Cycle-to-Cycle Variations in the Properties of Solar Energetic Particle Events
Thursday, 18 December 2014: 9:47 AM
We report on a cycle-to-cycle survey of Solar Energetic Particle (SEP) events using data from ACE, STEREO, GOES, SOHO, and other near-Earth spacecraft. We have compared the characteristics of solar proton events during cycles 21 – 24, including peak-intensity, fluence, spectral and compositional signatures. As of July 2014, a total of 72 cycle-24 events have met the NOAA criterion >10 protons/cm2-sr-s for >10 MeV proton intensities at either GOES, STEREO-A, or STEREO-B, including 33 at GOES. Most were observed at multiple longitudes. Compared to the first 5.7 years of cycles 22 and 23 the number of near-Earth GOES-class SEP events in cycle 24 is reduced by ~40%, and the total fluence of >10 MeV protons is a factor of ~4 lower. At >100 MeV the cycle 23 fluence is lower by a factor of >7. The two STEREOs make it possible to test whether Earth was simply in an anomalous location. There are also significant compositional differences between cycles 23 and 24 in abundance ratios such as Fe/O and Fe/H. We discuss factors that may have affected the intensity, spectra, and composition of SEPs during cycle 23, including the interplanetary magnetic field and solar wind properties, CME and shock properties, and seed particle abundances.