Measurements of the isotopic composition of solar energetic particles with the MAST instrument aboard the SAMPEX spacecraft

Citation

Williams, Daniel LeRoy (1998) Measurements of the isotopic composition of solar energetic particles with the MAST instrument aboard the SAMPEX spacecraft. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/P5H0-EB69. https://resolver.caltech.edu/CaltechETD:etd-10202005-144209

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. We report measurements of the isotopic composition of solar energetic particles (SEP) as observed in two gradual solar energetic particle events which began on 30 October and 2 November of 1992. These measurements were taken by the Mass Spectrometer Telescope (MAST), a charged-particle telescope on board the SAMPEX satellite in a polar, low-Earth orbit. The elements measured are carbon, nitrogen, oxygen, neon, magnesium and silicon, which are observed over an energy interval of ~15 to 70 MeV per nucleon. These are the first isotopic measurements of silicon, and only the second measurements of the other elements in individual SEP events. SAMPEX has access to interplanetary fluxes of energetic particles during the high latitude portions of its orbit. This access depends on the rigidity of the particles and therefore on their charge to mass ratio. Fortunately, the polar orbit of SAMPEX allows measurement of SEP charge states using the geomagnetic filtering technique, and a previous study reported measurements of the ionic charge states in the same two SEP events studied in this work. This was the first time that charge states of SEPs at these high energies have been measured, and it allows rigidity-dependent biases in the elemental and isotopic composition to be investigated in the present study. This is particularly important since acceleration/transport processes depend on rigidity, and because the observed composition is also affected by the Earth's magnetic field. The MAST instrument measures both elemental and isotopic composition using the [...] technique, which resulted in a typical mass resolution of ~0.24 amu. It was found that during times of high counting rates, the measured mass distributions exhibited high-mass tails. Through computer simulation of the instrument and its particle environment, it was found that these tails are caused by [...] MeV protons which hit the instrument hodoscope in coincidence with the heavy ions of interest. By choosing only data which occurred during periods of low to moderate counting rates, the probability of such coincidences could be reduced, and these tails could be effectively removed. The elemental composition was used to find and correct for the bias introduced by these restrictions. The energy spectra of the elements in each SEP event could be fit with exponentials in energy per nucleon, with a single, characteristic e-folding energy for each event. Such well-behaved spectra allowed elemental abundances to be calculated. These abundances exhibited a fractionation due to acceleration and propagation of the SEP source material, which could be ordered as a function of the charge to mass ratio of each element. The SEP source abundances exhibited a depletion of elements with a high first-ionization potential as compared to the composition of the solar photosphere, a phenomena which has been well-observed in SEP composition for many years. The depletion factor varied by a factor of ~2 between the two SEP events. The isotopic ratios of [...], [...], [...], [...] and [...] were found to be in generally good agreement with the solar system values of isotopic composition as tabulated by Anders and Grevesse (1989) based on terrestrial and meteoritic measurements. Earlier measurements of SEP isotopes found a [...] ratio which was ~80% higher than the [...] ratio measured in the solar wind, but close to the neon-A component found in meteorites, which had been adopted as the solar system standard by Cameron (1982). Our measurements of [...], however, are consistent with the [...] ratio found in the solar wind. This leads us to conclude that the underlying isotopic composition of the SEP source material is closer to that of the solar wind. There are only two cases in which the measured isotope ratios differ significantly from the Anders and Grevesse (1989) values, the [...] and the [...] ratio in the second SEP event, both of which are a factor of ~2 higher than the solar system value. There is no apparent pattern to these overabundances. While the isotopic ratios measured in the two SEP events generally support the assumption that SEPs and solar wind come from the same reservoir of material, and that SEP composition is consistent with the standard solar system abundances compiled by Anders and Grevesse (1989), SEP isotope measurements are still at an exploratory stage. The limited number of observations prevent us from making definite conclusions about the nature of event-to-event variations in SEP isotopic composition, or about possible differences between the coronal and solar system abundances.

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