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 will enable 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 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. Recent higher energy neutral atom images from NASA's POLAR spacecraft have provided further tantalizing glimpses of the power of neutral atom imaging.

The IMAGE mid-sized Explorer will provide 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.

Instrumentation

The TWINS instrumentation is essentially the same as the MENA instrument on the IMAGE mission. 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.

Orbits

TWINS will fly as a mission of opportunity on two high-inclination, high altitude spacecraft provided by a non-NASA US organization. Each spacecraft will be 3-axis stabilized and approximately nadir pointing, and will be placed in a Molniya orbit with 63.4° inclination and 7.2 R_E apogee, an ideal orbit for magnetospheric imaging. TWINS will provide a two year stereo imaging mission. Depending on the exact TWINS timing and the duration of the IMAGE science phase, the first TWINS spacecraft may overlap with the IMAGE mission, providing an even earlier oppurtunity for magnetospheric stereo imaging.