Temporal and spatial changes of the electron content of the ionosphere

Journalof Atmospheric andTerrestrial Physics,1970,Vol. 32,pp.1649-1660.PergamonPress. Printedin NorthernIreland

Temporal and spatial changes of the electron content of the ionosphere A. EBEL Institut fiir Geophysik und Meteorologie der Universittit zu K61n, Albertus-Magnus-PI&z, 5 Kijln 41, Federal Republic of Germany

(Received 24 March 1970) Ah&act--Measurements of the electron content of the ionosphere are analysed in order to study t,emporal and spatial variations of this parameter. Observations relating to a large part of a solar cycle are available for Australian and North American stations. Furthermore, measurements exist for a wide range of latitudes since October 1964 after the launching of Explorer 22. The analysis clearly shows the increase of the annual averages with solar activity and the decrease of the electron content with increasing geogmphic latitude. There is strong evidence for the existence of a semiannual variation of the electron content. Its amplitudes depend on solar activity and latitude similar to the mentioned annual averages. In general the v&&ion is more marked in the afternoon and evening than in the morning of the days around the equinoxes. A semiannual variation is also known for the maximum electron density of the ionosphere and the temperature of the high atmosphere, and thus the same v&Cation is expected for the equivalent slab thickness. But instead of the expected result there is only a semiannual variation of the daily change of this parameter. All these observed phenomena support the assumption that the semiannual variation in the studied ionospheric region is partly caused by plasma transport.

1. INTRODUCTION after the launching of the first Sputniks observations of the electron content of the ionosphere were made by means of artificial satellites. Published data covering long uninterrupted periods are available since May 1958. Measuring and transmission techniques have been steadily improved since then. The measuring methods are based on the Doppler and Faraday effect of the ionosphere. Both these effects have been used in a very different manner by various authors to calculate the electron content. The objective of this paper is an analysis of available data to determine temporal and spatial variations during the major part of a solar activity cycle. All data have been treated with equal weight without regard to methods of measurement or to the object of observation. This has been done although the early results derived from Sputnik 3 data are not as reliable as those from Explorer 22. HIBBERD and Ross (1966), TAYLOR (1966) and NELSON (196813) analysed the annual and seasonal anomalies. For this purpose either the daytime maximum electron content or noon observations were used. However such observations can only be made a few times a year for most satellites due to the slow rotation of the orbit plane. This entails renunciation of the many other observations of equal value. In this study the observation period data are used as mean values or as single values (cf. BURKARD, 1968). The mean values or medians are clearly attributed to a certain day of the year and a certain time of the day (cf. RAO et al., 1969). Seasonal or annual mean daily curves are not used in this context. Table 1 contains the analysed data including specifications relating to the period of observation, the observed satellite, the method of measurement and the frequency of the

SHORTLY

1649

a

The electron content of the ionosphere

1651

satellites’ signals. The symbols mentioned in the table are as follows: D Measurement of the Doppler effect lF, 23 Measurement of the Faraday effect using one or two frequencies P Faraday effect; continuous monitoring of the polarisation twist of radio signals from geostationary satellites. 2. METHOD OS ANALYSIS

