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The maintenance of the night-time ionosphere at mid-latitudes. I. The ionosphere above Malvern
002i 9169/8416300+ 00 t’, 1984 Pcrgamon PressLtd.
The maintenance of the night-time ionosphere at mid-latitudes. I. The ionosphere above Malvern P. J. Department
of Physics,
STANDLEY*
University
and
P. J. S. WrLLrAMst
College of Wales, (Adran Aberystwyth, U.K.
(Received injindform
Fftseg, Coleg Prifysgol
Cymru),
24 June 1983)
Abstract-The total rate of recombination in the night-time ionosphere above Malvern (at L = 2.6) was estimated using a model atmosphere, and the results were compared with the observed rate ofchange of total electron content to determine the net influx of plasma. Horizontal transport under the influence of electric fields was an important factor on a time-scale of an hour or less but when averaged throughout the night made little contribution. The main influx of plasma was a downward diffusion from the protonosphere, especially before midnight. The average downward flux increased steadily as the protonosphere filled after a magnetic storm, with a saturation time of at least 8 days.
(c) Plasma which escapes upwards during the day can diffuse back into the ionosphere as it cools during the night. The full process involves charge exchange between oxygen and hydrogen, so that the downward flux at night depends on the total content of the protonosphere. During magnetic storms the protonosphere is emptied and for a short period after such a storm the protonosphere contmues to fill day and night. Later, however, the protonosphere approaches saturation and thereafter acts as a reservoir of plasma which receives plasma from the ionosphere during the day and replenishes the ionosphere at night. The present paper examines these three factors in turnusingdatatakenatRSRE,Malvern,U.K.(52,1’N; 2.3”W). In a subsequent paper (JAIN and WILLIAMS, 1984) data taken above St. Santin. France is examined.
INTRODUCTION
theory would suggest that after sunset, when the thermosphere is no longer exposed to radiation from the sun, the total electron content of the ionosphere would decay steadily as recombination occurs. For many years, however, it has been recognised that the overalldropin totalcontent throughout the night is less than simple recombination theory would predict, and the decay of the ionosphere is frequently interrupted by periods when the total content appears to be constant or to increase (see, for example, JAIN et al., 1973). In the absence of any fresh ionisation, the continuity equation for N, the electron density in the ionosphere, can be written Simple
dN/& = -I - div (NV) where 1is the rate of recombination per unit volume and v is the plasma velocity. From this equation we can identify three factors that are able to contribute to the maintenance of the midlatitude ionosphere through the night. (a) After sunset, and especially during a substorm, the ionosphere rises in altitude and the corresponding density of the neutral atmosphere drops, so that the rate of recombination is reduced. (b) Electric fields cause a horizontal transport of plasma in the ionosphere, so that if there are horizontal gradients in electron density there is a net influx of plasma equal to -(a(Nc,) * Present address: London, U.K. t To whom reprint
OBSERVATIONS
The data were taken by the Malvern incoherent scatter radar, operating in the monostatic mode at 400.5 MHz (WILLIAMSand TAYLOK, 1974). The results were available as profiles ofelectron density N, electron temperature r, and ion temperature 7; taken at intervals of about 5 min for the altitude range 100-800 km. Whenever possible the system operated for 36 h each month, including one overnight run. Naturally, on some occasions the observations were incomplete. and during summer the observations were unsuitable for this study as the periods when the thermosphere was in total darkness were too short. As a result. eleven nights of data from 17 September 1968 to 6 November 1970 were available for analysis. For each night the electron density profiles were
;u+d(Nc,)/&).
London
Bible College.
Northwood.
requests should be addressed. 73
74
P. J. STANULCY
and
P. J. S. WILLIAMS
rose steadily until 0200 UT and 5 remained fairly constant throughout. This pattern was also observed on 30-3 1 October 1970. On other occasions, the decline in total content continued for a few hours after sunset but then stopped and N,, hmF2 and T remained more-or-less constant for several hours. Figure l(b) sho\vs that on the night of IS-16 October 1968, N, dropped steadily from 1900 UT to 0000 UT, but then remained constant until 0330 UT. Similarly hmF2 rose steadily until 0000 UT and then remained constant for about 3 h. A similar pattern was observed on 9--10 September 1969 and on 2&21 January 1970. Finally there were occasions when the total content ofthe ionosphere actually increased sharply during the middle of the night. Figure Ire) summarises the behaviour of the ionosphere on the night of 6-7 November 1970 when a rapid decay of the ionosphere was abruptly halted at 2230 UT. after which the totai
plotted. N,--the totai electron content of the ionosphere between 100 and 800 km-was summed in each case and near the peak of the F-region a “leastsquares” method was used to fit a parabola to the electron density profile and hence determine the peak electrondensity NmF2and theheightofthepeakhmF2. r-the equivalent slab thickness of the ionospherewas then derived by dividing the total content by the peak electron density (T = N,jNmF2). N,. hmF2 and r were plotted against time for each night. Figures l(a) to I(c) show results for three nights typical of the different patterns of behaviour observed. On a few occasions, when magnetic conditions were very quiet, the ionosphere did decay steadily through the night. For example, on the night of 17-18 September 1968 there was little trace of substorm activity and the average aurora1 electrojet index (AE) between 2130 UT and 0230 UT was 36. Figure I(a) shows how during this night N, droppedsteadily,hmFZ
17 -18 20 -
.
Sept
1968
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UT
I:‘&_l(a). The hehaviour of the ionosphere on a magneticaIly quiet night, showing the varldlion rrf totni electron content N,, height of the F-region peak hnz and the equwdent slab rhickns;\ 7
Maintenance of the night-time ionospherefollowing.
content more than doubled to reach a maximum value at 0110 UT. A similar increase in total content was observed on both 12-13 November 1968 and 28-29 January 1969. On all eleven nights the drop in N, throughout the night was less than would be predicted by simple recombination theory. To explain this the three factors listed in the introduction were considered in turn.
RECOMBINATION OF THE
0’
+o> so;
e-+0;
= k, Xf = k, = (L2
+o
=0+-O
,For recombination in this way, theoverail of free electrons is given by
rate ofloss
I = --.-_ r,cr,(k,CN,l+k,CO,I)N* ~,k$&l +~,k,IINJ +~,J,N
LAYER
the height
work, ion exchange
380
rate coefficient
e-+NO-=N+O
AND THE RAISING
15 - 16
NO’ -t N
0+ +N,=
range considered in the present followed by dissociative recombination is the most important mechanism of recombination, with the molecu~arconstituents N2 and O2 playing a major role. The stages involved are the Over
75
1
Ott
where [N2] and [O,] are the number densities of molecular nitrogen and oxygen. respectively. This formula was used in MI for all calculations, but in the F-region r,:Y >> k,[N,] and r,h- B k,[O,J so that the way in which recombination depends on [N2],
1968
r
Fig. l(b). The behaviour of the ionosphere on a magneucally dtsturbed night.
