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F-region effects following two severe magnetic storms over Delhi
,Journal of Atmospheric andTerrestrial Physics,1967,vol. 29,pp. 61-71.Pergamon PressLtd. PrintedInNorthern Ireland
F-region effects following two severe magnetic storms over Delhi Radio Propagation
K. K. MAHAJAN* Unit, National Physical Laboratory,
New Delhi-12, India
(Received 8 June 1966) Ah&act--F-region effects following two severe magnetic storms over Delhi are studied with N(h) profiles deduced from C4 ionograms. The major effects are a reduction in the electron density at heights above 200 km, and a considerable increase in the height of the maximum electron density, h,FZ, during the night; there being little or no change in h,F2 duringthesunlit hours. The effects are examined with the help of a parameter R,, which is the ratio of the loss coefficient, @ and the vertical transport velocity, w. The parameter R, is found to increase considerably on the storm nights. This increase is concluded as an increase in the loss coefficient and a decrease in the transport velocity. Further analysis of R, indicates that the transport velocity decreases with height on storm nights in contrast to an increase with height on quiet night,s. 1. INTRODUCTION THE F-region of the ionosphere is now known to be considerably affected during geomagnetic storms, exhibiting outstanding changes in the height and the electron density of the FZ-layer. These changes have, however, considerable latitudinal and seasonal dependence as well. Ionospheric effects following geomagnetic storms have been studied in two ways in the past. In one of these, systematic average features of the effects have been derived on the basis of statistical studies (e.g. APPLETON and PIGGOTT, 1952;
MARTYN, 1953;
SATO, 1956;
OBAYASHI, 1952, 1954a,
19543, 1954c;
SINKO, 1954a, 19543; MATSUSHITA, 1959, 1963a, 19633). In the second approach, the studies have been concentrated on some individual storms (e.g. MEEK, 1952; IAWRENCE, 1953 ; SATO, 1957 ; OBAYASHI, 1959 ; SOMAYAJULU, 1963 ; BECKER, 1964). Although most of the studies on the magnetic storms, in the past, were performed by the routine ionospheric parameter (i.e. f,,FZ, h’F2), during and after the IGY, the use of true heights has been more emphasized. MATSUSHITA (1963a, 1963b) and SOJ~AYAJULU (1963) have made a detailed study of some major ionospheric storms by using hourly N(h) profiles for several stations in North and South America. They have not only confirmed earlier results (i.e. a marked depression of the peak electron density at high and middle latitudes and an increase at the low latitudes), but also observed the following additional facts : (i) only regions above about 180 km are affected (ii) sub peak electron content exhibits changes similar to the peak electron density and (iii) the height of the F-layer peak and the thickness of the layer increase considerably. These results have been supplemented by information on the total electron content up to heights of 1000 km from satellite observations by YEH and SWENSON (1961) and GARRIOTT (1963) and by incoherent backscatter profiles by BOWLES (1962). During the IGY July
several
8, 1958 and September
severe geomagnetic
storms
3, 1958, have been selected
* Now at Arecibo Ionospheric Observatory,
occurred.
Arecibo, Puerto Rico. 61
Two
to investigate
such storms, the F®ion
K. K.
62
MAFLAJAN
effects over Delhi, (geographic coordinates, 28”33’N, 77’13’E; geomagnetic coordinates, 19”11’N, 24#‘N) with the help of hourly N(h) profiles deduced from C4 ionograms. The true height reductions have been made by using the SCHMERLINOVENTRICEcoefficients (1959) appropriate for Delhi. The parameters studied are: the peak electron density, N,F2, the height of the peak, /z,,P2, electron density at fixed heights, N(h) and height at fixed electron densities, h(N). All these parameters have been studied as function of time and compared with the quiet day behavior. We have selected the storms of July 8 and September 3, 1958, because there have been reports about an increase in temperature and atmospheric density on these days by JACCHIA(1959). Also published values for atmospheric density, in the ionospheric heights are available for these periods (JACCHIAand SLOWEY,1962). These values have been used in examining the effects with the help of a parameter R, (MITRA, RAO and -JAN, 1964), which is the ratio of the attachment coefiicient /.land the vertical transport velocity, w. 2. ANALYSISAND RESULTS Relevant data about the storms under study are summarized in Table 1. These data were obtained from the monthly bulletins of Ionospheric Data for Indian Table 1. Relevant
data about the storms studied
Sudden commenoement Amplitude
Storm time Ending Beginning Date
h
m
d
h
1958 July 8 srpt. 3
07 OS
51 38
9 July 5 Sept.
18 17
Type
SC SC
DH I
4 2
2 Y
Y
li6 61
58 24
Degree of nctivity
* s
Maximal activity day
8 July 4 Sept.
Range D
H
2
’
;r
y
710 532
131 92
17 11
All times are in U.T.
