Drifting patches of equatorial spread-F irregularities—experimental support for the spatial resonance mechanism in the ionosphere

~oumalof Armosphcric andMmvialPhysics, Vol.4O.p~. 1103-1112. @ PcrgmonRessLtd., 1978.prinkdinNmth.xnk&md

0021-9169/78/1101-1103$02.00/

Drifting patches of equatorial spread-F irregularitie+-experimental support for the spatial resonance mechanism in the ionosphere

Max-Planck-Institut (Received

fur Aeronomie,

14 November

3411 Katlenburg-Lindau,

1977; in revised form 6 February

Germany 1978)

Ab&aet--The spatial resonance mechanism which sets in when the plasma drift velocity matches the phase velocity of atmospheric gravity waves in the ionosphere is applied to explain large-scale structures of the equatorial spread-F. The resonance condition is examined for reasonable parameters of travelling ionospheric disturbances, plasma drift and neutral wind velocities. It is shown that the resonance condition can be fulfilled in equatorial regions during the post-sunset hours. The measured drift velocities of patches of range-type equatorial spread-F are similar to measured plasma drift velocities. Some spread-F structures observed with the RTI-technique at Jicamarca, range-type spread-F observed on ionograms of Huancayo as well as the occurrence of spread-F patches observed with transequatorial propagation experiments do occur after the vertical plasma drift reverses from upward to downward. The periods, wavelengths and velocities of the large-scale structure of the spread-F patches appear to be similar to those parameters for medium-scale TID’s. It is argued that a non-linear oreak-up of resonance-steepened TID’s gives rise to the quasi-periodic structures which are typical for the range-type equatorial spread-F.

1. INCRODUCI-ION

spatial resonance mechanism (WHITEHEAD, 1971) can give rise to a considerable amplification of the amplitude of travelling ionospheric disturbances (TID’s). The resonance condition is met if an ambient plasma drift velocity in the ionosphere matches the phase velocity of an atmospheric gravity wave perturbing the ionosphere. It was anticipated that the spatial resonance mechanism may be responsible for the quasi-periodic modulation of the macro-scale structure of the equatorial spread-F (BEER, 1972, 1973; R&TGER, 1973a, b, 1975a, 1976a). The considerations presented in this paper give further support to these suggestions. The equatorial spread-F phenomenon first was observed on vertical incidence ionograms from equatorial stations. It was reported that the normal F-layer traces after sunset spread into distinctly separated traces, which are transferred into strong scattered echoes (satellite traces) from patches of spread-F irregularities (CLEMEL~HA, 1964). This phenomenon, which is called range-spread-F, is typically observed at the equator during the postsunset to pre-midnight hours, but during the postmidnight to pre-sunrise period mostly frequencyspread-l; is observed (CLEMESHA and WRIGHT, 1966; S KINNER and KEJJ_EHJSR,1971). The theory of the spatial resonance mechanism appears not to be applicable to explain the frequency-spread-F. Thus, we do not treat frequency-spread-F in this paper, but confine the discussion to the premidnight or range-spread-F. The

1103

The pre-midnight spread-F causes non-greatcircle propagation on transequatorial HF radio paths (NIELSON and CROCHET, 1974). It was deduced from transequatorial propagation experiments (KELLEHER and R&-rGER, 1973; R~YITGER, 1973a, b) that the patches of equatorial spread-F irregularities drift mostly to the east at a velocity of 50-250 m s-‘. The median downward velocity component was observed to be 24ms-‘. A typical feature of this phenomenon observed with transequatorial propagation is the quasi-periodic structure of the patches with a horizontal wavelength of some hundred kilometers. The parameter values found in these observations led to the suggestion that atmospheric gravity waves could modulate the macro-scale structure of the equatorial spread-F since it had been proved by further experiments (STERLING er al., 1971; NAGPAL et&., 1973; RMTGER, 1976b) thattravelling ionospheric disturbances generated by atmospheric gravity waves occur regularly in the equatorial zone. 2. THE

