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Some characteristics of the ground diffraction pattern caused by the vertical reflection of radio waves from the ionosphere
Journalof Atmosphericand TerreatrialPhyaics, 1968, Vol. 28, pp. 99 to 102. PergamonPressLtd. Printedin Northern Ireland
SHORT PAPER
Some characteristics of the ground di!Tractionpattern caused by the vertical reflection of radio waves from the ionosphere R. F. KELLEFLER Physics Department, University College, Nairobi (Receive& 8 JuEy 1965) AbstrsCt-Ionospherically reflected pulses on a frequency of 4.85 MC/Swere recorded at Slough between September 1962 and August 1963 on three aerials at the corners of a triangle with sides of length about 1.6 wavelengths. The usual parameters referring to the size and rate of change of the ground diffraction pattern are presented in the tables mainly for echoes returned from the F-region. analysis of BRIUQS et al. (1950), as extended by PHILLIPS and SPENCER (1955). generally regarded as providing the most detailed information on the characteristics of ground diffraction patterns caused by radio waves reflected from, or transmitted through, an irregular ionosphere. The results of the analysis are usually given in terms of the following parameters: THE CORRELATION
is
r-axial ratio of the characteristic ellipse; V-drift velocity; (V,),/V--ratio characterising the relative importance of random variations and steady drift; d-size of characteristic ellipse (here taken as the length of the semi-minor axis of the ellipse p = O-5); y-orientation of the major axis of the characteristic ellipse; @-direction of drift vector. The correlation method is based on recordings of the time variations in the amplitude or phase of the reflected wave at three separated receiving points. An experiment using a more complex array of receiving aerials was carried out by the writer at the Radio and Space Research Station, Slough, in order to test some of the assumptions on which the correlation analysis is baaed. It is hoped to publish an account of this work in the near future. The purpose of the present note is to describe some of the results obtained by applying the conventional analysis to the amplitude fading records derived from the three most closely spaced aerials (separation of about 1.61). The sounding frequency of 465 MC/Slimited the study to the day-time ionosphere (OSOO1700 GMT). The transmitting aerial system consisted of two folded dipoles set at right angles to each other and fed 90” out of phase so as to produce a circularly polarized signal. The sense of the polarization could be easily reversed. The experiment was carried out between September 1962 and August 1963, and in all 109 records were analysed. Of these, 89 refer to F-region echoes (both ordinary and extraordinary) and 20 to sporadic-E echoes. The median values of the different pattern parameters are shown in Table 1. The parameter P, has been introduced to provide a quantitative measure of the tendency of the major axis of the characteristic ellipse to lie along the magnetic meridian. It is defined as the percentage number of occasions when the major axis is within &20” of the direction of the horizontal component of the magnetic field. No discussion of the direction of the drift vectors is given here since it proved impossible to determine any definite trends in this parameter. 99
The only significant difference which could be detected between the results for the ordinary and extraordinary echoes (F-region) was in the value of the-pattern drift velocity. The mean values obtained were: E’, 7 71 mjser : 1”, = 123 m/se<:. The overall data were examined for seasonal effects and changes with magnetic activity (K, from 0 to 4). The results are shown in Tables 2, 3 and 4 (the E, records were all obtained during the summer months). There is little previous work available on seasonal changes in the characteristics of the diffraction pattern. It is possible that some of the changes observed here might be due to the difference inthe mean values of K, corresponding to all the records in each season (cf. the last column in Table 2). As far as the effeets of magnetic activity are concerned, there seems to be little consistency between the present results and those obtained by RAO and RAO (19646). This may be explained by the fact that the two studies ware made at different, latitudes. On the other hand Table
Season
No. of Records
r
Spring Summer Autumn Winter
16 24 8 39
2.15 1.70 1.59 1.83
2
-. 75 33 50 63
1,’ (m/see) 85 71 142 89
( V,l,iV
----_
1.25 1.0 I.2 1.0
170 180 176 240
Table 3. P-Region
K ~-__9 1 2 3 4
1
2 3
f
5’ (m/see?
(V,),lV
td
-.
WI ..-._“_ _..._ ._.
2.0 1.8 2.0 1.6 1.5
80 31 59 33 29
90 s7 67 117 99
1.28 1.0 0.9 0.96 0.87
207
1.62 2.95 3-25
43 80 83
59 76 83
0.96 o-93 O-72
124 57 119
184 161 189 145
R I.3 2.2 2.4 1.9
Short paper
101
FORKS (1961), working at Cambridge, found a decrease in the F-region values of both (V&/V and d during magnetic storms, in agreement with the trends found here. There is no indication that F-region drift velocities vary in any definite way with magnetic activity indices below 5, although there does seem to be a marked increase in sporadic-E velocities with K values between 1 and 3. An investigation has been made of the short term changes in the characteristics of the diffraction patterns. On a number of occasions it was possible to analyse two or more sections of fading records having a time interval of 4 hr or less. In general it was found that one or more 00
o Es
400
t
. l
.
