Launched as missions of opportunity aboard classified, non-NASA U.S. government spacecraft, TWINS conducts stereoscopic imaging of Earth's magnetosphere. By imaging the charge exchange neutral atoms over a broad energy range using two identical instruments on two widely spaced high-altitude, high-inclination spacecraft, TWINS enables the 3-dimensional visualization and the resolution of large scale structures and dynamics within the magnetosphere for the first time. In contrast to traditional space experiments, which make measurements at only one point in space, imaging experiments provide simultaneous viewing of different regions of the magnetosphere. Stereo imaging, as done by TWINS, takes the next step of producing 3-D images, and provides a leap ahead in our understanding of the global aspects of the terrestrial magnetosphere. The ENA imagers observe energetic neutrals produced from the global magnetospheric ion population, over an energy range of 1 to 100 keV with high angular and time resolution. A Lyman-alpha geocoronal imager is used to monitor cold exospheric hydrogen atoms that produce ENAs from ions via charge exchange. Complementing these imagers are detectors that measure the localcharged particle environment around the spacecraft. The offset in the orbital phases of TWINS 1 and TWINS 2 means that in addition to stereo ENA imaging for several hours twice per day, the two TWINS instruments also obtain essentially continuous magnetospheric observations. The TWINS instrumentation is essentially the same as the MENA instrument on the IMAGE spacecraft. This instrumentation consists of a neutral atom imager covering the ~1-100 keV energy range with 4°x4° angular resolution and 1-minute time resolution, and a simple Lyman-alpha imager to monitor the geocorona. TWINS provides stereo imaging of the Earth's magnetosphere, the region surrounding the planet controlled by its magnetic field and containing the Van Allen radiation belts and other energetic charged particles. TWINS enables three-dimensional global visualization of this region, leading to greatly enhanced understanding of the connections between different regions of the magnetosphere and their relation to the solar wind.
Operation and storm events
Routine stereo imaging by TWINS began on 15 June 2008, during an extremely weak geomagnetic storm whose Dst index never fell below -40 nT, as compared to a nominal Dst of -100 nT for classification as a storm. During the TWINS prime mission, an extended and unprecedented solar minimum prevailed, bringing with it very calm magnetospheric conditions ranging from dead quiet to mildly disturbed. During this time period TWINS observed numerous weak storms, roughly once every 27 days. The strongest storm observed by TWINS during its prime mission was on 22 July 2009, with Dst reaching a moderate -79 nT. Throughout these extended quiet conditions TWINS images contained ENA signals from both high-altitude and low-altitude emission regions. The TWINS Mission of Opportunity maintains a library of selected storm events.
