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The nocturnal intermediate layer over South Georgia
J,,a,,,o,,q Pr,nted
Ammphm and in Grat Hrltaln
li,rrec/rroll’h~s,~,r,
Vol.
43, No.
IO. pp.
IO43
1050.
19x1
002, Y 16Y4: lOI 08$02.00~0 I‘: I’)81 Perpaman Press Ltd
The nocturnal intermediate layer over South Georgia A. S. RODGER, P. H. FITZGERALDand S. M. BROOM British Antarctic Survey, Natural Environment Research Council, Madingley Road, Cambridge CB3 OET, England (Received injnal.form
13 January
198 I)
Abstract Detailed analyses of ionograms from South Georgia (54”S, 37”W; L = 1.9) show that a nocturnal intermediate layer is frequently seen between the E- and F-regions in the height range 13G-180 km. The occurrence of the intermediate layer is almost totally restricted to the winter months and between the local hours of 1930 and 2300. The intermediate layer event is characterized by a prior downward movement of the F-layer, followed by the formation of the intermediate layer and its subsequent drift downwards to about 140 km. Initially, the layer is thick (- 1@20 km), but gradually thins as it evolves. The occurrence of the intermediate layer appears to be independent of the level of magnetic activity. Observations of intermediate layers from other localities, which are briefly reviewed, are compared with those made at South Georgia. Some similarities and differences are identified. Mechanisms which account for the formation of the intermediate layer at South Georgia are considered. It is shown that the main features of the diurnal and seasonal variations of occurrence can be adequately explained by the theory of convergence of ionization driven by the solar semi-diurnal tidal winds. Some limitations of this theorv are discussed, as it cannot account for all the features seen at this southern mid-
_
latitude observatory.
I. INTRODUCTION
These
observations
2 of this
paper,
are briefly together
with
summarized a short
which have been proposed to explain them. A nocturnal intermediate layer is frequently observed on ionograms from the southern mid-latitude station of South Georgia (geophysical parameters are given in Table 1). The phenology of the layer is described in detail for 1976 and 1977 in Section 3. Comparison of the South Georgia results with those from other localities shows that there are many similarities. Mechanisms for the formation of the layer at South Georgia are considered in the final part of this paper. It is shown that a mechanism, in which convergence of ionization under the action of a solar semi-diurnal tide occurs, can explain much of the observed phenology, but limitations of this mechanism are also described. mechanisms
The ionosphere at night may be conveniently divided into three regions : a night-time E-region, an F-region and an intermediate region between the E- and Fregions which is taken to cover the height range 12& 200 km for the purposes of this paper. Both theory and experiment indicate that the electron concentration in the intermediate region is usually considerably less than those in both the E- and F-regions, giving rise to a ‘valley’ in the electron concentration profile. Under normal conditions at night the maximum electron concentrations are of the order lOlo, lo8 and IO” m -’ for the E-, intermediate and F-regions, respectively. The night-time valley in electron concentration arises because the intermediate region is above the height where production of ionization by U.V. radiation, particularly Ly-/I and Ly-cc radiation, scattered by the geo-corona, is important. Also, fluxes of ionization from the protonosphere, which help to maintain the night-time F-layer, do not contribute significantly to the electron concentration of the intermediate region, whereas recombination processes are much more effective in removing ionization in the intermediate region than at F-region heights. Abnormal layers of much higher electron concentration ( - 10” mm”) have been observed at night in the intermediate region in various parts of the world. in Section review
of the
2. REVIEWOF OBSERVATIONSAND
Observations been reported
MECHANISMS of intermediate layers at night have from 12 stations. Three observing
Table I. Geophysical Geographic
parameters
co-ordinates
Centred dipole co-ordinates Magnetic dip Dip latitude Invariant latitude Total magnetic field strength at ground Electron gyro frequency at 200 km 1043
of South Georgia 54.8”s. 323S”E 44.2”S, 26.O”E - 52.3” 32.9”s 42.8”s 305OOy 0.87 MHz
1044
A. S. RODGER, P. H. FITZGERALD and S. M. BROOM
techniques have been employed : ionosondes operating at vertical incidence, rockets making in situ measurements, and, more recently, incoherent scatter radars. Both rockets and incoherent scatter radars can measure electron concentration throughout the intermediate region, whereas ionosondes give only limited information. Moreover, ionosondes are normally less sensitive than other techniques in the measurement of low electron concentrations, but they do have the advantage of much wider spatial and temporal coverage. lntercomparison of observations by these three techniques must be made with care. For convenience, the observations of the nocturnal intermediate layer can be grouped into four geographic regions; these are termed Equatorial, European/American mid-latitudes, Asian mid-latitude and High latitude. Table 2 summarizes the observations, the techniques employed, and the sources of the data. A positive correlation is reported from all locations between magnetic activity and either the occurrence of the nocturnal intermediate layer or the total electron content (TEC) of the intermediate region. There are no other features which are similar in all regions. This partly arises because not all the summarized features have been measured at each station. The differences observed, such as the time of occurrence of the intermediate layer at Equatorial and European/ American mid-latitude groups, have proved to be important evidence for identifying the mechanisms responsible for the formation of the layer. SMITH (1970) and CONSTANTINIDESand BEDINCER (I 971) have provided strong evidence that the intermediate layer in Equatorial and European/American mid-latitudes results from the convergence of ionization driven by meridional winds. This has been established using simultaneous electron concentration and wind measurements from rocket flights. Further analysis of similar data by FUJITAKA and TOHMATSU (1973) and FUJITAKA (1974) suggests that the occurrence of the intermediate layer can be explained by a neutral wind system driven by a solar semi-diurnal tide. These tidal theories have been further developed by HONG and LINDZEN (1976), and HARPER (1977, 1979) has shown that their theory agrees well with the observations. The occurrence of the intermediate layer at Asian mid-latitudes and at High latitudes has not been adequately explained. At Asian mid-latitudes, the intermediate layer can be observed throughout the night, which is inconsistent with a semi-diurnal tidal theory. At high latitudes, the amplitude of the semidiurnal tide should be small (CHAPMAN and LINDZEN, 1970), yet the maximum electron concentration re-
ported is higher than at other localities. For the latter zone, particle precipitation may be important in the formation process, but no conclusive evidence has been provided. MAEHLUM (1967), MANSON and MERRY (1970) and MANSON (I 97 1) have suggested that there is a drizzle precipitation of charged particles at mid-latitudes, even during periods of quiet magnetic activity. However, this precipitation alone is insufficient to form a layer, or to account for its subsequent downward movement. Hence, a mechanism involving transport of ionization is also required (SMITH et nl., 1974).
