On intra-cloud discharges and their accompanying electric field-changes

Journal of Atmosphericand Terratrial Physics, 1072,Vol.54,pp. 116-126.PargamonPrees.

On intra-cloud dischwges and their accompanying electric iIeld-changes S. R. KHASTCHR and S. K. SAEA* Visva-Bharati University, Santiniketan, West Bengal, India (Received7 June 1971) A-t-With regard to intro-cloud discharges opinions have been divided as to whether the discharges are initiated by 8 positive streamer from 8 cmtre in the upper P-region to 8 centre in the lower N-region of the thunder-cloud or by a negative streamer from e centre in the lower N-region to a centre in the upper P-region. The former view was supported by TAKAGI (1901) and by OC+AWA and BROOK(1904). The second alternative view W&Badvocated by SMITE (1967) and by PIERCE(1962). To decide the issue, explicit theoretical expressions have been obtained by us for the net electrostatic field-changes at different distances in the case of vertical I-C discharges,when (a) the positive streamer moves downwardsfrom the upper P-region of the cloud to the lower N-region, and when (b) the negative streamer moves upwards from the lower N-region to the upper P-region. In deriving the net electrostaticfield-changes, the field-changesdue to the upper P and the lower N-region have been separately considered and it has been assumed that the charge on the initiating centre is gradually reduced to zero, as the streamer reaches its destination, just before the flash. The curves representingthese theoreticel expressions are similar to the curves shown by OGAWA and BROOK (1964). By comparing these curves with the actual traces of the observed electrostatic field-changes, It, can be concluded that the dischargesare mostly by a positive streamer from the upper P-region to the lower N-region of the cloud. It is, however, considered reasonable to aesume that tht) charge on the initiating centre is maintained at a constant value by the replenishmentof the loss of charge due to the streamer movement from the neighbouringcharge-centres. We have thereforederived suitable expressionsfor the net electrostaticfield-changesat differentdistances in the cases of (a) positive streamer moving vertically downwards, and (b) negative streamer moving vertically upwards, assuming a constant charge on the centre initiating the streamer. These curves, when compared with the experimental traces of the net electrostaticfield-change show that both the processes, viz. the downward movement of a positive streamer from the upper P-region to the lower N-region of the cloud and the upward movement of a negative streamer from the lower N-region to the upper P-region are possible. The mechanism of the intra-cloud return-strokes is also given in the paper following the idea put forward by OGAWA and BROOK (1964). Diagrams showing the electrostatic potential gradient against time have been drawn to show the small and abrupt increasesin the potential gradient curves which are due to several intra-cloud return-strokes. INTRODUCTION THE OSCILLOGRAPHICrecords

of the wave-forms of electric field-changes due to intra-cloud discharges (briefly called I-C discharges) were obtained, perhaps for the first time, by SCHONLANDet al. (1938). They clearly mentioned that in these discharges there was no evidence of return-stroke pulses. In some of the waveforms (group 3) of atmospherics observed by LUTIUN (1934), the small isolated pulses of three half-cycles, separated by very short quiet intervals, were attributed to the discharges within the cloud. SCHONLANDet al. (1940) later pointed out that Lulkin’s waveforms of group 3 were very likely due to successive dart discharges within the cloud. PIERCE (1955) asserted quite definitely that there was no return-stroke phenomenon in intra-cloud field-changes. It was perhaps for the first time, KHASTGIR et al. (1957) put forward that the return-strokes do occur in I-C discharges. From a large number of oscillographic records taken at Banaras during 1952-56 and at Calcutta during * Present address: Tribeni College, Tribeni (Hooghly), West Bengal. 115

116

S. R. KHASTMH

mcl S. K. &WA

1959-61 by KHASTWR et al. (1957, 1962) with the automatic atmospherics recorder, constructed for the purpose (TANTRY, 1958), a few records showed unmistakeable evidence of return-stroke pulses in the waveforms associated with the intra-cloud discharges, while a very large number of such records showed that the nature of the intra-cloud discharges was indeed very complex. The complexity is due to various factors, viz. inhomogeneous nature of the electric charge in the P- and the N-regions of the thunder-cloud, the presence of a small pocket of positive-charge (the p-region) in the lower region of the cloud, the preponderance of the ice-particles in the higher parts and that of the water drops in the lower parts of the cloud, the local drift. inside the cloud, etc.

