- Email: [email protected]
A new form of representation for temporal and spatial variations in radio wave absorption in the ionosphere
A new fom of representationfor temporal and spatialvariations in radio wave absorptionin the ionosphere J. 0. QYINLOuE* Department of Physics, University of Ilorin, Ilorin, Nigeria (Receiued 23 August 1978; in revised
form
29
September
1979)
Al&a&-A general empirical formula has been obtained for successfully predicting and forecasting h.f. absorption on global basis. It is shown that by the explicit inclusion of an ionizing flux term in the formula, the necessity for two different cos x laws for the diurnal and seasonal variations, which had hitherto been the case, has been removed. Even though 1-8 A solar X-ray flux may not necessarily be considered a major source of ionization in the D region, particularly during quiet conditions, 2.2 MHz absorption has been found to respond significantly to changes in this band of X-ray flux and the threshold level of the X-ray flux for this response could be as low as 0.2 x lo-“ erg cmm2s-i. 1.INlRODUCiXON The purpose of this paper is to obtain an equation of the form
L = F(U, x, 0,
(1)
that will describe on a global basis, 2.2MHz absorption L (obtained at vertical incidence) at a given intensity level of the ionizing flux U, a given solar zenith angle x and at a given location having a magnetic dip angle I. It has been found that the ionizing flux could vary significantly within the course of a day and from month to month and that these variations are usually accompanied by similar variations in radio wave absorption (GNANALINGAM, 1974; OYrn~oYu, 1978). but earlier work on absorption prediction (e.g. GEORGE, 1971; GNANALINGAM, 1969) has underrated the importance of such flux variation on short-term variation in absorption. It is shown in the present work that indeed, this neglect has resulted in having two different cos x laws for the diurnal and seasonal variations in absorption. Once the absorption at a given wave frequency (e.g. 2.2MHz) is known, it is a straightforward matter to estimate the absorption at any other desired wave frequency (GEORGE, 1971; SAMUEL and BRADLEY, 1975) and the equivalent absorption at oblique incidence (GEORGE and BRADLEY, 1974). 2. -AL
DATA
U in the present work has been represented by l-8 8, solar X-ray flux measured by SATELLITE
* Formerly at the Department Ibadan, Nigeria.
of Physics, University of 437
SOLAM ~---EXPLORER 37 and listed in the ‘Solar Geophysical Data, Part I’ published by ESSA Research Laboratories. This is because of the fact that over a large range of heights in the ionosphere, absorption has been found to vary in sympathy with time changes in the intensity of l-8 A solar X-rays (QYINLOYn, 1978). In the case of absorption L, experimental data have been obtained from absorption bulletins issued by Colombo, Freiburg and Ibadan and also from “Absorption Data for the IGY/IGC and IQSY” issued by the World Data Centre A. Where there were no observed values at 2.2 MHz, absorption at this wave frequency has been deduced by the method described by GEORGE (1971) and SAMUEL and BRADLEY (1975). Absorption data have been obtained from the stations listed in Table 1. These stations cover a wide range of latitudes and longitudes. 3. rurauL.%.s Equation
(1) can be written in the form L = r(IM(U
x),
(2)
where r(I) is the latitude factor and $( U, x) represents the time variation at a reference station where r(1) is unity. In this paper, Colombo is adopted as the reference station because several years of absorption data are available at this station. The factor r(1) is therefore the ratio of the 2.2MHz absorption at a given station having dip angle I to that at Colombo under a corresponding condition. The latitude factor implicitly caters in an overall manner for such influences as geomagnetic and meteorological controls and incidence of high
Table
1. Information
on stations -.__---
Station ___~__._~
Grog. iat. idegi _-----
Gcog. long. ideg)
23.ON 17.w
32.2E 76.9E 58.3E 18.6E 264.0E
ideg) 32N
Ahmedabad Alma Ata Ashkabad Bangui Baker Lake Brisbane Colombo Churchill Debilt Freiburg Johannesburg Ibadan Moscow Penn. State U. Port Lockroy
1.6N 64.3X L7.SS 6.9N 58.8N 52.1N 48.ON 26.2s 7.4N 55.5N 4O.XN
153.OE
5:s
79.9E 265.XE S.2E 7.XE 28.1E 3.9E 37.3E 282.1E
5s X3N 68N 64N 49s 6S 7lN 72N
64.XS
296.SB
5%
Rostov
17.2~
39.7E 103.RE 139.6E 18.9E
64N 17s 49N 7XN
Singapore Tokyo Tromso
:7.1)!Y
! .3N 35.7N 69.7N
energetic particles that affect absorption in varying degrees from one latitude belt to another. Jl(U, x) has been determined by combining the empirical relationships between L and U at a fixed value of x and at a constant level of U. From plots of log,, L against logto U for a constant value of x = 30”, using Colombo diurnal data during 1969-1970, the relationships obtained between L and U in the range 0.2~ U 5 5.0 merg crn-‘s~’ are: L = 4~.0~‘“~‘43”o~oo5’ Morning,
@a)
L = 5 1.5 U”i~143*‘1~D”5) Afternoon.