It is possible to classify the observation stations mentioned in Table 1 in several groups of neighbouring and single stations. The data are so widely scattered that it is possible to cambine them within the groups without problems. An impression of the scattering can be obtained among others from the continuous measurement of the polarisation twist of Early Bird’s radio signal as made by KLOBUCHARand WHITNEY (1966) in Hamilton, Mass. The electroncontent8s determined by the various groups of stations have been plotted in a diagram with the solar zenith angle as the ordinate and the months as the abscissa. They were separated for the hours 0000-1200 and 1200-2400 LT. The solar zenith angle has been chosen instead of the local time, in order to eliminate the possible influence of different geographical latitudes of stations in one group and to reduce the possible influence of the solar zenith angle (BURXXW, 1967) on the following anaIysis. The medians of the electron content derived from several days’ observations in a small range of the solar zenith angle were used when single values were available (Groups 1 and 3). Otherwise published mean values or medians were applied. Then cnrves of equal electron content were drawn. Figures 1 and 2 give examples of different geographical latitudes and periods. Furthermore the figures contain the curves for ~o~t~t local time 1200, 0800,040O and 0000 br (upper part of the figure) and 1200, 1600, 2000 and 2400 br (lower part of the figure). The results have been smoothed twice namely by calculating the mean or median and by fitting these values to cnrves of equal electron content. From time to time it was necessary to interpolate over longer periods beoause observations by means of nonstationary satellites may oontain gaps. Normally two observ&tionsare obtained per station and day and they are gradually ~tribu~d throughout the day due to the rotation of the satellites’ orbit plane. Therefore the figures and all results derived from them can only show the temporal variation of the electron content of the ionosphere in a first approximation. In spite of the error of the basic data, the particularities of the temporal variation can be analysed. Their existence can be proven by comparing the various stations and various periods. 3. RESULTS Figures 1 and 2 clearly show a tendency towards higher electron content around the equinoxes. This tendency is more obvious in the afternoon than in the (U-S-B) the morning. For the group of stations Urbana-Stanford-Boulder maximum during the period from September 1958 to August 1959 is more pronounced in the spring than in the autumn while Hyderabad 1965 has almost equal maxima around the equinoxes. Although Group U-S-B lies 24 deg further north on the average than Hyderabad (17W) it shows e higher electron concentration. In this way the influence of the solar activity shows which in 1958-59 (Fig. 1, U-S-B) was much higher than in 1965 (Fig. 2, Hyderabad). The formation of maxima around the equinoxes can also be observed by almost all other groups of stations. Normally they are best seen between 1200 and 2400 LT and are However, some stations or periods of particularly prominent in the spring.

1652

A.

EBEL

Fig. 1. The electron content of the ionosphere (lOle rne2) over Urbana, Stanford and Boulder (U-S-B) from September 195%August 1959 between 0000 and 1200 LT (upper part) and 1200 and 2400 LT (lower part) and its dependence on the solar zenith angle and the month. Short-dashed lines indicate intermediate values, long-dashed lines indicate uncertain values. The thin continuous lines represent the constant local time 1200, 0800, 0400, 0000 (upper part) and 1200, 1600, 2000, 2400 (lower part).

observation may form an exception to the general rule as will be shown below by means of harmonic analysis. For instance, the Australian group of stations only showed a slight increase during the morning hours in the autumn of 1965. The peculiarities of the temporal variations of the electron content during the equinoxes are even more evident when the difference between the values of the second and The following examples are shown: the first halves of the day are calculated. differences AN, for the European group of stations and Hyderabad in 1965 (Figs. 3 and 4) and the differences for the North American group of stations in 195% 59 (Fig. 5). The units of AN, are 1015 m-2 for Fig. 3, and 10la m-2 for Figs. 4 and 5. The differences between AN, are approximately of one order of magnitude and also show clearly the dependence of the electron content on solar activity and geographical latitude. The clear appearance of a semiannual variation has been studied by means of harmonic analysis. For this purpose the monthly observations from all stations of the electron content at a constant solar zenith angle in the morning and afternoon were derived from the above mentioned figures. The lowest value at noon in

The electron content of the ionosphere

Month

Fig. 2. The electron content of the ionosphere (1016 m-2) over Hydembad 1965 between ~~QO-~~OQLT (upper p&r&of the fig~~~e)and ~~5~-24Q~ LT (tower pezb). See also text for Fig. I.

Month Fig. 3. Differences of the electron content from 1200-2400 LT and 0000-1200 LT in 1P mm2. European stations (group 1, L-G-K-V), 1965. See also text for Fig. 1.

1653

A. EBISL

1654

Month

-

YiFig. 4. Differences of the eIectron content from 1200-2400 LT and 0000-1200 LT in 10ls m-*.

Hydorabad,

1965. See Jso text for Fig. 1.

00

St

P .= .2

JO”

2

600

z

c%

909

CO0 150° 1800 Month

Fig. 6. Differences of the electron content from 1200-2400 LT and 0000-1200 LT in 1016 m-a. North American stations (group 2e, U-S-B), September 1959-August 1959. See also text for Fig. 1.

