Solar wind correlations are investigated with a spectral analysis. We employed magnetic field and plasma data from Wind and ACE for a whole year (1999), when the inter‐spacecraft separation varied over a wide range in all directions. We derived the amplitude ratio, coherence, and phase lag for periods between 16 and 2048 min, which allowed us to compare the dependence of the correlation on periods. The amplitude ratio lies typically between and in the whole period range, while the coherence is only large for periods longer than ∼128 min. When the coherence decreases at short periods the phase lag becomes random, indicating that it is hard to predict the arrival time of fluctuations over such short periods at downstream points. The power spectral density of short‐period fluctuations follows a −5/3 power law variation with frequency, indicative of turbulence. The average values of the coherence depend on the spacecraft separation in the direction parallel to the Sun‐Earth line as well as in the direction perpendicular to it. The coherence scale length parallel to the Sun‐Earth line is estimated as 125 RE, while that perpendicular to it is 44 RE. One possible reason for this difference might be the convective effect of the solar wind.
