Identification of backside solar proton events

Solar proton events, whose fluxes are larger than 10 particles cm-2 sec-1 ster-1 for >10 MeV protons, have been observed since 1976. NOAA proton event list from 1997 to 2006 shows that most of the events are related to both flares and CMEs but a few fraction of events (5/93) are only related with CMEs. In this study, we carefully identified the sources of these events. For this, we used LASCO CME catalog and SOHO MDI data. First, we examined the directions of CMEs related with the events and the CMEs are found to eject from the western hemisphere. Second, we searched a major active region in the front solar disk for several days before the proton events occurred by taking into account two facts: (1) The location of the active region is consistent with the position angle of a given CME and (2) there were several flares in the active region or the active region is the largest among several candidates. As a result, we were able to determine active regions which are likely to produce proton events without ambiguity as well as their longitudes at the time of proton events by considering solar rotation rate, $13.2^{\circ}$ per day. From this study, we found that the longitudes of five active regions are all between $90^{\circ}W$ and $120^{\circ}W$. When the flare peak time is assume to be the CME event time, we confirmed that the dependence of their rise times (proton peak time - flare peak time) on longitude are consistent with the previous empirical formula. These results imply that five events should be also associated with flares which were not observed because they occurred from back-side.