Nat Gopalswamy
Solar Physics Laboratory, NASA Goddard Space Flight Center
Space Weather Consequences of the Weak Solar Activity Cycle 24
The two main space weather consequences of solar activity are large solar energetic particle (SEP) events and major geomagnetic storms. The weak solar activity during solar cycle 24 seems to affect the SEP production in a peculiar way: the number of large SEP events is similar to that in cycle 23, but the particles are not accelerated to high energies, as evidenced by the dearth of ground level enhancement (GLE) events. The frequency and intensity of large geomagnetic storms are also at a historical low during solar cycle 24. Even though the Sun is in its maximum phase, there have been only a dozen major storms (Dst ≤ -100 nT) in cycle 24 so far, compared to 40 storms during the first 5 years of cycle 23. The storm intensity never exceeded 140 nT, whereas there were 9 storms in cycle 23 with Dst ≤ -200 nT (one of the storms had Dst ~ -301 nT). Both large SEP events and major geomagnetic storms are caused by energetic CMEs. When we examined the properties of the associated CMEs, we found that the average speed of SEP-producing CMEs is larger than that in cycle 23. Furthermore, all SEP-producing CMEs of cycle 24 are halos, compared to about 70% in cycle 23. The average speed of storm-producing CMEs was about 40% higher than that in the first half of cycle 23. The fraction of halo CMEs is also about 20% higher. In other words, cycle-24 CMEs seem to require more energy to produce similar or weaker space weather events. These observations indicate that there is something fundamentally different about cycle-24: CMEs seem to expand anomalously owing to the reduced heliospheric pressure. We suggest that such expansion dilutes the magnetic content of CMEs, thus resulting in weaker interaction with the magnetosphere. Another consequence of the weak cycle is the reduced magnetic field in the heliosphere, which might have reduced the efficiency of particle acceleration.