Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsm41a1120v&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #SM41A-1120
Physics
2720 Energetic Particles, Trapped, 2730 Magnetosphere: Inner, 2778 Ring Current, 2788 Storms And Substorms, 2794 Instruments And Techniques
Scientific paper
The Medium Energy Neutral Atom (MENA) imager, launched aboard the IMAGE spacecraft in 2000, was designed to perform remote sensing of the 1-70 keV ring current by measuring Energetic Neutral Atoms (ENAs). To isolate the remote-origin ENAs from the local plasma, a series of collimator plates are used to reject charged particles from entering the imager. The alternate collimator plates have high voltage applied to sweep out charged particles. The cut off energy for the charged particles is a function of the voltage applied to the collimator plates. The MENA collimator plates have been run at a reduced voltage, allowing energetic (>35 keV) ions to enter into the imager. For most of the mission, the signal from the entry of charged particles into the instrument is at levels of at least an order of magnitude below the ENA signal. However, there are times when high background (non-ENA) events are seen. At these times, the background flux can be significant when compared to the ENA signal. This elevated background has been observed during all times of the orbit while MENA is operational (L-shells > 9) and for all spin phases of the spacecraft. The background levels are higher on the dayside than on the night side. We present conclusive evidence that the MENA instrument's elevated background is from energetic charged particles passing through the collimators. The intensity of this charged particle flux into MENA is seen to correlate well with the energetic particle sensor (0.7 to 4 MeV) on the GOES-8 satellite. They also correlate well with changes in Dst and with solar wind dynamic pressure. Since the flux of ENAs is relatively weak, a large geometric factor was required in MENA. Therefore, MENA can play a dual role in observing geospace. During low-background conditions, MENA provides global images of the ring current plasma distribution. When high background signals exist, MENA can also serve as a sensitive, useful detector of the in situ plasma environment.
Goldstein Jeffrey Jay
McComas David John
Pollock C. J.
Valek P. W.
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