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 (~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° versus 58°), 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 ^3He/^4He 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 ^3He/^4He 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.