December in the northern hemisphere or in June in the southern hemisphere have been chosen as the constant angle. Table 2 contains the amplitudes and the time of the maximum of the harmonic components. The analysed annual curves have been formed by using the averages of the first and last 12 months of an observation period and its complete parts from January to December. For Sydney (1958-59) and Auckland (1965) no aomplete annual curve was available. For this reason the analysis has been done on the basis of a symmetrical annual curve relating to the 30 June. An indication of the maximum times is not warranted in such circumstances. The monthly values have been corrected before applying the harmonic analysis in order to eliminate approximately the trend caused by solar aotivity. A linear increase or decrease has been assumed. As in Figs. 1-5, the annual averages show the infiuence of solar activity and of the geographical latitude on the electron content. It rertches its maximum at the time of high solar

5c (near Auckland)

5b (S-R-H)

58 (SYdneYY)

near Hawaii 4 (~yder~b~d)

3 (D&C)

2b (Univ. Park) 20 (S-TBG-H)

2a (U-x--B)

1 (EG-E-V)

Croup of stations

11*64-10.65 l-65-12.65 l-6612.66 167-1267 168-12.68 @48-849 X2*58-11.69 ‘7.61-6.62 10.64-9.65 1*65-12.65 S-65-4.66 11~64-10~65 1*65-12.65 1,66-12.66 1.67-12.67 1.6~12.65 11.64-X0*65 1.65-12.65 1.66-12.66 3.66-2.67 7.58-12.58 11*64-10.65 1.65-12.65 4.65-3~66 7*65-12.65

Period

.~-

57O

58* 58’

44Os 38OS

34Ys

43O 42O

53O

29’N

2O*N 1’t’N

62O 64O

62O

41°N 41°N 4O*N

7o”

Solar xenith angle

50°N

Mean geogr. lat. 74 73 103 199 194 466 436 89 76 73 82 119 122 172 332 210 196 195 296 364 338 76 77 77 83 6 18 4 2 2 15 21 20 21 31 29 24 6 4 6 -

6 5 9 x7 39 14 25 33 10 16 13 5 4 8 13 13 18 10 14 11 9 10 9 9 2

3 3 19 12 8 9 11 8 16 26 25 4 3 4 2 13 11 8 7 5 -_ 3 5 5 -

10 12 9 5 18 11 13 4 99 101 107 177 181 281 479 319 278 283 423 484 440 96 96 99 110

82 80 123 222 260 498 477 140 14 9 13 35 29 21 7 6 17 16 6 9 8 20 x4 -

5 12 9 17 16 30 307 13 19 16 21 25 24 33 17 24 23 21 13 22 8 7 9 6

10 10 7 21 7 21 22 21

Amplitudes Morning Afternoon as aI a2 a3 a0 al a2

14 20 18 9 9 3 4 9 9 7 7 5 6 7 10 -

11 11 3 10 2 12 25 12

as

8.5 1.0 4.5 5.3 5.9 2.1 1.6 1.3 12.4 1.1 1.7 2.5 3.9 3.2 1.4 -

5-l 6.1 3.8 2.1 12,5 2.7 12.5 12,s

PI

4.1 4.7 4.9 4.2 4.1 6.2 5.6 5.0 4.1 4.4 5.2 4.9 4.3 4.5 5.4 -

6.5 5*7 5.6 5.2 6.8 2.1 5.1 5.2

232

Morning

3.0 2.9 2.7 3.4 3.7 4-O 1.8 3‘0 2.1 1.7 4.8 4.9 4.0 4.0 4.8 -

2.9 2.9 3.0 3 8 2.2 4.9 3.9 4.8

3%

PI

5.6 6.2 4.9 5.0 6.0 6.2 1.3 6.9 1.3 12.5 4.2 1.6 12.9 I.4 2.9 I

5.9 6.6 7.0 2.5 12.9 2.8 12.2 12.5

Phases

5.1 4.9 4.8 4.9 4.9 4.8 4.1 4.3 3.7 3.8 4.0 3.7 4.3 6.4 4.4 -

5.1 4.6 4.5 4.5 5.0 2.8 4.2 3.2

Pa

Pa

2.8 2.8 2.8 3.6 3.7 2.0 2.0 2.7 2*1 2.0 4.4 4.9 4.1 4.8 3.9 -

3.3 3.0 2.1 5.0 2,6 3.7 2.4 3.1

Afternoon

Table 2. Results of the Fourier analysis of the variation of the electron content of the ionosphere for constant solar zenith angle. Relative amplitudes aI - aa (%), related to a0 (1015 mV2). The time of maxima (phases) pI--p3 is given in month (1.0 stands for 1 January; the first maximum in the year is indicated)