300
A
F-region
l
.
200
.
.
0
0
.+
~
l
Fig. 1. Scatter plot of A against time interval between records.
r
400-
l * .
300-
.
*
A
‘
200 -
l* *
*
100 -
.
. .
0
08
I
I IO
I
I 12
I
I 14
I
i
I
16
GMT
Fig. 2. Scatter plot of A against time of day (F-region). pattern parame~~ might remain reasonably constant over this interval whilst the others varied considerably. In order to provide a measure of the total change, a quantity A was definedbeing the sum of the percentage changes in r, V, (V,),/V and d, plus the angular changes (degrees) in 1pand 0. When A is plotted against the timeGnterva1 between records (Fig. l), the Es rem&e seem to indicate a fairly regular increase, whereas the P-region points are scattered. If however the F-region results are plotted against time of day (Fig. 2), a reasonably smooth curve with e minimum around noon is obtained. It is possible that some of the variations in the different parameters with season and magnetic activity and the dBerence in the drift velocities for the ordinary and extraordinary echoes might, in fact, be explained in terms of changes in the height of reflection of the exploring wave (RAO and RAO, 1964a). work was done at the Radio and Spaoe Research Station of the Science Researoh Council and is published by permission of the Director. The help of Mr. J. A. RatcliiIe
Acknow&dpmae~t.+-This
and Dr. E. N. Bramley is gratefully acknowledged. receipt of’ a 1I.S.J.R. Studentship.
Lrhe author also wishes to acknowledge
REFl?&EKC‘ES BRIWS B. H.. P~XILLIPS (:. J. SHINN D. H. FOOKS G. P. PHILLIPS G. J . and SPENCER IN. RAO P. B. and RAO B. R. RAO I'. 13. and RAO B. R’.
und
1950 1961 1955 1964a 19640
t’roc. Phys.
Sot.
Land.
B63,
106.
J. Atmosph. Terr. Phys. 22,43. Proc. Phya. Sot. Lond. B08, 481. J. Atmosph. Terr. Phys. 26, 231. J. Atmosph. Terr. Ph?ys. 26, 445.
the
SHORT PAPER
Some characteristics of the ground di!Tractionpattern caused by the vertical reflection of radio waves from the ionosphere R. F. KELLEFLER Physics Department, University College, Nairobi (Receive& 8 JuEy 1965) AbstrsCt-Ionospherically reflected pulses on a frequency of 4.85 MC/Swere recorded at Slough between September 1962 and August 1963 on three aerials at the corners of a triangle with sides of length about 1.6 wavelengths. The usual parameters referring to the size and rate of change of the ground diffraction pattern are presented in the tables mainly for echoes returned from the F-region. analysis of BRIUQS et al. (1950), as extended by PHILLIPS and SPENCER (1955). generally regarded as providing the most detailed information on the characteristics of ground diffraction patterns caused by radio waves reflected from, or transmitted through, an irregular ionosphere. The results of the analysis are usually given in terms of the following parameters: THE CORRELATION
is
r-axial ratio of the characteristic ellipse; V-drift velocity; (V,),/V--ratio characterising the relative importance of random variations and steady drift; d-size of characteristic ellipse (here taken as the length of the semi-minor axis of the ellipse p = O-5); y-orientation of the major axis of the characteristic ellipse; @-direction of drift vector. The correlation method is based on recordings of the time variations in the amplitude or phase of the reflected wave at three separated receiving points. An experiment using a more complex array of receiving aerials was carried out by the writer at the Radio and Space Research Station, Slough, in order to test some of the assumptions on which the correlation analysis is baaed. It is hoped to publish an account of this work in the near future. The purpose of the present note is to describe some of the results obtained by applying the conventional analysis to the amplitude fading records derived from the three most closely spaced aerials (separation of about 1.61). The sounding frequency of 465 MC/Slimited the study to the day-time ionosphere (OSOO1700 GMT). The transmitting aerial system consisted of two folded dipoles set at right angles to each other and fed 90” out of phase so as to produce a circularly polarized signal. The sense of the polarization could be easily reversed. The experiment was carried out between September 1962 and August 1963, and in all 109 records were analysed. Of these, 89 refer to F-region echoes (both ordinary and extraordinary) and 20 to sporadic-E echoes. The median values of the different pattern parameters are shown in Table 1. The parameter P, has been introduced to provide a quantitative measure of the tendency of the major axis of the characteristic ellipse to lie along the magnetic meridian. It is defined as the percentage number of occasions when the major axis is within &20” of the direction of the horizontal component of the magnetic field. No discussion of the direction of the drift vectors is given here since it proved impossible to determine any definite trends in this parameter. 99
