The Solar Orbiter and Solar Probe Plus missions will provide observations of magnetic clouds closer to the Sun
than ever before, and it will be good preparation for these missions to make full use of the most recent in situ data
sets from the inner heliosphere—namely, those provided by MErcury Surface, Space ENvironment, GEochemistry,
and Ranging (MESSENGER) and Venus Express—for magnetic cloud studies. We present observations of the
same magnetic cloud made by MESSENGER at Mercury and later by Solar TErrestrial RElations Observatory-B
(STEREO-B), while the spacecraft were radially aligned in 2011 November. Few such radial observations of
magnetic clouds have been previously reported. Estimates of the solar wind speed at MESSENGER are also
presented, calculated through the application of a previously established technique. The cloudʼs flux rope has been
analyzed using force-free fitting; the rope diameter increased from 0.18 to 0.41 AU (corresponding to an rH
0.94
dependence on heliocentric distance, rH), and the axial magnetic field strength dropped from 46.0 to 8.7 nT (an -rH
1.84 dependence) between the spacecraft, clear indications of an expanding structure. The axial magnetic flux was
∼0.50 nT AU2 at both spacecraft, suggesting that the rope underwent no significant erosion through magnetic
reconnection between MESSENGER and STEREO-B. Further, we estimate the change in the cloudʼs angular width
by assuming helicity conservation. It has also been found that the rope axis rotated by 30° between the spacecraft
to lie close to the solar equatorial plane at STEREO-B. Such a rotation, if it is a common feature of coronal mass
ejection propagation, would have important implications for space weather forecasting.