A method for pre-processing ULF electric field disturbances observed by DEMETER and its case application analysis

This paper presents a method for pre-processing ULF electric field disturbances based on ULF waveforms observed by French DEMETER satellite. This method is simpler and easier to carry out. We analyze comprehensively ULF disturbances before the Chile M_S8.8 earthquake on 27 February 2010， which have been reported in the literature， by using our method. The results show that reported ULF disturbances in 20°S—40°S geomagnetic latitude bands do not just appear on the half-orbit that is the closest to the epicenter， but ULF disturbances are observed in a wider longitude range. The large range with ULF disturbances is located in the region of South Atlantic anomaly (SAA)， and ULF disturbances may be affected by the geomagnetic field anomaly in this region. ULF disturbances in the center of SAA region show a conjugate distribution， i.e.， electric field fluctuates with the amplitude of 5 mV/m at 20°—40° north and south geomagnetic latitudes. The amplitude of ULF disturbances at northern latitudes is weaker than those at southern latitudes. However， ULF disturbances outside the center of SAA region are only observed at 20°S—40°S geomagnetic latitudes， and there are no ULF fluctuations in the conjugate region. As ULF fluctuations outside the center of SAA region with the weaker amplitude are easier to be attenuated because of collision effects in the ionosphere， ULF disturbance cannot propagate to the conjugate region. The method and results in this paper can provide references for subsequently thorough analyses on ULF fluctuations recorded by satellites in the ionosphere.