ACE News Archives	ACE News #61 - April 16, 2002	ACE News Archives
ACE Science Nuggets		ACE Science Nuggets

Spectral Variability in Large Solar Energetic Particle Events

This figure shows representative heavy-ion spectra (color-coded by species) in three SEP events, using data from ACE/EPAM (~0.3-3 MeV/nuc), Wind/EPACT (~3-15 MeV/nuc), and ACE/SIS (>8 MeV/nuc). The center panel is the largest event so far in Cycle 23, and the left was the largest prior to that one. The right panel is the biggest ground-level event (GLE) to date in Cycle 23. These three events illustrate the challenge of SEP heavy-ion spectral variability:

• The left and center panels are log-linear plots, so that straight lines are exponentials. The right panel, however, is log-log, so that straight lines are power laws.
• On the left, Fe has a much softer spectrum than the other ions. On the right, the Fe spectrum is harder than the others. In the center, Fe starts out softer than the other species but hardens.

The left and center panels can be understood in terms of the exponential rollovers expected for shock-accelerated spectra. The e-folding energies scale linearly with an ions charge-to-mass (Q/A) ratio. In the left panel, Fe has a small e-folding energy (corresponding to Q~11) because it occurred in a period of very low flare activity, during which only solar-wind suprathermals were available as seed particles. In the center panel, the Fe ions where accelerated from a broader charge-state distribution, including a ~5% admixture of highly ionized suprathermals from numerous preceding large flares.

The right panel presents a different puzzle. Data from the Chicago/IMP8 instrument show that these power-laws continue to nearly ~500 MeV/nuc. The exponential rollovers are presumably at even higher energies. But simple shock-acceleration theory predicts that power-law indices should be the same for all species. So why does Fe show a harder power-law than lighter species? One intriguing but highly speculative possibility is that, at least while near the Sun, a shock might have finite width. If so, higher A/Q species would sample a larger compression ratio, hence giving them harder spectra (Eichler, 1979). Stripping during acceleration in the low corona would further complicate the picture. These results have implications for interpreting SEP composition and for assessing SEP radiation hazards.

Contributed by Allan Tylka (NRL), Christina Cohen (Caltech), Bill Dietrich (U. Chicago), Carol Maclennan (Lucent Technologies), Chee Ng (NASA/GSFC, U. Md.), and Don Reames (NASA/GSFC).

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Last modified 16 April 2002, by Andrew Davis