(3b)
Figures la and lb show a good fit of observed data over a long period to the predicted variations of equations (3a) and (3b) while Figs. l(c and d) demonstrate that the suggestion by GNANALINGAM (1974) that L is linearly related to V% could only be valid over a short interval of 0’. The 10.7 cm adjusted solar flux, Sa, has been used in the plots of Fig. 1 because of the inavailability of U data for the entire period of 1964-1970, From a plot of log, U against log, Sa, it has been found that U of equations (3a) and (3b} is related to Su in the form LJ= Sa
..--..- -.-
<-‘ip angie
i?ZN FhN 13s 86%
Years
of data
used
195x 1957--1959, 19&l-1965 19581959. 1964-196s 1958-1959 1957- 195s
1957-195x 1957-1959, 1964-197tl 1957-1959 1957-1958, 1964-1965 1958-1959, 1964-1965 I958 1957-1958, 3966-1968 1957-1958 1957-1958
1957-1958 1958-1959,
1964-1965 1964 1958-1959, 1964-1965 1957- 1958
the threshold value of U for this response can be as !ow as 0.2 x 10e4 erg cm ’ s ~’ (see Fig, 1). This result could be attributable to the following reasons. First, Lyman -_(y which is regarded as the dominant source of ion production below about 90 km varies very slowly in intensity compared to that of 1-8 A (or 31-lOO A) X-ray @WIDER, 1969). Secondly, most of the absorption at vertical incidence at 2.2 MHz takes place between about 85 and 95 km (GNANALINGAM and KANE, 1978; OYINLOYB, 1975; SMITH et al., 1978) and l-8 8, and 30-100 A X-rays contribute substantialiy to ionization in this height range (RATNASIRI, 1977; SWIDER, 1969). It is not unlikely that X-rays in the l-8 8, and 30100 A bands are correlated just as X-rays in the l-8 A and 8-20 8, have been found to be (C)YINLOYB, 1978). The cos x dependence of absorption at a constant flux intensity of 0.80 zz UC 1.20 merg cm-‘s-’ has been obtained for Colombo by plotting log, I.. against log,, cos x for the period 1969-1970. The relationships obtained between L and cos x are the following: L = 55.3(cos x)‘J~*“J~ for the monthly noon values,
(6a)
L = ss.l(cosX)0U6*““’ for 0730-l 130 h,
(6bJ
L = 57.6(cos x)‘.~~*(‘.“* for 1230-1630 h.
(6~)
It is significant that by considering
the influence of
Representation
for temporal and spatial variations in radio wave absorption Colombo,
439
1964-1970
60 -
I
I
I
I
I
,
,
I
I 200
] 220
Ll afternoon
x=30:
201
60
-4
80
I loo
I 120
I 140
l 160
I I80
I 200
I 220
I07 cm solar flux
60 So,
.I 80
I 100
lO-22 Wm-’
I 120
I !40
I 160
I 180
Hi’
1. A comparison of observed and computed variations of 2.2 MHz absorption at x = 30 with the 10.7 cm flux using for x = 30” (a) I_.= 48.0U”.*43 Morning (b) I.. = 51.5u0.‘43 Afternoon (c) L = 32.6+(13.9*0.8)& Morning (After GNANALINGAM,1974) (d) L = 37.6+(13.6*0.5)x@ Afternoon (After GNANALINGAM,1974) L 1, L2 correspond respectively to the upper and lower limits of the computed absorption arising from the error in the relationship between U and So.
