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Eclipse effects on the F-layer at sunrise
Journal of Atmosphericand TerrestrialPhpsics, l%O, Vol. IO,pp. T3 to 81. PergnnmnPwss Ltd. l’rinkxl in Northern Irdtnd
Eclipse effects on the F-layer at sunrise C. S.
G. K. SETTY
Department of Physics, Central College, Rangalore. India (Received 1 April1960) Abstract-Eclipse
effects on the F-layer at sunrise observed on 1 September 1951 at Derwood, Maryland,
U.S.A. are studied with some detail. The h’(f) curves taken during the eclipse period and on control days were analysed for N(h) and N(t) curves at a series of height,s. The eclipse variation of S computed from the control day data does not coincide with the observed variation. However, if the assumption is made that the electron production rates on the eclipse day were about 30 per cent higher than those on the succeeding control day the agreement between the observed and computed eclipse variations of N is improved. Further, if it is assumed that at the time of totality 30 per cent of the radiation was unobscured, in other words if the 30 per cent of the radiation originates in a region beyond the visible solar disc, then the agreement between the observed and computed eclipse variations is excellent. Some solar data in favour of both these assumptions is given and its validity in the light of several other known eclipses is discussed. 1.
INTRODUCTIOK
OIISERV~TIONS of the eclipse of 1 September 1951 were made at Derwood, Maryland, U.S.A. by Dr. H. W. Wells of the Department of TerrestriaI Magnetism, Washington who published details of the eclipse characteristics and made some deductions from the observed critical frequency variation (WELLS, 1952). The actual h’(S) records obtained at, Derwood and at two other stations nearby were made available by Dr. Wells, but since the critical frequency data indicated that the eclipse effect on the F-layer at the three stations was similar, it was decided that the analysis of records taken at any one of the three stations should be sufficient. Here the h’(f) records taken at 5 min intervals at Derwood are analysed using the method developed by SHIK~N (1955) to reduce the h’(f) curves into N(h) curves allowing for the earth’s magnetic field appropriate to the station. The geographic location of the station and t’he eclipse circumstances are as follows : ‘Table
Station
1 Geog. lat.
Geog. long.
Monthly
39.1”N
conditions / ,_ _._ ~____~_,__~_
~__
I
0537
._____
!
I
Greatest phase at, surface ~,~~. _~~~
,__~__.__ 54
0559
-~
_~
92
End of eclipse -._ -.
/
~_ ~
~
~. __
0707
average relative Zurich sunspot number: august 194 1
The obscuration used for Derwood. 1
77.2”W
I
G.S.R.
I
-___-
Derwood (Maryland)
1.
60 September 1941 80
function
computed
for Charlottesville
73
(WELLS, Fig. 2A) is also
C. S. Cr.K. SETTY
The effect of the solar eclipse on the P-layer at sunrise is of particular interest. The continuity equation for electrons at a given level is
At sunrise, y and dsjdt are the predominant terms in this equation, Ic, and it both being small, so there should be an al~preciable eclipse effect on the F-layer due to the large change in ~1. The crit’ical frequency data has in fact shown that there is a marked effect. The other interesting feature of this eclipse is that the critical frequency on the eclipse day is much higher than on the control day. Furthermore this annular eclipse, as is remarked by WELLS~ is especially favourable for a test of the effectiveness of ionizing radiation emanat,ing outside the visible disc of the sun. There are only two days of control data, one on 31 August and the other on 2 September 1951. Figs. la and lb show X(t) curves at 10 km intervals on the eclipse and control days. The increase in S at higher levels is fairly smooth and ~~~~~ can be co~~lp~lted -wit,h little u~~certai~~ty. At lower levels, 230-250 km, the ~uctuatio~s in M are rat,her large. Before studying the results of eclipse effects in detail it is interesting to check some of the conclusions made in connexion with the Cambridge sunrise observations (SETTY, 1956). 2. THE
SCNRISE PHENOMENON
One of the anomalies observed with the Cambridge critical frequency data near sunrise was “the seasonal sunrise anomaly,” when the critical frequency was found to increase near ground sunrise in summer but about 40 min to 1 hr before ground sunrise in wint,er and equinoxes. The Derwood data on control and eclipse days, Fig. 2, shows that the critical frequency began to increase at x = 96” which is in agreement with the Cambridge result. The N(t) curves also indicate that N began to increase before ground sunrise. 3. THE
ELECTRON
PRODUCTIOX
RATES
The electron production rates, q, are calculated by the use of the continuity equation (I). As the loss-coef-rieient, k,, is believed to depend on the solar-epoch the Loss-coefficients appropriate to this ease are not known, therefore two models based on the Can~bridge results are used (RATC~~I~FEe.t nl.,1956). Model I:
h, = 1 >: 10e4 exp
i
Model II: h, = 1 x 1O-4 exp i -
h -- 300 __-_ .- .30 h --
300 50
Figs. 3a and 3b show q(t) plots at 280 km computed by assuming k, to be 1.9 x 10-a and 1.5 x IO-" corresponding to models I and II respectively. There is little change in t,he shape of tShe two curves. 4 on model I reaches a steady 74
Time (75OWest med. Fig.
