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Increases of equatorial total electron content (TEC) during magnetic storms
Journalof .4tmoaphericand TerrestrialPhysics, Vol. 38, pp. 45 to 50. Pergamon Preea, 1076. Printed in Northern Ireland
Increases of equatorial total electron
content
(!t%X!)during magnetic storms
D. YEBOAH-AMANKWA~ Department of Physics, University of Ghana, Legon, Accra, Ghana (Received 10 March 1976; in ret&&form 23 May 1976)
Abstract--This paper is a report on the analysis of equatorial electron content, TEC, during magnetio storms. Storms between 1969 and 1972 have been examined as part of an on-going study of TEC morphology during magnetically disturbed days. The published magnetic KP indices and !l!EC data from the Legon observatory have been employed. The general picture arising from the analysis is that the total electron content of the ionosphereis signilloantly enhanced during magnetic storms. 1. IBTRODUC!CIOB During geomagnetic storms, characterized by large fluctuations in the Earth’s magnetic parameters measured on the surface of the Earth, the ionospheric total electron content, TEC, has been observed to undergo changes. Described as ionospheric storm, these changes have been studied by a large number of observers employing an sssortment of techniques. Data up to 1967 have been assembled by HIBBERD and ROSS (1967) who also show that investigators have employed techniques ranging from differential Doppler shift on signals from transit satellites to Faraday rotation messurements of satellite signals from Syncom 3 (NAKATA, 1966) and moon echo measurements on lunar radar (TAYLOR, 1961). Among the important studies recently conducted, we mention those of KATZ et al. (1972) on the variation of N,, F2 and h,,, during magnetic storm, MENDILLO (1973) on the variation of Kp index with TEC and N,,,, Low et al. (1973) on vertical movement of the P-layer during magnetic storm. In practically all the studies reported in the literature, data have come from mid-latitude regions of the globe. Even the work reported by WALKER (1973) and also Low et al. (1973) which are generally class&d as equatorial deal with measurements made in Hawaii and Hong Kong, Dip Latitudes 21’N and 30&N respectively. Of the few studies originating from equatorial ionosphere data, we mention that of S0H6DEL et d. (1974) on a single storm, occurring on 17 December 1971, which showed that there was a general global increase of TEC above quiet day levels. Earlier in a preliminary report on magnetically disturbed days, YEBOAH--WAH et al. (1972) found a similar behaviour for the equatorial TEC: the
daytime TEC during magnetic storms positively correlated with the sum planetary Kp index. Measurements of TEC during magnetic storm are very few for the equatorial region. The data reported for Legon (G. Lat. 6*63’N, G. Long. 0*19’W) present further additional information on the changes in the morphology of the equatorial TEC during magnetic storms, 2. OBSEEVATIOB The report covers the three year period 19691972. During that period, Faraday rotation measurements of the 137.350 MHz signals from the synchronous satellite AT&III have been made almost continuously at Legon. In Table 1 we give a list of the storms investigated, the position of the satellite at the time and the ionospheric point of the F-layer (taken as 350km) seen from the Legon observatory. The Faraday rotation angle has been taken to be proportional to the electron content. This assumption is quite good because the variability of the M-factor, the quantity which multiplies the rotation angle to give electron content, is much less than 10% over the 3-6 days that a typical storm occurs. In this report the changes dealt with are significantly larger than the variation in M so that the quality and conclusions of our results are not affected. The internationally published planetary Kp and Ap indices have been used in the analysis. The storms examined have been selected bssed on the availability of Faraday rotation data and subject to the criterion that the sum Kp index for the day of highest magnetio activity during the storm is 30 or greater. The storms analyzed are listed in Tables 1 and 2.
D. YEBOAH-AXANKWAH
46
Table1. Tableof storms, satellite position and ionospheric points . Satelli~ : 1-c Point. No. StO?lIl .:.... .....................*......... Eusitial.......:..... .Lcng..and:Lat..... :1
'21July- 4 $q.as& .l969' ;
42.5W
;
’ 3.6 W, 4.80N
Oct., -1969' :
42.5W
f
.3.9
6k3W
;
7.5'W, 4.8'N
42.9w
:
.3.00W, 4.8'N
:2 $7 Se&-15
,l970 I
:3
.6'19Auqxt,
:4
110- 20 Deok&er, .1970' 1
W, 4.8'N
.5. ;17Jan.-SF&.,
I.971 f
43.3w
:
. 3.00W, 4.8'N
:6 :l-lSOct&&,
1971
f
70.0w
:
9.0° w, 4.8O N
I18-'30 Noux&.r, .1971 :
7o.ow
f
9.0°W, 4.8'N
. :7
ii : :
.
