Advanced Composition Explorer provides new insights into particle sources



The January 15th issue of Geophysical Research Letters (GRL) includes a special section on results from Advanced Composition Explorer (ACE). ACE was launched in August 1997, and is now in orbit around the L1 Lagrangian point. ACE is instrumented to measure the elemental, isotopic, and ionic charge state composition of the heavy nuclei in the solar wind, solar energetic particles, and galactic cosmic rays.

As reported in the special section of GRL, a large solar particle event occurred on November 6, 1997. The energetic particle composition, as measured by ACE, was surprising with significant effects due to mass-dependent fractionation. The isotopic abundance ratios of 13C/12C, 22Ne/20Ne, and 26Mg/24Mg were about twice as large as typical solar system values, implying charge-to-mass dependent acceleration. Data from ACE and the Solar, Anomalous and Magnetospheric Particle Explorer (SAMPEX) show the ionic charge states of Mg, Si, and Fe increased with increasing energy for this event. This indicates that the higher energy nuclei may have come from a source millions of degrees hotter than typical coronal temperatures (∼ 1 million degrees). Several papers in the special section conclude that more than one acceleration process may have been present during the November 6 event.