Latitudinal Variability of Stratospheric 14C Production During Major Solar Energetic Particle Events

Radiocarbon measurements in annually-dated tree rings provide a proxy for past concentrations of 14CO2 in Earth’s troposphere. Since 14C is primarily produced by high-energy particles striking the stratosphere, this proxy can be further extrapolated to a record of cosmic ray modulation due to the 11-year solar cycle, as well as large short-term increases in particle flux, known as Miyake Events, which are thought to be solar particle events much stronger than any seen during the satellite era. Since modern solar events have all been below the noise floor of the 14C record, we lack a well-measured example to calibrate Miyake event estimates against. Previous estimates of the timing and particle flux during Miyake events have modeled the geomagnetic field as a static dipole, which may not yield the proper latitudinal distribution of 14C if the particle events coincided with major geomagnetic disturbances. Additionally, it is not known whether the energetic particles in Miyake events were primarily driven by flares or by CMEs, nor whether each Miyake event was a single large event or several smaller ones below the time resolution of tree ring measurements. We explore the variability of latitudinal 14C production during well-measured solar particle events from the past 30 years using simulated dose maps from the Nowcast of Atmospheric Ionizing Radiation for Aviation Safety (NAIRAS) model. By analyzing correlations between the 14C distribution and the spectra, duration, geoeffectiveness, and structure of the producing events, we quantify potential error sources in Miyake event particle flux estimates, and determine which event parameters could potentially be back-calculated using dendrochronological 14C measurements across a wide range latitudes.