stations, published by the Radio Research Committee, India. Both the storms were of the Sudden Commencement type and followed, at the usual l-day interval, the appearance of the flare of importance 3 on the Sun. Also both these storms occurred in the afternoon (local time). As already indicated, studies regarding the following parameters have been made (i) N,F2 (ii) Fy,F2 (iii) N(rh) and (iv) h(N). These are briefly described as follows: Changes in N,F2. Figure 1 shows a plot of N,F2 against time for the two cases. Quiet day average N,F2 values are also plotted for comparison. It can be seen that a decrease in N,F2 occurs following the sudden commencement, the decrease being more significant during the night than during the sunlit hours. From the magnitude of decrease, it appears that the maximum percentage decrease in N,F2 during the night occurs around 0300 hr and is about 70 per cent. It is interesting to observe that although the storms of July 8 and September 3 were different in their geomagnetic activity, the maximum percentage decrease in N,F2, during the night, was about the same for the two cases. The maximum effect during the sunlit hours appears to occur in the afternoon, around 1500 hr. It is found to be about 30 per cent for the storm of July 8 and about 15 per cent for the storm of September 3. The
F-region effects following two severe magnetic storms over Delhi
63
difference in the magnitude of percentage decrease in N,FZ during the sunlit hours, for the two cases could either be a seasonal effect or due to a change in geoma~etio activity for the two cases. We have examined N,l?2 values for some moderately severe storms as well, during the summer. We have found that the maximum percentage decrease in N,E’2 was again about 30 per cent (occurring around 1500
JULY
12
m
18
-JULY
”
8 - 9,1958
06
00
12 JULY
8 -
18
24
9
P
IO
0 00 -SEPT.3
I2
12
00 -
00
SEPT. 4 ---c--SEPT. 1S.T.
(U.T
+
53
24
12 5 _c
HRS)
Fig. 1. The effect of magnetic storms on the maximum electron density of the FZ-l&yer. The solid lines indicate the storm day values and the d&ed lines indicate the mean quiet values for the particular month.
hr) for these cases. Thus the comparatively smalIer decrease during the sunlit hours for the storm of September 3, is probably a seasonal elect. Changes in hrnF2. The height of the maximum ionization, h,FZ on the storm days is compared with the mean quiet values for the particular month in Fig. 2. ft can be noted that while h,B’2 shows a significant increase (sometimes as high as 300 km) during the night following the magnetic storm, there is little or no increase during the sunlit hours. There is, however, some evidence to believe that the daytime increase in A,P2 depends mainly on the activity of the storm ; for example although there was no change in h,F2 during the sunlit hours following the storm of September 3, it did show an increase on July 8, since the storm of July 8 was more intense than the one on September 3. We have also investigated the &,,F2 values during storms with lower activity and have found that the daytime behavior for such storms is
K. K. MAHAJAN
64
similar to that shown by the storm of September 3. The nighttime increase in lt,FZ, however, does not seem to depend entirely upon the intensity of the storm. We have tried to look for any “G” condition (measurement influenced because of the ionization density of the layer being too small (i.e. N,PI > N,FZ)) on the KM
500 JULY 8-9,
1958
400
300
16 -JULY
00
06
600
-
12
16
24
JULY 9-
SEPT. 3,485,
00
12
8
00
12
SEPT. 3 -SEPT. I.S.T.
1958
00 4 -
(U.T.
+ 5 +
12
24
SEPT. 5 HRS)
Fig. 2. The effect of magnetic storms on height of the maximum electron density
of the P&-layer. The solid lines indicate the storm day values and the dashed line indicate the mean quiet values for the particular month.
which could eventually give underestimates of i4mP2. However, no case of “G” condition was found for the cases under consideration. Changes in N(h). The electron density time curves can be very profitably used to look for the electron density changes during the sunlit hours, following a magnetic storm. However, these cannot be very conveniently utilized to study such changes during disturbed nights. This is so because h,P2 varies widely on the disturbed nights with a result that it becomes impossible to obtain value of electron density at any one height, at all the times. This introduces a discontinuity on the N-t curves and thus does not give a faithful picture of the electron density variation during nights. The daytime variations of electron density at fixed heights are shown in Fig. 3.
ionograms,
P-region effects following two severe magnetic storms over Delhi
65
These are compared with the quiet day average values during the particular month as well. It can be noted that although the electron density values remain unaffected up to heights of about 200 km, the values show a gradual decrease at heights above 200 km. It can also be noted that the magnitude of decrease for any one height, however, is not always the same at all the times. The maximum percentage decrease appears to occur in the afternoon. Changes in h(N). As pointed out above, N-t curves are not suitable for studying
-
JULY 9, MAG. STORM
-
-X-X-
JULY,
MEAN
X-x- SEPT., MEAN OUIET
12
14
IO
QUIET
16
18
SEPT 4 MAG STO$M
IO
I2
I4
“k
I6
..‘((
18
I.S T. (U T. + 5 $ I-IRS)
Fig. 3. A comparison of N-t curves on storm and quiet days. The zero of the electron density scale for each height is also marked.
the electron density changes at various heights during the storm nights, we have tried to study the effects with the help of isoionic contours, which represent the variation of true height at fixed plasma frequencies. These are shown in Figs. 4(a) and 4(b) for the two cases. The increase in height, during the night, for the various frequencies can be noted. The height changes are again shown in Fig. 5 for a plasma frequency of 4 MC/S,for one of the cases. These are compared with the average quiet day behavior of the heights at 4 MC/S. It can again be noted that although no changes in height occur during the sunlit hours (as expected since 4 MC/S would approximately refer to heights much below 200 km, for which no effects have been observed), the changes are significant during the night hours. 3. DISCUSSION The major ionospheric effects observed at Delhi, following geomagnetic storms can be summarized as follows: (i) Significant increase in 7t,P2 on disturbed nights, little or no change during the sunlit hours. 5
K.