FURTHERINVESTIGATIONS AND THE PRESENT POSITIONWITH REGARD TO ELECTRICFIELD-CHANGESDURING I-C DLWHAR~ES Various investigators studied about the same time, and also later, the electric field-changes accompanying the I-C discharges. They are: PIERCE (1955), MALAN (1956), ISHIKAWA (1956), KIMPAR (1957), SMITH (1957), KITAOAWA (1957), SZPOR and KOTLOWSIU(1957), KITAGAWA and KOBAYASHI (1958), KITAGAWA and BROOK (1960), PIERCE (1962), RAO et al. (1962), OGAWA and BROOK(1962,1964) and others In the opinion of Pierce, there is no similarity between the cloud-to-ground (or C-G) and the I-C discharges, except for the K-field changes that occur during both types of discharges. It may be mentioned in this connection that KITAGAWA (1957) and KITAGAWA and KOBAYASHI (1958) gave the special name, ‘K-field changes’, to the small electrostatic field-changes which they observed during the J or similar processes in either C-G or I-C discharges. It is to be noted that the K-changes are distinguished from the Malan or M-components which are small field-changes that occur superposed on the c-field change immediately after the return-stroke field-change in a C-G discharge. What, Pierce could concede was the presence of ‘recoil-streamers’ (see SCHONLAND, 1956; SZPOR and KOTLOWSKI, 1957) but not ‘return-streamers in an I-C discharge. The detailed investigations of OGAWA and BROOK ( 1964) have however confirmed the presence of the return-strokes in I-C discharges, although they have preferred to call the accompanying field-changes as K-field changes. The detailed nature of these K-changes has been discussed by them and it is interesting to note that Ogawa and Brook have used the term “intra-cloud return-strokes” to signify these K-changes. THE INTRA-CLOUDDISCHARGESAND THEIR COMPARISON WITH THE CLOUD-TO-GROUND DISCHARGES From the nature of the I-C field-change observed experimentally, and comparing it to fhe theoretical curves computed by Ogawa and Brook under certain specific conditions, they concluded that most frequently the discharge within the cloud would be initiated by a positive streamer from a centre in the upper P-region moving downwards to a centre in the lower N-region. Though this conclusion was in agreement with the work of TAKAGI (1961), it was in sharp contrast to the conclusion of SMITH (1957) who by making measurements simultaneously at two stations stated that the cloud discharges were initiated by a negative streamer going up from a

On i&m-cloud

discharges and their accompanying

electric field-changes

117

PIERCE (1962) also agreed with Smith’s conclusion. centre in the lower N-region. It appears from the work of BANDEL (1951) and LOEB (1953) that at heights where the positive centres in the P-region are located, the initiation of breakdown through corona is less likely because at these heights, the population of the ice-particles far exceeds that of water drops and the ice-particles give off very weak corona. Though this difference in the behaviour of corona from the ice-particles and the water drops does not go in favour of the conclusion of Ogawa and Brook, it has been utilized by them to explain the difference in the intra-cloud field-changes during the initial stages of the C-G and the I-C discharges. To illustrate this difference, it may be stated that the regular series of the first leader steps and the irregular discontinuities at the branching points in the first leader give rise to what are called ‘predischarges at the initial stage of a C-G discharge and that their conterparts in an intra-cloud discharge have quite a different pulse structure consisting of an ‘initial part’ and a ‘very active part’. The similarity between the C-G and the I-C field-change is in the later parts of both the types of discharge, the sudden changes in the field in the later part of the cloud-discharge being similar in appearence to the K-changes which occur during the J-process in the ground discharges. EXPLICIT THEORETICAL EXPRESSTONSFOR THE NET ELECTROSTATIC FIELD-CHANQES IN INTRA-CLOUD DISCHARQESAT DIFFERENT DISTANCES UNDER SPECIFIC CONDITIONS With regard to intra-cloud discharges it has already been mentioned that opinions have been divided as to whether the discharges are initiated by a positive streamer from a centre in the P-region on the top-side of the thunder-cloud to a centre in the N-region on the bottom-side or by a negative streamer from the lower N-region to the upper P-region. As already stated, the former view was supported by TAKAGI (1961) and by OGAWA and BROOK (1964) and the second alternative view was advocated by SMITH (1957) and by PIERCE (1962). To decide the issue, explicit theoretical expressions have been obtained by us for the net electrostatic fieldchanges at different distances in the case of vertical I-C discharges, when (a) the positive streamer moves downwards from the upper P-region of the cloud to the lower N-region, and when (b) the negative streamer moves upwards from the lower N-region to the upper P-region. In deriving the net electrostatic field-changes, the field-change due to the movement of the streamer and the field-changes, due to the upper P- and the lower N-regions have been separately considered and it has been assumed that the charge on the initiating centre is gradually reduced to zero, as the streamer reaches its destination just before the flash. We have also assumed for the sake of simplicity that the charges in the upper P-region and in the lower Nregion of the thunder-cloud are equal. Let the amount of this change be &Q, the distances of the centres of the upper P- and lower N-regions being respectively H and h from the ground. The centres of the P- and N-regions are also assumed to be in the vertical direction. In case I we have discussed the electrostatic field-changes due to an electrical discharge from the upper positive centre to the lower negative centre of the cloud, and in Case II the case of an electrical discharge from the lower negative centre to the upper positive centre of the cloud has been considered.

