1933 May 1 Goemagnetic Storm as an Example of Space weather Event with No Sunspot

When surveying records of space weather event from various records and using them, it is important to understand how to compare with modern observation parameters. In order to investigate long-term fluctuations of natural phenomena seamlessly linking research using materials with modern science, it is also important to utilize observation data in early modern observation. In the solar observation, there are data photographed by photographic plates etc from the end of the 19th century to the first half of the 20th century, and archivalization is progressing little by little.

As an example of utilizing such data, we have looked at sunspot images in the first half of the 20th century and collation of satellite images (CaK line heliograms) over the entire sun and geomagnetism observation data by spectro heliography.

Regarding the decades of the 1930s, when comparing photographic plate data of the solar CaK line heliogram photographed at Kwasan Observatory, Kyoto University, and the aa index which is one of the indicators of geomagnetic storms, it was found that a magnetic storm of a scale exceeding -100 nT, even when it occurred, it was recorded that none of the sunspots existed in nine cases. As an example, at the end of April 1933, there was a time when sunspots were not seen in the CaK line heliogram and sunspot drawing in various places, but on May 1, a geomagnetic storm was observed in Kakioka, the aa index was about -200 nT. The advantage of the CaK line heliogram is that not only black spots but also regions where relatively weak magnetic fields such as plague that is not visible with continuous light are distributed are visualized. This makes it possible to obtain information on the global magnetic neutral line where filaments causing corona mass ejection can exist.