3. OBSERVATIONSAT SOUTH
GEORGIA
Results of a detailed study of the intermediate layer, as recorded on South Georgia ionograms in the period January-November 1976 and April--September 1977 are presented in this section. Analysis has been restricted to the period between 2 h after sunset at 120 km and 2 h before sunrise at the same height, to prevent confusion in the data caused by ionization due to direct solar radiation. In particular, this restriction removes contamination of the data set by the high Elayer structures (EZ-layers) which are frequently observed at South Georgia near ground sunrise and sunset. The South Georgia ionosonde has a lower frequency limit of 0.7 MHz; this corresponds to an electron concentration of 6 x IO” m 3. Thus, only layers with higher concentration will be observed directly. Measurements in the frequency range in which the intermediate layer is normally observed (0.7-1.2 MHz) have not been possible on a small number of occasions due to either screening of the intermediate region by lower sporadic E-layers or equipment failure: allowance for this has been made in the statistics. A schematic diagram of a typical South Georgia intermediate layer event is shown in Fig. 1, in which the time origin is taken when the intermediate layer is first seen. The minimum virtual height of the F-layer (h’F) is seen to decrease prior to the event at a rate of between IO and 30 m s ‘. During this period there is very little difference between the virtual heights of the corresponding o- and x-mode echoes implying little ionization is underlying the F-layer (TITHERIDGE 1959a,b; 1975) before the event commences. The intermediate layer is first seen on the ionogram at a virtual height of 180 km. In every case, its height initially decreases at the same rate as did the F-layer, but then slows gradually until the layer stabilizes at between 150 and 130 km. The rate of downward movement and the final height of the intermediate layer are independent of season or magnetic activity.
name
Principal
references
with magnetic
79” Latitude
WAKAI (1967, 1968) SMXTH~~ al.(1974) AMAYENC and REDDY (1972) AMAY~NC, (I 974)
HARPER (1977, 1979)
WAKM and SAWADA (1964)
+ve correlation
Both TEC in valley and occurrence of layer reported to show +ve correlation Surrw (1970) WATTS and BROWN (19%)
only
FRENCH (1969)
+ ve with overhead aurorai activity, but not with K,
45 min
Not recorded
Not recorded
150 20
location.
(725 170E) I (78s 166E) I
Hours of darkness max. 01-04
- 75--
Cape Hallett Scott Base
4 x lOlO
TEC in valley i-ve correlated
ROWE (1973) SHEN et al. (1976)
1 I I I
in occurrence
night
(45N 141E) (39N 140E) (35N 139E) /31N 131E)
High latitude
the geographic
3 x itFJ
Throughout max. 02-04
20-35
Wakanai Akita Kokubunji Yamagawa
Not recorded
(39N 77W) I (38N 77W) R (4ON 1OSW) I (43N 72W) IS (45N 02E) IS
by letters following
Mid-latitude Asia
Upto6h
Relationship activity
1 near sunrise 1 near sunset
Upto6h
of events per night
Lifetime of intermediate layer
Number
1.5 near 150 km
120-1.50
Rate of downward of layer (m s- ’ )
variable in height range 40 km ; usually less 5 m SC’, but up to s-l
136160
Very 17&l than 15 m
Typical height (km)
movement
10’0
1010
Maximum electron concentration (mm31
42-5 7”
Washington Wallops Island Boulder Millstone Hill St Santin
After midnight
30”
(19N 47W) IS
Mid-latitude European/American
layer. The experimental technique used at each location is identified I = ionosonde; R = rocket: IS = incoherent scatter radar
After sunset and before sunrise
range of latitude
Arecibo
Equatorial latitude
of the intermediate
Time of occurrence (Local Zone Time, LZT)
Approximate geomagnetic
and
of morphology
Station name, location technique
Croup
Table 2. Summary
G.
H
3
P
?
A. S. RODGEK,P. H. FITZGERALIIand S. M. BROOM
I046
Initially, the semi-thickness of the intermediate layer, determined from the shapes of the E- and F-region traces on the ionograms, is similar to that of the normal E-region: typically 10-20 km. As the event evolves, it slowly decreases until the pattern finally resembles a conventional sporadic E- (Es-) trace. The maximum plasma frequency of the F-layer, j~F2, remains relatively unaffected throughout the event. The maximum plasma frequency of the intermediate layer,fi~lL, is constant at about 1 MHz. The layer normally lasts for about one hour then disappears rapidly between successive quarter-hourly soundings of the ionosondes. From the observational data, it is not possible to determine whether the disappearance of the layer is due to recombination or redistribution of ionization. The pattern described above has occasionally been repeated several times during the same night. In these cases, h’F rose between two events and each new event was preceded by the characteristic downward movement of the F-layer. Most of the features shown in Fig. 1 are seen on all events ; the main differences between events being the time of occurrence and the rate of downward movement of the layer. The probability of forming an
7 250 1
intermediate layer is very low when the minimum virtual height of the F2-layer is above 220 km, but rises rapidly as the height falls below this level. The very few occasions where an intermediate layer has been formed when h’F‘ is above 220 km occur during magnetically disturbed periods. The seasonal variation in the occurrence of the nocturnal intermediate layer, i.e. the percentage of nights on which at least one layer is observed, is shown in Fig. 2. The occurrence of the layer is most totally restricted to austral winter months with a maximum in June and July when a layer is observed on more than 20 nights in each month. The layer was seen marginally more frequently in 1977 than in 1976, but this difference is not regarded as significant. The diurnal variation of the occurrence of the intermediate layer (Fig. 3) shows that it is most likely to be observed between 1930 (i.e. from the analysis start time) and 2200 local zone time (LZT). Very few examples are observed after 2300 LZT. This diurnal pattern was remarkably constant in all the winter months analysed. There appears to be no significant correlation between the occurrence of the intermediate layer in winter and magnetic activity (Fig. 4). This conclusion
,h’F
\\/-
1
folL
I
,
1
1
-60
0
sequence
for a typical intermediate
1
60
MINUTES Fig. 1. Schematic
layer event observed
at South Georgia.
