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Effect of meteor activity on the E-region fading
JournalofAtmospheric andTerrestrial Physics,lQ66,Vol. 28,pp. 467-486. Perwnon Prm Ltd. Printedin Northernwand
Effect
of meteor
activity
on the E-region fading
P. S. KESAVA RAO and B. RAMACHANDRA RAO Ionospheric Research Laboratories, Physics Department, Andhra University, Waltair, India (Received 8 September 1965) Ah&act-The effect of meteor activity on the fading of a radiowave reflectedfrom the E-region in the ionosphere was studied during 1957-58 and 1963-64 at Waltair (17”43N, 83”183Geomag. Lat. 7~4”N). The fading frequency of a radiowave reflected from the E-region was found to increase during meteor shower periods. The annual variation of fading frequency in the E-region was studied in relation to sporadicmeteor activity. From a study of the variation of fading frequency and visual hourly meteor rate with time during the Geminid shower of 1963 and Leonid shower of 1964 it was concluded that both fading frequency and meteor rate attain maximum values at about the same time. 1. INTRODUCTION IS well known that meteors produce ionization in a region extending from 80 to 120 km above the earth’s surface. The suggestion that meteors might cause sufficient disturbance in the E-region to effect the propagation of radiowaves was first made by NAGOAKA (1929). Later on, several investigators have studied the effect of meteor activity on the ionization in E-region. LOVELL (1948) reviewed all the available literature on the E-region abnormalities and meteor ionization. PIERCE (1947) observed a reflecting layer which persisted for several hours on s, frequency of 3.5 MC/Sduring the 1946 Giacobinid shower. LOVELL (1950) pointed out from some theoretical considerations that the contribution of meteors to the E-region ionization is small, although at night, intense meteor showers might have an appreciable influence. Later on NICOLET (1955) after discussing the various possible atomic processes in the upper atmosphere concluded that meteor influx was the most likely source of ionization needed to maintain the night time level in the E-region, but pointed out that more information is necessary for a quantitative approach. LOVELL (1957) pointed out that some forms of sporadic-E are caused by meteors particularly in middle and high latitudes. Recently KOTADIA (1958) found instances of positive correlation between sporadic-E and meteor activity at Ahmedabad and Yamagawa. Very recently SIN~H (1963) observed many similarities in the E,-characteristics and meteor activity at Delhi. As previous investigators have not attempted to study the effect of meteor activity on the fading of a radiowave reflected from the E-region, it was considered worthwhile to examine this aspect in detail. For this purpose the fading frequency data obtained at Waltair (17”43’N; 83”lS’E Geomag. Lat. 7.4’N) during the International Geophysical Year (IGY) was utilized for a preliminary analysis. As these results were found to be interesting a more detailed experimental study was undertaken during 1963-64 and the results obtained are presented in the following sections. 457 IT
P. S.
458
KESAVARAO and B. R~~~CIXANDRA RAO
2.
EXPERIMENTAL TECHNIQUE AND METHODOF ANALYSIS The experimental technique used in the present investigation for taking spaced fading records is the same as that reported by RAO et al. (1956). Following . the method due to RICE (1945), the fading frequency is calculated by counting the number of peaks per minute. The operating radio frequency of the transmitter is 2.3 MC/S during 1957-58 and 2.0 MC/S during 1963-64. Echoes from the Eregion levels have been used for obtaining the fading records. A DuMont Oscilloscope Record Camera (Model 321-A) having a wide range of recording film speeds has been used for this purpose. The present investigation is confined to the data obtained during the periods December 1957-December 1958 and August X963November 1964. 3.