G? k? ~ $ 8 s 3 fj * g g w. 8 4 ff CD

A,

1666

EBEL

activity and decreases with increasing latitude, solar activity remaining unchanged. The formal separation of the annual variation into harmonic components permits the second periodic term of the Fourier series (amplitude a2, phase pz) to be interpreted as a semiannual variation. The amplitudes and phase angles of the first and third term have also been determined in order to make possible an estimation of the significance of the second one. The average position of the maximum shows that in most cases a good approximation of the semiannual variation has been derived. The mean position of the maximum appears around the 10 April and the 10 October and consequently corresponds to the semiannual variation of other geophysical elements. On most occasions the semiannual variation is predominant in the afternoon. Its average amplitude a2 differs clearly from the average amplitudes a3. On the other hand, it is less important in the morning on most occasions and is often barely evident at this time. Besides the decrease of the absolute amplitudes with solar activity there is a less obvious decrease of the relative amplitudes in the afternoon. However, in the morning the semiannual component seems to be relatively more important during times of low solar activity. The values for the North American group and Australia (group 5) are plotted in Fig. 6. The amplitudes shown are averages relating to solar zenith angles of less than 90 deg. The values for Fig. 7 are computed in the same way and are compared with the relative amplitudes of the noon electron content. They show a general decrease with increasing northern latitude in the afternoon of 1965. Most of the morning values are of minor significance and their variation in relation to the latitude is not very clear. No corresponding comment can be made for the southern hemisphere for lack of data. It has been mentioned above that the maximum of the electron content is often more apparent in the spring than in the autumn. This is formally contimed by the harmonic analysis which reveals that the maximum of the annual component appears on the average around the 1 March, about one month before the maximum of the semiannual component. a

-80 N

L”

‘E60 9 #Ii-40 D 0

-N 20 IO 0

Ll

gw

&--_oo-12 -12-24 D-4 o-12 *rlZ-24

2 2 5 5

.‘\.r

",A

'\

.\

o;j~~~~=.-,_.

-o---e~ '\

l

l

2 -

la”

20 10

Lt. ---&;Z._._.cl D---i

---c,

-‘c

0

-.-

0 1959

862 year

865

Fig. 6. Change of the average relative and absolute amplitudes of the semiannual component during the years of decreacling solar activity 1958/59-1965 according to measurements made in Australia (group 5) and North America (group 2).

1657

The electron content of the ionosphere

10

20 30 geographic

40 50° N latitude

Fig. 7. Dependence of the average relative amplitudes of the semiannual component on the geographic latitude. Northern hemisphere 1965.

solar

zenith

angle

Fig. 8. Relative amplitudes and position of the maxima of the semiannual component of the electron content over North American stations 1958/59 (a), over Hyderabad 1965 (b), and European stations 1965 (c). The position 1.0 of the maximum means the 1 January (1 July). l 0000-1200 LT; + 1200-2400 LT; 0 0000 LT; x 1200 LT. It may be of interest to study the variation of the amplitudes and the position of the maxima for various solar zenith angles. The results obtained for Group 1 (Europe 1965), 2a (N. America 1958/59) and Hyderabad (1965) are shown in Fig. 8. Formally the Fourier analysis always yields a semiannual component. If one defines the semiannual variation as a periodic change with maxima during March or April, September or October, it is evident that it is of minor importance in the morning over the North American stations and Hyderabad because of the amplitude and over Europe because of the phase angle. The results for constant local time (1200 and 2400 LT) are in principle similar to those for constant solar zenith angles. However, sometimes larger differences may exist (Fig. 8). Thus a question is raised as to how well the individual amplitudes as and phase angles pa represent the obviously existing semiannual effect. Figure 8 clearly shows that for instance the values of Table 2 for the smallest applicable zenith distance certainly represent the effect properly, but are nevertheless subject to some uncertainity. This justifies the application of mean values in Figs. 6 and 7. The percentage error of the results increases with decreasing electron content. is, therefore, considerable during dawn and for high latitude stations during Nevertheless, this fact does not fully explain the times of minor solar activity. It

changes

of the

relative

amplitudes

over

Europe

during

the

day

(Fig.

