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On the equatorial electrojet influence on geomagnetic pulsation amplitudes
1995 Copyright 0 1995ElsevierScienceLtd Printed in Great Britain. All rights reserved 00%9169/95 S9.50+0.00
Journal 01 nrmospnerrc and Terrestrial Physics, Vol. 57, No. 7, pp. 14%754,
0021-9169(94)00053-O
On the Equatorial electrojet influence on geomagnetic pulsation amplitudes S. V. National
Geophysical
(Received
S. Sarma and T. S. Sastry Research
Institute,
Hyderabad-500
007, India
infinal form 17 March 1994 ; accepted 22 April 1994)
Abstract-It is well known that several types of geomagnetic pulsations show a significant amplitude enhancement near the dip equator due to the daytime equatorial electrojet. In the present study, the dependence of this enhancement on the period and type of geomagnetic variations is examined. The results show that, in general, the amplitude enhancement appears to be more or less uniform, amounting to a factor of 2.~-2.5, over a wide range of periods. However, for pulsations, there is a fairly sharp cut-off of the equatorial enhancement around a 20 s period, the shorter period end of Pc3 pulsations. Further, shorter period pulsations (< 20 s) sometimes suffer an attenuation at the dip equator near noon. These results are discussed in the light of the transmission characteristics of the ionosphere, including the possible relation to the equatorial anomaly in the ionospheric F-region.
1. INTRODUCTION
et al. (1979) established the influence of the Sastry Equatorial Electrojet (EEJ) on Pc3 and Pc4 pulsations in the form of an enhancement of signal amplitudes at the dip equator from a study of pulsations simultaneously recorded at Choutuppal (geomagnetic latitude 7.5) and Etaiyapuram (geomagnetic latitude -0.6). It was shown that the enhancement factor attains a value of 2.e2.5 around 1100 h LT for both Pc3 and Pc4 pulsations. In a later investigation dealing with the daytime occurrence of Pi2 pulsations at low latitudes, Sastry et ul. (1983) not only established the occurrence of Pi2 pulsations during daytime at Choutuppal and Etaiyapuram, but also showed that these pulsations are also influenced by the EEJ in a manner similar to Pc3 and F’c4 pulsations. The EEJ is known to exercise considerable influence on geomagnetic variations over a wide range of periods (apart from Sq variations) from geomagnetic fluctuations with periods of several minutes to Pc3 pulsations having periods of some tens of seconds. The purpose of the present study is to examine if a similar influence extends to pulsations of periods well below 30 s and also to investigate the dependence, if any, of the enhancement factor on the period of geomagnetic pulsations. The investigation is based on data from the two stations Choutuppal, a sub-equatorial station, close to the dip equator. and Etaiyapuram, Choutuppal (about 60 km east of Hyderabad) and Etaiyapuram, the geographic locations of which are given in Fig. 1, form a unique pair of stations in the Indian Equatorial region recording magnetic pul-
Fig. 1. Map showing the location of the pulsation atories at Choutuppal (CHT) and Etaiyapuram India.
observ(ETT),
sations on identical system of recording equipment-a three-component induction magnetometer set-up with an analogue system of registration by photographic means (Sarma et al., 1969). The recording system covers the whole range of pulsation periods (0.21000 s). While the longer period band of pulsations with periods from 2&1000 s is recorded at a chart speed of 90 mm/h, the shorter period band of pulsations in the period range 0.2-20 s is registered sep749
750
S. V. S. Sanna and T. S. Sastry
_
1256UT
--
0829
ET
0928 UT
0227 ET
--
IS/2 / 78 CH
ET
a
I
0.20
nT
ot 0.5 H 2
*,-+,e,d
Cl-l 0700
0659
UT
0659
‘e-
ET
Fig. 2. Short-period magnetic pulsations from simultaneous recordings at Choutuppal and Etaiyapuram.
arately on a faster recorder (usually referred to as Ultra Quick Run, UQR) at a chart speed of 30mm/min.
