Low latitude field aligned ionization observed by the alouette topside sounder

Planet.

Spaa

Sci..

1963,

Vol. If,

pp.327

10 330.

Per.pmon

Pn?a

Ltd

Prlnled

RESEARCH

LOW LATITUDE

in Northern

Ireland

NOTE

FIELD ALIGNED IONIZATION OBSERVED ALOUETTE TOPSIDE SOUNDER

BY THE

(Received 7 Januaty 1963) Alouette, the artificial earth satellite 1962 Beta Alpha One, contains a sweep fr~uency sounder that can explore the ionization above the height of the maximum electron density of the Fre ton. Participating in the topside sounder project are the United States, Britain and Canada. The launch ve I?rcle and launch facilities were pxovided by the National Aeronautics and Space Administration, certain telemetry stations were provided by the Radio Research Station, and the satellite was designed and built by the Defence Research Board. Sample ionograms and some preliminary results from the analysis of the topside sounder records are given by Warren ItI, Petrieta’, Muldrew’*j, and Nehm?. Some unusual features are observed in topside ionograms recorded at low latitudes. The ionograms indicate that radio waves are reflected from an over-dense region of ionization which lies alon a magnetic field line. Fig. 1 shows a copy of the ionogram recorded at 03.10 GMT, October 11,1%2, at 92.4 W longitude and 0

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OCTOBER

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II, 1962

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FREOUENCY

Fro. 1. A VIRTUALHEICMTAM) REAL REGION

HEIOHT

PROFILE

OF IONIZATION

I

4

I 5

(MC/S) WHICH

AT LOW

CHARA-

THJI OVRR-DENSE

LATITUDES.

09 S latitude. The curve A’is the usual ordinary wave trace of the ionogram. The dashed part of the curve A’ represents that portion of the trace which is extrapolated from the extraordinary wave trace (not shown) and the gyrofrequencyCo*“. Branching from the curve A’ at a frequency of 1.85 MC/S is an abnormal trace B’ which represents the minimum range of the spread echoes. The shaded area represents tbe extent of the spread echoes observed. The curve A and tbe branch B are the real hei& ht f rotiles corr e$ondingtotbeapparentheightcurvesA’ and B’. The curve A is a normal real height pro e o the topside o the Ionosphere. However, the branch B indicates that the apparent height curve B’ is caused by an overdense region of ionization which exists at a 327

328

Research Note

particular height below the satellite. Since both spread echoes and fragments of the ordinary wave trace are observed on the ionogram at a greater range than the trace B’, the region is not uniformly over-dense but consists of over-dense irregularities. As the wave travels from the satellite to the irregularities, the retardation of the wave decreases as the wave frequency increases. Thus, the apparent height of reflection B’ approaches the real height I? at high frequencies. The difference in real height between the start of the branch B at 1.85 MC/S and its termination at 4.0 MC/S is an indication of the thickness of the region containing the irregularities. In the profile shown the difference in height is approximately 40 km. Also, the frequency of the termination of the curve B’ (4.0 MC/S) indicates the maximum electron density of the particular irregularity. Thus, the curves B and B’ are explained by a region of turbulence containing over-dense irregularities, which is located below the satellite. The variation in height of the irregularities as a function of latitude is depicted in Fig. 2. The ionogram in Fig. 2a, recorded at a geomagnetic latitude of 15.9” N indicates a turbulent region just above the F layer maximum. At lower geomagnetic latitudes (Figs. 2b and 2c) the turbulent region increases in height and approaches the height of the satellite at the geomagnetic equator (Fig. 2d). At latitudes south of the mag netic equator the turbulent region decreases in height as shown in Figs. 2e and 2f. These ionograms, as well as others, show that the ordinary and extraordinary wave traces are partially and sometimes completely obscured by the echoes reflected from the turbulent region. The ionograms of Fig. 2 were selected from a series of twenty-five ionograms recorded during a satellite pass. For each ionogram in the series the minimum range of the irregularity was determined. Fig. 3 shows this minimum range as a function of geomagnetic latitude. Calculations were made using the theoretical OCTOBER 18,1962

GEOMAGNETIC LATITUDE FIG. 3. A

COMPARISON

OF THE MEASURED

FROM THE SATELLITE

AND

CALCULAIXD

TO A MAGNETIC

FIELD

MMlMIJhi

DISTANCE

LINE.

