Observatories in India

Transcription

1 J. Geomag. Geoelectr., 26, , 1974 On the Occurrence of SSC(-+) Observatories in India at Geomagnetic R.G. RASTOGI* and N.S. SASTRI** Physical Research Laboratory, Ahmedabad, India* and Indian Institute of Geomagnetism, Colaba, Bombay, India** (Received July 22, 1974; Revised December 25, 1974) It is shown that about half of the sudden commencements in geomagnetic field at any equatorial station during the midday hours are preceded by a preliminary negative excursion. The negative excursions are seen in all the three components of the geomagnetic field. The amplitude of negative excursion decreases with increasing latitude much faster than the amplitude of main positive excursion. It is suggested that the mechanism of the generation and enhancement of the negative excursion over the magnetic equator is to be sought in the ionospheric region itself, and the height of the equivalent current system is slightly lower than that of the Sq current system. 1. Introduction Typical geomagnetic storms usually begin with a sudden increase of the H field almost simultaneously at most of the observatories in the world. This short impulse is known as Sudden Commencement, denoted as SC or SSC. NEWTON (1948) was first to note that there are three main types of SC; (i) the ordinary SC in which the sudden change in H is an increase, (ii) the other type (SC*) in which a small preliminary movement in the opposite direction immediately precedes the main impulse, and (iii) the inverted SC in which the movement of H is a decrease. There are also occasionally inverted SC* events. FERRARO and PARKINSON (1950) suggested that the occurrence of SC* is subjected to geomagnetic longitude, being least frequent around Honolulu and most frequent in European stations. WATSON and MCINTOSH (1950) found that about half of the sudden commencements at Lerwick were SC* type, and doubted the longitudinal effect in the frequency of occurrence of SC*. CHAKRABARTY (1951) did not find any SC* in the magnetograms of Alibag for the period NAGATA (1952) noted the occurrence of SC* to be maximum around 1600 LT and that the amplitude of reverse impulse to the main impulse increases markedly with geomagnetic latitude. SRINIVASAMURTHY (1959) observed that the number of SC* at the equatorial station Kodaikanal during the period were only 13 out of total number of SC of 229 and further the SC* occurred only around midday hours ( LT). MATSUSHITA (1962) studied the distribution of 529

2 530 R.G. RASTOGI and N.S. SASTRI Table 1. Coordinates of geomagnetic observatories in India. SC* during the IGY and found that within +15 dip angle SC* occurs between 0700 and 1700 LT, and that SC* are very rare at tropical latitudes. GouIN (1967) reported that for the period , 12% of all the SC events recorded at Addis Ababa were SC*; further most of the SC* occurred during daylight hours. NATARAJAN (1969) studied the magnetograms at Trivandrum for the period and found 17 SC* and 243 pure sc. All the SC* occurred between 0700 and 1400 LT. RASTOGI (1971) studied the occurrence of SSC-+ (SC*) at Kodaikanal over the period Both SSC- (pure SSC) as well as SSC-+ were found to be more frequent during years of high sunspots than the years of low sunspots. All the SSC-+ were observed between 0800 and 1600 LT. NISHIDA and JACOSS (1962) showed that the amplitude of preliminary reverse impulse is enhanced near the magnetic equator similar to the amplitude of main positive impulse. ROQUET (1968) showed that the number of SC* events are greatly reduced with increasing distance from the magnetic equator. Since the IGY, a close chain of geomagnetic observatories has been operating in India near the magnetic equator. The coordinates of the stations are given in Table 1. It is seen that the three observatories; Trivandrum, Kodaikanal and Annamalainagar are within the equatorial electrojet region while Hyderabad and Alibag are outside the electrojet region, Sabhawala is close to the northern Sq focus. It would be desirable to study the phenomenon like SC* by the use of rapidrun magnetograms, but due to the sparseness of the rapidrun magnetograms for the equatorial regions and due to the fact that the statistical properties of equatorial SC* have not been fully understood, we have studied equatorial SC* through normalrun magnetograms. It may be stressed here that the present study has been done directly on the basis of original magnetograms because it was felt that the finer portions of the traces are sometimes lost due to high contrast of the films used in the microfilm copies of these magnetograms. The period of study comprises to Diurnal and Solar Cycle Variation of SS-+ at Kodaikanal Since a comparatively larger series of magnetic data are available for Kodaikanal, the analysis reported by RASTOGI (1971) is extended upto 1970 and

