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Indication of Charge State Variation During the November 1997 Solar Particle Events

Oxygen count levels for 6-7 Nov, 1997

The ionic charge state of energetic particles is an indicator of their source temperature and greatly influences particle acceleration and transport. As a result, charge state measurements provide insight into physical processes in the acceleration regions on the Sun and in inter- planetary space. The primary objective of the SEPICA instrument on ACE is the determination of ionic charge states of a variety of elements in the energetic particle population with higher resolution and better statistical accuracy than has been possible before. The November 1997 solar particle events have provided the first opportunity to test and calibrate the sensor capabilities.

Shown above are the ionic charge state distributions of oxygen in the energy range 0.35 - 1 MeV/nucleon for the time periods November 6, 1200 - 2400 UT and November 7, 0000 - 2400 UT, 1997, as obtained with the high resolution sensor on SEPICA. The dominant presence of O6+ and O7+ is evident with a small contribution from O8+. This demonstrates the capability of SEPICA to distinguish the contribution of individual charge states. Note that O6+ dominates during the first time period, which contains the starting phase of the largest event. During the second period, which includes the passage of an interplanetary shock associated with a coronal mass ejection, O7+ becomes more prominent. The variation of charge states O6+ and O7+ is very sensitive to temperature around 2 x 106 K (Arnaud and Rothenflug, ApJ Suppl 60, 425, 1985). A rough estimate yields an estimated temperature of 2 x 106 K for the first period and 2.3 x 106 K for the second period. Although a complete quantitative interpretation has to await detailed modeling of the results with the sensor response, this appears to be the first measurement of a variation in the charge state composition during a solar particle event. The charge state distributions of other elements, such as He, C, N, Si and Fe, are currently under study.

Contributed by Eberhard Möbius and Mark Popecki of University of New Hampshire andBerndt Kleckerof Max-Planck-Institut für extraterrestrische Physik