2.
DATAANALYSISANDRESULTS
From an examination of simultaneous UQR records from the two stations Choutuppal and Etaiyapuram, over the three years from October 1975 to December 1978, intervals of clear pulsation signals in Hx component with periods less than 20 s covering a range of local time were identified. A few typical examples of these pulsations occurring simultaneously at the two stations are shown in Fig. 2 for different local times. The traces of the pulsation events refer to the Hx component at Choutuppal (upper) and Etaiyapuram (lower) as indicated. The progress of the event is from right to left as indicated by the successive minute time marks (a break in the trace), the spacing between which is 30 mm. The scale value is given at the lower left bottom portion of the figure and is ;:le same for both stations for the interval chosen for these events. A cursory glance at this figure shows that Hx amplitudes at Etaiyapuram are reduced (contrary to an expected enhancement under the jet as in the case of Pc3 and 4) compared with those at Choutuppal during the hours around local noon (the examples on the right half of the figure), while they show a tendency to be either equal to or slightly greater than those at
Choutuppal around dusk (those on the left half of the figure). To examine the local time dependence further, the amplitudes and periods of signals in Hx at both stations were measured. When trains of pulsations were observed at both stations, the amplitudes of three or more successive pulses were measured and the amplitudes averaged. In cases where only a single pulse is available, the amplitude of a crest or a trough, whichever is very clear on both the records, is measured. Then, the parameter R,,the ratio of the signal amplitudes in Hx at Etaiyapuram to that at Choutuppal, was computed for different events observed during different local time intervals. The events are more or less uniformly distributed over different local hour intervals with an average of about 15-20 values in each hourly interval, though they were relatively less in number between 07.30 and 10.30 LT and larger in number from 17.30 to 18.30 LT. The ratios for these shorter period pulsations (T < 20 set) falling in each of the local hourly intervals are averaged, and these average values along with the error bars plotted against local time are presented in Fig. 4. Similar plots obtained for Pc3, PC4 and Pi2 pulsations from earlier studies are also shown in Fig. 4. The smooth curves, which pass through most of the points in each of the plots in this figure, are obtained through a polynomial fit. In order to extend these studies further so as to
Equatorial geomagnetic pulsation amplitudes
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;;1.,. DAY
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Fig. 3. Examples of geomagnetic fluctuations (a) and Storm Sudden Commencements (b) recorded simultaneously at Choutuppal and Etaiyapuram.
include the longer period phenomena as well, the amplitudes of SSC,s and also other geomagnetic fluctuations in the H-component occurring during different local time intervals were measured from the La Cour magnetograms of Geomagnetic Observatories at Hyderabad (about 60 km west of Choutuppal) and at Etaiyapuram. A few examples of SSCs and also magnetic fluctuations (T z a few minutes to an hour) in H-component recorded simultaneously at Hyderabad and Etaiyapuram are presented in Fig. 3 for the local night and day hours. The enhancement of amplitudes at Etaiyapuram for daytime events can be readily seen from this figure. The computation of the enhancement factor (RH), in the case of these two
751
types of geomagnetic variations, was done in a similar way as in the case of pulsations, and the resulting plots of curves depicting local time dependence of R, are also shown in Fig. 4. Figure 4 thus presents a composite picture, demonstrating the local-time variation of the enhancement factor RH which represents the nature of electrojet influence on various types of geomagnetic phenomena covering a wide period range starting from the geomagnetic fluctuations of long periods down to pulsations of very short periods (T< 20 set). A common feature that could be immediately observed is a rise in R, during local noon hours around 11.0&12.00 h LT, with the mean value of RH reaching a maximum