@misc{indiciaec46a08c6adc6,
  title = {Annual and semiannual variations of the geomagnetic field at equatorial locations},
  author = {W.H. Campbell},
  year = {1981},
  doi = {10.1016/0021-9169(81)90123-9},
  url = {https://pubs.usgs.gov/publication/70012123},
  note = {Source identifier: 70012123}
}