On the field alignment of small ionospheric irregularities

Journal ofAtmospheric andTeneetrial Physics, 1969, Vol.51,PP.1499to1444.Pergamon Press.Printed inNorthern Ireland

SHORT PAPER

On the field alignment of small ionospheric irregularities J. E. T~HERID~E Radio Research Centre, University of Auckland, New ZeaIand (Reoeiaed 27 E’ebruary 1969; in wvisedfonn

24 May 1969)

transits of the sattelhtesBeB and BeC observed over a period of 3 yr were examined to find eases when the angle between the ray path and the megnetio field in the ionosphere became less than 10’. The amplitude and polarization acintillationa ocenrring at these times were compared with the ~intillatio~ occurring, on the same transits, when the ray path made an angle of 30’ to the magnetie field. In general the results showed only a slight tendency to field alignment, with a mean axis ratio for the irregularities of about 2 or 3 to 1. The field alignment i greatest near looal midnight, and almost disappears in the afternoon. On 10 per

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cent of the transits the a~int~~atio~ increased greatly near the longitude occasional existence of highly elongated irregularities.

point, showing the

THE OCCURRENCEof small irregularities in the ionosphere is shown by the spreading

of ionosonde traces, the scintillation of signals from radio stars and artificial satellites, the rapid fading of ionospherically reflected radio waves, and the scattering of radio waves. Simple theory suggests that such irregularities should be elongated in the direction of the Earth’s magnetic field, since the ionization can diffuse far more rapidly in this direction than it can across the field. Many experimental records are analysed on the assumption that the irregularities are highly elongated, with the major axis parallel to the Earth’s magnetic field. In a review of F-region irregularities, HERMAN (1966) concluded that the observational evidence showed conclusively the existence of irregula~ties in the ionosphere that are undoubtedly field aligned. This conclusion seems justified for equatorial irregularities, both in the E-region (EGAN, 1960; IRELAND and MAWDSLEY, 1962; BOWLES et al., 1963) and the F-region (COHEN and BOWLES, 1961; KOSTER, 1963; MWLDREW, 1963; RASTOGI et al., 1968). Scintillation and radar studies also strongly suggest the existence of field aligned irregularities in the polar ionosphere (LITTLE eEal., 1962;

WEAVER, 1965).

At medium latitudes, however, the situation is far less definite. Measurements showing highly elongated, field aligned irregularities can be obtained (SINGLETON and LYNCH, 1962; PARKIN, 1968) but these may not be typical of normal conditions. In a series of measurements designed specifically to detect field aligned irregrdarities in the F-region, BROWN and CHAPMAN (1967) concluded that they were not generally field aligned. Observations sizes down to 10 km have showu no tendency 1439

of 2600 individual irregularities with for the irregularities to be elongated

1440

J. E.

TITHERIDGE

in the direction of the magnetic field (TITHERIDGE, 1968). In the present note the tendency for small irregularities to be field aligned is examined, using records of the fluctuations in amplitude and polarization of the signals received from each of two different satellites over a period of 3 yr. 2. OBSERVATIONS Signals from the ionosphere beacon satellites BeB and BeC have been recorded at Auckland (37’5, 175’E) each day since October 1964 and March 1965 respectively. Computer programs were used to select all transits such that the angle between the ray path and the magnetic field in the ionosphere became less than 10”. This occurs once every 2 or 3 days for a given satellite. For the years 1965, 1966 and 1967 a total of 963 possible transits were found. On 525 of these occasions clear records were available of both the amplitude and polarization angle of the 40 MHz signal. These transits are spread uniformly over all seasons and all times of day, and include both the N-S passes of the satellite BeB and the E-W passes of the satellite BeC. They are therefore well suited to obtaining an unbiased measure of effects associated with longitudinal propagation at different times of day and different seasons. For each transit the fluctuations in amplitude and polarization were measured at the time when propagation was most nearly longitudinal, and at 16 min before and after this time. The mean results are given as a function of local time in Fig. 1. This shows the increase of scintillations which is normally observed at night, at all seasons. The change is more apparent in the polarization measurements, and is highly significant. The fluctuations in polarization angle average about &O-OS rad. during the day. This corresponds to a change of about 1Ol3 electrons/m2 in electron content, so that the day time irregularities cause fluctuations of the order of &O*Ol per cent in the total electron content of the ionosphere. At night the fluctuations increase to about f0.05 rad., or about 40.05 per cent in electron content. 3. THE ELONGATION OF THE IRREGULARITIES The fluctuations in phase and amplitude, caused by the presence of small irregularities in the ionosphere, will be larger when propagation is more nearly parallel to the major axis of the irregularity. For the commonly assumed case of a thick diffracting screen, containing a large number of individual field aligned irregularities, the fluctuations in the diffraction pattern are proportional to pl = (a2sin20

