How is the angular signature of SMOS brightness temperature different in the morning and evening?

Evening (6pm) and morning (6am) SMOS observations are different. The greatest difference is for incidence angles less than approximately 30°, but this difference can not be explained with physical models. There is a smaller but significant difference in evening and morning SMOS observations for incidence angles greater than approximately 50°. Evening values of the polarization index are higher than morning values. This difference does have two plausible explanations: either soil moisture has decreased overnight; or vegetation water content has increased overnight.