Fig.
IJ on short-time variations of absorption, the same cos x dependence of L has been found in (6a) and (6b) for both the seasonal and hourly variations during the morning period. Thus the discrepancy between the duirnal and seasonal cos x exponents (e.g. see GNANAUNOAN, 1969) has been removed. In further consideration of cos x exponent in this paper, (6b) will be used in preference to (6a) because the former has a greater spread in the cos x values and therefore less susceptible to errors than the latter. It is also to be noted that for x-* 0, equations (6a), (6b) and (6~) tend towards the same limit within the limit of experimental errors. Combining equations (3a) and (6b) and equations (3b) and (6~) we can now write Jl(U, x) in the form @(I.&x) = LJ_P~143cosm x and hence equation (1) in the form L = r(r)L,U0.‘43 COP x5
(7)
where m takes the values 0.96 and 0.78 for the morning and afternoon hours respectively. L,, has the physical significance that it gives the value of absorption for the reference station of Colombo when U= I.00 mergcm-* s-l and x = 0. Its magnitude is defined by equations (6b) and (6~). The latitude variation of r(1) with the dip angle I which also constitutes a normalized latitude variation of absorption at 2.2MHx is shown as Fig. 2. At low latitudes there is no significant seasonal or sunspot controlled variation of r(I) whereas in the middle and high latitudes, r(I) seem to be higher during low sunspot than during high sunspot period. The solar dependence of r(I) for the middle and high latitudes may on the surface took rather surprising. A tentative explanation for the observation may lie partially in the fact that cosmic rays responsible for ionizing the bottom f) region have higher intensity during the low sunspot period than
-140
.1. 0.
OYINL~~Y,~
pertcd * Low sunspot period
0 thigh sunspot
Fig. 3. A comparison of computed (continuous line) and observed (crosses) diurnal variation of 2.2 MHz absorption during summer over a wide range of dip angle I.
f,
06 0
20 Magnetic
I 40 dip angle,
/ 60
I 80
I degrees
Fig. 2. Normalized Latitude variation of 2.2 MHz noon absorption with the value at Colombo being 1.OO.
during the high sunspot period and more importantly an explanation may Iie in the occurrence pattern of midlatitudes-type sporadic E. At low latitudes, this type of sporadic E occurs more frequently during low than during high sunspot number (OYINLOYE, 1969) whereas the reverse is the case for the middle and high latitudes (REDDY and MATSUSHITA, 1968). According to SMITH et al. (1978), electron density enhanced in a narrow sporadic E layer tends to produce decreased absorption. Thus during high sunspot period, relatively greater reduction in absorption due to sporadic E layer is more likely in the middle latitudes than in the low latitudes with a resultant low r(f) and on the same reasoning, high r(f) would be expected during low sunspot number. 4.VALIDA~oFTHEN$w PoIMLRlLATION Figure 3 illustrates the good extent to which equation (7) predicts observed variation in absorption. It is evident that observed diurnal data during high and low sunspot periods have a good fit to computed diurnal variations for 1~72”. It is suspected that at Baker Lake (I= W&J), where the agreement between computed and observed variations is poor, there is a substantial cont~bution to ionization in the lower ionosphere by sources other
than direct larly, good have been seasons (not
U.V. and X-ray solar radiations. Simifit of observed to predicted variations obtained for the equinox and winter shown in Fig. 3). However while typi-
cal errors of about 20% were obtained in Winter at midlatitude stations, typical errors of about 10% and less were obtained during summer and equinox. Thus equation (7) can be used to predict absorption. The extent to which equation (7) can be used to forecast absorption over a short period is examined below. Computed and observed absorption for Colombo and Freiburg have further been compared on one to one basis on regular world days during the period 1969-1970. In all 214 comparisons, based on the data for 09, 12 and 15 h all through the 24 months, were made for Colombo while only 44 such comparisons would be made for Freiburg because only noon data were published in the bulletins for the period January 1969-July 1970. The choice of the stations and the period under examination has been dictated by the simultaneous availability of observed absorption data and 1-8 8, solar X-ray flux used in the calculation. Defining percentage deviation of computed from observed value as
(
computed value-observed computed value
value 1
x 100.
