The NASA TWINS that are seeing double in the Earth’s magnetosphere are a new set of instruments that have been working since June 15, 2008. They are stereoscopic imaging satellites, and they’re called TWINS because the acronym stands for Two Wide-angle Imaging Neutral-atom Spectrometers.
The magnetosphere surrounds the Earth and is a dynamic region always in a state of flux. Magnetic and electric forces govern the region, incoming energy and material from the sun, and a vast zoo of waves and processes very different from what is normally experienced in Earth-bound physics.
A doughnut of charged particles generally aligned with Earth’s equator, the ring current, lies nestled inside this constantly changing magnetic bubble.
The waxing and waning of the ring current is a crucial part of the space weather surrounding our planet. It is able to induce magnetic fluctuations on the ground and also transmit disruptive surface charges onto spacecraft.
The TWINS of NASA orbit in widely separated planes. They have provided the first and only stereo view of the ring current within the Earth’s magnetosphere. The two satellites map the energetic neutral atoms that shoot away from the ring current when created by ion collisions.
How are the NASA TWINS helping us?
With the use of the three-dimensional TWINS maps and Earth’s observatories scientists can track how the magnetosphere responds to space weather storms, characterize global information such as temperature and shape of various structures within the magnetosphere, and improve models of the magnetosphere that can be used to simulate a vast array of events.
According to Mei-Ching Fok, the project scientist for TWINS at NASA’s Goddard Space Flight Center in Greenbelt, Md.:
With two satellites, with two sets of simultaneous images we can see things that are entirely new. This is the first ever stereoscopic energetic neutral atom mission, and it’s changed the way we understand the ring current.”
Each of the NASA TWINS is in a highly elliptical orbit. The orbits are called Molniya orbits.
During the Molniya orbits, the spacecraft spend most of their time around 20,000 miles above Earth, where they get a great view of the magnetosphere.
TWINS was initially launched for a two-year mission, but was formally extended in 2010 for three more years. Another multi-year extension is pending. TWINS has worked hand-in-hand over those five years with other NASA missions that provide information about Earth’s magnetosphere.
David McComas, the principal investigator for TWINS at the Southwest Research Institute in San Antonio, Texas, stated:
We’ve done some fantastic new research in the last five years. As a mission of opportunity, it is a very inexpensive mission and it continues to return incredible science.”
Two instruments that can track neutral atoms provide TWINS with their useful information that they then translate into maps.
The first is a neutral atom imager that records the atoms that naturally stream away when a neutral atom collides with an ion. This allows the instrument to map the original ions from far away instead of only collecting data from the areas of space it passes through.
According to McComas:
Over the course of the last 20 years a completely new technique evolved so we can observe charged particles, such as those in the ring current, remotely. The charged particles sometimes collide with a slow-moving neutral particle, in this case from a population of neutrals from Earth’s highly extended atmosphere, the geocorona.”
The collision of these particles causes an electron to hop from the slow neutral atom to the fast ion, so now the former becomes charged, and the latter neutral.
The new neutral speeds off in a straight direction. The magnetic field lines around Earth that guide and control the motion of charged particles don’t have an effect on it.
TWINS collects such fast neutral particles and from that data scientists can work backward to map out the location and movement of the original ions.
A Lyman alpha detector is the other instrument on TWINS. Both of the satellites have one, and with them, the density of hydrogen can be measured from afar. For instance, the Lyman alpha detectors can measure the density of the geocorona, the hydrogen cloud around Earth.
The NASA TWINS enable scientists to watch these neutrals from two viewpoints. This allows scientists to analyze not only speed and number of particles, but also to determine the angles at which the particles left their original collisions.
How does the stereo vision of the NASA TWINS help scientists?
The stereo vision of the NASA TWINS contributed to the detailed perspectives on how the magnetosphere reacts to space weather storms, those that are due to the impact of a coronal mass ejection that traveled from the sun toward Earth, and those due to an incoming twist in the solar wind known as a co-rotating interaction region.
The fact that the pitch angle at which the ions travel around Earth is different on each side of the planet was also revealed by TWINS. This sort of information helps scientists determine if the ions are more likely to escape from the ring current out into space or to ultimately funnel down toward Earth.
According to Natalia Buzulukova, a magnetospheric scientist at Goddard who works with TWINS data:
TWINS is a stereo mission, providing the first observations of the neutral atoms from two vantage points, but two spacecraft give us another advantage. Two spacecraft provide continuous coverage of the ring current, as one set of instruments always has a view.”
By not being in sync, the orbits of the NASA TWINS provide stereoscopic imaging for a few hours each day. At least one spacecraft is always keeping tabs on how events are unfolding.
A spacecraft prior to TWINS might see a mysterious process taking place in the ring current for only a short while before its orbit took it out of view, and the event might well have finished before the spacecraft came back around for its second look.
The continuity in the images that the TWINS provide has proved useful in determining what governs whether particles in the ring current will precipitate downward toward Earth.
Also, the stereoscopic images provide a global temperature map of the magnetic tail trailing behind Earth, known as the magnetotail. Previously, such maps been inferred from models and statistical analysis, but never from a comprehensive data set of what was actually observed.
McComas mentioned further that:
We get two really unique things with two spacecraft: stereo imaging and continuous coverage. Together the observations we get are fantastic. It’s an incredibly powerful combination of tools.”
These are a few of the many benefits that the NASA TWINS that are seeing double in the magnetosphere provide scientists and researchers. Seeing double in the case of humans may mean you’re inebriated, but in the case of the NASA TWINS, it’s definitely a good thing and enables scientists to have a much clearer picture of the ring current in Earth’s magnetosphere.
Written by: Douglas Cobb