ACE News #16
June 15, 1998

High Time Resolution Measurements of a CME: SWICS/ACE Observations

During a period of about two days in early May, 1998, the ACE spacecraft was immersed in plasma associated with a coronal mass ejection (CME). The SWICS instrument on ACE, which determines unambiguously the mass and charge of solar wind ions with high time resolution, has produced a new and very exciting data set from this event. These data carry a wealth of information on the origin and the thermal history of the CME ejecta and have the potential of being the missing link between solar and in situ observations.

Here, we concentrate on the iron charge state distribution, which has been fully measured once per hour during the entire CME period. These distributions are imprints of the electron temperature distribution function within a few solar radii from the solar surface where the ejecta was accelerated. During this event the observed charge states range from the very rare Fe3+, during phases of very cold plasma, to Fe16+, when the plasma is extraordinarily hot, indicating an enormous range in electron temperatures from ~105 K to several 106 K.

The movie shows the time history of this event: After a period of standard slow solar wind, a very hot charge state distribution arrives just after the beginning of Day 122. This is followed near the end of Day 122 by one of the coldest components ever measured in situ,including what is (to our knowledge) the first observation of Fe3+ in solar wind. After about 12 hours of this cold plasma, the ``hot'' Fe16+ shows up again for a short time. Notice, that the transitions between ``hot'' and ``cold'' components are not abrupt: there are clearly time-periods where very ``cold'' Fe6+ and very ``hot'' Fe16+ co-exist.

These data provide a unique view of a CME associated with a cold plasma component, perhaps originating from an erupted filament. They provide valuable information about the CME between the time of solar observations with imaging instruments on SOHO and in situ plasma observations at 1 AU, i.e. electron and proton distributions measured by ACE-SWEPAM.

Contributed by S. Hefti and T. H. Zurbuchen, University of Michigan.

See The SWICS and SWIMS Home Page at the University of Michigan for more information on these ACE instruments.

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Last modified 12 June 1998, Andrew Davis
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