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Measurement of Faraday rotation of radar meteor echoes for the modelling of electron densities in the lower ionosphere
Journal of Armospherii
and Solar-Terresrrial
Physics,
PII:SOO21-9169(96)00083-9
Vol. 59, No. 9. pp. 1021-1024, 1997 tD 1997 Elsev~er Science Ltd reserved. Prmted in Great Britam 136&668?6!97 $17.00+0.00
All rights
Measurement of Faraday rotation of radar meteor echoes for the modelling of electron densities in the lower ionosphere W. G. Elford and A. D. Taylor Department
of Physics and Mathematical
Physics,
University
of Adelaide.
Adelaide
5005, South Australia
(Received infinalform 10 May 1996; accepted 24 May 1996) Abstract-Narrow beam VHF radars are able to detect transient ionization at heights between 80 and 120 km produced by meteoroids entering the atmosphere ‘down-the-beam’. Using a vertically directed beam, observations of this new type of meteor echo on two orthogonal receiving arrays enable the orientation of the plane of polarization to be measured. Significant Faraday rotation is observed to occur during daytime at heights above 80 km. Such observations make possible the measurement of ionospheric electron density profiles between heights of 80 and 120 km. 0 1997 Elsevier Science Ltd
INTRODUCTION
A linearly polarized radio wave passing through the ionosphere undergoes Faraday rotation of the plane of polarization due to the presence of the Earth’s magnetic field. Measurements of the amount of rotation leads to estimates of the total electron content of the ionosphere between the source and the receiver (Garriott et al., 1970; Ratcliffe, 1972). The sources for such observations are usually satellites, some of which have been equipped with transmitters and antennas specially designed for the purpose. The fact that radio waves reflected from meteor trails undergo Faraday rotation was first discussed by Billam and Browne (1956) in their study of the polarization characteristics of radar echoes. They were chiefly concerned as to whether the amount of rotation due to the ionosphere needed to be taken into account, and their calculations showed that in their situation it could be ignored. However they did add the following proviso, “for aerial directions close to that of the terrestrial magnetic field...the rotation can be of the order of one radian during daytime”. In retrospect, it is clear that this statement has been overlooked by almost all who have been involved in radar meteor astronomy and radar studies of the atmosphere using meteors. One exception is an article by Baggaley (1979) who estimated the effect of Faraday rotation on a meteor forward scatter link. It is now evident that Faraday rotation can have quite profound effects on the interpretation of radar meteor rates (Elford, 1993). However, that topic is addressed elsewhere (Elford, 1997). Here we are concerned with a simple application of the Faraday rotation of echoes
from meteor trails to the measurement of electron densities in the lower ionosphere. Recently, a new type of meteor echo has been detected using a narrow beam VHF radar. The radar reflections have been observed from the ‘head’ of meteors moving almost directly down the beam and appearing in successive range gates. During daytime the plane of polarization of the echoes is observed to rotate between successive range gates. For a vertically directed beam this leads to estimations of the electron content within each height interval, and thence to the electron density profile over the length of the meteor trail.
