THE LONGITUDINAL DEPENDENCE OF HEAVY-ION COMPOSITION IN THE 2013 APRIL 11 SOLAR ENERGETIC PARTICLE EVENT

On 2013 April 11 active region 11719 was centered just west of the central meridian; at 06:55 UT, it erupted with an M6.5 X-ray flare and a moderately fast (similar to 800 km s(-1)) coronal mass ejection. This solar activity resulted in the acceleration of energetic ions to produce a solar energetic particle (SEP) event that was subsequently observed in energetic protons by both ACE and the two STEREO spacecraft. Heavy ions at energies >= 10 MeV nucleon(-1) were well measured by SEP sensors on ACE and STEREO-B, allowing the longitudinal dependence of the event composition to be studied. Both spacecraft observed significant enhancements in the Fe/O ratio at 12-33 MeV nucleon(-1), with the STEREO-B abundance ratio (Fe/O = 0.69) being similar to that of the large, Fe-rich SEP events observed in solar cycle 23. The footpoint of the magnetic field line connected to the ACE spacecraft was longitudinally farther from the flare site (77 degrees versus 58 degrees), and the measured Fe/O ratio at ACE was 0.48, 44% lower than at STEREO-B but still enhanced by more than a factor of 3.5 over average SEP abundances. Only upper limits were obtained for the He-3/He-4 abundance ratio at both spacecraft. Low upper limits of 0.07% and 1% were obtained from the ACE sensors at 0.5-2 and 6.5-11.3 MeV nucleon(-1), respectively, whereas the STEREO-B sensor provided an upper limit of 4%. These characteristics of high, but longitudinally variable, Fe/O ratios and low He-3/He-4 ratios are not expected from either the direct flare contribution scenario or the remnant flare suprathermal material theory put forth to explain the Fe-rich SEP events of cycle 23.