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Irregularities in the lower ionosphere deduced from beacon satellite observations
Joumal
ofAtmospheric
and
TerrestrialPhysics,1969,vol. 31, pp.
$1 to i9.
Pergamon Press.
Printedin Worthern Ireland
Irregularities in the lower ionosphere deduced from beacon satellite observations J. A. D. HOLBROOK and G. F. LYON Centre for Radio Science, University of Western Ontario, London, Ontario, Canada (Receiwed 15 May 1968) Abstract-Received field strength measurement,s of the 20 MHz. transmission from Explorer 22 have been made at London, Ontario, Canada (43”N, Slow). It was possible to perform such measurements unambiguously only during quiet ionospheric conditions. Most daytime observations showed evidence of t.he presence of electron density fluctuations whereas night-time observations were nearly always smooth. Interpretation of the results in terms of non-deviative absorption along the ray paths suggests that the irregularities observed are occurring at the 70 km height level and are probably the same as those thought to be responsible for the continuous signal received on high power T’HF forward scatter circuits. INTRODUCTION MOST observations
of ionospheric irregularities utilizing beacon satellite transmissions have been based upon either the Faraday rotation or the differential Doppler effect. These effects are most pronounced in the region of highest electron density so that the irregularities observed are located in the F-region. If the nondeviative absorption of the satellite signal along its ray path can be measured this should yield information concerning the lower ionosphere since the absorption is greatest in regions of high collision frequency. For a single observing station the ray path to the satellite sweeps through a large spatial extent of the ionosphere as the vehicle moves in its orbit; any changes in the received signal strength which can be attributed to absorption may be interpreted as structure in the lower ionosphere. This is in contrast to the riometer method of measuring absorption which yields an integrated absorption figure over a large volume of the ionosphere. While it is somewhat difficult to make absolute measurements of received signal strength, it is comparatively simple to perform relative signal strength measurements given a controlled stable power transmission. In practice the difficulty lies in interpreting the measurements, since not only is the received signal strength affected by absorption, but also by several other factors such as Faraday rotation, scintillation, satellite spin, etc. However, as will be described later, it is possible by the careful application of certain criteria to the received signal records to select those records for which it is reasonably certain the amplitude changes are due to absorption alone. It must be emphasized that no attempt is made to obtain an absolute magnitude for the absorption at any time, or even to compare the absorption from day to day, but only for selected records are the relative amplitude changes within the record interpreted as being due to absorption. KAZANTSEV (1959) determined F-region absorption coefficients by measuring signal strengths when a satellite was first above and then below the region. VA~SY (1961) proposed a method, somewhat similar to the one used here, for determining 71
J. A. D. HOLBROOKand G. F. LPOK
72
ionospheric absorption coefficients but confined his discussion largely to the geometry of the situation. The present study suggests that, because of the variability of the absorption along successive ray paths, the use of a satellite-borne beacon could yield only integrated absolute values of the absorption coefficient, but the integration may be over a smaller spatial extent than is possible with a riometer. The beacons used in this investigation were the 20 MHz. and 40 MHz. transmissions from Explorer 22 (1964-648) which is in a near polar, near circular orbit at a height of about 1000 km. The transmissions are continuous wave, linearly polarized and of good short term amplitude stability; the vehicle has been de-spun and is attitude controlled so that the transmitting antennae are maintained at right angles to the Earth’s magnetic field. The ground receiving station was situated at London, Ontario (43*O”N, 81.3W). The receiving antennae employed were horizontal folded dipoles a quarter wavelength above the ground plane. Conventional receivers employing either automatic frequency control or sufficient flat bandpass to accommodate the signal Doppler shift were used. The receiver IF outputs were detected and recorded directly on strip chart recorders and each system was calibrated immediately after each observation using crystal oscillators and precision step attenuators. METHOD OF ANALYSIS In the absence of any anomalous effects the received signal strength is dependent upon a number of easily determinable factors. These are the range, the zenith angle (slant path length through a ‘normal’ ionosphere) the receiving antenna pattern, and the transmitting antenna pattern. The effect of each of these factors is different for every satellite position, and because the satellite position is continually changing the expected received signal amplitude is also continuously changing. For every satellite record obtained the expected amplitude variation, normalized to the maximum, is calculated by computer using the orbital information regularly published by NASA. The predicted amplitude versus time curve is then fitted to the experimental amplitude versus time record and any differences noted. Other factors being equal, the relative absorption along various slant paths will simply be proportional to the secant of the zenith angle for the ray. However, it is of some interest to compute an approximate absolute value for the absorption and to estimate what proportion occurs in which region of the ionosphere. The well known expression for the non-deviative absorption, derived from magneto-ionic theory using the quasi-longtitudinal approximation, may be written : A = (4.59 x 10-b) where A is absorption
in decibels
iV is electron number density Y is electron collision frequency ds is an element of path length
f
is the frequency.
