  The Interplanetary Magnetic Field shows complex spatial and temporal
variations. Single spacecraft measurements reveal only a one dimensional
section of this rich four dimensional phenomenon. Multi-point measurements of
the four Cluster spacecraft provide a unique tool to study the spatiotemporal
structure of the field. Using Cluster data we determined three dimensional
correlation functions of the fluctuations. By means of the correlation function
one can describe and measure field variations. Our results can be used to
verify theoretical predictions, to understand the formation and nature of solar
wind turbulence. We found that the correlation length varies over almost six
orders of magnitude. The IMF turbulence shows significant anisotropy with two
distinct populations. In certain time intervals the ratio of the three axes of
the correlation ellipse is 1/2.2/6 while in the remaining time we found
extremely high correlation along one axis. We found favoured directions in the
orientation of the correlation ellipsoids.
