Geomagnetic secular variation in India-regional and local features.

Abstract

A study of the secular variation in the geomagnetic elements H, Z, F and D at Colaba (Bombay)-Alibag for the period 1848-1973, has been made. Fifth degree polynomials are fitted to the data of antiual mean values of H, Z and F, and third degree to D, and the residuals discussed. The trends are also examined at the six Indian observatories using the data for [1960][1961][1962][1963][1964][1965][1966][1967][1968][1969][1970][1971][1972][1973][1974].The increasing trend of Z at Alibag is found to decrease from about 1937, while that of H and F from 1965 at 20-30 nT/year, it being of the same order at Sabhawala and Hyderabad but smaller at the equatorial stations, particularly for Z component. The westward annual change in D swings eastward again around 1965 at all the Indian stations. This reversal of the secular variation trend in India after 1965 emerges as an important regional feature connected with a southward migration of the dip equator in India from 1968.The secular change in D at Alibag (+0.4'/year) is somewhat anomalous in the sense that it is reduced as compared to Hyderabad and other stations (+1.6'/year), probably due to the local magnetic anomaly of the Deccan lavas, and calls for detailed investigations.

Trends in the solar quiet geomagnetic field variation linked to the Earth's magnetic field secular variation and increasing concentrations of greenhouse gases

“…The equatorial electrojet phenomenon is linked closely with the dip equator, so the enhanced geomagnetic signature on the horizontal component due to equatorial electrojet currents is intimately tied to the location of the dip equator [Rastogi, 1989], which migrates in different directions for different regions [Deka et al, 2005]. A quasiperiodicity of about 80 years in the migration of the dip equator was pointed out by Srivastava and Abbas [1977]. As a preliminary test of the possibility that the secular migration of the equatorial electrojet may influence Sq trends in the case of Apia, the latitude of the dip equator at 188.2°E was derived from the IGRF model for the period 1960-2001.…”

Migration of the dip equator in the Indian region

“…Further, it is noteworthy that if the present trend of southward migration (figure 2) of the dip equator continues, it would reach its 1909 latitude by the year 1990, apparently representing one full cycle of migration in the Indian region with its mean position approximately at 8 ° 35' N. Lat, with a periodicity of about 80 yr as pointed out by Srivastava and Habiba Abbas (1977). This coincides with the known 80 yr spectral peak in the secular variation of the geomagnetic field at Alibag (Bhargava and Yacob 1970).…”

“…on the longitude 77.5°E for the epoch 1971. The recent study, by Srivastava and Habiba Abbas (1977), of the secular variation of Z at the Indian equatorial stations oiz., ANR, TaD and KOD suggests that the dip equator migrated to the south from 1968 onwards, though the actual estimates for 1970 and 1971 mentioned earlier, indicate a northward movement up to 1971. The latest position determined for the epoch 1981 based on atleast 30 equatorial stations with near-zero dip values, distributed over different longitudinal zones, is about 24 km to the south of the 1971 position along the 77.5°E longitude.…”

“…The long-term shifts of the dip equator in the Indian region have been studied earlier, among others, by Chatterjee (1970), Sanker Narayan and Ramanujachary (1971), IIG, NGRl and sol (1972) and Srivastava and Habiba Abbas (1977). These studies were based on either magnetic charts, data from a few repeat stations, or on magnetic recordings at a few equatorial stations in selected longitudinal zones.…”

Secular trend of geomagnetic elements in the Indian region

“…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).…”