Some geomagnetic effects in the equatorial F2-region

PreesLtd., London Jourpalof Atmospheric andTerrestrial Phyalcs,1955,Vol. 6, pp. 177 to 188. Pergamon

Some geomagnetic effects N.

in the

equatorial

Qresion

J. SKINNER and R. W. WRIQHT University College, Ibadan, Nigeria

(Received6 Jzcne1954) ABSTRACT The diurnal vari&tionsof the V8h0S of m8ny ionospheric paremeters obtained 8t Ibed8n, Nigeria, close to the mrrgnetioequetor, 8re investigated on magneticelly quiet days and on m8gIietiC8llydisturbed deys. It is found that the middsy minimum of the maximum concentration of ionization N, in the F, layer disappears 8s the days become more disturbed. On quiet days there is very little scatter of values of N,, but on disturbed days this is much increesed. Furthermore, both N, and the tot81 ion content, R, of 8 column of unit cross-section below the height of maximum ionizcltionare greater at 811times of the day on disturbed days than they are on quiet da,ys. The perturbation of the quiet-dey v8riation introduced on disturbed days, SD, has not only 8 diurnal (twenty-four hours) component, but 81~0 a semidiurmd component. Whilst there is general 8greement in the beheviour of the results described here and the theory of ionospheric disturbances proposed by MARTYN (1953), our data imply th8t the phases of the motions differ appreciably from those suggested by MARTYN. 1. INTRODUCTION

It is we11 known that the earth’s magnetic activity exerts an influence on the ionospheric parameters f,,F, and h’F, over a wide range of latitudes. APPLETON, NAISMITH, and INGRAM (1937) established a close correlation between the noon and midnight critical frequencies in the F,-layer (fOF2) and the magnetic activity at places of high latfitude. For the Polar Year 1932-33 they obtained a correlation coefficient of --O-34 (for Tromso) and -0.40 (for Slough) betweenf,F, at noon and t,he daily magnetic activity. In addition, they noted the presence of a seasonal variation in the relation between magnetic and ionospheric disturbances which has recently been confirmed by APPLETON and PIG~OTT (1952). A rather different technique was used by BERKNER and SEATON (1940), who considered the correlation between t’he mean electron density throughout t)he day and magnetic activity for the stations Watheroo and Huancayo. The temperate station gave results agreeing in general wit,h APPLETON et al., but it was found that for the equatorial station the correlation was reversed in sign and f,,F, increased during magnetically disturbed periods. More recently MARTYN (1953) has investigated in a more systematic manner how the diurnal variations of f,,F, and the virtual height h’F, may be affected by magnetic activity. He considered the three moderate latitude stations, Watheroo, Washington. and Canberra, and found that the mean level of foF, was depressed during magnetic disturbance in the summer and at the equinoxes, but that there was a very much smaller effect in winter, f,,Fz actually being slightly raised for Canberra and Watheroo. We have examined below the difference between the magnetically quiet and disturbed day variations of the maximum ion concentration N, (proportional to (foF2)2) for t,he equatorial station Ibadan (lat. 7’ 26’ N, long. 3” 54’ E, magnetic lat. about 2&OS). The difference is so marked that we have extended the analysis to include most of the other ionospheric parameters connected with the F,-layer. Explanations are suggested which seem to explain many of our experimental results. 177

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J. SKINNER and R. W.

Ukmm

2. TREATMENT OF DATA The results of ionospheric records obtained at Ibadan in 1952 have been utilized. The days have been divided into three groups according to the sums of the geomagnetic planetary three-hour-range indices Kp. Table 1 shows how the division was made and gives the percentage number of days in each group. Table 1

For each ionospheric characteristic considered, the average quiet (Q) day variation has been plotted on the same axes as the average disturbed (I)9 day variation. In some cases, the diurnal variation has also been evaluated for H days. For the evaluation of N,, not all the available data were used, because we were particularly interested in comparing its behaviour to that of y, . IV,,,, (which is proportional to n, the total ion content in a column of unit cross-section below the height of maximum ionization in the F,-layer). Accordingly, only values of N, obtained on hours for which Y,,, (the semi-thickness of the P,-layer) had been measured, were used. This had little effect on the shape of the mean N, curve, except that at night, when it is often impossible t,o measure y=, t~here are few values of fOPz and the averages are somewhat more erratic. There are some obvious objections to the above method of analysis. It will be seen that we have taken the magnetic activity of the whole day as governing the ionospheric state throughout the day. This will be discussed further in the next section. Also, it is well known that there is a correlation bet,ween sunspot numbers, magnetic activity, and ionization in the ionosphere. It might be suggested that D days and Q days represent days of high and low sunspot act#ivity respectively. Table I, however, shows that t>here is a negligible difference between t-he mean sunspot number for Q, H, and D days in 1952. 3. THE MAXIMUM COKCENTRATION OF ELECTRONS N, The maximum concentration of electrons in the F,-layer, N,, is proportional the square of the critical frequency. When f,,F, is measured in MC/S N,

