Two Exceptions in the Large SEP Events of Solar Cycles 23 and 24

Abstract

We discuss our findings from a survey of all large solar energetic particle (SEP) events of Solar Cycles 23 and 24, i.e. the SEP events where the intensity of > 10 MeV protons observed by GOES was > 10 pfu. In our previous work (Gopalswamy et al. in Geophys. Res. Lett. 41, 2673, 2014) we suggested that ground level enhancements (GLEs) in Cycles 23 and 24 also produce an intensity increase in the GOES > 700 MeV proton channel. Our survey, now extended to include all large SEP events of Cycle 23, confirms this to be true for all but two events: i) the GLE of 6 May 1998 (GLE57) for which GOES did not observe enhancement in > 700 MeV protons intensities and ii) a high-energy SEP event of 8 November 2000, for which GOES observed > 700 MeV protons but no GLE was recorded. Here we discuss these two exceptions. We compare GLE57 with other small GLEs, and the 8 November 2000 SEP event with those that showed similar intensity increases in the GOES > 700 MeV protons but produced GLEs. We find that, because GOES > 700 MeV proton intensity enhancements are typically small for small GLEs, they are difficult to discern near solar minima due to higher background. Our results also support that GLEs are generally observed when shocks of the associated coronal mass ejections (CMEs) form at heights 1.2 – 1.93 solar radii [\(\mathrm{R}_{\odot}\)] and when the solar particle release occurs between 2 – 6 \(\mathrm{R}_{\odot}\). Our secondary findings support the view that the nose region of the CME-shock may be accelerating the first-arriving GLE particles and the observation of a GLE is also dependent on the latitudinal connectivity of the observer to the CME-shock nose. We conclude that the GOES > 700 MeV proton channel can be used as an indicator of GLEs excluding some rare exceptions, such as those discussed here.