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The height of large ionospheric irregularities
Joumalof Atmosphericand TeRe%trial
Phyeicn,1966,Vol. 28, PP.266-258. PergamonPreseLtd. Printedin NorthernIreland
SHORT PAPER In future, Short Papera will replace the Reeearch Notes which appeared in previous Nwnbera. For detaile about the Short Papers see L’Mesmge From the Editor-in-Chief”( Vol.27.No. 10,~. 1033,1965)and “Informaation for Contributora” (Inside back cover)
The height of large ionospheric irregularities G. F.
STUART
and J. E.
TITHERIDQE
Radio Research Centre, University of Auckland, New Zealand (Received 3 Augwrt 1965)
&&&-The amplitude of the 54 MC/Ssignal from the satellite Tran8it 4A was recorded at two stations, 100 km apart, from March to August 1964. The time difference between observing similar fluctuations in the fading period at the two atationa was used to determine the height of ionospheric irregularities. 44 irregularities were observed, predominantly between the hours 0800 and 1800 local time, at heights from 180 to 750 km. The horizontal sizes varied from 75 to 520 km. The denser irregularities occurred only near the peak of the ionosphere, between 250 and 400 km, so that the mean percentage density fluctuations were approximately the same at all heights. Two examples of trains of irregularities were found, in which the irregularities lay on a straight line tilted down to the south at an angle of about 15 degrees. There was some evidence that the individual irregularities were also tilted down to the south by this amount. THE Faraday fading of satellite signals gives a sensitive method of observing ionospheric irregularities. This fading is caused by the rotation of the plane of polarisation of the signals as they pass through the ionosphere. Irregular fluctuations in the electron content along the ray path produce variations in the number of rotations, and therefore irregular variations in the fading period of the received signal. If the fading period is plotted a,s a function of the number of rotations the content and size of the irregularities can be determined directly (TFEIERIDQE, 1963). In addition, the height of the irregularities can also be determined if records from two stations situated along the line of the satellite are compared. The height of the irregularities responsible for satellite scintillations have been determined by this method (FRIHAOEN and TROIM, 1960; and others). This note gives some results of six months observations (March-August, 1964) of the 64 MC/S signal from the satellite Transit 4A. The amplitude of the signals was recorded at Auckland (latitude 37*0’S, longitude 176.O’E) and at Cambridge (New Zealand) which is 100 kilometres south in the direction of the prime sweep of the satellite. For accurate measurements clear records showing at least seven fades are required. Because of the slow fading rate at night moat of the observations have therefore been restricted to the daylight hours, between 0800 and 1800 local time, and only 44 irregularity heights have been determined. The height of an irregularity is obtained from the time difference between similar fluctuations at the two stations, and this can be determined more accurately 256 8
256
0. F. STUART
and J. E. TITHERIDCJE
if the fading period (T) is plotted against time. The content is still determined from the areas of the fluctuations on the plot, but by a less accurate relation than that given by Titheridge. The equation for AL>, which is the change oaueckd by the irregularity in the number of rotations. becomes AQ = ! 2’ - ‘lo at = & 2s TT,
0
where To is the fading period at the centre of the fluctuation
LOCAL
and A is the area of the
OC
TIME
Fig. 1. The determination of the height of irregularities from Faraday fading records at two stations. (a) The 54 MC/S Transit 4A signal strength record taken at Cambridge on April 15, 1904; (b) the variation of the total number of rotations with time, from (a); (0) fading period plotted as a function of time, from (a); (d) the corresponding fading period plot at Auckland.
fluctuation. The maximum is then given by
increase in electron content,
c=_
A!2 KHL
=
A 2KHLTo2
e/m2
caused by the irregularity,
(1)
where K = 1.63 x lo-ls m.k.s. units at 54 MC/S and HL is the component of the Earth’s magnetic field along the ray path. The size of the irregularity is taken as the horizontal distance between the half-density points. These correspond to the
The height of large ionospheric irregularities
257
half-area points on the fading period plots. If the separation of these points is seconds (Fig. l(c)) the size of the irregularity is S = V&/h,
km,
D (2)
where hi is the height of the irregularity and h, and I’ are the height and velocity of the satellite. The height of the irregularity is determined from the time difference between observing the centre of the irregularity at the two stations. If this difference is t set, and the stations are separated by a distance of x km, we have
hi =
4 (tv
+
km.
x)
An error of dt set in the measurement of the time difference results in a height error of 6it, = ‘2 6t km. z&
(4) ,
‘\\
700, 6~x3.:
.
