Long-term variation in the solar F10.7 index and Sq amplitude. Long-term variation in the solar F10.7 index (a) and Sq amplitude obtained at Addis Ababa (b), Bangui (c), Guam (d), and Trivandrum (e) in the equatorial region. The Addis Ababa and Trivandrum stations are located near the dip equator. The vertical dashed lines indicate the times of solar F10.7 minima. The time intervals of these plots are between 1945 and 2015. The plots show a clear variation of Sq amplitude with the 11-y period corresponding to the solar F10.7 index variation.