Abstract
Abstract We report the probable detection of energetic neutral hydrogen atoms (ENAs) at >0.8 MeV in several large solar energetic particle events observed between 1997 and 2004. The low Earth orbiting SAMPEX satellite detected transient increases of quasi-trapped equatorial protons beginning typically ∼3 hr after the X-ray flare and lasting for up to several hours. Since the magnetic cutoff rigidity is >10 GV at the magnetic latitude where the particles were observed, we interpret the signal as due to ENAs that penetrate Earth’s magnetic field and charge exchange in the upper atmosphere, whereupon the charged particles may become trapped. One event outside our survey period (2006 December 5) had previously reported solar flare ENAs, the only example of this phenomenon of which we are aware. Although the statistics are limited, the events we report suggest that the ENAs are produced as the flare-associated coronal mass wjection moves through the corona, as concluded previously for the 2006 December 5 event. The finding of ENAs emitted in conjunction with large solar flares opens a new avenue to understanding these events.
Highlights
Solar energetic particle (SEP) events accelerate ions over the full range of the periodic table, at energies up to hundreds of megaelectronvolts per nucleon or more
Points plotted in red are for geomagnetically undisturbed periods (Dst > −40), while blue points are for disturbed periods (Dst < −40)
Quasi-trapped protons >0.5 MeV observed near the geomagnetic equator have been described previously, wherein particles in the ring current were neutralized by charge exchange with the geocorona, enabling them to penetrate Earth’s magnetic field
Summary
Solar energetic particle (SEP) events accelerate ions over the full range of the periodic table, at energies up to hundreds of megaelectronvolts per nucleon or more. Mewaldt et al (2009) pointed out that ENAs would be expected to be copiously produced near the solar surface, they would be stripped as they moved through the corona They concluded that the ENAs they detected were more likely produced beyond ∼1.6 Rs, including the associated coronal mass ejection (CME)-driven shock. The radiation belts were quiescent in these periods, and Greenspan et al were unable to describe a mechanism by which the protons could reach this location since the magnetic cutoff rigidity exceeds 10 GV It is clear the events reported by Greenspan et al may have been caused by ENAs. The ENAs could pass through Earth’s magnetic field, and some could interact with the upper atmosphere and be stripped. We discuss the observational evidence in these events, and statistical properties of both the solar sources, and details of the detection in low Earth orbit
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