ACE News #24
November 2, 1998

A prolonged Enhancement of He+ in a CME : ACE/SWEPAM Observations

During the May 2-4, 1998 CME and magnetic cloud, the SWEPAM instrument measured a significant enhancement of singly charged (He+) for an interval of more than 24 hours. This is the longest reported He+ enhancement in the solar wind, and is only the sixth such event observed in the past 27 years.

The plasma during this CME was extremely cold, so different E/q states are well resolved, allowing SWEPAM to be used for composition measurements. The figure shows stacked 20-minute averaged E/q flux spectra from the SWEPAM ion detector. The usual solar wind protons (m/q=1) and He++ ions (m/q=2) are seen beginning on May 2 at E/q values of 1.6 and 3.2 keV/q, respectively. In addition, a peak is seen at 6.4 keV/q (m/q=4) that grows larger as time progresses. This peak is most likely due to He+. In contrast to typical solar wind values of 10-6 (for a coronal temperature of ~106 K), the He+/He++ ratio was elevated above 0.5% for all of May 3, and reached a maximum hourly-averaged value of 45%, from 12-13 UT. These ratios indicate material coming from a source with a temperature of ~105 K. A shock was observed at 17 UT on May 3, propagating into the back of the CME, with He+ observed both before and after this shock.

Low ionization temperature ions, such as He+, are generally assumed to be due to solar prominence material which somehow remains thermally isolated from the solar corona as it is ejected into the solar wind. The May 2-4 He+ enhancement was observed during the latter half of a CME (05 UT/May 2 — 01 UT/May 4) and magnetic cloud(13 UT/May 2 — 12 UT/May 3), and in fact extended beyond the back of the cloud. This is the first observation of cold prominence material filling a large fraction of the volume of a CME in the solar wind, and contrasts with the previous He+ events, in which the He+ was observed for short periods of time at either the beginning or end of a CME/cloud.

Contributed by Ruth Skoug of the Los Alamos National Laboratory.

See The LANL SWEPAM Home Page for more information about the SWEPAM instrument.

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Last modified 2 november 1998, Andrew Davis
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