On the Source Field Geometry and Geomagnetic Induction in Southern India.

Abstract

The effect of source field geometry on the response function (AZ/OH), used in geomagnetic depth sounding to decipher lateral conductivity distribution within the earth is highlighted with the help of contour plots of OZ/OH ratios for day and nighttime short-period fluctuations recorded through a largescale magnetometer array operated in the equatorial region of southern India. The representation of the difference between day (non-uniform) and night (uniform) time ratios in terms of the spatial derivatives of source field suggests that the reduction in daytime OZ/OH ratios at stations close to the periphery of the electrojet is due to the mutual balance of external and internal parts in OZ. The reduction in daytime AZ/A.Hratios near the center of electrojet axis is interpreted to indicate the weakening of the intensity of induced currents due to the presence of second and higher order spatial derivatives in the non-uniform source field.

On latitudinal profile of Storm Sudden Commencement in H, Y and Z at Indian Geomagnetic Observatory chain

“…However, their figure 3(b) in which they have shown latitudinal variation of H from about 9 • dip latitude to the equator during nighttime indicates a decrease in H towards the equator. The progressive decrease of SSC amplitudes in H from a wider latitudinal range shown here coupled with the results of Sekhar and Arora (1994), can be explained if one assumes the source current is far off and uniform during the night hours thereby the induced currents, over a large latitudinal extent, may extend to the north and south sides of the axis of the electrojet. These induced currents may deviate towards the highly conducting narrow belt, just south of Trivandrum resulting the broad decrease of H field in shorter wavelengths.…”

“…The three sub-surface conductors were (1) The Indo-Ceylon Graben and Pondicherry failed arm together with thick sediments (2) Crustal alteration and mantle uprise with thick sediments underneath or near the Comorin Ridge and (3) West coast rift owing to injection of mantle material or mantle uprise and sediments. Sekhar and Arora (1994), while studying the latitudinal variation of short period fluctuations (not SSC frequencies) in H and Z fields, have identified two zones of significant differences between day and nighttime ratios of Z / H , one near the central axis and the other close to the periphery of the equatorial electrojet. They have explained the decrease in daytime ratios at the central axis of the electrojet as due to the second order spatial derivatives of the source field.…”

“…It was suggested that the abnormal behaviour at Trivandrum was due to the concentration of induced currents over a wide range of low latitudes north and south of the dip equator through the conducting graben in the sub-surface region between India and Sri Lanka, besides the channeling of induced ocean currents through Palk Strait. Sekhar and Arora (1994) have dealt the problem of Geomagnetic Induction in South India by working out the response function ( Z / H ) for short period fluctuations during day and night hours separately. They attributed the reduction in the response function during daytime near the centre of electrojet axis to the weaking of the intensity of induced currents due to second and higher order spatial derivatives.…”

Response of ionospheric disturbance dynamo and electromagnetic induction during geomagnetic storm

“…It was also reported that auroral electrojet, substorm onsets, geomagnetic pulsation, and sudden impulses are responsible for large induction current [32,33]. Chandrasekhar and Arora [34] also confirmed that the reduction in daytime ratio ⌬Z/⌬H for stations close to the electrojet may be due to mutual balance of external and internal parts of ⌬Z while a reduction noticed near the center of the electrojet axis indicates weakening of the intensity of induced current due to the presence of the higher order spatial derivatives in non-uniform source field [20]. Also, this view has been supported with model calculations showing that shape and thickness of sedimentary layers contribute strongly to induced current variation.…”