K.
MAHAJAN
KM
I
DELHI STORM
I
OF JULY 8, 1958
500 FIGURES
DENOTE
FREOUENCIES
IN
08
16
MC/S
400
: P 300 9!
200
100
0 00
16
08
24
JULY 9 -
JULY 8
-
I.S.T. (U.T. + 5 +Fig. 4(a).
24
HRS)
Isoionic contours for the storm of July 8,1958. the night may be noted.
Increase in height during
KM
600
500 ;I
FREOUENCIES
IN
MC/~
400 & $ P 300
200
-I
100
It
0
. I6
-SEPT.
I
24
3 I.S.T.
Fig. 4(b).
I
a
-1
06 16 SEPT. 4 (U.T.
+5+
a
1.
24
” ” I6 06 SEPT. 5 -
1, 24
1
HRS)
Isoionic contours for the storm of September 3, 1958. during the night may be noted.
Increase in height
P-region effects following two severe magnetic storms over Delhi
67
(ii) Decrease in electron density at heights above 200 km, the magnitude of percentage decrease being more during the nights than during the sunlit hours. Two major theories have been proposed in the past to explain these effects-one is based on the electrodynamic drift motions of electrons (~RTYN, 1953) and the other on the changes in electron loss rate by an enhanced thermal effect in the Pregion (NAGATA, 1954; SEATON, 1956; BECKER, 1964). It is not possible to explain the observed effects on the basis of one theory alone ; since an increase in loss rates, csn explain the decrease in electron density, but cannot by itself explain the increase KM I
600
I
3.4,5, 1958
-
SEPT.
---
MEAN QUIET.SEPT.
1956
4 MC/S
I I
500
1
DELHI
I
400 !i !z !i 300
200
100
00 -
06
16
SEPT. 3
24
06
16
24
SEPT. 4
06
16
24
SEPT. 5 -
I.S.T. (U.T. + 5 4 HRS 1
Fig. 5. Comparison of height changes at a, plasma frequency of 4 MC/S, on the storm (solid lines) and quiet (dashed lines) days. in height
(sometimes as high as 300 km). On the other hand an increase in the upward motion cannot explain the decrease in electron density although it can explain the height increase. However, it is quite possible that both the processes (increased electron loss rates and the upward motion) occur simultaneously in a manner that there is a net decrease in the electron density accompanied or followed by an upward motion of the layer. An evidence to this is presented below from the study of a parameter R,, which is the ratio of attachment coefficient, /? and transport velocity, w. We will first briefly describe this parameter. A detailed description is given elsewhere (MITRA, RAO and M&AJAN, 1964). From a study of electron density variation during the nights, it is found that the electron density versus time curve (N-t) for any height, very often, passes through maxima and minima. And while the times of these maxima and minima shift from day to day there is no appreciable height variation of these times, for any one night.
K. K.
68
&hEAJAN
If one concentrates at these times alone and approximates production term to zero, the continuity equation reduces to : /gijJ-N!+$
(1)
where p is the coefficient of attachment and w the resultant vertical transport velocity under the action of electromagnetic drift and ambipolar diffusion. Equation (1) can be written as: _ /l + awlah 1 aN =---. (2) W N ah MITRA, RAO and MAHAJAN(1964) have shown that, under these conditions, a plot of log l/N aN/ah against height is linear for a considerable range of heights. It has
300
350
400 HEIGHT
450
500
Fig. 6. Effect of magnetic storms on the parameter R,. A considerableincrease in R, on the storm nights, as compared to the maximum quiet day values, may be noted.
