4.3 SEPs: neutrons
At the biggest flares protons are accelerated to energies of several GeV. Upon interaction with other
atoms they cause nuclear reactions that release Gamma-rays and relativistic neutrons. Chupp
et al. (1982, 1987) reported the first direct detection of solar neutrons from a satellite-borne instrument
following an impulsive solar flare. Identification of the n-p capture line at 2.223 MeV observed at energetic
flares confirms the existence of flare-associated neutrons. Combining these observations, it is possible to
study the solar neutron energy spectrum from ∼1 MeV to several GeV. Ramaty et al. (1983) pointed out
that this allows us to investigate the time history of particle acceleration in solar flares and to
determine the total number and the energy spectrum of the accelerated ions up to the highest
energies.
Due to the absence of charge, solar neutrons can reach the Earth fully unhindered, provided their
lifetime is long enough. That is the case only for relativistic neutrons. However, it is difficult to differentiate
solar neutrons from those neutrons that are generated in the Earth’s atmosphere. In fact, when a high
flux of relativistic SEPs strikes the Earth’s atmosphere, spallation of atmospheric atoms sets
on that lets nuclear byproducts cascade down such that enhanced neutron fluxes reach the
ground. For monitoring these so-called ground level events (GLEs), a network of neutron monitors
has been installed all over the globe, e.g., the “Spaceship Earth” observing network (Bieber
et al., 2004). It provides an effective means of studying the angular distribution and energy
spectrum of the most energetic SEPs. The problem is that these SEPs and the solar neutrons
are generated simultaneously and, thus, should arrive at the Earth almost simultaneously as
well.
During the Halloween events three GLEs were observed. The one on October 28 was of particular
interest: About 8 minutes before the main GLE seen by several stations, the one in Tsumeb, South Africa
saw a very weak but distinct neutron flux (Bieber et al., 2005), simultaneously with the SONG neutron
detector on the CORONAS-F spacecraft (Miroshnichenko et al., 2005
), as is shown in Figure 24.
Assuming that the first solar neutrons were emitted at the onset of the most conspicuous electromagnetic
emissions from the Sun, the travel time to Earth allows an estimate of the neutrons’ energy of 400 to
500 MeV.
.
Assuming that the first solar neutrons were emitted at the onset of the most conspicuous electromagnetic
emissions from the Sun, the travel time to Earth allows an estimate of the neutrons’ energy of 400 to
500 MeV.