EFFECTOF METEORACTIVITYONFADINGOF E-REGION REFLECTIONS DURINCJ1957-58
While analyaing the spaced fading records taken during International Geophysical Year (IGY), it was observed that the fading frequency increased considerably at night time during certain periods of the year. As the occurrence of such high values of fading frequency is rare in the large wealth of available data, such records were discarded during the fading analysis (whose results were published by RAO and RAO, 1964). On a closer examination it was noticed that these high values of fading frequency were obtained during well known meteor shower periods. The present study was undertaken with a view to see whether any relation exists between meteor activity and fading frequency. For this purpose all the available fading frequencies taken during night time (1800-0600 L.M.T.) during meteor shower periods are averaged. The average values of fading frequency thus obtained during Ursid, Quadrantid and Perseid showers are presented in Table 1. For comparison, the average values of night time fading frequency taken during non-shower days around the shower period were also presented in Table 1. Table 1. Average values of night time E-region fading frequencies during different periods Average value of night time E-region fading frequency in elmin
Shower -. Ursid Quadrantid Per&d
Shower days ----
Non-shower days
16.9 20.2 22.5
11.7 14.6 12.7
In this analysis meteor showers under magnetically active conditions were not considered, as RAO and RAO (1964) reported that the E-region fading frequency increases during disturbed days. From a perusal of Table 1, it will be noticed that the average E-region fading frequency observed during night time on meteor shower days is higher than that during non-shower nights. It will be also noticed that during meteor shower days, the average night time E-region fading frequency attained a maximum value
Effect of meteor aotivity on the E-regionfading
469
in the c8se of Perseid shower snd minimum value in the c8se of Ursid shower, which is in 8greement with the activities of the respective showers. The annual variation of fading frequency is next examined in relation to sporadic meteor activity. Monthly mean values of night time fading frequency for the E-region were calculated by sveraging all fading frequencies available during the month excluding those taken during meteor shower periods. To study 14r (A)
a
LONGITUDE
OF
SUN
Fig. 1. Annual variation of fading frequency and sporadic meteorrates.
P. S. KESAVARAO
460
and B. HAMACHANDRA KAO
the annual variation of fading frequency, a smoothed curve is drawn through the monthly mean observations as shown in Fig. l(A). From this variation it is clear that the average fading frequency reaches a maximum during Autumn From the visual sporadic meteor data and minimum during Spring seasons. available for a period of five years, MURAKAMI (1955) concluded that the annual variation of sporadic meteors exhibits a maximum around autumn and minimum around spring. For a better comparison, the annual variation curve of MURAKAMI From forward scatter radio observations, (1955) is reproduced in Fig. l(B). VOGAN and CAMPBELL (1957) arrived at a similar conclusion although they did not eliminate the shower meteors from their data. Their annual variation curve is also reproduced in Fig. l(C). Such annual variation of sporadic meteors is to be expected as the apex of the Earth’s way is above the horizon of an observer in the northern hemisphere for more hours each day during September than during March (MCKINLEY, 1961). Comparing fading rate and hourly meteor rate curves, it can be seen that there is a general agreement in the trend of variation. Although the minimum and maximum in the fading rate curve cannot be definitely located, it can be seen that the maximum values occur in the autumnal equinox and minimum values in the vernal equinox. The positions of maximum and minimum could not be located definitely as the observations are available only for about five or six days in a month. In contrast, the curve showing the hourly meteor rate by visual methods shows a prominent and well defined maximum and minimum. By comparing the extent of variation from minimum to maximum in the fading rate and hourly meteor rate curves, it will be evident that they are not of the same order. The maximum deviation of the fading frequency is 28 per cent over the yearIy average value. On the other hand, these deviations in the case of visual and radio meteor data have values of 48 and 65 per cent respectively. From this study, it appears that, although the fading frequency is related to hourly sporadic meteor rate in its annual variation, it is less sensitive as an index meteor activity. The above investigation is based on the fading frequency data taken during one year and perhaps an extended investigation covering t,he data over several years may yield more definite and fruitful results. 4.
EFFECT OF METEOR ACTIVITY ON FADING OF E-REGION REFLECTIONS DURING 1963-64
Although the fading frequency data during the period 1957-58 on meteor shower days is meagre, yet it provided sufficient evidence of the positive correlation between fading frequency and meteor activity to enable further and more detailed investigation. As such the investigation of Fading frequency versus Meteor activity has been undertaken in more detail during the period 1963--64. For this purpose fading records were taken at night time during the meteor showers of Perseid, Orionids, Geminids, Ursids, Quadrantids and Leonids during the period August 1963-November 1964. Observations were also t.aken on 3-4 non-shower days in each of the shower months for comparison purposes. In this analysis also meteor showers under magnetically active conditions were not considered.
Effect of meteor activity on the E-region fading ----
6
461
NON -SHOWER DAYS METEOR SHOWER DAYS
__ I
3
0 IL 3 FADING
FREQUENCY
(~YCLES/MINUTE)
Fig. 2. Histogram of fading frequenoy during meteor shower day@ and non-shower days.