I28ktc+-- 9 Nov:l.972 i 9.0°W, 4.8'N 7o.ow i ’ .: . . . . . . . . . . . ..*......... ._......._..................................
Table2. ObservedTECincreases during storms
21July - 4 Aug::l969 17 Sept.- 15 Oct.l.969 6-19August,l970
51
xl.5
4
10 - 20 m?&r,
1970 40
65 I
75
26
130
5 -
17Jan. - 5 Feb.,1971
90
22
120
6 -
1 - 15 Octdxr, 1971
50
30
150
7
18 -'30 Novznixx, 1971 38
45'
i3 -
26 Oct.- 9 Nov.,I.972 46 +
98
-
N.B. -
l?he~ercmtageincreasesarecalcuiatedforthermimun &viatFcnoccurrfn gduringea&stmm. InsaTeentries vmereu~eha~dashedlines,~Mlriati~wasjudged iniignificantor umztain.
Increases of equatorial total electron content (TEC) during magnetic storms The Faraday rotation angle is measured for every 10 mm. Using the planetary Kp record, 10 quietest days, five before and five after each storm, were seleoted and averaged to obtain the equivalent of the quietest day TEC, Qg. The Q. was then subtracted from the storm-time Faraday rotation angle to obtain deviations from the quiet-day levels. The deviations have been plotted as Aa in all the plots shown. To investigate what happens during day-time, night-time and early morning pre-sunrise period, AIR was averaged over IPI6 hr, 22-24 hr and 3-5 hr, respectively. They appear in the plots as QD, a, and QM respectively. In addition, we have plotted the daily planetary AZ, index in the graph. For easy reference, the symbols used are listed together as follows: An = deviation of storm time Faraday rotation angle from mean quiet day Faraday rotation. sl, = mean of As1 taken over the daytime period 14-16 hr. R, = mean of A51 over night-time period 22-24 hr. R, = mean of Aa over pre-sunrise period 3-5 hr. ZKp = sum of planetary magnetic Kp index over one day. Ap = daily planetary magnetic index.
8. RESULTS The general results are listed in Table 2. From the plots the following comments can be made on the various storms studied. Each graph has an error bar indicating the standard deviation calculated for the days used in calculating the mean quiet day rotation levels. All changes equal to or larger than the standard deviation should be regarded as significant.
47
Storm 1: 21 July-4 August 1968 The storm shown in Fig. 1 is a marginal one with a maximum EKp index of 31 and lasting for 4 days. s2, shows an increase during the storm while R, and Sz, do not show any significant trend except for the large fluctuations in Sz, in the post storm period.
“s? -60 MO ?: yx) ‘N CJ G 0 4; *“o -Pm 29
21
1
2
&)&
Fig. 1. Storm-time changes in total electron content for magnetic storm occurring between 21 July4 August 1969. Stomz 2:
17 September-15 October1969
With a maximum TZKp of 46, this is a strong storm lasting for approximately 5 days. There is a marked increase in sl,, a0 and R, during the storm as can be seen from Fig. 2. Storm 3: 6-19 August 1970 The maximum ZKp is 61 indicating a very strong storm. Several days before the storm, Fig. 3, there was a general depression observed in SIN, R, and Sz,. However, in the middle of the storm, they all show an increasing trend, eventually becoming positive at the tail end of the storm. The record shows a set of storms at the end of July preceding the main storm and it is not known
Fig. 2. Storm-time changes in total electron content for the magnetic storm occurring between 17 September-16 October 1969.
D. YEBOAE-AMANKWAH
48 whether
this has infiuenced
during the major
storm,
the behaviour
observed
of TEC
storm while in fi,
and &,, the increase disappears
soon after the storm ae shown in Fig. 6.
in August.
This storm is unusual for it shows a slight depreasion in Q,
and no signif%cant values
of s2, and
s.9 “D.;
s2, during the storm.
