Measurements of the nine largest solar energetic particle (SEP) events observed to date by the SIS instrument have shown that several isotope abundance ratios, including ²²Ne/²⁰Ne, can vary by a factor of ~3 to ~4 from event to event -- considerably more than had been expected based on earlier SEP elemental composition studies. SEP composition variations are commonly assumed to indicate that the acceleration and/or transport mechanism depends on the charge-to-mass ratio (Q/M) of the ions. Available charge-state measurements in large SEP events show that ions with Z>=8 are typically not fully stripped; the mean Q/M values indicate a source temperature of ~2 x 10⁶ K, similar to the corona.

If the observed ²²Ne/²⁰Ne variations result from varying degrees of Q/M-dependent fractionation, elemental abundance variations should be correlated with ²²Ne/²⁰Ne if the mean charge states remain about the same from event to event. Although there are no high-energy measurements of SEP charge states in these events, Arnaud and Rothenflug have calculated equilibrium charge-state distributions for key elements over a range of temperatures. Over the range from 2 to 4 x 10⁶ K the mean charge states of ₁₁Na and ₁₂Mg do not vary significantly from +9 and +10, respectively, because of their He-like electron configuration. Thus, the abundance of ²³Na (Q/M = 0.39) relative to Mg (mean Q/M = 0.41 taking into account isotopic composition) should be correlated with ²²Ne/²⁰Ne. Indeed, we might expect the ²²Ne/²⁰Ne ratio (10% difference in M and therefore in Q/M, assuming the same Q for isotopes) to vary by twice as much as the Na/Mg ratio (~5% difference in Q/M expected). The correlation shown above is consistent with the assumption that a common Q/M-dependent fractionation process produces both the elemental and isotopic variations, but it remains as a challenge for theoretical models to specify the actual mechanisms involved. These data should also result in improved determinations of the underlying coronal element and isotopic abundances.

Contributed by R. A. Mewaldt, C. M. S. Cohen, R. A. Leske of Caltech.

See the SIS Home Page at Caltech for more information about the SIS instrument.

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Last modified 12 April 1999, Steve Sears
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