FARADAY ROTATION OF METEOR ECHOES
The amount of rotation of the plane of polarization of a linearly polarized radio wave propagated between the ground and a meteor trail (assuming quasi-longitudinal approximation to the refractive index) is given by R = 2.36 x 104f--ZJp”th Bcos~N(s)ds where f is the wave frequency (Hz), B is the local geomagnetic field strength (Wb m--2), x is the angle between the wave propagation direction and the direction of B, N is the electron density (mm3), ds is a path element (m). To assess the expected magnitude of the rotation of the plane of polarization of a radio signal back-scattered from meteors at different heights, values of the 1021
1022
W. G. Elford and A. D. Taylor
angle rZ were calculated for the 54 MHz radar at the Buckland Park Research Station (40 km north of Adelaide, Australia) for a typical mid-latitude, midday, low sunspot number ionosphere. The distribution of electron density below 115 km was taken from Mechtly et al. (1972) and extended to 120 km by joining smoothly to profiles given by Hargreaves (1979). The geomagnetic field at Adelaide has an intensity of 5.9 x 10m5Wbm-* and a dip angle of 67”. Table 1 lists the total amount of rotation (2 a) to be expected for back-scattering from meteors occurring at heights between 80 and 120 km in daytime. A glance at Table 1 suggests that Faraday rotation should be readily detectable for trails occurring above 95 km. OBSERVATIONS Observations of Faraday rotation were carried out using the VHF radar located at Buckland Park and operating on 54.1 MHz, during 5-l 3 September 1995. The transmitter generated 12.6 /JS pulses at a repetition rate of 1024 Hz with a peak power of 28 kW. A large collinear coaxial (CoCo) transmitting array, 89 m square, produces a 3.2” (full width half power) wide beam directed vertically, giving a gain of about 3200 relative to an isotropic source. The antenna efficiency is 29%. Two groups of 16 vertically directed Yagi antennas (each 4 x 4) were used for reception. The gain of each array was 150. One receiving array was aligned parallel with the transmitting array, while the other could be placed parallel or orthogonal to the transmitting array. The receivers are phase coherent, and the echo amplitude and phase are calculated from the in-phase and quadrature output signals. Speed limitations in the data acquisition equipment restricted sampling of the two receiver channels to 20 ps intervals giving a range resolution of 3 km. The Table 1. Total Faraday rotation for backscatter from meteors at 54MHz at Buckland Park (midday, sunspot minimum) Height (km)
Rotation (deg)
80 85 90 95 100 105 110 115 120
0 1 3 8 19 34 53 17 103
necessity to use a 12.6~s transmitter pulse implies that, of those meteors present, some were not detected since the echo would have fallen in the range-scan gaps. The meteor detection criteria required that the echo amplitude at a given range exceeded twice the mean noise for four successive transmitter pulses. Only meteor echoes exhibiting evidence of coming ‘down-the-beam’ were analysed, thereby removing the more commonly observed transverse meteor trails which for a vertically directed beam have a high probability of occurring in a side lobe. These criteria translated into echoes of short duration in successively decreasing range intervals, and a rapid but consistent change in measured phase. Of the 75 ‘down-the-beam’ echoes detected over the eight days, two were rejected because the amplitude showed complex rapid changes which were assumed to be due to beating between scattering from multiple trails associated with a fragmented particle, and one echo was excluded because of an abnormally long range which was assumed to be from a meteor detected in a side lobe. A total of 23 ‘down-the-beam’ meteors (giving 36 samples from separate heights) were detected in the three days spent checking the calibration, while the echo polarization angle was determined for 40 meteors (61 heights) in the other five days. These echo rates are about 10% of the rate of ‘down-the-beam’ meteors when the CoCo array is used for both and transmission and reception. As the measurement of the polarization angle depends on the comparison of the amplitude of the echoes from the two Yagi arrays, it is essential that the two receiving systems have the same or known gains. This was achieved by recording the mean sky noise in both systems and using the measured values to continuously determine the relative gains. An independent check on the gains of the systems was carried out by observing ‘down-the-beam’ meteors for three days with the both Yagi arrays aligned parallel with the transmitting array. The relative gains were determined by comparing the recorded sky noise in both systems, and the echoes were analysed as if they were detected in orthogonally aligned systems. Under these conditions the apparent polarization angle should be 45”. The results are shown in Fig. 1, from which it is clear that the gain comparison method works well. Moreover, the plot indicates an uncertainty of a few degrees in the measurement of the echo polarization angles. POLARIZATION MEASUREMENTS To measure the polarization of the meteor echoes, the two Yagi arrays were aligned one parallel to the
Meteor
ionisation
1023
in the lower ionosphere
I
t
1
t
t
*I b3
o % w
6-
.
