Energetic (approx 10-65 MeV) protons observed by ERNE on August 13-14, 1996: Eruption on the solar back side as a possible source of the event

摘要

The onset of the > 10-MeV proton event of August 13-14, 1996, revealed a velocity dispersion, which is a signature of its solar origin, but no associated soft X ray flare was observed. The LASCO CME observations, the presence of AR 7981 beyond the west limb, and type II and IV radio burst timing with respect to the proton event onset indicate that the parent solar eruption may be centered on the back side of the Sun, at approx 150 deg W. In such a case, expanding CME-associated wave can reach the Earth-connected interplanetary magnetic field line in approx 1 hour and so give rise to the > 10-MeV proton event observed with the Energetic and Relativistic Nuclei and Electron (RENE) instrument onboard SOHO. We verify this hypothesis against observational data and conclude that a solar back side eruption is the most plausible explanation of the August 13, 1996, event. We compare the August 13, 1996, event with events associated with Earth directed CMEs and show that the August 13, 1996, event reveals many properties common to > 10-MeV proton events originating from solar eruptions centered approx 90 deg away from the root of the Earth-connected interplanetary magnetic field line. In such events, the first detected protons are released approx 1 hour after the start time of type II and IV radio bursts. The first injection spectrum is essentially harder than the spectrum at the intensity maximum; that is, the hard but less intensive proton production is followed by the major soft-spectrum production when CME expands farther from the Sun.