Multi-point Observation of SEP Events: Longitudinal and Radial Effects

The rising phase of solar cycle 24 has been providing multiple examples of solar energetic particle (SEP) events observed not only by spacecraft well separated in heliolongitude (e.g., STEREO, ACE) but also by the MESSENGER spacecraft moving toward, and later in, orbit around Mercury at heliocentric distances ranging from 0.3 to 0.4 AU. Two of the largest events observed so far (18 August 2010 and 4 June 2011) occurred when STEREO-A and MESSENGER were to a good approximation magnetically connected to the same region of the Sun. This nominal connection between both spacecraft allows us to analyze radial gradients of SEP intensities along field lines. A lower limit for the gradient of the SEP peak intensities along the same magnetic field line can be inferred from these observations. Similarly, during the decay phase of the events, periods of similar intensities are observed by both spacecraft, indicating that there is a null gradient of SEP intensities along a field line, i.e., they both were observing an energetic particle "reservoir" at different helioradii. Observations of radial intensity gradients and energetic particle reservoirs made by the Helios spacecraft during solar cycle 21 provide an important point of comparison.