of about 2.0-2.5 for all the types of variations considered except in the case of the last one, namely, the pulsations with periods less than 20 s. It may also be seen that the rise in RH, which is broad for the longer period phenomena like fluctuations and Pi2 pulsations (excepting the SSCs), becomes narrower for pulsations with decreasing period and finally is absent in the case of very short period pulsations (< 20 s) during a major part of the daylight hours. Thus, while R, reaches a value of 2.G2.5 for longer period variations from geomagnetic fluctuations down to pulsations of Pc3 class with periods greater than 30 s, this parameter for the shorter periods (T < 20 set) diminishes rapidly to a value close to 1.O (0.95) near 11.OOh LT and tends to decrease thereafter towards afternoon hours, suggesting an attenuation of these very short period signals at the dip equator. Another feature that may be noticed from this figure is that the RH for these short period pulsations shows a minor rise (- 1.8) at the dawn hours around 05.00 h LT and again shows a tendency to rise during the dusk hours (18.0&19.00 h LT). In order to bring out the feature more clearly, the R, value for the local time interval centred at 12.00 h LT for each type of phenomenon picked up from Fig. 4 is plotted against period (each value having been plotted at the dominant period observed in the range corresponding to each type of phenomenon, indicated by the long solid bars at the bottom of the figure) and shown in Fig. 5. These points, which are discrete values, are joined by a smooth dashed curve. It is immediately apparent from this plot that the RH representing the enhancement factor, which is more or less uniform and amounting to around 2.s2.5 over a wide range of periods up to those of Pc4 and Pc3 pulsations, declines to a value of 0.95 for pulsations of short periods (< 20 s), decreasing further to a value of about 0.70 at periods around 5.0 s. Thus, the results of this study reveal the existence of a cut-off in the enhancement factor at the dip equator under the equa-
S. V. S. Sarma and T. S. Sastry
752
IW
2 -
2eo 2.00
I
I
I
2.00
r 0.00
II 4.00
PC,PI ( 5-20s)
6.00
12.00 klrr
16.00
20.00
24.00
LT
Fig. 4. Composite picture depicting the local-time variation of RH (Hx component amplitudes at Etaiyapuram normalised with respect to those at Choutuppal) for different types of geomagnetic phenomena.
Fig. 5. Mid-day
values of the RH factor for different
geomagnetic
phenomena
over a wide range of periods.
Equatorial geomagnetic pulsation amplitudes torial electrojet, with the cut-off located around periods a little less than 20 s.
3. DISCUSSION
The Pc3 and 4 magnetic pulsations observed in the equatorial and low latitude regions are generally considered to correspond to the magnetospheric signals of the poloidal mode that could propagate across the magnetic lines of force towards the low and equatorial region (e.g. Orr, 1’373; Yumoto, 1986). However, it was shown from recent studies that pulsations at low latitudes also exhibit characteristics of toroidal mode resonances (e.g. Waters et al., 1991). The theoretical studies of Poulter et al., (1988) further suggest that the ULF pulsations in the Pc3 and 4 range at lower latitudes exhibit a diurnal variation of periods, with longer periods appearing during daytime and relatively shorter periods during night-time, reflecting a dominant effect of O+ ions of daytime ionosphere at very low latitudes. It is also more or less established that some of the very short period pulsations (T N 5 set) like pearl pulsations (PP) reach the equatorial region with signals propagating from higher to lower latitudes (during night-time through a duct centred on the F-region peak of the ionosphere (Tepley and Landshoff, 1966). Also, in the case of some geomagnetic phenomena like SSCs, the possibility of an instantaneous transmission of polar electric field signals from higher to lower latitudes through a zero order TM mode of propagation has been suggested (Kikuchi and Araki, 1979). Whatever may be the mode of transmission of pulsation signals towa.rds the equator, it is a feature of common observation that a wide spectrum of geomagnetic phenomena including magnetic pulsations have enhanced amplitudes at the dip equator during the daytime hours, indicating an enhancement of the ionospheric current