S. R. KHASTGIR and S. K. SAHA

118

(a)

/-

0

--I

(b) Fig. 1. Vertical dipole in the cloud-positive charge in the upper P-region and negative charge in the lower N-region. (a) Case I: Positive streamer from the upper P-region moving vertically downwards. (b) Case II: Negative streamer from the lower N-region moving vertically upwards.

Cme I: Electrical discharge from the upper positive to the lower negative charge (see Fig. l(a)) (i) The electrostatic field-change due to the vertical movement of the positive streamer from a distance x to (x + dx) km as measured from the upper positive centre is given by 2p dx

(H -

Cc)*+ Da * 1/(H

H-x -

2p(H 342 + 02 = [(H -

2) dx

x)2 + oy

(1)

where p is the charge per unit length of the streamer and D the distance of the receiving site from the point, A, where the vertical line joining the positive and the negative centres of charge meets the ground. The factor 2 is due to the effect of the electric images of the centres of the upper positive and the lower negative charges. The electrostatic field-change when the streamer from the upper positive

On intro-cloud discharges and their accompanying

centre moves vertically downwards to a distance x km is then given by El=

2p(H - 2) dx = s 0 [(H - x)* + PJs/s = 2P d(H

119

electric field-change%

- z)~ + 1

1

Da - dHa + Da

1’ (2)

(ii) Taking into account the image effect in the ground, the electrostatic field due to the upper positive charge is given by (3)

E~=2.~~~~~.l/H*~D’=~~~~~~~.

Since we have assumed that the charge on the upper positive centre is gradually reduced to zero as the streamer goes down and reaches the lower negative centre (just before the flash), we can write: & = p(H - h), so that E = ~P(H - h - NH 2 (Ha + D*)3/2 ’

(4)

(iii) Considering the image effect, the electrostatic field due to the lower negative charge is given by

A

--2Q

Es = ha + Ds ’ 2/hs + D” =

-2p(H - A)h (hs + D8)3/8 ’

(5)

Thus the net electrostatic field change at the receiving site is given by E = E, + E, + E, =2

1

’ [ d(H

1

(H - h - x)H (H - h)h _ x)8 + 02 - 1/Hs + D8 + (HB + DI)W - (h* + Da)V

1’ @)

Let the value of E, as calculated from (6) for the values of x = 0, 1, 2, 3 km be (E),, (E),, (E)2, (E)3 . . . . The field-changes at x = 0, 1,2,3 . . . km are then obtained from the values of (E)l - (E)o, (E), - (E),, (E)3 - (E)% . . . . The potential gradients at these successive distances are then known by assigning negative sign to the successive values of(E), - (E),, (E), - (E),, (E), - (E)z . . . . It is to be noted that the time, t, to cover the distance, x, is proportional to, x, as the velocity of the streamer is taken as constant. Case II: Electrical discharge from th.e lower negative to the upper negative charge (see Fig. l(b)) (i) The electrostatic field-change due to the vertical movement of the negative streamer from a distance x to (x + dx) km, as measured from the lower negative centre is given by - 2pdx

-2p(h + x) dx h+x (h + x)a + Da ’ 2/(h + xy + Ds = [(h + x)* + Dz]3’2

(7)

where p and D have the same significance as given for the expression (1). The factor 2 is due to the image effect. Thus the electrostatic field-change, when the negative streamer from the lower

120

S.