to

= 1.24 x lo4 x (f,,FJ2

The behaviour of N, is more relevant to the layer processes than fOF2, so we have accordingly considered N, (expressed in terms of ( ,foF2)2 in (Mc/s)a) in detail, ratsher than the critical frequency. Fig. 1 shows the diurnal variations of N, on L) and Q days. The differences between the two groups are very great. This is very especia.lly as a bout half of all days fa,ll important in radio communications, definitely into these ext.reme groups with about equal numbers in each. Fig. 2, 178

Some geomagnetic

effects in the equatorial

F,-region

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N. J. SKINNERend R. W. Waroar

Curve VII, shows the ratio D/Q of the hourly values of N, on D and Q days. The increase of about 40 per cent during the daytime on disturbed days is in the opposite direction to that which is encountered in temperate latitudes. Although the large peak at 0500 hr occurs at the local time showing maximum ratio at higher latitudes, curve suggests that the peak may not be reliable. the shape of the N,(D)/N,(Q) In practice this is the most difficult time of the day at Ibadan to measure N,, and the number of observations is therefore abnormally small. In view of the practical importance of this parameter, a more detailed investigation has been made into its behaviour. The days have been redivided into smaller

50

0

IO

20

30

40

so

DAILY I: Kp.

Fig. 3

groups, with daily total Kp figure of O-10,1 I-15,16-20,21-25,26-30,31-35,36-40, and those greater than, or equal to, 41. Whilst the number of days in each group is made rather small, the trend of the change as the day becomes more disturbed is very marked, though crossovers do occur. The complete figure is not reproduced here, but in Fig. 3 the mean value of N, between the hours of 1000 and 1400 inclusively are plotted for each of these groups of days against t,he mean dP;ly Kp The monotonic increase which is obtained is all the more figure of the group. striking when it is considered that data at all seasons have been included. An analysis into seasons has been made, but the overall diurnal variations of the Q and the D days are little changed and discussion of this point is deferred until a later date. The most marked fact about the change in the diurnal variation as the days become more and more disturbed is the manner in which the midday minimum of ionization on Q days gradua,lly disappears and on very disturbed days is replaced by a pronounced maximum. It would be interesting to know whet,her t,he geomagnetic anomaly of a minimum of noon fnF, at other stations along t)lle magnetic equator is also removed by a similar consideration of very disturbed days alone. The distribution of the values off,,F, is shown in Fig. 4. where the number of times a value off,F, is obtained in a small range of values is plotted against t,he median value of t’he range. Curves are given showing (1) all days. (2) quiet days. and (3) disturbed days. Again it: must be realized t,llat, all seasons are included in these curves. The width of the distribution curve for quiet, days may be almost completely explained by the variation which must occur throughout the ye;\r due 1so

Some geomagnetic effects in the equatorial F,-region

to seasonal changes. Disturbed day values are very much more scattered, but about a much higher median value. It is mentioned above that we have used the daily Kp figure to characterize the whole day from 0000 hr to 2400 hr. A more exact analysis would seem to involve using the Kp figure for the twenty-four hours preceding the hour in question. The probable effects of such a treatment have been considered in two ways. Firstly, we 1

6

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a 9 IO II NOON VALUE OF to 5 IN MC/S

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13

Fig. 4

have investigated the diurnal behaviour of N, on the days which follow D days, and the days which follow Q days, called briefly L) + 1 and Q + 1 days. It is to be expected that the D + 1 days would resemble the L) days since appreciable magnetic activity usually lasts for periods longer than twenty-four hours. In fact, the average Kp figure for a D day is 36.9, whilst for a D + 1 day it is 31.5. There is no significant deviation between D + 1 and U days or Q + 1 and Q days. Secondly, the correlation coefficient. has been worked out not only between the noon values of (foF2)2 and the daily Kp figure, rl. but also between the noon values of (f,,FJ2 and the Kp figures for the period 0600-1200 hr on the day in question, r2. It is found that rl = +O-43, and r2 = +O-44, both values being obtained from 274 pairs of values. The difference is hardly significant. From both the above considerations it will be seen that the method of classifying the day does not lead to any marked differences in the results obtained. MARTYN in his discussion of the behaviour of the ionosphere during periods of magnetic activity has studied t’he behaviour of foF, and h’F, as a function of storm time. At Ibadan during 1952 we have records for only seven storms classified severe or moderately severe. the majority of which commenced around midnight. However, even with so few storms the variation off,F, with storm time is marked. Upon removal of the normal quiet-day variations it is found that foFz is given a 181

N.