Y400. 3 z 300. z
.
.
‘\
.
. .
.
0. . . 0..
l.
l
+*
‘\
‘\
.
‘\ .
l*
. .
‘\ ‘\ . \\ .
.
‘*
5 c3200. P
‘\
. .
ti500. E
‘\
.
,% .-
, ‘;
‘\
.\
.
‘,
..*:*’
.
/_==
,/ ,OO.____-_---1
OO
0.1 C/s
0.2
0.3
0.4
in lo” electrons/m3
0.5
0.6
Fig. 2. The variation with height of the ratio of the content (C) in electrons/m2 to the size (8) in kilometres. The broken line is a Chapman profile having a scale height of 80 km at the peak and increasing to 130 km at a height of 750 km. The density of the profile is 10 per cent of the mean ambient density at the time of the observations.
Figure l(a) shows the signal strength record taken at Cambridge on April 15, 1964. The corresponding variation in the number of Faraday rotations is shown, with an arbitrary zero, in Fig. l(b). The departures from the smooth curve, at A, B and C, are due to clouds of electrons. The size and content of these clouds could be obtained from this plot but can be determined more accurately from Fig. l(c), which is a plot of the fading period with time. Figure l(d) is the corresponding fading period plot for Auckland and it shows the same three irregularities but at later times. Using equations (l), (2) and (3), the content, size and height of the irregularities A, B and C are 5.9 x 1016e/m2, 230 km and 690 km; 4.5 x 1015e/m2. 310 km and 580 km; and 2-S x 1Ol5e/m2, 190 km and 360 km, respectively, Because of the slow fading of the 54 MC/Ssignal the fading period plots are not very
258
G.
F.
STUART
and
J. E. TI~HERIDCE
detailed and the time intervals can be determined only to within about three seconds. This results in an error (from equation (4)) of 25 km in the height of an irregularity situated near the peak of the ionosphere. Records suitable for analysis were obtained on 72 occasions. 27 of these showed similar fluctuations at both stations, and yielded a total of 44 irregularities. The calculated heights ranged from 180 to 750 km, with a median value of 370 km. Most of the irregularities had sizes between 100 and 300 km, although values from 75 to 620 km were observed. There was a definite increase in the size of the irregularities with height, from a mean size of about 100 km at h = 200 km, to 300 km at h = 600 km. This change is, however, caused entirely by experimental limitations, since the size corresponding to a given time interval increases with height. Thus, at all heights, sizes were obtained throughout the observable range. The total content of the irregularities varied from 4 x 1014to 8 x 1O1”e/ma. The content (C) generally increases with the size (8) of the irregularity, and the ratio C/S for the individual irregularities is shown in Fig. 2. The broken line in this figure gives 10 per cent of the probable mean electron density of the ionosphere at the time of the observations. The values of C/S show some correlation with the mean density, with the denser irregularities occurring near the peak of the ionosphere, so that the percentage density fluctuations are approximately the same at all heights. If the vertical extent of the irregularities is taken as one third of the horizontal size (TITHERIDQE,1963) then the density of the irregularities ranges from about 2 to 42 per cent of the ambient density. The three irregularities shown in Fig. 1, with heights of 690, 580 and 360 km, lie roughly on a line with a tilt of 15 degrees down to the south. Only one other record showed a similar train of irregularities, and these lay close to a straight line with a tilt of 18 degrees down to the south. There is also some evidence that the individual irregularities are tilted. The sizes observed from Cambridge were, on the average, 10 per cent less than the sizes (of the same irregularities) observed from Auckland, while the contents were 5 per cent greater. The differences are about 95 per cent significant. Assuming that the irregularities are flattened with a vertical thickness of less than half the horizontal size, this corresponds to a mean tilt of around 15 degrees down to the south. These tilts may indicate a tendency for the irregularities to become field aligned. They also agree in direction with observations of trsvelling ionospheric disturbances, which show large irregularities or fronts tilted down to the south as they move towards the equator (HEISLER,1960). Aciinowledgements-This work was carried out under Research Grant No. NsG-54-60 from the National Aeronautics and Space Administration, Washington. We are grateful to Mr J. E. WYLDE for operating the recorder at Cambridge. One of us (G. F. S.) was on a D.S.I.R. Study Award from the Physics and Engineering Laboratory, New Zealand. REFERENCES FRIHAQEN J. and TROIM J. HEISLER L. H. TITHERIDGE J. E.
