A regular and sustained wave in the ionosphere

J-l Pm

of AaMIphaicand TmrrrriolPhysics. Vol. 42, RecaLtd. 1980. Rhed in Northern Wend

pp. 513-515.

Short paper

A

regular and sustained

wave

in the

A. P. VAN EYKEN, P. J. S. WILLIAMS, A. D.

MAUDE

ionosphere and A. MAIID*

University College of Wales, Aberystwyth, Wales (Receiued 24 August 1979; in revised form 8 October 1979)

IlWRODUCI’lON

In 1968 T~THERDDGEreported observations of periodic changes in the total electron content of the ionosphere. On many occasions he observed a sustained wave motion with period and amplitude remarkably constant for up to 12 complete cycles. The diurnal variation in occurrence of such periodic disturbances showed peaks at mid-day and midnight, but an aimost complete absence at sunrise and sunset. Periods from about 15 to 80 mins were detected, with a sharp cut-off below 15 and a more gradual cut-off above 80 min. The highly periodic nature of the changes in total electron content indicate large scale waves travelling through tne inosphere and T~THZSRIDGEsuggested that these corresponded to internal gravity waves propagating through the neutral atmosphere. In 1972 MAJID reported similar examples of sustained periodic disturbances in the ionosphere. From obviations of the refraction of signals from strong astronomical radio sources he was able to determine the east-west gradient of total electron content. Wave-like disturbances were observed on many occasions, and on at least seven occasions the period, amplitude and phase of the waves observed were remarkably constant during the transit of Cygnus and Cassiopeia, suggesting regular sinusoidal disturbances lasting for 5 h or more. Figure 1 shows two examples of such regular disturbances. A recent analysis of a series of electron density profiles has shown an even more remakable example of a periodic disturbance sustained in period and phase for at least 18 cycles, a total elapsed time of over 12 h. oBsERvA~oNs

AND ANALYSIS

Profiles of electron density in the height range 100-1000 km were taken at intervals of about 6 min by the incoherent scatter radar operating at * Present address: SUPARCO, Karachi, Pakistan.

Malvem in the monostatic mode (WILLIAMS and TAYLOR, 1974). In order to detect small disturbances in electron density, the differences between the average of 11 successive profiles and the central profile of that series were plotted as a function of height, AN(h). This profile of AN was then scanned to indicate every height at which the curvature of the profile, at a local m~imum or minimum, exceeded a given value, the resuits distinguishing between positive and negative curvature. While these heights usually correspond to the absolute maxima and minima in AN(h), the curvature method can also detect several waves on the same profile. Figure 2 shows a typical section of the records; by comparing points of similar phase on successive profiles we have evidence of a wave with phase-fronts descending through the ionosphere, as would be expected in the case of an internal gravity wave where the energy flow is upwards from the lower atmosphere. By plotting the sampled values of height against time for a given phase-front, and using a least squares regression to fit the best straight line over the height range 200-400 km, as shown on Fig. 2, the exact time at which the phase front passed through a point 300 km above Malvern can be determined. Figure 3 presents these results for the night of 23-24 June 197 1. Each point represents in turn a positive or negative curvature of the profile of AN, corresponding to a change of phase of ?r between successive fronts. Plotting phase against time we see that the points lie on a straight line for the time interval 22.00 on the 23rd to 10.15 on the 24th June. This corresponds to a regular period of 41 min sustained for at least 18 cycles. There are two gaps in the sequence where the profiles of AN suggest that several waves may be present simultaneously, and the procedure outlined above does not provide a clear indi~tion of the position of the phase-front. When such a gap exists we have to assume that a suitable number of phase-fronts have

513

Short

paper

Cygnus A

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;

:

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Cassiopeia

A

Observed refraction Fitted sinusoidal curve

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1000

/

II00

12.00

U.T.

Fig. 1.

Evidence

waves from measurements of ionsopheric refraction (MAIZD, 1972).

for sustained

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Fig. 2. Profile differences from means. (Each mean formed from 11 profiles.) passed during the gap before allocating a phase to the resumed wave. This might introduce a phase ambiguity but this can be eliminated by visual inspection of a three-dimensional representation of the variation of AN with height and time, sampled at intervals of 6 min as shown in Fig. 4. Further

confi~ation of this is obtained by anaiysing the three intervals 22.00-01.15, 03.0065.45 and 07.45-10.15 independently. The results are summarized in Table 1, which shows that the period of the disturbance was the same in each interval, and the same as the period derived for the 18 cycles as a whote. The table also shows that the vertical wavelength remained constant over the 12 h. CONCLXJ!+IONS

l Positive ONepotive

.’ 2200

I oac?O

i 0200

II

/I/l

0450 UT

Fig. 3.

curvature curvature

I

OSGQ

08.00

I

IO00

,

These observations confirm the existence of regular waves propagating through the atmosphere, and the downward propagation of the pha~-fronts suggests internal gravity waves with energy propagating upward from the lower atmosphere. Two gaps occur in the observations where the main wave is confused by other disturbances. It is noticed that the first gap begins about three hours after sunset in the ozone layer and that the second gap begins about three hours after sunrise in that layer. CHERNYSHEVA ef at. (1977) have observed the effects of

515

Short paper fN

(arbitrary units)

Time

400 km. 300 km. 200 km. 100 km. 22.0023.0000.00Dl.0002.000300 04.0005.0006.0007.0008.000900 10.00

Fig. 4. Electron density perturbations 23-24 June 1971.

gravity waves generated in the ozone layer by the motion of the terminator and it is perhaps waves of similar origin which confuse the records at these times. These will be discussed in a later paper. The present observations show that after the two periods of confusion, however, the underlying periodic disturbance re-appears unchanged. Several authors have suggested that the atmosphere might offer a narrow pass-band for the propagation of internal gravity waves. Various effects such as reflection to the ground in the middle atmosphere, molecular viscosity, ion drag and the Brunt-Vaisala cut-off (at a period of 15 min in the F-region on this particular night) combine to select a narrow band of waves with wavelengths and periods suitable for propagation. For example, HINF,S (1964) has shown that waves with a horizon-

tal wavelength of 350 km can only propagate to the F-region if the period is 40 min and the vertical wavelength is 160 km, which agrees closely with the wave observed. Nevertheless it is difficult to explain how such an exactly regular wave can be generated and sustained in the atmosphere over the time span reported in this paper. Moreover, a preliminary study of other observations suggest that regular and sustained waves may be common though no other example has been found where the wave is maintained for so many cycles.

Acknowledgements-We wish to thank the Director and Staff of the Royal Signals and Radar Establishment, Malvem for the data from the incoherent scatter facility and Dr. B. H. BRKGSfor helpful discussiorrsduring his stay at Aberystwyth.

Table 1 Time interval (UT) Period of wave (mm) Vertical wavelength (km)

CIIERNYSHEVA S. P., SHEFTELV. M. and E.G. Hrrms c. 0. MAJIDA. TITHER~GEJ. E. Wus P. J. S. and TAYL.ORG. N.

22.00-01.15 41.0*0.7 119*22

03.00-05.4s 41.9* 1.2 156zt 10

07.4510.15 42.1& 1.2 139* 15

Average 41.4zto.5 147*8

1977

Geomag. Aeron. 17, 633

1964 1972 1974 1974

Can. J. Rhys. 42, 1424. Ph.D. Thesis, U.C.W., Aberystwyth. J. geophys. Res. 73, 243.

SHCHARENSUAYA

Radio Sci. 9, 85.