It was found that at Colombo cumulative 84.5 and 56.5% of the computed values had respectively between 0.0-20.0 and O.O-10.0% deviations from
Representation
for temporal and spatial variations in radio wave absorption
the observed. At Freiburg, the figures corresponding to 0.0-20.0 and O&20.0% deviations were respectively 74.7 and 48.9% of the computed values. This result suggests that equation (7) can reasonably be used for forecasting absorption val-
441
ues. It is however suspected that for stations showkg Iarge errors in the latitude factor t(I), during winter (see Fig. 2) the use of equation (7) for forecasting purposes may be limited to summer and equinox.
REFERENCES GEORGE, GEORGE,
P. L. P. L. and BRADLEY, P. A. GNANAL~NGAM, S.
1971 1974 1969
GNANAUNGAM,S. GNANALINGAM,S. and KANE, J. A. OYINLOYIZ,J. 0. OYINLO~E, J. 0. OMNLOYE, J. 0. RATNASIRI,P. A. J. REDDY, C. A. and MATSUSHITA,S. SAMUFIL,J. C. and BRADLEY,P. A. Sm, L. G., WALTON, E. K. and MECHTLY,E. K. SWIDER, W.
1974 1978 1969 197s 1978 1977 1968 1975 1978
J. atnzos. terr. Phys. 33, 1893. P.O. Te~ecomm~~. J. 41, 307. Proc. Third International Symposium on Quatoriaf Aeronomy, P. 47. J. atmos. terr. Phys. 36, 1335. J. atmos. ten. Phys. 40, 629. Radio Sci. 4, 765. J. amtos. terr. Phys. 37, 1. 3. atoms. rerr. Phys. 45, 793. J. ammos.terr. Pkys. 39, 999. J. Geophys. Resr. 73, 1641. J. atmos. ten. Phys. 37, 131. J. atmos. tew. Phys. 40, 1185.
1969
Review Geophys. 7, 573.
form
29
September
1979)
Al&a&-A general empirical formula has been obtained for successfully predicting and forecasting h.f. absorption on global basis. It is shown that by the explicit inclusion of an ionizing flux term in the formula, the necessity for two different cos x laws for the diurnal and seasonal variations, which had hitherto been the case, has been removed. Even though 1-8 A solar X-ray flux may not necessarily be considered a major source of ionization in the D region, particularly during quiet conditions, 2.2 MHz absorption has been found to respond significantly to changes in this band of X-ray flux and the threshold level of the X-ray flux for this response could be as low as 0.2 x lo-“ erg cmm2s-i. 1.INlRODUCiXON The purpose of this paper is to obtain an equation of the form
L = F(U, x, 0,
(1)
that will describe on a global basis, 2.2MHz absorption L (obtained at vertical incidence) at a given intensity level of the ionizing flux U, a given solar zenith angle x and at a given location having a magnetic dip angle I. It has been found that the ionizing flux could vary significantly within the course of a day and from month to month and that these variations are usually accompanied by similar variations in radio wave absorption (GNANALINGAM, 1974; OYrn~oYu, 1978). but earlier work on absorption prediction (e.g. GEORGE, 1971; GNANALINGAM, 1969) has underrated the importance of such flux variation on short-term variation in absorption. It is shown in the present work that indeed, this neglect has resulted in having two different cos x laws for the diurnal and seasonal variations in absorption. Once the absorption at a given wave frequency (e.g. 2.2MHz) is known, it is a straightforward matter to estimate the absorption at any other desired wave frequency (GEORGE, 1971; SAMUEL and BRADLEY, 1975) and the equivalent absorption at oblique incidence (GEORGE and BRADLEY, 1974). 2. -AL
DATA
U in the present work has been represented by l-8 8, solar X-ray flux measured by SATELLITE
* Formerly at the Department Ibadan, Nigeria.
