The neutron flux at balloon altitudes during a solar proton event

Abstract

A large increase in the atmospheric neutron flux was observed at balloon altitudes on September 3, 1966, during a solar proton event. The flux and energy spectrum of the solar particles are known with some precision from satellite detectors, as well as from our balloon measurements. From these data quantitative comparisons have been made with the predictions of Lingenfelter and Flamm (1964) on the production of neutrons in the atmosphere by p•oton interactions. The theory is in satisfactory agreement with the observations, although there is considerable uncertainty about the fraction of neutrons that leaked out of the atmosphere. The distribution of neutrons produced in the atmosphere by solar protons has been derived by Lingenfelter and Flarnm [1964a]. These authors [1964b] have also calculated the flux of albedo neutrons of energy less than 10 Mev resulting from solar protons. This work is important for the determination of the inject]on rate of geomagnetically trapped protons by the SPAND process (see, for example, Dragt et al. [1966]) and for estimates of the carbon-14 production rate during solar proton events. Tests of the theory of Lingenfelter and Flamm are complicated by an error in their calculations (R. E. Lingenfelter, private communication, 1968) that has the effect of approximately doubling the predicted values of all atmospheric fluxes, although it does not significantly affect the form of the altitude or energy distributions. The effect of this error on the leakage flux calculations [Lingenfelter and Flamm, 1964b] is unknown. Observations by balloon-borne detectors released from Bemidji, Minnesota (X --57øN) by Smith et al. [1962] tend to support the theory, but there is uncertainty of a factor of 5 in the interpretation of the observations because of lack of information about the cutoff rigidity at the time of the flight. Chupp et al. [1967] observed a large neutron flux increase with a moderated borcn tr[fiuoride counter in a rocket flight from a low-!atitude station during the solar-flare event of November 15, 1960. They found that the neutron flux was 10-15 times

Neutron measurements in space

“…The early observations by Korff and Haymes (1960) of enhanced neutron intensities during an auroral display indicated that neutrons were produced in the atmosphere by protons with E> 50 MeV. More recent observations by Lockwood and Friling (1968), Greenhill et al (1970), Light et aI. (1970 of several increases in the fast neutron flux subsequent to solar particle events have confirmed the earlier predictions of Lingenfelter and Flamm (1964a), but there is some discrepancy between the predicted and measured fluxes.…”

“…It is conceivable that the estimated discrepancy could be reduced to less than a factor of two. Lockwood and Friling (1968) and Greenhill et al (1970) observed enhanced neutron fluxes at high latitudes from neutron production by solar protons. A day after the large solar flare event on September 25, 1966, Greenhill and coworkers observed a fourfold increase in count rate of a moderated BF3 detector flown at a low cutoff rigidity.…”

“…Inclusion of possible temporal variations in the proton flux might reduce this disagreement. Greenhill et al (1970) did not apparently make any detailed evaluation of the effects of angular dependence of the solar proton produced neutron intensity when converting from counting rates to fluxes. Lingenfelter and Flamm (1964a) indicate that the angular dependence of the neutron flux near the top of the atmosphere goes as cos z 0, where 0 is the angle with respect to the zenith.…”