Speaker
Dr
Ian Richardson
(University of Maryland)
Description
Solar energetic particle events, specifically those including ~25 MeV protons, observed by the STEREO spacecraft and/or at the Earth since 2006 and cataloged by Richardson et al. (2014), show evidence of clustering in time during the rise of Solar Cycle 24, with groups of events occurring at intervals of around 6 months. By considering separately the occurrence rates of events with sources in the north or south solar hemisphere, we suggest that these “periodicities” in the SEP rates have continued through much of the cycle so far, but that the rates vary independently in each hemisphere. In particular, they are typically in anti-phase except early in the cycle, when they were closer in phase. We examine how these variations in the SEP rate are related to other phenomena such as the sunspot number and area in each hemisphere, rates of coronal mass ejections at the Sun and interplanetary coronal mass ejections at Earth, the interplanetary magnetic field, solar magnetic field (“Sun as a star”), geomagnetic activity and the galactic cosmic ray intensity. Most obviously, and not unexpectedly, the SEP rate closely follows the sunspot number and area in the same hemisphere, including the sunspot maximum in the north that was followed two years later by the southern maximum. We also note the remarkably different time-development of activity in the northern and southern hemispheres during Cycle 24 compared to Cycle 23, where the rise and fall of activity in each hemisphere was more symmetrical. The ~6 month periodicities are similar to the “~150 day” periodicities reported in various solar and interplanetary phenomena during other solar cycles. Understanding the underlying processes driving such variations may help to improve space weather forecasting on time scales of many months.
Author
Dr
Ian Richardson
(University of Maryland)
Co-authors
Dr
Hilary Cane
(University of Tasmania)
Tycho von Rosenvinge
(NASA/Goddard Space Flight Center)