system through the influence of the equatorial electrojet. In fact, it was clearly established from earlier studies (Sastry et al., 1979) that both Pc3 and Pc4 pulsations (in the period range 3&150 s) show a significant enhancement during the daytime at the dip equator, pointing to the influence of the enhanced ionospheric conductivity in the equatorial electrojet region. On the other hand, the results of the present study suggest that pulsations with periods less than 20 s provide a conspicuously different picture, namely, a (cut-off in the enhancement factor corresponding to this shorter period range during midday. It was shown from theoretical considerations that
753
the ionosphere plays a major role in the modification of pulsation characteristics during their transmission through the magnetosphereionosphere system (Hughes, 1974; Hughes and Southwood, 1976; Newton et al., 1978). Considering this aspect and the observational results pertaining to the local time variation of the enhancement factor for various types of geomagnetic variations, including pulsations at the dip equator, presented here as a composite picture, it may be conjectured that the anomalous behaviour in the case of short period pulsations (T < 20 s) should also in some way be related to the influence of the equatorial ionosphere. In particular, the observed result, namely, the decrease of the RH factor at the equator at shorter periods in contrast to pulsations above a 20 s period during a major part of the daytime lends credence to such an inference. Further, the tendency for a rise in R,, particularly during dawn hours, indicates the additional control exercised by the ionospheric conditions. It is important to recall in this context the existence of the “equatorial ionization anomaly” in the F-region, the “foF2 anomaly” (Rastogi, 1960,199O). Rastogi (1990) showed that thecrest of this foF2 anomaly, located on the dip equator around sunrise, moves towards the sub-equatorial zone, reaching a latitude of about 35” by midday ; the anomaly crest then moves back towards the equator during the afternoon hours. It is interesting to note that the variation of the R, factor in the case of short period pulsations also exhibits a similar pattern, namely a rise around dawn, a relatively low value (1 .O) around midday and again a minor rise during the evening hours (15-18 h), corresponding to the movement of the crest of the foF2 towards the equator. The results of the study thus clearly bring out the distinct period dependence of the amplitude enhancement of pulsations under the equatorial electrojet, the enhancement factor showing a fairly sharp cut-off around 20 s. The results indicate a clear-cut boundary between the longer (T > 20 s) and shorter period pulsations (T < 20 s) from the viewpoint of the influence of the equatorial ionosphere during their transmission towards the equator. The amplitude modulation of the short period pulsations at the dip equator also shows a close correlation with the pattern of latitudinal movements of the foF2 equatorial ionisation anomaly, suggesting a possible role of the F-region in the transmission of shorter period pulsations. Acknowledgements-The authors are grateful to the Director, National Geophysical Research Institute, Hyderabad, for giving permission to publish the results of this study.
154
S. V. S. Sarma and T. S. Sastry REFERENCES
Hughes W. J.
1974
Hughes W. J. and Southwood D. J.
1976
Kikuchi T. and Araki T.
1979
Newton R. S., Southwood D. J. and Hughes W. J.
1978
Orr D.
1973
Poulter E. M., Allan W. and Bailey G. J.
1988
Rastogi R. G.
1960
Rastogi R. G.
1990
Sarma Y. S., Sankemarayan P. V., Ramanujachary K. R., Sarma S. V. S., Zybin K. Yu and Makaraov V. I.
1969
Sastry T. S., Sarma Y. S. and Sarma S. V. S.
1979
Sastry T. S., Sarma Y. S., Sarma S. V. S. and Sanker Narayan P. V. Tepley L. R. and Landshoff R. K.
1983
Waters C. L., Menk F. W. and Fraser B. J.
1991
Yumoto K.
1986
The effect of the atmosphere and ionosphere on long period magnetospheric micropulsations. Planer. Space Sci. 22, 1157-l 172. The screening of micropulsation signals by the atmosphere and ionosphere. .I. geophys. Res. 81, 3234 3240.
Horizontal transmission of the polar electric field to the equator. J. atmos. terr. Phys. 41,927-936. Damping of geomagnetic pulsations by the ionosphere. Planet. Space Sci. 26,201-209.