Histograms are drawn showing the variation of percentage occurrence of number of observations versus fading frequency obtained during non-shower and shower days, with a view to see if there is any difference in the distributions. It can be seen from the histogram shown in Fig. 2, that the E-regionfadingfrequency during meteor shower days falls in the range of 4-52 c/min with a most probable value of 18 cjmin. The fading frequency for non-shower days falls in the range O-24 c/min with a most probable value of 12 c/min. The average value of fading frequency during meteor shower periods and quiet periods came out to be 23.9 and 12-6 c/min respectively. Thus it is evident that, during meteor shower periods, the range of variation of fading frequency increases and the most probable and average values are higher than those on non-shower days. It may therefore be concluded that meteor activity has the effect of increasing the fading frequency of radiowaves reflected from the E-region.
P. S. KEEXAVARAO and B. RAMACHANDEA RAO
462
Table 2. Average night time E-region fading frequency for d&rent
Sl. x0. __--__-
Shower
Hourly meteor rate ._.
meteor showers
Average fading frequency (c/min) ---_-..____..__
1
Perseids
.‘,u
28.0
2
Geminids
50
27.5
3 4
Quadrantids Orionids
40 25
24.x 20.3
5
Leonids
1R
16.7
6
Ursids
15
17.5
1
8
I
I
I
24 32 HOURLY METEOR 16
1
40 RATE
I
48
I
56
Fig. 3. Vmiation of fading frequency with hourly meteor rate.
Effect of meteor activity on the E-region fading
463
With a view to study if there is any variation in the fading rate for different types of showers and to see if the fading rate depends on the hourly meteor rate, the fading records obtained during each shower were analysed and the mean fading rate for each meteor shower was evaluated separately. The values thus obtained for the night time meteor showers during the period 1963-64 were presented in Table 2, along with the hourly meteor rates, taken from The Observers Halzd Boolc, 1963. At a glance it can be seen from the table that the fading frequency is generally high for those showers which have high hourly meteor rate. Figure 3 shows the plot of fading frequency and hourly meteor rate presented in Table 2. It can be seen from this plot that all the points fall fairly well on a straight line. Statistical methods (FISHER,1958) were used to determine the value of correlation coefficient between these two variates and it came out to be 0.99 which is highly significant (P = 0.01 level). This high level of significance suggests that there is a linear relationship between hourly meteor rate and the average fading frequency observed during the shower. According to the “Method of Least Squares”, a line of best fit was drawn and presented in Fig. 3. The coincidence of the intercept of value 12.6 c/min with the fading frequency on non-shower days is significant. With a view to see if there is any good correlation between short time variations of fading frequency versus meteor activity during meteor shower periods, observations were taken on the nights of 12-13 December 1963 and 18 November 1964 when the Geminid and Leonid showers are active. Figure 4(A) shows the nocturnal variation of fading frequency on the night of 12-13 December 1963. For comparison the nocturnal variation of visual hourly meteor rate for a standard group of six observers for the same night (taken from the unpublished work of LOKANADHAMand RAO, 1965) and the nocturnal variation of radio meteor rate (compiled from the data published by EVANS,1960) for 12-13 December 1958 are also presented in Fig. 4(A). There is a remarkable agreement between the time of occurrence of peak value of the visual meteor rate and fading frequency, both of which attain maximum values at about 0300 L.T. But the radio meteor rate attains maximum at 0100 L.T. It may be noted that while the visual meteor rate increases almost linearly from 2100 to 0300 L.T., the fading frequency rises more or less exponentially during the same interval. Figure 4(B) shows the nocturnal variation of fading frequency on the night of 18 November 1964. As there were no visual meteor observations taken at Waltair, a direct comparison could not be made. However, the nocturnal variation of visual hourly meteor rate during the Leonid shower in the year 1961 was reported by LOKANADHAM (1965) and so his curve was reproduced in Fig. 4(B). From these curves it can be noticed that both the fading frequency and visual meteor rate attain maximum values at about 0300 hr L.T. Figure 5(A) and (B) show the diurnal variation of fading frequency on the nights of 12-13 August 1963 and 21-22 October 1964. As no visual meteor observations were available at this station for these showers, the nocturnal variation of fading frequency could not be compared with that of the visual meteors. However, the diurnal variation of radio meteor rate at Jodrell Bank (compiled from the data published by EVANS, 1960) for 11-12 August 1958 and 20-21 October 1958 have been presented in Figs. 5(A) and (B). It can be seen 3
P. S. KESAVA RAO and B. RABIACHANDRA RAO
464
from Fig. 5(A), that the fading frequency attains a maximum at 0200 L.T. whereas the radio meteor rate attains maximum at about 0600 L.T. In Fig. 5(B), the fading frequency reaches maximum value at about 0200 L.T. while the radio meteor rate attains maximum at 0500 L.T. In general, it is found that the time of maximum fading frequency coincides with the time of peak visual meteor rate. There appears to be a delay of 3-4 hr between the peak fading frequency and M
A-.--A 37-
O----O .
