Snodgrass, H.; Ulrich, R. (1990). «Rotation of Doppler features in the solar photosphere». Astrophysical Journal. 351: 309–316. Bibcode:1990ApJ...351..309S. doi:10.1086/168467
Bartels, J. (1934), «Twenty-Seven Day Recurrences in Terrestrial-Magnetic and Solar Activity, 1923-1933», Terrestrial Magnetism and Atmospheric Electricity, 39 (3): 201–202a, Bibcode:1934TeMAE..39..201B, doi:10.1029/TE039i003p00201
Snodgrass, H.; Ulrich, R. (1990). «Rotation of Doppler features in the solar photosphere». Astrophysical Journal. 351: 309–316. Bibcode:1990ApJ...351..309S. doi:10.1086/168467
Bartels, J. (1934), «Twenty-Seven Day Recurrences in Terrestrial-Magnetic and Solar Activity, 1923-1933», Terrestrial Magnetism and Atmospheric Electricity, 39 (3): 201–202a, Bibcode:1934TeMAE..39..201B, doi:10.1029/TE039i003p00201