Measurement and analysis of diurnal variations of the geomagnetic field

Abstract

Measurements of the geomagnetic field show that the Earth's field is not constant but varies temporally and spatially. Radiation coming from the sun that is incident on Earth causes the atmosphere's upper regions to become ionized. This ionized atmosphere is acted upon thermal and tidal forces. These, in tum, generate electrical currents in the ionosphere at altitudes of around 90 to 130 km and in addition, they generate a magnetic field. This magnetic field, in conjunction with the main geomagnetic field generated in the core and crust, make up the observed geomagnetic field. The electrical currents' magnetic field varies in time at any point on Earth. Indeed it has a 24 hour cycle. This variation in the geomagnetic field is known as the solar quiet variations (Sq) or Diurnal Variation. It has an effect on navigation, explorational geophysics and its accurate measurement is of fundamental practical importance. The tidal effect of the moon on the atmosphere also introduces a lunar component to the variation in the geomagnetic field. This lunar perturbation (Lq) has a magnitude of about tenth of the solar quiet variations and a period of about 12.4 hours i.e. semi diurnal. The aim of this project is to measure and analyse such variations by means of an Overhauser magnetometer, GSM-19. A detailed literature review about the main aspects of the geomagnetic field, its variations and means of measuring such field is given to introduce the topic and helps in further understanding. Measurements of the geomagnetic field were continuously taken over a period of one month at a stationary location using an Overhauser magnetometer. This is followed by an analysis of the results obtained. By using the power spectrum based on the autoregressive technique, these variations were analysed for their spectral characteristics. The diurnal variation of the geomagnetic field was successfully isolated in the spectral analysis. Furthermore, other cycles attributed to the lunar effect and other phenomena were observed and identified.