STORM3:6-19AUWST “Me;
:
, .
np JO
1970.
40
A
_ T
T
%
40”
=I
T
Kloo
An
0 -DO
FEBRUARY
JANUARY
Fig. 6. Storm-time changes in total electron content for the magnetic storm occurring between 17 Jannary6 February 1971. St0rr.n 6:
l-16
October 1971
The maximum storm Fig. 3. Storm-time changes in total electron content for the magnetic storm occurring between 6-19 August 1970. Stoma 4: A Ap
lo-20
December
1970
strong
storm,
modestly
nM and Q,.
increases
in the caee of
are decaying
during the storm and in fact reach negative just after the storm.
loo
STORM
10
The provides
a neat
during storms.
values
4 is shown in Fig.
16 14 DECEMBER
lost
the storm.
values of sl,,
It appears
positive
for
main storm has passed. is not a neat one, very present.
The
after
by
there is an increase
shown by the positive remain
one and some
as indicated
gaps, aa
Sz, and Q,
that all the param-
several
days
after
the
On the whole,
this storm
large variations
in AhR are
effects
curring on 27 September
the
in TEC
of an earlier
storm
oc-
1971, may be responsible
for this behaviour.
4.
16
Fig. 6. Storm-time for the magnetic
20
changes in total electron content storm occurring between 10-20 December 1970.
17 Januaqp6
maximum
away
were
for a day of the
is a weak
4:IO;20DE
I2
Fig. 4. Storm-time for the magnetic
Storm 6:
Storm
and
IZKp = 40
X.&u occurring
The storm
6, however,
during
It is observed
S& and $,, that the increases
data
in Fig.
eters
= 116; this storm records large TEC
in sl,,
TEC
is 36.
February
XKp
for
example QN, sl,
this
of an increase
In the case of R,,
to remain
for some
days
It
of TEC
the increase appears
after
The maximum rise
is 37.
and fi2, all show marked
increases.
18 Noventber-2 EKp
Fig. 7, was 38. !+,
1971 storm
Storna 7:
changes in total electron content storm occurring between 1-15 October 1971.
the period
of the
during
the
particularly
December
negative maximum
The
all show a positive increase
and indicates
crease in the background the smallest
during the storm,
occurring
Sa, and R,
storm.
large
TEC.
but it is signi&ant.
values
at the beginning
positive
1971
R,
is
a substantial
in
in-
The rise in a, sl,
grows
of the storm
values at the middle.
is
from to
49
Increases of equatorial total electron content (TEC) during magnetio storms STORM7:18-30 NOVEMBER 100 %_i “0
7 -50 LOO
"N _; 20 0 20 0
*P Ma
T loo
An
0
-00 16
22
20
24
28
30
NOVEMBER
Fig. 7. Storm-time changes in total electron for the
Storm
magnetic storm occurring between vember-2 December 1971. 8:
26 October-9
content 18 No-
November 1972
The maximum Kp observed for the storm was 46 oorresponding to Ap = 98 which represents a strong storm. SZD end QN show large positive values before and during the storm but are observed to be decaying away at the tail-end of nM shows a simple large positive the storm. increase during the period of the storm. The An shows very large fluctuations in the pre-storm period.
I
STORM% 26 OCT-9 NO” 1972
OCTOBER
6, the increase persists after the storm has died away. (b) The night-time background TEC change, characterised by Sz,, appears to show the greatest percentage increase during a magnetio storm. Generally, the inoreaee is of the order of 100% or more. (c) When a combination of minor and a major storm occurs with the minor preceding the major storm by a few days, the TEC may fluotuate widly or be inhibited. Storms 3 and 6 illustrate this behaviour. It appears that the average increase of TEC during times of intense magnetio activity is a well established phenomenon. In an unpublished study, KOSTER (1975) has calculated hourly correlation coefficients using TEC and planetary Kp indices over 3 year periods. As his results in Fig. 9 show, there is a positive correlation between Kp and TEC for all hours.
Y
OO
I
6
I
I
12
18
Fig. 9. Variation of correlation coefficient with time. The dotted line is the 99% confidence level.
