Measurement of the Energetic Ion Population With the Medium Energetic Neutral
Atom (MENA) Imager

P.W. Valek, J. Goldstein, D.J. McComas, and C.J. Pollock Southwest Research
Institute, San Antonio, TX


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.

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Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004