The First Ground Level Event of Solar Cycle 24 and Its Longitudinal Distribution in the Inner Heliosphere

Ground level events (GLEs) are the most energetic solar particle events (SEPs) that are detected not only by space borne instrumentation but also by ground-based instruments like neutron monitors. On May 17 2012 at 01 :25 UT a M5. 1 X-ray flare from the active region 1476 (N07, W88) was detected. The event was accompanied by a type III radio burst starting at 1 :30 UT and a coronal mass ejection heading towards STEREO A. The corresponding shock wave passed STEREO A on May 18 at 12:43 UT but missed the Earth. The event onsets of near relativistic electrons have been detected at 06:05 UT, 03:38 UT, and 01:51 UT aboard STEREO A and B (125-335 keV) and at SOHO (250 -700 keV), respectively. In contrast to observations close to the Earth no strong anisotropies have been observed at both, STEREO A and B. The neutron monitor network recorded the first GLE for solar cycle 24. The Electron Proton Helium INstrument on board SOHO measured protons with energies of more than 600 MeV (rigidities of more than 1.2 GV). The interplanetary field direction was such that neutron monitor stations with asymptotic direction in the 1 to 2 GV range over Australia were connected best and recorded the biggest increase of 17% (Apatity and Oulu) with an onset time of 1:52 UT. Data observed close to and at Earth will be presented and the longitudinal structure of the event in the inner heliosphere will be discussed.