TWINS

TWINS (Two Wide-Angle Imaging Neutral-Atom Spectrometers) consists of two NASA instruments mounted on a pair of classified U.S. National Reconnaissance Office satellites. The satellites travel in Molniya orbits, and the instruments were designed to capture stereo images of Earth’s ring current. TWINS-1 launched on 28 June 2006 aboard USA-184, and TWINS-2 followed on 13 March 2008 aboard USA-200.

Each instrument carries an energetic neutral atom imager and a Lyman alpha detector. The imager indirectly senses ring current ions, while the Lyman alpha detector measures the neutral hydrogen cloud around Earth, called the geocorona. The prime mission ran from 2008 to 2010, and an extended mission continues.

The project was a mission of opportunity on non-NASA, classified U.S. government spacecraft. By imaging charge-exchange neutral atoms across an energy range of about 1 to 100 keV, using two identical instruments on widely spaced, high-altitude, high-inclination spacecraft, TWINS provides three-dimensional views of large-scale structures and dynamics in the magnetosphere. Unlike single-point space experiments, imaging experiments observe different regions simultaneously. Stereo imaging, as TWINS does, produces 3-D images and advances understanding of global magnetospheric behavior.

The ENA imagers observe energetic neutrals from the global magnetospheric ion population with 4° angular resolution and about one-minute time resolution. The Lyman-alpha geocoronal imager tracks cold exospheric hydrogen atoms that produce ENAs when ions exchange charge. Local charged particle detectors complement these imagers. Because the two spacecraft have offset orbital phases—apogees at different times—they provide stereo ENA imaging for several hours twice daily and nearly continuous magnetospheric observations otherwise. The TWINS instruments are essentially identical to the MENA instrument on the IMAGE spacecraft.

Routine stereo imaging began on 15 June 2008, during a weak geomagnetic storm with a Dst index near -50 nT (storms are typically classified at -100 nT). The prime mission coincided with an extended, unprecedented solar minimum from solar cycle 23, bringing calm to mildly disturbed magnetospheric conditions. TWINS observed weak storms roughly every 27 days, tied to the solar rotation period and corotating interaction regions.