Estimating the anomalously large fluences of solar flare protons with tritium data from 1900 to 1952

Abstract

An analysis is presented for estimating the “anomalously large” fluences from solar flare protons by measuring tritium deposited between 1900 and 1952 in a high‐latitude ice core. Most of the tritium from cosmic rays and solar flare protons comes by way of indirect (p, n) and (n, t) reactions rather than from direct (p, t) reactions. Therefore the neutron spectrum produced by protons of the August 1972 flare is estimated first. (The August 1972 flare is the most recent “anomalous” fluence, and it was accurately measured by spacecraft detectors.) Then the calculated neutron spectrum is compared with that known from galactic cosmic rays. Finally, the tritium produced by (n, t) reactions is compared for the solar protons and cosmic rays. We thus calculate that the August 1972 flare‐produced tritium was 67 times larger than the cosmic ray‐produced tritium in all of 1972. Range tables of energetic protons in nitrogen and oxygen gases show that tritium from solar protons is produced in the stratosphere, while that from galactic cosmic rays is produced in both the stratosphere and troposphere. Other studies estimate that the stratosphere residence half‐life time for tritium is 16–20 months and the troposphere residence time is short (on the order of 10 days.) In any event, the tritium will scavange out of the atmosphere, and if it is locked in snow that does not melt in summer months, such as that in glaciers at very high latitudes and elevations, it will remain for many years. Therefore tritium frozen in a high‐latitude glacier would give a good measure of the fluences from the very largest solar flares.