Phase Properties of Ulf Waves Observed in the African Sector

Abstract:

ULF waves have been investigated in Lagos, Nigeria. Events were recorded with a ground magnetometer in Abuja (Geographic: 7.39°E, 8.99°N; Dip latitude -1.53). To understand the propagation mode and propose source mechanisms for the observed ULF wave events, we analyzed data from four other spatially distributed magnetometer stations in the African region namely Medea (Algeria), Adigrat (Ethiopia), Yaounde (Cameroun), and Tsumeb (Namibia). Five days when a ULF wave event was registered simultaneously in the HF-Doppler receiver and magnetometer in Abuja, Nigeria were analyzed. These events were also registered in at least two other magnetometer stations in Africa (subject to data availability from the stations). Results from phase analyses showed two distinct patterns. We propose the effects of solar heating as the source mechanism for the first set of events while for the second set of events we propose any or all of: enhanced solar activity, disturbed magnetic conditions, or a strong downward acting component of the interplanetary magnetic field (IMF Bz). Observed events are not likely to be Field Line Resonances (FLRs) because the geometry of magnetic field lines at the equator and the sampling rate of data do not support observation of FLRs at the low L – shell locations. Also, the same events have been registered on different L-shell values or across field lines and these are unlikely features of FLRs.