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The radial expansion of coronal mass ejections (CMEs) is known to occur fromremote observations; from the variation of their properties with radialdistance; and from local in situ plasma measurements showing a decreasing speedprofile throughout the magnetic ejecta (ME). However, little is known on howlocal measurements compare to global measurements of expansion. Here, wepresent results from the analysis of 42 CMEs measured in the inner heliosphereby two spacecraft in radial conjunction. The magnetic field decrease withdistance provides a measure of their global expansion. Near 1 au, the decreasein their bulk speed provides a measure of their local expansion. We find thatthese two measures have little relation with each other. We also investigatethe relation between characteristics of CME expansion and CME properties. Wefind that the expansion depends on the initial magnetic field strength insidethe ME, but not significantly on the magnetic field inside the ME measured near1 au. This is an indirect evidence that CME expansion in the innermostheliosphere is driven by the high magnetic pressure inside the ME, while by thetime the MEs reach 1 au, they are expanding due to the decrease in the solarwind dynamic pressure with distance. We also determine the evolution of the MEtangential and normal magnetic field components with distance, revealingsignificant deviations as compared to the expectations from force-free fieldconfigurations as well as some evidence that the front half of MEs expand at afaster rate than the back half.

Inconsistencies Between Local and Global Measures of CME Radial Expansion as Revealed by Spacecraft Conjunctions