1960 1980 1963
J. Atmosph. Terr. Phya. 18, 75. Awt. J. Phya. 18, 656. J. Cfeophys. Rea. 68, 3399.
Phyeicn,1966,Vol. 28, PP.266-258. PergamonPreseLtd. Printedin NorthernIreland
SHORT PAPER In future, Short Papera will replace the Reeearch Notes which appeared in previous Nwnbera. For detaile about the Short Papers see L’Mesmge From the Editor-in-Chief”( Vol.27.No. 10,~. 1033,1965)and “Informaation for Contributora” (Inside back cover)
The height of large ionospheric irregularities G. F.
STUART
and J. E.
TITHERIDQE
Radio Research Centre, University of Auckland, New Zealand (Received 3 Augwrt 1965)
&&&-The amplitude of the 54 MC/Ssignal from the satellite Tran8it 4A was recorded at two stations, 100 km apart, from March to August 1964. The time difference between observing similar fluctuations in the fading period at the two atationa was used to determine the height of ionospheric irregularities. 44 irregularities were observed, predominantly between the hours 0800 and 1800 local time, at heights from 180 to 750 km. The horizontal sizes varied from 75 to 520 km. The denser irregularities occurred only near the peak of the ionosphere, between 250 and 400 km, so that the mean percentage density fluctuations were approximately the same at all heights. Two examples of trains of irregularities were found, in which the irregularities lay on a straight line tilted down to the south at an angle of about 15 degrees. There was some evidence that the individual irregularities were also tilted down to the south by this amount. THE Faraday fading of satellite signals gives a sensitive method of observing ionospheric irregularities. This fading is caused by the rotation of the plane of polarisation of the signals as they pass through the ionosphere. Irregular fluctuations in the electron content along the ray path produce variations in the number of rotations, and therefore irregular variations in the fading period of the received signal. If the fading period is plotted a,s a function of the number of rotations the content and size of the irregularities can be determined directly (TFEIERIDQE, 1963). In addition, the height of the irregularities can also be determined if records from two stations situated along the line of the satellite are compared. The height of the irregularities responsible for satellite scintillations have been determined by this method (FRIHAOEN and TROIM, 1960; and others). This note gives some results of six months observations (March-August, 1964) of the 64 MC/S signal from the satellite Transit 4A. The amplitude of the signals was recorded at Auckland (latitude 37*0’S, longitude 176.O’E) and at Cambridge (New Zealand) which is 100 kilometres south in the direction of the prime sweep of the satellite. For accurate measurements clear records showing at least seven fades are required. Because of the slow fading rate at night moat of the observations have therefore been restricted to the daylight hours, between 0800 and 1800 local time, and only 44 irregularity heights have been determined. The height of an irregularity is obtained from the time difference between similar fluctuations at the two stations, and this can be determined more accurately 256 8
256
0. F. STUART
and J. E. TITHERIDCJE
if the fading period (T) is plotted against time. The content is still determined from the areas of the fluctuations on the plot, but by a less accurate relation than that given by Titheridge. The equation for AL>, which is the change oaueckd by the irregularity in the number of rotations. becomes AQ = ! 2’ - ‘lo at = & 2s TT,
0
where To is the fading period at the centre of the fluctuation
LOCAL
and A is the area of the
OC
TIME
Fig. 1. The determination of the height of irregularities from Faraday fading records at two stations. (a) The 54 MC/S Transit 4A signal strength record taken at Cambridge on April 15, 1904; (b) the variation of the total number of rotations with time, from (a); (0) fading period plotted as a function of time, from (a); (d) the corresponding fading period plot at Auckland.
fluctuation. The maximum is then given by
increase in electron content,
c=_
A!2 KHL
=
A 2KHLTo2
e/m2
caused by the irregularity,
(1)
where K = 1.63 x lo-ls m.k.s. units at 54 MC/S and HL is the component of the Earth’s magnetic field along the ray path. The size of the irregularity is taken as the horizontal distance between the half-density points. These correspond to the
The height of large ionospheric irregularities
257
half-area points on the fading period plots. If the separation of these points is seconds (Fig. l(c)) the size of the irregularity is S = V&/h,
km,
D (2)
where hi is the height of the irregularity and h, and I’ are the height and velocity of the satellite. The height of the irregularity is determined from the time difference between observing the centre of the irregularity at the two stations. If this difference is t set, and the stations are separated by a distance of x km, we have
hi =
4 (tv
+
km.
x)
An error of dt set in the measurement of the time difference results in a height error of 6it, = ‘2 6t km. z&
(4) ,
‘\\
700, 6~x3.:
.
Y400. 3 z 300. z
.
.
‘\
.