FADING VISUAL RADIO
FREQUENCY DATA METEOR DATA METEOR DATA
,i ,a,\‘\
36-
:\ \ ii I
32I I
I i I
28-
c
’
.’
\
\
./
.;
\/
,*’
-22
‘\
,’
\
i
-p_
3
j
2-o
.P
/ IY
-26
\
,,q+_-_
I
,I r? 24
(A) 12”
,.
/.
P
‘\
y
\\ .‘\. 414
z0 25’ W a
u.
$206 i? IS-
OL 2100
t 2200
, 2300
I
OQOO LOCAL
I
0100 TIME
1
0200
I
0300
1
0400
I OS&
(HOURS)
Fig. 4. Nocturnal variation of fading frequency and hourly meteor rates.
Effect of meteor aativity on the E-region fading
-
FADING
D----El
RADIO
fREOUENCY METEOR
Sl-
I’: 0: I I , I ;o
466
DATA
DATA
(A) “‘O
B
-I20 : I ’ I \ :
- I00
cl : I ,
i,
a
LOCAL
TIME
(HOURS)
Fig. 6. Diurnal variation of fading frequency and radio meteor rate.
P. S. KESAVA RAO and B. RAMACHAXDRARAO
466
the peak meteor rate as determined by radio methods. It might be possible that the fading frequency increase is due to large meteors which can be seen with th.e naked eye and which can reach lower altitudes in the ionosphere causing enhanced fading. On the other hand the radio meteor data relate to all meteors including a substantial number of smaller meteors which attain a maximum at a later time and which may not reach these heights to produce enhanced fading. .‘,.
CONCLUSIONS
The following are the main conclusions drawn from the present investigation: (1) It is observed for the first time that the fading of a radio wave reflected from the E-region increases during meteor shower nights. (2) It is found that the annual variation of fading frequency of E-region reflections taken during night Oime va,ries in a similar manner to that of the annual variation of sporadic meteors that enter the earth’s atmosphere. (3) From the studies during Geminid shower 1963 and Leonid shower 1964, it was found that both visual meteor rate and E-region fading frequency attain maximum values at about the same time. Acknowledgements--We discussions. New
are thankful
to Dr. M. SRIRAMA RAO for some helpful
One of us (I?. S. K. RAO) is grateful to University
Delhi for the award
of Junior Research
Grants Commission,
Fellowship.
REFERENCES EVANS G. C. FISHER R. 9.
1960 1958
KOTADIA K. M. LOKANADHAMB.
1958 1965
LOVELL A. C. B. LOVELL A. C. B. LOVELL A. C. B.
1948 1950 1957
MCKINLEY D. W. H.
1961
MUWKAMI T. NAaOm H. NWOLET M. PIERCE 5. A. RAO B. R., RAO M. S. and MURTY D. S. X. RAO P. S. K. and RAO B. R. RICE S. 0. SINGHR. N. The Observers Hand Book
1955 1929 1955 1947 1956 1964 1945 1963 1963
VO~EN E. L. and CAMPBELLL. L.