of Physics, University of 437
SOLAM ~---EXPLORER 37 and listed in the ‘Solar Geophysical Data, Part I’ published by ESSA Research Laboratories. This is because of the fact that over a large range of heights in the ionosphere, absorption has been found to vary in sympathy with time changes in the intensity of l-8 A solar X-rays (QYINLOYn, 1978). In the case of absorption L, experimental data have been obtained from absorption bulletins issued by Colombo, Freiburg and Ibadan and also from “Absorption Data for the IGY/IGC and IQSY” issued by the World Data Centre A. Where there were no observed values at 2.2 MHz, absorption at this wave frequency has been deduced by the method described by GEORGE (1971) and SAMUEL and BRADLEY (1975). Absorption data have been obtained from the stations listed in Table 1. These stations cover a wide range of latitudes and longitudes. 3. rurauL.%.s Equation
(1) can be written in the form L = r(IM(U
x),
(2)
where r(I) is the latitude factor and $( U, x) represents the time variation at a reference station where r(1) is unity. In this paper, Colombo is adopted as the reference station because several years of absorption data are available at this station. The factor r(1) is therefore the ratio of the 2.2MHz absorption at a given station having dip angle I to that at Colombo under a corresponding condition. The latitude factor implicitly caters in an overall manner for such influences as geomagnetic and meteorological controls and incidence of high
Table
1. Information
on stations -.__---
Station ___~__._~
Grog. iat. idegi _-----
Gcog. long. ideg)
23.ON 17.w
32.2E 76.9E 58.3E 18.6E 264.0E
ideg) 32N
Ahmedabad Alma Ata Ashkabad Bangui Baker Lake Brisbane Colombo Churchill Debilt Freiburg Johannesburg Ibadan Moscow Penn. State U. Port Lockroy
1.6N 64.3X L7.SS 6.9N 58.8N 52.1N 48.ON 26.2s 7.4N 55.5N 4O.XN
153.OE
5:s
79.9E 265.XE S.2E 7.XE 28.1E 3.9E 37.3E 282.1E
5s X3N 68N 64N 49s 6S 7lN 72N
64.XS
296.SB
5%
Rostov
17.2~
39.7E 103.RE 139.6E 18.9E
64N 17s 49N 7XN
Singapore Tokyo Tromso
:7.1)!Y
! .3N 35.7N 69.7N
energetic particles that affect absorption in varying degrees from one latitude belt to another. Jl(U, x) has been determined by combining the empirical relationships between L and U at a fixed value of x and at a constant level of U. From plots of log,, L against logto U for a constant value of x = 30”, using Colombo diurnal data during 1969-1970, the relationships obtained between L and U in the range 0.2~ U 5 5.0 merg crn-‘s~’ are: L = 4~.0~‘“~‘43”o~oo5’ Morning,
@a)
L = 5 1.5 U”i~143*‘1~D”5) Afternoon.
(3b)
Figures la and lb show a good fit of observed data over a long period to the predicted variations of equations (3a) and (3b) while Figs. l(c and d) demonstrate that the suggestion by GNANALINGAM (1974) that L is linearly related to V% could only be valid over a short interval of 0’. The 10.7 cm adjusted solar flux, Sa, has been used in the plots of Fig. 1 because of the inavailability of U data for the entire period of 1964-1970, From a plot of log, U against log, Sa, it has been found that U of equations (3a) and (3b} is related to Su in the form LJ= Sa
..--..- -.-
<-‘ip angie
i?ZN FhN 13s 86%
Years
of data
used
195x 1957--1959, 19&l-1965 19581959. 1964-196s 1958-1959 1957- 195s
1957-195x 1957-1959, 1964-197tl 1957-1959 1957-1958, 1964-1965 1958-1959, 1964-1965 I958 1957-1958, 3966-1968 1957-1958 1957-1958
1957-1958 1958-1959,
1964-1965 1964 1958-1959, 1964-1965 1957- 1958
the threshold value of U for this response can be as !ow as 0.2 x 10e4 erg cm ’ s ~’ (see Fig, 1). This result could be attributable to the following reasons. First, Lyman -_(y which is regarded as the dominant source of ion production below about 90 km varies very slowly in intensity compared to that of 1-8 A (or 31-lOO A) X-ray @WIDER, 1969). Secondly, most of the absorption at vertical incidence at 2.2 MHz takes place between about 85 and 95 km (GNANALINGAM and KANE, 1978; OYINLOYB, 1975; SMITH et al., 1978) and l-8 8, and 30-100 A X-rays contribute substantialiy to ionization in this height range (RATNASIRI, 1977; SWIDER, 1969). It is not unlikely that X-rays in the l-8 8, and 30100 A bands are correlated just as X-rays in the l-8 A and 8-20 8, have been found to be (C)YINLOYB, 1978). The cos x dependence of absorption at a constant flux intensity of 0.80 zz UC 1.20 merg cm-‘s-’ has been obtained for Colombo by plotting log, I.. against log,, cos x for the period 1969-1970. The relationships obtained between L and cos x are the following: L = 55.3(cos x)‘J~*“J~ for the monthly noon values,
(6a)
L = ss.l(cosX)0U6*““’ for 0730-l 130 h,
(6bJ
L = 57.6(cos x)‘.~~*(‘.“* for 1230-1630 h.
