can also be produced at ground level, primarily by cosmic rays that penetrate the atmosphere as far as the earth's surface, but also by spontaneous fission of naturally occurring uranium.ratio having remained the same over the preceding few thousand years.
The known fluctuations in the strength of the earth's magnetic field match up quite well with this oscillation: cosmic rays are deflected by magnetic fields, so when there is a weaker magnetic field, more production and an older apparent age.
A secondary oscillation is thought to be caused by variations in sunspot activity, which has two separate periods: a longer-term, 200-year oscillation, and a shorter 11-year cycle.
A polarity excursion, which can be either global or local, is a shorter-lived version of a geomagnetic reversal.
A local excursion would not significantly affect 14C production.
Two different trends can be seen in the tree ring series.
First, there is a long-term oscillation with a period of about 9,000 years, which causes radiocarbon dates to be older than true dates for the last 2,000 years and too young before that.