le.
Fig.
lb.
N(I)
hr
curves at a stories of height.rj un the eclipse day 1 September
75
1961.
C. S. G. K. SETTV
6055-50 -454.03.530c ii
.
’
l
.
.
~~~~//:/~:~/~~:!!~!,~l~:~/~l~/,,,l~~,,,,~~~,~,
04
06
05
07
Time (75O West mer), Fig.
08
09
II
IO
hr
2. Plots of f$‘%-_t on the eclipse and control days.
(a) 31 August 1951. (b) 1 September 1951.
Station: Dervuood, 394’N. (c) 2 September 1951.
I,--
)14c
,i2C
IImc
)-
t )-
cl- 4c)-
2<)-
(lI 06
L_
1 07
Time,
I
I
08
09
hr (b)
Pig.
3. The variation of q at 280 km height on t,he control day, the observed and computed
eclipse day. O-0 N = qe - qc.
Eclipse da,y. *---*
Control day. H, = 40 km; = 30km; K,(300)
(a) Assumed KI-model: K,-model:
H,
78
Computed eclipse day. - - - X,(300) = 10--4. (b) Assumed = lo-*.
Eclipseeffectson the P-layer at sunrise
value which is 20 per cent higher than the corresponding cl calculated Rearing this in mind only model I is used for further calculations.
on model II.
4. EXPECTED ECLIPSE EFFECT OE; THE TIME VARIATION OF q FROM COXT
THE
ROL DATA
Let qC be the product,ion rate at a given height at, any instant on the eont,rol day and let ‘1, be the corresponding quantity on the eciipse day. Assuming that the ionizing radiation arises uniformly over the visible solar disc q, is proportional to the unobscured fraction of the solar disc. Then if,!(t) is the obscuration function WP have the relation cl, = ~,.~(~) where *f(t) is the area unobscured. Using the computed f(t) (WELLS’ -Fig. 2A), the expected q, from the control day data of 2 September is calculated and shown in Figs. 3a and 310. Figs. 3a and 3b also show 4 plotted against time on the control day and the eclipse day. It is immediately apparent. from this figure that although the expected and observed variations of q, are similar, the entire curve on the eclipse day is shifted by a constant factor above the computed eclipse curve. This leads us to believe that the production rate itself must have been higher by a factor of 1.3 on t,he eclipse day compared with the succeeding control day value. Vertical movement as a possible source of the increased value of 4 immediately after the eclipse was considered to be very unlikely. The control day records on the preceding day, 31 August 1951, were, u~lfort~lnately, not available after 0800 hours, but the computed eclipse variation of N using August 31 as the control day at 270 and 260 km agreed with the observed variation of hTduring the eclipse. This supports the suggestion made that on the eclipse day and the preceding day the sun must have been quite active and returned to normal on 2 September 1951. If this were correct, the Ff critical frequency must be higher on days of high solar activity as it is believed t,hat f,( P.2) varies regularly with solar activity. It was in fact noticed that the critical frequencies of the FI-layer on 1 September were higher than the corresponding values on 2 September by about 0.3 MC/S. This leads to a value of q,, which is 1.3 times bigger on the eclipse day than on 2 September. This would therefore explain the fact that the production rate at sunrise was enhanced by the same factor.. Figs. 4a and 4b show t.he observed variation of 4 at 270 km on t,he eclipse day and the control day, and the computed variation on the eclipse day. t,he latter two being multiplied by a factor of 1.4. Although there is a good fit between the observed and computed eclipse curves, except for a bump at 0600 hours on the eclipse curve, q, on the observed eclipse curve at the time of totality is much higher t~han the expected value. This seems to suggest8 &at there must have been an additional source of ionization. This could either be due to another source of ionizing radiation lying beyond t,he visible disc (e.g. in t,lre corona) and therefore uncovered during the eclipse or it could be only an apparents additional ionizing source due to vertical movements. To decide between these two possibilities the following procedure was adopted: the observed values of q, on tZhe eclipse day and 4, on the succeeding control day were considered. A constant factor was subtracted from the actual values of 4, on the eclipse day so that at t,he 77