J.

snd R. W.

SKINNER

WRICX3T

pronounced decrease at the beginning of an ionospheric storm for a period of about ten hours. After this, f,,F, rises above normal, as would be expected in a period of disturbed days. This is the opposite of MARTYN’S general results obtained when discussing temperate stations, as is to be expected. It must be remembered, that not much reliance should be placed upon the results from only seven storms, and a further analysis is desirable. 4. OTHER IONOSPHERIC PARAMETERS (a) The semi-thickness

of the F,-layer

-

ym

Fig. 5 shows the average Q and D diurnal variations for y,. The values of y,,, have been deduced for the most part by APPLETON and BEYNON’S method. However, when an individual hourly record showed that a non-parabolic type of

Fig. 5

electron distribution existed in the F,-region, an equivalent semi-thickness Teg. was evaluated instead, using techniques evolved by RATCLIFFE (1951). This expedient was frequently resorted to during the daylight hours 0800-1200, 1500-1800. It is seen in Fig. 5 that there is a general tendency for y,(Q) to exceed y,(D) for the morning hours 0900-1300, whereas in the aft.ernoon from 1300 onwards y,(D) is markedly greater than y,(Q). In Fig. 2, Curve V, the ratio of y,,, on D days to that on Q days is plotted for each hour of the day. There is seen to be quite a pronounced semi-diurnal oscillation. (b) Total ion content in unit column below h, - n RATCLIFFE ( 1951) has described a quick method of analyzing h’f records whereby n, the total ion content of a column of unit cross-section below the height, of maximum ionization in the F,-layer, can be obtained. The authors (1954) found that for Ibadan, the mean diurnal variation of n was of a much more regular nature than that for N,. In part,icular, consideration of n largely removed the characteristic midday “equatorial bite-out” that is present throughout the year in the daily variations of N,.

182

Some geomagnetic effects in the equatorial F,-region

The value of n is related to N,

by the expression n = 213 y,N,

so that one would expect from our individual graphs for yrn and N, that n would be larger on D days than & days. Fig. 6 show& the mean diurnal variation of no

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Fig. 6

and n, (in units of km(Mc/s)2) and it is seen that no is greater than no for every hour. In Fig. 2, Curve VI, the variation of the ratio n,/no is illustrated. (c) Percentage ridge occurrence-R Ridges and thick-layer phenomena in the F,-region are found to occur almost every day at Ibadan and have been the subject of an earlier communication

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20 22 24

(SKINNER, BROWN, and WRIGHT, 1954). The variation of the percentage probability of ridge occurrence R with time of day showed a pronounced maximum at 1000 hr followed by a dip at 1300 hr. There were usually one or more peaks in the

183

N. J. SW

and R. W. WBIQET

afternoon, but ridges were not so pronounced then as in the morning periods. Fig. 7 shows the mean diurnal variation of R, and R,, and it is clear that there is a greater probability (about 14 per cent) of ridge occurrence at 1000 hr on quiet However, afternoon ridges appear to be more day8 than on disturbed days. frequent on disturbed days. Fig. 2, Curve IV, shows the diurnal RJR0 variation, which is seen to consist of a negative phase followed by a positive phase.

Previous workers have found that for temperate- and high-latitude stations, the virtual height of the F ,-layer h’F a was inoreased d&ing magnetically disturbed periods. In agreement with the general tendency for correlation of the opposite sign in equatorial regions, we find that h’F, tend8 to be decreased during magnetic disturbance for the daylight hours when the P,-layer is separated from the

Fig. 8 F, (Fig. 8). This tendency has been found to persist at all seasons of the year. Fig, 2, Curve II, show8 the ratio of the values of h’F, on D days to those on & days. It is interesting to notice that the near-unity ratio between 1500 hr and 2000 hr strongly supports the authors’ suggestion that the sunset maximum of h’F, observed at Ibadan is largely accounted for by difficulties of reduction at that time. A short analysis has been effected for variations during storms, as is mentioned above. With the limited data available it is still possible to observe that there is an initial increase of k’F, at the beginning of a storm, lasting about fifteen hours, followed by an overall decrease. However, since h’F, seems larger during the night on D days, this can be linked with the fact that most of the storms used Much more data are required, but it is felt that a more began at midnight. 184

Some geomagnetic effects in the equatorial F,.region

relkble parameter than h’F, no single simple meaning.