s WZ:VZ
as
Irregularities in the lower ionosphere deduced from beacon satellite observations
Writing equation
73
(1) in the form
‘4-59 x 10-5 XY
AA =
(02 + yz
AS
(2)
enables an estimate to be made of the absorption per unit path length for any Figure 1 shows & plot of absorption per unit path length region of the ionosphere. against height for a 20 MHz. tra.nsmission and was computed from average values
Daytime
I
absorption
I
,
1
0’
Absorption,
Fig. 1. Absorption
I
10.’
db/lO
km
per 10 km height interval vs. height for the average mid-solar-cycle ionosphere.
midday
of electron density and collision frequency with height at local noon at the middle of the sunspot cycle. The integrated vertical absorption to 1000 km. is l-3 dB. and it is clear that most of the absorption occurs in one or all of three principle maxima corresponding roughly to the D-, E- or F-regions. It might be argued that small signal amplitude changes could result from asymmetries or irregularities in the receiving antenna pattern. Although, in fact, the actual antenna pattern was not determined experimentally, the received signals effectively map the complete pattern since the satellite appears to move westward each succeeding day and in the course of time the antenna pattern in a large number of planes through the observing station is obtained. It would be expected that any asymmetry in the radiation pattern would be constant in time and space so that such au effect would consistently give an increase or decrease in signal amplitude every time the ray path to the satellite passed close to that point in space. Such results are usually attributed to site effects and have been observed by some experimenters (IRELAND and PREDDY, 1966). In the present experiment sufficient records
74
J. A. D. HOLBROOKand G. F. LYON
were obtained to identify radiation pattern anomalies if present, but none were detected. If the satellite were spining, received signal amplitude variations would result. The vehicle is altitude controlled along the earth’s magnetic field lines and is nominally despun. Any residual spin would cause the transmitting antenna orientation to Many experimenters using this satellite for Faraday change during an observation. rotation observations have indicated that the residual spin rate is too slow to be of any consequence ; see, for example, Ross and KUNTMAN, (1967). Since both the receiving and transmitting antennae are linearly polarized the received signal amplitude exhibits a quasi-regular fading pattern due to Faraday rotation. It is only at the maxima of this fading pattern, where the incoming wave polarization and receiving antenna polarization are parallel, that the experimental amplitude can be compared with the predicted amplitude. The Faraday effect thus limits the spatial resolution of the measurements, which may be somewhat of a disadvantage at higher frequencies. At 20 MHz. this limitation corresponds to a resolution of about 0.5 km. at a height of 70 km. The Faraday fading pattern, however, is a useful indicator of the presence of unwanted effects. A regular quasisinusoidal pattern suggests the absence of any polarization anomalies; since the nulls in the pattern indicate zero signal level any noise or spurious signal is easily detected by a raising of the null above the calibration zero point. Scintillation, other than very long period scintillation, is obvious from the records, and those showing such effects were discarded for the purposes of this investigation. Finally, in order to minimize refraction effects, analysis was generally restricted to those portions of a satellite pass for which line of sight zenith angles were less than 45”. EXPERIMENTAL RESULTS Since it is necessary to have a reasonable assurance that the signal amplitude changes can be attributed to absorption alone, the observations are necessarily restricted to times when ‘quiet’ ionospheric conditions prevail. All of the quiet night-time observations made yielded a smooth received signal envelope and showed very little evidence of any structure in the ionization. In contrast, the majority of the quiet daytime observations had a received signal envelope which departed from the smooth predicted envelope at many points, thus suggesting considerable ionospheric structure. It must be borne in mind that the spatial resolution is much larger for the night-time observations since then the electron content is lower and the maxima in the Faraday fading pattern are farther apart than is the case for daytime observations. Nevertheless, the night observations under quiet conditions were consistently smooth and closely followed the predicted signal envelope. A typical quiet daytime record is shown in Fig. 2 in which time runs from right to left and top to bottom. The upper trace of the pair is the WWV time signal showing the seconds markers, and the lower trace is the received amplitude of the 20 MHz. satellite signal showing the expected rapid Faraday fading and having the predicted received signal amplitude superimposed. It is readily seen that the actual received signal departs from that expected at many points. These departures in signal amplitude, expressed in decibel absorption referred to the
, ORBIT 1tmt lf#04#64/A
2/2re4
/
ORBIT 11290 -_--
1984/64/A
O/l/67
1*
Fig. 4. 20 MHz received
“I
amplitude
record
for Orbit
11290, 9th January
1967.
