In the 1940s, long before modern sensors, instruments on Earth were already catching particles from space. When a powerful flare erupted on the Sun, it ejected a stream of charged particles — solar energetic particles. They reached Earth and left their mark in old ionization chambers, which were like metal cans sensitive to electric charge. Four such events, occurring in 1942 and 1946, long remained without precise measurements: the data were scattered across archives and not digitized.

Now researchers have gathered old records from stations around the world and converted them into digital form. This allowed them to reconstruct how the particle flux changed in 1–15 minute intervals. It turned out that two storms built up slowly, over 45 and 105 minutes, while the other two struck abruptly, in just 15 minutes. This difference suggests that the particle acceleration mechanisms on the Sun were different: some flares acted like a slow heating, others like an instant shock.

By comparing instrument readings at different latitudes, scientists estimated the "hardness" of the particle spectrum: that is, the fraction of the most energetic particles. Two storms turned out to be extremely hard, and two moderately so. This is important for space weather forecasting: such particles can damage satellites and disrupt radio communications. Old data, like photographs found in an archive, help us see the whole picture.