(6~)
It is significant that by considering
the influence of
Representation
for temporal and spatial variations in radio wave absorption Colombo,
439
1964-1970
60 -
I
I
I
I
I
,
,
I
I 200
] 220
Ll afternoon
x=30:
201
60
-4
80
I loo
I 120
I 140
l 160
I I80
I 200
I 220
I07 cm solar flux
60 So,
.I 80
I 100
lO-22 Wm-’
I 120
I !40
I 160
I 180
Hi’
1. A comparison of observed and computed variations of 2.2 MHz absorption at x = 30 with the 10.7 cm flux using for x = 30” (a) I_.= 48.0U”.*43 Morning (b) I.. = 51.5u0.‘43 Afternoon (c) L = 32.6+(13.9*0.8)& Morning (After GNANALINGAM,1974) (d) L = 37.6+(13.6*0.5)x@ Afternoon (After GNANALINGAM,1974) L 1, L2 correspond respectively to the upper and lower limits of the computed absorption arising from the error in the relationship between U and So.
Fig.
IJ on short-time variations of absorption, the same cos x dependence of L has been found in (6a) and (6b) for both the seasonal and hourly variations during the morning period. Thus the discrepancy between the duirnal and seasonal cos x exponents (e.g. see GNANAUNOAN, 1969) has been removed. In further consideration of cos x exponent in this paper, (6b) will be used in preference to (6a) because the former has a greater spread in the cos x values and therefore less susceptible to errors than the latter. It is also to be noted that for x-* 0, equations (6a), (6b) and (6~) tend towards the same limit within the limit of experimental errors. Combining equations (3a) and (6b) and equations (3b) and (6~) we can now write Jl(U, x) in the form @(I.&x) = LJ_P~143cosm x and hence equation (1) in the form L = r(r)L,U0.‘43 COP x5
(7)
where m takes the values 0.96 and 0.78 for the morning and afternoon hours respectively. L,, has the physical significance that it gives the value of absorption for the reference station of Colombo when U= I.00 mergcm-* s-l and x = 0. Its magnitude is defined by equations (6b) and (6~). The latitude variation of r(1) with the dip angle I which also constitutes a normalized latitude variation of absorption at 2.2MHx is shown as Fig. 2. At low latitudes there is no significant seasonal or sunspot controlled variation of r(I) whereas in the middle and high latitudes, r(I) seem to be higher during low sunspot than during high sunspot period. The solar dependence of r(I) for the middle and high latitudes may on the surface took rather surprising. A tentative explanation for the observation may lie partially in the fact that cosmic rays responsible for ionizing the bottom f) region have higher intensity during the low sunspot period than
-140
.1. 0.