1047
The nocturnal intermediate layer over South Georgia
x
1976
F
J
M
A
M
J
J
A
s
N
0
Fig. 2. The seasonal variation in occurrence of the intermediate layer at South Georgia for JanuaryNovember 1976 and April-September 1977.
must be treated with some caution, however, because the data samples at high and low levels of magnetic activity are small. The K index used to construct Fig. 4 was the mean value of K for the period 190&0100 LZT determined at the local magnetic observatory at South Georgia. No better correlation was found by using either Kp or the local K indices up to 12 h prior to the occurrence of the intermediate layer. Also, none of the local magnetometers (La Cour, fluxgate and
0
19
01
22 LOCAL
Fig. 3. Thediurnal
variation
rubidium vapour) show any consistent magnetic signatures before, during, or after the periods in which the intermediate layer is observed. The lack of correlation between the occurrence of the intermediate layer and magnetic activity at South Georgia is contrary to resuhs from all other observatories (see Section 2). The nocturnal intermediate layer is observed much more frequently at South Georgia than at the other
ZONE
04
TIME
in occurrence ofthe intermediate layer at South Georgia for 1976 and 1977
A. S. RODGER,
P. H. FITZGERALD and S. M. BROOM
. 0 0
*
.
0
0
.
0
0 ~10
Data
points
l
Data
points
>lO
I
I
I
I
I
I
0
1
2
3
4
5
MAGNETIC
INDEX
K
Fig. 4. The variation in occurrence of the intermediate layer at South Georgia as a function of magnetic activity, K. See the text for a definition of K.
localities reported in Section 2. To some extent, this results from the natural advantages of the South Georgia site for these studies. At most mid-latitude observatories, there are considerable difficulties in interpreting ionograms below 1.6 MHz at night because of strong medium-wave broadcast interference. The South Georgia observatory is encircled by mountains which effectively screen the aerial array from most broadcast interference. Also, such signals must travel over 3500 km to reach South Georgia and are, therefore, comparatively weak. Thus, the signal-tonoise ratio of the ionosonde system is high in the frequency range in which the intermediate layer is observed. The properties of the nocturnal intermediate layer at South Georgia are summarized in Table 3. These show similarities to those seen at other localities (see Table 2). For example, the rate at which the layer descends is remarkably similar to observations made at Arecibo ;
and maximum electron concentration observed at Equatorial latitudes, European/American midlatitudes and South Georgia are the same. However, the most significant difference is the relationship between magnetic activity and the occurrence of the intermediate layer.
4. DISCUSSION
Any mechanism which explains the formation of the nocturnal intermediate layer at South Georgia must account for both the observed increase in electron concentration in the intermediate region and for the regular height and thickness variations described in Section 3. In principle, the drizzle precipitation of charged particles at magnetic mid-latitudes (references in Section 2) could be responsible for the additional ionization in the intermediate region, especially as
Table 3. Summary of the morphology of the nocturnal intermediate layer at South Georgia Seasonal occurrence : winter only Rate of downward
movement
of the layer: 15 m s-r near 180 km
Diurnal occurrence : 193&2300 LZT
Maximum electron concentration: 10” mm3
Number of events per night : Lifetime of layer: about 1 h usually 1, but up to 4
Height range : 180-130 km Magnetic correlation : probably none
The nocturnal
intermediate
should be comparatively greater near South Georgia (TORR et al., 1975 ; MASSAMBINI,1978) as it lies within the South Atlantic Geomagnetic Anomaly. However, no other signatures of particle precipitation such as aurora1 Es (PIGGOTT, 1975) are observed on ionograms when the intermediate layer is observed. The remarkably consistent behaviour of the F-layer immediately prior to the occurrence of the intermediate layer strongly suggests that the ionization enhancements observed in the intermediate region result from downward transport from the F-layer. Both theory (HONG and LINDZEN, 1976) and observations (AMAYENCand REDDY, 1972; AMAYENC,1974; HARPER,1979) have shown that the 2,2 mode of the solar semi-diurnal tide can be effective in redistributing ionization in the intermediate region above about 130 km and in the lower F-region (see Section 2). This tidal mechanism can account for many, but not all, of the observations at South Georgia. For example, the time of occurrence of the intermediate layer is almost totally restricted to the period 1930 (the start time of the analysis) to 2300 LZT. This is consistent with the theory of FUJITAKA and TOHMAXSU (1973) for a mid-latitude station in the southern hemisphere. The intermediate layer is observed much more frequently in the first half of this period. This may be expected: we have determined from the model of KOHL and KING (1967) that the effect of the upward redistribution of ionization in the intermediate region by the diurnal thermospheric wind (RISHBETH,1972) is comparatively small, near 2000, but becomes increasingly important towards midnight. Also, towards midnight. the height of the F-layer rises above the height where the semi-diurnal tide has a downward phase velocity; this height has been determined to be between 225 km (AMAYENC, 1974) and 300 km (HARPER, 1979). Thus, there is little ionization available to be transported into the intermediate region near midnight. However, if a layer were formed at this time, then it is likely to be of low electron concentration and not observable by the South Georgia ionosonde. The semi-diurnal tidal mechanism cannot account for the intermediate layers observed after midnight. These are comparatively rare and it is not possible to determine whether these layers exhibit different features from those occurring before midnight. The intermediate layer is most often observed about mid-winter (June/July). This is to be expected because production and recombination will dominate the electron concentration distribution with height in the intermediate region at other seasons. Also, AMAYENC (1974) showed that the amplitude of the semi-diurnal precipitation
layer over South Georgia
I049