P. J.
76
[O,j
STANDLEY
and P. J. S. WILLIAMS
k, = 2.82 x lo- l7 -7.74 x lo- ~s(~~3~)
and N is best shown in the approximation
f 1.073 x io-‘s(T/300)*
f = (k,EN,I + MO,lW.
-5.17 x 10-z0(Ti300)3
Various estimates have been made of the rate coefficients and the way in which they vary with temperature. In the present work we have used the valuessummarised by TORR and TORR (1979) as follows
+9.65 x 10-Z2(T~300)4m” s-l for
For the night-time ionosphere we have assumed that temperature and the ion temperature were measured by the incoherent scatter radar, but the errors were large f- 300 K) and in the present work we used the neutral temperature T predicted by a model atmosphere.
k, = 1.533x 10-18-_;.92 x 10-‘9(T/300)
T, = K = I: The electron
+8.6x 10~~~“(~/3~)*m3s~~1 for
300 < T<
17OOK:
IO 9= 8
-’
7-
..*
b-7
Nov
1970
f
6”. 5-
-.
.
4.* 3-
-.
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*
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*
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. I
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. . 200
-
2000
300 -C Tc6OOOK.
I
i
I
I
I
2100
2200
2300
00
OIOO
1 0200
I 0300
UT
Fig. l(c). The behaviour of the ionosphere on an extremely disturbed night.
Maintenance
of the night-time
The model atmospherealso provided values for [NJ and [Or] which were then used in the calculation of recombination. Various model atmospheres were available, but we have used MSIS 79 (HEDIN et al., 1977a, 1977b, 1979). Using this model to determine the profiles of 1T;[NJ and [0,] and the incoherent scatter radar to provide the profiles of N, we were able to calculate /,-the total rate of recombination in the height range 10&800 km-for each profile of electron density. The average value of I, over a period At was then compared with the rate of change in total electron content for the same height range and over the same period to determine F-the net flux of plasma into the ionosphere above Malvern-where
II
ionosphere-I
. 0
I 100 Auroral
200 elecirqet
300
400
Index
500
AE
Fig. 2. Rate of increase of hmF2 for different values of AE.
F = ANJAt + 1,. In calculating I, it was clear that the most important factors were the concentrations of molecular nitrogen and oxygen. In the F-region these had scale heights of about 25-30 km, so that any increase in the height ofthe F-region of this order significantly reduced the rate of recombination. On all eleven nights the peak of the F-region rose in height after sunset, and the average increase was just over 80 km. In the absence of substorms this increase was partly due to a southward neutral wind driving the plasma upward along the magnetic field line. Models of the neutral wind in the F-region suggest that the raising of the layer would be a gentle process lasting for 6 or 7 h, and this indeed occurred on 17-18 September 1968 and on 3G-31 October 1970. On both occasions hmF2 increased steadily by about 10 km/h. On the nine other nights, when there was substorm activity, the rise in the layer was more sudden. On 67 November 1970, for example, hmF2 increased by 125 km in just over 4 h--an average increase of 30 km/h. This suggests that the substorms were associated with an enhanced southward neutral wind or with strong west-to-east electric fields which drove the plasma sharply northwards and upwards. The relationship between magnetic activity and the raising of the layer is summarised in Fig. 2, where the average rate of increase in hmF2 is plotted against the mean aurora1 electrojet index(AE). The raising of the layer certainly caused a reduction in the rate of decay of the ionosphere, but it did not explain the long periods when no decay was observed. Neither did it explain the periods when the total content actually increased. By calculating the total recombination for each hour in turn and comparing this with the change in total content over the same hour, we calculated the net influx ofplasma necessary to balance the continuity equation and so maintain the ionosphere through the night. The results are listed in Table 1 for
the period from 2130 UT to 0230 UT on each night. In every case, taking the average of the 5 h, there was a net influx of plasma ranging from 0.2 x 10’ 2 m _ 2 s- ’ to 4.9x lOi* mm2 s-l. However, the results from hour to hour showed that the net flux into the ionosphere fluctuated from positive to negative and back. These sudden fluctuations were almost certainly due to the second factor controlling the night-time ionosphere.
ELECTRIC
FIELDS
AND HORIZONTAL
TRANSPORT
This second factor was the horizontal transport of plasma under the influence of electric fields. When an electric field is applied to the plasma in the F-region, the first response is for the plasma to move perpendicular to the magnetic field with a velocity v equal to E x B/B2 where E is the electric field and B is the magnetic induction. This velocity has a vertical and a horizontal component, but after a time the effects of diffusion and induced ion drag counter the vertical component and only the horizontal component can be sustained. Evidence that electric fields were indeed driving the night-time ionosphere came from a close examination of the electron density profiles during the period when the steady decay of the ionosphere came to a sudden end. This always occurred abruptly and it was usually possible to determine the exact moment ofchange from decay to maintenance within a minute or so. Moreover, the change always occurred at precisely the same time at all heights. Electric fields associated with substorms would explain this behaviour. These fields are known to change abruptly and the effect would certainly occur simultaneously at all heights. A west-to-east electric field would move plasma northwards, and hence the plasma above Malvern would be replaced by plasma from further south. As there is usually a considerable
P. J.
78
STANDLEY
and P. J. S.
K’ILLIAMS
Table 1. Average flux into the ionosphere above Malvern (to” rn--‘s- ‘) Date at beginning of night -___. 17-09-68 1S- 1O-68 12-11-68 28-01-69 26-08-69
____-_2200
Universal time _.__~~_.__ 0100 2300 O@Xl
-
___~ 0200
Mean
S.D.
it:,
0.36 1.82 0.22 0.12 0.18
0.76 1.54 0.70 0.25 3.37
40 54 29 64 41s
1.68 0.76 3.00 3.50 0.59 4.48
242 67 166 104 40 341
2.4 3.2 1.3 0.4 5.0
0.6 0.9 1.3 0.4 3.2
-0.4 1.7 -0.3 0.4 0.0
-0.1 1.8 -0.5
-4.2
-0.7 1.5 -0.7 -0.5 _ 3.1
09-09-69 20-Ol-70 17-02-70 17-03-70 30- 10-70 06-l l-70
4.4 1.7 7.6 10.8 4.7 -0.8
-0.6 1.2 1.2 2.8 1.6 3.2
0.6 1.5 4.8 -0.1 0.5 3.2
0.9 -0.1 4.3 3.5 0.9 4.3
-0.7 -0.9 0.7 4.2 0.1 - 8.2
1.32 0.56 4.92 4.24 1.56 0.34
Mean (SD.)
