Longitudinal variation of the equatorial anomaly

Planet. Space Sci. 1972, Vol. 20, pp. 2093 to 2098.

LONGITUDINAL

Department

Pcrsamon Press. Printed in Northern Ireland

VARIATION OF THE EQUATORIAL ANOMALY

H. D. HOPKINS of Space Research, University of Birmingham,

England

(Received in final form 20 June 1972) Abstract-The present analysis concerns the longitudinal variation in the development and decay of the equatorial anomaly as observed by the Ariel 3 satellite. Data from about seven hundred equatorial passes of the satellite were used to give a broad longitudinal and local time coverage. 1. INTRODUCTION

The Arie13 satellite was the third satellite in the US-UK cooperative space programme. It was launched on May 5,1967 into a polar orbit of inclination 81” with perigee and apogee at 500 km and 600 km respectively. One of the experiments on board the satellite was a capacitance probe for measuring ambient electron densities. Full details of this Birmingham University experiment can be found in Sayers et al. (1969). The present analysis examines the behaviour of the low-latitude topside ionosphere using the ambient electron densities observed by the Arie13 experiment. It is well-established that shortly after sunrise the latitudinal variation of electron density near the F-layer peak shows a single maximum situated above the magnetic equator, whereas at later times during the day, when the P2-layer equatorial anomaly is present, two maxima occur, one on each side of the equator (Appleton, 1946). In the topside ionosphere the anomaly takes the form of a magnetically field-aligned arch along which the electron density is enhanced. During the development of the anomaly the top of the arch moves upwards to greater and greater heights such that, when it is above the height of the satellite, enhancements of electron density are observed by the satellite at points where the arch intersects the satellite path. These enhancements or ‘crests’ occur at higher latitudes as the top of the arch moves to greater altitudes above the magnetic equator (Eccles and King, 1969). There seems to be considerable longitudinal variation in the development and decay of the anomaly. Lockwood and Nelms (1964) showed that near 75”W the anomaly is first seen at local times between 1100 and 1730, whereas King et al. (1964) concluded that the anomaly starts developing between 0800 and 1000 LMT near 105”E. Similar broad differences between the Asian and American zones were found in studies of the location of crests by King and Sayers (1971) using Ariel 1 data. Goldberg (1966) has offered an explanation of such effects based on differences in magnetic declination between the two geographic regions. Fitzenreiter et al. (1967), however, found closer agreement between the morphology of ionization density in the American and Asian zones than comparison of the results of Lockwood and Nelms (1964) and King et al. (1964) would indicate; they suggested that the discrepancy between the results was due to a shortage of data obtained between 0930 and 1100 LMT in the work of Lockwood and Nelms. 2. ANALYTICAL

METHOD

of Ariel 3 were spaced almost exactly 24” apart in longitude, so that 15 orbits were completed each day apart from a slip of about 2” in longitude (O-15 hr in LMT) between corresponding orbits on successive days. All southbound passes between Successive

orbits

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H. D, HOPKINS

JdY 7 and October 18, 1967 were investigated. During this period, the local time of the equatorial crossings of the satellite changed through 154 hr from 2300 to 0730. Fifteen local time intervals of one hour and twelve longitude zones were considered. Thus, 1000 refers to the period 0930 to 1030 and the southbound passes of the satellite remained in each of these local time zones for one week. In order to examine the morphology of the equatorial F region under magnetically quiet conditions, all passes at a time when a, was greater than 15 (i.e. &_,> 3) were ignored. Electron density values obtained from the Bi~ngh~ instrumentation whilst the satellite was in a particular local time zone and in a particular longitude zone were then plotted against magnetic latitude. The plots in each zone were then averaged by considering all points in bands of latitude 2’ wide, and the averaged electron density values were also plotted against magnetic latitude. This was repeated for all local time zones and all longitude zones, thus giving 180 averaged passes, On a few occasions it was found that the local time ‘width’ of one hour contained a statistic~ly inadequate amount of data at low a,, and the width was increased to two hours only for these particular cases. During periods when the electron density exceeded l-15 x 1012m-3 the Birmingham electron density experiment saturated and, if one of the relevant passes contained any saturated points, it was noted that the resulting plot was an underestimate at that point. The data remained reliable up to the limitation of the saturation level. The electron density meas~ements are those obtained at the satellite height above the F2 peak and it should be stressed that during a typical equatorial pass not only will the altitude of the satellite change, but also the height of the F2 maximum will change. Hence care should be taken in relating any occurrence at the satellite height with one at the F2 peak. The full effects of the height variations will be discussed later. Figure 1 summarises the frequency of occurrence of the equatorial anomaly in the various sectors. The figure was obtained by plotting the individual passes in each zone against magnetic latitude, and subsequently allocating a ‘frequency of occurrence of anomaly’ value to each sector, according to the percentage of passes showing clear equatorial anomalies. It was seldom difficult to decide whether an anomaly was present or not; the limiting conditions used were a crest: trough ratio greater than l-1, with the trough having to be situated within 2” of the magnetic equator. The figure shows that the anomaly is first observed at the height of the satellite (500600 km) in the Asian sector (about 100”E) at about 1000 LMT, but the single peak persists in the region -150 to +60” at this time. By 1200 LMT the anomaly is consistently observed in the region -30 to +120”, whereas in the other regions its presence is more variable. By 1400 LMT the anomaly is consistently observed at practically all longitudes. The afternoon plots were difficult to interpret because most of them show saturation for several degrees on either side of the magnetic equator. The plots without extensive saturation invariably showed a very strong equatorial anomaly. Figure 1 shows that the anomaly persists until 1800 LMT at practically all longitudes although it has occasionally disappeared from the -60 to 3-60” region. By 2000 LMT, the equatorial anomaly can still be seen in the Asian sector, but it is not very well-defined, the overall level of ionization at low latitudes having decreased considerably. However, closer examination showed that in the American sector the anomaly is still clearly prevalent at the height of the satellite. The 2200 LMT profiles (not reproduced here) exhibit little latitudinal variation; the twin