Irregularities in the lower ionosphere deduced from beacon satellite observations
75
direction are plotted against sub-satellite latitude in Fig. 3. Each data point is derived from an amplitude scaled at a Faraday maxima. The decibel absorption scale is relative, the zero point representing the predicted signal level and positive decibel readings representing levels below that predicted. If the resulting fluctuating absorption pattern is interpreted as being due to spatial fluctuations in electron density t,hen considerable ionospheric structure is implied. The horizontal size that can be attributed to this structure depends upon the height assigned to the electron it is clear from Fig. 1 that the most probable heights are density ~u~tuations; 65 km., 100 km. or 350 km. It is usual to associate abnormal absorption effects with the lower ionosphere where the cohision frequency is highest. To confirm that the observed effects are zenith
Orbit
I
I
I 38
I
Sub-satellite
Fig,
3. Relative
signal strength
I
42 latitude,
11618
I
I
46 beg
vs. sub sat,ellite latitude
for Orbit
11618.
not occurring in the P-region some of the field strength observations were compared with simultaneous electron content measurements made at 40 and 41 MHz. using the Faraday rotation method and made available through the courtesy of Mr. C. 8. Chen. Figure 4 shows the 20 MHz. record obtained from Orbit 11290 on the 9th January 1967. Figure 5 shows the absorption deduced from this record and also shows the total electron content observations for the same orbit. The spatial resolution of the electron content measurements is less than that of the 20 MHz. absorption measurements so that it is not expected that the fine structure appearing in the latter would be reproduced exactly in the former. Nevertheless, if the two sets of observations relate to phenomena in the same region of the ionosphere at least some of the features would appear on both. This is evidently not the case. The Faraday rotation effect occurs predominantly in the F-region so that any structure in the total electron content curve presumably results from F-region irregularities. As the structure in the absorption curve is not even partly reproduced in the electron content curve, it is inferred that the absorption curve structure is caused by irregularities below the F-region, that is to say, by irregularities at a height of 70 km or 100 km Evident in one section of the record illustrated in Fig. 4 is a marked change in the Faraday rotation rate accompanied by an amplitude enhancement; this section
J. A. D. HOLBROOK and G. F. LYON
16
of the record is not included in the analysis shown in Fig. 5. This type of rotation rate event has been noted several times during the concurrent 40 and 41 i%fHz. Faraday observations (&EN, 1967) and is the subject of a separate study. It is thought that such events are due to travelling ionospheric disturbances. In an attempt to confirm that the 20 MHz amplitude fluctuations were indeed due to non-deviative absorption some simultaneous, identical field strength measurements were made on the 40 MHz beacon transmission from the same satellite. Because tlhe Faraday rotation at 40 MHz is much slower than that at 20 MHz, a
2 oo\
Orbit
“E \
--*
3 -0 ;
II290
\ \
270l
I 0
.
*_*_. \
l
< o-
l
=I
I
38
Sub-satellite I%de.
deg
46
Fig. 5. Relative signal strength vs. sub slttellite latitude for Orbit 11290, together with total electron content figures.
direct comparison of amplitude at the two frequencies can only be made at a few points on the records. Furthermore the 40 MHz amplitude ffuctuations were, as might be expected, only of the order of O-I-O=4 dB, making accurate scaling of the records difficult. It usually proved to be the case that when the 40 MHz amplitude fluctuations were approaching 0.5 dB the 20 MHz signal was scintillating and could not be used for amplitude measurements. On the few occasions when a comparison could be made, the ratio of the decibel absorption at the two frequencies lay between 3 and 4. The expected ratio would be 4 in regions where collision frequency could be neglected and less than 4 when the collision frequency could not be considered small compared to the wave frequency. The received amplitudes can be measured only at the maxima in the Faraday fading pattern. There is thus the possibility that the amplitude fluctuations result from some anomalous polarization phenonomena. To check this possibility some observations were made of the 20 MHz amplitudes on separate systems utilizing orthogonal linear receiving antennae. Here again a direct comparison of the amplitudes on each of the two systems is complicated by the fact that the data
Irregularities in the lower ionosphere deduced from beacon satellite observations
77
points are not coincident in time. The amplitudes being read at the Faraday maxima, the data point from one system falls midway between two data points from the orthogonal system. Figure 6 shows the absorption plot obtained directly from one system together with an absorption plot obtained by interpolating midway between adjacent data points on the orthogonal system. The fluctuations in the two plots are very similar, lending more weight to the supposition that the observed amplitude changes are resulting from absorption. During the period January to August 1967, some 50 useable records were obtained. As previously noted, all of these are for ‘quiet’ ionospheric conditions,
Orbit
I
14118
/
/ 40
Sub- satellite
lotltude,
I
42
deg