SC) and

A.

1668

EBEL

during the years 19651968 (Table 2). Special ionospheric conditions must exist over these stations, since the ionosphere behaves more regularly over Hyderabad and Delhi. However, the observations made at the last mentioned stations also do not fully exclude the possibility, that the relative amplitudes a2 do not regularly increase with solar activity in the afternoon as it was assumed in Fig. 6 with respect to the error of the computed values. The existence of a closer relation than can be shown on the basis of the available data is supported by the results of YONEZAWA (1959) for the maximum electron density of the ionosphere. The applied data are not sufficient to determine the influence of the geomagnetic latitude on the long-term temporal and spatial variation of the electron content. 4. DISCUSSION Despite considerable gaps and differences in the basic material the dependence of the semiannual variation of the electron content on the geographical latitude and the solar activity can be considered as qualitatively proven. There is a close relationship to similar variations of the temperature of the upper atmosphere (PAETZOLD, 1960; JACCEIA, 1965) and the maximum electron density of the F2-layer (BURKARD, 1951; YONEZAWA, 1959 and 1967, BECKER, 1966). The semiannual variation of the last mentioned parameter behaves similarly to the variation of the electron content. Supposing that the electron density distribution can be described as a Chapman profile one has N,

= k . H . n,.

N, is the electron content (m-2), H the scale height (m) and n= the maximum of the electron density (m-3). k is 4.1 for the a-Chapman profile (WRIGHT, 1960) and 2.7

for the B-Chapman profile. Small corrections have to be made to account for the fact that the electron content can only be observed up to a final average height of 1000 km. Since H is proportional to T, + T, (electron and ion temperature), there is a possibility of determining the temporal variation of this sum on the condition that the composition of the upper atmosphere is constant. It is expected that T, + T, reflects the known semiannual variation of the neutral gas temperature. Therefore the quantity d = N,/fo2 cc N,/nz

= D

(equivalent slab thickness) has been analysed for the stations of Lindau and Delhi in the same manner as N,. Yet d by no means shows the expected variation. Thus it follows that D # k . H for the annual change of N,. However, at the Lindau station was it observed that with respect to 1200 LT d is more symmetric

during or near the time of the equinoxes than at any other time. As a measure of symmetry the mean absolute difference 1AdI was chosen. The results (Fig. 9) show a clear semiannual variation of this parameter over Lindau. Over Delhi it is more evident in the single values than in the running mean values (curve). The applied measure of symmetry is more suitable for the measurements made at Lindau than for those made at Delhi. The solar flux 5i0., as measured at Ottawa and the differences 1Ad1 apparently show no clear correlation. The observed phenomenon can only be interpreted physically

in such a way

The electron content of the ionosphere

1669

.

lo -

/

n

1965

1966

1967

vear Fig. 9. Mean sbsolute differences of the parameter andDelhi (X---X).

1968 d over Lindau

(0-0

that the parameter d not only depends on the plasma temperature, but that the varying composition of the atmosphere or motions of ionisation prevent the appearance of the semiannual variation. Observations made at the University of Hawaii (YUEN and ROELOFS, 1967) and at Table Mountain (HIBBERD and Ross, 1966) also show that the equivalent slab thickness varies very irregularly. With regard to the composition of the neutral atmosphere its regular and large scale variations should be reflected by the slab thickness. However, it is known that ionospheric transport phenomena especially depend on the geographic and geomagnetic position. This may lead to irregular variations of the divergence term of the continuity equation for the electrons and thus considerably influence the slab thickness. Furthermore, if motions of plasma-for instance periodically increased transport towards the pole- are considered to be the cause, the close relation between the total electron content N, and the maximum electron density n, is plausible. Because of the inclination of the geomagnetic field lines a downward component of the motion must appear. This component naturally influences n, in the same way as N,. The link between them shown by jadl should therefore increase with an increasing dip angle. The different behaviour of the slab thickness over Lindau (53’N geomagnetic lat.) and Delhi (19”) seems to confirm this conclusion. A plasma flow only from layers above 1000 km, the mean height of the satellite orbits, cannot explain the large variation of often more than 25 per cent. For such increases the electron content of the upper layers is not sufficient. The interpretation of the semiannual variation of N, as an effect of horizontal plasma transport is also an interpretation of the semiannual variation of the maximum electron density. It is an indirect effect of the solar radiation and reaches its maximum when the sun passes the equator. The sometimes supposed direct relationship with the ionizing radiation cannot be confirmed when comparing the solar flux i&., and the electron content. The poleward decrease of amplitudes also supports the proposed explanation. When comparing the variation of the mean electron content in relation to the latitude with that of the content during the equinoxes, the mean values seem to be shifted 3-7” towards the pole in the years 1965 and 1966 in the northern