I
The origin of these increases in TEC is generally accepted to be complex in nature and requires more research to illuminate it. It is however expected that a correct physical explanation of the phenomenon should take account of the roles of electromagnetic E x B drift of the Pa-layer (RAS~OUI et al. 1971; RAGEIAVAEAOet al.,1973; WOODU et al., 1972), and the differential heating of the ionosphere which leads to atomic and molecular mixing of oxygen (THOMAS et al.. 1966). Both of these affect the recombination of ions in the ionosphere and therefore affect the life times of electrons during storms.
NOVEMBER
Fig. 8. Storm-time changes in total electron content for the magnetic storm occurring between 26 October9 November 1972. 4. COBCLUSIOa
The summary of the results may be made as follows: (a) There is a general rise in TEC during equatorial storms. Sometimes, as exemplitkl by storm
AcknoluledgementThe author’s thanks are due to Drs. J. R. KOSTEBand T. BEERwith whom he has had very useful ditIoua3ions. The work reported in this paper wae supported in part by the Air Force Cambridge Laboratories, through the European Office of Aerospace Research (O.A.R.), United States Air-Force contract No. AFOSR-72-2268.
REFERENCES
HIBBEEDF. H. and Ross W. J. KATZ A. H. and PAPAoIAXNIsM. D.
1969 1972
Low N. C. and ROELOFS T. H.
1973
4
t
24
HOURS
J. gwphy8. Rw. 72, 6331. J. attnw. terr. Phya. 84,626. Planet. &me Sci. 21, 1806.
60
D. YEBOAE-UKWAE
MENDILLO M. NAC+ATAY. RACUXAVARAO R. and SIVARAIUN M. R. RAsTOoI R. G., CEANDRA H. and Mrsaa R. K. SOFIODELJ. P. et d. TAYLOX G. N. THOU L. end NORTON R. B. WALKES G. 0. WOODMAN R. F. STERLINO D. L. snd HANSON W. B. YEBOAH-AIUANKWAH D. a4d KOSTER J. R.
1973 1966 1973 1971 1974 1961 1961 1973 1972
Pz4nW.t.S$lace Soi. 21,345. Radio Sci. 1,1145. J. atmo8. tew. Phya. 85,209l. N&we., Land. a88, 13. J. atmoe. kw. Phga. 86, 1121. Natura, Lvnd. I@, 7401 J. awchva. Res. 71.227. J. &k~ tew. Phyi. 85, 1573. Radio Sci. 7, 739
1972
J. atmos. terr. Phyu. 20,395.
Reference ia al80 made to the following unpublished material: KOSTEX J. R.
1976
Data provided from an on-going study.
Increases of equatorial total electron
content
(!t%X!)during magnetic storms
D. YEBOAH-AMANKWA~ Department of Physics, University of Ghana, Legon, Accra, Ghana (Received 10 March 1976; in ret&&form 23 May 1976)
Abstract--This paper is a report on the analysis of equatorial electron content, TEC, during magnetio storms. Storms between 1969 and 1972 have been examined as part of an on-going study of TEC morphology during magnetically disturbed days. The published magnetic KP indices and !l!EC data from the Legon observatory have been employed. The general picture arising from the analysis is that the total electron content of the ionosphereis signilloantly enhanced during magnetic storms. 1. IBTRODUC!CIOB During geomagnetic storms, characterized by large fluctuations in the Earth’s magnetic parameters measured on the surface of the Earth, the ionospheric total electron content, TEC, has been observed to undergo changes. Described as ionospheric storm, these changes have been studied by a large number of observers employing an sssortment of techniques. Data up to 1967 have been assembled by HIBBERD and ROSS (1967) who also show that investigators have employed techniques ranging from differential Doppler shift on signals from transit satellites to Faraday rotation messurements of satellite signals from Syncom 3 (NAKATA, 1966) and moon echo measurements on lunar radar (TAYLOR, 1961). Among the important studies recently conducted, we mention those of KATZ et al. (1972) on the variation of N,, F2 and h,,, during magnetic storm, MENDILLO (1973) on the variation of Kp index with TEC and N,,,, Low et al. (1973) on vertical movement of the P-layer during magnetic storm. In practically all the studies reported in the literature, data have come from mid-latitude regions of the globe. Even the work reported by WALKER (1973) and also Low et al. (1973) which are generally class&d as equatorial deal with measurements made in Hawaii and Hong Kong, Dip Latitudes 21’N and 30&N respectively. Of the few studies originating from equatorial ionosphere data, we mention that of S0H6DEL et d. (1974) on a single storm, occurring on 17 December 1971, which showed that there was a general global increase of TEC above quiet day levels. Earlier in a preliminary report on magnetically disturbed days, YEBOAH--WAH et al. (1972) found a similar behaviour for the equatorial TEC: the