SPATIAL RJBONANCE MECHANISM IN THE EQUATORIAL IONOSPHERE

The spatial resonance mechanism is an appropriate physical mechanism to explain the spread-F modulation, i.e. the wave-like structure of the macro-scale of the equatorial spread-F. This resonance effect is expected to operate most effectively in the post-sunset equatorial F-region (BEER, 1974: R&TGER, 1975a). Here one finds the

J. R&TGER

1104

essentially caused by zonal winds blowing across the magnetic field, we have to consider their influence on the gravity wave propagation. Depending on assumptions about polarization and E-layer fields transferred to the F-layer, the plasma drift velocity u,. might be somewhat larger than the zonal wind velocity w, during the day, and somewhat smaller during the night (Rrsrmrrrrr, 1971). Reasonable values of the wind velocity u, are: 0.8 uy < WY< 1.2 l+. Inserting (3) into (2), the vertical plasma velocity co,,-k-n=0 (1) u, as a function of the eastward plasma velocity u, 1 is found for the spatial resonance mechanism: where o,, is the angular frequency and k is the wave vector of the atmospheric gravity wave, and u is the velocity vector of the drifting ionospheric plasma. Introducing a fixed coordinate system with the x(4) axis pointing to the south, the y-axis to the east, and the z-axis pointing in the upward direction, the Since only zonal winds are taken into account: angular frequency is

ionospheric plasma drifting downward and eastward (WOODMAN, 1970, 1972) so that the matching condition of the spatial resonance effect, demanding equality of plasma drift and phase velocity of atmospheric gravity waves, can be properly fulfilled. Neglecting in this approach production, diffusion, loss processes of ionization and non-linear effects in TID-generation, the resonance condition is given by (WHITEHEAD, 1971):

o,=~xk~sinIL+~yk,~~sIL+yk,.

(2)

Here kh is the horizontal and k, the vertical component of the wave vector k. y, u,. and u, are the x, y, z-components of u, and $ is the angle between the horizontal component k,, and the y-axis. The EXB plasma drift in the vertical E-W plane, i. e. u, and y have been measured by W~~DMAN (1970, 1972). For simplicity, the N-S plasma motions, which can be comparable in magnitude to the E-W motions, have been neglected in the following calculations. The general picture is not changed by this simplification, so that we put u, = 0. The relation between k,, and k,

w,-k,,SW,,. o. is the angular frequency measured in the fixed frame of reference, and w,, is the horizontal component of the wind velocity (also measured in the fixed frame of reference). Except for this definition of the Dopplershifted frequency, the vertical wind velocity component is always negligible compared to the horizontal wind component. The dispersion relation becomes k,'

=

Ob coo-kh ‘w,,

((

khz

+bo-kh

;)*-d. (3)

Here q 0, C Since

.. . .. .. = Brunt-Vaisala frequency, = acoustic cut-off frequency, = speed of sound. the EXB plasma drift at the equator

is

In Fig. 1, the resonance curves uZ = f(q), defined by equation (4), are shown for gravity waves with typical periods 7’ of 15 and 30 min and horizontal phase trace velocities of 100 m s-l (corresponding to the horizontal wavelengths 90 and 180 km) and 200 ms-’ (horizontal wavelengths 180 and 360 km). The frame of reference is fixed with respect to the ground. Referring to the CIRA 1972 model (COSPAR, 1972), the following values of the atmospheric parameters at 200 km height were used in the computations: o, = 9.1 x 10e3 s-l, % = 8.2 x 10V3 s-l, C = 705 m s-l. The zonal wind was assumed to be w, = 1.2 y. For decreased zonal wind velocity (e.g. w, =0.8 4), the resonance curves are shifted to slightly larger values of u, and u,, but the general picture is not changed considerably. The calculations indicate that at least the necessary condition u,
RON, 1976). Only those plasma drift velocities which are greater than the boundaries given by the JI 2 0” curves can give rise to the spatial resonance

