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THEMIS (Time History of Events and Macroscale Interactions during Substorms) is a NASA Medium-class Explorer Program (MIDEX) mission consisting of five identical satellites studying the Earth's magnetic field. It was launched in 2007 and in 2010 when the primary mission was complete, two of the satellites were repurposed. Instruments:
The name ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun) was given to a mission repurposing two of the satellites, to study the Moon's effect on the Earth's magnetic field. It placed the two in Moon-Earth Lagrangian points, then later in lunar orbit. As far as I can tell, the mission is still active in December 2024. This mission is now called THEMIS-ARTEMIS to distinguish it from the Artemis program, the name NASA chose for its program (in development) of future manned missions to the Moon.
Note that THEMIS is also the name of an instrument on the Mars Odyssey mission.