Temperature-dependent $H_{c2}$ anisotropy in MgB$_2$ as inferred from   measurements on polycrystals
  We present data on temperature-dependent anisotropy of the upper critical field of MgB$_2$ obtained from the analysis of measurements on high purity, low resistivity polycrystals. The anisotropy decreases in a monotonic fashion with increase of temperature. 
Sergey L. Bud'ko, and Paul C. Canfield