3.9 (1.0)
2.0 (0.6)
1.1 10.5)
0.9
-0.7 (0.9)
1.44
north-south gradient in total content, this would be equivalent to a net input of flux. Similarly a north-tosouth electric field would move the plasma eastwards and during the earlier part of the night this would also lead to a net input of flux. At the same time we must remember that east-to-west and south-to-north electric fields wouid have the opposite effect, deplenishing the ionosphere. The north-south gradient in total content was estimated using values of the F-region critical frequency foF2 measured by a north-south chain of ionosondes (see JAIN and WILLIAMS, 1984). The values measured at each station were interpolated to give values at the same local time, and a regression analysis between NmF2 and latitude yielded dNmFZj& This was then multiplied by the equivalent slab thickness measured at the same local time at Malvern to give an estimate of the north-south gradient of total content a N,/?.Y. The averageeast-west gradient was derived from the time variation of N, above Maivern. assuming that the variation of IV, on a given day was a function oflatitude and local time rather than a function of UT. This is a reasonable assumption at mid-latitude. The results are listed in Table 2. It is clear that, throughout the night there is a strong north-south gradient in total content with an average value of 2.4 x IO’” m -2 m ~ I, so that a northwards velocity of about 40 m s-l would cause an equivalent flux input of t x IO’* m-* s- ’ and so make a significant contribution to the observed changes in total electron content. The east-west gradient is much smaller, with an average value of zero, and only before midnight would an eastward velocity make any significant contribution to the maintenance of the ionosphere.
-0.1
(0.8)
tinfortunately the monostatic radar at Malvern did not measure plasma velocity. On other occasions, however. a multistatic radar was used and this frequentlymeasurednorth-southvelocitiesof40ms-’ or more (THOMAS and WILLIAMS, 1975); velocities of this magnitude would certainly make a significant contribution to the observed changes in total content. North-south velocities at Mafvern are driven by eastwest electric fields which are highly correlated with aurora1 activity (GOEL and JAW 1980). It foflows that if fluctuations in the influx of plasmaare indeed the resuh ofelectric fields we wouldexpect such fluctuations to be linked with aurora1 activity. To test this, the influx of plasma was first averaged over all eleven nights to give a reference variation with time. For each night in turn the reference was scaled to match the total influx for that particular night and this was then compared with the hourly values actually observed to determine a standard deviation as a measure of the fluctuations. When the total flux was plotted against (AE) for each night there appeared to be no correlation, but when the standard deviation was plotted against (AE) there was a correlation of 76”:s (Fig. 3). In other words, the total flux into the ionosphere did not appear to depend on aurora1 electric fields but on a quiet night the
Table 2. Average gradient in the total electron content (l()‘Omw~Lm+‘)
_---.North-south East-west
Universal time 2200 2300 0000 OlW 0200 Mean ________ 02.3 01.7 02.5 02.6 02.7 02.4 00.1 00.3 -0.1 -0 I -0.1 00.0
Maintenance
Average
auroral
Fig. 3. Standard dewation ionosphere from the scaled
electrqet
of the night-time
Index
of the observed flux into the reference tlux as a function of (A.%.
influx varied smoothly with time while on a disturbed night the flux varied considerably from hour to hour. The evidence suggests, therefore, that electric fields affected the behaviour of the ionosphere on a time scale of an hour or so, but they did not explain the maintenance of the ionosphere through the night. For this we must turn to the third factor.
DOWNWARD
FLOW OF PL4ShlA
The transfer protonosphere
of plasma between the ionosphere and was first proposed by HANSON and ORTENKIRGER (1961). A mathematical model of the coupling between the F-region and the protonosphere was published by MWFETT and ML~RIWY(1973) and this model was developed in a series of subsequent papers (e.g. MURPHY et ul., 1976, 1980; BAILEYet al., 1977, 1978,1979 ; MURPHY and MOFFETT, 1978). The full process involves charge exchange between oxygen and hydrogen O++HzO+H+. During the day oxygen ions are produced in the F-region and when these diffuse upwards they expertence charge exchange with hydrogen atoms to generate protons which enter the protonosphere. During a magnetic storm, the protonosphere is sudden]) emptied and for a short period after such a storm the protonosphere continues to fill day and night. However, when the content of the protonosphere has increased sufficiently the process is reversed at night. After sunset the ionosphere cools, recombination takes place and oxygen ions ditTuse downwards. This changes the balance of the charge-exchange equation at the
ionosphere-
I
79
transition height and protons experience chargeexchange with oxygen atoms to generate more oxygen ions. This in turn causes a downward diffusion of protons. In this way charge-exchange provides a ioose coupling between the oxygen and hydrogen populations. The downward flux of oxygen ions at 800 km is not wholly correlated with the protonospheric Rux ; it has been shown, for example, that on occasions counter-streaming can occur with oxygen ions moving downward and protons upward (BAILEY ct ul.. 1977). Nevertheless. for the night as a whole the tlux of oxygen ions downwards into the height range measured in the present paper (l@&SOO km) corresponds closely to the llux from the protonosphere. Theory suggests that the downward flux should be greatest in the period between sunset and midnight, dropping to a low level after midnight (MURPHY et nl., 1976 ; BAILEY et al., 1979). This prediction is confirmed by the results given in Table 1, where the mean flux into the ionosphere over all eleven nights is listed for each hour between 2130 UT and 0230 UT. The same theory predicts that the protonospheric flux at night increases steadily as time elapses after a major storm, until either the protonosphere is saturated or else there is another magnetic storm. The Saturation time depends on the total volume of the protonospheric reservoir and hence increases sharply with magnetic latitude. KRINRERG and TASHCHILJN (1982) have predicted that the saturation timr r, - O.! 7 I.:’ days where L is rhe McIlwain L-parameter. .\t Malvern I, = 2.6 so that I, is predicted to be about 8 days. Detailed accounts of the recovery of the protonosphere after a geomagnetic storm have been published by PARK (1974), KERSLEY and KLWCJCH.XR (198o)and Po[!tni~ CI a/.(19811. We therefore plotted the mean Rux mto the ionosphere between 2130 UT and 0230 lJT against the time elapsed since the previous magnetic storm. The mean fluxes are listed in Table 1 and the errors for these fluxes correspond to two sources of uncertainty : firstly the fluctuations which we have assosiated with electric fields and secondly the limitations In the MSIS 79 model of the neutral atmosphere. The uncertainty due to the electric tieids can be derived from the measured standard deviation in the hourly values of flux. The uncertainty in the MSIS 79 model can only be estimated. The errors quoted for the \,arious parameters in the model are the errors in mean value and do not represent day-to-day variations. HF~XS c’t CL/.(1977a) do suggest a 154, error in the concentration of molecular nitrogen. the major constituent. Molecular oxygen is a minor constituent and therefore the error in concentration is likely to be