“0 !is 43 z -
2-
0
Angle (degree) Fig. 1. Distribution of the measurements of the ‘apparent polarization angle’ deduced from meteors observed simultaneously on two receiving arrays with parallel polarization. The measurements cluster about 45” as expected. The spread of the results gives an estimate of the precision to be expected in the actual measurements presented in Fig. 2.
transmitting array and the other orthogonal to it. The echo amplitude profiles were smoothed using a lopoint (10 ms) moving average and the peak amplitudes determined and adjusted for the relative gains of the
two systems. The arctangent of the resultant amplitude ratio gave the polarization angle. The results are shown in Fig. 2 where the short bars represent the polarization of the meteor echo observed at the
,,.‘...‘.*,‘...‘.I.‘..,
II5
-it
110 -
0
1.. 4
* I. 8
* = I. 12
”
I
16
*.
.
1.. 20
. 24
Hour (CST) Fig. 2. The observed polarization (short bars) of radar signals reflected from meteors at the heights where the bar lines are plotted. The centre of each bar is set at the time of the observation (Central Australian Time, CST, is very close to local solar time). The observation of an individual meteor at several heights is indicated by a dotted line linking the bar lines. The ‘noon model’ plot at the right is calculated for a model ionosphere as described in the text.
1024
W. G. Elford and A. D. Taylor
ground after reflection from a meteor at the height indicated. Observations at several heights from the one meteor are indicated by linking the polarization bars with a dotted vertical line. The results are summarized as follows:
COMMENTS
of the 43 night-time polarization measurements, two show a small non-zero angle, all daytime measurements at heights above 97 km show significant rotation, midday measurements show rotation down to a height of 90 km, the one daytime measurement at 85 km shows no measurable rotation. the rotation observed at several heights from a single meteor always increases with height. For comparison, the polarization predicted by the model described earlier and listed in Table 1, is shown in bar form at the right of Fig. 2. There is substantial similarity between the model and the observations, and in fact very close agreement is achieved if the mode1 data is shifted 3 km lower.
The preliminary observations of Faraday rotation of radar echoes reflected from meteors using a narrow beam VHF wind-profiler radar in the vertical mode shows that such systems can be used to determine the daytime electron density profile at heights between 90 and 115km for localities at medium and high magnetic latitudes. By the addition of a second receiving antenna of the same size as the transmitting array, and oriented orthogonally, ‘down-the-beam’ meteor echoes would give polarization measurements up to heights of 120 km, with many individual meteors giving instantaneous profiles over 1520 km in height, As mentioned earlier, VHF radars with full-size arrays detect at least 100 down-the-beam echoes per day, with most occurring between midnight and noon. A second CoCo array is being added to the VHF radar at Buckland Park, oriented orthogonally to the existing array, and it is intended that the measurement of electron density profiles between 80 and 120 km will become part of the routine operation of the system.
REFERENCES
Baggaley W. J.
1979
Billam E. R. and Browne I. C
1956
Elford W. G.
1993
Elford W. G.
1997
Garriott
1970
0. K.. Da Rosa A. V. and Ross W. J.
Hargreaves
J. K.
1979
Mechtly
E. A., Bowhill S. A. and Smith L. G.
1972
Ratcliffe
J. A.
1972
D-region absorption and forward-scatter radio-meteor data. J. atmos. lerr. Phys. 41, 671-676. Characteristics FO radio echoes from meteor trails. IV: Polarization effects. Proc. Phys. Sot. B69, 98-l 13. Radar observations of meteors. Meteoroids and Their Parent Bodies, &oh1 J. and Williams I. P., eds) Astronomical Inst., Slovak Acad. Sci. Bratislava, pp. 235-244. Scattering of radio waves from meteor trails. Interplanetary Dust, (Griin, E., Dermott, S. F., Fechtig, H. and Gustafson, B. A. S., eds). Univ. of Arizona Press, Tuscan. Electron content obtained from Faraday rotation and phase path length variations. J. atmos. terr. Phys. 32, 705-727. The Upper Atmosphere and Solar-Terrestrial Relutions. Van Nostrand Reinhold, New York. Changes of lower ionosphere electron concentrations with solar activity. J. atmos. terr. Phys. 34, 1899. An Introduction to the Ionosphere and Magnetosphere, Cambridge University Press, 256 pp.
and Solar-Terresrrial
Physics,
PII:SOO21-9169(96)00083-9
Vol. 59, No. 9. pp. 1021-1024, 1997 tD 1997 Elsev~er Science Ltd reserved. Prmted in Great Britam 136&668?6!97 $17.00+0.00
All rights
Measurement of Faraday rotation of radar meteor echoes for the modelling of electron densities in the lower ionosphere W. G. Elford and A. D. Taylor Department
of Physics and Mathematical
Physics,
University
of Adelaide.
