Secular trend of geomagnetic elements in the Indian region
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
Received: 13 May 2013
Accepted: 2 September 2013
Published: 6 December 2013
In the present study, secular trends and jerks in the geomagnetic elements D, H and Z are investigated at the six Indian magnetic observatories using annual and monthly mean values for all days, quiet days and night base (night time mean). The residuals of all-day annual and monthly means are computed by removing a polynomial fit from their best fitting curves. The residuals of D, H and Z curves do not show any parallelism with the 11-year sunspot cycle. At Alibag, the D residual shows a periodicity of 2 solar cycles, whereas the H and Z residuals indicate a quasi-periodicity of 3 solar cycles for the period 1921–2009. At the Indian stations, an in-phase solar cycle component is observed for 2 of the solar cycles in the D and Z residuals, while the H residual shows out-of-phase variations with the sunspot cycle for the period 1958–2009. Two geomagnetic jerks, 1970 and 1991, are well reflected in the monthly and annual mean values in the Indian region, as observed globally.
Gilbert, in 1600 A.D., suggested a model Earth called Terrella in his book De Magnete. According to this model, the planet Earth is a huge magnet and is approximated to a magnetic dipole placed at its center. This geomagnetic field changes over a wide range of time scales from a fraction of a second to millions of years. The geomagnetic field consists of an internal and external field; the internal field is the main field on which external field variations such as Sq, storms and sub storms are superimposed. The main field variations (more than 90%) are generated by the Earth’s liquid outer core. The core-mantle interactions lead to secular changes. The external source (less than 10%) is located in the Earth’s ionized upper atmosphere and is due to highly-penetrating radiations from cosmic rays, solar, planetary or lunar origin. Among all these external sources, the Sun is the major energy source, causing upper atmosphere ionization that sets up currents responsible for short-period variations, such as Sq, storms and sub storms in the Earth’s magnetic field.
The annual mean values of magnetic observations are used to determine the secular variations of the geomagnetic field components. These variations differ from place to place and vary with time. The secular variation covering a period of ~100 to 1000 years in which the major part of the geomagnetic field does not undergo reversals comes under this type of study. Moos (1910) studied the secular variation of the geomagnetic field components at Bombay for the period 1871–1905 and, after reducing Bombay (Colaba) data to Alibag, Pramanik (1952) extended these secular variation curves up to 1949. Rao and Bansal (1969) fitted polynomials of a third order to the observed annual mean values of H, Z and D for Alibag from 1905–1965. Bhardwaj and Rangarajan (1997) further extended these curves up to 1990 for Alibag. In the present study, secular variations, long-period oscillations in the residuals, and secular jerks, are discussed using annual and monthly mean values of observations from six Indian magnetic observatories.
2. Data and Technique Used
List of stations along the Indian Sector, their Geographic and Geomagnetic coordinates, and the period of data used.
Period of data used
Annamalainagar / Pondicherry
The data sets for Alibag are from 1921–2009, and, for other equatorial electrojet stations, from 1958–2009. As Annamalainagar and Trivandrum observatories are closed, the data for these stations have been reduced from Pondicherry and Tirunelveli by applying correction factors to all three elements D, H and Z. Data sets for Hyderabad and Sabhawala are from 1965–2009. Data for Kodaikanal station are from 1965–2005 for D and H, whereas data for Z are from 1965 to 1997 due to technical problems. It is to be pointed out that the declination (D) at Alibag was easterly up to 1926, and from 1927 has continued to be westerly and, at present, is again swinging towards the east.
3. Results and Discussion
The results are based on the annual and monthly mean values of the geomagnetic field components D, H and Z and are discussed in terms of (a) trends in secular variations (b) long-period oscillations in the residuals, and (c) secular jerks.
3.1 Secular trends at Alibag (1921–2009)
Percentage of the variance in Alibag quiet-day annual means accounted for by polynomial fits.