tide in winter was about twice as large as that in summer. There is an asymmetry in the seasonal variation in the occurrence of the intermediate layer about mid-winter which cannot be adequately explained. During periods of enhanced magnetic activity, an increased semi-diurnal temperature oscillation is expected (CHAPMAN and LINDZEN, 1970). Our simplistic approach suggests that the effects of the consequent increased driving force will at least be partly offset by increased ion-drag due to increases in electron concentration in the intermediate region (SMUH e( al., 1974) resulting in relatively small variations in occurrence of the intermediate layer under differing magnetic conditions. The observations from South Georgia are in agreement with this idea, but differ from those from all other localities. This is one aspect of semi-diurnal tidal theory requiring further study. The average rate of descent of the intermediate layer at South Georgia is 15 m s- ’ near 180 km. This is consistent with the theory for a 2,2 mode (FUJITAKA and TOHMATSU,1973). The average wavelength of the tidal mode observed at South Georgia, determined from the period and vertical phase velocity, is more than 100 km in the height range 25(&l SOkm. This, too, is in agreement with the observations of AMAYENC (1974). Both theory and observation show that higherorder tidal modes are more important in the transport of ionization below about 130 km (BERNARD, 1974; SALAH et ul., 1975 ; LINDZEN, 1976; HONG and LINDZEN, 1976). The height to which the layer descends at South Georgia may be related to the height where there is a change in the dominant tidal mode. This, too, may account for the rapid disappearance of the intermediate layer near this height. The maximum electron concentration of the intermediate layer observed at South Georgia is about 1 MHz. This value is similar to that determined by HARPER (1977) using a simple model in which he assumed that that convergence of ionization is balanced by recombination. Some of the weaknesses in the present semi-diurnal tidal mechanism for the formation of the intermediate layer at South Georgia have already been indicated. There are other features which cannot be adequately explained, such at the variability from one intermediate layer event to the next in both the time of occurrence of the layer and the rate of descent of the layer. Also, the occurrence of multiple intermediate layers on the same night cannot be accommodated by the present theory. We suggest that gravity waves in the thermosphere may contribute significantly to the redistribution of ionization on these nights. Variations of .fi)F2 about a smoothed mean show that gravity
A. S. RODGEK, P. H. FITZGERALDand S. M. BROOM
IO50
waves with the necessary common at South Georgia
(iii) Initially the intermediate layer is thick (- 20 km), but gradually thins till it resembles an Es layer.
period (1-3 h) are fairly in all seasons.
5. CONCLUSIONS
Ionosonde observations at South Georgia from an epoch near sunspot minimum have shown that a nocturnal intermediate layer is frequently seen between 1930 and 2300 LZT in winter over the height range 180-130 km. At other times, and during other seasons, nocturnal intermediate layers are rarely seen. The observations at this southern hemisphere midlatitude station show similarities to those from other areas in the world. The main features which characterize the intermediate layer event at South Georgia are : (i) The downward movement of the F-layer prior to the formation of the intermediate layer; (ii) The continued downward movement of the intermediate layer, initially at the same rate as the Flayer, but slowing and eventually stopping at between 150 and 130 km ; and that
The principal features of the diurnal and seasonal variations in the occurrence of the intermediate layer can be adequately explained by the theory of convergence of ionization driven by solar semi-diurnal tidal winds. Some limitations to this mechanism have been identified as it cannot account for all the observed features. Particular advantages of the South Georgia site allow the intermediate layer to be observed much more frequently by ionosonde than elsewhere. Thus South Georgia data should be valuable in testing the theories of semi-diurnal tides. For example, our analysis has shown the need to examine early morning data to determine what effects the other cycle of the semidiurnal tide produces. Also, variations in the occurrence of the intermediate layer through a solar cycle would provide observational evidence to test tidal theories such as those of GARRET-~ and FORES
(1978).
REFERENCES AMAYENCP. AMAYENCP. and REDDYC. A. BERNARDR. CHAPMANS. and LINDZENR. S. CONSTANTINIDES E. and BEDINGERJ. F. FRENCHA. G. FUJITAKAK. FUJITAKAK. and TOHMATSU T. GARRETTJ. M. and FORBE~H. B. HARPERR. M. HARPERR. M. HONGS. S. and LINDZENR. S. KOHL H. and KING J. W. LINDZENR. A. MAEHLUMB. MANXINA. H. MANXINA. H. and MERRY M. W. J. MA~AMBANIP. PIGC;OTTW. R.
RISHBETHH. ROWEJ. F. SALAHJ. E., WANDR. H. and EVANSJ. R. SHENJ. S., SWARTZW. E. and FARLEVD. T. SMITHL. G. SMITHL. G., GELLERM. A. and Voss J. D. TITHERIDGE J. R. TITHERIDGE J. R. TITHERIDGE J. R. TORRD. G., TORRM. R., WALKERJ. C. G. and HOFFMAN R. A. WAKAIN. WAKAIN. WAKAIN. and SAWADAK. WATTSJ. M. and BROWNJ. N.
1974 1972 1974 1970 1971 1969 1974 1973 1978 1977 1979 1976 1967 1976 1967 1971 1970 1978 1975
1972 1973 1975 1976 1970 1974 1959a 1959b 1975 1975 1967 1968 1964 1954
Rndio Sci. 9, 28 1. Planet. Space Sci. 20, 1269. Radio Sci. 9, 295. Atmospheric Tides D. Reidel, Dordrecht. J. atmos. tert. Phys. 33,461. .I. atmos. terr. Phys. 31, 1435. J. atmos. terr. Phys. 36, 1883. .I. atmos terr. Phys. 35,425. J. atmos. terr. Phys. 40, 657. J. geophys. Res. 82, 3243. J. geophys. Res. 84,411. J. atmos. Sci. 33, 135. J. atmos. terr. Phys. 29, 1045. J. geophys. Res. 81, 2923. J. geophys. Res. 72,2287. Planet. Space Sci. 19, 270. J. atmos. terr. Phys. 32, 1169. J. atmos. terr. Phys. 40, 1143. High latitude supplement to the URSI Handbook of Ionogram Interpretation and Reduction Z/AC-50 World Data Center A, Boulder, Colorado, U.S.A. J. atmos. terr. Phys. 34, 1. J. geophys. Res. 78, 681 I. Radio Sci. 10, 347. J. geophys. Res. 81, 5517. J. atmos. terr. Phys. 32, 1247. J. atmos. terr. Phys. 36, 1601. J. atmos. terr. Phys. 17, 96. J. atmos. terr. Phys. 17, 110. J. atmos. terr. Phys. 37, 1517. Planet. Space Sci. 23, 15. J. geophys. Res. 72,4507. J. Radio Rex Labs 15, 109. J. Radio. Res. Labs 11, 1. J. geophys. Res. 59, 71.