C. S. G. K.
SETTY
220 I2OQ-
160 -
160 -
140 -
140 -
-i :
I20 -
;s
loo-
i tl
100 -
60 -
E k
80-
60 -
”
60-
40 -
_ w
40-
G
120 -
__/---
20 -
20 o-
O-
: I 05
I 06
I 07 Time,
I 06
I 09
I 05
c6
I 07 Time.
hr
/
I Oa
hr
(b)
b)
Fig. 4a. Computed eclipse variation of q from the control variation with the intensity of the ionizing radiation enhanced by a factor 1.4. 270 km height. K, = 4.4 x 10m4. 00 1 September 1951. 0-0 2 September 1951 (Factor 1.4 is allowed). - -Computed eclipse from 2 September 1951. - - - - M. Fig. 4b. Computed eclipse variation of g from the control day witsh the intensity of the ionizing radiation enhanced by a factor 1.4. 280 km height. 0-a 2 September 1951. O-0 Observed eclipse. - -Computed eclipse. - - - - M.
25_260i 05
1 06
‘1
1 07 Time,
1
1 06
1
1 09
1
hr
Fig. 5. Computed eclipse variation of q at a series of heights allowing for the radiation height from outside the visible solar disc. (a) 260 km height. (b) 270 km height. (c) 280 km height. x---x Observed eclips-. + - + Eclipse computed from 31 August 1951. --Eclipse computed from 2 September 1951.
78
09
Eclipse effects on the F-layer
at sunrise
time of totality the actuaf values of q, and that computed from t’he succeeding control day were the same. Thus at 270 km height q, observed and computed were made each equal to 7 at 0600 hours. The actual observed value of q, on 2 Beptember 1951 at 0600 hours is 44, and the steady value of 4, on 2 September 1951 is 100. It is therefore necessary to assume that the extra radiation coming from outside the visible dise on the eclipse day should be about 30 per cent of the entire radiation coming from the visible disc. This is the same as the earlier conclusion that y,, on the eclipse day is greater by a factor of 1.3 compared mit’h q0 on 2 September. In addition to this it has to be assumed that t’he extra radiation comes from outside the visible disc so t,h;tt$it is uncovered during t.he time of totalit,g. When this assumption is made it is seen from Fig. 5 t,hat the agreement bet,ween the observed and computed curves is very much improved. The computed eclipse variatzion of N from data for 31 August, reduced by a fact’or of 1.3 is also shown in the same figure. This curve fits closely wit-h the observed eclipse curves and the other computed curve of :! Sept,ember 1951. It may therefore be concluded t.hat t.he sun was active on 31 August and 1 Sept,ember and had returned t,o normal ~ondit’ion on 2 September. Also it is necessary to conclude that the extra radiation dne 00 t’be enhanced activ%y came from out,side the visible disc. 5. SOLAR I)ATa
IN
Fnvoun
OF THE (;OX-cLl-SIOXS
PRECEDING
OF THE
SECTION
Evidence in favour of the conclusions arrived at in the fastr section was sought from the published solar data in the ~~~~r~er~~~~~~~~~~n on 8oZar Activit;~ published in Zurich. It was found that there was no significant difference in coronal line intensity on the eclipse day and the preceding control day, compared with the day following the eclipse. There was no significant difference in tot,al intensity on t’hese days. However, t’he dates 31 August and 1 September 1951 were in fact n~entiol~ed amongst the days when solar eruptio~ls were observed, and 2 September 1!)51 was not mentioned. The following are the characteristics of the eruptions observed on 31 August and 1 September 1951. Table
Dat,e
Time of observation (U.T.)
