ACE News #7: Early Data from the Solar Wind Ion Composition Spectrometer (SWICS)

The Solar Wind Ion Composition Spectrometer measures the energy-per-charge, time-of-flight and total energy of solar wind ions. These observables are converted into Mass and Mass per Charge using algorithms based on calibration data. The Figure shows the M/Q distribution for solar wind Fe ions observed on 30 Nov 1997 (0800-2400 UT). The iron charge states corresponding to the M/Q peaks are also indicated. Solar wind observations from the Proton Monitor on the SOHO spacecraft indicate that the solar wind at this time was likely a coronal hole-associated high speed stream. The displayed M/Q distribution was obtained by integrating over a mass range that maximizes the efficiency for Fe ions. However, other ions can be detected with much lower efficiency in this mass range; for this reason the abundances at charge states +12 and +14 should be regarded as upper limits. For example, the peak near M/Q=4 may contain some counts from Ca(+10), S(+8) and Si(+7). More detailed analysis is necessary to separate these ions. The dominant observed charge states for Fe are +9 and +10, in excellent agreement with the few previous measurements in coronal hole-associated solar wind flows in the ecliptic plane made by instruments on the ISEE and ULYSSES spacecraft. These charge state distributions correspond to a coronal "freezing-in" temperature of about 1.1x10^6 K. Interestingly, the dominant Fe charge states observed by the ULYSSES/SWICS instrument at large heliographic latitudes are consistently +10 and +11, higher by about 1 charge state unit than the in-ecliptic measurements.

The charge state distribution in the above figure was derived using only pre-launch calibration data. Ground calibration data for elements such as Fe at solar wind energies are very difficult to obtain. We anticipate that the flight data set being accumulated by ACE/SWICS will lead to a much improved knowledge of the instrument response function and ultimately to improved resolution in both Mass and Mass per Charge.

The ACE/SWICS sensor was originally the ULYSSES flight spare, developed by the Univ. of Maryland, the Max-Planck-Institut/Lindau, and the Univ. of Bern; it was refurbished for ACE by the Univ. of Md.

....contributed by Fred Ipavich, University of Maryland

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Last modified 22 December 1997, mrt
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