For the H-component, the percentage of variance accounted by a 5th-order polynomial is 99.4% and fitting well with the observed annual mean values in comparison with other low-order polynomials. The secular trend of H shows that this field has increased continuously since 1921, reaching a maximum by about 1965, and thereafter decreased at a rate of about 20 nT/year. This H-field has a quasi-periodicity of around 90–100 years and is consistent with earlier results.
The percentage of variance of the Z-component for the 5th-order polynomial is 96.9%. The secular trend of Z shows a near-sinusoidal variation with a periodicity of ~80 years, known as the Gleissberg cycle (Gleissberg, 1965). The continuously increasing trend of the Z-field at present denotes another periodicity in the coming years.
Figure 2(b) shows the plots of D, H and Z for monthly mean values at Alibag for all days. There is no distinct difference between the monthly mean and the annual mean plots for all days and quiet days. Also, the cumulative percentage of variance accounted for by polynomial fits in successive orders is almost the same for the annual and monthly means of all days and quiet days.
3.2 Secular trends in the Indian chain of observatories
Three out of the six stations—TRD, KOD and ANN— are under the influence of the daytime equatorial electrojet. This causes an enhancement of the daily variation and short-period fluctuations in H, and though the electrojet does not contribute in any measurable way to the secular trends of H or D, it may introduce significant departures from the trend (Bhardwaj and Rangarajan, 1997). The diurnal variation in the vertical component close to the dip equator is expected to be small, but the analysis of south India magnetic array data suggests a significant internal contribution due to the channeling of induced currents through a subsurface conductor between India and Sri Lanka (Rajaram et al., 1979) and a regional south Indian offshore conductivity anomaly (Arora and Subba Rao, 2002). The three stations HYB, ABG, and SAB, are the low- and mid-latitude stations. The westerly declination (D-trend) is decreasing at all six Indian stations (the sign of SAB has been reversed to indicate westerly) and, at Alibag, it has again been swinging easterly from 2009, as shown in Fig. 3(a). The cumulative percentage of variance accounted for by the 5th-order polynomials varies between 97.0 to 99.6 for the D component, 98.7 to 99.6 for the H component, and 98.5 to 99.5 for the Z component, at all six Indian stations.
Secular trends for the horizontal component (H) at the six Indian stations for quiet day annual means are shown in Fig. 3(b). A distinct difference between equatorial and low-latitude stations is that the H-field increases rapidly after 1990 for equatorial stations and is linear for non-equatorial electrojet stations. The parallelisms in the secular trend of Hyderabad, Alibag and Sabhawala indicate that the feature has a broad regional coverage, whose southern latitudinal extent may be just above the edge of the equatorial electrojet belt. A quasi-periodicity of nearly 40–50 years is suggested close to the equator. In broader terms, we can say that all six stations show comparable trends without large local anomalies.
The secular trends of the vertical component (Z) for the six Indian stations for quiet day annual means is shown in Fig. 3(c). The Z-field decreased since 1958, reaching a minimum value around 1970, and then it increased again. The increase in Z from 1970 is more rapid at equatorial stations (denoting the southward migration of the dip equator) as compared with mid-latitude stations. The southward migration of the dip equator was suggested by Srivastava (1992) and confirmed by Rangarajan and Deka (1991). The maximum speed of the southward migration of the dip equator is estimated to be ~5 km/year during 1980–1990 (Deka et al., 2005). The southward migration of the dip equator is also shown in Fig. 1 for different years from 1975 to 2005. Srivastava and Abbas (1977) have found a quasi-periodicity of ~80 years in the migration of the dip equator. In our analysis, the Z-field also completes a half-cycle in 40 years; hence, the period of the secular change of the position of the dip equator is of the order of ~80 years, attributable to the Gleissberg cycle (Gleissberg, 1965).
Night base plots at three equatorial, and three non-equatorial, stations for the D, H and Z components, which are shown in Figs. 4(a)–4(c), are not much different compared with all days and quiet days plots. Hence, the equatorial electrojet current does not contribute to secular variations. Also, the cumulative percentage of variance of the 5th-order polynomials for D, H and Z is almost the same for all days, quiet days and night base at the six Indian stations.
