Dependence of the main ionospheric trough position on local time, longitude and geomagnetic activity in the southern winter hemisphere

Based on the meticulous identification of ionization troughs, performed earlier from the CHAMP satellite data, two additional issues were resolved: (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 the MIT position on geomagnetic activity was determined for all local time 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 MIT position changes from ∼ 3° to ∼ 5° in the course of the day. The shape of the longitudinal effect varies with local time, however, the MIT in the eastern hemisphere is consistently located 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 activity was determined as a linear regression Λ_T = Λ₀ − aKp, where Λ is the geomagnetic latitude, and the Kp index is considered for the previous 6  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 also depends on longitude. Consequently, coefficient a was determined separately for the eastern and western hemispheres. The average values of a vary from 1.3 − 1.4° during the day to 1.8 − 1.9° at night. The difference between the eastern and western hemispheres is ∼ 0.3°.