P. J.
80
STANDLEY
and
larger; a comparison of MSIS 77 with rocket measurements would suggest an error of about 25% (HEDIN et al., 1977b). The time elapsed since the previous magnetic storm is defined as follows. (a) The magnetic storms listed in the NOAA Prompt Reports of Solar-Geophysical Data were examined. These storms vary considerably in size and duration but all the storms listed were recognised provided they were recorded by at least two stations and had a Kp index greater than 3. (b) For each night of observation the previous magnetic storm was identified and the period following the storm carefully examined to determine the number of quiet days between the end of the storm and the night when observations were made. The criterion adopted counted all days when the sum of Kp was less than 24. The results are shown in Fig 4. Regression analysis was used to plot the best straight line to thedata and the result gave qualitative agreement with the theoretical model of MURPHY et al. For the first night after a magnetic storm there was a net flux out of the ionosphere, but as the protonosphere was filled the average downward flux increased steadily and there was a correlation coefficient of 83% between the mean flux into the ionosphere and the number ofdayselapsed since the last magnetic storm. The quantitative agreement is also reasonable. The
Murphy, Baiky
and
1
3 R E
I
5
2
4 N E ra s 0 5 s
t : z i r
15 Number of
quiet days fence last mognet!c storm Theory ~ Observations
-2
Fig. 4. Mean flux into the inosphere (2130 UT-0230 UT) as a function of time since the preceding magnetic storm.
P. J. S.
WILLIAMS
theoretical model predicts that for an L-value of 3.2 the downward flux between 2130 UT and 0230 UT would increase on average by 0.56 x 10” mm2 s-’ per day. The observations showed that at an L-value of 2.6 the average downward flux increased by 0.34x lo’* mm2 s-l per day, but we must remember that a fair comparison would reduce the flux predicted by the model to allow for the lower L-value above Malvern. What the results certainly show is that the protonospheric flux increased steadily with time that had elapsed since the previous magnetic storm and that at an L-value of 2.6 at least 8 days were required for saturation of the protonosphere.
CONCLUSIONS
We can summarise the behaviour ionosphere at night as follows.
of the mid-latitude
(a) After sunset the F-region peak rose by an average of 80 km and as a result the rate of recombination was reduced. During magnetically disturbed conditions the layer rose more rapidly. (b) During disturbed conditions electric fields were also responsible for moving the plasma horizontally and in the presence of strong horizontal gradients in total electron content this was equivalent to a net flux of plasma into-or out of-the ionosphere above a given point. The correlation between rapid changes in the net influx of plasma and the aurora1 electrojet index suggests that such electric fields dominated the behaviour of the ionosphere on an hour-to-hour time scale but did not contribute a net input for the night as a whole. (c)The main process responsible for the maintenance of the ionosphere through the night was downward diffusion from the protonosphere. After a strong magnetic storm the protonsophere was empty and there followed a period of refilling until either the protonosphere was full or there was another magnetic storm. As the total content of the protonosphere increased so did the downward flux into the ionosphere in the period before midnight. At an L-value of 2.6 the time for saturation of the protonosphere was at least 8 days.
AcknowledgemenrsThe authors would like to thank the Director of the Royal Signals and Radar Establishment for the data used in this paper. We would also like to thank Dr G. N. TAYLOR for his interest,and encouragement and Dr R. J. MOFFETT for valuable comments. One of us (PJS) was supported by an SERC Research Studentship during the period when much of this work was completed.
Maintenance
of the night-time
ionosphere
-I
REFERENCES
BAILEYG. J., MOFFE~~R. J. and MURPHYJ. A. BAILEYG. J., MOFFEIT R. J. and MURPHY J. A. BAILEYG. J., MOFFETTR. J. and MURPHY J. A. GOEL M. K. and JA~NA. R. HANSONW. B. and ORTENBURGER I. B. HEDINA. E., SALAHJ. E., EVANSJ. V., REBERC. A., NEWMANG. P., KAYSETD. C., ALCAYDED., BAUERP., C~CGER L. and MCCLURI~J. P. HEDINA. E., REBERC. A., NEWTONG. P., SPENCER N. W., BIUNTONH. C. and MAYR H. G. HEDINA. E., REBERC. A., SPENCERN. W. and BRINTONH. C. JAINA. R., TAYLORG. N. and WILLIAM P. J. S. JAIN4. R. and WILLIAMSP. J. S. KER~LEYL. and KL~BIJCHARJ. A. KRINBERG1. A. and TASHCHILIN A. V. MOFFE~ R. J. and MURPHYJ. A. MURPHYJ. A., BAILEYG. J. and MOFFETTR. J. MURPHYJ. A., BAILEYG. J. and Mom R. J. MURPHYJ. A. and Momrr R. J. PARK C. C. POULTERE. M., HARGREAVE~ J. K., BAILEYG. J. and MOFFETTR.J. THOMASD. P. and WILLIAMSP. J. S. TORR D. G. and TORR M. R. WILLIAMSP. J. S. and TAYLORG. N.
1977 1978 1979 1980 1961 1977a
J. atmos. terr. Phys. 39, 105. Planet. Space Sci. 26, 753. J. atmos. terr. Phys. 41,417. J. atmos. lerr. Phys. 42, 357. J. geophys. Res. 66, 1425. J. yeophys. Res. 82, 2 139.
1977b
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1973 1984 1980 1982 1973 1976 1980 1978 1974 198 1
J. atmos. terr. Phys. 35, 1717. J. atmos. terr. Phys. 46, (ATP 43). Planet. Space Sci: 28,453. Annls Gkophys. 38, 25. Planer. Space Sci. 21,43. J. atmos. terr. Phys. 38, 351. J. oeonhvs. Res. 85. 1979. Plnne;. Space Sci. k, 281. J. geophys. Res. 79, 169. Planet. Space Sci. 29, 1281
1975 1979 1974
J. atmos. terr. Phys. 37, 127 1. J. armos. terr. Phys. 41, 797. Radio Sci. 9, 85.
81
The maintenance of the night-time ionosphere at mid-latitudes. I. The ionosphere above Malvern P. J. Department
of Physics,
STANDLEY*
University
and
P. J. S. WrLLrAMst
College of Wales, (Adran Aberystwyth, U.K.
