Balloon observations of the Galactic Center 511 keV line and their relationship to satellite results.

Results on the Galactic Center (GC) electron-positron annihilation line obtained from experiments flown on high-altitude balloons are reviewed. A historical overview of the early balloon work is first presented. Then recent results from new generation balloon-borne instruments are presented and discussed. These balloon observations are responsible for most of our knowledge of the narrow component of the GC annihilation radiation. They suggest that narrow 511 keV line is time-variable, thus providing an important constraint on the size of the source. A new generation balloon instrument (GRIS) has, for the first time, resolved the narrow 511 keV line. The balloon results are compared with new results from the CGRO/OSSE and GRANAT/SIGMA experiments.