Ammphm and in Grat Hrltaln
li,rrec/rroll’h~s,~,r,
Vol.
43, No.
IO. pp.
IO43
1050.
19x1
002, Y 16Y4: lOI 08$02.00~0 I‘: I’)81 Perpaman Press Ltd
The nocturnal intermediate layer over South Georgia A. S. RODGER, P. H. FITZGERALDand S. M. BROOM British Antarctic Survey, Natural Environment Research Council, Madingley Road, Cambridge CB3 OET, England (Received injnal.form
13 January
198 I)
Abstract Detailed analyses of ionograms from South Georgia (54”S, 37”W; L = 1.9) show that a nocturnal intermediate layer is frequently seen between the E- and F-regions in the height range 13G-180 km. The occurrence of the intermediate layer is almost totally restricted to the winter months and between the local hours of 1930 and 2300. The intermediate layer event is characterized by a prior downward movement of the F-layer, followed by the formation of the intermediate layer and its subsequent drift downwards to about 140 km. Initially, the layer is thick (- 1@20 km), but gradually thins as it evolves. The occurrence of the intermediate layer appears to be independent of the level of magnetic activity. Observations of intermediate layers from other localities, which are briefly reviewed, are compared with those made at South Georgia. Some similarities and differences are identified. Mechanisms which account for the formation of the intermediate layer at South Georgia are considered. It is shown that the main features of the diurnal and seasonal variations of occurrence can be adequately explained by the theory of convergence of ionization driven by the solar semi-diurnal tidal winds. Some limitations of this theorv are discussed, as it cannot account for all the features seen at this southern mid-
_
latitude observatory.
I. INTRODUCTION
These
observations
2 of this
paper,
are briefly together
with
summarized a short
which have been proposed to explain them. A nocturnal intermediate layer is frequently observed on ionograms from the southern mid-latitude station of South Georgia (geophysical parameters are given in Table 1). The phenology of the layer is described in detail for 1976 and 1977 in Section 3. Comparison of the South Georgia results with those from other localities shows that there are many similarities. Mechanisms for the formation of the layer at South Georgia are considered in the final part of this paper. It is shown that a mechanism, in which convergence of ionization under the action of a solar semi-diurnal tide occurs, can explain much of the observed phenology, but limitations of this mechanism are also described. mechanisms
The ionosphere at night may be conveniently divided into three regions : a night-time E-region, an F-region and an intermediate region between the E- and Fregions which is taken to cover the height range 12& 200 km for the purposes of this paper. Both theory and experiment indicate that the electron concentration in the intermediate region is usually considerably less than those in both the E- and F-regions, giving rise to a ‘valley’ in the electron concentration profile. Under normal conditions at night the maximum electron concentrations are of the order lOlo, lo8 and IO” m -’ for the E-, intermediate and F-regions, respectively. The night-time valley in electron concentration arises because the intermediate region is above the height where production of ionization by U.V. radiation, particularly Ly-/I and Ly-cc radiation, scattered by the geo-corona, is important. Also, fluxes of ionization from the protonosphere, which help to maintain the night-time F-layer, do not contribute significantly to the electron concentration of the intermediate region, whereas recombination processes are much more effective in removing ionization in the intermediate region than at F-region heights. Abnormal layers of much higher electron concentration ( - 10” mm”) have been observed at night in the intermediate region in various parts of the world. in Section review
of the
2. REVIEWOF OBSERVATIONSAND
Observations been reported
MECHANISMS of intermediate layers at night have from 12 stations. Three observing
Table I. Geophysical Geographic
parameters
co-ordinates
Centred dipole co-ordinates Magnetic dip Dip latitude Invariant latitude Total magnetic field strength at ground Electron gyro frequency at 200 km 1043
of South Georgia 54.8”s. 323S”E 44.2”S, 26.O”E - 52.3” 32.9”s 42.8”s 305OOy 0.87 MHz
1044
A. S. RODGER, P. H. FITZGERALD and S. M. BROOM
techniques have been employed : ionosondes operating at vertical incidence, rockets making in situ measurements, and, more recently, incoherent scatter radars. Both rockets and incoherent scatter radars can measure electron concentration throughout the intermediate region, whereas ionosondes give only limited information. Moreover, ionosondes are normally less sensitive than other techniques in the measurement of low electron concentrations, but they do have the advantage of much wider spatial and temporal coverage. lntercomparison of observations by these three techniques must be made with care. For convenience, the observations of the nocturnal intermediate layer can be grouped into four geographic regions; these are termed Equatorial, European/American mid-latitudes, Asian mid-latitude and High latitude. Table 2 summarizes the observations, the techniques employed, and the sources of the data. A positive correlation is reported from all locations between magnetic activity and either the occurrence of the nocturnal intermediate layer or the total electron content (TEC) of the intermediate region. There are no other features which are similar in all regions. This partly arises because not all the summarized features have been measured at each station. The differences observed, such as the time of occurrence of the intermediate layer at Equatorial and European/ American mid-latitude groups, have proved to be important evidence for identifying the mechanisms responsible for the formation of the layer. SMITH (1970) and CONSTANTINIDESand BEDINCER (I 971) have provided strong evidence that the intermediate layer in Equatorial and European/American mid-latitudes results from the convergence of ionization driven by meridional winds. This has been established using simultaneous electron concentration and wind measurements from rocket flights. Further analysis of similar data by FUJITAKA and TOHMATSU (1973) and FUJITAKA (1974) suggests that the occurrence of the intermediate layer can be explained by a neutral wind system driven by a solar semi-diurnal tide. These tidal theories have been further developed by HONG and LINDZEN (1976), and HARPER (1977, 1979) has shown that their theory agrees well with the observations. The occurrence of the intermediate layer at Asian mid-latitudes and at High latitudes has not been adequately explained. At Asian mid-latitudes, the intermediate layer can be observed throughout the night, which is inconsistent with a semi-diurnal tidal theory. At high latitudes, the amplitude of the semidiurnal tide should be small (CHAPMAN and LINDZEN, 1970), yet the maximum electron concentration re-
ported is higher than at other localities. For the latter zone, particle precipitation may be important in the formation process, but no conclusive evidence has been provided. MAEHLUM (1967), MANSON and MERRY (1970) and MANSON (I 97 1) have suggested that there is a drizzle precipitation of charged particles at mid-latitudes, even during periods of quiet magnetic activity. However, this precipitation alone is insufficient to form a layer, or to account for its subsequent downward movement. Hence, a mechanism involving transport of ionization is also required (SMITH et nl., 1974).