/
(OC,
2.
C!oordinates ~_______~ j Distance from central meridian
1
(“E)
- ___ _--._-
/
31.viii.1951 31.viii.1951 l.ix.1951 Remarks
Estimated importance (max. = 3)
against 1 September 1951: A few brights spots. 31 August 1951: Eruptions accompanied
2, 3 2,-i
by large and fast radial filaments.
The latter remark is of particular significance. There is a suggestion of not only enhanced activity on 31 August and 1 September, I951 but also the possibility of the ionizing radiation having originated from outside the visible disc of the sun. 79
C. X. G. K. SETTY It may be concluded, therefore, that the enhanced production of ionization on the eclipse day is due to the extra ionizing radiation caused by enhanced activity on the sun. This conclusion has two important consequences. The day-to-day fluctuations in the critical frequency of the F-layers may be caused by fluctuations in the ionizing radiation of the sun. Solar flares which are accompanied by the enhancement of H, radiation may have an effect on the F-layer critical freyuenties. The other important effect would be on t’he deductions made from the eclipse effects on the FI-layer. In deducing the recombination coefficient the assumption is made that the production rate is reduced to zero at the time of totalit’y. Such an assumption may be wrong if the radiat,ion comes from outside the visible disc. While summarizing the eclipse effects on the E- and PI-layers observed by various workers in the past,, RlzTczrFFe (1%X5) came to the conclusion that about 15 per cent of the radiation should come from outside the visible disc. The COP elusions reached here from the eclipse effect at sunrise in the F-region observed at Derwood supports this suggestion. 6. CoXcLrrsIo~s The main conclusions reached from the analysis of t’he eclipse are summarized below. (i) To account for enhanced ionization on the eclipse day and t,he preceding day it is necessary to assume that t,here was ext’ra ionizing radiation on these days of about 30 per cent of Dhe total radiation arising from the entire solar disc as observed on 2 September. It is also necessary to assume that this additional source should be beyond the visible disc of the sun. There is solar data, which favours both these conclusions. (ii) The ionization in the Fl- and FZ-layers go together. IZ small increase of 0.3 MC/S inf,,(F1) is equivalent to a comparatively big change of 1 NC/S in f,,( F2). This is expected as the loss-coefficient in the F1-layer is believed to be bigger than that in the FL’-layer. (iii) The day-to-day fluctuations in f,( El) and f& FZ) may therefore be associated closely with the fluctuations of the solar radiation producing the ionization. This variable part of the radiation is likely to originate in the corona, but the intensity of coronal lines may not be a very good indication of t’he correlation between the two phenomena. (iv) The foregoing conclusions support the suggestion by RATCLIFFE (I 966) that the recombination coefficients of E- and Fl-layers deduced from the eclipse results need reconsideration. Acknowledgements-I have pleasure in expressing thanks t’o Dr. H. 117. WELLS of the Department of Terrestrial Magnetism, Carnegie Institution, Washington for the loan of eclipse records and to Dr. J. 0. THO~IASof the Radio Section, Cavendish Laboratory for the computing help made available to me through his computing staff. I would like to take this opportunity to extend my grateful thanks to the J. N. TATA Endowment, Bombay for a grant, and to the Government of India and the University of Mysore for an overseas scholarship.
Eclipse effects on the F-layer
at sun&o
REFERENCES RATCLIFFE
J. A.
19.55
Boceedings of the URSI Eclipse London, August, 1955. Phil. Trans. A 248, 621.
Sym-
posium,
RATCLIFFE J. A., SCHINERLING E. R., SETTY C. S. G. K. and THOMAS J. 0. SE~CTP C. S. G. K.
1956
SHINN D. H.
19.55
WICLLS H. \I-.
195“ %Y
19%
81
Some Illvestigatiorls of the F-rcgiorj of the Io,losplcere. Ph.D. thesis, Cartlbridac. Ptipics of the Ionosphew, paper X1. Physical Society, London. J. Geophys. Res. 57, 291.
