ACE News #10: First measurements of the solar wind composition with the high resolution mass spectrometer SWIMS

The Solar Wind Ion Mass Spectrometer (SWIMS) on ACE measures the elemental as well as the isotopic composition of solar wind ions. The figure above shows SWIMS data covering the entire mass range from He up to Fe. The relative heights of C, N, O, etc. have not been corrected for efficiencies which are very important for the interpretation of the data. The figure demonstrates the excellent mass resolution of the sensor. This becomes particularly evident when looking at the Mg peaks. The three isotopes of Mg clearly show up even in this low-count, non-optimized survey mode of the instrument.

SWIMS consists of an electrostatic deflection system and a time-of-flight section. Because of the harmonic potential in the time-of-flight section of the instrument, the mass resolution is remarkably high, of the order M/dM ~ 100 or better for most elements. The combination of ACE/SWIMS with the ionic charge spectrometer SWICS, the plasma package SWEPAM, the magnetometer, and the higher energy particle spectrometers on ACE is a first in solar wind research. Because of the excellent mass resolution, SWIMS will be used to ``zoom'' in on interesting mass ranges and study compositional aspects with high precision.

SWIMS is similar to the mass spectrometer MTOF on SOHO, also in a halo orbit about L1, and the MASS instrument on Wind, which is in orbit closer to Earth. The combined data from these sensors is therefore expected to be very useful for purposes of cross-calibration and for the study of spatial variations of features in the solar wind.

In summary, we report that the SWIMS sensor on ACE is performing nominally. The mass resolution is as expected and the Mg isotopes are easily separated. In the near future the operation of the SWIMS sensor will be changed from ``survey'' to a more constrained ``zoom'' mode to provide unique data which will be analyzed in combination with the SWICS sensor and other ACE instruments.

....contributed by Thomas H. Zurbuchen and Simon Hefti, University of Michigan

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Last modified 27 February 1998, mrt
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