The Seasonal Dependence of the Unexpected Afternoon Westward Equatorial Electrojet During Quiet Time.

The equatorial electrojet is a strong ionospheric current along the dip equator at 110 km. It usually flows eastward and peaks at noon. However, it sometimes reverses westward. Using 10 years of horizontal geomagnetic field observations from a ground station, this work investigated the counter electrojet (CEJ) in the afternoon at Indian longitudes during geomagnetically quiet time, which has a substantial occurrence rate of 46.7%. A total of 41.7% of the afternoon CEJ is related to the westward electric field during substorms. The intensity of the afternoon CEJ has significant seasonal dependences: peaks in June‐September and December‐February. A similar seasonal variation is established in the westward electric field, indicating its key roles. The occurrence rate reaches its peak solely in June‐September, which might also be attributed to the activity of substorms. Plain Language Summary: Ground‐ and space‐based observations show that the daytime equatorial electrojet forms a precise coincidence of the peak at the dip equator and usually peaks at the noon sector. This work found that the quiet‐time counter electrojet has a high occurrence rate, nearly half the time, in the afternoon over Indian longitudes. Knowledge of the associated drivers is still lacking. The study of its behaviors is key to understanding equatorial space weather. Key Points: The afternoon counter electrojet (CEJ) has a strong occurrence rate of 46.7% during quiet timeThe westward electric field during the substorm was responsible for 41.7% of the afternoon CEJBoth the occurrence rate and intensity of the CEJ peak in June‐September [ABSTRACT FROM AUTHOR]