Composition variations and their implications for SEP acceleration and transport processes

The composition of solar energetic particle (SEP) events varies significantly from event to event and with energy within individual events.  For example, although the ‘typical’ Fe/O ratio is considered to be 0.134, it is observed to vary from <0.01 to >1 and often shows strong dependence on energy.  Several processes and conditions may contribute to this variability, including the dominant acceleration process (e.g., diffusive shock acceleration versus magnetic reconnection), the properties of the seed population being accelerated, the conditions of the interplanetary medium  as the particles travel from the acceleration region to the observer.  Here we examine the composition variability as measured by older missions such as ACE and STEREO, but also include more recent observations from Parker Solar Probe, Solar Orbiter and the newly launch Interstellar Mapping and Acceleration Probe (IMAP).  The combination of these measurements also provides the capability of examining the composition of individual events as a function of radius and longitude.