daytime TEC during magnetic storms positively correlated with the sum planetary Kp index. Measurements of TEC during magnetic storm are very few for the equatorial region. The data reported for Legon (G. Lat. 6*63’N, G. Long. 0*19’W) present further additional information on the changes in the morphology of the equatorial TEC during magnetic storms, 2. OBSEEVATIOB The report covers the three year period 19691972. During that period, Faraday rotation measurements of the 137.350 MHz signals from the synchronous satellite AT&III have been made almost continuously at Legon. In Table 1 we give a list of the storms investigated, the position of the satellite at the time and the ionospheric point of the F-layer (taken as 350km) seen from the Legon observatory. The Faraday rotation angle has been taken to be proportional to the electron content. This assumption is quite good because the variability of the M-factor, the quantity which multiplies the rotation angle to give electron content, is much less than 10% over the 3-6 days that a typical storm occurs. In this report the changes dealt with are significantly larger than the variation in M so that the quality and conclusions of our results are not affected. The internationally published planetary Kp and Ap indices have been used in the analysis. The storms examined have been selected bssed on the availability of Faraday rotation data and subject to the criterion that the sum Kp index for the day of highest magnetio activity during the storm is 30 or greater. The storms analyzed are listed in Tables 1 and 2.
D. YEBOAH-AXANKWAH
46
Table1. Tableof storms, satellite position and ionospheric points . Satelli~ : 1-c Point. No. StO?lIl .:.... .....................*......... Eusitial.......:..... .Lcng..and:Lat..... :1
'21July- 4 $q.as& .l969' ;
42.5W
;
’ 3.6 W, 4.80N
Oct., -1969' :
42.5W
f
.3.9
6k3W
;
7.5'W, 4.8'N
42.9w
:
.3.00W, 4.8'N
:2 $7 Se&-15
,l970 I
:3
.6'19Auqxt,
:4
110- 20 Deok&er, .1970' 1
W, 4.8'N
.5. ;17Jan.-SF&.,
I.971 f
43.3w
:
. 3.00W, 4.8'N
:6 :l-lSOct&&,
1971
f
70.0w
:
9.0° w, 4.8O N
I18-'30 Noux&.r, .1971 :
7o.ow
f
9.0°W, 4.8'N
. :7
ii : :
.
I28ktc+-- 9 Nov:l.972 i 9.0°W, 4.8'N 7o.ow i ’ .: . . . . . . . . . . . ..*......... ._......._..................................
Table2. ObservedTECincreases during storms
21July - 4 Aug::l969 17 Sept.- 15 Oct.l.969 6-19August,l970
51
xl.5
4
10 - 20 m?&r,
1970 40
65 I
75
26
130
5 -
17Jan. - 5 Feb.,1971
90
22
120
6 -
1 - 15 Octdxr, 1971
50
30
150
7
18 -'30 Novznixx, 1971 38
45'
i3 -
26 Oct.- 9 Nov.,I.972 46 +
98
-
N.B. -
l?he~ercmtageincreasesarecalcuiatedforthermimun &viatFcnoccurrfn gduringea&stmm. InsaTeentries vmereu~eha~dashedlines,~Mlriati~wasjudged iniignificantor umztain.
Increases of equatorial total electron content (TEC) during magnetic storms The Faraday rotation angle is measured for every 10 mm. Using the planetary Kp record, 10 quietest days, five before and five after each storm, were seleoted and averaged to obtain the equivalent of the quietest day TEC, Qg. The Q. was then subtracted from the storm-time Faraday rotation angle to obtain deviations from the quiet-day levels. The deviations have been plotted as Aa in all the plots shown. To investigate what happens during day-time, night-time and early morning pre-sunrise period, AIR was averaged over IPI6 hr, 22-24 hr and 3-5 hr, respectively. They appear in the plots as QD, a, and QM respectively. In addition, we have plotted the daily planetary AZ, index in the graph. For easy reference, the symbols used are listed together as follows: An = deviation of storm time Faraday rotation angle from mean quiet day Faraday rotation. sl, = mean of As1 taken over the daytime period 14-16 hr. R, = mean of A51 over night-time period 22-24 hr. R, = mean of Aa over pre-sunrise period 3-5 hr. ZKp = sum of planetary magnetic Kp index over one day. Ap = daily planetary magnetic index.
