A 42,000 year old tree trunk was unearthed from a bog in New Zealand. The tree lived for 1700 years, including the period of magnetic field reversal. Cave records from China had already dated the magnetic field reversal, but the tree's rings give better resolution. The tree rings tell us that the reversal lasted for five hundred years. The rings store radioactive carbon 14. This tells us how ionized the atmosphere was, which in turn lets us know that the magnetic field went down to a low of 6% of what it is today. The aurora polaris was visible everywhere on Earth. If the power grid of today had existed the induced excess electrical charges would have "knocked it out."
https://www.sciencemag.org/news/2021/02/ancient-kauri-trees-capture-last-collapse-earth-s-magnetic-field