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The Solar Wind Ion Mass Spectrometer (SWIMS) on ACE has successfully measured the elemental abundance of nitrogen in the solar wind. The figure shows data acquired in a high speed stream lasting 2.5 days and demonstrates the clean mass resolution of SWIMS. The peak to peak ratio of N/O needs to be corrected for detection efficiencies and preliminary analysis yields N/O ~ 0.121 +/- 0.014 which is in excellent agreement with the photospheric value of N/O ~ 0.123 and with the coronal value derived from solar energetic particles.
The abundance of nitrogen in the heliosphere is an enigma. Laboratory analysis of lunar soils shows that trapped nitrogen is overabundant in them by about one order of magnitude relative to the heavy noble gases, which in turn are efficiently trapped in the lunar regolith. The measurements reported here show no sign of enrichment of N in the contemporary solar wind, at least with respect to O. Together with earlier solar wind measurements of Ne/O, which do not show any significant depletion of Ne, this observation provides further evidence for a non-solar origin of nitrogen in lunar soils.
Nitrogen is not readily measured in the solar wind because it is not very abundant and it is neighbored in mass and in mass per charge by the most abundant heavy ions, oxygen and carbon. For this reason, previous elemental abundance determinations of nitrogen in the solar wind have had large intrinsic uncertainties. However, as is evident from the figure, with SWIMS, nitrogen is cleanly separated from its neighbors and its abundance can be accurately measured.
Contributed by Robert F. Wimmer-Schweingruber, University of Bern, Switzerland
See The SWIMS Home Page for more information on ACE SWIMS.
Last modified November 5, 1999.
Andrew Davis