Adelaide
5005, South Australia
(Received infinalform 10 May 1996; accepted 24 May 1996) Abstract-Narrow beam VHF radars are able to detect transient ionization at heights between 80 and 120 km produced by meteoroids entering the atmosphere ‘down-the-beam’. Using a vertically directed beam, observations of this new type of meteor echo on two orthogonal receiving arrays enable the orientation of the plane of polarization to be measured. Significant Faraday rotation is observed to occur during daytime at heights above 80 km. Such observations make possible the measurement of ionospheric electron density profiles between heights of 80 and 120 km. 0 1997 Elsevier Science Ltd
INTRODUCTION
A linearly polarized radio wave passing through the ionosphere undergoes Faraday rotation of the plane of polarization due to the presence of the Earth’s magnetic field. Measurements of the amount of rotation leads to estimates of the total electron content of the ionosphere between the source and the receiver (Garriott et al., 1970; Ratcliffe, 1972). The sources for such observations are usually satellites, some of which have been equipped with transmitters and antennas specially designed for the purpose. The fact that radio waves reflected from meteor trails undergo Faraday rotation was first discussed by Billam and Browne (1956) in their study of the polarization characteristics of radar echoes. They were chiefly concerned as to whether the amount of rotation due to the ionosphere needed to be taken into account, and their calculations showed that in their situation it could be ignored. However they did add the following proviso, “for aerial directions close to that of the terrestrial magnetic field...the rotation can be of the order of one radian during daytime”. In retrospect, it is clear that this statement has been overlooked by almost all who have been involved in radar meteor astronomy and radar studies of the atmosphere using meteors. One exception is an article by Baggaley (1979) who estimated the effect of Faraday rotation on a meteor forward scatter link. It is now evident that Faraday rotation can have quite profound effects on the interpretation of radar meteor rates (Elford, 1993). However, that topic is addressed elsewhere (Elford, 1997). Here we are concerned with a simple application of the Faraday rotation of echoes
from meteor trails to the measurement of electron densities in the lower ionosphere. Recently, a new type of meteor echo has been detected using a narrow beam VHF radar. The radar reflections have been observed from the ‘head’ of meteors moving almost directly down the beam and appearing in successive range gates. During daytime the plane of polarization of the echoes is observed to rotate between successive range gates. For a vertically directed beam this leads to estimations of the electron content within each height interval, and thence to the electron density profile over the length of the meteor trail.
