Solar eruptive events, such as solar flares and coronal mass ejections, can accelerate protons and other ions to high energies (> 30 MeV). The relative roles of both acceleration mechanisms and how we can discriminate them remains a key problem in solar and solar-terrestrial physics.

When relativistic SEPs (> 500 MeV) reach the Earth, they can enter the Earth’s atmosphere and produce cascades of secondary particles similar to galactic cosmic rays (GCRs) that can be observed by ground based neutron monitors (NMs). These events are known as Ground Level Events or Ground Level Enhancements (GLEs).

The figure below shows as an example the relative count rate increase of selected NM stations during the GLE on 13 December 2006. Since the observation of the first GLE on 28 February 1942 a total of about 70 GLEs have been detected until mid of 2016, i.e. about one GLE per year.

Why GLE investigations

The interest in GLEs is manifold:

• There is a practical interest in GLEs owing to their significance for space weather. Solar cosmic rays (SCRs) can damage spacecraft electronic components and are a significant radiation hazard to astronauts.
• Although detailed case studies have been conducted on a number of individual GLEs, the conditions and processes that are responsible for these extreme SCR events are still not fully understood.