@misc{indiciae704f654999a8,
  title = {Semiannual variation of geomagnetic activity.},
  author = {Russell, C. T. and Mcpherron, R. L.},
  year = {1973},
  url = {https://ntrs.nasa.gov/citations/19730034432},
  note = {Source identifier: 19730034432}
}