Energetic Neutral Atom Imaging of Space Plasmas
The Earth's magnetosphere is primarily a hydrogen plasma. Since bare protons only radiate through bremstrahlung or synchrotron emission, only extremely high-energy protons can be imaged by optical means. Because a tenuous bubble of cold hydrogen gas surrounds the Earth, energetic plasma ions can charge exchange with the cold geocoronal hydrogen gas. The end result is an outward flux of energetic neutral atoms that can be imaged if a "neutral atom camera" with sufficient sensitivity, background rejection, and temporal response can be built.
As part of the development of a neutral atom camera, we demonstrated that sub-micron period gratings can be used to block UV light. The gratings are used to eliminate six orders of magnitude of background photon signal in a medium energy neutral atom imager (2 - 60 keV). The gratings block the UV light while permitting the passage of neutral atoms. The imager was built for the IMAGE spacecraft and successfully launched in early 2000. Since then, we have published the first medium energy neutral atom (MENA) images of the Earth's magnetosphere and demonstrated that the plasma ion temperature can be remotely determined using techniques pioneered for magnetic fusion plasmas. These remote ion temperature measurements (shown below on the left) have demonstrated, for the first time, that quantitative measurements can be obtained directly from neutral atom images. Comparison to in-situ measurements made at the same time (shown below on the right) confirmed the effectiveness of this technique.

Comparison of the HENA deduced Ion equatorial temperature (red diamonds with error bars) with the ion temperature inferred from geosynchronous MPA measurements (solid line) and MENA observations (black solid circles with ( 0.5 keV error bars) at 12:00 UT, 12:30 UT and 13:00 UT during the 12 August 2000 storm.

More recently, we demonstrated that we can create ion temperature images for each phase of a geomagnetic storm.

(a) pre-storm, (b) main, (c) early recovery, (d) late recovery ion temperature images averaged over 39 storms. In each Earth-centered image, the Sun is to the right. L = 2 and L = 4 representative magnetic field lines are shown centered on the Earth. The white arrows show the direction of the spacecraft-Earth vector for each interval used in the superposed image. The length of the arrows is the projection of the spacecraft-Earth vector in the equatorial plane. The ENA fluxes used to create (b) are shown in Fig. 6. L = 2 and L = 4 representative magnetic field lines are shown centered on the Earth for scale and a 1 RE x 1 RE grid is shown in (d) for reference

Our work with the ENA imaging continues with the recent launch of the first TWINS spacecraft in 2006 and we look forward to the launch of TWINS II in 2007 or 2008.