Fig. 6. Signal amplitudes for Orbit 14118, 3rd August 196’7, observed simultaneously on antennae of orthogonal polarization. yet s,ll exhibit fluctuations similar to those shown in Figs. 3 and 5. The method of observation does not distinguish between temporal fluctuations and spatial variations. It is usually assumed that in the case of a rapidly moving satellite borne source the observed fluctuations are spatial. The horizontal size assigned to a fluctuation then depends only upon the height assumed for the irregularity in electron density. If this height is taken to be 70 km the spatial resolution of the experiment is approximately 0.5 km and the sizes observed range from O-5 km to 10 km. If this height is taken to be 100 km the resolution is 0.7 km and the size range O-7 km to 14.0 km. DISCUSSION The structure observed is not reproduced in total electron content measurements, which are most sensitive to P-region effects. This suggests that the signal strength observations relate to one of the lower ionospheric regions, but there is no direct evidence in the present measurements to indicate whether the D- or E-region is responsible. If the observed structure was occurring in the E-region then it would be reasonable to suppose that the irregularities were of the sporadic-E type. Unfortunately there is no vertical ionosonde in operation in the immediate vicinity of London so that it was not possible to make a direct comparison of events with locally observed fa, or fbEs. However, a comparison was made with sporadic-E data
78
J. A. D. HOLBROOKand G. F. LYON
from the Billerica, Massachusetts, field station (Lat. 42*3”N, Long. 71-2” W), published in the Geophysics and Space Data Bulletin (1967). No correlation with either foEa or fbEg was found. In this connection it may be noted that the signal strength observations are specifically restricted to times when no scintillation is occurring. AARONS and WHITNEY (1968), in a scintillation study utilizing a 136 MHz transmission from a synchronous satellite, have noted that the occurrence of scintillation is often accompanied by the occurrence of sporadic-E, thus giving further indirect evidence that the present observations do not refer to sporadic-E type irregularities. If the D-region is considered as the location of the observed structure there is an indirect explanation of the fact that the signal amplitude fluctuations were rarely seen at night but were almost always present during the day since the D-region of the ionosphere is present o&y in the daytime. This, and the lack of any other evidence to the contrary, suggests that the observed phenomenon is occurring in the D-region. It must be emphasized that all of the observations reported here refer to quiet conditions, that is to say, to times when there was no evidence of scintillation when there were no local riometer absorption events, and when the magnetic activity index was low. The observations suggest that even under quiet conditions there are often present in the daytime lower ionosphere, inhomogenieties in the electron density with sizes ranging from 0.5 to 10.0 km. The strength of the electron density fluctuations may be estimated from the relative amplitude of the received signal strength excursions if the latter are assumed to arise because of non-deviative absorption. Using a 10 km height interval and a collision frequency appropriate to a mean height of 70 km (KANE, 1961) yields electron density ~u~tuation in the range of 15-100 per cent of the ambient density. It is somewhat difficult to envision electron density fluctuations of the order of 100 per cent in the upper regions of the ionosphere under quiet conditions, when no external cause, such as a stream of precipitating particles, can be invoked. At the lower heights in the daytime ionosphere pronounced vertical gradients in electron density are known to exist over small height ranges. For example, AIKIN et al. (1964), reporting an afternoon rocket flight at Wallops Island, Virginia, show gradients of approximately threefold increases in electron density over height intervals of about 2 km at heights of 70 km and at 85 km. Such sharp vertical gradients may readily produce horizontal gradients under the influence of turbulence or wind shear effects. It has long been considered that the almost continuous signal received on high power VHF forward scatter circuits is due to scattering from irregularities in the lower ionosphere arising from turbulent mixing (see, for example, BAILEY et al. (1955) and WHEELON (1960)). Such signals show a daytime enhancement centered at noon; Wheelon has suggested that the daytime scattered signals arise from scattering at the 70 km level. Thus it seems reasonable to suppose that the irregularities observed in the present investigation are due to turbulence and occur at about 70 km height, The present study was initiated with the thought that a similar technique might allow the monitoring of t’he extent and structure in absorbing regions associated
Irregularities in the lower ionosphere deduced from beacon satellite observations
79
with amoral disturbances and polar cap absorption events. It is now evident that the technique is only useful under quiet conditions for 20 MHz trsnsmissions at middle latitudes. It seems likely that if similar measurements were attempted at higher latitudes where it would be possible to utilize tt higher frequency (say, 40 MHz) transmission to detect the greater absorption associated with disturbed conditions, the accompanying increase in scintillation occurrence frequency would restrict use of the system to quiet conditions alone. Acknowledgements-This work was supported by Defence Research Board of Canada Grant K-0. 9511-58. The paper is based in part upon material which will be submitted in an M.Sc. Thesis by one of the authors (J. A. D. H.) to the Faculty of Graduate Studies of the University of Western Ontario. Personal financial assistance provided (J. A. D. H.) by a Province of Ontario Graduat,e Fellowship is gratefully acknowledged. REFERENCES AARONYJ. and WHITKEY H. E. AIKIN A. C., KANE J. A. and TROIM J. BAILEY D. K., BATEMANR. and KII~BY R.C. IRELAND W. and PREDDY G. F. KANE J. A. KAZANTSEVA. N. Ross W. .J. and KUNTMAND. VASSY E. T. WHEELONA. D. The following
unpublished
document
1968 1964 1955
J. Planet Space Sci. 19, 21. J. Geophys. Res. 69, 4621. Proc. IRE 43, 1181.
1966 1961 1959 1967 1961 1960
J. J. J. J. J. J.
is also referred
GEOPHYSICSAND SPACE DATA BULI~ETIN
1967
Atmosph. Terr. Phys. 28, 481. Atmosph. Terr. Phys. 23, 338. Planet Space 6%. 1, 130. Geophys. Res. 72, 1041. Atmosph. Terr. Phys. 23, 85. Res. Natn. Bur. Stand. 64D, 301.
to:
Vol. IV, Nos. 2 and 3, Space Physics Lab., AFCRL, L.G. Hanscom Field, Massachusetts.
ofAtmospheric
and
TerrestrialPhysics,1969,vol. 31, pp.