. .
.
0. . . 0..
l.
l
+*
‘\
‘\
.
‘\ .
l*
. .
‘\ ‘\ . \\ .
.
‘*
5 c3200. P
‘\
. .
ti500. E
‘\
.
,% .-
, ‘;
‘\
.\
.
‘,
..*:*’
.
/_==
,/ ,OO.____-_---1
OO
0.1 C/s
0.2
0.3
0.4
in lo” electrons/m3
0.5
0.6
Fig. 2. The variation with height of the ratio of the content (C) in electrons/m2 to the size (8) in kilometres. The broken line is a Chapman profile having a scale height of 80 km at the peak and increasing to 130 km at a height of 750 km. The density of the profile is 10 per cent of the mean ambient density at the time of the observations.
Figure l(a) shows the signal strength record taken at Cambridge on April 15, 1964. The corresponding variation in the number of Faraday rotations is shown, with an arbitrary zero, in Fig. l(b). The departures from the smooth curve, at A, B and C, are due to clouds of electrons. The size and content of these clouds could be obtained from this plot but can be determined more accurately from Fig. l(c), which is a plot of the fading period with time. Figure l(d) is the corresponding fading period plot for Auckland and it shows the same three irregularities but at later times. Using equations (l), (2) and (3), the content, size and height of the irregularities A, B and C are 5.9 x 1016e/m2, 230 km and 690 km; 4.5 x 1015e/m2. 310 km and 580 km; and 2-S x 1Ol5e/m2, 190 km and 360 km, respectively, Because of the slow fading of the 54 MC/Ssignal the fading period plots are not very
258
G.
F.
STUART
and
J. E. TI~HERIDCE
detailed and the time intervals can be determined only to within about three seconds. This results in an error (from equation (4)) of 25 km in the height of an irregularity situated near the peak of the ionosphere. Records suitable for analysis were obtained on 72 occasions. 27 of these showed similar fluctuations at both stations, and yielded a total of 44 irregularities. The calculated heights ranged from 180 to 750 km, with a median value of 370 km. Most of the irregularities had sizes between 100 and 300 km, although values from 75 to 620 km were observed. There was a definite increase in the size of the irregularities with height, from a mean size of about 100 km at h = 200 km, to 300 km at h = 600 km. This change is, however, caused entirely by experimental limitations, since the size corresponding to a given time interval increases with height. Thus, at all heights, sizes were obtained throughout the observable range. The total content of the irregularities varied from 4 x 1014to 8 x 1O1”e/ma. The content (C) generally increases with the size (8) of the irregularity, and the ratio C/S for the individual irregularities is shown in Fig. 2. The broken line in this figure gives 10 per cent of the probable mean electron density of the ionosphere at the time of the observations. The values of C/S show some correlation with the mean density, with the denser irregularities occurring near the peak of the ionosphere, so that the percentage density fluctuations are approximately the same at all heights. If the vertical extent of the irregularities is taken as one third of the horizontal size (TITHERIDQE,1963) then the density of the irregularities ranges from about 2 to 42 per cent of the ambient density. The three irregularities shown in Fig. 1, with heights of 690, 580 and 360 km, lie roughly on a line with a tilt of 15 degrees down to the south. Only one other record showed a similar train of irregularities, and these lay close to a straight line with a tilt of 18 degrees down to the south. There is also some evidence that the individual irregularities are tilted. The sizes observed from Cambridge were, on the average, 10 per cent less than the sizes (of the same irregularities) observed from Auckland, while the contents were 5 per cent greater. The differences are about 95 per cent significant. Assuming that the irregularities are flattened with a vertical thickness of less than half the horizontal size, this corresponds to a mean tilt of around 15 degrees down to the south. These tilts may indicate a tendency for the irregularities to become field aligned. They also agree in direction with observations of trsvelling ionospheric disturbances, which show large irregularities or fronts tilted down to the south as they move towards the equator (HEISLER,1960). Aciinowledgements-This work was carried out under Research Grant No. NsG-54-60 from the National Aeronautics and Space Administration, Washington. We are grateful to Mr J. E. WYLDE for operating the recorder at Cambridge. One of us (G. F. S.) was on a D.S.I.R. Study Award from the Physics and Engineering Laboratory, New Zealand. REFERENCES FRIHAQEN J. and TROIM J. HEISLER L. H. TITHERIDGE J. E.
1960 1980 1963
J. Atmosph. Terr. Phya. 18, 75. Awt. J. Phya. 18, 656. J. Cfeophys. Rea. 68, 3399.