A Miyake event is an observed sharp enhancement of the production of cosmogenic isotopes by cosmic rays. It can be marked by a spike in the concentration of radioactive carbon isotope C in tree rings, as well as Be and Cl in ice cores, which are all independently dated. At present, five significant events are known (7176 BCE, 5259 BCE, 660 BCE, 774 CE, 993 CE) for which the spike in C is quite remarkable, i.e. above 1% rise over a period of 2 years, and four more events (12,350 BCE, 5410 BCE, 1052 CE, 1279 CE) need independent confirmation. It is not known how often Miyake events occur, but from the available data it is estimated to be every 400–2400 years.