Large Energetic Particle Pressures in Solar Cycles 23 and 24

We study periods of elevated energetic particle intensities observed at the L1 Sun-Earth Lagrangian point when the partial energy density associated with energetic (≥80 keV) particles (P_EP) dominates that of the local magnetic field (P_B) and thermal plasma populations (P_PLS). These periods are not uncommon and are frequently observed prior to the passage of interplanetary (IP) shocks. Because of the significant decreases in key solar wind parameters observed during solar cycle 24 [e.g., 1], we were motivated to perform a comparative statistical analysis to determine if the occurrence rate of periods when P_EP exceeded P_B or P_PLS, or both, differed between solar cycles 23 and 24. We find that the general decrease of P_B and P_PLS in solar cycle 24 was also accompanied by a general decrease of periods with elevated P_EP. The result is that solar cycle 24 showed a lower number of time intervals dominated by P_EP. We analyze whether these differences can be related to the properties of the IP shocks observed at L1. Incomplete datasets of shock parameters do not show significant differences between solar cycles 23 and 24 that would allow us to explain the difference in the number of periods with P_EP>P_B and P_EP>P_PLS. We analyze then the averaged plasma parameters measured in the upstream region of the shocks and find significantly lower solar wind proton temperatures and magnetic field magnitude upstream of IP shocks in solar cycle 24 compared with those in solar cycle 23. These factors, together with the lower level of solar activity, may explain the lower particle intensities in solar cycle 24 and hence the fewer events with P_EP>P_B and P_EP>P_PLS.