1957
Jodrell Bnk. Ann. 1, 280 Statistical Methods for Research Workers, Oliver and Boyd, Edinburgh. .f. Sci. In&&r. Res. Al.7, 46. Ph.D. Thesis, submitted to the Andhra University Waltair, India. Rep. Prog. Phys. 11, 415. A%. Progr. 38, 22. Hand&h der Physik, Geophiscs II, 48, 427. Meteor Science and Engineering, McGrawHill, New York. Publ. As&on. Sot. Japan 7, 49. Proc. Imp. Acad. Tokyo 5, 233. Spl. Supp. J. Atmoaph. Terr. Phys. 2,99. Phys. Rev. 71, 88. J. Sci. Industr. Res. A15, 75. .I. Atmosph. Terr. Phys. 26, 841. Bell. Syst. Tech. J. 25, 282. J. Atmosph. Terr. Phys. 25, 589. Published by the Royal Astronomical Society of Canada, 55th Publication. Can. J. Phys. 35, 1176.
Effect
of meteor
activity
on the E-region fading
P. S. KESAVA RAO and B. RAMACHANDRA RAO Ionospheric Research Laboratories, Physics Department, Andhra University, Waltair, India (Received 8 September 1965) Ah&act-The effect of meteor activity on the fading of a radiowave reflectedfrom the E-region in the ionosphere was studied during 1957-58 and 1963-64 at Waltair (17”43N, 83”183Geomag. Lat. 7~4”N). The fading frequency of a radiowave reflected from the E-region was found to increase during meteor shower periods. The annual variation of fading frequency in the E-region was studied in relation to sporadicmeteor activity. From a study of the variation of fading frequency and visual hourly meteor rate with time during the Geminid shower of 1963 and Leonid shower of 1964 it was concluded that both fading frequency and meteor rate attain maximum values at about the same time. 1. INTRODUCTION IS well known that meteors produce ionization in a region extending from 80 to 120 km above the earth’s surface. The suggestion that meteors might cause sufficient disturbance in the E-region to effect the propagation of radiowaves was first made by NAGOAKA (1929). Later on, several investigators have studied the effect of meteor activity on the ionization in E-region. LOVELL (1948) reviewed all the available literature on the E-region abnormalities and meteor ionization. PIERCE (1947) observed a reflecting layer which persisted for several hours on s, frequency of 3.5 MC/Sduring the 1946 Giacobinid shower. LOVELL (1950) pointed out from some theoretical considerations that the contribution of meteors to the E-region ionization is small, although at night, intense meteor showers might have an appreciable influence. Later on NICOLET (1955) after discussing the various possible atomic processes in the upper atmosphere concluded that meteor influx was the most likely source of ionization needed to maintain the night time level in the E-region, but pointed out that more information is necessary for a quantitative approach. LOVELL (1957) pointed out that some forms of sporadic-E are caused by meteors particularly in middle and high latitudes. Recently KOTADIA (1958) found instances of positive correlation between sporadic-E and meteor activity at Ahmedabad and Yamagawa. Very recently SIN~H (1963) observed many similarities in the E,-characteristics and meteor activity at Delhi. As previous investigators have not attempted to study the effect of meteor activity on the fading of a radiowave reflected from the E-region, it was considered worthwhile to examine this aspect in detail. For this purpose the fading frequency data obtained at Waltair (17”43’N; 83”lS’E Geomag. Lat. 7.4’N) during the International Geophysical Year (IGY) was utilized for a preliminary analysis. As these results were found to be interesting a more detailed experimental study was undertaken during 1963-64 and the results obtained are presented in the following sections. 457 IT
P. S.
458
KESAVARAO and B. R~~~CIXANDRA RAO
2.
EXPERIMENTAL TECHNIQUE AND METHODOF ANALYSIS The experimental technique used in the present investigation for taking spaced fading records is the same as that reported by RAO et al. (1956). Following . the method due to RICE (1945), the fading frequency is calculated by counting the number of peaks per minute. The operating radio frequency of the transmitter is 2.3 MC/S during 1957-58 and 2.0 MC/S during 1963-64. Echoes from the Eregion levels have been used for obtaining the fading records. A DuMont Oscilloscope Record Camera (Model 321-A) having a wide range of recording film speeds has been used for this purpose. The present investigation is confined to the data obtained during the periods December 1957-December 1958 and August X963November 1964. 3.
