FOR RELEASE: 9:00 a.m. CDT, June 11, 1996


Space Physicists are announcing today the first observations of rare elements and isotopes in the solar wind. The results are important because they will help determine how neutral atoms in the relatively cool photosphere become the highly ionized atoms in the corona and then ultimately the supersonic plasma we call the solar wind. The results will also help establish the isotopic composition of the primordial solar nebula.

The report is being presented by Dr. Antoinette Galvin of the University of Maryland, College Park MD, representing Dr. Fred Ipavich (also of the University of Maryland, and the Lead Investigator for the MTOF sensor); the Co-Principal Investigators Prof. Peter Bochsler (University of Bern, Switzerland), and Dr. Dietrich Hovestadt (Max Planck Institute, Garching, Germany); and the other members of the Charge, Element, Isotope Analysis System (CELIAS) Team on the SoHO spacecraft.

CELIAS uses three "time of flight" (TOF) sensors to make composition measurements. 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.


Figure 1 showing mass 12 through 62 amu

The first Figure displays a mass spectrum obtained from MTOF during a 3-day period in February 1996. Unlike data previously available, the elements of nitrogen, sulfur, argon, and calcium are now easily distinguished from their more abundant neighboring elements. The rare elements phosphorus, chlorine, potassium, titanium, chromium, manganese 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 abundance enhancement (or depletion) relative to photospheric values that is correlated with the amount of energy required to strip off the first electron from the atom. 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.), which will help distinguish the physical mechanisms involved in the atom ion separation process. 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 element Phosphorus (P) is also of interest because its FIP of 10.48 eV places it in a transition region between low and high FIP elements. The FIP effect is not the same for all types of solar wind. The temporal resolution of MTOF 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.


Figure 2 showing mass 51 through 61 amu

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 the solar wind, in solar energetic particle populations, or spectroscopically. Among the brand new isotopes are those of silicon, sulfur, calcium, chromium, iron, and nickel; the second Figure demonstrates the presence of isotopes of Cr, Fe and Ni. 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 some 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.

This work was supported by NASA and the Swiss NSF.