(Received injindform
Fftseg, Coleg Prifysgol
Cymru),
24 June 1983)
Abstract-The total rate of recombination in the night-time ionosphere above Malvern (at L = 2.6) was estimated using a model atmosphere, and the results were compared with the observed rate ofchange of total electron content to determine the net influx of plasma. Horizontal transport under the influence of electric fields was an important factor on a time-scale of an hour or less but when averaged throughout the night made little contribution. The main influx of plasma was a downward diffusion from the protonosphere, especially before midnight. The average downward flux increased steadily as the protonosphere filled after a magnetic storm, with a saturation time of at least 8 days.
(c) Plasma which escapes upwards during the day can diffuse back into the ionosphere as it cools during the night. The full process involves charge exchange between oxygen and hydrogen, so that the downward flux at night depends on the total content of the protonosphere. During magnetic storms the protonosphere is emptied and for a short period after such a storm the protonosphere contmues to fill day and night. Later, however, the protonosphere approaches saturation and thereafter acts as a reservoir of plasma which receives plasma from the ionosphere during the day and replenishes the ionosphere at night. The present paper examines these three factors in turnusingdatatakenatRSRE,Malvern,U.K.(52,1’N; 2.3”W). In a subsequent paper (JAIN and WILLIAMS, 1984) data taken above St. Santin. France is examined.
INTRODUCTION
theory would suggest that after sunset, when the thermosphere is no longer exposed to radiation from the sun, the total electron content of the ionosphere would decay steadily as recombination occurs. For many years, however, it has been recognised that the overalldropin totalcontent throughout the night is less than simple recombination theory would predict, and the decay of the ionosphere is frequently interrupted by periods when the total content appears to be constant or to increase (see, for example, JAIN et al., 1973). In the absence of any fresh ionisation, the continuity equation for N, the electron density in the ionosphere, can be written Simple
dN/& = -I - div (NV) where 1is the rate of recombination per unit volume and v is the plasma velocity. From this equation we can identify three factors that are able to contribute to the maintenance of the midlatitude ionosphere through the night. (a) After sunset, and especially during a substorm, the ionosphere rises in altitude and the corresponding density of the neutral atmosphere drops, so that the rate of recombination is reduced. (b) Electric fields cause a horizontal transport of plasma in the ionosphere, so that if there are horizontal gradients in electron density there is a net influx of plasma equal to -(a(Nc,) * Present address: London, U.K. t To whom reprint
OBSERVATIONS
The data were taken by the Malvern incoherent scatter radar, operating in the monostatic mode at 400.5 MHz (WILLIAMSand TAYLOK, 1974). The results were available as profiles ofelectron density N, electron temperature r, and ion temperature 7; taken at intervals of about 5 min for the altitude range 100-800 km. Whenever possible the system operated for 36 h each month, including one overnight run. Naturally, on some occasions the observations were incomplete. and during summer the observations were unsuitable for this study as the periods when the thermosphere was in total darkness were too short. As a result. eleven nights of data from 17 September 1968 to 6 November 1970 were available for analysis. For each night the electron density profiles were
;u+d(Nc,)/&).
London
Bible College.
Northwood.
requests should be addressed. 73
74
P. J. STANULCY
and
P. J. S. WILLIAMS
rose steadily until 0200 UT and 5 remained fairly constant throughout. This pattern was also observed on 30-3 1 October 1970. On other occasions, the decline in total content continued for a few hours after sunset but then stopped and N,, hmF2 and T remained more-or-less constant for several hours. Figure l(b) sho\vs that on the night of IS-16 October 1968, N, dropped steadily from 1900 UT to 0000 UT, but then remained constant until 0330 UT. Similarly hmF2 rose steadily until 0000 UT and then remained constant for about 3 h. A similar pattern was observed on 9--10 September 1969 and on 2&21 January 1970. Finally there were occasions when the total content ofthe ionosphere actually increased sharply during the middle of the night. Figure Ire) summarises the behaviour of the ionosphere on the night of 6-7 November 1970 when a rapid decay of the ionosphere was abruptly halted at 2230 UT. after which the totai
plotted. N,--the totai electron content of the ionosphere between 100 and 800 km-was summed in each case and near the peak of the F-region a “leastsquares” method was used to fit a parabola to the electron density profile and hence determine the peak electrondensity NmF2and theheightofthepeakhmF2. r-the equivalent slab thickness of the ionospherewas then derived by dividing the total content by the peak electron density (T = N,jNmF2). N,. hmF2 and r were plotted against time for each night. Figures l(a) to I(c) show results for three nights typical of the different patterns of behaviour observed. On a few occasions, when magnetic conditions were very quiet, the ionosphere did decay steadily through the night. For example, on the night of 17-18 September 1968 there was little trace of substorm activity and the average aurora1 electrojet index (AE) between 2130 UT and 0230 UT was 36. Figure I(a) shows how during this night N, droppedsteadily,hmFZ
17 -18 20 -
.
Sept
1968
I. .
.:
. .
. . . -.
G‘E
.
***. ...
to-
9-
El-
-... ..
0
-
z
7-
i
6-
-. -.
.
. .
.* *
*
.
5-
..
*.
4I
I
I
1
1
1
I
380 T E 2
340
E .c
320
I
I
*1.
‘.
I
.
,
.*
360
..
. -
300
280
‘;; E ” I-
260
r
.. *.. . ..
240
.
.. .
.
. .
....
..< .’
. .
200 J800
.
. :
:. .
. . ‘...
220 1 1900
I
2000
I
2100
I
2200
1 2300
I 00
I 0100
I
0200
.
* I
0300
.. I
G4C?
I 0500
UT
I:‘&_l(a). The hehaviour of the ionosphere on a magneticaIly quiet night, showing the varldlion rrf totni electron content N,, height of the F-region peak hnz and the equwdent slab rhickns;\ 7
Maintenance of the night-time ionospherefollowing.
content more than doubled to reach a maximum value at 0110 UT. A similar increase in total content was observed on both 12-13 November 1968 and 28-29 January 1969. On all eleven nights the drop in N, throughout the night was less than would be predicted by simple recombination theory. To explain this the three factors listed in the introduction were considered in turn.