3 On the Occurrence of SSC(-+) at Geomagnetic Observatories in India 531 Fig. 1. The occurrence frequency of sudden commencement of storms with the time of the day and the diurnal variation of the present number of SSC-+ events within them. is described below. The number of SSC-+ was found to be directly related with sunspot number. No SSC-+ event is found during minimum sunspot years while about 7 to 8 events are found during maximum sunspot years. Figure 1 illustrates the average solar daily variation of the total number of SSC events (i.e., pure SSC +- and SSC -+ as well as the fraction of SSC -+ events out of these total SSCs. The SSC -+ are seen to occur only between 0700 and 1800 LT. However, it is to be noted that about 50% of SSCs occurring around midday hours are SSC-+ type, i.e. main positive impulse is preceded by a negative impulse. ARAKI and ISHIZAKI (1973) have shown that the ratio of occurrence frequency of SCE to that of SC at Guam has a maximum value of about unity at 1300 LT. Thus the occurrence of SSC-+ should not be considered as rare event at low latitudes. 3. Characters of SSC -+ at Trivandrum In Fig. 2 are reproduced a portion of the magnetogram at Trivandrum on 28 October A sudden commencement type of storm started at 0649 UT. The main positive portion of SSC in H field was preceded by a very clear negative impulse. In the Z component too, the main positive impulse was preceded by preliminary negative impulse. The sudden commencement in D was very small but the evidences of both positive and negative deflections were evident. One of the largest sudden commencements of the storm at Trivandrum was recorded at 0644 UT on 25 March The H, Z and D traces at Trivandrum are reproduced in Fig. 3. One can clearly see remarkable example of SSC-+ in all the three components. The preliminary negative impulse was about 50. The SSC impulses in H and Z are very clear while in D component the deflections are very small but undoubtedly clear. In Fig. 4 are reproduced the tracings of a few SSC -+ events at Trivandrum.

4 532 R.G. RASTOGI and N.S. SASTRI Fig. 2. The magnetogram tracing on 28 October 1958 at Trivandrum showing Sc-+ event in all the three components of the geomagnetic field. Fig. 3. An exceptionally large SSC-+ event in all the three components of geomagnetic field recorded at Tirvandrum on 25 March In all these cases the simultaneous impulses in H as well as Z components are very clearly seen. In order to find whether the comparable ratio of negative and positive deflection in H is related to similar ratio in Z component, we have gathered in

5 On the Occurrence of SSC(-+) at Geomagnetic Observatories in India 533 Fig. 4. A few examples of SC-+ events on H and Z magnetograms at Trivandrum. Fig. 5. Mass plot of points showing the relation between the negative to positive excursions of the H and the same for the Z field at Trivandrum. Fig. 5 mass plot of the two quantities. It is seen that if the ratio of negative to positive impulse in H is large, then the ratio of negative to positive impulse in Z is also large. Thus it is concluded, that the magnitude of SSC-+ in H and Z components is related to each other. 4. Latitudinal Variation of SSC-+ in H All the original SSC magnetograms at Indian magnetic observatories for the period were examined for the presence of negative impulses during sudden commencements. In Fig. 6 are shown the tracings of the magnetograms at the four Indian observatories during the sudden commencement events on 23 September 1966, 1 April 1967 and 16 November These events were of SSC-+ type. It is seen from the figure that the negative excursions were clearly recorded at the three equatorial observatories Trivandrum, Kodaikanal and Annamalainagar.

6 534 R.G. RASTOGI and N.S. SASTRI At Alibag, the negative excursions were generally not seen. Alibag is outside the electrojet region and the negative excursion of SC at this station is either not present or too small to be detected in the normal magnetograms. A critical examination of the magnetograms at Alibag revealed two cases of SSC-+ recorded there. The tracing of H, Z and D magnetograms at Alibag and Trivandrum for the SSC at 0649 UT on 28 October 1958 is reproduced in Fig. 7. This event had produced an abnormally large negative excursion of the field at Trivandrum both in H and the Z field; the D field also indicated the complex nature of the SSC. At Alibag, the negative and positive changes of the geomagnetic field were clearly recorded in the D and Z traces and rather weakly in H trace. Another remarkably large SSC -+ event occurred on 25 March The magnetogram tracings for this event at Trivandrum are already shown in Fig. 3. In Fig. 8 are reproduced the magnetogram tracings for the I APRIL SEPTEMBER NOVEMBER 1968 Fig. 6. The magnetograms of the different observatories in India showing SSC-+ event in H field. 28 OCTOBER 1958 Fig. 7. Example of SSC-+ event in H recorded at equatorial (Trivandrum) as well as at tropical latitude (Alibag).

