Table 1 Summary of Study Period Geomagnetic Activity.

From: Long-Term Study of Heart Rate Variability Responses to Changes in the Solar and Geomagnetic Environment

Category Ap index range Days, n
Quiet <8 41
Unsettled 8–16 48
Active 16–30 35
Minor storm 30–50 17
Major storm 50–100 9
Severe storm >100 3