FARADAY ROTATION OF METEOR ECHOES
The amount of rotation of the plane of polarization of a linearly polarized radio wave propagated between the ground and a meteor trail (assuming quasi-longitudinal approximation to the refractive index) is given by R = 2.36 x 104f--ZJp”th Bcos~N(s)ds where f is the wave frequency (Hz), B is the local geomagnetic field strength (Wb m--2), x is the angle between the wave propagation direction and the direction of B, N is the electron density (mm3), ds is a path element (m). To assess the expected magnitude of the rotation of the plane of polarization of a radio signal back-scattered from meteors at different heights, values of the 1021
1022
W. G. Elford and A. D. Taylor
angle rZ were calculated for the 54 MHz radar at the Buckland Park Research Station (40 km north of Adelaide, Australia) for a typical mid-latitude, midday, low sunspot number ionosphere. The distribution of electron density below 115 km was taken from Mechtly et al. (1972) and extended to 120 km by joining smoothly to profiles given by Hargreaves (1979). The geomagnetic field at Adelaide has an intensity of 5.9 x 10m5Wbm-* and a dip angle of 67”. Table 1 lists the total amount of rotation (2 a) to be expected for back-scattering from meteors occurring at heights between 80 and 120 km in daytime. A glance at Table 1 suggests that Faraday rotation should be readily detectable for trails occurring above 95 km. OBSERVATIONS Observations of Faraday rotation were carried out using the VHF radar located at Buckland Park and operating on 54.1 MHz, during 5-l 3 September 1995. The transmitter generated 12.6 /JS pulses at a repetition rate of 1024 Hz with a peak power of 28 kW. A large collinear coaxial (CoCo) transmitting array, 89 m square, produces a 3.2” (full width half power) wide beam directed vertically, giving a gain of about 3200 relative to an isotropic source. The antenna efficiency is 29%. Two groups of 16 vertically directed Yagi antennas (each 4 x 4) were used for reception. The gain of each array was 150. One receiving array was aligned parallel with the transmitting array, while the other could be placed parallel or orthogonal to the transmitting array. The receivers are phase coherent, and the echo amplitude and phase are calculated from the in-phase and quadrature output signals. Speed limitations in the data acquisition equipment restricted sampling of the two receiver channels to 20 ps intervals giving a range resolution of 3 km. The Table 1. Total Faraday rotation for backscatter from meteors at 54MHz at Buckland Park (midday, sunspot minimum) Height (km)
Rotation (deg)
80 85 90 95 100 105 110 115 120
0 1 3 8 19 34 53 17 103
necessity to use a 12.6~s transmitter pulse implies that, of those meteors present, some were not detected since the echo would have fallen in the range-scan gaps. The meteor detection criteria required that the echo amplitude at a given range exceeded twice the mean noise for four successive transmitter pulses. Only meteor echoes exhibiting evidence of coming ‘down-the-beam’ were analysed, thereby removing the more commonly observed transverse meteor trails which for a vertically directed beam have a high probability of occurring in a side lobe. These criteria translated into echoes of short duration in successively decreasing range intervals, and a rapid but consistent change in measured phase. Of the 75 ‘down-the-beam’ echoes detected over the eight days, two were rejected because the amplitude showed complex rapid changes which were assumed to be due to beating between scattering from multiple trails associated with a fragmented particle, and one echo was excluded because of an abnormally long range which was assumed to be from a meteor detected in a side lobe. A total of 23 ‘down-the-beam’ meteors (giving 36 samples from separate heights) were detected in the three days spent checking the calibration, while the echo polarization angle was determined for 40 meteors (61 heights) in the other five days. These echo rates are about 10% of the rate of ‘down-the-beam’ meteors when the CoCo array is used for both and transmission and reception. As the measurement of the polarization angle depends on the comparison of the amplitude of the echoes from the two Yagi arrays, it is essential that the two receiving systems have the same or known gains. This was achieved by recording the mean sky noise in both systems and using the measured values to continuously determine the relative gains. An independent check on the gains of the systems was carried out by observing ‘down-the-beam’ meteors for three days with the both Yagi arrays aligned parallel with the transmitting array. The relative gains were determined by comparing the recorded sky noise in both systems, and the echoes were analysed as if they were detected in orthogonally aligned systems. Under these conditions the apparent polarization angle should be 45”. The results are shown in Fig. 1, from which it is clear that the gain comparison method works well. Moreover, the plot indicates an uncertainty of a few degrees in the measurement of the echo polarization angles. POLARIZATION MEASUREMENTS To measure the polarization of the meteor echoes, the two Yagi arrays were aligned one parallel to the
Meteor
ionisation
1023
in the lower ionosphere
I
t
1
t
t
*I b3
o % w
6-
.