7 On the Occurrence of SSC(-+) at Geomagnetic Observatories in India 535 ALIBAG 25-MARCH, 1969 HYDERABAD Fig. 8. An exceptional example of SSC-+ event in all the three components of geomagnetic field at tropical latitude stations Alibag and Hyderabad on 25 March Fig. 9. Histograms of the ratio of negative to positive excursions in H field at different stations in India. SSC event on 25 March 1969 at Alibag and Hyderabad. Very clear negative and positive excursions are seen in all the three elements at Hyderabad. The H trace at Alibag was too faint to be recorded during SSC, there was a gap in the record during SSC; but the Z and D traces clearly indicate the complex nature of SSC. No negative excursion was evident in the magnetogram traces at Sabhawala. It is thus seen that the SSC -+ events are not completely inhibited at tropical latitude stations, though these events are comparatively rare and thus is in difference with MATSUSHITA's (1962) remark that SC-+ are seen only in the electrojet region and not seen at low-latitudes outside the electrojet. The ratio of negative to the positive excursions of all the SSC-+ events was computed for each station. Histograms of this ratio for each station are shown in Fig. 9. At Trivandrum, the ratio ranges from 0.1 to 0.5; at Kodaikanal, the ratio ranges from 0.05 to 0.35, while at Annamalainagar it ranges from 0.01 to Thus, the ratio of negative to positive excursion goes on decreasing very rapidly with the increasing latitude of the station. The average ratio of the two events at Alibag was around It may be mentioned that ARAKI and ISHIZAKI

8 536 R.G. RASTOGI and N.S. SASTRI Fig. 10. Latitudinal variations of the amplitude in SSC+ and the ratio of negative to positive excursions in SSC-+ events in Hfor Indian zone. (1973) found that when a decrease of negative impulse of SC* is steep, the following main increase is also steep. In Fig. 10 are shown the latitudinal variations of the positive excursions of the SSC normalized to the corresponding amplitudes at Trivandrum. This is compared with the latitudinal variation of the average ratio of negative to the positive excursion in H. It is seen from the figure that SSC amplitude decreases rapidly with latitude such that the SSC amplitude at Alibag is about half of the corresponding amplitude at Trivandrum. The ratio of negative and positive excursion decreases much more rapidly with latitude such that it is reduced from its value of about 0.20 at 0 dip to about 0.08 at 5 dip within the electrojet region. This is consistent with the results of NISHIDA and JACOBS (1962) who showed that the rate of equatorial enhancement of SCE is larger than that of the ordinary SC. 5. Conclusion The present result, that the reverse impulse is greatly enhanced over the magnetic equator undoubtedly suggest that these preliminary impulses are due

9 On the Occurrence of SSC(-+) at Geomagnetic Observatories in India 537 to westward currents in the region of the ionosphere of greatly enhanced conductivity. The latitudinal variation of negative excursion being sharper than that of the positive excursion suggests that the former current system may be at a lower level than the height of normal Sq current system. Sincere thanks are due to Dr. M.K. Vainu Bappu, Director, Indian Institute of Astrophysics, Kodaikanal; Mr. B.N. Bhargava, Director, Indian Institute of Geomagnetism, Bombay; Dr. Hari Narain, Director, National Geophysical Research Institute, Hyderabad and Mr. J. Chatterjee, Director, Geodetic and Research Branch, Survey of India, Dehra Dun for their cooperation in providing the facilities to examine the original magnetograms of their observatories. Thanks are also due to Prof. K.R. Ramanathan and Mr. B.N. Bhargava for their keen interest and useful suggestions during the course of work. REFERENCES ARAKI, T. and H. ISHIZAKI, Characteristics of SC* in the low latitude region, IAGA Bulletin No. 34, pp. 138, CHAKRABARTY, S.K., Sudden commencements in geomagnetic field variations, Nature, 167, 31, FERRARO, V.C.A. and W.C. PARKINSON, Sudden commencement in geomagnetism their dependence on local time and geomagnetic latitude, Nature, 165, , GOUIN, P., A note on the number of geomagnetic storm sudden commencements recorded at Addis Ababa, Bull. Geophysical Observatory, Addis Ababa, 10, 3-13, MATSUSHITA, S., On geomagnetic sudden commencements, sudden impulses and storm durations, J. Geophys. Res., 67, , NAGATA, T., Sudden commencements preceded by the preliminary reverse impulse in a geomagnetic field, Nature, 169, , NATARAJAN, R., A study of sudden commencements of geomagnetic storm at Trivandrum, Ind. J. Meteor. Geophys., 20, , NEWTON, H.W., Sudden commencements in the Greenwich magnetic records ( ) and related sunspot data, Mon. Not. R. Astr. Soc. Geophys. Suppl., 5(6), , NISHIDA, A. and J.A. JACOBS, Equatorial enhancement of worldwide changes, J. Geophys. Res., 67, , RASTOGI, R.G., On the occurrence of SSC(-+) at Kodaikanal, Planet. Space Sci., '19, , ROQUET, J., Sur l'amplification des debuts brusques d'orages magnetiques a Addis Ababa, Bull. Geophysical Observatory, Addis Ababa, 12, 11-18, SRINIVASAMURTHY, B., Sudden commencements and impulses in Kodaikanal magnetogramstheir hourly frequency, Ind. J. Meteor. Geophys., 10, , WATSON, R.A, and D.H. MCINTOSH, Sudden commencements in geomagnetism, Nature, 165, 1018, 1950.