$1 to i9.
Pergamon Press.
Printedin Worthern Ireland
Irregularities in the lower ionosphere deduced from beacon satellite observations J. A. D. HOLBROOK and G. F. LYON Centre for Radio Science, University of Western Ontario, London, Ontario, Canada (Receiwed 15 May 1968) Abstract-Received field strength measurement,s of the 20 MHz. transmission from Explorer 22 have been made at London, Ontario, Canada (43”N, Slow). It was possible to perform such measurements unambiguously only during quiet ionospheric conditions. Most daytime observations showed evidence of t.he presence of electron density fluctuations whereas night-time observations were nearly always smooth. Interpretation of the results in terms of non-deviative absorption along the ray paths suggests that the irregularities observed are occurring at the 70 km height level and are probably the same as those thought to be responsible for the continuous signal received on high power T’HF forward scatter circuits. INTRODUCTION MOST observations
of ionospheric irregularities utilizing beacon satellite transmissions have been based upon either the Faraday rotation or the differential Doppler effect. These effects are most pronounced in the region of highest electron density so that the irregularities observed are located in the F-region. If the nondeviative absorption of the satellite signal along its ray path can be measured this should yield information concerning the lower ionosphere since the absorption is greatest in regions of high collision frequency. For a single observing station the ray path to the satellite sweeps through a large spatial extent of the ionosphere as the vehicle moves in its orbit; any changes in the received signal strength which can be attributed to absorption may be interpreted as structure in the lower ionosphere. This is in contrast to the riometer method of measuring absorption which yields an integrated absorption figure over a large volume of the ionosphere. While it is somewhat difficult to make absolute measurements of received signal strength, it is comparatively simple to perform relative signal strength measurements given a controlled stable power transmission. In practice the difficulty lies in interpreting the measurements, since not only is the received signal strength affected by absorption, but also by several other factors such as Faraday rotation, scintillation, satellite spin, etc. However, as will be described later, it is possible by the careful application of certain criteria to the received signal records to select those records for which it is reasonably certain the amplitude changes are due to absorption alone. It must be emphasized that no attempt is made to obtain an absolute magnitude for the absorption at any time, or even to compare the absorption from day to day, but only for selected records are the relative amplitude changes within the record interpreted as being due to absorption. KAZANTSEV (1959) determined F-region absorption coefficients by measuring signal strengths when a satellite was first above and then below the region. VA~SY (1961) proposed a method, somewhat similar to the one used here, for determining 71
J. A. D. HOLBROOKand G. F. LPOK
72
ionospheric absorption coefficients but confined his discussion largely to the geometry of the situation. The present study suggests that, because of the variability of the absorption along successive ray paths, the use of a satellite-borne beacon could yield only integrated absolute values of the absorption coefficient, but the integration may be over a smaller spatial extent than is possible with a riometer. The beacons used in this investigation were the 20 MHz. and 40 MHz. transmissions from Explorer 22 (1964-648) which is in a near polar, near circular orbit at a height of about 1000 km. The transmissions are continuous wave, linearly polarized and of good short term amplitude stability; the vehicle has been de-spun and is attitude controlled so that the transmitting antennae are maintained at right angles to the Earth’s magnetic field. The ground receiving station was situated at London, Ontario (43*O”N, 81.3W). The receiving antennae employed were horizontal folded dipoles a quarter wavelength above the ground plane. Conventional receivers employing either automatic frequency control or sufficient flat bandpass to accommodate the signal Doppler shift were used. The receiver IF outputs were detected and recorded directly on strip chart recorders and each system was calibrated immediately after each observation using crystal oscillators and precision step attenuators. METHOD OF ANALYSIS In the absence of any anomalous effects the received signal strength is dependent upon a number of easily determinable factors. These are the range, the zenith angle (slant path length through a ‘normal’ ionosphere) the receiving antenna pattern, and the transmitting antenna pattern. The effect of each of these factors is different for every satellite position, and because the satellite position is continually changing the expected received signal amplitude is also continuously changing. For every satellite record obtained the expected amplitude variation, normalized to the maximum, is calculated by computer using the orbital information regularly published by NASA. The predicted amplitude versus time curve is then fitted to the experimental amplitude versus time record and any differences noted. Other factors being equal, the relative absorption along various slant paths will simply be proportional to the secant of the zenith angle for the ray. However, it is of some interest to compute an approximate absolute value for the absorption and to estimate what proportion occurs in which region of the ionosphere. The well known expression for the non-deviative absorption, derived from magneto-ionic theory using the quasi-longtitudinal approximation, may be written : A = (4.59 x 10-b) where A is absorption
in decibels
iV is electron number density Y is electron collision frequency ds is an element of path length
f
is the frequency.