also been shown (M~TRA, RAO and -JAN, 1966) that the term awlah is negligible in comparison to the magnitude of /? in this height range. Thus an extrapolation of the quantity l/N aN/ah in this linear portion to other heights, yields the ratio B/w. We will subsequently refer to this ratio as R,. Since w is found to be negative during the night (MITRA, RAO and MAHAJAN,1966), consequently the parameter R, is always positive. A plot of R,, for quiet and disturbed nights is shown in Fig. 6. We have also indicated the maximum and minimum values observed for R, on quiet nights. A
P-region effects following two severe magnetic storms over Delhi
69
comparison of the values of R, on storm nights, with the maximum observed R, values on quiet nights, indicates that R, increases considerably on the storm nights. This implies that either ,!? increases or w decreases or there is simultaneously an increase in /3 and a decrease in w. The enhancement in the loss coefficient /? could occur due to an increase in the atmospheric temperature, which can result in an increase of atmospheric density. An increase in the atmospheric density would result in an increase of 0, or N, conFrom the centration (the molecule through which the loss proceeds in the P-region). satellite dragperturbations during storms, JACCECIA (1959, 1961a) and GROVES(1961) have inferred that there is a general heating of the atmosphere from 200 to 700 km JACCHIA (1961a, 1961b, 1963), from satellite drag during geomagnetic storms. variations, has shown that during magnetic storms the increase, AT, in temperature is approximately proportional to the increase, ha,, in the three hourly geomagnetic planetary index a9. JACCIHA and SLOWEY (1963) have shown that AT increases by l-1*5’K for Aa, = 1. For the storms under consideration, it appears that the temperature and the atmospheric density increased by about 20 per cent in the F-region heights as evidenced from the published data by JACCHIAand SLOWEY (1962) for the drag of satellite 1958 tc (perigee height 355 km). If the increase in 0, or N, is taken to be proportional to the increase in atmospheric density, the loss coefficient will also increase by about 20 per cent in the P-region heights. It can, however, be seen from Fig. 6 that R, has increased by a factor of 3 for July 8 and by a factor of 5 for September 5 at a height of 350 km. It would be difficult to explain such a large increase in R, due to an increase in ,!?alone. This indicates that there is in addition a decrease in the transport velocity w so as to enhance the ratio /?/w on the storm nights. It can also be seen from Fig. 6 that the ratio (R,),/(R,), (where the subscripts & and D refer to quiet and disturbed values) progressively increases with increasing height. Assuming an increase of 20 per cent in temperature and atmospheric density at 350 km level, one could estimate the changes in the density and hence in /l at different levels. Then the transport velocity on the storm nights can be determined at various heights from the equations:
(R,), -=-
(B/w)o
(R,),
(B/w)Q
(3)
or
If the transport velocity on quiet nights is known, wg can be calculated. We have taken values of WQ obtained by MITRA, RAO and MAHAJ~ (1966) for Delhi. The ,disturbed day values of the transport velocity obtained for July 9,1959, and September 5, 1958 are summarized in Table 2 and compared with the quiet day values in Fig. 7. It can be noted that although transport velocity increases with increasing height on quiet nights, it decreases with height on the storm nights. Since there is little or no change in h,B’2 during the sunlit hours, it appears that
70
K. K. MAHAJAN Table 2. Vertical transport velocity during magnetic storms July 9, 1958, 0000 hr Height 300 350 400 450
Sept. 5, 1958, 0200 hr
‘WD
WC)
(%)DI(Ro)o
-6.5 -8.5 -11.0 -14.0
1.3 3.0 7.0 18.0
WD
m/set
m/set
WD/%)Q
-5.0 -3.4 -2.4 -1.5
-4.3 -2.3 -1.4 -0.7
1.5 4.4 12.0 36.0
DELHI MAGNETIC
STORM EFFECT
ON TRANSPORT
16’1 300
1
’
’
’
’
’
’
’
’
’ 400
350
’
VELOCITY
’
’
’
J
450
HEIGHT ( KM)
Fig. 7. Comparison of transport velocity on quiet and storm nights.
the transport velocity remains unaffected during storms, in the daytime. The decrease in electron density could result from an increase in atmospheric temperature alone. A relatively low decrease in electron density could possibly be due to an increase of the production rates as well, since atomic oxygen concentration is also expected t.o increase due to an increase in the atmospheric temperature. Acknowledgements-I wish to acknowledge the benefit of many valuable discussions with Dr. A. P. MITRA. I am thankful to Dr. A. K. SARA, Dr. Y. V. SOMAYAJULUand Dr. W. BECKER for very critically going through the manuscript and suggestions. I am grateful to Mrs. I. M. MII~ANDA,Mr. H. M. CAJIGAS and Mr. G. GILMOUR of the Arecibo Ionospheric Observatory, for typing this paper, for making drawings and for making photographic prints respectively.
F-region effects following two severe magnetic storms over Delhi
71
REFERENCES
APPLETON E. V. and P~caoa W. BECKER W.
R.
1952 1964
BOWLES K. L. GARRIOTT 0. K. GROVES 0. V. JACCHIA L. G. JACCHIA L. G. JACCHIA L. G. JACCHIA L. G. JACCHIA L. G. and SLOWEY J. JACCHIA L. G. and SLO~EY J. LAWRENCE R. S. MARTYN D. F. MATSUSHITA,S. MATSUSHITA S. MATSUSHITA S.
1962 1960 1961 1959 1961a 1961b 1963 1962 1963 1953 1953 1959 1963a 19638
MEEK J. H. MITRA A. P.,RAO B. C. N. and MAHAJAN K. K. MITRA A. P., RAO B. C. N. and MAHAJAN K. K. NA~ATA T. OBAYASHI T. OBAYASHI T. OBAYASHI T. OBAYASHI T. OBAYASHI T. SATO T. SATO T. SCHMERLING E. R. a;ndVENTRICE C. A. SEATON :M.J. SINNO K. SINNO K. SOMAYAJULU Y. V. YEH K. C. and SWENSON G. W.