Eclipse effects on the F-layer at sunrise C. S.
G. K. SETTY
Department of Physics, Central College, Rangalore. India (Received 1 April1960) Abstract-Eclipse
effects on the F-layer at sunrise observed on 1 September 1951 at Derwood, Maryland,
U.S.A. are studied with some detail. The h’(f) curves taken during the eclipse period and on control days were analysed for N(h) and N(t) curves at a series of height,s. The eclipse variation of S computed from the control day data does not coincide with the observed variation. However, if the assumption is made that the electron production rates on the eclipse day were about 30 per cent higher than those on the succeeding control day the agreement between the observed and computed eclipse variations of N is improved. Further, if it is assumed that at the time of totality 30 per cent of the radiation was unobscured, in other words if the 30 per cent of the radiation originates in a region beyond the visible solar disc, then the agreement between the observed and computed eclipse variations is excellent. Some solar data in favour of both these assumptions is given and its validity in the light of several other known eclipses is discussed. 1.
INTRODUCTIOK
OIISERV~TIONS of the eclipse of 1 September 1951 were made at Derwood, Maryland, U.S.A. by Dr. H. W. Wells of the Department of TerrestriaI Magnetism, Washington who published details of the eclipse characteristics and made some deductions from the observed critical frequency variation (WELLS, 1952). The actual h’(S) records obtained at, Derwood and at two other stations nearby were made available by Dr. Wells, but since the critical frequency data indicated that the eclipse effect on the F-layer at the three stations was similar, it was decided that the analysis of records taken at any one of the three stations should be sufficient. Here the h’(f) records taken at 5 min intervals at Derwood are analysed using the method developed by SHIK~N (1955) to reduce the h’(f) curves into N(h) curves allowing for the earth’s magnetic field appropriate to the station. The geographic location of the station and t’he eclipse circumstances are as follows : ‘Table
Station
1 Geog. lat.
Geog. long.
Monthly
39.1”N
conditions / ,_ _._ ~____~_,__~_
~__
I
0537
._____
!
I
Greatest phase at, surface ~,~~. _~~~
,__~__.__ 54
0559
-~
_~
92
End of eclipse -._ -.
/
~_ ~
~
~. __
0707
average relative Zurich sunspot number: august 194 1
The obscuration used for Derwood. 1
77.2”W
I
G.S.R.
I
-___-
Derwood (Maryland)
1.
60 September 1941 80
function
computed
for Charlottesville
73
(WELLS, Fig. 2A) is also
C. S. Cr.K. SETTY
The effect of the solar eclipse on the P-layer at sunrise is of particular interest. The continuity equation for electrons at a given level is
At sunrise, y and dsjdt are the predominant terms in this equation, Ic, and it both being small, so there should be an al~preciable eclipse effect on the F-layer due to the large change in ~1. The crit’ical frequency data has in fact shown that there is a marked effect. The other interesting feature of this eclipse is that the critical frequency on the eclipse day is much higher than on the control day. Furthermore this annular eclipse, as is remarked by WELLS~ is especially favourable for a test of the effectiveness of ionizing radiation emanat,ing outside the visible disc of the sun. There are only two days of control data, one on 31 August and the other on 2 September 1951. Figs. la and lb show X(t) curves at 10 km intervals on the eclipse and control days. The increase in S at higher levels is fairly smooth and ~~~~~ can be co~~lp~lted -wit,h little u~~certai~~ty. At lower levels, 230-250 km, the ~uctuatio~s in M are rat,her large. Before studying the results of eclipse effects in detail it is interesting to check some of the conclusions made in connexion with the Cambridge sunrise observations (SETTY, 1956). 2. THE
SCNRISE PHENOMENON
One of the anomalies observed with the Cambridge critical frequency data near sunrise was “the seasonal sunrise anomaly,” when the critical frequency was found to increase near ground sunrise in summer but about 40 min to 1 hr before ground sunrise in wint,er and equinoxes. The Derwood data on control and eclipse days, Fig. 2, shows that the critical frequency began to increase at x = 96” which is in agreement with the Cambridge result. The N(t) curves also indicate that N began to increase before ground sunrise. 3. THE
ELECTRON
PRODUCTIOX
RATES
The electron production rates, q, are calculated by the use of the continuity equation (I). As the loss-coef-rieient, k,, is believed to depend on the solar-epoch the Loss-coefficients appropriate to this ease are not known, therefore two models based on the Can~bridge results are used (RATC~~I~FEe.t nl.,1956). Model I:
h, = 1 >: 10e4 exp
i
Model II: h, = 1 x 1O-4 exp i -
h -- 300 __-_ .- .30 h --
300 50
Figs. 3a and 3b show q(t) plots at 280 km computed by assuming k, to be 1.9 x 10-a and 1.5 x IO-" corresponding to models I and II respectively. There is little change in t,he shape of tShe two curves. 4 on model I reaches a steady 74
Time (75OWest med. Fig.