s WZ:VZ
as
Irregularities in the lower ionosphere deduced from beacon satellite observations
Writing equation
73
(1) in the form
‘4-59 x 10-5 XY
AA =
(02 + yz
AS
(2)
enables an estimate to be made of the absorption per unit path length for any Figure 1 shows & plot of absorption per unit path length region of the ionosphere. against height for a 20 MHz. tra.nsmission and was computed from average values
Daytime
I
absorption
I
,
1
0’
Absorption,
Fig. 1. Absorption
I
10.’
db/lO
km
per 10 km height interval vs. height for the average mid-solar-cycle ionosphere.
midday
of electron density and collision frequency with height at local noon at the middle of the sunspot cycle. The integrated vertical absorption to 1000 km. is l-3 dB. and it is clear that most of the absorption occurs in one or all of three principle maxima corresponding roughly to the D-, E- or F-regions. It might be argued that small signal amplitude changes could result from asymmetries or irregularities in the receiving antenna pattern. Although, in fact, the actual antenna pattern was not determined experimentally, the received signals effectively map the complete pattern since the satellite appears to move westward each succeeding day and in the course of time the antenna pattern in a large number of planes through the observing station is obtained. It would be expected that any asymmetry in the radiation pattern would be constant in time and space so that such au effect would consistently give an increase or decrease in signal amplitude every time the ray path to the satellite passed close to that point in space. Such results are usually attributed to site effects and have been observed by some experimenters (IRELAND and PREDDY, 1966). In the present experiment sufficient records
74
J. A. D. HOLBROOKand G. F. LYON
were obtained to identify radiation pattern anomalies if present, but none were detected. If the satellite were spining, received signal amplitude variations would result. The vehicle is altitude controlled along the earth’s magnetic field lines and is nominally despun. Any residual spin would cause the transmitting antenna orientation to Many experimenters using this satellite for Faraday change during an observation. rotation observations have indicated that the residual spin rate is too slow to be of any consequence ; see, for example, Ross and KUNTMAN, (1967). Since both the receiving and transmitting antennae are linearly polarized the received signal amplitude exhibits a quasi-regular fading pattern due to Faraday rotation. It is only at the maxima of this fading pattern, where the incoming wave polarization and receiving antenna polarization are parallel, that the experimental amplitude can be compared with the predicted amplitude. The Faraday effect thus limits the spatial resolution of the measurements, which may be somewhat of a disadvantage at higher frequencies. At 20 MHz. this limitation corresponds to a resolution of about 0.5 km. at a height of 70 km. The Faraday fading pattern, however, is a useful indicator of the presence of unwanted effects. A regular quasisinusoidal pattern suggests the absence of any polarization anomalies; since the nulls in the pattern indicate zero signal level any noise or spurious signal is easily detected by a raising of the null above the calibration zero point. Scintillation, other than very long period scintillation, is obvious from the records, and those showing such effects were discarded for the purposes of this investigation. Finally, in order to minimize refraction effects, analysis was generally restricted to those portions of a satellite pass for which line of sight zenith angles were less than 45”. EXPERIMENTAL RESULTS Since it is necessary to have a reasonable assurance that the signal amplitude changes can be attributed to absorption alone, the observations are necessarily restricted to times when ‘quiet’ ionospheric conditions prevail. All of the quiet night-time observations made yielded a smooth received signal envelope and showed very little evidence of any structure in the ionization. In contrast, the majority of the quiet daytime observations had a received signal envelope which departed from the smooth predicted envelope at many points, thus suggesting considerable ionospheric structure. It must be borne in mind that the spatial resolution is much larger for the night-time observations since then the electron content is lower and the maxima in the Faraday fading pattern are farther apart than is the case for daytime observations. Nevertheless, the night observations under quiet conditions were consistently smooth and closely followed the predicted signal envelope. A typical quiet daytime record is shown in Fig. 2 in which time runs from right to left and top to bottom. The upper trace of the pair is the WWV time signal showing the seconds markers, and the lower trace is the received amplitude of the 20 MHz. satellite signal showing the expected rapid Faraday fading and having the predicted received signal amplitude superimposed. It is readily seen that the actual received signal departs from that expected at many points. These departures in signal amplitude, expressed in decibel absorption referred to the
, ORBIT 1tmt lf#04#64/A
2/2re4
/
ORBIT 11290 -_--
1984/64/A
O/l/67
1*
Fig. 4. 20 MHz received
“I
amplitude
record
for Orbit
11290, 9th January
1967.
