Electron density irregularities in the equatorialD andE regions have been studied using ground-based and rocket-borne experiments. In this paper this subject is reviewed with an emphasis on the rocket-borne measurements of electron density and electric field structures and other parameters relevant for identifying processes responsible for generation of electron density irregularities. These studies have led to the identification of various causative mechanisms, in terms of plasma and neutral dynamical processes for producing irregularities. Based on these mechanisms, the irregularities are classified and discussed. There are many observations which have eluded explanation and hence call for sustained efforts.

Various experiments involving the measurement of new, regenerated and total productivity using ¹⁵N and ¹³C tracers were carried out in the Bay of Bengal (BOB) and in the Arabian Sea. Results from ¹⁵N tracer experiments indicate that nitrate uptake can be underestimated by experiments with incubation time > 4 hours. Indirect evidence suggests pico- and nano-phytoplankton, on their dominance over microphytoplankton, can also influence the f-ratios. Difference in energy requirement for assimilation of different nitrogen compounds decides the preferred nitrogen source during the early hours of incubation. Variation in light intensity during incubation also plays a significant role in the assimilation of nitrogen. Results from time course experiments with both ¹⁵N and ¹³C tracers suggest that photoinhibition appears significant in BOB and the Arabian Sea during noon. A significant correlation has been found in the productivity values obtained using ¹⁵N and ¹³C tracers.