Magnetic pulsations within the Magnetosphere ; A review. J. atmos. terr. Phys. 35, l-50. ULF pulsation Eigen periods within the Plasmasphere. Planet. Space Sci. 36, 185-196.
A synoptic study of the F2 region of the ionosphere in the Asian zone. J. atmos. terr. Phys. l&315-331. Equatorial ionosphere. Ind. J. Rad. Space Phys. 19, 41@423. Three component induction magnetometer for recording geomagnetic micropulsations at the Geoelectric observatory at Choutuppal. Bull. NGRI 7, 51-65.
Equatorial electrojet effects on Geomagnetic pulsations. Ind. J. Rad. Space Phys. 8,249-253. Daytime Pi pulsations at equatorial latitudes. J. atmos. terr. Phys. 45,733-741.
1966
Wave-guide theory for ionospheric propagation of hydromagnetic emission. J. geophys. Res. 71,1499-1504. The resonance structure of low latitude Geomagnetic pulsations. Geophys. Res. Lett. l&2293-2296. Generation mechanism of Pc3 magnetic pulsations at very low latitudes. Planet. Space. Sci. 34, 1329-1334.
Journal 01 nrmospnerrc and Terrestrial Physics, Vol. 57, No. 7, pp. 14%754,
0021-9169(94)00053-O
On the Equatorial electrojet influence on geomagnetic pulsation amplitudes S. V. National
Geophysical
(Received
S. Sarma and T. S. Sastry Research
Institute,
Hyderabad-500
007, India
infinal form 17 March 1994 ; accepted 22 April 1994)
Abstract-It is well known that several types of geomagnetic pulsations show a significant amplitude enhancement near the dip equator due to the daytime equatorial electrojet. In the present study, the dependence of this enhancement on the period and type of geomagnetic variations is examined. The results show that, in general, the amplitude enhancement appears to be more or less uniform, amounting to a factor of 2.~-2.5, over a wide range of periods. However, for pulsations, there is a fairly sharp cut-off of the equatorial enhancement around a 20 s period, the shorter period end of Pc3 pulsations. Further, shorter period pulsations (< 20 s) sometimes suffer an attenuation at the dip equator near noon. These results are discussed in the light of the transmission characteristics of the ionosphere, including the possible relation to the equatorial anomaly in the ionospheric F-region.
1. INTRODUCTION
et al. (1979) established the influence of the Sastry Equatorial Electrojet (EEJ) on Pc3 and Pc4 pulsations in the form of an enhancement of signal amplitudes at the dip equator from a study of pulsations simultaneously recorded at Choutuppal (geomagnetic latitude 7.5) and Etaiyapuram (geomagnetic latitude -0.6). It was shown that the enhancement factor attains a value of 2.e2.5 around 1100 h LT for both Pc3 and Pc4 pulsations. In a later investigation dealing with the daytime occurrence of Pi2 pulsations at low latitudes, Sastry et ul. (1983) not only established the occurrence of Pi2 pulsations during daytime at Choutuppal and Etaiyapuram, but also showed that these pulsations are also influenced by the EEJ in a manner similar to Pc3 and F’c4 pulsations. The EEJ is known to exercise considerable influence on geomagnetic variations over a wide range of periods (apart from Sq variations) from geomagnetic fluctuations with periods of several minutes to Pc3 pulsations having periods of some tens of seconds. The purpose of the present study is to examine if a similar influence extends to pulsations of periods well below 30 s and also to investigate the dependence, if any, of the enhancement factor on the period of geomagnetic pulsations. The investigation is based on data from the two stations Choutuppal, a sub-equatorial station, close to the dip equator. and Etaiyapuram, Choutuppal (about 60 km east of Hyderabad) and Etaiyapuram, the geographic locations of which are given in Fig. 1, form a unique pair of stations in the Indian Equatorial region recording magnetic pul-
Fig. 1. Map showing the location of the pulsation atories at Choutuppal (CHT) and Etaiyapuram India.