dipole field to determine the minimum range from the satellite to the surface defined by the rotation of a magnetic field line about the magnetic polar axis. The minimum range of the surface which crosses the geomagnetic equator at a height of 1000 km agrees well with the minimum range of the irregularities. The close agreement between the minimum range to the irregularity and the minimum distance from the satellite to the surface defined by the magnetic field line is evidence that the irregularities are located on a surface defined by the rotation of a magnetic field line about the magnetic polar axis. At present, other records indicate that at the magnetic equator the minimum range of the echoes reflected from the irregularities does not exceed 400 km. Another feature of the spread echoes observed is the welldetined maximum range of the echoes reflected by the irregularities. The phenomenon is illustrated in Figs. 1 and 2d by the straight line C’ which indicates the maximum or limiting range of the spread echoes as a function of frequency. The maximum range of the spread echoes can be explained by a model involving direct backscatter from irregularities. Fig. 4 represents the geometry of the backscatter at the magnetic equator where the irregularities are located in a horizontal plane. In Fig. 4, f.is the lowest frequency at which the wave is reflected vertically from the irregularity, r.

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0” -

FIG.

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Research Note

FIG.~. THEGEOMETRY

FOR DIRECTBACKSCAITER EMBEDDED

IN THE

329

FROM THE IRREGULARI~

IONOSPHERES.

the maxiis the range of the echo reflected at the frequency f. (ray path 1). At a frequency f,greater than f*, mum angle #I~ at which a wave can penetrate down to the irregularities (ray path 2, is given by From the geometry of Fig. 4,

where r,,, is the maximum range at the frequency f. Combining (1) and (2) one obtains the linear relationship

For angles greater than I#++,the wave will be reflected obliquely at a height above the irregularities (ray path 3). At the higher frequencies the absence of echoes at the maximum range indicates only that the scattering of the wave back towards the satellite becomes less efficient at greater angles from the vertical. At higher latitudes, where the surface containing the irregularities is inclined, ray tracing is required to determine the maximum range of the backscatter. The latitude at which the irregularities reach the F layer maximum (about i20’) magnetic latitude corresponds to the latitude of the equatorial anomaIym, I.e. . the latitude at which the critical frequency of the Flayer reaches a maximum. The irregularities are observed at about 1900-2130 hr local time when the equatorial anomalytat is pronounced. The irregularities are not observed at about 08OO-0900 hr local time when the equatorial anomaly is poorly detined. Observations have not yet been made at other local times. The observations suggest that the irregularities along the field line are part of the mechanism involved in the maintenance of the equatorial anomaly. Mitra w has suggested that the equatorial anomaly is maintained by a source of electrons at the geomagnetic equator at heights EOtH200 km. The movement of the electrons along a field line from the magnetic equator to the F-layer maximum could cause the region to become turbulent. The following points summarize some of the phenomena observed in topside ionograms recorded near the magnetic equator, and the conclusions made from them: 1. The minimum range of the spread echoes is explained by the reflection of radio waves from over-dense. irregularities located below the satellite. 2. The irregularities are located on a surface defined by the rotation of a magnetic field line about the magnetic polar axis. 3. The maximum range of the spread echoes is a result of the backscatter of the wave, at a maximum angle &,, from the vertical from a plane containing the irr~~iti~. 4. The similar diurnal behaviour of the equatorial anomaly and the spread echoes indicates that they have a common origin.

330

Research

Note

REFERENCES 1. E. S. WARREN, Camf. J. Whys.40, 1692 (1962). 2. L. E. Pm, Cam&. J. Whys.41 (1963) (to be published). 3. D. B. MULDREW, Cunad. J. Phys. 41 (1963) (to be published). 4. G. L. NELMS, Cunad. J. Phys. 41 (1963) (to be published). 5. E. L. HAGG, Cunad. J. Phys. 41 (1963) (to be published). 6. G. E. K. LOCKWOOD,Cunud. J. Phys. 41 (1963) (to be published). 7. E. V. ~PLETON, Nature, Land. 157, 691 (1946). 8. D. F. MARTYN. Phys. Sot. Rept. Ionosphere Conference 260 (1955). 9. S. K. MITRA, Nature, Land. 158, 668 (1946). Department of National Defence, Defence Research Board Defence Research Telecommunications Establishment, Shirley Bay, Ottawa 4, Ontario

G. E. K. LOCKWOOD L. E. PETRIE