8. RESULTS The general results are listed in Table 2. From the plots the following comments can be made on the various storms studied. Each graph has an error bar indicating the standard deviation calculated for the days used in calculating the mean quiet day rotation levels. All changes equal to or larger than the standard deviation should be regarded as significant.
47
Storm 1: 21 July-4 August 1968 The storm shown in Fig. 1 is a marginal one with a maximum EKp index of 31 and lasting for 4 days. s2, shows an increase during the storm while R, and Sz, do not show any significant trend except for the large fluctuations in Sz, in the post storm period.
“s? -60 MO ?: yx) ‘N CJ G 0 4; *“o -Pm 29
21
1
2
&)&
Fig. 1. Storm-time changes in total electron content for magnetic storm occurring between 21 July4 August 1969. Stomz 2:
17 September-15 October1969
With a maximum TZKp of 46, this is a strong storm lasting for approximately 5 days. There is a marked increase in sl,, a0 and R, during the storm as can be seen from Fig. 2. Storm 3: 6-19 August 1970 The maximum ZKp is 61 indicating a very strong storm. Several days before the storm, Fig. 3, there was a general depression observed in SIN, R, and Sz,. However, in the middle of the storm, they all show an increasing trend, eventually becoming positive at the tail end of the storm. The record shows a set of storms at the end of July preceding the main storm and it is not known
Fig. 2. Storm-time changes in total electron content for the magnetic storm occurring between 17 September-16 October 1969.
D. YEBOAE-AMANKWAH
48 whether
this has infiuenced
during the major
storm,
the behaviour
observed
of TEC
storm while in fi,
and &,, the increase disappears
soon after the storm ae shown in Fig. 6.
in August.
This storm is unusual for it shows a slight depreasion in Q,
and no signif%cant values
of s2, and
s.9 “D.;
s2, during the storm.
STORM3:6-19AUWST “Me;
:
, .
np JO
1970.
40
A
_ T
T
%
40”
=I
T
Kloo
An
0 -DO
FEBRUARY
JANUARY
Fig. 6. Storm-time changes in total electron content for the magnetic storm occurring between 17 Jannary6 February 1971. St0rr.n 6:
l-16
October 1971
The maximum storm Fig. 3. Storm-time changes in total electron content for the magnetic storm occurring between 6-19 August 1970. Stoma 4: A Ap
lo-20
December
1970
strong
storm,
modestly
nM and Q,.
increases
in the caee of
are decaying
during the storm and in fact reach negative just after the storm.
loo
STORM
10
The provides
a neat
during storms.
values
4 is shown in Fig.
16 14 DECEMBER
lost
the storm.
values of sl,,
It appears
positive
for
main storm has passed. is not a neat one, very present.
The
after
by
there is an increase
shown by the positive remain
one and some
as indicated
gaps, aa
Sz, and Q,
that all the param-
several
days
after
the
On the whole,
this storm
large variations
in AhR are
effects
curring on 27 September
the
in TEC
of an earlier
storm
oc-
1971, may be responsible
for this behaviour.
4.
16
Fig. 6. Storm-time for the magnetic
20
changes in total electron content storm occurring between 10-20 December 1970.
17 Januaqp6
maximum
away
were
for a day of the
is a weak
4:IO;20DE
I2
Fig. 4. Storm-time for the magnetic
Storm 6:
Storm
and
IZKp = 40
X.&u occurring
The storm
6, however,
during
It is observed
S& and $,, that the increases
data
in Fig.
eters
= 116; this storm records large TEC
in sl,,
TEC
is 36.
February
XKp
for
example QN, sl,
this
of an increase
In the case of R,,
to remain
for some
days
It
of TEC
the increase appears
after
The maximum rise
is 37.
and fi2, all show marked
increases.