LONGITUDINAL

VARIATION

OF THE EQUATORIAL

ANOMALY

2095

.:.:.:.:.::, Y AYOHALV

FIG. 1.

A

BLOCKDIAGRAM ASOBSERVED

COYSISILYILV

OBSERVED.

OF THE FREQUENCY OF OCCURRENCEOF THE EQUATORIAL BYTHEARIEL 3 SATELLITE (ALTITUDE: X0-6ookm).

ANOMALY

The diagram refers to the period July 7-October 18, 1967, during which the local time of the equatorial crossings of the satellite changed through 15) hr from 2300 to 0730.

crests of the equatorial anomaly are rarely observed at any longitude. It is important to note that each local time zone contains data accrued over a period of only seven days. Another feature of interest in Fig. 1 is that the anomaly appears to decay early in the 30 to 60” zone. This is not a zone with any particular geophysical abnormalities in that the geographic and magnetic equators are parallel and near each other, but the frequency of anomaly occurrence during the afternoon is significantly less than that at the two neighbouring zones. Figure 2 shows four of the 180 averaged plots obtained as described previously. In the Asian sector the single profile is replaced by the twin peaks of the equatorial anomaly at about 1000 LMT, so that at 1200 LMT a very strong anomaly is present. In this region, the electron density at the crests of the anomaly is often 100 per cent greater than that at the trough 7

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H. D. HOPKINS

I

30” s MAGNETIC

FIG. 2. A

COMPARISON THEhERICAN(-

0

30’N

O0

-

MAGNETIC

LATITUDE. OF THE MORNING 120 TO -90’)

I

I

00” s

0’

30*N

LATITUDE.

(12OOLMT) AND

&UN

AND EVENING (2OOOLMT) PASSES FOR (6oTO 90”) LONGITUDESECTORS.

The profiles are obtained by averaging seven days data and plotting against magnetic latitude. The regions of saturation of the Birmingham experiment are indicated by a bar.

above the magnetic equator, and the density at the crests often reaches the saturation point of the Birmingham experiment. The equatorial profile in the -120 to -90” zone at 1200 LMT is saturated in the region marked, but an analysis of the individual passes concerned showed that the anomaly was rarely present here at this time. The averaged plots illustrate that the situation is somewhat changed in the evening. Examination of the individual passes showed that the anomaly is weak in the Asian sector and sometimes it has totally disappeared. However, in the American sector the equatorial anomaly is still well-defined at the satellite height, the electron density at the crests being typically 50 per cent greater than the corresponding trough densities. This longitudinal variation in the formation and decay of the anomaly results in important longitudinal gradients in the topside ionosphere. An example of this can be seen in Fig. 3, which compares the averaged latitudinal variation of electron density at -90 to -60” with that at -60 to -30” for 1800 LMT. A detailed examination of the 180 profiles shows that the anomaly is best-defined in the Asian sector at 1400 LMT. In the American sector, however, the anomaly is most developed at 2000 LMT, although it is not as well-developed as in the Asian sector at 1400 LMT. Thus, assuming that the altitude variation of the anomaly is similar in the two sectors, it can be concluded that the equatorial anomaly forms a few hours earlier in the Asian sector than in the American sector and possibly it may also decay at an earlier hour in the Asian sector. The anomaly is first regularly observed at the height of the satellite at 1000 LMT in the Asian sector and at 1300 LMT in the American sector. This is consistent firstly

LONGITUDINAL

VARIATION

OF THE EQUATORIAL

MAGNETIC FIG. 3. COMPARISONOFTHE

ANOMALY

2097

LATITUDE.