OYINL~~Y,~
pertcd * Low sunspot period
0 thigh sunspot
Fig. 3. A comparison of computed (continuous line) and observed (crosses) diurnal variation of 2.2 MHz absorption during summer over a wide range of dip angle I.
f,
06 0
20 Magnetic
I 40 dip angle,
/ 60
I 80
I degrees
Fig. 2. Normalized Latitude variation of 2.2 MHz noon absorption with the value at Colombo being 1.OO.
during the high sunspot period and more importantly an explanation may Iie in the occurrence pattern of midlatitudes-type sporadic E. At low latitudes, this type of sporadic E occurs more frequently during low than during high sunspot number (OYINLOYE, 1969) whereas the reverse is the case for the middle and high latitudes (REDDY and MATSUSHITA, 1968). According to SMITH et al. (1978), electron density enhanced in a narrow sporadic E layer tends to produce decreased absorption. Thus during high sunspot period, relatively greater reduction in absorption due to sporadic E layer is more likely in the middle latitudes than in the low latitudes with a resultant low r(f) and on the same reasoning, high r(f) would be expected during low sunspot number. 4.VALIDA~oFTHEN$w PoIMLRlLATION Figure 3 illustrates the good extent to which equation (7) predicts observed variation in absorption. It is evident that observed diurnal data during high and low sunspot periods have a good fit to computed diurnal variations for 1~72”. It is suspected that at Baker Lake (I= W&J), where the agreement between computed and observed variations is poor, there is a substantial cont~bution to ionization in the lower ionosphere by sources other
than direct larly, good have been seasons (not
U.V. and X-ray solar radiations. Simifit of observed to predicted variations obtained for the equinox and winter shown in Fig. 3). However while typi-
cal errors of about 20% were obtained in Winter at midlatitude stations, typical errors of about 10% and less were obtained during summer and equinox. Thus equation (7) can be used to predict absorption. The extent to which equation (7) can be used to forecast absorption over a short period is examined below. Computed and observed absorption for Colombo and Freiburg have further been compared on one to one basis on regular world days during the period 1969-1970. In all 214 comparisons, based on the data for 09, 12 and 15 h all through the 24 months, were made for Colombo while only 44 such comparisons would be made for Freiburg because only noon data were published in the bulletins for the period January 1969-July 1970. The choice of the stations and the period under examination has been dictated by the simultaneous availability of observed absorption data and 1-8 8, solar X-ray flux used in the calculation. Defining percentage deviation of computed from observed value as
(
computed value-observed computed value
value 1
x 100.
It was found that at Colombo cumulative 84.5 and 56.5% of the computed values had respectively between 0.0-20.0 and O.O-10.0% deviations from
Representation
for temporal and spatial variations in radio wave absorption
the observed. At Freiburg, the figures corresponding to 0.0-20.0 and O&20.0% deviations were respectively 74.7 and 48.9% of the computed values. This result suggests that equation (7) can reasonably be used for forecasting absorption val-
441
ues. It is however suspected that for stations showkg Iarge errors in the latitude factor t(I), during winter (see Fig. 2) the use of equation (7) for forecasting purposes may be limited to summer and equinox.
REFERENCES GEORGE, GEORGE,
P. L. P. L. and BRADLEY, P. A. GNANAL~NGAM, S.
1971 1974 1969
GNANAUNGAM,S. GNANALINGAM,S. and KANE, J. A. OYINLOYIZ,J. 0. OYINLO~E, J. 0. OMNLOYE, J. 0. RATNASIRI,P. A. J. REDDY, C. A. and MATSUSHITA,S. SAMUFIL,J. C. and BRADLEY,P. A. Sm, L. G., WALTON, E. K. and MECHTLY,E. K. SWIDER, W.
1974 1978 1969 197s 1978 1977 1968 1975 1978
J. atnzos. terr. Phys. 33, 1893. P.O. Te~ecomm~~. J. 41, 307. Proc. Third International Symposium on Quatoriaf Aeronomy, P. 47. J. atmos. terr. Phys. 36, 1335. J. atmos. ten. Phys. 40, 629. Radio Sci. 4, 765. J. amtos. terr. Phys. 37, 1. 3. atoms. rerr. Phys. 45, 793. J. ammos.terr. Pkys. 39, 999. J. Geophys. Resr. 73, 1641. J. atmos. ten. Phys. 37, 131. J. atmos. tew. Phys. 40, 1185.
1969
Review Geophys. 7, 573.