“0 !is 43 z -
2-
0
Angle (degree) Fig. 1. Distribution of the measurements of the ‘apparent polarization angle’ deduced from meteors observed simultaneously on two receiving arrays with parallel polarization. The measurements cluster about 45” as expected. The spread of the results gives an estimate of the precision to be expected in the actual measurements presented in Fig. 2.
transmitting array and the other orthogonal to it. The echo amplitude profiles were smoothed using a lopoint (10 ms) moving average and the peak amplitudes determined and adjusted for the relative gains of the
two systems. The arctangent of the resultant amplitude ratio gave the polarization angle. The results are shown in Fig. 2 where the short bars represent the polarization of the meteor echo observed at the
,,.‘...‘.*,‘...‘.I.‘..,
II5
-it
110 -
0
1.. 4
* I. 8
* = I. 12
”
I
16
*.
.
1.. 20
. 24
Hour (CST) Fig. 2. The observed polarization (short bars) of radar signals reflected from meteors at the heights where the bar lines are plotted. The centre of each bar is set at the time of the observation (Central Australian Time, CST, is very close to local solar time). The observation of an individual meteor at several heights is indicated by a dotted line linking the bar lines. The ‘noon model’ plot at the right is calculated for a model ionosphere as described in the text.
1024
W. G. Elford and A. D. Taylor
ground after reflection from a meteor at the height indicated. Observations at several heights from the one meteor are indicated by linking the polarization bars with a dotted vertical line. The results are summarized as follows:
COMMENTS
of the 43 night-time polarization measurements, two show a small non-zero angle, all daytime measurements at heights above 97 km show significant rotation, midday measurements show rotation down to a height of 90 km, the one daytime measurement at 85 km shows no measurable rotation. the rotation observed at several heights from a single meteor always increases with height. For comparison, the polarization predicted by the model described earlier and listed in Table 1, is shown in bar form at the right of Fig. 2. There is substantial similarity between the model and the observations, and in fact very close agreement is achieved if the mode1 data is shifted 3 km lower.
The preliminary observations of Faraday rotation of radar echoes reflected from meteors using a narrow beam VHF wind-profiler radar in the vertical mode shows that such systems can be used to determine the daytime electron density profile at heights between 90 and 115km for localities at medium and high magnetic latitudes. By the addition of a second receiving antenna of the same size as the transmitting array, and oriented orthogonally, ‘down-the-beam’ meteor echoes would give polarization measurements up to heights of 120 km, with many individual meteors giving instantaneous profiles over 1520 km in height, As mentioned earlier, VHF radars with full-size arrays detect at least 100 down-the-beam echoes per day, with most occurring between midnight and noon. A second CoCo array is being added to the VHF radar at Buckland Park, oriented orthogonally to the existing array, and it is intended that the measurement of electron density profiles between 80 and 120 km will become part of the routine operation of the system.
REFERENCES
Baggaley W. J.
1979
Billam E. R. and Browne I. C
1956
Elford W. G.
1993
Elford W. G.
1997
Garriott
1970
0. K.. Da Rosa A. V. and Ross W. J.
Hargreaves
J. K.
1979
Mechtly
E. A., Bowhill S. A. and Smith L. G.
1972
Ratcliffe
J. A.
1972
D-region absorption and forward-scatter radio-meteor data. J. atmos. lerr. Phys. 41, 671-676. Characteristics FO radio echoes from meteor trails. IV: Polarization effects. Proc. Phys. Sot. B69, 98-l 13. Radar observations of meteors. Meteoroids and Their Parent Bodies, &oh1 J. and Williams I. P., eds) Astronomical Inst., Slovak Acad. Sci. Bratislava, pp. 235-244. Scattering of radio waves from meteor trails. Interplanetary Dust, (Griin, E., Dermott, S. F., Fechtig, H. and Gustafson, B. A. S., eds). Univ. of Arizona Press, Tuscan. Electron content obtained from Faraday rotation and phase path length variations. J. atmos. terr. Phys. 32, 705-727. The Upper Atmosphere and Solar-Terrestrial Relutions. Van Nostrand Reinhold, New York. Changes of lower ionosphere electron concentrations with solar activity. J. atmos. terr. Phys. 34, 1899. An Introduction to the Ionosphere and Magnetosphere, Cambridge University Press, 256 pp.