Solar Particle Ionic Charge States

In large, long-duration solar energetic particle events (SEPs), heavy ions are not fully stripped of their electrons. Instead, they have ionic charge states characteristic of the relatively low-density, ~2 million degree plasma from which they arose. For example, the mean ionic charge state of Fe nuclei in these events is <Q>~14. Moreover, these partially-ionized charge states persist to the very highest energies.

In this regard, these SEP heavy ions differ from galactic cosmic rays, which are fully stripped of electrons and thus have ionic charge equal to the atomic number (Q=26 for Fe, for example). As a result, these SEP heavy ions can appear in low-Earth orbits to which galactic cosmic rays of the same energies cannot penetrate.

For example, a large flux of Fe ions appeared at LDEF (28.5^o ,475 km orbit) at ~150-500 MeV/nuc during the historic SEP events of September-October 1989. Galactic cosmic-ray Fe nuclei with energies below ~800 MeV/nuc are excluded from this orbit by Earth's magnetic field.

The heavy-ion charge states in these large, gradual SEP events also differ from those in the short-duration, relatively low-energy events, in which particle acceleration occurs near a flare site on the Sun. In these events, the mean ionic charge state of Fe is <Q>~20. This difference in charge states is one of the key pieces of evidence in establishing that particle acceleration in these large, long-duration SEP events is not due to flares, but rather, due to interplanetary shocks driven by fast coronal mass ejections.

For more information on solar particle charge states, see:

• "The Mean Ionic Charge State of Solar Energetic Fe Ions above 200 MeV/nuc", by A.J. Tylka, P.R. Boberg, J.H. Adams, Jr., L.P. Beahm, W.F. Dietrich, & T. Kleis, Astrophysical Journal Letters 444, L109-113 (1995).
• "Solar Energetic Fe Charge State Measurements: Implications for Acceleration by Coronal Mass Ejection-Driven Shocks", by P.R. Boberg, A.J. Tylka, & J.H. Adams, Jr., Astrophysical Journal Letters, 471, L65-68 (1996).
• "Measurements of the Ionic Charge States of Solar Energetic Particles Using the Geomagnetic Field", by R.A. Leske, J.R. Cummings, R.A. Mewaldt, and E.C. Stone, Astrophysical Journal Letters, 452, L149-52 (1995).

For more information on coronal mass ejections and their central role in major transient disturbances in the near-Earth space environment, including SEP events, see:

• 'The Solar Flare Myth", by J. T. Gosling, Journal of Geophysical Research, 98, 18937-18949 (1993).
• "Solar Flares and Coronal Mass Ejections", by S.W. Kahler, Annual Reviews of Astronomy and Astrophysics, 30, 113-141 (1992).
• "Non-Thermal Particles in the Interplanetary Medium", by Donald V. Reames, Advances in Space Research, 13 (9)331-(9)339 (1991).
• "Injection Profiles of Solar Energetic Particles as Functions of Coronal Mass Ejection Heights", by S. Kahler, Astrophysical Journal 428, 837-842 (1994).