The TWINS Mission
The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission provides a new capability for stereoscopically imaging the magnetosphere. By imaging the charge exchange neutral atoms over a broad energy range (~1-100 keV) using two identical instruments on two widely spaced high-altitude, high-inclination spacecraft, TWINS enabes 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 will provide a leap ahead in our understanding of the global aspects of the terrestrial magnetosphere.
History
The feasibility of magnetospheric imaging using energetic neutral atoms (ENAs), which arise from the charge exchange process between cold geocoronal neutral hydrogen and local energetic ion populations, was first demonstrated over a decade ago. Since then, technologies have been developed to provide higher sensitivity, better angular resolution, and, most importantly, extend the observable ENA energy range downward below several tens of keV to ~1 keV. Higher energy neutral atom images from NASA's POLAR spacecraft provided further tantalizing glimpses of the power of neutral atom imaging.
The IMAGE mid-sized Explorer was NASA's first dedicated platform for making ENA measurements of the Earth's magnetosphere. IMAGE, launched on March 25, 2000, was developed by a team of researchers led by TWINS Co-Investigator, Dr. J. Burch. TWINS will extend our understanding of magnetospheric structure and processes well beyond what is achievable from IMAGE by providing simultaneous images from two widely separated locations over a broad ENA energy range.
In 1996, NASA released an Announcement of Opportunity to study advanced mission concepts which would be suitable for future Sun-Earth Connections missions. One of the winners of that competition was MRI-VIDEOS, led by TWINS Co-Investigator, Dr. D. Mitchell, which proposed to study the next logical step in magnetospheric imaging: stereo imaging of the magnetosphere. The TWINS concept benefitted substantially from the MRI-VIDEOS study. In addition, this study convinced the scientific community of the value of a stereo imaging mission, and the 1996 Sun-Earth Connections Roadmap included a dedicated "Stereo Magnetospheric Imager" mission in its Solar-Terrestrial Probes mission set. The TWINS mission provides a unique opportunity to obtain stereo images of the magnetosphere and achieve many of the scientific goals of MRI-VIDEOS and the Stereo Magnetospheric Imager in the near future at low cost.
Orbits
TWINS flies as a mission of opportunity on two high-inclination, high altitude spacecraft provided by a non-NASA US government organization. Each spacecraft is 3-axis stabilized and approximately nadir pointing, and is placed in a Molniya orbit with 63.4° inclination and 7.2 RE apogee, an ideal orbit for magnetospheric imaging. TWINS will provide a two year stereo imaging mission.
The TWINS instrumentation is essentially the same as the