TY  - RPRT
TI  - Annual and semiannual variations of the geomagnetic field at equatorial locations
AU  - W.H. Campbell
PY  - 1981
DO  - 10.1016/0021-9169(81)90123-9
UR  - https://pubs.usgs.gov/publication/70012123
ID  - 70012123
ER  -