A relationship between rise times and decay times of relativistic solar particle events observed by neutron monitors

K.-L. Klein (Obs. Paris, France), S. Musset (ESA, ESTEC, The Netherlands), N. Fuller (Obs. Paris, France), G. Khreich (Obs. Paris, France), A. Wargnier (Obs. Paris, France)

The most energetic particles accelerated in solar eruptive events are protons with energies up to a few tens of GeV (Ground-Level Enhancements, GLEs). Their study is relevant on the one hand because the high particle energies pose particularly strong challenges on the understanding of the acceleration processes. On the other hand, the secondary particles from the atmospheric cascade constitute a source of irradiation in the atmosphere that may temporarily exceed the permanent dose rate from galactic cosmic rays. The monitoring of radiation doses received by aircrew from GLEs is one issue of space weather services for aviation, which since 2019 has also been established as a permanent real-time service for international aviation under the auspices of the United Nations agency ICAO. In this contribution we report an empirical investigation of the rise times and decay times of historical GLEs, focussing on the time profile (corrected to sea-level) observed by the neutron monitor of the worldwide network with the strongest response, which is presumably best-connected to the Sun by the interplanetary magnetic field at the time of the event. Data from the neutron monitor database ( www.nmdb.eu) at the University of Kiel (Germany) and the GLE database at the University of Oulu ( https://gle.oulu.fi/#/, Finland) are used. As previous studies by Strauss and coworkers (Solar Phys. 2017) we find an empirical correlation between the observed rise times and decay times of the neutron monitor count rate profiles. We identify possible evidence of different types of events and compare them with previous work (Vashenyuk, McCracken, Moraal and coworkers) on the contribution of prompt and delayed particle releases to GLEs. Ideas on the origin of the relationship between rise times and decay times and on its usefulness for space weather services will be discussed.