In 2008, the NASA mission called TWINS, or Two Wide-angle Imaging Neutral-atom Spectrometers, began obtaining the first stereo images of the near-Earth region of space. An article recently published in Reviews of Geophysics reviews the last several years of science results from TWINS. The editors asked the authors to explain what the instruments have been measuring, and describe some of the most interesting findings.
What and where is “geospace”?
Geospace is the region of space close to the Earth and dominated by the Earth’s own magnetic field. This region contains a variety of neutral gases and plasmas (ionized gases) which are the major players in geomagnetic storms—disturbances in near-Earth space that strike our magnetic field, and that can disrupt our technology.
Our review article considers two constituents of geospace. The first is the ring current, which is made of energetic ions and electrons that are trapped by Earth’s magnetic field. During geomagnetic storms, the ring current’s magnetic field is strong enough to measure on the ground with an ordinary refrigerator magnet. The second is the exosphere, which is made of neutral hydrogen atoms. The hydrogen exosphere helps control the duration and strength of geomagnetic storms by neutralizing the ions of the ring current.
What information about geospace has NASA’s TWINS mission been collecting?
NASA’s TWINS mission has used two separate spacecraft to perform stereo imaging of the ring current and the exosphere. Each TWINS spacecraft carries two specialized cameras. The first is an energetic neutral atom (ENA) camera. This measures the particles emitted when the exosphere neutralizes the ions of the ring current. The second is a camera that measures the ultraviolet glow of the hydrogen atom exosphere—called the “geocorona.” Using these cameras TWINS observes the big picture, that is, the large-scale (global) response to storms—just as a weather satellite can show the progress of a hurricane. Since 2008, the two TWINS observatories have recorded the global response to over 200 storms ranging from mild to major.
What have been some of the most significant or surprising scientific findings from TWINS?
Geospace has been imaged before but TWINS was the first mission to perform stereo imaging—with cameras on two separate spacecraft—of the ring current and exosphere. This stereo imaging has revealed extra dimensionality and structure that was previously undetected by monoscopic imaging. Continuous coverage by two imagers has allowed monitoring of storms from start to finish. Geocoronal imaging has revealed a surprising amount of time variability and structure in the exosphere—surprising because the exosphere has been widely thought to be quiescient and symmetric.
What are some other important results from this mission?
TWINS has led the innovation of several important techniques for quantitative interpretation of geospace ENA images, which in general entails very complicated analysis. The development of a method to interpret the ENA signal from low altitudes (a few hundred kilometers), where ions are lost, is a significant advance. TWINS has been at the forefront of recent advances in producing global maps of the temperature and composition (hydrogen versus oxygen) of the ions of geospace.
What are some of the unresolved questions where additional research and data is needed?
A lot of fine-scale structure in the ring current has been observed by spacecraft taking local measurements. Big regions of the ring current are actually made up of many small islands and blobs, some of them quite intense. By analogy with a storm on Earth, if the ring current makes up the storm, then these smaller structures are like individual gusts of wind, or even individual rain drops. Next we need to investigate how important these structures are, and how they affect the evolution of storms in space.
The TWINS mission was designed to last for two years from June 2008, when it started delivering simultaneous stereo imaging of geospace. The mission provided these data through the fall of 2016, when TWINS 2 suffered an unexpected failure. The ENA and geocoronal cameras on TWINS 1 continue to operate well and return high-quality data to support TWINS mission science, although with single-perspective viewing going forward.
—Jerry Goldstein, Southwest Research Institute, Texas; email: [email protected]