Drifting patches of equatorial

spread-F

1105

irregularities

Fig. 1. Resonance curves (-) for gravity waves with period T propagating with the horizontal phase trace velocity u,, at an angle 1,4measured with respect to the east direction. The zonal background wind is wY = 1.2 u,,, where uY is the horizontal (zonal) component and u, the vertical component of the are the mean plasma drift velocities measured plasma drift velocity. The closed curves (_) in the epoch 1968-1976 with the Incoherent Scatter Radar of Jicamarca/Peru (WOODMAN and CALDERON. 1976). The dots on the curves indicate the time of observation (LT75’W). The shaded

areas enclose 90% of the drift velocities, when supposing that the values of uY scatter *75% around their mean values, respectively u, scatter f 100%.

mechanism. The mean values of the measured plasma drift indicate that this condition is fulfilled only between about 19.30 and 03.00 LT. Depend-

ing on the gravity wave period and phase speed, we thus expect the resonance mechanism to come into operation only during this time interval which coincides with the occurrence time of the post-sunset equatorial spread-F, i.e. the range-spread-F (LYON et al., 1961; CLJSES HA and WRIGHT, 1966). In the shaded area (in Fig. 1) which is limited by the resonance curves t+Q 2 O”, we find from the calculations that the vertical wavelength of the gravity waves is smaller than the horizontal wavelength. This indicates that gravity waves with longer horizontal wavelengths, i.e. mostly longer-period gravity waves, are preferred to excite the resonance mechanism. Further proof for this assumption is given by the evidence that the hatched area cut out by the resonance curves, which is a measure of the probability how often the resonance mechanism works, increases with increasing gravity wave period. It is found from Fig. 1 that gravity waves with longer periods and smaller horizontal phase speeds are preferred to seed the spatial resonance mechanism. The period distribution of TID’s influenced by the spatial resonance mechanism is given by the period distribution corresponding to the sources and the propagation of gravity waves, multiplied by a function increasing with T, due to the resonance

effect. Due to diffusion and recombination processes, an upper limit of the amplitude growth is given at periods of several hours. For the longer-period waves, the propagation angle $ might be considerably greater than 0”. In view of the assumptions mentioned above and considering the measured plasma drift values, gravity waves with a westward component of propagation (4 >900) cannot excite the spatial resonance mechanism in the equatorial onosphere. One might argue that, because the wind is approximately equal to the horizontal plasma drift, the gravity waves undergo critical layer effects (BOOKER and BRETHERTON,1967; HINES, 1968; MCKENZIE, 1972) before they reach the level of resonance. The condition that gravity waves generated at low altitudes can propagate to F-region heights without undergoing critical layer effects is wo--k-w,

>O

with

0<,>0.

(5)

In the case when k-w,,+ oo, it follows from (3) that k: -+ 00, i.e. A, + 0. Gravity waves can excite the spatial resonance mechanism if w. -k *u = 0 (equation (1)) holds. Thus, for any height z below the level of resonance, the relation k*u(z,)>k*w,(z)

(6)

has to be fulfilled, where z, is the height of the level of resonance. It follows from (1) that the left-hand

J. R~TTGER

1106

side of relation (6) is always positive since oO > 0, and for reasonable cases, like those shown in the diagrams of Fig. 1, is greater than the right-hand side. In Fig. 1 only those parts of the resonance curves are shown which obey the condition (6). Otherwise the resonance curves would cross the abscissa and continue into the upper quadrants where u, >O. The following example will explain why, in reasonable cases, (6) is fulfilled. Assuming that the wind has only a zonal component w,, which linearly increases with height and becomes w, = ay, at the level of the spatial resonance, and assuming that the plasma drift is given by the eastward and downward velocities u, and u,, we find at the resonance level:

Consequently

the condition

must hold. Since k, is negative for gravity waves in the thermosphere, and k, is positive, the right-hand side of (7) must be negative. During the night-time hours when the spatial resonance mechanism operates, u,,>O and cu> 1 (RISHBETH, 1971). Consequently, u, has to be negative, which is the limiting condition mentioned earlier. During the premidnight hours, (Y = 1.2 (RISHBETH, 1971) and u, = -5u, (WOODMAN, 1972) are representative values. Thus, critical reflection will not occur for those gravity waves where -k, > b.