should be used (such as h,), since h’F,

(e) Bifurcation

-

of the F-layer

is capable of

L

The symbol L is internationally used in routine reduction of ionospheric records instead of numerical values of fOF, and k’F, when there is no clear maxiThe percentage occurrence of L thus mum of ionization density in the F,-layer. presents a convenient method of assessing the degree of bifurcation of the F-layer at any hour, and it has been examined to see whether it is subject to geomagnetic control (Fig. 9). It is found that there is a consistently greater probability of L occurrence during magnetic disturbance, i.e., during disturbed periods the degree of bifurcation is smaller. Fig. 2, Curve III, shows the ratio of the percentage occurrence of L on disturbed and quiet days. At temperate latitudes this effect is reversed, bifurcation being greatest in disturbed periods.

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htll

Values of h, were reduced by APPLETON and BEYNON’S method whenever a parabolic distribution of electron density existed. Values of h, were also calculated for non-parabolic distributions, provided that the deviation from parabolism was slight. The average diurnal variations of h,( D) and h,(Q) are shown in Fig. 10. The general shapes are seen to be similar to those for ynz and R with a “cross over” at 1300 hr. The morning increase of ~~(~~ above la,,,(D) may be accentuated by the fact that more ridges occur on quiet days than on disturbed, so that values of h, will appear to be larger, due to group retardation in the ridges. Fig. 2, Curve I, shows the ratio of values of h, obtained on disturbed days and quiet days. 185

N.

J.

SKINNER

and

R. W. WRXGHT

6. DISCUSSION MARTYN (1953) has developed a theory, according to which all ionospheric disturbance variations are due to the effect of an electrostatic field which is developed in the aurora1 zone. This field spreads over the earth through the ionosphere in such a way as to produce the current responsible for the disturbed days diurnal magnetic variation, and the combined effect of the current and the earth’s magnetic field is to produce a vertical ionic drift velocity which perturbs the normal quiet-clay (So) variations of the ionospheric parameters. MARTYN computed the disturbance daily variations (S,) of the characteristics~~~~ and h’F, by subtrae~,ing, for each month, the average daily variation for the five International -magnetically quiet days from its average daily variation on the five Internat,ional magnetically disturbed days, and found a strong diurnal (24-hour) harmonic present’ in the S, (f,F,) and S,(h’F,) variations for stations of moderately low latitude. The D/Q variations shown in Fig. 2 [which represent (S, i_ S,)jS,] should give the shape of the S, variat.ions, and it is seen that whereas h’l”, shows a strong diurnal component for Tbadan, the y, and N, curves have marked semi-diurnal harmonics in addition. The quiet-day variation So of the parameter is considered by MARTYN’S theory (1947) to be due to solar semi-diurnal tides in the ionosphere which produce vertical ionic drifts. Unfortunately, the phases of these So motions cannot as yet be said to be definitely known, particularly close to the magnetic equator, and this However, it is fairly simple to discuss the complicates the present discussion. re ative phases of the effects of the two motions So and S,. One of the effects of the quiet-day tidal variation, So, at low latitudes is to assist the separation of the F,-layer from the F,. Fig. 9 shows that at Ibadan bifurcation is less marked on disturbed days. It would follow that the S, and So fields are in opposition during the daylight period at least. Confirmation of this is afforded by consideration of the following experimental evidence: (1) h’F, is lower on disturbed days, indicating that the F,-F,-layer separation is less (the F,-layer is relatively unaffected by magnetic disturbances). (2) Both h, and yrn are increased in the morning and reduced in the afternoon by the So movements on quiet days. On disturbed days t’he rate of increase in the mornings and the rate of decrease in the aft,ernoons are both less than on quiet days. (3) The probability of morning-ridge occurrence is decreased on disturbed days. Ridges are obviously a dynamic phenomenon, and thus fewer ridges indicate less motion. The afternoon anomaly of R, greater than R, is more difficult to explain. However, ridges are far less pronounced in the afternoon, being mainly “thick layers.” On disturbed days with little bifurcation the whole F-layer is not far removed from one thick layer, so that little motion will be required to produce the occurrence of R. (It is seen from (2) above that the layer is already thicker in the afternoons of disturbed days.) (4) Evidence of the N, variation depends upon the absolute phases of So, and will be discussed later. The night-time situation is complicated by several factors. It seems likely that the S, variation has a fairly large semi-diurnal, as well as diurnal, component. The latter could reinforce the So semi-diurnal motions over a period of about twelve hours at night, and still oppose them during the day. There is slight 186

