The idea to use soft X-rays to study the Earth’s magnetic field came from contaminated measurements and initially puzzled astronomers. In the early 1990s, astronomers had carefully developed soft X-ray detectors for the ROSAT spacecraft and were eagerly awaiting the first results to study the local galaxy. When measurements started coming to Earth the signal showed a significant background that varied on time scales of minutes and hours – far shorter that the hundreds of years time scales at which the local galaxy was though to be changing at. The figure below shows an all sky map of the initial measurements (left) and cleaned ones (right).
This signal, shown as streaks in the left image above, was not anticipated and initially caused great concern. Eventually researchers identified the signal was coming from the plasma environment surrounding the Earth or the magnetosheath. The time variability was a result of the short-term variation of the solar wind flux and its interaction with the Earth’s neutral atmosphere. The emissions were a result of charge-exchange. Although this was an unwanted background for some, it’s a valuable tool to image the plasma environment surrounding the Earth.
Heritage of Wide Field-of-View soft X-ray Imagers
Soon after recognizing a valuable signal was coming from the Earth’s magnetosheath and cusps, there were efforts to image it. The first flight of a wide field-of-view soft X-ray occurred in 2012 on a sounding rocket launched from White Sands, New Mexico. The detector, called STORM, was developed at NASA Goddard and flew on the Diffuse X-ray emission from the Local galaxy (DXL) sounding rocket (PI: Massimiliano Galeazzi, U. of Miami). The successful flight demonstrated the innovative capabilities of the STORM instrument.
Currently under development is the CuPID Cubesat Observatory. The free-flying 6U spacecraft will carry a 5x5deg FOV soft X-ray telescope and a radiation sensor in low Earth orbit to image the entry of solar wind plasma into the magnetospheric cusps. The X-ray telescope will use the same lobster-eye optics used on the STORM instrument and being used on LEXI. The CuPID spacecraft launched in 2021.
Outside of the United States, a joint European Space Agency (ESA) and Chinese Academy of Sciences (CAS) mission called SMILE (Solar Wind, Magnetosphere, Ionosphere Link Explorer) is also designed to measure soft X-rays coming from the Earth’s magnetosphere. This mission is currently set to launch into orbit in 2024.