A Study On The Current Sources And The Period Ranges Of The Long Period Induced Geoelectric Field During The Magnetic Storms

The long-period electromagnetic field could be used to explore the mantle conductivity.During the magnetic storms,the induced geoelectric field is controlled by both the current sources in in the space and the underground electrical structures.However,the space current sources and their effects on the magnitude,direction,and the period ranges of the induced long-period field remain unknown.This paper aims to analyze the current sources in the magnetosphere and in the ionosphere,which produce this long-period(>10~5s)electromagnetic field in the magnetic storms.The period ranges and the contributing factors are also studied.We present a case study and a statistical study on the long-period(>10~5 s)induced geoelectric field disturbance during the magnetic storms.A set of continuous 33-day measurements of the geoelectric field,geomagnetic field at Chang Chun observatory,the geomagnetic indices,and the interplanetary magnetic field are analyzed as a case study.The geoelectric field and geomagnetic field measurements at Memambetsu observatory in Japan from 1989 to 2008 are studied by the statistical methods to examines the period upper limit and its contributing factor.The SYM-H index is used to reflect the level of the magnetic disturbance and the enhancement of the ring current.The auroral image and the AE index are used to check the enhancement of the field-aligned current.To exclude the effects from the different underground electrical structures,the geomagnetic and geoelectric field measurements from the same ground observatory are studied.The LP electromagnetic(EM)field is identified by the wavelet coherence analysis.Besides,Space Weather Modeling Framework is adopted to calculate the global geomagnetic field disturbances and the contribution from different current systems.The wavelet power spectra analysis reveals that the long-period geoelectric field disturbance appears only during magnetic storms.Especially,stronger magnetic storm generates weaker geoelectric disturbances at the same observatory.The opposite-direction eastward geomagnetic field disturbance,which is generated mainly by the field-aligned current at different magnetic local time,significantly changes the magnitude and direction of the induced geoelectric field via the underground impedance tensor.The longest period of LP geoelectric field caused by the investigated magnetic storms is 9*10~5 s.The eastward geoelectric field is stronger than northward geoelectric field obviously.The power and period upper limit of LP field grow in trend with the stronger level of magnetic storm.And the enhancement of eastward geoelectric field show a great asymmetry varying with magnetic local time.Above all,both the effects caused by the ring current and the field-aligned current should be considered in analyzing the ground induced geoelectric field.The asymmetric features of the geoelectric field produced by the field-aligned current play an important role in the induction process.This LP EM field satisfies the uniform plane-wave assumption for layered structure underground medium.And it has a skin depth of 1500 km which could explore the electrical conductivity in the whole upper mantle.Excluding the effect from the geospace current sources,the more accurate electrical conductivity in the upper mantle would be obtained in future.