3. OBSERVATIONSAT SOUTH
GEORGIA
Results of a detailed study of the intermediate layer, as recorded on South Georgia ionograms in the period January-November 1976 and April--September 1977 are presented in this section. Analysis has been restricted to the period between 2 h after sunset at 120 km and 2 h before sunrise at the same height, to prevent confusion in the data caused by ionization due to direct solar radiation. In particular, this restriction removes contamination of the data set by the high Elayer structures (EZ-layers) which are frequently observed at South Georgia near ground sunrise and sunset. The South Georgia ionosonde has a lower frequency limit of 0.7 MHz; this corresponds to an electron concentration of 6 x IO” m 3. Thus, only layers with higher concentration will be observed directly. Measurements in the frequency range in which the intermediate layer is normally observed (0.7-1.2 MHz) have not been possible on a small number of occasions due to either screening of the intermediate region by lower sporadic E-layers or equipment failure: allowance for this has been made in the statistics. A schematic diagram of a typical South Georgia intermediate layer event is shown in Fig. 1, in which the time origin is taken when the intermediate layer is first seen. The minimum virtual height of the F-layer (h’F) is seen to decrease prior to the event at a rate of between IO and 30 m s ‘. During this period there is very little difference between the virtual heights of the corresponding o- and x-mode echoes implying little ionization is underlying the F-layer (TITHERIDGE 1959a,b; 1975) before the event commences. The intermediate layer is first seen on the ionogram at a virtual height of 180 km. In every case, its height initially decreases at the same rate as did the F-layer, but then slows gradually until the layer stabilizes at between 150 and 130 km. The rate of downward movement and the final height of the intermediate layer are independent of season or magnetic activity.
name
Principal
references
with magnetic
79” Latitude
WAKAI (1967, 1968) SMXTH~~ al.(1974) AMAYENC and REDDY (1972) AMAY~NC, (I 974)
HARPER (1977, 1979)
WAKM and SAWADA (1964)
+ve correlation
Both TEC in valley and occurrence of layer reported to show +ve correlation Surrw (1970) WATTS and BROWN (19%)
only
FRENCH (1969)
+ ve with overhead aurorai activity, but not with K,
45 min
Not recorded
Not recorded
150 20
location.
(725 170E) I (78s 166E) I
Hours of darkness max. 01-04
- 75--
Cape Hallett Scott Base
4 x lOlO
TEC in valley i-ve correlated
ROWE (1973) SHEN et al. (1976)
1 I I I
in occurrence
night
(45N 141E) (39N 140E) (35N 139E) /31N 131E)
High latitude
the geographic
3 x itFJ
Throughout max. 02-04
20-35
Wakanai Akita Kokubunji Yamagawa
Not recorded
(39N 77W) I (38N 77W) R (4ON 1OSW) I (43N 72W) IS (45N 02E) IS
by letters following
Mid-latitude Asia
Upto6h
Relationship activity
1 near sunrise 1 near sunset
Upto6h
of events per night
Lifetime of intermediate layer
Number
1.5 near 150 km
120-1.50
Rate of downward of layer (m s- ’ )
variable in height range 40 km ; usually less 5 m SC’, but up to s-l
136160
Very 17&l than 15 m
Typical height (km)
movement
10’0
1010
Maximum electron concentration (mm31
42-5 7”
Washington Wallops Island Boulder Millstone Hill St Santin
After midnight
30”
(19N 47W) IS
Mid-latitude European/American
layer. The experimental technique used at each location is identified I = ionosonde; R = rocket: IS = incoherent scatter radar
After sunset and before sunrise
range of latitude
Arecibo
Equatorial latitude
of the intermediate
Time of occurrence (Local Zone Time, LZT)
Approximate geomagnetic
and
of morphology
Station name, location technique
Croup
Table 2. Summary
G.
H
3
P
?
A. S. RODGEK,P. H. FITZGERALIIand S. M. BROOM
I046
Initially, the semi-thickness of the intermediate layer, determined from the shapes of the E- and F-region traces on the ionograms, is similar to that of the normal E-region: typically 10-20 km. As the event evolves, it slowly decreases until the pattern finally resembles a conventional sporadic E- (Es-) trace. The maximum plasma frequency of the F-layer, j~F2, remains relatively unaffected throughout the event. The maximum plasma frequency of the intermediate layer,fi~lL, is constant at about 1 MHz. The layer normally lasts for about one hour then disappears rapidly between successive quarter-hourly soundings of the ionosondes. From the observational data, it is not possible to determine whether the disappearance of the layer is due to recombination or redistribution of ionization. The pattern described above has occasionally been repeated several times during the same night. In these cases, h’F rose between two events and each new event was preceded by the characteristic downward movement of the F-layer. Most of the features shown in Fig. 1 are seen on all events ; the main differences between events being the time of occurrence and the rate of downward movement of the layer. The probability of forming an
7 250 1
intermediate layer is very low when the minimum virtual height of the F2-layer is above 220 km, but rises rapidly as the height falls below this level. The very few occasions where an intermediate layer has been formed when h’F‘ is above 220 km occur during magnetically disturbed periods. The seasonal variation in the occurrence of the nocturnal intermediate layer, i.e. the percentage of nights on which at least one layer is observed, is shown in Fig. 2. The occurrence of the layer is most totally restricted to austral winter months with a maximum in June and July when a layer is observed on more than 20 nights in each month. The layer was seen marginally more frequently in 1977 than in 1976, but this difference is not regarded as significant. The diurnal variation of the occurrence of the intermediate layer (Fig. 3) shows that it is most likely to be observed between 1930 (i.e. from the analysis start time) and 2200 local zone time (LZT). Very few examples are observed after 2300 LZT. This diurnal pattern was remarkably constant in all the winter months analysed. There appears to be no significant correlation between the occurrence of the intermediate layer in winter and magnetic activity (Fig. 4). This conclusion
,h’F
\\/-
1
folL
I
,
1
1
-60
0
sequence
for a typical intermediate
1
60
MINUTES Fig. 1. Schematic
layer event observed
at South Georgia.