1952 1964
J. Atmosph. TeTT. Phys. 2, 236. Electron Distribution in the Ionosphere and Exosphere, p. 152, North-Holland, Amsterdam. US-NBS Rep. 7633. J. Geophys. Res. 65, 1139. Rep. Univ. College, London. Nature, Land. 183,1662. Space Res. 2, 747. Nature, Lond. 192,1147. Space Res. 3, 3. Smith. Astrophys. Obs. Spec. Rep. 100. Smith. Astrophys. Obs. Spec. Rep. 125. J. Geophys. Res. 58, 219. PTOC. R. Sot. &?18, 1. J. Geophys. Res. 64, 305. J. Geophys. Res. 68, 2595. Proc. Int. Conj., The Ionosphere, p. 120, Inst. Phys. and Phys. Sot. London. J. Geophys. Res. 57, 177. J. Atmosph. Terr. Phys. 26, 525.
1967
J. Atmosph.
1954 1952 1954a 1954b 1954c 1959 1956 1957 1959 1956 1954a 19548 1963 1961
Rep. Ionosph. Res. Japan 8, 39. Rep. Ionosph. Res. Japan 8, 79. Rep. Ionosph. Res. Japan 8, 135. Rep. Ionosph. Res. Japan 8, 165. J. Geomagn. Geoelect., Kyoto 6, 57. J. Radio Res. Labs Japan 6, 375. J. Geomugn. Geoelect., Kyoto 8, 129. J. Geomagn. Geoelect., Kyoto 9, 1. J. Atmosph. Terr. Phys. 14, 249. J. Atmosph. TeTT. Phys. 8, 122. J. Geomagn. Geoelect., Kyoto 6, 120. J. Radio Res. Labs Japan 1, 127. J. Geophys. Res. 68, 1899. J. Geophys. Res. 60, 1061.
Terr. Phys.
29, 43.
F-region effects following two severe magnetic storms over Delhi Radio Propagation
K. K. MAHAJAN* Unit, National Physical Laboratory,
New Delhi-12, India
(Received 8 June 1966) Ah&act--F-region effects following two severe magnetic storms over Delhi are studied with N(h) profiles deduced from C4 ionograms. The major effects are a reduction in the electron density at heights above 200 km, and a considerable increase in the height of the maximum electron density, h,FZ, during the night; there being little or no change in h,F2 duringthesunlit hours. The effects are examined with the help of a parameter R,, which is the ratio of the loss coefficient, @ and the vertical transport velocity, w. The parameter R, is found to increase considerably on the storm nights. This increase is concluded as an increase in the loss coefficient and a decrease in the transport velocity. Further analysis of R, indicates that the transport velocity decreases with height on storm nights in contrast to an increase with height on quiet night,s. 1. INTRODUCTION THE F-region of the ionosphere is now known to be considerably affected during geomagnetic storms, exhibiting outstanding changes in the height and the electron density of the FZ-layer. These changes have, however, considerable latitudinal and seasonal dependence as well. Ionospheric effects following geomagnetic storms have been studied in two ways in the past. In one of these, systematic average features of the effects have been derived on the basis of statistical studies (e.g. APPLETON and PIGGOTT, 1952;
MARTYN, 1953;
SATO, 1956;
OBAYASHI, 1952, 1954a,
19543, 1954c;
SINKO, 1954a, 19543; MATSUSHITA, 1959, 1963a, 19633). In the second approach, the studies have been concentrated on some individual storms (e.g. MEEK, 1952; IAWRENCE, 1953 ; SATO, 1957 ; OBAYASHI, 1959 ; SOMAYAJULU, 1963 ; BECKER, 1964). Although most of the studies on the magnetic storms, in the past, were performed by the routine ionospheric parameter (i.e. f,,FZ, h’F2), during and after the IGY, the use of true heights has been more emphasized. MATSUSHITA (1963a, 1963b) and SOJ~AYAJULU (1963) have made a detailed study of some major ionospheric storms by using hourly N(h) profiles for several stations in North and South America. They have not only confirmed earlier results (i.e. a marked depression of the peak electron density at high and middle latitudes and an increase at the low latitudes), but also observed the following additional facts : (i) only regions above about 180 km are affected (ii) sub peak electron content exhibits changes similar to the peak electron density and (iii) the height of the F-layer peak and the thickness of the layer increase considerably. These results have been supplemented by information on the total electron content up to heights of 1000 km from satellite observations by YEH and SWENSON (1961) and GARRIOTT (1963) and by incoherent backscatter profiles by BOWLES (1962). During the IGY July
several
8, 1958 and September
severe geomagnetic
storms
3, 1958, have been selected
* Now at Arecibo Ionospheric Observatory,
occurred.