Some geomagnetic

effects in the equatorial

P,-region

experimental evidence for this, as examination of the curves will show. There might also be a change in the phase-lag between the field variations and the resulting ionospheric effects from night to day. In addition, the considerable differences of height of the layer between night and day may result in different velociby gradients, so that in all it, is difficult to draw any unique set of conclusions from the night-time results. The question of the absolute phases of the S, and S, motions may now be considered. MARTYN (1948), from an analysis of a wide range of stations, has suggested that there may be a phase-lag of from 7~12to n between maximum drift motion and the maximum resulting effect on the ionospheric parameters. In order to simplify the discussion we shall consider first the phases of the effects produced by the motions. MARTYN has suggested that the maximum effect of the upwards ionic drift will occur at 1200 hr, in the region of the equator. This will produce a maximum value of N,, at noon. Hence, in order to explain the increased values of N, on D days, the motion must be increased. This contradicts the evidence so far discussed, and, moreover, such a phase does not explain the midday minimum of N,. Some additional effect is therefore required to explain this midday minimum, and if this effect is opposed by the S, motions, or not present on D days, the difficulty is overcome. Such an additional mechanism has, in effect, been suggested by MCNISH and GAUTIER (1949), depending upon the large values of dH/dt at the equator before and after noon. A vertical ionic motion downwards just before noon and upwards soon after noon is produced. At first glance this would seem to account for the absence of the “bite-out” on disturbed days, since dH/dt would not change so smoothly as on quiet days, and so no continuous motion would be set up. Consideration of the S, and S, magnetic variations for an equatorial station (Huancayo-BARTELS and JOHNSTON, 1939) would seem to indicate that the perturbation of dH/dt on disturbed days is not sufficient to justify this explanation. Nevertheless, this choice of phase satisfactorily explains the experimental facts, provided that some such additional mechanism is postulated. The variations of n may be accounted for by considering that on Q days the morning upward motion carries a considerable proportion of ionization above h,. We consider, however, that changing the phase of the S, variation better explains the experimental facts. A tide whose phase is such that the maximum effect’ of the downward velocity occurs at noon will explain the following points: (1) There is a midday minimum of N, which disappears when S, opposes S,. (2) 1% and h’F, increase to a maximum at noon on Q days (provided that an appropriate velocity gradient is assumed). (3) There is a maximum of y, at noon on Q days. (4) h,, Y/,> and h’F, are smaller before noon on D days than on Q da,ys and larger after noon. (5) n is larger on D days than on Q days. (This is because on Q days, when the downward motion to regions of high a is larger, there is a greater loss of ions by recombination. This effect may be partly cancelled out by the downward motion bringing a greater proportion of ionization below the maximum.) (6) The behaviour of R and L is explained in the same way as for the alternative choice of phase. 187

N. J. SKINNERand FL W. WRIGHT:Somegeomagneticeffectain the equatorialF,-region

The phases of the actual motions of the ions will depend upon the phase-lag between the ionic velocity end the effects considered. If MARTYN is correct and the phase-lag in the P,-layer is 7r, then his analysis indicates thet the maximum downward motion at the equator should occur at noon. We suggest instead a maximum upward velocity at noon with the same phase-lag. If the phase-leg is 7112then these suggestions are f 1500 hr and 4 0900 hr, respectively. We would suggest that either of the above theories would explain the experimental results obtained at Ibadan. It must be remembered that this station is very close to the magnetic equator, and effects which are of secondary importance in higher magnetic latitudes may here become predominant. It would be instructive to analyze results from stations slightly fsrther from the equator, especially those in the intermediate zone where N, is neither increased nor decreased during magnetio disturbance. Seasonal effects would then become significant in determining phases. The authors would like to acknowledge the constant encouragement of Professor N. S. ALEXANDER and the very helpful advice given by Mr. W. R. PIQOOTT, of the Radio Research Station, Slough. REFERENCES APP~TON, E. V., and BEYNON, W. J. G. APPLETON, E. V.. NAISMITH, R., md INGRAM,L. J. APPLETON,E. V., and PICXIOT,W. R. Bmmm, J., and JOHNSTON,H. F. BERKNER,L. V., and SEATON,S. L. MARTYN, D. F. MARTYN, D. F. MARYN, D. F. MCNISH, A. G., and GAUTIER,T. N. RATCLIFFE,J. A. SKINNER, N. J., BROWN, R. W., and WRIcET, R. W. SKINNER,N. J., and WRIQHT, R. W.

1940

Proc. Phys. Sot. 53, 518

1937 1952 1939 1940 1947 1948 1953 1949 1951

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1954 1954

J. Atnwsph. J. Atmmph.

18S

Terr. Phys. li, 92 Ten-. Phys. 5, 290