  Measurements of coronal mass ejections (CMEs) by multiple spacecraft at small
radial separations but larger longitudinal separations is one of the ways to
learn about the three-dimensional structure of CMEs. Here, we take advantage of
the orbit of the Wind spacecraft that ventured to distances of up to 0.012
astronomical units (au) from the Sun-Earth line during the years 2000 to 2002.
Combined with measurements from ACE, which is in a tight halo orbit around L1,
the multipoint measurements allow us to investigate how the magnetic field
inside magnetic ejecta (MEs) changes on scales of 0.005 - 0.012 au. We identify
21 CMEs measured by these two spacecraft for longitudinal separations of 0.007
au or more. We find that the time-shifted correlation between 30-minute
averages of the non-radial magnetic field components measured at the two
spacecraft is systematically above 0.97 when the separation is 0.008 au or
less, but is on average 0.89 for greater separations. Overall, these newly
analyzed measurements, combined with 14 additional ones when the spacecraft
separation is smaller, point towards a scale length of longitudinal magnetic
coherence inside MEs of 0.25 - 0.35 au for the magnitude of the magnetic field
but 0.06 - 0.12 au for the magnetic field components. This finding raises
questions about the very nature of MEs. It also highlights the need for
additional "mesoscale" multi-point measurements of CMEs with longitudinal
separations of 0.01 - 0.2 au.
