Solar Wind and Energetic Particle composition data is the venue of the Charge, Element, Isotope Analysis System (CELIAS) on the SOHO spacecraft. CELIAS uses "time of flight" (TOF) technology to make composition measurements. To get an overview of the entire experiment, visit the CELIAS HOMPAGE.

The CELIAS solar wind mass spectrometer (MTOF, Mass Time-of-Flight sensor) has unprecedented mass resolution for solar wind composition studies, and has already measured rare elements and isotopes that were previously not resolvable from more abundant neighboring species, or were not previously observable at all. For example, as seen in the figure, the elements of sulfur, argon, and calcium are now easily distinguished from the neighboring species of silicon and iron, as is nitrogen from carbon and oxygen. The rare elements phosphorus, chlorine, potassium, titanium, chromium, and nickel are being measured in the solar wind for the first time.

Some of these elements (P, Cl, K, Ti, Cr, and Mn) have no coronal spectroscopic measurements available. The determination of the elemental abundances of these rarer species will allow us to fill in the "blanks" of the solar wind versus photospheric abundance tables. This is important in obtaining a much better analysis of the solar wind feeding and acceleration processes in the chromosphere and inner corona. The solar wind and coronal elemental abundances indicate an ordering of relative abundance enhancement (or depletion) to photospheric values partially correlated with the the amount of energy required to strip off the first electron from the atom, that is to become ionized. This is called the "FIP effect", after the name for the energy involved - the First Ionization Potential. The newly observed solar wind elements have different chemical properties from each other and previously measured elements (first ionization potentials, first ionization times, charge state equilibrium times, atomic mass, etc.). Knowledge of their relative abundances serve as diagnostic tools for determining conditions in the chromosphere/transition region where ions are separated from neutrals. The elemental abundance determination of potassium (K) will be particularly interesting since its first ionization potential at 4.34 eV makes it the lowest FIP species observed to date in the solar wind.

The FIP effect is not the same for all types of solar wind. The temporal resolution of CELIAS means that abundance variations in different types of solar wind (e.g., coronal hole-associated vs. slow solar wind) may be better traced to varying conditions in the source regions of the solar wind.

The MTOF sensor is routinely measuring isotopic abundance variations for several elements (neon, magnesium, silicon, sulfur, argon, calcium, iron, and nickel), some of which have never been previously observed in either the solar wind, solar energetic particles, or spectroscopically. Among the new solar wind isotopes are those of silicon, sulfur, calcium, chromium, iron, and nickel. Other isotopes are being measured with a much finer temporal resolution than previously available (on the order of minutes/hours instead of months/years).

Matter in the corona and solar wind is derived from the outer convective zone (OCZ) of the Sun. Isotopic abundances of the less volatile elements in the solar atmosphere are probably very similar to terrestrial, lunar and meteoritic abundances. From such elements it is possible to infer the amount of isotopic fractionation under varying conditions in the solar wind source region. For many species, the solar wind provides the only source of information, which is important for many cosmochemical and astrophysical applications. Knowledge of the isotopic composition of the OCZ will yield information on the early solar nebula and the history of the solar system.

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