 The flux of particles must be estimated by calculating production rates of radiocarbon, and then modelling the behaviour of the CO2  once it has entered the carbon cycle; the fraction of the created radiocarbon taken up by trees depends to some extent on that cycle.The energetic particle spectrum of a solar flare varies considerably between events; one with a 'hard' spectrum, with more high-energy protons, will be more efficient at producing a 14C increase.The most powerful flare which also had a hard spectrum that has been observed instrumentally took place in February 1956 (the beginning of nuclear testing obscures any possible effects in the 14C record); it has been estimated that if a single flare were responsible for the AD 774/5 event it would need to be 25-50 times more powerful than this.