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Energetic Neutral Atom Images of Substorm InjectionsKey Data Sets Polar/IPS Energetic Ions, LANL Geosynchronous Energetic Particles, Polar/VIS Auroral Imager Key Results First ever image of a substorm injection Contacts Mike Henderson, Los Alamos National Lab., 505/665-7425, mhenderson@lanl.gov Geoffrey D. Reeves, Los Alamos National Lab., 505/665-3877, reeves@lanl.gov Harlan Spence, Boston University, (617) 353-7412, spence@buasta.bu.edu More Information Henderson, M. G., G. D. Reeves, H. E. Spence, R. B. Sheldon, A. M. Jorgensen, J. B. Blake, and J. F. Fennell, First energetic neutral atom images from Polar CEPPAD/IPS, Geophys. Res. Lett., submitted, 1997. http://nis-www.lanl.gov/~mgh/ENA.shtml Abstract Energetic Neutral Atoms (ENAs) are produced when an ion in the magnetosphere is neutralized by charge exchange with a neutral atom from the atmosphere (or geocorona). Once neutralized the ENA is no longer trapped by the earth's magnetic field. Therefore ENAs can be collected from any point in space. Since charge exchange essentially preserves the particle's energy the ENAs retain `knowledge' of their source in the magnetosphere. By collecting ENAs at some remote location and determining their point of origin one can construct an `image' of the magnetosphere - an image of the otherwise invisible ions in the magnetosphere. 
Polar is particularly well suited to produce ENA images. Because of its orbit it spends up to half of its time in the northern polar cap where it can `look down' on the magnetosphere from `above'. The polar cap is also relatively free of trapped energetic particles which can complicate the measurements. The IPS instrument on Polar (J. B. Blake, PI) maps out a full sphere in 9 polar by 32 azimuthal look directions. The spatial resolution, sensitivity, and orbit of Polar/IPS allows us to produce images of the magnetosphere never seen before. One particularly striking opportunity came on July 31, 1996 and is shown in the figure to the left. Polar was in the northern polar cap near dawn in local time. It observed a burst of energetic neutral atoms that appeared suddenly and then slowly decayed away. We used a ray-tracing program to produce the images and to define all the correct geometries. In the figure the sun is to the right (the +X-SM axis is shaded red), the view is from the position of Polar at the time of measurement, and a 2Rex2Re grid in the X-Y plane is shown for reference. This sequence of images show direct counts of energetic
neutral atoms. No modeling or smoothing has yet been applied. Between 0044
and 0054 UT one can clearly see the intensification ion fluxes (and hence
ENA fluxes) on the night side of the Earth between 4 and 8 Re. This is exactly
the region of space where substorms produce injections of energetic ions.
(These images have also been made into a video of this event - another first.) Confirmation that these are indeed images of substorm injections came from the Polar visible auroral imager (VIS - L. A. Frank PI) and from the LANL geosynchronous satellites (SOPA - R. D. Belian PI). Geosynchronous orbit passes directly through the region of substorm injections and the plot below clearly shows two strong substorm injections at the time that the ENA images show their injection. The auroral images from this time also show the classic development of an auroral substorm. The image to the right shows the substorm in progress when the auroral ring has become expanded on the night side. The development of an expanded auroral bulge is an unambiguous substorm signature. While this is the first ever image of a substorm injection it is by no means unique. Polar/IPS has observed many substorm injections. Further studies are underway to combine this unique global imaging tool with other in situ measurements to develop a more comprehensive and complete understanding of substorms and geomagnetic storms.
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