1047
The nocturnal intermediate layer over South Georgia
x
1976
F
J
M
A
M
J
J
A
s
N
0
Fig. 2. The seasonal variation in occurrence of the intermediate layer at South Georgia for JanuaryNovember 1976 and April-September 1977.
must be treated with some caution, however, because the data samples at high and low levels of magnetic activity are small. The K index used to construct Fig. 4 was the mean value of K for the period 190&0100 LZT determined at the local magnetic observatory at South Georgia. No better correlation was found by using either Kp or the local K indices up to 12 h prior to the occurrence of the intermediate layer. Also, none of the local magnetometers (La Cour, fluxgate and
0
19
01
22 LOCAL
Fig. 3. Thediurnal
variation
rubidium vapour) show any consistent magnetic signatures before, during, or after the periods in which the intermediate layer is observed. The lack of correlation between the occurrence of the intermediate layer and magnetic activity at South Georgia is contrary to resuhs from all other observatories (see Section 2). The nocturnal intermediate layer is observed much more frequently at South Georgia than at the other
ZONE
04
TIME
in occurrence ofthe intermediate layer at South Georgia for 1976 and 1977
A. S. RODGER,
P. H. FITZGERALD and S. M. BROOM
. 0 0
*
.
0
0
.
0
0 ~10
Data
points
l
Data
points
>lO
I
I
I
I
I
I
0
1
2
3
4
5
MAGNETIC
INDEX
K
Fig. 4. The variation in occurrence of the intermediate layer at South Georgia as a function of magnetic activity, K. See the text for a definition of K.
localities reported in Section 2. To some extent, this results from the natural advantages of the South Georgia site for these studies. At most mid-latitude observatories, there are considerable difficulties in interpreting ionograms below 1.6 MHz at night because of strong medium-wave broadcast interference. The South Georgia observatory is encircled by mountains which effectively screen the aerial array from most broadcast interference. Also, such signals must travel over 3500 km to reach South Georgia and are, therefore, comparatively weak. Thus, the signal-tonoise ratio of the ionosonde system is high in the frequency range in which the intermediate layer is observed. The properties of the nocturnal intermediate layer at South Georgia are summarized in Table 3. These show similarities to those seen at other localities (see Table 2). For example, the rate at which the layer descends is remarkably similar to observations made at Arecibo ;
and maximum electron concentration observed at Equatorial latitudes, European/American midlatitudes and South Georgia are the same. However, the most significant difference is the relationship between magnetic activity and the occurrence of the intermediate layer.
4. DISCUSSION
Any mechanism which explains the formation of the nocturnal intermediate layer at South Georgia must account for both the observed increase in electron concentration in the intermediate region and for the regular height and thickness variations described in Section 3. In principle, the drizzle precipitation of charged particles at magnetic mid-latitudes (references in Section 2) could be responsible for the additional ionization in the intermediate region, especially as
Table 3. Summary of the morphology of the nocturnal intermediate layer at South Georgia Seasonal occurrence : winter only Rate of downward
movement
of the layer: 15 m s-r near 180 km
Diurnal occurrence : 193&2300 LZT
Maximum electron concentration: 10” mm3
Number of events per night : Lifetime of layer: about 1 h usually 1, but up to 4
Height range : 180-130 km Magnetic correlation : probably none
The nocturnal
intermediate
should be comparatively greater near South Georgia (TORR et al., 1975 ; MASSAMBINI,1978) as it lies within the South Atlantic Geomagnetic Anomaly. However, no other signatures of particle precipitation such as aurora1 Es (PIGGOTT, 1975) are observed on ionograms when the intermediate layer is observed. The remarkably consistent behaviour of the F-layer immediately prior to the occurrence of the intermediate layer strongly suggests that the ionization enhancements observed in the intermediate region result from downward transport from the F-layer. Both theory (HONG and LINDZEN, 1976) and observations (AMAYENCand REDDY, 1972; AMAYENC,1974; HARPER,1979) have shown that the 2,2 mode of the solar semi-diurnal tide can be effective in redistributing ionization in the intermediate region above about 130 km and in the lower F-region (see Section 2). This tidal mechanism can account for many, but not all, of the observations at South Georgia. For example, the time of occurrence of the intermediate layer is almost totally restricted to the period 1930 (the start time of the analysis) to 2300 LZT. This is consistent with the theory of FUJITAKA and TOHMAXSU (1973) for a mid-latitude station in the southern hemisphere. The intermediate layer is observed much more frequently in the first half of this period. This may be expected: we have determined from the model of KOHL and KING (1967) that the effect of the upward redistribution of ionization in the intermediate region by the diurnal thermospheric wind (RISHBETH,1972) is comparatively small, near 2000, but becomes increasingly important towards midnight. Also, towards midnight. the height of the F-layer rises above the height where the semi-diurnal tide has a downward phase velocity; this height has been determined to be between 225 km (AMAYENC, 1974) and 300 km (HARPER, 1979). Thus, there is little ionization available to be transported into the intermediate region near midnight. However, if a layer were formed at this time, then it is likely to be of low electron concentration and not observable by the South Georgia ionosonde. The semi-diurnal tidal mechanism cannot account for the intermediate layers observed after midnight. These are comparatively rare and it is not possible to determine whether these layers exhibit different features from those occurring before midnight. The intermediate layer is most often observed about mid-winter (June/July). This is to be expected because production and recombination will dominate the electron concentration distribution with height in the intermediate region at other seasons. Also, AMAYENC (1974) showed that the amplitude of the semi-diurnal precipitation
layer over South Georgia
I049
tide in winter was about twice as large as that in summer. There is an asymmetry in the seasonal variation in the occurrence of the intermediate layer about mid-winter which cannot be adequately explained. During periods of enhanced magnetic activity, an increased semi-diurnal temperature oscillation is expected (CHAPMAN and LINDZEN, 1970). Our simplistic approach suggests that the effects of the consequent increased driving force will at least be partly offset by increased ion-drag due to increases in electron concentration in the intermediate region (SMUH e( al., 1974) resulting in relatively small variations in occurrence of the intermediate layer under differing magnetic conditions. The observations from South Georgia are in agreement with this idea, but differ from those from all other localities. This is one aspect of semi-diurnal tidal theory requiring further study. The average rate of descent of the intermediate layer at South Georgia is 15 m s- ’ near 180 km. This is consistent with the theory for a 2,2 mode (FUJITAKA and TOHMATSU,1973). The average wavelength of the tidal mode observed at South Georgia, determined from the period and vertical phase velocity, is more than 100 km in the height range 25(&l SOkm. This, too, is in agreement with the observations of AMAYENC (1974). Both theory and observation show that higherorder tidal modes are more important in the transport of ionization below about 130 km (BERNARD, 1974; SALAH et ul., 1975 ; LINDZEN, 1976; HONG and LINDZEN, 1976). The height to which the layer descends at South Georgia may be related to the height where there is a change in the dominant tidal mode. This, too, may account for the rapid disappearance of the intermediate layer near this height. The maximum electron concentration of the intermediate layer observed at South Georgia is about 1 MHz. This value is similar to that determined by HARPER (1977) using a simple model in which he assumed that that convergence of ionization is balanced by recombination. Some of the weaknesses in the present semi-diurnal tidal mechanism for the formation of the intermediate layer at South Georgia have already been indicated. There are other features which cannot be adequately explained, such at the variability from one intermediate layer event to the next in both the time of occurrence of the layer and the rate of descent of the layer. Also, the occurrence of multiple intermediate layers on the same night cannot be accommodated by the present theory. We suggest that gravity waves in the thermosphere may contribute significantly to the redistribution of ionization on these nights. Variations of .fi)F2 about a smoothed mean show that gravity
A. S. RODGEK, P. H. FITZGERALDand S. M. BROOM
IO50
waves with the necessary common at South Georgia
(iii) Initially the intermediate layer is thick (- 20 km), but gradually thins till it resembles an Es layer.