(8)

Because k, 5 k,,, these gravity waves have periods greater than about 15 min (YEH and LIU, 1974). If the Doppler-shift is taken into account, it follows from (3) that, for o0 + k -wh, k,’ increases. This implies that the condition (8) -k, > k, is fulfilled a fortiori. When the resonance. effect becomes operative, even very small TID amplitudes will be strongly amplified by the resonance to reach the non-linear regime (RINGER, 1975a, 1976a; KLOSTE~YER, 1978). Steep gradients of the ionospheric electron density appear as a consequence. The corresponding depletions of ionization can be accepted as source regions of rising equatorial spread-F bubbles (SCANNAPIECOand OSSAKOW, 1976) and the ionization biteouts reported by MCCLURE et al. (1977). The generated electron density gradients eventually break up into smaller-scale irregularities which decay in a non-linear cascading process (OTT and FARLEY, 1974). As reported by MATHEWS and

HARPER (1972). range-spread-F in mid-latitudes is also observed to be associated with tilts in the ionosphere, and this too might be associated with the resonance mechanism. The post-sunset premidnight period when the spatial resonance mechanism can operate is the time when the maximum activity of range-spread-F is observed at the equator (LYON et al., 1961; KELLEHER and SKINNER, 1971; SKINNER and KELLEHER, 1971). Thus, it is argued that the spatial resonance mechanism can be responsible as one triggering mechanism to seed spread-F irregularities. It is assumed that this mechanism is the initial phase responsible for quasi-periodical spread-F structures with large scales of several tens of kilometers to hundred kilometers. This phase is followed by the second phase when primary irregularities are generated by means of plasma instabilities (FARLEY et al., 1970; BALSLEY et al., 1972; HUDSON and KENNEL, 1974a, b) which then cascade into irregularities with small scales of several meters (FARI-EY, 1974; OTT and FARLEY, 1974). 3. DRIFIS OF SPREAD-P PATCHES AND BACKGROUNDIONIZATION To find further experimental support for the spatial resonance mechanism, measurements of plasma drift in the equatorial ionosphere (BALSLEY and WOODMAN, 1969; WOODMAN, 1970,1972; WOODMAN and CALDERON, 1976) are used for comparison with the drift velocities of irregularity patches deduced from transequatorial propagation experiments (TEP). For this purpose, drift data from the Jicamarca Radio Observatory were selected for the period when the transequatorial experiments were carried out. It must be stressed here that we can only provide a statistical comparison since the Jicamarca data are from South America whereas the TEP data are from Africa (R~LITGER, 1973a, b). However, it is reasonable to expect the mean drift conditions to be comparable at both locations. Anyway, a correct comparison would have been impossible at the same location since drift measurements with the incoherent scatter technique often fail during spread-F conditions (BALSLEY and WOODMAN, 197 I; WOODMAN, private communication, 1976). The results of evaluations of the drift data, available from Jicamarca for the time period MarchJuly 1971, are shown in Fig. 2. Additional’-*, the median value of the drift velocities of spread-F patches measured by transequatorial propagb -ion is indicated. The TEP results show that the median value of the vertical velocity (R~LITGER, 1973a, b)

1107

Drifting patches of equatorial spread-F irregularities 1--

li----March-April

Median Median(TEP)