EFFECTOF METEORACTIVITYONFADINGOF E-REGION REFLECTIONS DURINCJ1957-58
While analyaing the spaced fading records taken during International Geophysical Year (IGY), it was observed that the fading frequency increased considerably at night time during certain periods of the year. As the occurrence of such high values of fading frequency is rare in the large wealth of available data, such records were discarded during the fading analysis (whose results were published by RAO and RAO, 1964). On a closer examination it was noticed that these high values of fading frequency were obtained during well known meteor shower periods. The present study was undertaken with a view to see whether any relation exists between meteor activity and fading frequency. For this purpose all the available fading frequencies taken during night time (1800-0600 L.M.T.) during meteor shower periods are averaged. The average values of fading frequency thus obtained during Ursid, Quadrantid and Perseid showers are presented in Table 1. For comparison, the average values of night time fading frequency taken during non-shower days around the shower period were also presented in Table 1. Table 1. Average values of night time E-region fading frequencies during different periods Average value of night time E-region fading frequency in elmin
Shower -. Ursid Quadrantid Per&d
Shower days ----
Non-shower days
16.9 20.2 22.5
11.7 14.6 12.7
In this analysis meteor showers under magnetically active conditions were not considered, as RAO and RAO (1964) reported that the E-region fading frequency increases during disturbed days. From a perusal of Table 1, it will be noticed that the average E-region fading frequency observed during night time on meteor shower days is higher than that during non-shower nights. It will be also noticed that during meteor shower days, the average night time E-region fading frequency attained a maximum value
Effect of meteor aotivity on the E-regionfading
469
in the c8se of Perseid shower snd minimum value in the c8se of Ursid shower, which is in 8greement with the activities of the respective showers. The annual variation of fading frequency is next examined in relation to sporadic meteor activity. Monthly mean values of night time fading frequency for the E-region were calculated by sveraging all fading frequencies available during the month excluding those taken during meteor shower periods. To study 14r (A)
a
LONGITUDE
OF
SUN
Fig. 1. Annual variation of fading frequency and sporadic meteorrates.
P. S. KESAVARAO
460
and B. HAMACHANDRA KAO
the annual variation of fading frequency, a smoothed curve is drawn through the monthly mean observations as shown in Fig. l(A). From this variation it is clear that the average fading frequency reaches a maximum during Autumn From the visual sporadic meteor data and minimum during Spring seasons. available for a period of five years, MURAKAMI (1955) concluded that the annual variation of sporadic meteors exhibits a maximum around autumn and minimum around spring. For a better comparison, the annual variation curve of MURAKAMI From forward scatter radio observations, (1955) is reproduced in Fig. l(B). VOGAN and CAMPBELL (1957) arrived at a similar conclusion although they did not eliminate the shower meteors from their data. Their annual variation curve is also reproduced in Fig. l(C). Such annual variation of sporadic meteors is to be expected as the apex of the Earth’s way is above the horizon of an observer in the northern hemisphere for more hours each day during September than during March (MCKINLEY, 1961). Comparing fading rate and hourly meteor rate curves, it can be seen that there is a general agreement in the trend of variation. Although the minimum and maximum in the fading rate curve cannot be definitely located, it can be seen that the maximum values occur in the autumnal equinox and minimum values in the vernal equinox. The positions of maximum and minimum could not be located definitely as the observations are available only for about five or six days in a month. In contrast, the curve showing the hourly meteor rate by visual methods shows a prominent and well defined maximum and minimum. By comparing the extent of variation from minimum to maximum in the fading rate and hourly meteor rate curves, it will be evident that they are not of the same order. The maximum deviation of the fading frequency is 28 per cent over the yearIy average value. On the other hand, these deviations in the case of visual and radio meteor data have values of 48 and 65 per cent respectively. From this study, it appears that, although the fading frequency is related to hourly sporadic meteor rate in its annual variation, it is less sensitive as an index meteor activity. The above investigation is based on the fading frequency data taken during one year and perhaps an extended investigation covering t,he data over several years may yield more definite and fruitful results. 4.
EFFECT OF METEOR ACTIVITY ON FADING OF E-REGION REFLECTIONS DURING 1963-64
Although the fading frequency data during the period 1957-58 on meteor shower days is meagre, yet it provided sufficient evidence of the positive correlation between fading frequency and meteor activity to enable further and more detailed investigation. As such the investigation of Fading frequency versus Meteor activity has been undertaken in more detail during the period 1963--64. For this purpose fading records were taken at night time during the meteor showers of Perseid, Orionids, Geminids, Ursids, Quadrantids and Leonids during the period August 1963-November 1964. Observations were also t.aken on 3-4 non-shower days in each of the shower months for comparison purposes. In this analysis also meteor showers under magnetically active conditions were not considered.