Arecibo, Puerto Rico. 61
Two
to investigate
such storms, the F®ion
K. K.
62
MAFLAJAN
effects over Delhi, (geographic coordinates, 28”33’N, 77’13’E; geomagnetic coordinates, 19”11’N, 24#‘N) with the help of hourly N(h) profiles deduced from C4 ionograms. The true height reductions have been made by using the SCHMERLINOVENTRICEcoefficients (1959) appropriate for Delhi. The parameters studied are: the peak electron density, N,F2, the height of the peak, /z,,P2, electron density at fixed heights, N(h) and height at fixed electron densities, h(N). All these parameters have been studied as function of time and compared with the quiet day behavior. We have selected the storms of July 8 and September 3, 1958, because there have been reports about an increase in temperature and atmospheric density on these days by JACCHIA(1959). Also published values for atmospheric density, in the ionospheric heights are available for these periods (JACCHIAand SLOWEY,1962). These values have been used in examining the effects with the help of a parameter R, (MITRA, RAO and -JAN, 1964), which is the ratio of the attachment coefiicient /.land the vertical transport velocity, w. 2. ANALYSISAND RESULTS Relevant data about the storms under study are summarized in Table 1. These data were obtained from the monthly bulletins of Ionospheric Data for Indian Table 1. Relevant
data about the storms studied
Sudden commenoement Amplitude
Storm time Ending Beginning Date
h
m
d
h
1958 July 8 srpt. 3
07 OS
51 38
9 July 5 Sept.
18 17
Type
SC SC
DH I
4 2
2 Y
Y
li6 61
58 24
Degree of nctivity
* s
Maximal activity day
8 July 4 Sept.
Range D
H
2
’
;r
y
710 532
131 92
17 11
All times are in U.T.
stations, published by the Radio Research Committee, India. Both the storms were of the Sudden Commencement type and followed, at the usual l-day interval, the appearance of the flare of importance 3 on the Sun. Also both these storms occurred in the afternoon (local time). As already indicated, studies regarding the following parameters have been made (i) N,F2 (ii) Fy,F2 (iii) N(rh) and (iv) h(N). These are briefly described as follows: Changes in N,F2. Figure 1 shows a plot of N,F2 against time for the two cases. Quiet day average N,F2 values are also plotted for comparison. It can be seen that a decrease in N,F2 occurs following the sudden commencement, the decrease being more significant during the night than during the sunlit hours. From the magnitude of decrease, it appears that the maximum percentage decrease in N,F2 during the night occurs around 0300 hr and is about 70 per cent. It is interesting to observe that although the storms of July 8 and September 3 were different in their geomagnetic activity, the maximum percentage decrease in N,F2, during the night, was about the same for the two cases. The maximum effect during the sunlit hours appears to occur in the afternoon, around 1500 hr. It is found to be about 30 per cent for the storm of July 8 and about 15 per cent for the storm of September 3. The
F-region effects following two severe magnetic storms over Delhi
63
difference in the magnitude of percentage decrease in N,FZ during the sunlit hours, for the two cases could either be a seasonal effect or due to a change in geoma~etio activity for the two cases. We have examined N,l?2 values for some moderately severe storms as well, during the summer. We have found that the maximum percentage decrease in N,E’2 was again about 30 per cent (occurring around 1500
JULY
12
m
18
-JULY
”
8 - 9,1958
06
00
12 JULY
8 -
18
24
9
P
IO
0 00 -SEPT.3
I2
12
00 -
00
SEPT. 4 ---c--SEPT. 1S.T.
(U.T
+
53
24
12 5 _c
HRS)
Fig. 1. The effect of magnetic storms on the maximum electron density of the FZ-l&yer. The solid lines indicate the storm day values and the d&ed lines indicate the mean quiet values for the particular month.
hr) for these cases. Thus the comparatively smalIer decrease during the sunlit hours for the storm of September 3, is probably a seasonal elect. Changes in hrnF2. The height of the maximum ionization, h,FZ on the storm days is compared with the mean quiet values for the particular month in Fig. 2. ft can be noted that while h,B’2 shows a significant increase (sometimes as high as 300 km) during the night following the magnetic storm, there is little or no increase during the sunlit hours. There is, however, some evidence to believe that the daytime increase in A,P2 depends mainly on the activity of the storm ; for example although there was no change in h,F2 during the sunlit hours following the storm of September 3, it did show an increase on July 8, since the storm of July 8 was more intense than the one on September 3. We have also investigated the &,,F2 values during storms with lower activity and have found that the daytime behavior for such storms is
K. K. MAHAJAN
64
similar to that shown by the storm of September 3. The nighttime increase in lt,FZ, however, does not seem to depend entirely upon the intensity of the storm. We have tried to look for any “G” condition (measurement influenced because of the ionization density of the layer being too small (i.e. N,PI > N,FZ)) on the KM
500 JULY 8-9,
1958
400
300
16 -JULY
00
06
600
-
12
16
24
JULY 9-
SEPT. 3,485,
00
12
8
00
12
SEPT. 3 -SEPT. I.S.T.
1958
00 4 -
(U.T.