18 Noventber-2 EKp
Fig. 7, was 38. !+,
1971 storm
Storna 7:
changes in total electron content storm occurring between 1-15 October 1971.
the period
of the
during
the
particularly
December
negative maximum
The
all show a positive increase
and indicates
crease in the background the smallest
during the storm,
occurring
Sa, and R,
storm.
large
TEC.
but it is signi&ant.
values
at the beginning
positive
1971
R,
is
a substantial
in
in-
The rise in a, sl,
grows
of the storm
values at the middle.
is
from to
49
Increases of equatorial total electron content (TEC) during magnetio storms STORM7:18-30 NOVEMBER 100 %_i “0
7 -50 LOO
"N _; 20 0 20 0
*P Ma
T loo
An
0
-00 16
22
20
24
28
30
NOVEMBER
Fig. 7. Storm-time changes in total electron for the
Storm
magnetic storm occurring between vember-2 December 1971. 8:
26 October-9
content 18 No-
November 1972
The maximum Kp observed for the storm was 46 oorresponding to Ap = 98 which represents a strong storm. SZD end QN show large positive values before and during the storm but are observed to be decaying away at the tail-end of nM shows a simple large positive the storm. increase during the period of the storm. The An shows very large fluctuations in the pre-storm period.
I
STORM% 26 OCT-9 NO” 1972
OCTOBER
6, the increase persists after the storm has died away. (b) The night-time background TEC change, characterised by Sz,, appears to show the greatest percentage increase during a magnetio storm. Generally, the inoreaee is of the order of 100% or more. (c) When a combination of minor and a major storm occurs with the minor preceding the major storm by a few days, the TEC may fluotuate widly or be inhibited. Storms 3 and 6 illustrate this behaviour. It appears that the average increase of TEC during times of intense magnetio activity is a well established phenomenon. In an unpublished study, KOSTER (1975) has calculated hourly correlation coefficients using TEC and planetary Kp indices over 3 year periods. As his results in Fig. 9 show, there is a positive correlation between Kp and TEC for all hours.
Y
OO
I
6
I
I
12
18
Fig. 9. Variation of correlation coefficient with time. The dotted line is the 99% confidence level.
I
The origin of these increases in TEC is generally accepted to be complex in nature and requires more research to illuminate it. It is however expected that a correct physical explanation of the phenomenon should take account of the roles of electromagnetic E x B drift of the Pa-layer (RAS~OUI et al. 1971; RAGEIAVAEAOet al.,1973; WOODU et al., 1972), and the differential heating of the ionosphere which leads to atomic and molecular mixing of oxygen (THOMAS et al.. 1966). Both of these affect the recombination of ions in the ionosphere and therefore affect the life times of electrons during storms.
NOVEMBER
Fig. 8. Storm-time changes in total electron content for the magnetic storm occurring between 26 October9 November 1972. 4. COBCLUSIOa
The summary of the results may be made as follows: (a) There is a general rise in TEC during equatorial storms. Sometimes, as exemplitkl by storm
AcknoluledgementThe author’s thanks are due to Drs. J. R. KOSTEBand T. BEERwith whom he has had very useful ditIoua3ions. The work reported in this paper wae supported in part by the Air Force Cambridge Laboratories, through the European Office of Aerospace Research (O.A.R.), United States Air-Force contract No. AFOSR-72-2268.
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D. YEBOAE-UKWAE
MENDILLO M. NAC+ATAY. RACUXAVARAO R. and SIVARAIUN M. R. RAsTOoI R. G., CEANDRA H. and Mrsaa R. K. SOFIODELJ. P. et d. TAYLOX G. N. THOU L. end NORTON R. B. WALKES G. 0. WOODMAN R. F. STERLINO D. L. snd HANSON W. B. YEBOAH-AIUANKWAH D. a4d KOSTER J. R.
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Pz4nW.t.S$lace Soi. 21,345. Radio Sci. 1,1145. J. atmo8. tew. Phya. 85,209l. N&we., Land. a88, 13. J. atmoe. kw. Phga. 86, 1121. Natura, Lvnd. I@, 7401 J. awchva. Res. 71.227. J. &k~ tew. Phyi. 85, 1573. Radio Sci. 7, 739
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Reference ia al80 made to the following unpublished material: KOSTEX J. R.
1976
Data provided from an on-going study.