4

1

1971

19-2LLT

March-July1971 19-2LLl 15 i

N

Median Median (TEPI

/

”

uy-ms

-1

u,-ms

-1

Fig. 2. Occurrence frequency of ionospheric drift velocities measured with the Jicamarca Incoherent Scatter Radar. N gives the number of events when the velocities were observed in the specified

intervals. The evaluation bases on radar measurements of the vertical drift velocity u, from four nights (negative u, means downwardvelocity) and the eastward velocity u,, from ten nights during the indicated time periods. Median (TEP) indicates the median values of the drift velocities q and iiL of irregularity patches measured with the transequatorial propagation experiment in March 1971. is equal to the median value of the vertical plasma drift velocities. The median eastward velocity deduced from the TEP experiments is slightly greater than the median eastward plasma drift velocity, but equality still exists within the error bars of the measurements. From the agreement of the results of both measurements it is deduced that the patches of irregularities drift with the velocity of the background plasma. The irregularity patches with scale sizes of the gravity wavelength around some 100 km have been found to consist of smaller-scale irregularities (CLEMEEZHA,1964; R&I-~GER, 1975b). Due to turbulent electric fields (MCCLURE and WOODMAN, 1972) or due to gravitational instability (SCANNAPIECO and OSSAKOW, 1976; WOODMAN and LA Hoz, 1976; HUDSON, 1977), the smallscale irregularities can move with different velocities than the background ionization or the patches, which are regarded as the envelope of the small-scale irregularities. The median values of the plasma and the patch velocities (4 = -25 m s-r and & = 100 m s-r) are evidently comparable to the phase velocities of travelling ionospheric disturbances. If the spatial resonance mechanism is generating the irregularity patches, it is deduced from lfi,1<11?,.1 that j~l>lk,,l. This points to a preference for gravity waves with longer periods as predicted by the earlier calculations.

As pointed out before, the equality of the plasma and the TID velocities is the essential supposition for the spatial resonance mechanism. Since one observes a considerable similarity of plasma and patch velocities during the lifetime of the spread-F patches, it is assumed that the equality of plasma and TID velocities is a necessary condition not only for the spatial resonance mechanism but also for the generation of the periodically occurring irregularity patches. This assumption sounds reasonable if one supposes the small-scale irregularities to be generated by means of an appropriate plasma instability mechanism at the steep electron density gradient which is caused by the spatial resonance mechanism. Thus, the leading edge of the irregularity patch, which is the region ahead of the crest of the non-linearly steepened TID, is moving with the background and the TID velocity. The phases of TID’s propagate from larger to smaller heights which yield electron density contours in TIDs that are tilted with respect to the horizontal propagation direction. By means of the non-linear steepening, the contours become even more steepened so that besides strong vertical electron density gradients also strong horizontal gradients are set up. These give rise to side-reflection of the HF-waves as observed as range-spread-F with ionosondes (C-HA and WRIGHT, 1966) and with transequatorial HF-propagation (R~~TGER, 1975a). It

1108

J. RC~TGER

was reported (CLEMESHA, 1964; KELLEHER and SKINNER, 1971; RC%TGER, 1973b) that the leading

