A technique to distinguish the mean magnetic field from the fluctuations in the solar wind? My question is:
Is there a technique to distinguish the mean magnetic field from the fluctuations in the solar wind?
Thanks
 A: 
Is there a technique to distinguish the mean magnetic field from the fluctuations in the solar wind?

Yes, but the definition of mean field is scale-dependent.  That is, in the solar wind the plasma is turbulent.  Different frequency fluctuations will care about different spatial scale sizes, e.g., higher frequency fluctuations will adjust to shorter duration changes in the magnetic field.  There are several methods for determining this but the most important thing is to be clear and consistent with how you implement the method.  That is, make sure the reader, and especially your colleagues, know what you did, why you did it, and why it's okay to do what you did.
The problem is that in regions where the fluctuations become large amplitude, the fluctuation amplitude can meet or exceed the surrounding background magnetic field magnitude.  These types of fluctuations can cause headaches because they become the background field, if you will, due to their huge amplitudes.  A region of space where this consistently occurs is the terrestrial foreshock.  Again, it is possible to define a mean magnetic field but it should be scale-dependent, i.e., there will not be one mean field but many because different frequency/wavelength fluctuations will care about different local magnetic field vectors.
