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. (19821987) 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., 2005Jump To The Next Citation Point), as is shown in Figure 24View Image. 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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Figure 24: Increase profiles of the GLE observed with 5-min averages by Tsumeb and Moscow neutron monitors. The vertical arrow indicates the start of bright radio emission at 1102 UT. The horizontal bar marks the time when neutrons were detected by the SONG instrument at CORONAS-F spacecraft. From Miroshnichenko et al. (2005).

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