Effect of meteor activity on the E-region fading ----
6
461
NON -SHOWER DAYS METEOR SHOWER DAYS
__ I
3
0 IL 3 FADING
FREQUENCY
(~YCLES/MINUTE)
Fig. 2. Histogram of fading frequenoy during meteor shower day@ and non-shower days.
Histograms are drawn showing the variation of percentage occurrence of number of observations versus fading frequency obtained during non-shower and shower days, with a view to see if there is any difference in the distributions. It can be seen from the histogram shown in Fig. 2, that the E-regionfadingfrequency during meteor shower days falls in the range of 4-52 c/min with a most probable value of 18 cjmin. The fading frequency for non-shower days falls in the range O-24 c/min with a most probable value of 12 c/min. The average value of fading frequency during meteor shower periods and quiet periods came out to be 23.9 and 12-6 c/min respectively. Thus it is evident that, during meteor shower periods, the range of variation of fading frequency increases and the most probable and average values are higher than those on non-shower days. It may therefore be concluded that meteor activity has the effect of increasing the fading frequency of radiowaves reflected from the E-region.
P. S. KEEXAVARAO and B. RAMACHANDEA RAO
462
Table 2. Average night time E-region fading frequency for d&rent
Sl. x0. __--__-
Shower
Hourly meteor rate ._.
meteor showers
Average fading frequency (c/min) ---_-..____..__
1
Perseids
.‘,u
28.0
2
Geminids
50
27.5
3 4
Quadrantids Orionids
40 25
24.x 20.3
5
Leonids
1R
16.7
6
Ursids
15
17.5
1
8
I
I
I
24 32 HOURLY METEOR 16
1
40 RATE
I
48
I
56
Fig. 3. Vmiation of fading frequency with hourly meteor rate.
Effect of meteor activity on the E-region fading
463
With a view to study if there is any variation in the fading rate for different types of showers and to see if the fading rate depends on the hourly meteor rate, the fading records obtained during each shower were analysed and the mean fading rate for each meteor shower was evaluated separately. The values thus obtained for the night time meteor showers during the period 1963-64 were presented in Table 2, along with the hourly meteor rates, taken from The Observers Halzd Boolc, 1963. At a glance it can be seen from the table that the fading frequency is generally high for those showers which have high hourly meteor rate. Figure 3 shows the plot of fading frequency and hourly meteor rate presented in Table 2. It can be seen from this plot that all the points fall fairly well on a straight line. Statistical methods (FISHER,1958) were used to determine the value of correlation coefficient between these two variates and it came out to be 0.99 which is highly significant (P = 0.01 level). This high level of significance suggests that there is a linear relationship between hourly meteor rate and the average fading frequency observed during the shower. According to the “Method of Least Squares”, a line of best fit was drawn and presented in Fig. 3. The coincidence of the intercept of value 12.6 c/min with the fading frequency on non-shower days is significant. With a view to see if there is any good correlation between short time variations of fading frequency versus meteor activity during meteor shower periods, observations were taken on the nights of 12-13 December 1963 and 18 November 1964 when the Geminid and Leonid showers are active. Figure 4(A) shows the nocturnal variation of fading frequency on the night of 12-13 December 1963. For comparison the nocturnal variation of visual hourly meteor rate for a standard group of six observers for the same night (taken from the unpublished work of LOKANADHAMand RAO, 1965) and the nocturnal variation of radio meteor rate (compiled from the data published by EVANS,1960) for 12-13 December 1958 are also presented in Fig. 4(A). There is a remarkable agreement between the time of occurrence of peak value of the visual meteor rate and fading frequency, both of which attain maximum values at about 0300 L.T. But the radio meteor rate attains maximum at 0100 L.T. It may be noted that while the visual meteor rate increases almost linearly from 2100 to 0300 L.T., the fading frequency rises more or less exponentially during the same