le.
Fig.
lb.
N(I)
hr
curves at a stories of height.rj un the eclipse day 1 September
75
1961.
C. S. G. K. SETTV
6055-50 -454.03.530c ii
.
’
l
.
.
~~~~//:/~:~/~~:!!~!,~l~:~/~l~/,,,l~~,,,,~~~,~,
04
06
05
07
Time (75O West mer), Fig.
08
09
II
IO
hr
2. Plots of f$‘%-_t on the eclipse and control days.
(a) 31 August 1951. (b) 1 September 1951.
Station: Dervuood, 394’N. (c) 2 September 1951.
I,--
)14c
,i2C
IImc
)-
t )-
cl- 4c)-
2<)-
(lI 06
L_
1 07
Time,
I
I
08
09
hr (b)
Pig.
3. The variation of q at 280 km height on t,he control day, the observed and computed
eclipse day. O-0 N = qe - qc.
Eclipse da,y. *---*
Control day. H, = 40 km; = 30km; K,(300)
(a) Assumed KI-model: K,-model:
H,
78
Computed eclipse day. - - - X,(300) = 10--4. (b) Assumed = lo-*.
Eclipseeffectson the P-layer at sunrise
value which is 20 per cent higher than the corresponding cl calculated Rearing this in mind only model I is used for further calculations.
on model II.
4. EXPECTED ECLIPSE EFFECT OE; THE TIME VARIATION OF q FROM COXT
THE
ROL DATA
Let qC be the product,ion rate at a given height at, any instant on the eont,rol day and let ‘1, be the corresponding quantity on the eciipse day. Assuming that the ionizing radiation arises uniformly over the visible solar disc q, is proportional to the unobscured fraction of the solar disc. Then if,!(t) is the obscuration function WP have the relation cl, = ~,.~(~) where *f(t) is the area unobscured. Using the computed f(t) (WELLS’ -Fig. 2A), the expected q, from the control day data of 2 September is calculated and shown in Figs. 3a and 310. Figs. 3a and 3b also show 4 plotted against time on the control day and the eclipse day. It is immediately apparent. from this figure that although the expected and observed variations of q, are similar, the entire curve on the eclipse day is shifted by a constant factor above the computed eclipse curve. This leads us to believe that the production rate itself must have been higher by a factor of 1.3 on t,he eclipse day compared with the succeeding control day value. Vertical movement as a possible source of the increased value of 4 immediately after the eclipse was considered to be very unlikely. The control day records on the preceding day, 31 August 1951, were, u~lfort~lnately, not available after 0800 hours, but the computed eclipse variation of N using August 31 as the control day at 270 and 260 km agreed with the observed variation of hTduring the eclipse. This supports the suggestion made that on the eclipse day and the preceding day the sun must have been quite active and returned to normal on 2 September 1951. If this were correct, the Ff critical frequency must be higher on days of high solar activity as it is believed t,hat f,( P.2) varies regularly with solar activity. It was in fact noticed that the critical frequencies of the FI-layer on 1 September were higher than the corresponding values on 2 September by about 0.3 MC/S. This leads to a value of q,, which is 1.3 times bigger on the eclipse day than on 2 September. This would therefore explain the fact that the production rate at sunrise was enhanced by the same factor.. Figs. 4a and 4b show t.he observed variation of 4 at 270 km on t,he eclipse day and the control day, and the computed variation on the eclipse day. t,he latter two being multiplied by a factor of 1.4. Although there is a good fit between the observed and computed eclipse curves, except for a bump at 0600 hours on the eclipse curve, q, on the observed eclipse curve at the time of totality is much higher t~han the expected value. This seems to suggest8 &at there must have been an additional source of ionization. This could either be due to another source of ionizing radiation lying beyond t,he visible disc (e.g. in t,lre corona) and therefore uncovered during the eclipse or it could be only an apparents additional ionizing source due to vertical movements. To decide between these two possibilities the following procedure was adopted: the observed values of q, on tZhe eclipse day and 4, on the succeeding control day were considered. A constant factor was subtracted from the actual values of 4, on the eclipse day so that at t,he 77