observ(ETT),
sations on identical system of recording equipment-a three-component induction magnetometer set-up with an analogue system of registration by photographic means (Sarma et al., 1969). The recording system covers the whole range of pulsation periods (0.21000 s). While the longer period band of pulsations with periods from 2&1000 s is recorded at a chart speed of 90 mm/h, the shorter period band of pulsations in the period range 0.2-20 s is registered sep749
750
S. V. S. Sanna and T. S. Sastry
_
1256UT
--
0829
ET
0928 UT
0227 ET
--
IS/2 / 78 CH
ET
a
I
0.20
nT
ot 0.5 H 2
*,-+,e,d
Cl-l 0700
0659
UT
0659
‘e-
ET
Fig. 2. Short-period magnetic pulsations from simultaneous recordings at Choutuppal and Etaiyapuram.
arately on a faster recorder (usually referred to as Ultra Quick Run, UQR) at a chart speed of 30mm/min.
2.
DATAANALYSISANDRESULTS
From an examination of simultaneous UQR records from the two stations Choutuppal and Etaiyapuram, over the three years from October 1975 to December 1978, intervals of clear pulsation signals in Hx component with periods less than 20 s covering a range of local time were identified. A few typical examples of these pulsations occurring simultaneously at the two stations are shown in Fig. 2 for different local times. The traces of the pulsation events refer to the Hx component at Choutuppal (upper) and Etaiyapuram (lower) as indicated. The progress of the event is from right to left as indicated by the successive minute time marks (a break in the trace), the spacing between which is 30 mm. The scale value is given at the lower left bottom portion of the figure and is ;:le same for both stations for the interval chosen for these events. A cursory glance at this figure shows that Hx amplitudes at Etaiyapuram are reduced (contrary to an expected enhancement under the jet as in the case of Pc3 and 4) compared with those at Choutuppal during the hours around local noon (the examples on the right half of the figure), while they show a tendency to be either equal to or slightly greater than those at
Choutuppal around dusk (those on the left half of the figure). To examine the local time dependence further, the amplitudes and periods of signals in Hx at both stations were measured. When trains of pulsations were observed at both stations, the amplitudes of three or more successive pulses were measured and the amplitudes averaged. In cases where only a single pulse is available, the amplitude of a crest or a trough, whichever is very clear on both the records, is measured. Then, the parameter R,,the ratio of the signal amplitudes in Hx at Etaiyapuram to that at Choutuppal, was computed for different events observed during different local time intervals. The events are more or less uniformly distributed over different local hour intervals with an average of about 15-20 values in each hourly interval, though they were relatively less in number between 07.30 and 10.30 LT and larger in number from 17.30 to 18.30 LT. The ratios for these shorter period pulsations (T < 20 set) falling in each of the local hourly intervals are averaged, and these average values along with the error bars plotted against local time are presented in Fig. 4. Similar plots obtained for Pc3, PC4 and Pi2 pulsations from earlier studies are also shown in Fig. 4. The smooth curves, which pass through most of the points in each of the plots in this figure, are obtained through a polynomial fit. In order to extend these studies further so as to
Equatorial geomagnetic pulsation amplitudes
Q
1lf3flWO
d-
14/10/l980
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WO pii&
I
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ETT
W/?11@80 3011214S80
+
/
IO UT
HYO14(lnT -J-J_ “yo
‘+fETTknl NIGHT
TIME:
;;1.,. DAY
TIME
Fig. 3. Examples of geomagnetic fluctuations (a) and Storm Sudden Commencements (b) recorded simultaneously at Choutuppal and Etaiyapuram.