+ 5 +
12
24
SEPT. 5 HRS)
Fig. 2. The effect of magnetic storms on height of the maximum electron density
of the P&-layer. The solid lines indicate the storm day values and the dashed line indicate the mean quiet values for the particular month.
which could eventually give underestimates of i4mP2. However, no case of “G” condition was found for the cases under consideration. Changes in N(h). The electron density time curves can be very profitably used to look for the electron density changes during the sunlit hours, following a magnetic storm. However, these cannot be very conveniently utilized to study such changes during disturbed nights. This is so because h,P2 varies widely on the disturbed nights with a result that it becomes impossible to obtain value of electron density at any one height, at all the times. This introduces a discontinuity on the N-t curves and thus does not give a faithful picture of the electron density variation during nights. The daytime variations of electron density at fixed heights are shown in Fig. 3.
ionograms,
P-region effects following two severe magnetic storms over Delhi
65
These are compared with the quiet day average values during the particular month as well. It can be noted that although the electron density values remain unaffected up to heights of about 200 km, the values show a gradual decrease at heights above 200 km. It can also be noted that the magnitude of decrease for any one height, however, is not always the same at all the times. The maximum percentage decrease appears to occur in the afternoon. Changes in h(N). As pointed out above, N-t curves are not suitable for studying
-
JULY 9, MAG. STORM
-
-X-X-
JULY,
MEAN
X-x- SEPT., MEAN OUIET
12
14
IO
QUIET
16
18
SEPT 4 MAG STO$M
IO
I2
I4
“k
I6
..‘((
18
I.S T. (U T. + 5 $ I-IRS)
Fig. 3. A comparison of N-t curves on storm and quiet days. The zero of the electron density scale for each height is also marked.
the electron density changes at various heights during the storm nights, we have tried to study the effects with the help of isoionic contours, which represent the variation of true height at fixed plasma frequencies. These are shown in Figs. 4(a) and 4(b) for the two cases. The increase in height, during the night, for the various frequencies can be noted. The height changes are again shown in Fig. 5 for a plasma frequency of 4 MC/S,for one of the cases. These are compared with the average quiet day behavior of the heights at 4 MC/S. It can again be noted that although no changes in height occur during the sunlit hours (as expected since 4 MC/S would approximately refer to heights much below 200 km, for which no effects have been observed), the changes are significant during the night hours. 3. DISCUSSION The major ionospheric effects observed at Delhi, following geomagnetic storms can be summarized as follows: (i) Significant increase in 7t,P2 on disturbed nights, little or no change during the sunlit hours. 5
K.
K.
MAHAJAN
KM
I
DELHI STORM
I
OF JULY 8, 1958
500 FIGURES
DENOTE
FREOUENCIES
IN
08
16
MC/S
400
: P 300 9!
200
100
0 00
16
08
24
JULY 9 -
JULY 8
-
I.S.T. (U.T. + 5 +Fig. 4(a).
24
HRS)
Isoionic contours for the storm of July 8,1958. the night may be noted.
Increase in height during
KM
600
500 ;I
FREOUENCIES
IN
MC/~
400 & $ P 300
200
-I
100
It
0
. I6
-SEPT.
I
24
3 I.S.T.
Fig. 4(b).
I
a
-1
06 16 SEPT. 4 (U.T.
+5+
a
1.
24
” ” I6 06 SEPT. 5 -
1, 24
1
HRS)
Isoionic contours for the storm of September 3, 1958. during the night may be noted.
Increase in height
P-region effects following two severe magnetic storms over Delhi
67
(ii) Decrease in electron density at heights above 200 km, the magnitude of percentage decrease being more during the nights than during the sunlit hours. Two major theories have been proposed in the past to explain these effects-one is based on the electrodynamic drift motions of electrons (~RTYN, 1953) and the other on the changes in electron loss rate by an enhanced thermal effect in the Pregion (NAGATA, 1954; SEATON, 1956; BECKER, 1964). It is not possible to explain the observed effects on the basis of one theory alone ; since an increase in loss rates, csn explain the decrease in electron density, but cannot by itself explain the increase KM I
600
I
3.4,5, 1958
-
SEPT.
---
MEAN QUIET.SEPT.
1956
4 MC/S
I I
500
1
DELHI
I
400 !i !z !i 300
200
100
00 -
06
16
SEPT. 3
24
06
16
24
SEPT. 4
06
16
24
SEPT. 5 -
I.S.T. (U.T. + 5 4 HRS 1
Fig. 5. Comparison of height changes at a, plasma frequency of 4 MC/S, on the storm (solid lines) and quiet (dashed lines) days. in height
(sometimes as high as 300 km). On the other hand an increase in the upward motion cannot explain the decrease in electron density although it can explain the height increase. However, it is quite possible that both the processes (increased electron loss rates and the upward motion) occur simultaneously in a manner that there is a net decrease in the electron density accompanied or followed by an upward motion of the layer. An evidence to this is presented below from the study of a parameter R,, which is the ratio of attachment coefficient, /? and transport velocity, w. We will first briefly describe this parameter. A detailed description is given elsewhere (MITRA, RAO and M&AJAN, 1964). From a study of electron density variation during the nights, it is found that the electron density versus time curve (N-t) for any height, very often, passes through maxima and minima. And while the times of these maxima and minima shift from day to day there is no appreciable height variation of these times, for any one night.
