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ACE News #52 - March 1, 2001

ACE News Archives

High Charge States of Iron as a Tracer of Interplanetary Coronal Mass Ejections

The solar wind emits electrons, protons and heavier ions into interplanetary space. Iron (Fe) ions make up a small percentage of solar wind particles. Generally, the Fe atoms found in the solar wind have a charge state of 9+ or 10+. Occasionally the charge states reach up to 16+ and higher. The charge states observed at ACE are related to the coronal temperature of the source region, based on the physics of solar wind expansion. For example, in a 1 million degree plasma the mean Fe charge state is ~9+, while in a 5 MK plasma the mean charge state is ~16+. Solar wind data from ACE can therefore be used to infer temperatures at the Sun.

The figure above shows the Fe charge state distribution for the time period of October 18th through noon on October 20th, 1998. A shock was observed around 1900 UT, October 18th, by the MAG instrument on ACE, followed by the passage of a magnetic cloud, as labeled on the figure. The increased relative abundance, or enhancement, of high charge state (Q 16) ions coincides well with the passage of the magnetic cloud and subsides shortly before the trailing edge of the cloud passes ACE.

Statistical analysis of periods with enhanced high charge states of Fe reveal a strong correlation between these events and Interplanetary Coronal Mass Ejections (ICMEs). As magnetic clouds are a subset of ICMEs, the figure illustrates an example of this correlation. In fact, it has recently been determined that periods with enhanced high charge states lasting ~20 hours correlate with ICMEs more than 90% of the time. However, the number of ICMEs that show high charge states of Fe exhibit only ~50% correlation. Hence, it appears that high charge states of Fe provide sufficient but not necessary conditions for the identification of ICMEs.

Contributed by Susan Lepri ( and Thomas Zurbuchen of the University of Michigan.

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Last modified 01 March 2001, by Andrew Davis