include the longer period phenomena as well, the amplitudes of SSC,s and also other geomagnetic fluctuations in the H-component occurring during different local time intervals were measured from the La Cour magnetograms of Geomagnetic Observatories at Hyderabad (about 60 km west of Choutuppal) and at Etaiyapuram. A few examples of SSCs and also magnetic fluctuations (T z a few minutes to an hour) in H-component recorded simultaneously at Hyderabad and Etaiyapuram are presented in Fig. 3 for the local night and day hours. The enhancement of amplitudes at Etaiyapuram for daytime events can be readily seen from this figure. The computation of the enhancement factor (RH), in the case of these two
751
types of geomagnetic variations, was done in a similar way as in the case of pulsations, and the resulting plots of curves depicting local time dependence of R, are also shown in Fig. 4. Figure 4 thus presents a composite picture, demonstrating the local-time variation of the enhancement factor RH which represents the nature of electrojet influence on various types of geomagnetic phenomena covering a wide period range starting from the geomagnetic fluctuations of long periods down to pulsations of very short periods (T< 20 set). A common feature that could be immediately observed is a rise in R, during local noon hours around 11.0&12.00 h LT, with the mean value of RH reaching a maximum of about 2.0-2.5 for all the types of variations considered except in the case of the last one, namely, the pulsations with periods less than 20 s. It may also be seen that the rise in RH, which is broad for the longer period phenomena like fluctuations and Pi2 pulsations (excepting the SSCs), becomes narrower for pulsations with decreasing period and finally is absent in the case of very short period pulsations (< 20 s) during a major part of the daylight hours. Thus, while R, reaches a value of 2.G2.5 for longer period variations from geomagnetic fluctuations down to pulsations of Pc3 class with periods greater than 30 s, this parameter for the shorter periods (T < 20 set) diminishes rapidly to a value close to 1.O (0.95) near 11.OOh LT and tends to decrease thereafter towards afternoon hours, suggesting an attenuation of these very short period signals at the dip equator. Another feature that may be noticed from this figure is that the RH for these short period pulsations shows a minor rise (- 1.8) at the dawn hours around 05.00 h LT and again shows a tendency to rise during the dusk hours (18.0&19.00 h LT). In order to bring out the feature more clearly, the R, value for the local time interval centred at 12.00 h LT for each type of phenomenon picked up from Fig. 4 is plotted against period (each value having been plotted at the dominant period observed in the range corresponding to each type of phenomenon, indicated by the long solid bars at the bottom of the figure) and shown in Fig. 5. These points, which are discrete values, are joined by a smooth dashed curve. It is immediately apparent from this plot that the RH representing the enhancement factor, which is more or less uniform and amounting to around 2.s2.5 over a wide range of periods up to those of Pc4 and Pc3 pulsations, declines to a value of 0.95 for pulsations of short periods (< 20 s), decreasing further to a value of about 0.70 at periods around 5.0 s. Thus, the results of this study reveal the existence of a cut-off in the enhancement factor at the dip equator under the equa-
S. V. S. Sarma and T. S. Sastry
752
IW
2 -
2eo 2.00
I
I
I
2.00
r 0.00
II 4.00
PC,PI ( 5-20s)
6.00
12.00 klrr
16.00
20.00
24.00
LT
Fig. 4. Composite picture depicting the local-time variation of RH (Hx component amplitudes at Etaiyapuram normalised with respect to those at Choutuppal) for different types of geomagnetic phenomena.
Fig. 5. Mid-day
values of the RH factor for different
geomagnetic
phenomena
over a wide range of periods.
Equatorial geomagnetic pulsation amplitudes torial electrojet, with the cut-off located around periods a little less than 20 s.
