Separating E from B

In a microwave background polarization map that covers only part of the sky, it is impossible to separate the E and B components perfectly. This difficulty in general makes it more difficult to detect the B component in a data set. Any polarization map can be separated in a unique way into "pure E", "pure B" and "ambiguous" components. Power that resides in the pure E(B) component is guaranteed to be produced by E(B) modes, but there is no way to tell whether the ambiguous component comes from E or B modes. A polarization map can be separated into the three components either by finding an orthonormal basis for each component, or directly in real space by using Green functions or other methods. (C) 2003 Elsevier B.V. All rights reserved.