edge of travelling irregularity patches is much more pronounced to scatter and reflect HF-waves than the trailing edge. These observations fit with the explanation of a TID steepened due to the resonance mechanism which gives rise to seed smallerscale irregularities at the steepened electron density contours. Results of further evaluations, which are shown in Fig. 3, clearly support the introduction of a limiting condition for the generation of spread-F patches. As pointed out by WOODMAN and LA Hoz (1976), one has to discriminate between different types of the equatorial spread-F as observed with the range-time intensity technique (RTI) when using the VI-E-radar of Jicamarca. The bottom-type spread-F is generated after sunset at the steep electron density gradient of the F-layer. This type of spread-F often is succeeded by a wider and filamented layer which eventually breaks into more complicated structures like plumes and tail fins extending into the topside F-layer. The upper diagram of Fig. 3 demonstrates the transition into the regime of the structured spread-F by plotting the number of vertically striated spread-F layers observed with the Jicamarca RTI (e.g. Fig. 1 in WOODMAN and LA Hoz, 1976). Until around 20.00 h only one spread-F layer, i.e. the bottomtype spread-F, is observed each night. After 20.00 h, one identifies a high number of structured echoes extending to the topside of the F-region. This effect is most pronounced between 21.00 and 23.00 h, which is shown in Fig. 1 to be the most probable time interval during which the resonance mechanism works. A picture rather similar to that observed with the Jicamarca RTI-technique can be seen on the ionograms of Huancayo (about 170 km east of Jicamarca) where we find only one spread-F trace until 20.00 h (see middle diagram of Fig. 3). A splitting into many satellite traces is observed after 20.00 h indicating a strong evidence for rangespread-F. It is appropriate to assume that the VI-IPradar detects the vertical structure of irregularities whereas the ionosonde is more sensitive to oblique total reflections caused by horizontally striated irregularity patches. The similarity of the results presented in the two upper diagrams of Fig. 3 demonstrates a correspondence of the spread-F activity in the vertical and the horizontal planes. It can be furthermore deduced that the range-type spread-F structure is normally not observed before 20.00 h, which is (in November) about one hour

JIC November1970

1

_ November 1970

Fig. 3. Upper diagram: Number N of striated layers of spread-F irregularities observed on range-time intensity diagrams recorded with the Jicamarca VHF-radar during N,,=6 nights of RTJ schedules (3,5,7,9,19 and 21 November 1970). Middle diagram: Number of satellite traces observed on Huancayo ionograms during those nights mrresponding to the Jicamarca RTJ observations. Lower diagram: Vertical drift velocity u, (upward positive) measured with the Incoherent Scatter Radar of Jicamarca. The straight line is the median value. The drift reversal is at 20.00 LT when we find the tirst structures of range-spread-F (No>6 in the two upper diagrams). after sunset in the ionosphere

over Jicamarca and Huancayo. This finding is consistent with TEP results (Fig. 4) indicating that the most pronounced side-reflections due to spread-F patches are observed later than around 30-60 min after local sunset in the equatorial ionosphere. The type- of pre-midnight spread-F appears to be closely correlated with height changes of the equatorial F-layer. SKINNER and NR (1971) report on strong spread-F occurrences when about one hour after sunset the F-layer descended rapidly

Drifting patches of equatorial spread-F irregularities

30

t

1109

TEP March 1970 18.2 MHz

i

20

N 10

0 _I

0

t-tS5 -h Fig. 4. Occurence distribution of spread-l; patches measured with the TEP technique. t-6 is the time difference between the time of commencement of spread-F patches and the time of sunset in F-region heights at the location of the observed patch.

from a height of at least 350 km. These observations of Skinner and Kelleher consistently support the idea of the spatial resonance mechanism which needs a downward F-layer drift as a necessary condition. A further confirmation is found from the vertical drift data measured at Jicamarca during the same time of the year when the RTI- and ionogram records were taken. In the lower part of Fig. 3 we plotted all drift data available from November 1970, 1971 and 1972 and fihd that the median vertical drift reverses from upward to downward around 20.00 h. As mentioned before, the downward plasma drift is a strong matching condition for the onset of the spatial resonance mechanism. The necessary condition that the vertical plasma drift has to be downward is found from the results shown in the lower diagram of Fig. 3 to be normally not assured before 20.00 h, which coincides with the time when the structured range-spread-F is observed. This statement is supported by observations at Jicamarca (BALSLEY and WOODMAN, 1971) that drift measurements with the incoherent scatter method sometimes are obscured by spread-F or even fail due to the generation of the strong spread-F irregularities after the vertical velocity changes its direction from upward to downward. Examples of the failure of vertical drift measurements after the onset of strong spread-F irregularities are found on the 18th and 19th November 1970 (Fig. 3) when no drift data could be taken after about 20.30 LT. The lower diagram in Fig. 3 confirms that between 21.00 h and 23.00 h the downward drift velocity reaches its largest values. This is just the time when the structured or 2