Irregularities in the lower ionosphere deduced from beacon satellite observations
75
direction are plotted against sub-satellite latitude in Fig. 3. Each data point is derived from an amplitude scaled at a Faraday maxima. The decibel absorption scale is relative, the zero point representing the predicted signal level and positive decibel readings representing levels below that predicted. If the resulting fluctuating absorption pattern is interpreted as being due to spatial fluctuations in electron density t,hen considerable ionospheric structure is implied. The horizontal size that can be attributed to this structure depends upon the height assigned to the electron it is clear from Fig. 1 that the most probable heights are density ~u~tuations; 65 km., 100 km. or 350 km. It is usual to associate abnormal absorption effects with the lower ionosphere where the cohision frequency is highest. To confirm that the observed effects are zenith
Orbit
I
I
I 38
I
Sub-satellite
Fig,
3. Relative
signal strength
I
42 latitude,
11618
I
I
46 beg
vs. sub sat,ellite latitude
for Orbit
11618.
not occurring in the P-region some of the field strength observations were compared with simultaneous electron content measurements made at 40 and 41 MHz. using the Faraday rotation method and made available through the courtesy of Mr. C. 8. Chen. Figure 4 shows the 20 MHz. record obtained from Orbit 11290 on the 9th January 1967. Figure 5 shows the absorption deduced from this record and also shows the total electron content observations for the same orbit. The spatial resolution of the electron content measurements is less than that of the 20 MHz. absorption measurements so that it is not expected that the fine structure appearing in the latter would be reproduced exactly in the former. Nevertheless, if the two sets of observations relate to phenomena in the same region of the ionosphere at least some of the features would appear on both. This is evidently not the case. The Faraday rotation effect occurs predominantly in the F-region so that any structure in the total electron content curve presumably results from F-region irregularities. As the structure in the absorption curve is not even partly reproduced in the electron content curve, it is inferred that the absorption curve structure is caused by irregularities below the F-region, that is to say, by irregularities at a height of 70 km or 100 km Evident in one section of the record illustrated in Fig. 4 is a marked change in the Faraday rotation rate accompanied by an amplitude enhancement; this section
J. A. D. HOLBROOK and G. F. LYON
16
of the record is not included in the analysis shown in Fig. 5. This type of rotation rate event has been noted several times during the concurrent 40 and 41 i%fHz. Faraday observations (&EN, 1967) and is the subject of a separate study. It is thought that such events are due to travelling ionospheric disturbances. In an attempt to confirm that the 20 MHz amplitude fluctuations were indeed due to non-deviative absorption some simultaneous, identical field strength measurements were made on the 40 MHz beacon transmission from the same satellite. Because tlhe Faraday rotation at 40 MHz is much slower than that at 20 MHz, a
2 oo\
Orbit
“E \
--*
3 -0 ;
II290
\ \
270l
I 0
.
*_*_. \
l
< o-
l
=I
I
38
Sub-satellite I%de.
deg
46
Fig. 5. Relative signal strength vs. sub slttellite latitude for Orbit 11290, together with total electron content figures.
direct comparison of amplitude at the two frequencies can only be made at a few points on the records. Furthermore the 40 MHz amplitude ffuctuations were, as might be expected, only of the order of O-I-O=4 dB, making accurate scaling of the records difficult. It usually proved to be the case that when the 40 MHz amplitude fluctuations were approaching 0.5 dB the 20 MHz signal was scintillating and could not be used for amplitude measurements. On the few occasions when a comparison could be made, the ratio of the decibel absorption at the two frequencies lay between 3 and 4. The expected ratio would be 4 in regions where collision frequency could be neglected and less than 4 when the collision frequency could not be considered small compared to the wave frequency. The received amplitudes can be measured only at the maxima in the Faraday fading pattern. There is thus the possibility that the amplitude fluctuations result from some anomalous polarization phenonomena. To check this possibility some observations were made of the 20 MHz amplitudes on separate systems utilizing orthogonal linear receiving antennae. Here again a direct comparison of the amplitudes on each of the two systems is complicated by the fact that the data
Irregularities in the lower ionosphere deduced from beacon satellite observations
77
points are not coincident in time. The amplitudes being read at the Faraday maxima, the data point from one system falls midway between two data points from the orthogonal system. Figure 6 shows the absorption plot obtained directly from one system together with an absorption plot obtained by interpolating midway between adjacent data points on the orthogonal system. The fluctuations in the two plots are very similar, lending more weight to the supposition that the observed amplitude changes are resulting from absorption. During the period January to August 1967, some 50 useable records were obtained. As previously noted, all of these are for ‘quiet’ ionospheric conditions,
Orbit
I
14118
/
/ 40
Sub- satellite
lotltude,
I
42
deg
