Michael McCarthy is a physicist with an experimental bent and interests centered on the dynamics and physical interactions of low-density gases and plasmas. That kind of material is found in Earth’s upper atmospheric regions and the near-Earth space environment. An on-going research effort of McCarthy’s has been to understand what physical processes remove high energy electrons from the Van Allen radiation belts, which are layers of energetic charged particles held in place around the Earth by the planet’s magnetic field. While in the radiation belts, these electrons dose geosynchronous satellites, thereby aging them; and when lost to Earth’s upper atmosphere, these electrons introduce unusual chemical and electrical effects. While it is known that radiation belt electron fluxes are highly variable, the processes leading to their generation and loss are still not understood and of intense interest. To obtain the measurements needed for such studies, he and his students use instrumentation that they design, build, calibrate, and then deploy on balloon, rocket, or spacecraft platforms.