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 annual 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 1960-1974.
The increasing trend of Z at Alibag is found to decrease from about 1937, while that of H and F from 1965 at 20-30nT/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.
