Based on the meticulous identification of ionization troughs, performed earlier from the CHAMP satellite data, twoadditional issueswereresolved: (1) the longitudinal effect characteristics in the position of the main ionospheric trough (MIT) were corrected, and (2) for the first time,the dependence of theMITposition on geomagnetic activity was determined foralllocaltime hours. A large dataset from the CHAMP satellite in the southern winter hemisphere under high solar activity was utilized. According to the refined data the amplitude of the longitudinal effect in the MITposition changes from ∼ 3° to ∼ 5° in the course of the day. The shape of the longitudinal effectvarieswith local time, however, the MIT in the eastern hemisphere isconsistentlylocated at higher latitudes than in the western hemisphere. The main reason for the longitudinal effect is the dependence of the equatorward boundary of auroral diffuse precipitation on the tilt angle of the Earth’s dipole. The dependence on geomagnetic activitywas determined as a linear regression ΛT = Λ0 − aKp, where Λ is the geomagnetic latitude, and the Kp indexisconsidered for the previous6 h. The latitude Λ0 and coefficient a exhibited pronounced dependence on local time, with Λ0 increasing and a decreasing when moving from night to day. Because the amplitude of the longitudinal effect decreases with increasing magnetic activity, the value of a alsodependson longitude. Consequently, coefficienta wasdetermined separatelyforthe eastern and western hemispheres. Theaveragevaluesof a vary from 1.3 − 1.4° during the day to 1.8 − 1.9° at night. Thedifference between theeastern and western hemispheres is ∼ 0.3°.
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