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.
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