The radial expansion of coronal mass ejections (CMEs) is known to occur from
remote observations; from the variation of their properties with radial
distance; and from local in situ plasma measurements showing a decreasing speed
profile throughout the magnetic ejecta (ME). However, little is known on how
local measurements compare to global measurements of expansion. Here, we
present results from the analysis of 42 CMEs measured in the inner heliosphere
by two spacecraft in radial conjunction. The magnetic field decrease with
distance provides a measure of their global expansion. Near 1 au, the decrease
in their bulk speed provides a measure of their local expansion. We find that
these two measures have little relation with each other. We also investigate
the relation between characteristics of CME expansion and CME properties. We
find that the expansion depends on the initial magnetic field strength inside
the ME, but not significantly on the magnetic field inside the ME measured near
1 au. This is an indirect evidence that CME expansion in the innermost
heliosphere is driven by the high magnetic pressure inside the ME, while by the
time the MEs reach 1 au, they are expanding due to the decrease in the solar
wind dynamic pressure with distance. We also determine the evolution of the ME
tangential and normal magnetic field components with distance, revealing
significant deviations as compared to the expectations from force-free field
configurations as well as some evidence that the front half of MEs expand at a
faster rate than the back half.