range-spread-F is most pronounced (see upper diagrams of Fig. 3) and the resonance condition, defined by equation (4), is fulfilled (see Fig. 1). Thus, we deduce also from the observations that the spatial resonance mechanism is more effective the larger the downward velocity. This inference appears to be straight-forward because the probability of finding medium-scale TID’s with the vertical trace velocity approaching the horizontal velocity increases at periods around 20 min (FRANCIS, 1974); this trace velocity is typically observed to be the equator (RINGER, 100-200 m SC’ near 1975a). However, the spatial resonance condition demands also that the horizontal plasma drift velocity be greater than about 20 m s-r (Fig. 1) since the downward drift velocity very seldom was observed to exceed 30 ms-‘. It is evident from the Jicamarca drift measurements (WOODMAN, 1972) that the horizontal E-W drift normally reverses in the late afternoon hours and reaches its maximum eastward velocity between about at 120ms-’ 20.00 h and 23.00 h. Again this evidence fits properly with the observed patch velocities, the horizontal trace. velocities of TIDs and the time of maximum occurrence of the range-type equatorial spread-F. In further statistical evaluations of RTI data an examination was made to see whether those quasiperiodic layers observed with the Jicamarca radar (see Fig. la in WOODMAN and LA Hoz (lY76)) are consistent with the resonance mechanism. This examination indicates that the mean vertical separation of the striated spread-F layers is about 50 km, but does not show a significant height dependence

J. R~TGER

1110

20

15

N

March

TEP 1971 18.2 MHz

L2

LA

IO

5

0 I2

18

2L

30

36

60

5~

T-min Fig.

5. Period

distribution

of wave-trains of spread-F patches HF-propagation experiment.

(R~ITGERand WOODMAN, 1976). Since the vertical wavelength

in the with height

of gravity

phere increases

MEYER, 1972), may seem the layered ture of irregularities at above the maximum does reflect the wavelength of gravity wave. these striated layers occur to heights 800 km, is the height where were observed 1968). Furthermore, mean vertical of 50 fits appropriately with an (neglecting loss of the wavelength A,. estimate follows equation (2):

1 ---r

where

A,

wavelength

u,

AJcos J, is the apparent horizontal in the W-E direction. The period dis-

tribution of wave-trains of irregularity patches which was obtained from evaluations of the TFP experiments (see Fig. 5) shows the most frequent period T to be around 24 minutes. As indicated in Fig. 2, the median vertical patch velocity U, is - 24 m ss’ and the median horizontal patch velocity The median wavelength of the u, is llOms-‘. patch wave-trains in the W-E direction was deduced to be A,,= 380 km (R~TI‘GER, 1973a). Rquation (9) then yields the vertical wavelength A, to be 60 km, which is reasonably close to the observed

mean vertical separation of 50 km. Equation (4) yields Ah =3.8 A, for the above mentioned values of the parameters wr,, o,, T, C and w, = 1.2 u,,. Since AhI A, = 380 km, the vertical wavelength, as derived from the dispersion relation (3), is A, < 100 km which, in the frame-work of the present experimental results, also fits acceptably with the estimate that A, ==50 km.

observed

with

the transequatorial

From the period distribution of patch wave-trains

shown in Fig. 5, one finds that the maximum occurrence between 15 min and 30 min is at longer periods than the typical periods of TID’s observed near the equator (R&~-~GER, 1977), which supports the suggestion that the resonance effect preferably operates at the longer period gravity waves. The mean horizontal wavelength of the patch structure at 380 km is longer than the mean horizontal wavelength of equatorial TID’s, which might also indicate a preference for TID’s with longer wavelengths. Another explanation for this observation is that the atmospheric gravity waves propagate at angles JI considerably greater than 0” so that the apparent wavelength A, in eastward direction is

