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Ground level events (GLEs) occupy the high-energy end of gradual solarenergetic particle (SEP) events. They are associated with coronal massejections (CMEs) and solar flares, but we still do not clearly understand thespecial conditions that produce these rare events. During Solar Cycle 23, atotal of 16 GLEs were registered, using ground-based neutron monitor data. Wefirst ask if these GLEs are clearly distinguishable from other SEP eventsobserved from space. Setting aside possible difficulties in identifying allGLEs consistently, we then try to find observables which may unmistakablyisolate these GLEs by studying the basic properties of the associated eruptionsand the active regions (ARs) that produced them. It is found that neither themagnitudes of the CMEs and flares nor the complexities of the ARs givesufficient conditions for GLEs. It is possible to find CMEs, flares or ARs thatare not associated with GLEs but that have more extreme properties than thoseassociated with GLEs. We also try to evaluate the importance of magnetic fieldconnection of the AR with Earth on the detection of GLEs and their onset times.Using the potential field source surface (PFSS) model, a half of the GLEs arefound to be well-connected. However, the GLE onset time with respect to theonset of the associated flare and CME does not strongly depend on howwell-connected the AR is. The GLE onset behavior may be largely determined bywhen and where the CME-driven shock develops. We could not relate the shocksresponsible for the onsets of past GLEs with features in solar images, but thecombined data from the Solar TErrestrial RElations Observatory (STEREO) and theSolar Dynamics Observatory (SDO) have the potential to change this for GLEsthat may occur in the rising phase of Solar Cycle 24.

What Are Special About Ground-Level Events? Flares, CMEs, Active Regions And Magnetic Field Connection