3. DISCUSSION
The Pc3 and 4 magnetic pulsations observed in the equatorial and low latitude regions are generally considered to correspond to the magnetospheric signals of the poloidal mode that could propagate across the magnetic lines of force towards the low and equatorial region (e.g. Orr, 1’373; Yumoto, 1986). However, it was shown from recent studies that pulsations at low latitudes also exhibit characteristics of toroidal mode resonances (e.g. Waters et al., 1991). The theoretical studies of Poulter et al., (1988) further suggest that the ULF pulsations in the Pc3 and 4 range at lower latitudes exhibit a diurnal variation of periods, with longer periods appearing during daytime and relatively shorter periods during night-time, reflecting a dominant effect of O+ ions of daytime ionosphere at very low latitudes. It is also more or less established that some of the very short period pulsations (T N 5 set) like pearl pulsations (PP) reach the equatorial region with signals propagating from higher to lower latitudes (during night-time through a duct centred on the F-region peak of the ionosphere (Tepley and Landshoff, 1966). Also, in the case of some geomagnetic phenomena like SSCs, the possibility of an instantaneous transmission of polar electric field signals from higher to lower latitudes through a zero order TM mode of propagation has been suggested (Kikuchi and Araki, 1979). Whatever may be the mode of transmission of pulsation signals towa.rds the equator, it is a feature of common observation that a wide spectrum of geomagnetic phenomena including magnetic pulsations have enhanced amplitudes at the dip equator during the daytime hours, indicating an enhancement of the ionospheric current system through the influence of the equatorial electrojet. In fact, it was clearly established from earlier studies (Sastry et al., 1979) that both Pc3 and Pc4 pulsations (in the period range 3&150 s) show a significant enhancement during the daytime at the dip equator, pointing to the influence of the enhanced ionospheric conductivity in the equatorial electrojet region. On the other hand, the results of the present study suggest that pulsations with periods less than 20 s provide a conspicuously different picture, namely, a (cut-off in the enhancement factor corresponding to this shorter period range during midday. It was shown from theoretical considerations that
753
the ionosphere plays a major role in the modification of pulsation characteristics during their transmission through the magnetosphereionosphere system (Hughes, 1974; Hughes and Southwood, 1976; Newton et al., 1978). Considering this aspect and the observational results pertaining to the local time variation of the enhancement factor for various types of geomagnetic variations, including pulsations at the dip equator, presented here as a composite picture, it may be conjectured that the anomalous behaviour in the case of short period pulsations (T < 20 s) should also in some way be related to the influence of the equatorial ionosphere. In particular, the observed result, namely, the decrease of the RH factor at the equator at shorter periods in contrast to pulsations above a 20 s period during a major part of the daytime lends credence to such an inference. Further, the tendency for a rise in R,, particularly during dawn hours, indicates the additional control exercised by the ionospheric conditions. It is important to recall in this context the existence of the “equatorial ionization anomaly” in the F-region, the “foF2 anomaly” (Rastogi, 1960,199O). Rastogi (1990) showed that thecrest of this foF2 anomaly, located on the dip equator around sunrise, moves towards the sub-equatorial zone, reaching a latitude of about 35” by midday ; the anomaly crest then moves back towards the equator during the afternoon hours. It is interesting to note that the variation of the R, factor in the case of short period pulsations also exhibits a similar pattern, namely a rise around dawn, a relatively low value (1 .O) around midday and again a minor rise during the evening hours (15-18 h), corresponding to the movement of the crest of the foF2 towards the equator. The results of the study thus clearly bring out the distinct period dependence of the amplitude enhancement of pulsations under the equatorial electrojet, the enhancement factor showing a fairly sharp cut-off around 20 s. The results indicate a clear-cut boundary between the longer (T > 20 s) and shorter period pulsations (T < 20 s) from the viewpoint of the influence of the equatorial ionosphere during their transmission towards the equator. The amplitude modulation of the short period pulsations at the dip equator also shows a close correlation with the pattern of latitudinal movements of the foF2 equatorial ionisation anomaly, suggesting a possible role of the F-region in the transmission of shorter period pulsations. Acknowledgements-The authors are grateful to the Director, National Geophysical Research Institute, Hyderabad, for giving permission to publish the results of this study.
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