3.3 Solar cycle component in the monthly means at Alibag
3.4 Solar cycle component in the annual means at the Indian observatories
From Fig. 6(c), an in-phase solar-cycle component can be seen for 2 of the solar cycles in the Z residuals, with minima near 1963, 1986 and 2009, i.e. the Z residuals appear in phase with the double sunspot cycle at all the Indian stations (except KOD). The equatorial and lower-latitude group of stations do not indicate different patterns, so these variations are due to external sources. The minimum in Z in phase with the solar activity minimum is consistent with the expectations of the signature of the equatorial ring current in the northern hemisphere, since this source (ring current) would create an increase in Z (a downward field) in the northern hemisphere and a decrease in Z (an upward field) in the southern hemisphere as the ring current (as well as the sunspot number) increases.
3.5 Secular impulse or Jerk
A sudden change in the slope of the magnetic secular variation is known as a secular impulse, or geomagnetic jerk, that arises from sources inside the Earth (Cafarella and Meloni, 1995; Macmillan, 1996; Le Huy et al., 1998). Recently, these jerks have been suggested as geomagnetic rapid secular fluctuations (Olsen and Mandea, 2008; Mandea and Olsen, 2009; Qamili et al., 2013) that have periods ranging from several months to a few years (Macmillan, 2007). These events are observed in magnetic data as sudden V-shaped changes in the slope of the secular variation (Mandea et al., 2010). Geomagnetic jerks have been observed around 1901, 1913, 1925, 1932, 1949, 1958, 1969, 1978, 1986, 1991, 1999, and 2003, at a number of observatories around the world (e.g. Malin and Hodder, 1982; Courtillot and Le Mouël, 1984; Alexandrescu et al., 1996; Mandea et al., 2000; Mandea et al., 2010).
Mandea et al. (2000) reported jerks around 1970 and 1999 by using data from Chambon la Foret (~118 years) and Niemegk (~111 years) observatories’ monthly mean values of the East magnetic component (Y). The jerk of ~1970 was recorded all over the globe in declination data. 1932, 1949 and 1970 jerks were reported by Alexandrescu et al. (1996) by using 97 European observatories, and Kerridge and Barraclough (1985) by using data of annual mean values from worldwide observatories. Gubbins and Tomlinson (1986) also noticed the signature of the 1969–70 jerk at Apia and Amberley magnetic observatories in New Zealand.
The secular change of D curves show a decreasing trend in the westward direction at all the Indian observatories, and, at Alibag, at present it again swings back towards the eastward direction.
The secular trends of the H field show a distinct difference between the equatorial and non-equatorial stations.
Increasing trends of the Z field at equatorial stations since 1970 indicates the southward migration of the dip equator, which has a periodicity of ~80 years— the so-called Gleissberg cycle.
At the Indian chain of stations, D and Z residuals show a periodicity of ~2 solar cycles, while H residuals show out-of-phase variations with the sunspot cycle. At Alibag, the D residuals show a periodicity of ~2 solar cycles, whereas the H and Z residuals are found to be opposite in phase with a quasi-periodicity of 3 solar cycles for the period 1921–2009.
As observed globally, geomagnetic jerks are observed in the Indian region. The 1970 and 1991 jerks are well reflected in the 3 components of the monthly mean values.
We are grateful to Prof. G. K. Rangarajan and Prof. B. R. Arora for their motivation and fruitful suggestions. We would like to thank Dr. B. Veenadhari, Observatory and Data chairperson, Indian Institute of Geomagnetism, Navi Mumbai, India, for providing us with the monthly means database and for helpful discussions. We would like to thank Prof. S. Gurubaran, Director-in-Charge, Indian Institute of Geomagnetism, Navi Mumbai, India, for his keen interest and encouragement for carrying out this work. The authors would also like to thank two anonymous reviewers for their constructive comments and suggestions.
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