Upper Limits on Heliosheath ENA Intensities (4-70 keV) from STEREO/STE

Details Upper Limits on Heliosheath ENA Intensities (4-70 keV) from STEREO/STE Upper Limits on Heliosheath ENA Intensities (4-70 keV) from STEREO/STE

May 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agusmsh24a..07w&link_type=abstract

American Geophysical Union, Spring Meeting 2009, abstract #SH24A-07

Other

2114 Energetic Particles (7514), 2124 Heliopause And Solar Wind Termination, 2151 Neutral Particles (7837)

Scientific paper

An upper limit on heliosheath ENA intensities provides a crucial limitation on the dynamic range for ENA detection and simulation. Here, we present an upper limit estimate for heliosheath ENAs at ˜4-70 keV based on measurements by STEREO/STE (SupraThermal Electron) sensors during March 2007 - March 2009. STE has recently demonstrated its capability to detect ENAs with images from the Earth's magnetosphere. From March 2007 to March 2009, the 8 detectors of STE (four on STEREO A and four on B) performed in total almost 16 full scans of the heliosphere in longitude. However, STE didn't detect any significant ENA enhancements above the background (mainly interplanetary electrons) in directions outside the heliosphere nose region (220-300° ecliptic longitude), where STE is contaminated by X-rays from galactic sources. In this study, we assume that heliosheath ENAs don't vary with time. Then we obtain the typical lowest flux level f0 in STE at ˜4-70 keV as a function of the source direction excluding the nose. The preliminary results show f0 (cm-2 s-1 str-1 eV-1)= 0.9-3.2×106 [E(eV)]-2.3. If heliosheath ENAs are spatially uniform, f0 would be a good estimate of their upper limit from the heliosphere flank or tail direction. If they are localized with an angular width <20-30° in longitude, then they would show high fluxes only in one detector since the other three detectors simultaneously look in other directions. The absence of such anisotropy suggests that for a localized ENA source, the upper limit would be less than 20%- 30% of f0.

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