R. KEASTUIRand S. K. SAHA

negative centre moves vertically E,’

=

upwards to a distance x km is then given by

1 4p(h + z 2p I’0 [(h+ x)2+ 021212= d(h 1. (8) +x)2 +IF -d&+D2 1

x) dz

-

(ii) Taking into account the image effect in the ground, the electrostatic to the lower negative charge is given by E,'

h = -2(Q - f4 h2 + D2 *Z/j3773=

-2p(H (h2 +

(iii) Considering the image effect, the electrostatic positive charge is given by E

,

h -

x)h

(9)

D2)3/2 ’

field change due to the upper

2pP-J- h)H

=

3

Thus the net electrostatic

-

field due

(10)

(H2 + D2)3/2’

field change is given by

E’ = E,’ f E,’ + E,’ zz.T 2P

1

1 d(h

+ x)~ + 02 -

d/h-

(H - h) H + (H2 + D2)3/2 (h2 + 02)3/z

(11- h - z)k -

’

(11)

The potential gradients at successive distances x = 0, 1,2,3,. . . km from the lower negative charge are then obtained in the same way as in Case I. For the purpose of comparing the theoretical curves of Ogawa and Brook showing potential gradient against time with our computed curves, we have taken the following values taken by Ogawa and Brook, viz. Q = 20 C, H = 7.5km and F, = 2.5km. Our computed curves (Case I-when the positive streamer from the upper positive charge moves vertically downwards to the lower negative charge in the thunder-cloud for different values of D) are shown by solid lines and the corresponding curves (Case II-when the negative streamer from the lower negative charge moves vertically upwards to the upper positive charge) are shown for different values of D, by dotted lines in Fig. 2. It is interesting to observe that our curves representing the theoretical expressions, as given in (6)and (11)are similar to the curves drawn by Ogawa and Brook. By comparing our curves with the actual traces of the observed electrostatic potential gradients, it can be concluded that the discharges are mostly by positive streamer from the upper P-regions to the lower N-regions of the thunder-cloud.

EXPRESSIONS FOR THE POTENTIAL GRADIENT WHEN THE CHARGE ON THE INITIATINQCENTRE REMAINS CONSTANT BY THE REPLENISHMENT OF TEE Loss OF CHARUE DUE TO STREAMER MOVEMENT FROM THE NEIOHBOURINO CHABOE-CENTRES It is considered very reasonable that the loss of charge in the initiating chargecentre due to the movement of the streamer from it, is often replenished by similar charges from the neighbouring charge-oentres. The expressions for the net electro-

121

On &m-cloud disoharges and their aocompenying eleotricfield-changes

Dhtcmce,

km

Fig. 2. Potential gradient variation with vertical distance from the initiating charge-cemtre,when the charge is gradually reduced to zero, when the streamer from it rtxches its destination, just before the f&u&. Q = 20 C, H = 7.6 km and h = 2.5 km. 4~3 I; ---Cease II. The numerals shown against the curves give the values of the distance D in km.

static field-changes are then given by 1

E = 2p +(a

- 2)’ + I?” -

1

(H -h)H

~/HZ + ~)a + (Ifs + I)a)V

-

(H - h)h (h* + Ds)*t*

1

(I’)

for Case 1, when the discharge takes place from the upper positive to the lower negative charge in the cloud. The expressions for the net electrostatio field changes for Case II, when the discharge takes place from the lower negative to the upper position are given by E = 2p

1 (h + %)a + D’ -

1 y/h% + ~2 -

(H -

h)li

H-&H

(h% + Da)sla + (Ha + D’)sla

1 .

(13)

The field-changes at x = 0, 1, 2, 3 . . . km are known from the dif&rences in the

122

s.

R.

and S. K. Sa~ll

&USTaIR

xK

2

/ 22 zc iE

10

\‘-_-_===_rr==~~~~~-I-I_z~~o I\ (I’ ‘,‘ --_____----_____-,I’\ --___________------__----J_ II \

1: I \

, \ I

-10

I

._______--/f-_/--

-+I; _c

.4 \ /’

\.__A--

-12 -14 -16 -16 -20 -22 -24 -26 -28

/

I

I

2

3

I

I

/

4

5

6

Dlstmcr , km

Fig. 3. Potent&l gradient variation with vertical distence from the &it&kg charge-centre, when the charge rem&a the same. & = 20 C, H = 7.5 km and h = 2.5 km. ----cf%se II. I; - - - C&r%3 The numerals shown against the curves give the vslues of the distance D in km.

