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We compare the properties of halo coronal mass ejections (CMEs) thatoriginate close to the limb (within a central meridian distance range of 60 to90 deg) during solar cycles 23 and 24 to quantify the effect of theheliospheric state on CME properties. There are 44 and 38 limb halos in thecycles 23 and 24, respectively. Normalized to the cycle-averaged total sunspotnumber, there are 42 percent more limb halos in cycle 24. Although the limbhalos as a population is very fast (average speed 1464 km s-1), cycle-24 halosare slower by 26 percent than the cycle-23 halos. We introduce a new parameter,the heliocentric distance of the CME leading edge at the time a CME becomes afull halo; this height is significantly shorter in cycle 24 (by 20 percent) andhas a lower cutoff at 6 Rs. These results show that cycle-24 CMEs become halossooner and at a lower speed than the cycle-23 ones. On the other hand, theflare sizes are very similar in the two cycles, ruling out the possibility oferuption characteristics contributing to the differing CME properties. Insummary, this study reveals the effect of the reduced total pressure in theheliosphere that allows cycle-24 CMEs expand more and become halos sooner thanin cycle 23. Our findings have important implications for the space-weatherconsequences of CMEs in cycle 25 (predicted to be similar to cycle 24) and forunderstanding the disparity in halo counts reported by automatic and manualcatalogs.

The State of the Heliosphere Revealed by Limb Halo Coronal Mass Ejections in Solar Cycles 23 and 24