greater than the horizontal wavelength Ah (see explanation of Equation (9)). The observations presented appear to be acceptable arguments to support the suggestion that the spatial resonance mechanism is responsible for the quasi-periodical structure of the range-type equatorial spread-F. It is stated that, if the limiting condition of velocity matching is fulfilled after the reversal of the vertical drift direction, the spatial resonance mechanism causes steep electron density gradients which trigger or modulate the generation of smaller-scale spread-F irregularities. These behave quite differently in the drift pattern from the large-scale background ionosphere, e.g. move upward in bubbles (WOODMAN and LA Hoz, 1976) or, due to electrical fields or other influences, form those structures (RO?TGER and WOODMAN, 1976; MCCLURE et al., 1977) which are somewhat different from the originating TID structure. The seeding structure of the resonance mechanism, featured by periodicities with typical gravity wave periods between 15 and 40 min, still can be identified on spread-F maps (e.g. Figs. 3c, 3e and 3j in WOODMAN and LA Hoz (1976). and MCCLURE et

Drifting patches of equatorial

al., 1976). One also can find that the quasi-periodic structures on the spread-F maps are tilted. At the bottomside of the F-layer the higher part of the spread-F structure is often observed earlier than the lowest part. This is in accordance with TID structures which have the typical downward phase progression. 4. CONCLUSION

was shown that the vertical and horizontal velocity of large-scale wave-like structures of equatorial spread-F patches appears to be similar to the drift velocity of the background ionization. The periodical range-type spread-F structure occurs after the vertical ionization drift reverses from upward to downward. By this event, the limiting condition of the spatial resonance mechanism, i.e. equality of ionization drift and phase velocity of atmospheric gravity waves, is fulfilled. As a consequence of the spatial resonance mechanism, electron density gradients are produced which act as a seeding mechanism for plasma instabilities generating equatorial spread-F irregularities. From the analysis presented we find strong support for considering the spatial resonance mechanism to be responsible for some of the relevant macro-scale structures of the pre-midnight equatorial spread-F, such as the moving wave-trains of irregularity patches. In this context, however, we It

spread-F

1111

irregularities

recommend again a clear distinction between the micro-scale structure which has its origin in appropriate plasma instability mechanisms, and the macro-scale structure modulated by travelling ionospheric disturbances originating from atmospheric gravity waves. If the spatial resonance mechanism is accepted for modulating the macrostructure of the equatorial spread-F, we are on the way to explaining some of the features of the diurnal, seasonal, sunspot and geographical variations of the range-spread-F by means of the corresponding variations of the ionospheric plasma drift as well as the variation in the occurrence of travelling ionospheric disturbances. Acknowledgements-The author appreciates the cooperation and discussions with the former Directors, Drs. CARLOS CALDERON and RONALD F. WOODMAN, and the Staff of the Radio Observatorio de Jicamarca. He gratefully acknowledges the provision of the drift and RTI data from Jicamarca as well as the ionosonde data obtained from the Observatotio de Huancayo by courtesy of the Director General of the Instituto Geotisico de1 Peru, ING. A. GIESIXKE. The author also wishes to thank Drs. J~RGEN KLOSTERMEYERand C. H. Ltu for valuable discussions. This paper is an extension of the contribution ‘Transequatorial Propagation Path Deviations--Implications to Equatorial Spread-F Irregularities’, which the author presented at the 5th International Symposium on Equatorial Aeronomy, held on 25-31 August 1976 at Townsville, Australia.

REFERENCES

J. atmos. ten. Phys. 31, 865. Ionospheric Drift Velocity Measurements at Jicamarca, Peru (July 1%7-March 1970), Report UAG-17, World Data Center A, Washington. J. geophys. Res. 77, 5625.

BAL~LEY B. B. and WCEJDMAN R. F. BAL~LEY B. B. and W~~DMAN R. F.

1969 1971

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