North/South Hemispheric Periodicities in the ${>}\,25\mbox{ MeV}$ Solar Proton Event Rate During the Rising and Peak Phases of Solar Cycle 24

Abstract

We present evidence that > 25 MeV solar proton events show a clustering in time at intervals of ≈ six months that persisted during the rising and peak phases of Solar Cycle 24. This phenomenon is most clearly demonstrated by considering events originating in the northern or southern solar hemispheres separately. We examine how these variations in the solar energetic particle (SEP) event rate are related to other phenomena, such as hemispheric sunspot numbers and areas, rates of coronal mass ejections, and the mean solar magnetic field. Most obviously, the SEP event rate closely follows the sunspot number and area in the same hemisphere. The ≈ six-month variations are associated with features in many of the other parameters we examine, indicating that they are just one signature of the episodic development of Cycle 24. They may be related to the "≈150 day" periodicities reported in various solar and interplanetary phenomena during previous solar cycles. The clear presence of ≈six-month periodicities in Cycle 24 that evolve independently in each hemisphere conflicts with a scenario suggested by McIntosh et al. (2015, Nature Com. 6, 6491) for the variational time scales of solar magnetism.

25 MeV solar proton events in Cycle 24 and previous cycles

“…Other intervals when sun spots and SEP events from a particular hemisphere are dominant are evident, including a brief interval of enhanced southern activity in 2012 that follows the northern hemisphere activity peak and included the largest SEP event observed (at STEREO A) so far in this cycle (Russell et al, 2013). In addition, there are quasi-periodic ∼ 6-7 month (i.e., data point) variations in the SEP rates in each hemisphere that are also evident in the respective sunspot number and areas and are present during much of the rise and peak phases of Cycle 24, as discussed in more detail by Richardson et al (2016).…”

“…For Solar Cycle 24, we use an updated version (Richardson et al, 2016) of the catalog of 25 MeV proton events compiled by Richardson et al (2014) using observations from the High Energy Telescopes (von Rosenvinge et al, 2008) on the STEREO A ("Ahead") and B ("Behind") spacecraft (Kaiser et al, 2008) and from near-Earth spacecraft, since launch of the STEREO spacecraft on October 26, 2006. STEREO A and B moved ahead or behind the Earth in its orbit, respectively, advancing at ∼ 22 • /year, and passed each other on the far side of the Sun in March 2015.…”

“…By individual events, we mean that an event observed at more than one location is only counted once. No rate is shown for 2015 since contact was lost with STEREO B on October 1, 2014, and data were only received intermittently from STEREO A, located behind the Sun as viewed from Earth (Richardson et al, 2016).…”

“…Unlike the event rate at Earth, which peaked in 2014, the combined near-Earth and STEREO rate peaked in 2012. However, it is likely that the 2014 rate is depressed by the loss of STEREO B data and intermittent STEREO A observations when on the far side of the Sun (c.f., Richardson et al (2016)). …”

“…These are separated by a temporary decrease in the occurrence of 25 MeV proton events that is probably a manifestation of the "Gnevyshev gap" (GG), a temporary decrease in energetic solar activity, including SEP events, often found near solar maximum (e.g., Gnevyshev (1967), Gnevyshev (1977), Storini et al (2003); Norton and Gallagher (2010)). This feature is examined in more detail in Figure 3 where we show the 25 MeV proton event rate/month for Cycle 24 (and also Cycle 23) in the top panel, separating the events by whether they originated in the northern (black) or southern (red) solar hemisphere based on the location of the related solar activity (see Richardson et al (2016) for further details). To increase the SEP event statistics in Cycle 24, we include here events detected by the STEREO spacecraft as well as those observed at the Earth.…”