  Multiferroic TbMnO3 is investigated using x-ray diffraction in high magnetic
fields. Measurements on first and second harmonic structural reflections due to
modulations induced by the Mn and Tb magnetic order are presented as function
of temperature and field oriented along the a and b-directions of the crystal.
The relation to changes in ordering of the rare earth moments in applied field
is discussed. Observations below T_N(Tb) without and with applied magnetic
field point to a strong interaction of the rare earth order, the Mn moments and
the lattice. Also, the incommensurate to commensurate transition of the wave
vector at the critical fields is discussed with respect to the Tb and Mn
magnetic order and a phase diagram on basis of these observations for magnetic
fields H||a and H||b is presented. The observations point to a complicated and
delicate magneto-elastic interaction as function of temperature and field.
