NOPR Community:

Some features of low latitude second and third harmonic tweeks

Tue, 01 Dec 1992 00:00:00 GMT

Title: Some features of low latitude second and third harmonic tweeks Authors: Singh, R N; Singh, R P; Singh, U P; Singh, AK Abstract: Third harmonic tweeks have been observed for the first time and reported in this communication. The fundamental and second harmonic tweeks are observed, at times along with accompanying whistlers. The earth-ionosphere waveguide dispersion features play an important role in propagation of broad band lightning generated signals and their occasional observation as higher harmonic tweeks. Page(s): 377-380

Variability of the time of peak occurrence in electron content at low latitudes

Tue, 01 Dec 1992 00:00:00 GMT

Title: Variability of the time of peak occurrence in electron content at low latitudes Authors: Bhuyan, P K Abstract: A winter-to-summer delay in the occurrence of diurnal peak in total ionospheric electron content (lEC) was observed at Indian and Pacific equatorial and low latitude stations. The time delay exhibits latitudinal variation with a minimum near the crest of the 'equatorial anomaly'. A comparison with the data from American near-equatorial stations showed that the time delay is reversed from summer to winter indicating a longitudinal asymmetry between Asian and American sectors. Page(s): 374-376

Model calculations of the OI 844.6 nm emission in evening twilight

Tue, 01 Dec 1992 00:00:00 GMT

Title: Model calculations of the OI 844.6 nm emission in evening twilight Authors: Singh, Vir Abstract: The volume emission rate profile and intensity of the atomic oxygen multiplet emission at 844.6 nm have been studied in evening twilight. The zenith intensity has been calculated as a function of solar zenith angle (SZA) by using the photoelectron flux model of Richards and Torr [J Geophys Res (USA). 88 (1983) 2155] for Jacchia-71 and MSIS-77 model neutral atmospheres. It has been found that the intensity results obtained from Jacchia-71 model neutral atmosphere do not agree with the measurements. The intensity results obtained from MSIS-77 model neutral atmosphere have been found to be in agreement with the measurements only up to 96° SZA. Above 96° SZA both models highly overestimate the measurements. Page(s): 370-373

Qualitative appraisal of the modified Tikhonov-Cagniard equation in electromagnetic•induction

Tue, 01 Dec 1992 00:00:00 GMT

Title: Qualitative appraisal of the modified Tikhonov-Cagniard equation in electromagnetic•induction Authors: Reddy, C D Abstract: The modified Tikhonov-Cagniard equation is seldom used in magnetotelluric (MT) and magnetovariational (MV) studies, mainly because of the complexities it introduces in numerical solution. Most MT and MV calculations are based on the plane-wave approximation of the inducing field. The region around the tip of peninsular India is uniquely located to quantify the source field effect, and to estimate the contribution of the spatial gradients of various orders. Upon analysis of geomagnetic data collected in this region it is concluded that, over most of the earth's surface, source-field geometry affects only the second-order spatial gradients, and hence is significant only under the auroral electrojet (AEJ), and under the equatorial electrojet (EEJ) during daytime. Page(s): 366-369