period (1-3 h) are fairly in all seasons.
5. CONCLUSIONS
Ionosonde observations at South Georgia from an epoch near sunspot minimum have shown that a nocturnal intermediate layer is frequently seen between 1930 and 2300 LZT in winter over the height range 180-130 km. At other times, and during other seasons, nocturnal intermediate layers are rarely seen. The observations at this southern hemisphere midlatitude station show similarities to those from other areas in the world. The main features which characterize the intermediate layer event at South Georgia are : (i) The downward movement of the F-layer prior to the formation of the intermediate layer; (ii) The continued downward movement of the intermediate layer, initially at the same rate as the Flayer, but slowing and eventually stopping at between 150 and 130 km ; and that
The principal features of the diurnal and seasonal variations in the occurrence of the intermediate layer can be adequately explained by the theory of convergence of ionization driven by solar semi-diurnal tidal winds. Some limitations to this mechanism have been identified as it cannot account for all the observed features. Particular advantages of the South Georgia site allow the intermediate layer to be observed much more frequently by ionosonde than elsewhere. Thus South Georgia data should be valuable in testing the theories of semi-diurnal tides. For example, our analysis has shown the need to examine early morning data to determine what effects the other cycle of the semidiurnal tide produces. Also, variations in the occurrence of the intermediate layer through a solar cycle would provide observational evidence to test tidal theories such as those of GARRET-~ and FORES
(1978).
REFERENCES AMAYENCP. AMAYENCP. and REDDYC. A. BERNARDR. CHAPMANS. and LINDZENR. S. CONSTANTINIDES E. and BEDINGERJ. F. FRENCHA. G. FUJITAKAK. FUJITAKAK. and TOHMATSU T. GARRETTJ. M. and FORBE~H. B. HARPERR. M. HARPERR. M. HONGS. S. and LINDZENR. S. KOHL H. and KING J. W. LINDZENR. A. MAEHLUMB. MANXINA. H. MANXINA. H. and MERRY M. W. J. MA~AMBANIP. PIGC;OTTW. R.
RISHBETHH. ROWEJ. F. SALAHJ. E., WANDR. H. and EVANSJ. R. SHENJ. S., SWARTZW. E. and FARLEVD. T. SMITHL. G. SMITHL. G., GELLERM. A. and Voss J. D. TITHERIDGE J. R. TITHERIDGE J. R. TITHERIDGE J. R. TORRD. G., TORRM. R., WALKERJ. C. G. and HOFFMAN R. A. WAKAIN. WAKAIN. WAKAIN. and SAWADAK. WATTSJ. M. and BROWNJ. N.
1974 1972 1974 1970 1971 1969 1974 1973 1978 1977 1979 1976 1967 1976 1967 1971 1970 1978 1975
1972 1973 1975 1976 1970 1974 1959a 1959b 1975 1975 1967 1968 1964 1954
Rndio Sci. 9, 28 1. Planet. Space Sci. 20, 1269. Radio Sci. 9, 295. Atmospheric Tides D. Reidel, Dordrecht. J. atmos. tert. Phys. 33,461. .I. atmos. terr. Phys. 31, 1435. J. atmos. terr. Phys. 36, 1883. .I. atmos terr. Phys. 35,425. J. atmos. terr. Phys. 40, 657. J. geophys. Res. 82, 3243. J. geophys. Res. 84,411. J. atmos. Sci. 33, 135. J. atmos. terr. Phys. 29, 1045. J. geophys. Res. 81, 2923. J. geophys. Res. 72,2287. Planet. Space Sci. 19, 270. J. atmos. terr. Phys. 32, 1169. J. atmos. terr. Phys. 40, 1143. High latitude supplement to the URSI Handbook of Ionogram Interpretation and Reduction Z/AC-50 World Data Center A, Boulder, Colorado, U.S.A. J. atmos. terr. Phys. 34, 1. J. geophys. Res. 78, 681 I. Radio Sci. 10, 347. J. geophys. Res. 81, 5517. J. atmos. terr. Phys. 32, 1247. J. atmos. terr. Phys. 36, 1601. J. atmos. terr. Phys. 17, 96. J. atmos. terr. Phys. 17, 110. J. atmos. terr. Phys. 37, 1517. Planet. Space Sci. 23, 15. J. geophys. Res. 72,4507. J. Radio Rex Labs 15, 109. J. Radio. Res. Labs 11, 1. J. geophys. Res. 59, 71.