The Charge-to-Mass Dependence of SEP Fluences Over Wide Longitudes

Accurate characterization of the transport of energetic particles throughout the inner heliosphere is important for the planning of space missions and the development and testing of space weather forecasting tools. How particles are distributed in both radius and longitude during a solar energetic particle (SEP) event has been the subject of a number of studies. Initially these studies were performed through statistical analysis of single-spacecraft measurements of many different SEP events. Later multi spacecraft observations of individual events were examined, most notably using data from Helios and, very recently, MESSENGER. Currently by combining measurements from near-Earth spacecraft and the twin STEREO spacecraft, particle distributions can be examined as a function of longitude separately from radial dependences. Additionally, while previous studies concentrated on protons and electrons, the SEP sensors on STEREO and ACE allow heavy ions to be examined as well. We have analyzed 5 large SEP events in 2011 and 2012 that were clearly observed by both STEREOs and ACE and determined the longitudinal distribution of the event-integrated fluences for H, He, O at 3.6-5 MeV/nuc and for H, He, O, and Fe at 0.32-0.45 MeV/nuc. We find no consistent charge-to-mass dependence in the longitudinal distributions at either energy suggesting rigidity is not a controlling factor in the particle spread in longitude. We find that typically lower energy ions have a wider longitudinal spread than higher energy ions suggesting a velocity dependence. Both of these results are consistent with the possibility that magnetic field line meandering and/or co-rotation is a primary means of longitudinally transporting particles.