+ cos20)-1/4

(1)

where tl is the axis ratio of the irregularities and 0 is the angle between the ray path and the magnetic field (BRIGGS and PARKIN, 1963). This gives an increase by a factor a1f2 from transverse to longitudinal propagation. If the fluctuations are caused by a comparatively small number of isolated irregularities, however, they will be proportional to QI = (x2 sin2 13+ cos2 ey (2) giving an increase by a factor a for propagation parallel to the magnetic field. In the present measurements the angle between the ray path and the magnetic

On the field alignment of small ionospheric irregularities

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1441

1442

J. E.

TITHERIDGE

field in the ionosphere, at the closest approach to longitudinal propagation, averaged 5.3”. 14 min before and after this time, the angle was about 30”. To obtain a measure of the elongation of the irregularities, the fluctuations occurring near the longitudinal point were divided by the average of the fluctuations at the 30” points. The results are shown in histogram form on Fig. 2. The ratios observed

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Fig. 2. The ratio of the fluctuations in amplitude (or polarization) near the longitudinal point, to the fluctuations occurring when the ray path made an angle of 30’ to the magnetic field. The dotted lines show the symmetrical pattern which would be expected if the irregularities had no tendency to be elongated in the direction of the Earth’s magnetic field.

ranged from about 0.2 to 10, and were not consistently greater than 1. Thus the small irregularities in the ionosphere are not consistently field aligned. The histograms peak at about 1.3, indicating a median axial ratio a of about 3.0 from equation (l), or a = 1.7 from equation (2). These results do not include any allowance for the variation of scintillation depth with zenith angle. For the thick diffracting screen considered in equation (l), the scintillation depth is proportional to (see i)l12, where i is the angle between the ray path and the vertical in the ionosphere (BRIGGS and PARKIN, 1963). For the E-W transits of the satellite BeC this gives a decrease of 5 per cent in the scintillation depth at the longitudinal point, compared with the average depth at the 30” points. For the satellite BeB the decrease is 6 per cent. For the case considered in equation (2), where scintillation is caused by isolated irregularities, there is no variation with zenith angle. The effect has therefore been ignored, since it does not seriously affect the present conclusions. The tails of the histograms are quite unsymmetrical. Scintillation increases of more than 7 times were observed on about 10 per cent of the transits, while similar decreases were seldom observed. This indicates that for about 10 per cent of the time field aligned irregularities with large axial ratios were present. The average

On the field alignment of small ionospheric irregularities

1443

value of 8 for these large increases was 4-l’. This is not sufficiently small to give a s~int~latio~ increase of 7 times on simple theory (using real values of a in equations (1) or (Z)), so some ducting meehanism must be operative. The increase in the amplitude scintillations, near the longitudinal point, is shown as a function of local time in Fig. 3. This gives the average (on a logarithmic

Local Time, Hours Fig. 3. The average (on a logarithmic scale) of the ratios giving the increase of amplitude scintillations near the longitudinal point. The vertical line shows the standard error in the mean.

scale) of the values in each 2 hr L.T.. There is a slight diurnal variation, with the mean ratio having a minimum near sunset and a maximum near midnight. 4. CoNcLusIoas N-S transits of the satellite BeB, and E-W transits of the satellite BeC, were examined to find cases where the angle between the ray path and the magnetic field became less than 10”. 525 useful cases were found, over a period of 3 yr. The amplitude and polarization scintillations occurring near the longitudinal point were compared with the scintillations occurring on the same transits, when the ray path made an angle of 30” to the magnetic field. This showed that there was no consistent increase in scintillations near the longitudinal point. Thus, at medium latitude8 the small irregularities commonly present in the ionosphere are not highly elongated in the direction of the magnetic field. There is a slight tendency to field alignment, but the mean axis ratio is only about 2 or 3 to 1. This tendency is greatest near local midnight, and almost disappears in the afternoon. On about 10 per cent of the transits, the scintillations increased by a factor of 7 or more near the longitudinal point. Corresponding decreases were not observed. This suggests the occasional existence of highly elongated irregularities in the ionosphere. The scintillation increases are too large to be explained on first order theory, and suggest the existence of some ducting mechanism.

1444

J. E. TITHERIDGE

Acknowledgements-This work was carried out under National Aeronautics and Space Administration Research Grant Number NGR52-001-001. The records were analysed by Mr. A. H. BRASH. REFERENCES BOWLES K. L., BALSLEY B. B. and COHEN R. BRIGGS B. H. and PARKIN I. A. BROWN G. M. and CHAPMAN J. W. COHEN R. and BOWLES K. L. EGAN R. D. HERMAN J. R. IRELAND W. and MAWDSLEY J. KOSTER J. R. LITTLE C. B., REID G. C., STILTNER E. and MERRITT R. P. MULDREW D. B. PARKIN I. A. RASTOGI R. G., DESHPANDE M. R. and HARISCHANDRA SINGLETON D. G. and LYNCH G. J. E. TITHERIDGE J. E. WEAVER P. F.

1963

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