1660

A. EBEL

hemisphere. As expected the smaller values appear in higher latitudes. In this context it would be of great interest to prove the influence of the geomagnetic field. However, for this purpose more or denser measurements are required for the higher latitudes. Acknowledgements-The author gratefully acknowledges that Professor Dr. 0. BURKARD and Dr. R. LEITINGER of the Institut fiir Meteorologie und Geophysik of the University of Grez end Professor Dr. W. DIEMINGER, Dr. G. HAR~N, Mr. G. SCHMIDT and Mr. J. P. SCH~DEL of the Max-Planck-Institut fiir Aeronomie, Lindau, were so kind es to make their observations available before publication. REFERENCES BECKJ~RW. BURKARD 0. M. BTJRKARD0. M.

1966

1951 1967

Kleinhwb. Ber. 11,45. Arch. Met. Geophys. Bioklim. A4,391. Space Research VII, p. 1364. North-

BUREARD 0. M. G~RRIOTT 0. K. GARRIOTT 0. K., SMITE III F. L. and YU?zN P. C. HARNISCHMACIQ~RE., HESS H. A., KR~T R., M~NTHER CH. and RARER K. HIBBERD F. H. and Ross W. J. JACCRIA L. G. KLOBUCRAR J. A. and WHITNEY H. E. LAB. PHYS. L’ATM.

1968 1960 1965

Sber. 6st. Akad. W&s. Abt. II, J. geophys. Res. 65, 1139. Planet. Space Sci. 13, 829.

1965

Arch.

Holland, Amsterdam.

1966 1965 1966 1967-68

elekt.

obertr.

177,

123.

19,589.

J. geophys. Res. 71, 2243. CIRA 1965, 293. Radio Sci. 1,1149. Laboratoire de Physique de I’Atmosphere.

Facultb des Science de Paris. LAWRENCE R. S., POS~ONY D. J., GARRIOTT 0. K. and HALL S. C. NELSON G. J. NELSON G. J. PAETZOLD H. K. and ZSCH~~RNERH.

1963

J. geophys.

Res. 68, 1889.

1968s 1968b 1960

J. Atmosph. Terr. Phys. 30, 513. J. Atmosph. Tern. Phys. 30, 1591. Space Research II, p. 958. North-

1969

J. Atmosph.

1966

Electron density pro$les in ionosphere and exosphere; (Edited by J. FRIHAGEN) p. 543. North-Holland, Amsterdam. J. Atmosph. Terr. Phys. 28, 1135. J. Atmosph. Terr. Phys. 30, 1857. Radio Propagation Unit; National Phys. Lab., Delhi, Scient. Rep. No. 32 (1966), No. 33 (19673, No. 44 (1968). Radio Sci. 1, 1125. J. geophys. Res. 65, 185. J. geophys. Res. 66, 1061. J. Radio Res. Labs Japan 6, 651. J. Radio Res. Labs Japan 14, 1. J. Atmosph. Terr. Phys. 29, 321.

Holland, Amsterdam. RAO C. R., RAO E. B. end SUBBARAJU G. C. TAYLOR G. N.

TITHERIDGE J. E. TITHERIDGE J. E. TYAGI T. R.

TYAGI T. R. and SOMAYAJULU Y. V. WRIGHT J. W. YEII K. C. and SWENSON G. W. YONEZAWA T. YONEZAWA T. YUEN P. C. and ROELEFS T. H. Reference

is also made to the following

MPI AF.RONOMIE

1966 1968 1966-68

1966 1960 1961 1959 1967 1967 unpublished 1969

Tew.

Phys.

31, 1197.

material: Abteilung f. Weltraumphysik, Max Planck-Institutfiir Aeronomie, Linduu/

Ham, Germany.