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The anisotropy of the critical current density (Jc) and its influence onmeasurement of irreversibility field (Birr) has been investigated for highquality, in-situ MgB2 strands. Comparison of transport and magnetizationmeasurements has revealed the onset of a regime where large differences existbetween transport and magnetically measured values of the critical currentdensity and Birr. These effects, initially unexpected due to the lack ofcrystalline texture in these in-situ processed strands, appear to be due to afibrous microstructure, connected with the details of the wire fabrication andMgB2 formation reactions. Scanning electron micrographs of in-situ-processedMgB2 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 textureat a larger length scale, with stringers of MgB2 (~ 60 {\mu}m long and ~5{\mu}m in diameter) partially separated by elongated pores -- the spacespreviously occupied by stringers of elemental Mg. This leads to aninterpretation of the differences observed in transport and magneticallydetermined critical currents, in particular a large deviation between the twoat higher fields, in terms of different transverse and longitudinalconnectivities within the strand. The different values of connectivity alsolead to different resistive transition widths, and thus irreversibility fieldvalues, as measured by transport and magnetic techniques. Finally, theseconsiderations are seen to influence estimated pinning potentials for thestrands.

Anisotropic Connectivity and its Influence on Critical Current Densities, Irreversibility Fields, and Flux Creep in In-Situ-Processed MgB2 Strands