successive values of E for x = 0, 1, 2, 3 . . . km. The potential gradients are then obtained by assigning negative sign to the electrostatic field-changes. The computed curves showing potential grtiient against x (or against t) for different values of D are shown in Fig. 3 by solid lines for Case I and by dotted lines for Case fI. These curves, when compared with the experimental traces of the net potential gradient show that both the processes, viz. the downward movement of a positive streamer from the upper P-region to the lower iv-region of the cloud and the upward movement of a negative streamer from the lower N-region of the cloud to the upper P-region sre possible. Thus it is concluded that since the loss of charge on the initiating charge-centre, due to the movement of the streamer from it, is often replenished by similar charges from the neighbouring charge-oentres, the downward movement of the positive streamer from the upper P-region and the upward movement of a negative streamer from the lower N-region are both likely to take place

On intro-cloud diachargea and their accompanying

electric field-&ages

123

during intra-cloud discharges. In some cases, however, when the charge on the initiating charge-centre is not maintained at a constant value but is gradually reduced to zero as the streamer reaches its destination, just before the flash, only the downward passage of the positive streamer from the upper P-region would take place. THE MECHANISM OR TEE INTRA-CLOUDRETURN-STROKES

Let us now explain the mechanism of the intra-cloud return-strokes. As has already been mentioned, O~AWA and BROOE (1964) looked upon the intra-cloud return-strokes as K-field changes, when a positive streamer encounters in its downward passage a negative charge concentration. Based on this idea, the explanation of the intra-cloud return-strokes can be detailed as follows: We have already given our reasona to believe that an ix&a-cloud discharge can take place between a positive charge in the P-region on the topside and a negative charge in the N-region on the bottom-side of a b&polar cloud or vice versa. When the moving positive or negative streamer from the P- or N-region proceeds towards the N- or P-region, it may encounter a small concentration or a ‘blob’ of opposite charge. On such an encounter, there would be a sudden increase in the velocity of the streamer. Also the ‘blob’ would move very rapidly in a direction opposite to that of the advancing streamer like a small return-stroke. Thus there would be a current-surge in the streamer channel for an extremely small time. Further, the ‘blob’ of opposite charge, while moving through the streamer channel, would lose its charge by partially neutralizing the positive or the negative charge of the advancing streamer. The streamer would then move onward with a slight loss of charge-density, depending on the amount of charge on the ‘blob’. Later the advancing streamer may encounter a number of ‘blobs’ of opposite charge in its onward journey at irregular intervals, giving rise to a number of current-surges. Each such successive current-surge would cause a small but sharp rise in the electrostatic potential gradient at each encounter. This explains how the intra-cloud return-strokes would occur, causing several small but sharp rises in the potential gradient curve. ILLUSTRATIVE POTENTIALGRADIENTCURVESS~owx~a K-CEANGIES IN I-C DISCHAROES We shall now draw curves showing the potential gradient variation with the vertical distance (as measured from the initiating centre of charge) along with the R-changes for three different distances-D = 11, 10 and 9 km. Here we take the charge Q = 20 C on the initiating centre of charge as constant. Referring to Fig. 3, it can be seen that the potential gradient would slowly decrease as z (or t) increases for each of these three distances for both the Cases I and II, i.e. when the positive streamer from the upper P-region goes vertically down and also when the negative streamer from the lower N-region goes vertically upwards. Let us assume quite arbitrarily that the positive or the negative streamer encounters successively a ‘blob’ of opposite charge at z = 2, 3 and 4 km. It has already been mentioned that at each successive encounter, the ‘blob’ would move in a direction opposite to that of the advancing streamer, causing thereby a small return-stroke within the cloud. It has also been already stated that there would be an increase in the velocity of the

S. R. KFIABTCD and S. K.

124

SAHA

FbsitivestreamergoingdowI7

II km

2

5

15 -

G

IO -.

____----___;6 -

5

10 km

___------_______

15 IO -

9

km

S-

0

I I

I 2

5

4

3

Distance.km Fig. 4, Potential gradient variation w&h vertical distance or time for D = 9, 10 and 11 km wltb K-changes (Case I). It is assumed that the advancing streamer loses 10 per cent of the original charge-dsnsity at each encounter with a ‘blob’ of opposite chargo.

Negative

streamer

going

up

llkm

TT&&$&e v-e____------

~~:--_=.&+X-~ ---------.-_________ ------ __--9km I I

I 2 Chtance,

I 3

I

I

4

5

km

Fig. 6. Potential gradient variation with vertical diitance as tima for D = 9, 10 and 11 km with K-changes (Case II). It is aesumed that the dvitncing strmer loses 10 per cant of the originalcharge-density at each encountef with a ‘blob’ of opposite charge.

