Variability of N/O in the Solar Wind

The Solar Wind Ion Mass Spectrometer (SWIMS) on ACE has successfully measured the elemental abundance of nitrogen in the solar wind, N/O ≈ 0.121 +/- 0.014, in excellent agreement with the photospheric value of N/O ≈ 0.123 and with the SEP-dervied coronal value. 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. In this work we investigate the variability of N/O in the solar wind. Since Ne/O is known to vary little, variations in N/O would translate directly to variations of N with respect to noble gases. Small variations in N/O will therefore provide further evidence for a non-solar origin of most 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, with SWIMS, nitrogen is cleanly separated from its neighbors and its abundance can be accurately measured.