearized theory should provide a good approximation even at these elevations. Vector I I I does not come close to accounting for either the wind speed fluctuation or the vertical size. In fact the vertical wavelength is imaginary. However vector I I I is interesting from another standpoint. Note that the speed is about 83 mps and the period is 207 sec. It is o£ interest to compare these figures with the

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dispersion characteristics of atmospheric waves calculated b y I~FEFFER and ZA_RICHNY (1963). T h e y used a computer solution to obtain the complete dispersion relationships for m a n y acoustic and g r a v i t y wave modes for a realistic atmosphere extending to 300 km and having two upper sound channels. Their crucial results from our standpoint are shown in Fig. 8. The reader's attention is especially directed to the first gravity wave mode. Note t h a t the observed period for vector I I I and its corresponding velocity fall nicely on the dashed line shown in the figure. F o r this part of the dispersion curve, where the phase velocity is beginnn~ng to change rapidly with frequency the kinetic energy resides mainly in the 85 k m sound channel as shown b y the 199 and 255 sec excitation curves in the b o t t o m frame of t h e figure.

896

EARL E. GOSSARD and M. R. PAULSON

CONCLUSIONS

We are therefore faced with the dilemma of deciding whether to believe vector I I which is in reasonable agreement with some of the fluctuation characteristics of the wind sounding, considering the large time displacement of the sounding, or of believing vector I I I which agrees well with wave t h e o r y for the modal excitation of the whole atmosphere. I n an effort to resolve the problem one final bit of evidence is presented in Fig. 9. This figure shows the cross correlogram between the amplitude records of the Midway Wells receiver a n d the Ogilby R o a d receiver for the time period during which the waves occurred. The a m b i g u i t y represented b y vectors I I and I I I depends on whether we interpret t h e time difference between the records as a delay in going from 1VIW to OR or a delay in going from OR to M W - - i n other words, whether the time delay between records is a lag, r, or a lead, r -- T. The two interpretations correspond to the two peaks on the correlogram which are so labeled. I t is seen t h a t the over-all correlation between the records is significantly better for vector I I I t h a n for vector II. The evidence as a whole therefore favors interpretation I I I , and we t e n t a t i v e l y conclude t h a t perturbations on the observed wind profile are unrelated to the waves in the radio records. REFERENCES

GOSSARDE.E.

1967

P~EFFER R. L. and Z~RiCm~r J.

1963

J. Geophys. Res. 72, 1563. Geofi~ca pura ~. Appl. 55, 175.