On ix&a-cloud discharges and their accompanying

electric field-changes

125

streamer at each encounter giving rise to a current-surge which would cause a small but sharp rise in the electrostatic field or potential gradient. On the other hand, the charge on the ‘blob’, when the ‘blob’ moves through the streamer channel, would also spend itself by partially neutralizing the advancing streamer. The streamer would therefore, after each encounter would proceed onward with a slight loss of its charge density. We shall now suppose that at each encounter the advancing streamer loses, say, 10 per cent of the original charge-density. The small but sharp rise in the potential gradient curve after each encounter would soon be followed by a rapid fall and the potential gradient curve would then correspond to the advancing streamer having 90 per cent of the charge-density that it had before each encounter. Figure 4 shows the potential gradient variation with K-changes for three distances, 11, 10 and 9 km at 2 = 2, 3 and 4 km, when the positive streamer going vertically downwards (Case I) loses 10 per cent of the original charge-density at each encounter. Figure 5 illustrates the potential gradient variation with the K-changes for the same three distances, when the negative streamer going vertically upwards (Case II) loses 10 per cent of the original charge-density. AckmowZedgement-We sincerely thank Dr. M. RAO for certain ideas which arc embodied in his doctoral thesis (1967). We also thank Mr. D. GROSHfor his help in drawing the diagrams. Our grateful thanks are also due to the ESSA for a PL-480 Grant and to the CSIR for a suitable grant which enabled us take some electric field-change records during intra-cloud discharges. The authors are also indebted to CSIR, New Delhi, the first named one (S. R. K) for the assignment as Retired Scientist, attached to Vieva-Bharati University and the second-named one (S. K. 5.) for the Senior Research Assistantship in a CSIR-scheme. REFERENCES APPLETON E. V. et al. BANDEL R. W. ISHIICAWA H. KEASTQIR S. R., T-Y SRI~ASTAVA R. S.

1926 1951 1956 1957

Proc. 22, Sot. IIIA, 654. J. appl. Phya. B, 984. J. Geomag. Geoelect. 8, 136. J. ecient. ind. Rec. 16B, 318.

KITAQAWA N. KITAQAWA N. and BROOK M. KITAOAWA N. and KOBAYASHI M.

1957 1957 1960 1958

LOEB L. B.

1953

Mem. Fat. Engng Nagoya Univ. 9.1. Met. Ueophye. (Tokyo) 7, 416. J. geophya. Res. 65, 1189. Recent Advancea in Atmospherics (Edited by L. G. SMITE), p. 485. Pergamon Press. London. Thunde&orm Electricity (Edited by H. H. BYERS),p. 150. Chicago University Press. Proc. R. Sot 171A, 285. Geojls.pra. appl84, 221. J. geophys. Rec. 69, 6141. Q. Jl R. met. Sot. 81, 21 and 229. Monograph on Radio Noise of Terrestrial O&gin (Edited by F. HORNER), p. 55.

B. A. P. and

KIMPAR~A.

LUTKIN M.ALAN OQAWA PIERCE PIleaCE

F. D. T. E. E.

E. J. and BROOK M. T. T.

1939 1956 1904 1955 1962

Elsevier, Amsterdam. RAO M., BEA~ACEA~~~A H. and KEASTGIR S. R. SCHONUND B. F. J., ELDER J. S., HODQES D. B., PHILLIPS W. E. and VAN WYK

1962

J. Atmoeph. Terr. Phys. 84, 989.

1940

Proo. R. Sot. 176A, 180.

126

S. R. KEASTCXR and S. K.

SCHONLAND B. F. J., HODGES D. and CO&ENS H. SMITHL. G. SZPOR S. and KOTLOWSKI J. TANTRY B. A. P. TANTRY B. A. P., SRIVASTAVA R. md KHA~TGIR S. R.

SAFIA

B.

1938

Proc. R. Sot. 1%6A, 56. Q. Jl R. met. Sot. 88, 103. Arch. elektro. 6, 21.

S.

1957 1957 1958 1957

Id J. Phys. 8$.3,267. PT~C. dfh. ht. sci. 178di088& 499.

Reference is also made to the following unplsblished material: OGAWA T. and BROOK M.

1962

RAO M. RAO M.

1967 1965

Presented at Am. Geophys. Un., Pacific South-West Regional Meeting, 1 Feb. 1962, Tuscan, Arizona. D. Phil. Thesis, C&utta University. Unpublished.