K. K.
68
&hEAJAN
If one concentrates at these times alone and approximates production term to zero, the continuity equation reduces to : /gijJ-N!+$
(1)
where p is the coefficient of attachment and w the resultant vertical transport velocity under the action of electromagnetic drift and ambipolar diffusion. Equation (1) can be written as: _ /l + awlah 1 aN =---. (2) W N ah MITRA, RAO and MAHAJAN(1964) have shown that, under these conditions, a plot of log l/N aN/ah against height is linear for a considerable range of heights. It has
300
350
400 HEIGHT
450
500
Fig. 6. Effect of magnetic storms on the parameter R,. A considerableincrease in R, on the storm nights, as compared to the maximum quiet day values, may be noted.
also been shown (M~TRA, RAO and -JAN, 1966) that the term awlah is negligible in comparison to the magnitude of /? in this height range. Thus an extrapolation of the quantity l/N aN/ah in this linear portion to other heights, yields the ratio B/w. We will subsequently refer to this ratio as R,. Since w is found to be negative during the night (MITRA, RAO and MAHAJAN,1966), consequently the parameter R, is always positive. A plot of R,, for quiet and disturbed nights is shown in Fig. 6. We have also indicated the maximum and minimum values observed for R, on quiet nights. A
P-region effects following two severe magnetic storms over Delhi
69
comparison of the values of R, on storm nights, with the maximum observed R, values on quiet nights, indicates that R, increases considerably on the storm nights. This implies that either ,!? increases or w decreases or there is simultaneously an increase in /3 and a decrease in w. The enhancement in the loss coefficient /? could occur due to an increase in the atmospheric temperature, which can result in an increase of atmospheric density. An increase in the atmospheric density would result in an increase of 0, or N, conFrom the centration (the molecule through which the loss proceeds in the P-region). satellite dragperturbations during storms, JACCECIA (1959, 1961a) and GROVES(1961) have inferred that there is a general heating of the atmosphere from 200 to 700 km JACCHIA (1961a, 1961b, 1963), from satellite drag during geomagnetic storms. variations, has shown that during magnetic storms the increase, AT, in temperature is approximately proportional to the increase, ha,, in the three hourly geomagnetic planetary index a9. JACCIHA and SLOWEY (1963) have shown that AT increases by l-1*5’K for Aa, = 1. For the storms under consideration, it appears that the temperature and the atmospheric density increased by about 20 per cent in the F-region heights as evidenced from the published data by JACCHIAand SLOWEY (1962) for the drag of satellite 1958 tc (perigee height 355 km). If the increase in 0, or N, is taken to be proportional to the increase in atmospheric density, the loss coefficient will also increase by about 20 per cent in the P-region heights. It can, however, be seen from Fig. 6 that R, has increased by a factor of 3 for July 8 and by a factor of 5 for September 5 at a height of 350 km. It would be difficult to explain such a large increase in R, due to an increase in ,!?alone. This indicates that there is in addition a decrease in the transport velocity w so as to enhance the ratio /?/w on the storm nights. It can also be seen from Fig. 6 that the ratio (R,),/(R,), (where the subscripts & and D refer to quiet and disturbed values) progressively increases with increasing height. Assuming an increase of 20 per cent in temperature and atmospheric density at 350 km level, one could estimate the changes in the density and hence in /l at different levels. Then the transport velocity on the storm nights can be determined at various heights from the equations:
(R,), -=-
(B/w)o
(R,),
(B/w)Q
(3)
or
If the transport velocity on quiet nights is known, wg can be calculated. We have taken values of WQ obtained by MITRA, RAO and MAHAJ~ (1966) for Delhi. The ,disturbed day values of the transport velocity obtained for July 9,1959, and September 5, 1958 are summarized in Table 2 and compared with the quiet day values in Fig. 7. It can be noted that although transport velocity increases with increasing height on quiet nights, it decreases with height on the storm nights. Since there is little or no change in h,B’2 during the sunlit hours, it appears that
70
K. K. MAHAJAN Table 2. Vertical transport velocity during magnetic storms July 9, 1958, 0000 hr Height 300 350 400 450
Sept. 5, 1958, 0200 hr
‘WD
WC)
(%)DI(Ro)o
-6.5 -8.5 -11.0 -14.0
1.3 3.0 7.0 18.0
WD
m/set
m/set
WD/%)Q
-5.0 -3.4 -2.4 -1.5
-4.3 -2.3 -1.4 -0.7
1.5 4.4 12.0 36.0
DELHI MAGNETIC
STORM EFFECT
ON TRANSPORT
16’1 300
1
’
’
’
’
’
’
’
’
’ 400
350
’
VELOCITY
’
’
’
J
450
HEIGHT ( KM)
Fig. 7. Comparison of transport velocity on quiet and storm nights.
the transport velocity remains unaffected during storms, in the daytime. The decrease in electron density could result from an increase in atmospheric temperature alone. A relatively low decrease in electron density could possibly be due to an increase of the production rates as well, since atomic oxygen concentration is also expected t.o increase due to an increase in the atmospheric temperature. Acknowledgements-I wish to acknowledge the benefit of many valuable discussions with Dr. A. P. MITRA. I am thankful to Dr. A. K. SARA, Dr. Y. V. SOMAYAJULUand Dr. W. BECKER for very critically going through the manuscript and suggestions. I am grateful to Mrs. I. M. MII~ANDA,Mr. H. M. CAJIGAS and Mr. G. GILMOUR of the Arecibo Ionospheric Observatory, for typing this paper, for making drawings and for making photographic prints respectively.
F-region effects following two severe magnetic storms over Delhi
71
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