RECOMBINATION OF THE
0’
+o> so;
e-+0;
= k, Xf = k, = (L2
+o
=0+-O
,For recombination in this way, theoverail of free electrons is given by
rate ofloss
I = --.-_ r,cr,(k,CN,l+k,CO,I)N* ~,k$&l +~,k,IINJ +~,J,N
LAYER
the height
work, ion exchange
380
rate coefficient
e-+NO-=N+O
AND THE RAISING
15 - 16
NO’ -t N
0+ +N,=
range considered in the present followed by dissociative recombination is the most important mechanism of recombination, with the molecu~arconstituents N2 and O2 playing a major role. The stages involved are the Over
75
1
Ott
where [N2] and [O,] are the number densities of molecular nitrogen and oxygen. respectively. This formula was used in MI for all calculations, but in the F-region r,:Y >> k,[N,] and r,h- B k,[O,J so that the way in which recombination depends on [N2],
1968
r
Fig. l(b). The behaviour of the ionosphere on a magneucally dtsturbed night.
P. J.
76
[O,j
STANDLEY
and P. J. S. WILLIAMS
k, = 2.82 x lo- l7 -7.74 x lo- ~s(~~3~)
and N is best shown in the approximation
f 1.073 x io-‘s(T/300)*
f = (k,EN,I + MO,lW.
-5.17 x 10-z0(Ti300)3
Various estimates have been made of the rate coefficients and the way in which they vary with temperature. In the present work we have used the valuessummarised by TORR and TORR (1979) as follows
+9.65 x 10-Z2(T~300)4m” s-l for
For the night-time ionosphere we have assumed that temperature and the ion temperature were measured by the incoherent scatter radar, but the errors were large f- 300 K) and in the present work we used the neutral temperature T predicted by a model atmosphere.
k, = 1.533x 10-18-_;.92 x 10-‘9(T/300)
T, = K = I: The electron
+8.6x 10~~~“(~/3~)*m3s~~1 for
300 < T<
17OOK:
IO 9= 8
-’
7-
..*
b-7
Nov
1970
f
6”. 5-
-.
.
4.* 3-
-.
. *. Z-
*
.
*
.
. I
I
I
I
I
I
I
.* -’
*
.
. ..
.
*.. .
.
0
I.
. . 200
-
2000
300 -C Tc6OOOK.
I
i
I
I
I
2100
2200
2300
00
OIOO
1 0200
I 0300
UT
Fig. l(c). The behaviour of the ionosphere on an extremely disturbed night.
Maintenance
of the night-time
The model atmospherealso provided values for [NJ and [Or] which were then used in the calculation of recombination. Various model atmospheres were available, but we have used MSIS 79 (HEDIN et al., 1977a, 1977b, 1979). Using this model to determine the profiles of 1T;[NJ and [0,] and the incoherent scatter radar to provide the profiles of N, we were able to calculate /,-the total rate of recombination in the height range 10&800 km-for each profile of electron density. The average value of I, over a period At was then compared with the rate of change in total electron content for the same height range and over the same period to determine F-the net flux of plasma into the ionosphere above Malvern-where
II
ionosphere-I
. 0
I 100 Auroral
200 elecirqet
300
400
Index
500
AE
Fig. 2. Rate of increase of hmF2 for different values of AE.
F = ANJAt + 1,. In calculating I, it was clear that the most important factors were the concentrations of molecular nitrogen and oxygen. In the F-region these had scale heights of about 25-30 km, so that any increase in the height ofthe F-region of this order significantly reduced the rate of recombination. On all eleven nights the peak of the F-region rose in height after sunset, and the average increase was just over 80 km. In the absence of substorms this increase was partly due to a southward neutral wind driving the plasma upward along the magnetic field line. Models of the neutral wind in the F-region suggest that the raising of the layer would be a gentle process lasting for 6 or 7 h, and this indeed occurred on 17-18 September 1968 and on 3G-31 October 1970. On both occasions hmF2 increased steadily by about 10 km/h. On the nine other nights, when there was substorm activity, the rise in the layer was more sudden. On 67 November 1970, for example, hmF2 increased by 125 km in just over 4 h--an average increase of 30 km/h. This suggests that the substorms were associated with an enhanced southward neutral wind or with strong west-to-east electric fields which drove the plasma sharply northwards and upwards. The relationship between magnetic activity and the raising of the layer is summarised in Fig. 2, where the average rate of increase in hmF2 is plotted against the mean aurora1 electrojet index(AE). The raising of the layer certainly caused a reduction in the rate of decay of the ionosphere, but it did not explain the long periods when no decay was observed. Neither did it explain the periods when the total content actually increased. By calculating the total recombination for each hour in turn and comparing this with the change in total content over the same hour, we calculated the net influx ofplasma necessary to balance the continuity equation and so maintain the ionosphere through the night. The results are listed in Table 1 for
the period from 2130 UT to 0230 UT on each night. In every case, taking the average of the 5 h, there was a net influx of plasma ranging from 0.2 x 10’ 2 m _ 2 s- ’ to 4.9x lOi* mm2 s-l. However, the results from hour to hour showed that the net flux into the ionosphere fluctuated from positive to negative and back. These sudden fluctuations were almost certainly due to the second factor controlling the night-time ionosphere.
ELECTRIC
FIELDS
AND HORIZONTAL
TRANSPORT
This second factor was the horizontal transport of plasma under the influence of electric fields. When an electric field is applied to the plasma in the F-region, the first response is for the plasma to move perpendicular to the magnetic field with a velocity v equal to E x B/B2 where E is the electric field and B is the magnetic induction. This velocity has a vertical and a horizontal component, but after a time the effects of diffusion and induced ion drag counter the vertical component and only the horizontal component can be sustained. Evidence that electric fields were indeed driving the night-time ionosphere came from a close examination of the electron density profiles during the period when the steady decay of the ionosphere came to a sudden end. This always occurred abruptly and it was usually possible to determine the exact moment ofchange from decay to maintenance within a minute or so. Moreover, the change always occurred at precisely the same time at all heights. Electric fields associated with substorms would explain this behaviour. These fields are known to change abruptly and the effect would certainly occur simultaneously at all heights. A west-to-east electric field would move plasma northwards, and hence the plasma above Malvern would be replaced by plasma from further south. As there is usually a considerable
P. J.
78
STANDLEY
and P. J. S.