Fig. 6. Signal amplitudes for Orbit 14118, 3rd August 196’7, observed simultaneously on antennae of orthogonal polarization. yet s,ll exhibit fluctuations similar to those shown in Figs. 3 and 5. The method of observation does not distinguish between temporal fluctuations and spatial variations. It is usually assumed that in the case of a rapidly moving satellite borne source the observed fluctuations are spatial. The horizontal size assigned to a fluctuation then depends only upon the height assumed for the irregularity in electron density. If this height is taken to be 70 km the spatial resolution of the experiment is approximately 0.5 km and the sizes observed range from O-5 km to 10 km. If this height is taken to be 100 km the resolution is 0.7 km and the size range O-7 km to 14.0 km. DISCUSSION The structure observed is not reproduced in total electron content measurements, which are most sensitive to P-region effects. This suggests that the signal strength observations relate to one of the lower ionospheric regions, but there is no direct evidence in the present measurements to indicate whether the D- or E-region is responsible. If the observed structure was occurring in the E-region then it would be reasonable to suppose that the irregularities were of the sporadic-E type. Unfortunately there is no vertical ionosonde in operation in the immediate vicinity of London so that it was not possible to make a direct comparison of events with locally observed fa, or fbEs. However, a comparison was made with sporadic-E data
78
J. A. D. HOLBROOKand G. F. LYON
from the Billerica, Massachusetts, field station (Lat. 42*3”N, Long. 71-2” W), published in the Geophysics and Space Data Bulletin (1967). No correlation with either foEa or fbEg was found. In this connection it may be noted that the signal strength observations are specifically restricted to times when no scintillation is occurring. AARONS and WHITNEY (1968), in a scintillation study utilizing a 136 MHz transmission from a synchronous satellite, have noted that the occurrence of scintillation is often accompanied by the occurrence of sporadic-E, thus giving further indirect evidence that the present observations do not refer to sporadic-E type irregularities. If the D-region is considered as the location of the observed structure there is an indirect explanation of the fact that the signal amplitude fluctuations were rarely seen at night but were almost always present during the day since the D-region of the ionosphere is present o&y in the daytime. This, and the lack of any other evidence to the contrary, suggests that the observed phenomenon is occurring in the D-region. It must be emphasized that all of the observations reported here refer to quiet conditions, that is to say, to times when there was no evidence of scintillation when there were no local riometer absorption events, and when the magnetic activity index was low. The observations suggest that even under quiet conditions there are often present in the daytime lower ionosphere, inhomogenieties in the electron density with sizes ranging from 0.5 to 10.0 km. The strength of the electron density fluctuations may be estimated from the relative amplitude of the received signal strength excursions if the latter are assumed to arise because of non-deviative absorption. Using a 10 km height interval and a collision frequency appropriate to a mean height of 70 km (KANE, 1961) yields electron density ~u~tuation in the range of 15-100 per cent of the ambient density. It is somewhat difficult to envision electron density fluctuations of the order of 100 per cent in the upper regions of the ionosphere under quiet conditions, when no external cause, such as a stream of precipitating particles, can be invoked. At the lower heights in the daytime ionosphere pronounced vertical gradients in electron density are known to exist over small height ranges. For example, AIKIN et al. (1964), reporting an afternoon rocket flight at Wallops Island, Virginia, show gradients of approximately threefold increases in electron density over height intervals of about 2 km at heights of 70 km and at 85 km. Such sharp vertical gradients may readily produce horizontal gradients under the influence of turbulence or wind shear effects. It has long been considered that the almost continuous signal received on high power VHF forward scatter circuits is due to scattering from irregularities in the lower ionosphere arising from turbulent mixing (see, for example, BAILEY et al. (1955) and WHEELON (1960)). Such signals show a daytime enhancement centered at noon; Wheelon has suggested that the daytime scattered signals arise from scattering at the 70 km level. Thus it seems reasonable to suppose that the irregularities observed in the present investigation are due to turbulence and occur at about 70 km height, The present study was initiated with the thought that a similar technique might allow the monitoring of t’he extent and structure in absorbing regions associated
Irregularities in the lower ionosphere deduced from beacon satellite observations
79
with amoral disturbances and polar cap absorption events. It is now evident that the technique is only useful under quiet conditions for 20 MHz trsnsmissions at middle latitudes. It seems likely that if similar measurements were attempted at higher latitudes where it would be possible to utilize tt higher frequency (say, 40 MHz) transmission to detect the greater absorption associated with disturbed conditions, the accompanying increase in scintillation occurrence frequency would restrict use of the system to quiet conditions alone. Acknowledgements-This work was supported by Defence Research Board of Canada Grant K-0. 9511-58. The paper is based in part upon material which will be submitted in an M.Sc. Thesis by one of the authors (J. A. D. H.) to the Faculty of Graduate Studies of the University of Western Ontario. Personal financial assistance provided (J. A. D. H.) by a Province of Ontario Graduat,e Fellowship is gratefully acknowledged. REFERENCES AARONYJ. and WHITKEY H. E. AIKIN A. C., KANE J. A. and TROIM J. BAILEY D. K., BATEMANR. and KII~BY R.C. IRELAND W. and PREDDY G. F. KANE J. A. KAZANTSEVA. N. Ross W. .J. and KUNTMAND. VASSY E. T. WHEELONA. D. The following
unpublished
document
1968 1964 1955
J. Planet Space Sci. 19, 21. J. Geophys. Res. 69, 4621. Proc. IRE 43, 1181.
1966 1961 1959 1967 1961 1960
J. J. J. J. J. J.
is also referred
GEOPHYSICSAND SPACE DATA BULI~ETIN
1967
Atmosph. Terr. Phys. 28, 481. Atmosph. Terr. Phys. 23, 338. Planet Space 6%. 1, 130. Geophys. Res. 72, 1041. Atmosph. Terr. Phys. 23, 85. Res. Natn. Bur. Stand. 64D, 301.
to:
Vol. IV, Nos. 2 and 3, Space Physics Lab., AFCRL, L.G. Hanscom Field, Massachusetts.