Abstract:
We attempt to understand the influence of the heliospheric state on the expansion
behavior of coronal mass ejections (CMEs) and their interplanetary counterparts (ICMEs)
in solar cycles 23 and 24. Our study focuses on the distributions of the radial sizes and
duration of ICMEs, their sheaths, and magnetic clouds (MCs). We find that the average
radial size of ICMEs (MCs) at 1 AU in cycle 24 is decreased by ∼33% (∼24%) of its value in
cycle 23. This is unexpected as the reduced total pressure in cycle 24 should have
allowed the ICMEs in cycle 24 to expand considerably to larger sizes at 1 AU. To
understand this, we study the evolution of radial expansion speeds of CME-MC pairs
between the Sun and Earth based on their remote and in situ observations. We find that
radial expansion speeds of MCs at 1 AU in solar cycles 23 and 24 are only 9% and 6%,
respectively, of their radial propagation speeds. Also, the fraction of radial propagation
speeds as expansion speeds of CMEs close to the Sun are not considerably different for
solar cycles 23 and 24. We also find a constant (0.63 ± 0.1) dimensionless expansion
parameter of MCs at 1 AU for both solar cycles 23 and 24. We suggest that the reduced
heliospheric pressure in cycle 24 is compensated by the reduced magnetic content
inside CMEs/MCs, which did not allow the CMEs/MCs to expand enough in the later
phase of their propagation. Furthermore, the average radial sizes of sheaths are the same
in both cycles, which is also unexpected, given the weaker CMEs/ICMEs in cycle 24. We
discuss the possible causes and consequences of our findings relevant for future
studies.