K’ILLIAMS
Table 1. Average flux into the ionosphere above Malvern (to” rn--‘s- ‘) Date at beginning of night -___. 17-09-68 1S- 1O-68 12-11-68 28-01-69 26-08-69
____-_2200
Universal time _.__~~_.__ 0100 2300 O@Xl
-
___~ 0200
Mean
S.D.
it:,
0.36 1.82 0.22 0.12 0.18
0.76 1.54 0.70 0.25 3.37
40 54 29 64 41s
1.68 0.76 3.00 3.50 0.59 4.48
242 67 166 104 40 341
2.4 3.2 1.3 0.4 5.0
0.6 0.9 1.3 0.4 3.2
-0.4 1.7 -0.3 0.4 0.0
-0.1 1.8 -0.5
-4.2
-0.7 1.5 -0.7 -0.5 _ 3.1
09-09-69 20-Ol-70 17-02-70 17-03-70 30- 10-70 06-l l-70
4.4 1.7 7.6 10.8 4.7 -0.8
-0.6 1.2 1.2 2.8 1.6 3.2
0.6 1.5 4.8 -0.1 0.5 3.2
0.9 -0.1 4.3 3.5 0.9 4.3
-0.7 -0.9 0.7 4.2 0.1 - 8.2
1.32 0.56 4.92 4.24 1.56 0.34
Mean (SD.)
3.9 (1.0)
2.0 (0.6)
1.1 10.5)
0.9
-0.7 (0.9)
1.44
north-south gradient in total content, this would be equivalent to a net input of flux. Similarly a north-tosouth electric field would move the plasma eastwards and during the earlier part of the night this would also lead to a net input of flux. At the same time we must remember that east-to-west and south-to-north electric fields wouid have the opposite effect, deplenishing the ionosphere. The north-south gradient in total content was estimated using values of the F-region critical frequency foF2 measured by a north-south chain of ionosondes (see JAIN and WILLIAMS, 1984). The values measured at each station were interpolated to give values at the same local time, and a regression analysis between NmF2 and latitude yielded dNmFZj& This was then multiplied by the equivalent slab thickness measured at the same local time at Malvern to give an estimate of the north-south gradient of total content a N,/?.Y. The averageeast-west gradient was derived from the time variation of N, above Maivern. assuming that the variation of IV, on a given day was a function oflatitude and local time rather than a function of UT. This is a reasonable assumption at mid-latitude. The results are listed in Table 2. It is clear that, throughout the night there is a strong north-south gradient in total content with an average value of 2.4 x IO’” m -2 m ~ I, so that a northwards velocity of about 40 m s-l would cause an equivalent flux input of t x IO’* m-* s- ’ and so make a significant contribution to the observed changes in total electron content. The east-west gradient is much smaller, with an average value of zero, and only before midnight would an eastward velocity make any significant contribution to the maintenance of the ionosphere.
-0.1
(0.8)
tinfortunately the monostatic radar at Malvern did not measure plasma velocity. On other occasions, however. a multistatic radar was used and this frequentlymeasurednorth-southvelocitiesof40ms-’ or more (THOMAS and WILLIAMS, 1975); velocities of this magnitude would certainly make a significant contribution to the observed changes in total content. North-south velocities at Mafvern are driven by eastwest electric fields which are highly correlated with aurora1 activity (GOEL and JAW 1980). It foflows that if fluctuations in the influx of plasmaare indeed the resuh ofelectric fields we wouldexpect such fluctuations to be linked with aurora1 activity. To test this, the influx of plasma was first averaged over all eleven nights to give a reference variation with time. For each night in turn the reference was scaled to match the total influx for that particular night and this was then compared with the hourly values actually observed to determine a standard deviation as a measure of the fluctuations. When the total flux was plotted against (AE) for each night there appeared to be no correlation, but when the standard deviation was plotted against (AE) there was a correlation of 76”:s (Fig. 3). In other words, the total flux into the ionosphere did not appear to depend on aurora1 electric fields but on a quiet night the
Table 2. Average gradient in the total electron content (l()‘Omw~Lm+‘)
_---.North-south East-west
Universal time 2200 2300 0000 OlW 0200 Mean ________ 02.3 01.7 02.5 02.6 02.7 02.4 00.1 00.3 -0.1 -0 I -0.1 00.0
Maintenance
Average
auroral
Fig. 3. Standard dewation ionosphere from the scaled
electrqet
of the night-time
Index
of the observed flux into the reference tlux as a function of (A.%.
influx varied smoothly with time while on a disturbed night the flux varied considerably from hour to hour. The evidence suggests, therefore, that electric fields affected the behaviour of the ionosphere on a time scale of an hour or so, but they did not explain the maintenance of the ionosphere through the night. For this we must turn to the third factor.
DOWNWARD
FLOW OF PL4ShlA
The transfer protonosphere
of plasma between the ionosphere and was first proposed by HANSON and ORTENKIRGER (1961). A mathematical model of the coupling between the F-region and the protonosphere was published by MWFETT and ML~RIWY(1973) and this model was developed in a series of subsequent papers (e.g. MURPHY et ul., 1976, 1980; BAILEYet al., 1977, 1978,1979 ; MURPHY and MOFFETT, 1978). The full process involves charge exchange between oxygen and hydrogen O++HzO+H+. During the day oxygen ions are produced in the F-region and when these diffuse upwards they expertence charge exchange with hydrogen atoms to generate protons which enter the protonosphere. During a magnetic storm, the protonosphere is sudden]) emptied and for a short period after such a storm the protonosphere continues to fill day and night. However, when the content of the protonosphere has increased sufficiently the process is reversed at night. After sunset the ionosphere cools, recombination takes place and oxygen ions ditTuse downwards. This changes the balance of the charge-exchange equation at the
ionosphere-
I
79
transition height and protons experience chargeexchange with oxygen atoms to generate more oxygen ions. This in turn causes a downward diffusion of protons. In this way charge-exchange provides a ioose coupling between the oxygen and hydrogen populations. The downward flux of oxygen ions at 800 km is not wholly correlated with the protonospheric Rux ; it has been shown, for example, that on occasions counter-streaming can occur with oxygen ions moving downward and protons upward (BAILEY ct ul.. 1977). Nevertheless. for the night as a whole the tlux of oxygen ions downwards into the height range measured in the present paper (l@&SOO km) corresponds closely to the llux from the protonosphere. Theory suggests that the downward flux should be greatest in the period between sunset and midnight, dropping to a low level after midnight (MURPHY et nl., 1976 ; BAILEY et al., 1979). This prediction is confirmed by the results given in Table 1, where the mean flux into the ionosphere over all eleven nights is listed for each hour between 2130 UT and 0230 UT. The same theory predicts that the protonospheric flux at night increases steadily as time elapses after a major storm, until either the protonosphere is saturated or else there is another magnetic storm. The Saturation time depends on the total volume of the protonospheric reservoir and hence increases sharply with magnetic latitude. KRINRERG and TASHCHILJN (1982) have predicted that the saturation timr r, - O.! 7 I.:’ days where L is rhe McIlwain L-parameter. .\t Malvern I, = 2.6 so that I, is predicted to be about 8 days. Detailed accounts of the recovery of the protonosphere after a geomagnetic storm have been published by PARK (1974), KERSLEY and KLWCJCH.XR (198o)and Po[!tni~ CI a/.(19811. We therefore plotted the mean Rux mto the ionosphere between 2130 UT and 0230 lJT against the time elapsed since the previous magnetic storm. The mean fluxes are listed in Table 1 and the errors for these fluxes correspond to two sources of uncertainty : firstly the fluctuations which we have assosiated with electric fields and secondly the limitations In the MSIS 79 model of the neutral atmosphere. The uncertainty due to the electric tieids can be derived from the measured standard deviation in the hourly values of flux. The uncertainty in the MSIS 79 model can only be estimated. The errors quoted for the \,arious parameters in the model are the errors in mean value and do not represent day-to-day variations. HF~XS c’t CL/.(1977a) do suggest a 154, error in the concentration of molecular nitrogen. the major constituent. Molecular oxygen is a minor constituent and therefore the error in concentration is likely to be
