Close Features of Annual and Semiannual Variations Derived from the Global Ionospheric Maps of Total electron Content,
Zhao, B., W. Wan, L. Liu, T. Mao, Z. Ren, M. Wang, A. B. Christensen (2007),
Annales Geophysicae, 25 p. 2513–2527,
10.5194/angeo-25-2513-2007.