Proton velocity data from identical plasma instruments on board NASA's two Stereo spacecraft are used to investigate the correlation length of large‐scale radial velocity fluctuations in the solar wind in the direction tangent to the Earth's orbit. The data cover the period from February 2007 through August 2007, near solar minimum in 2007. Estimates of the spatial correlation function are based on simultaneous 10 min averages of the solar wind speed at the two spacecraft that are binned according to the distance between the spacecraft. The results for the correlation function are noisy due, in part, to the modest number of data points in each bin. Linear least squares fits are used to extract the correlation length from noisy data. It is found that the transverse correlation length (along the Y‐direction) is 0.25 ± 0.02 AU which is approximately equal to the longitudinal correlation length (along the X‐direction) 0.28 ± 0.05 AU measured using the same data. Assuming that the autocorrelation time of the velocity signal at one spacecraft is determined by the rate of rigid rotation of solar wind source structures past the Earth‐Sun line, the observed equivalence of longitudinal and transverse integral scales can be understood as a rotation effect. It must be emphasized that the correlation length studied here pertains to large‐scale solar wind fluctuations associated with the large‐scale flow, not to the smaller‐scale fluctuations associated with solar wind turbulence.