AVERAGED ELECTRON DENSITYPLOTS AT TWO ADJACENT SECTORS, (-90 TO -60”) AND (-60 TO -30”).

The profiles refer to August 3-17,1967 when the satellite was in the local time zone 1730-1830. The profiles are again plotted against magnetic latitude.

with the observation of the onset of the anomaly in the Asian sector at 0800-1000 LMT (Eccles and King, 1969) and secondly, also with the observation of the onset of the anomaly at times after 1100 LMT on quiet days in the American sector (Lockwood and Nelms, 1964). Both these latter observations refer to the F2 peak where the anomaly would be observed earlier than at the satellite height. Thus, the suggestion by Fitzenreiter et al. (1967) that there are no significant longitudinal differences is not substantiated by the present results. The present conclusions are not invalidated by the variation in satellite altitude. In order to observe the equatorial regions at constant local time only southbound passes in the vicinity of the equator have been analysed. The variation of satellite altitude with latitude will remain practically constant from one southbound pass to the next. Thus the variation of satellite altitude during a pass will be the same regardless of the longitude of the equatorial crossing. This variation, however, can be anything up to 35 km during a pass from 20”N to 2O”S, so that it is difficult to draw any conclusions concerning the asymmetry of the anomaly with an altitude variation of such magnitude.

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Goldberg (1966) has suggested that the anomaly would be observable in the American zone at the same early hours it becomes observable in the Asian zone provided data points along a path of constant magnetic declination were considered. Zones of only 30” in longitude have been considered in the present work so that the declination variation within any one zone is reasonably small. It can be seen from Fig. 1 that the anomaly is not seen consistently until 1300 LMT in the American sector and this assessment is based on individual satellite passes. There is very little change in longitude during an equatorial pass and consequently the change in declination must also be small. Hence, Goldberg’s suggestion cannot be invoked to explain the absence of an anomaly in the present results. 4. SUMMARY

For some time it was suspected that there were differences in the development and decay of the equatorial anomaly at different longitudes, but the longitudinal effects were not sufficiently well documented to confirm this, as stated by Goldberg (1969). The Birmingham electron density experiment on board the Ariel 3 satellite, in conjunction with the satellite tape recorder, gave world-wide coverage of the electron density at about 550 km at a local time which slowly varied. An analysis of a very large number of equatorial passes has shown that the anomaly was found most distinctly in the Asian sector and also that the anomaly develops earlier and possibly decays earlier in the Asian sector than in the American sector. This work establishes that there are, in the equatorial F2 ionosphere, definite longitudinal differences which have not yet been fully explained. Obvious differences relating to the varying shape and strength of the magnetic field must be considered in any attempt to understand the observed longitudinal variation in the development and decay of the anomaly. Acknowledgements-I wish to thank Professor J. Sayers (Birmingham University) and Dr. J. W. King (Radio and Space Research Station, Slough) for their active interest and advice. I am grateful to Mr. D. Eccles (R.S.R.S., Slough) for assistance with the production of Fig. 1, and I also wish to acknowledge the assistance of Mr. W. R. Piggott and his group at Slough for providing the Arie13 data in a readily accessible form. REFERENCES APPLETON,E. V. (1946). Nature 157, 691. ECCLES,D. and KING, J. W. (1969). Proc. IEEE 57,1012. FITZENREITER,R., GOLDBERG,R. A. and KRISHNAMURTHY, B. V. (1967). Magnetic declination and solar control of the topside ionosphere. NASA Rept. X-615-67-21. GOLDBERG,R. A. (1966). AnnIs Geophys. 22,588. GOLDBERG,R. A. (1969). Proc. IEEE 57, 1119. KING, J. W. and SAYERS, J. (1971). J. atmos. terr. Phys. 33, 355. KING, J. W., SMITH, P. A., ECCLES,D., FORKS, G. F. and HELM, H. (1964). Proc. R. Sot., London A281, 464. LOCKWOOD,G. E. K. and NELMS, G. L. (1964). J. atmos. terr. Phys. 26,569. SAYERS,J., WILSON, J. W. G. and Loprus, B. (1969). Proc. R. Sot. A311,501.