C. S. G. K.
SETTY
220 I2OQ-
160 -
160 -
140 -
140 -
-i :
I20 -
;s
loo-
i tl
100 -
60 -
E k
80-
60 -
”
60-
40 -
_ w
40-
G
120 -
__/---
20 -
20 o-
O-
: I 05
I 06
I 07 Time,
I 06
I 09
I 05
c6
I 07 Time.
hr
/
I Oa
hr
(b)
b)
Fig. 4a. Computed eclipse variation of q from the control variation with the intensity of the ionizing radiation enhanced by a factor 1.4. 270 km height. K, = 4.4 x 10m4. 00 1 September 1951. 0-0 2 September 1951 (Factor 1.4 is allowed). - -Computed eclipse from 2 September 1951. - - - - M. Fig. 4b. Computed eclipse variation of g from the control day witsh the intensity of the ionizing radiation enhanced by a factor 1.4. 280 km height. 0-a 2 September 1951. O-0 Observed eclipse. - -Computed eclipse. - - - - M.
25_260i 05
1 06
‘1
1 07 Time,
1
1 06
1
1 09
1
hr
Fig. 5. Computed eclipse variation of q at a series of heights allowing for the radiation height from outside the visible solar disc. (a) 260 km height. (b) 270 km height. (c) 280 km height. x---x Observed eclips-. + - + Eclipse computed from 31 August 1951. --Eclipse computed from 2 September 1951.
78
09
Eclipse effects on the F-layer
at sunrise
time of totality the actuaf values of q, and that computed from t’he succeeding control day were the same. Thus at 270 km height q, observed and computed were made each equal to 7 at 0600 hours. The actual observed value of q, on 2 Beptember 1951 at 0600 hours is 44, and the steady value of 4, on 2 September 1951 is 100. It is therefore necessary to assume that the extra radiation coming from outside the visible dise on the eclipse day should be about 30 per cent of the entire radiation coming from the visible disc. This is the same as the earlier conclusion that y,, on the eclipse day is greater by a factor of 1.3 compared mit’h q0 on 2 September. In addition to this it has to be assumed that t’he extra radiation comes from outside the visible disc so t,h;tt$it is uncovered during t.he time of totalit,g. When this assumption is made it is seen from Fig. 5 t,hat the agreement bet,ween the observed and computed curves is very much improved. The computed eclipse variatzion of N from data for 31 August, reduced by a fact’or of 1.3 is also shown in the same figure. This curve fits closely wit-h the observed eclipse curves and the other computed curve of :! Sept,ember 1951. It may therefore be concluded t.hat t.he sun was active on 31 August and 1 Sept,ember and had returned t,o normal ~ondit’ion on 2 September. Also it is necessary to conclude that the extra radiation dne 00 t’be enhanced activ%y came from out,side the visible disc. 5. SOLAR I)ATa
IN
Fnvoun
OF THE (;OX-cLl-SIOXS
PRECEDING
OF THE
SECTION
Evidence in favour of the conclusions arrived at in the fastr section was sought from the published solar data in the ~~~~r~er~~~~~~~~~~n on 8oZar Activit;~ published in Zurich. It was found that there was no significant difference in coronal line intensity on the eclipse day and the preceding control day, compared with the day following the eclipse. There was no significant difference in tot,al intensity on t’hese days. However, t’he dates 31 August and 1 September 1951 were in fact n~entiol~ed amongst the days when solar eruptio~ls were observed, and 2 September 1!)51 was not mentioned. The following are the characteristics of the eruptions observed on 31 August and 1 September 1951. Table
Dat,e
Time of observation (U.T.)
