Earth Magnetic Fields Evolution over Nusa Tenggara Region from Intensity and Power Spectral Density Changes on Lombok Geomagnetic Observatory

Abstract

Earth's magnetic field is a physical quantity that always changes both spatially and temporally. Its long-term change is closely related to internal mechanisms within the Earth's core, whereas changes in the medium-term may be related to events occurring in the Earth's crust. Continuous observationsat many locations are needed to determine the magnitude and direction of the Earth's magnetic field. A geomagnetic observatory is an ideal means for conducting such observations, whereby its location is required to be very quiet in order to limit noise and interference (mainly due to human activities). On the island of Lombok, a geomagnetic observatory has been established in 2014 and it has produced high quality and continuous data since 2015. This paper (together with another paper presented in this conference) presents the results of observations of the Earth's magnetic field at the Lombok geomagnetic observatory during 3 (three) years of its initial operation. This paper specifically presents the results of declination and inclination measurements. The rapid changes in the values of intensity are observed clearly, with a marked decrease of −40 nT/yr, while power spectral density shows a decrease in average density values of 18.56 nT2/Hz at a frequency <0.0025 Hz every year. Compared to the 12th IGRF model, it shows a very noticeable drop in intensity, which for this region should only be around −25 nT/yr. The results of the observation provide clues that we have to pay more attention and scrutiny to the geomagnetic evolution over Nusa Tenggara region. Earth's magnetic field over this region drops rapidly at a rate almost equal to that of the famous South Atlantic anomaly region, while the spectral density changes open up the possibility of accumulation of geomagnetic energy released as a result of earthquake events. Further continuous observations on the island of Lombok are necessary to confirm the tendency of long-term intensity changes, which will be most useful for integration with global observations (to know the evolution of the Earth's magnetic field, especially related to magnetic pole reversal). Meanwhile, fluctuating intensity and spectral density changes can be integrated with regional observation results (for earthquake precursor study purposes).