This article, the second in a series of three on the GEM program to appear in Eos, reports on the GEM observing campaign and data/model comparisons. The first article by William Lotko provided an overview of the program organization and current scientific activities. The third article by George Siscoe will report on GGCM development.

The Geospace Environment Modeling (GEM) program is a National Science Foundation program designed to produce a comprehensive understanding of the interactions of the solar wind with the Earth's magnetosphere and ionosphere. It is also structured to help develop the first quantitative global circulation model of the magnetosphere and ionosphere that focuses on the dynamic response to the changing solar wind energy and particle inputs. The concept of a GEM program was conceived at workshop in 1987, and NSF has provided modest funding over the past 2 years.