/
(OC,
2.
C!oordinates ~_______~ j Distance from central meridian
1
(“E)
- ___ _--._-
/
31.viii.1951 31.viii.1951 l.ix.1951 Remarks
Estimated importance (max. = 3)
against 1 September 1951: A few brights spots. 31 August 1951: Eruptions accompanied
2, 3 2,-i
by large and fast radial filaments.
The latter remark is of particular significance. There is a suggestion of not only enhanced activity on 31 August and 1 September, I951 but also the possibility of the ionizing radiation having originated from outside the visible disc of the sun. 79
C. X. G. K. SETTY It may be concluded, therefore, that the enhanced production of ionization on the eclipse day is due to the extra ionizing radiation caused by enhanced activity on the sun. This conclusion has two important consequences. The day-to-day fluctuations in the critical frequency of the F-layers may be caused by fluctuations in the ionizing radiation of the sun. Solar flares which are accompanied by the enhancement of H, radiation may have an effect on the F-layer critical freyuenties. The other important effect would be on t’he deductions made from the eclipse effects on the FI-layer. In deducing the recombination coefficient the assumption is made that the production rate is reduced to zero at the time of totalit’y. Such an assumption may be wrong if the radiat,ion comes from outside the visible disc. While summarizing the eclipse effects on the E- and PI-layers observed by various workers in the past,, RlzTczrFFe (1%X5) came to the conclusion that about 15 per cent of the radiation should come from outside the visible disc. The COP elusions reached here from the eclipse effect at sunrise in the F-region observed at Derwood supports this suggestion. 6. CoXcLrrsIo~s The main conclusions reached from the analysis of t’he eclipse are summarized below. (i) To account for enhanced ionization on the eclipse day and t,he preceding day it is necessary to assume that t,here was ext’ra ionizing radiation on these days of about 30 per cent of Dhe total radiation arising from the entire solar disc as observed on 2 September. It is also necessary to assume that this additional source should be beyond the visible disc of the sun. There is solar data, which favours both these conclusions. (ii) The ionization in the Fl- and FZ-layers go together. IZ small increase of 0.3 MC/S inf,,(F1) is equivalent to a comparatively big change of 1 NC/S in f,,( F2). This is expected as the loss-coefficient in the F1-layer is believed to be bigger than that in the FL’-layer. (iii) The day-to-day fluctuations in f,( El) and f& FZ) may therefore be associated closely with the fluctuations of the solar radiation producing the ionization. This variable part of the radiation is likely to originate in the corona, but the intensity of coronal lines may not be a very good indication of t’he correlation between the two phenomena. (iv) The foregoing conclusions support the suggestion by RATCLIFFE (I 966) that the recombination coefficients of E- and Fl-layers deduced from the eclipse results need reconsideration. Acknowledgements-I have pleasure in expressing thanks t’o Dr. H. 117. WELLS of the Department of Terrestrial Magnetism, Carnegie Institution, Washington for the loan of eclipse records and to Dr. J. 0. THO~IASof the Radio Section, Cavendish Laboratory for the computing help made available to me through his computing staff. I would like to take this opportunity to extend my grateful thanks to the J. N. TATA Endowment, Bombay for a grant, and to the Government of India and the University of Mysore for an overseas scholarship.
Eclipse effects on the F-layer
at sun&o
REFERENCES RATCLIFFE
J. A.
19.55
Boceedings of the URSI Eclipse London, August, 1955. Phil. Trans. A 248, 621.
Sym-
posium,
RATCLIFFE J. A., SCHINERLING E. R., SETTY C. S. G. K. and THOMAS J. 0. SE~CTP C. S. G. K.
1956
SHINN D. H.
19.55
WICLLS H. \I-.
195“ %Y
19%
81
Some Illvestigatiorls of the F-rcgiorj of the Io,losplcere. Ph.D. thesis, Cartlbridac. Ptipics of the Ionosphew, paper X1. Physical Society, London. J. Geophys. Res. 57, 291.