P. J.
80
STANDLEY
and
larger; a comparison of MSIS 77 with rocket measurements would suggest an error of about 25% (HEDIN et al., 1977b). The time elapsed since the previous magnetic storm is defined as follows. (a) The magnetic storms listed in the NOAA Prompt Reports of Solar-Geophysical Data were examined. These storms vary considerably in size and duration but all the storms listed were recognised provided they were recorded by at least two stations and had a Kp index greater than 3. (b) For each night of observation the previous magnetic storm was identified and the period following the storm carefully examined to determine the number of quiet days between the end of the storm and the night when observations were made. The criterion adopted counted all days when the sum of Kp was less than 24. The results are shown in Fig 4. Regression analysis was used to plot the best straight line to thedata and the result gave qualitative agreement with the theoretical model of MURPHY et al. For the first night after a magnetic storm there was a net flux out of the ionosphere, but as the protonosphere was filled the average downward flux increased steadily and there was a correlation coefficient of 83% between the mean flux into the ionosphere and the number ofdayselapsed since the last magnetic storm. The quantitative agreement is also reasonable. The
Murphy, Baiky
and
1
3 R E
I
5
2
4 N E ra s 0 5 s
t : z i r
15 Number of
quiet days fence last mognet!c storm Theory ~ Observations
-2
Fig. 4. Mean flux into the inosphere (2130 UT-0230 UT) as a function of time since the preceding magnetic storm.
P. J. S.
WILLIAMS
theoretical model predicts that for an L-value of 3.2 the downward flux between 2130 UT and 0230 UT would increase on average by 0.56 x 10” mm2 s-’ per day. The observations showed that at an L-value of 2.6 the average downward flux increased by 0.34x lo’* mm2 s-l per day, but we must remember that a fair comparison would reduce the flux predicted by the model to allow for the lower L-value above Malvern. What the results certainly show is that the protonospheric flux increased steadily with time that had elapsed since the previous magnetic storm and that at an L-value of 2.6 at least 8 days were required for saturation of the protonosphere.
CONCLUSIONS
We can summarise the behaviour ionosphere at night as follows.
of the mid-latitude
(a) After sunset the F-region peak rose by an average of 80 km and as a result the rate of recombination was reduced. During magnetically disturbed conditions the layer rose more rapidly. (b) During disturbed conditions electric fields were also responsible for moving the plasma horizontally and in the presence of strong horizontal gradients in total electron content this was equivalent to a net flux of plasma into-or out of-the ionosphere above a given point. The correlation between rapid changes in the net influx of plasma and the aurora1 electrojet index suggests that such electric fields dominated the behaviour of the ionosphere on an hour-to-hour time scale but did not contribute a net input for the night as a whole. (c)The main process responsible for the maintenance of the ionosphere through the night was downward diffusion from the protonosphere. After a strong magnetic storm the protonsophere was empty and there followed a period of refilling until either the protonosphere was full or there was another magnetic storm. As the total content of the protonosphere increased so did the downward flux into the ionosphere in the period before midnight. At an L-value of 2.6 the time for saturation of the protonosphere was at least 8 days.
AcknowledgemenrsThe authors would like to thank the Director of the Royal Signals and Radar Establishment for the data used in this paper. We would also like to thank Dr G. N. TAYLOR for his interest,and encouragement and Dr R. J. MOFFETT for valuable comments. One of us (PJS) was supported by an SERC Research Studentship during the period when much of this work was completed.
Maintenance
of the night-time
ionosphere
-I
REFERENCES
BAILEYG. J., MOFFE~~R. J. and MURPHYJ. A. BAILEYG. J., MOFFEIT R. J. and MURPHY J. A. BAILEYG. J., MOFFETTR. J. and MURPHY J. A. GOEL M. K. and JA~NA. R. HANSONW. B. and ORTENBURGER I. B. HEDINA. E., SALAHJ. E., EVANSJ. V., REBERC. A., NEWMANG. P., KAYSETD. C., ALCAYDED., BAUERP., C~CGER L. and MCCLURI~J. P. HEDINA. E., REBERC. A., NEWTONG. P., SPENCER N. W., BIUNTONH. C. and MAYR H. G. HEDINA. E., REBERC. A., SPENCERN. W. and BRINTONH. C. JAINA. R., TAYLORG. N. and WILLIAM P. J. S. JAIN4. R. and WILLIAMSP. J. S. KER~LEYL. and KL~BIJCHARJ. A. KRINBERG1. A. and TASHCHILIN A. V. MOFFE~ R. J. and MURPHYJ. A. MURPHYJ. A., BAILEYG. J. and MOFFETTR. J. MURPHYJ. A., BAILEYG. J. and Mom R. J. MURPHYJ. A. and Momrr R. J. PARK C. C. POULTERE. M., HARGREAVE~ J. K., BAILEYG. J. and MOFFETTR.J. THOMASD. P. and WILLIAMSP. J. S. TORR D. G. and TORR M. R. WILLIAMSP. J. S. and TAYLORG. N.
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1977b
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81