The only significant difference which could be detected between the results for the ordinary and extraordinary echoes (F-region) was in the value of the-pattern drift velocity. The mean values obtained were: E’, 7 71 mjser : 1”, = 123 m/se<:. The overall data were examined for seasonal effects and changes with magnetic activity (K, from 0 to 4). The results are shown in Tables 2, 3 and 4 (the E, records were all obtained during the summer months). There is little previous work available on seasonal changes in the characteristics of the diffraction pattern. It is possible that some of the changes observed here might be due to the difference inthe mean values of K, corresponding to all the records in each season (cf. the last column in Table 2). As far as the effeets of magnetic activity are concerned, there seems to be little consistency between the present results and those obtained by RAO and RAO (19646). This may be explained by the fact that the two studies ware made at different, latitudes. On the other hand Table
Season
No. of Records
r
Spring Summer Autumn Winter
16 24 8 39
2.15 1.70 1.59 1.83
2
-. 75 33 50 63
1,’ (m/see) 85 71 142 89
( V,l,iV
----_
1.25 1.0 I.2 1.0
170 180 176 240
Table 3. P-Region
K ~-__9 1 2 3 4
1
2 3
f
5’ (m/see?
(V,),lV
td
-.
WI ..-._“_ _..._ ._.
2.0 1.8 2.0 1.6 1.5
80 31 59 33 29
90 s7 67 117 99
1.28 1.0 0.9 0.96 0.87
207
1.62 2.95 3-25
43 80 83
59 76 83
0.96 o-93 O-72
124 57 119
184 161 189 145
R I.3 2.2 2.4 1.9
Short paper
101
FORKS (1961), working at Cambridge, found a decrease in the F-region values of both (V&/V and d during magnetic storms, in agreement with the trends found here. There is no indication that F-region drift velocities vary in any definite way with magnetic activity indices below 5, although there does seem to be a marked increase in sporadic-E velocities with K values between 1 and 3. An investigation has been made of the short term changes in the characteristics of the diffraction patterns. On a number of occasions it was possible to analyse two or more sections of fading records having a time interval of 4 hr or less. In general it was found that one or more 00
o Es
400
t
. l
.
300
A
F-region
l
.
200
.
.
0
0
.+
~
l
Fig. 1. Scatter plot of A against time interval between records.
r
400-
l * .
300-
.
*
A
‘
200 -
l* *
*
100 -
.
. .
0
08
I
I IO
I
I 12
I
I 14
I
i
I
16
GMT
Fig. 2. Scatter plot of A against time of day (F-region). pattern parame~~ might remain reasonably constant over this interval whilst the others varied considerably. In order to provide a measure of the total change, a quantity A was definedbeing the sum of the percentage changes in r, V, (V,),/V and d, plus the angular changes (degrees) in 1pand 0. When A is plotted against the timeGnterva1 between records (Fig. l), the Es rem&e seem to indicate a fairly regular increase, whereas the P-region points are scattered. If however the F-region results are plotted against time of day (Fig. 2), a reasonably smooth curve with e minimum around noon is obtained. It is possible that some of the variations in the different parameters with season and magnetic activity and the dBerence in the drift velocities for the ordinary and extraordinary echoes might, in fact, be explained in terms of changes in the height of reflection of the exploring wave (RAO and RAO, 1964a). work was done at the Radio and Spaoe Research Station of the Science Researoh Council and is published by permission of the Director. The help of Mr. J. A. RatcliiIe
Acknow&dpmae~t.+-This
and Dr. E. N. Bramley is gratefully acknowledged. receipt of’ a 1I.S.J.R. Studentship.
Lrhe author also wishes to acknowledge
REFl?&EKC‘ES BRIWS B. H.. P~XILLIPS (:. J. SHINN D. H. FOOKS G. P. PHILLIPS G. J . and SPENCER IN. RAO P. B. and RAO B. R. RAO I'. 13. and RAO B. R’.
und
1950 1961 1955 1964a 19640
t’roc. Phys.
Sot.
Land.
B63,
106.
J. Atmosph. Terr. Phys. 22,43. Proc. Phya. Sot. Lond. B08, 481. J. Atmosph. Terr. Phys. 26, 231. J. Atmosph. Terr. Ph?ys. 26, 445.
the