CME Expansion as Revealed by Joint Measurements by STEREO, Wind, MESSENGER and Venus Express

The radial expansion of magnetic ejecta (ME) has been investigated through the analysis of remote observations, the variation of their properties with radial distance and from local in situ plasma measurements showing a decreasing speed profile, as first discussed almost 40 years ago. However, little is known on how local measurements compare to global measurements of expansion and what causes the different expansion properties of different CMEs. In order to correctly forecast CME properties at Earth from measurements below 0.9 AU CME expansion must be considered, and first, understood.  Here, we take advantage of 42 CMEs being measured by two spacecraft in radial conjunction to determine how the magnetic field decrease with distance, as a measure of their global expansion. As all these CMEs are also measured near 1 AU by STEREO or Wind, we are able to determine their local expansion from the speed decrease. We find that these two measures have little relation with each other, even when looking only at the events with the closest conjunctions (in term of angular separation). We also determine the relation between measures of the CME expansion and the CME properties. Lastly, we also determine the evolution of the ME radial, azimuthal and north-south magnetic field with distance, which allow us to compare their evolution with the expectations from force-free field configurations.