  The anisotropy of the critical current density (Jc) and its influence on
measurement of irreversibility field (Birr) has been investigated for high
quality, in-situ MgB2 strands. Comparison of transport and magnetization
measurements has revealed the onset of a regime where large differences exist
between transport and magnetically measured values of the critical current
density and Birr. These effects, initially unexpected due to the lack of
crystalline texture in these in-situ processed strands, appear to be due to a
fibrous microstructure, connected with the details of the wire fabrication and
MgB2 formation reactions. Scanning electron micrographs of in-situ-processed
MgB2 monocore strands have revealed a fibrous microstructure. Grains (~100 nm)
are randomly oriented, and there is no apparent local texture of the grains.
However, this randomly oriented polycrystalline material has a fibrous texture
at a larger length scale, with stringers of MgB2 (~ 60 {\mu}m long and ~5
{\mu}m in diameter) partially separated by elongated pores -- the spaces
previously occupied by stringers of elemental Mg. This leads to an
interpretation of the differences observed in transport and magnetically
determined critical currents, in particular a large deviation between the two
at higher fields, in terms of different transverse and longitudinal
connectivities within the strand. The different values of connectivity also
lead to different resistive transition widths, and thus irreversibility field
values, as measured by transport and magnetic techniques. Finally, these
considerations are seen to influence estimated pinning potentials for the
strands.
