Correlation of ICME Magnetic Fields at Radially Aligned Spacecraft

The B-field structures of two ICMEs each observed by a pair of spacecraft close to radial alignment have been analysed. The ICMEs were observed in situ by MESSENGER and STEREO-B in Nov. 2010 and Nov. 2011, while the spacecraft were separated by more than 0.6 AU in heliocentric distance, less than 4{\deg} in heliographic longitude, and less than 7{\deg} in heliographic latitude. Both ICMEs took around 2 days to travel between the spacecraft. The ICME B-field profiles observed by MESSENGER have been mapped to the heliocentric distance of STEREO-B and compared directly to the profiles observed by STEREO-B. Figures that result from this mapping allow for easy qualitative assessment of similarity in the profiles. Macroscale features in the profiles that varied on timescales of 1 hour, and which corresponded to the underlying flux rope structure of the ICMEs, were well correlated in the solar east-west and north-south directions, with Pearson's correlation coefficients of around 0.85 and 0.95, respectively; microscale features with timescales of 1 minute were uncorrelated. Overall correlation values in the profiles of one ICME were increased when an apparent change in the flux rope axis direction between the observing spacecraft was taken into account. The high degree of similarity seen in the B-field profiles may be interpreted in in two ways. If the spacecraft sampled the same region of each ICME (i.e. if the spacecraft angular separations can be neglected), the similarity indicates that there was little evolution in the underlying structure of the sampled region during propagation. Alternatively, if the spacecraft observed different, nearby regions within the ICMEs, it indicates that there was spatial homogeneity across those different regions. The field structure similarity observed in these ICMEs points to the value of placing in situ space weather monitors well upstream of the Earth.