interval. Figure 4(B) shows the nocturnal variation of fading frequency on the night of 18 November 1964. As there were no visual meteor observations taken at Waltair, a direct comparison could not be made. However, the nocturnal variation of visual hourly meteor rate during the Leonid shower in the year 1961 was reported by LOKANADHAM (1965) and so his curve was reproduced in Fig. 4(B). From these curves it can be noticed that both the fading frequency and visual meteor rate attain maximum values at about 0300 hr L.T. Figure 5(A) and (B) show the diurnal variation of fading frequency on the nights of 12-13 August 1963 and 21-22 October 1964. As no visual meteor observations were available at this station for these showers, the nocturnal variation of fading frequency could not be compared with that of the visual meteors. However, the diurnal variation of radio meteor rate at Jodrell Bank (compiled from the data published by EVANS, 1960) for 11-12 August 1958 and 20-21 October 1958 have been presented in Figs. 5(A) and (B). It can be seen 3
P. S. KESAVA RAO and B. RABIACHANDRA RAO
464
from Fig. 5(A), that the fading frequency attains a maximum at 0200 L.T. whereas the radio meteor rate attains maximum at about 0600 L.T. In Fig. 5(B), the fading frequency reaches maximum value at about 0200 L.T. while the radio meteor rate attains maximum at 0500 L.T. In general, it is found that the time of maximum fading frequency coincides with the time of peak visual meteor rate. There appears to be a delay of 3-4 hr between the peak fading frequency and M
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Effect of meteor aativity on the E-region fading
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466
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P. S. KESAVA RAO and B. RAMACHAXDRARAO
466
the peak meteor rate as determined by radio methods. It might be possible that the fading frequency increase is due to large meteors which can be seen with th.e naked eye and which can reach lower altitudes in the ionosphere causing enhanced fading. On the other hand the radio meteor data relate to all meteors including a substantial number of smaller meteors which attain a maximum at a later time and which may not reach these heights to produce enhanced fading. .‘,.
CONCLUSIONS
The following are the main conclusions drawn from the present investigation: (1) It is observed for the first time that the fading of a radio wave reflected from the E-region increases during meteor shower nights. (2) It is found that the annual variation of fading frequency of E-region reflections taken during night Oime va,ries in a similar manner to that of the annual variation of sporadic meteors that enter the earth’s atmosphere. (3) From the studies during Geminid shower 1963 and Leonid shower 1964, it was found that both visual meteor rate and E-region fading frequency attain maximum values at about the same time. Acknowledgements--We discussions. New
are thankful
to Dr. M. SRIRAMA RAO for some helpful
One of us (I?. S. K. RAO) is grateful to University
Delhi for the award
of Junior Research
Grants Commission,
Fellowship.
REFERENCES EVANS G. C. FISHER R. 9.
1960 1958
KOTADIA K. M. LOKANADHAMB.
1958 1965
LOVELL A. C. B. LOVELL A. C. B. LOVELL A. C. B.
1948 1950 1957
MCKINLEY D. W. H.
1961
MUWKAMI T. NAaOm H. NWOLET M. PIERCE 5. A. RAO B. R., RAO M. S. and MURTY D. S. X. RAO P. S. K. and RAO B. R. RICE S. 0. SINGHR. N. The Observers Hand Book
1955 1929 1955 1947 1956 1964 1945 1963 1963
VO~EN E. L. and CAMPBELLL. L.
1957
Jodrell Bnk. Ann. 1, 280 Statistical Methods for Research Workers, Oliver and Boyd, Edinburgh. .f. Sci. In&&r. Res. Al.7, 46. Ph.D. Thesis, submitted to the Andhra University Waltair, India. Rep. Prog. Phys. 11, 415. A%. Progr. 38, 22. Hand&h der Physik, Geophiscs II, 48, 427. Meteor Science and Engineering, McGrawHill, New York. Publ. As&on. Sot. Japan 7, 49. Proc. Imp. Acad. Tokyo 5, 233. Spl. Supp. J. Atmoaph. Terr. Phys. 2,99. Phys. Rev. 71, 88. J. Sci. Industr. Res. A15, 75. .I. Atmosph. Terr. Phys. 26, 841. Bell. Syst. Tech. J. 25, 282. J. Atmosph. Terr. Phys. 25, 589. Published by the Royal Astronomical Society of Canada, 55th Publication. Can. J. Phys. 35, 1176.