The geophysical significance of the thin nitrate-rich layers that have been found in both Arctic and Antarctic firn and ice cores, dating from the period 1561-1991, is examined in detail. It is shown that variations of meteorological origin dominate the record until the snow has consolidated to high-density firn some 30 years after deposition. The thin nitrate layers have a characteristic short timescale (<6 weeks) and are highly correlated with periods of major solar-terrestrial disturbance, the probability of chance correlation being less than 10-9. A one-to-one correlation is demonstrated between the seven largest solar proton fluence events that have been observed since continuous recording of the cosmic radiation started in 1936, and the corresponding thin nitrate layers for the event date. The probability of this occurring by chance is <10-6. This high degree of statistical correlation, together with the modeling studies of Jackman, Vitt, and coworkers, is interpreted as establishing that the impulsive nitrate events are causally related to the generation of energetic particles by solar activity. The timescale of the nitrate events is too short to be understood in terms of transport mechanisms in the gaseous phase and indicates that the nitrate must be precipitated to the polar caps by the gravitational sedimentation of stratospheric solid particles. A conversion factor is established between the impulsive transient nitrate concentrations and the >30 MeV solar proton fluence. The proton fluences (omnidirectional fluence cm-2) derived from the 70 largest impulsive nitrate events between 1561 and 1950 are tabulated. The proton fluence probability distribution derived from these large impulsive nitrate events are in good agreement with earlier studies of the cumulative probabilities of solar proton events and with the observation of cosmogenic isotopes in moon rocks. The cumulative probability curve derived from the impulsive nitrate events indicates a rapidly decreasing probability of occurrence of >30 MeV solar proton events having an omnidirectional fluence exceeding 6×109cm-2. It is concluded that the impulsive nitrate events are reliable indicators of the occurrence of large fluence solar proton events and that they provide a quantitative measure of these events. It is further concluded that the impulsive nitrate events will permit the study of solar activity for many thousands of years into the past.