Friday, January 17, 2003, 16:00
WHGA Auditorium
Prof. R. von Steiger, Univ. Bern
Abstract:
The heliosphere is permeated by the solar wind, a tenuous thermal plasma
with energies of ~1 keV/nucleon, which streams out radially from the Sun
at a speed of ~300-800 km/s. The properties of this plasma, in
particular its elemental and charge state composition, can be used to
infer its sources in the solar atmosphere an the conditions and
processes that prevail there.
The elemental composition deviates from the solar composition in a characteristic way: it is enhanced in elements with a low first ionisation potential (FIP). The enhancement is caused by an atom-ion separation process in the chromosphere (where the solar atmosphere is partially neutral), so the FIP fractionation factor probes the conditions deep down in the solar atmosphere. The enhancement is found to be relatively small (1.5-2) and constant in fast solar wind streams, proving their origin from coronal holes. Larger, and much more variable, enhancements are found in the slow solar wind from the streamer belt, which provides hints to its origin that is still poorly known.
The charge state composition, on the other hand, is determined by hot electrons higher up in the solar atmosphere and thus probes coronal conditions. The ratio of two charge states of an element in the solar wind, O7+/O6+, say, depends on the coronal temperature at the altitude where the expansion time scale of the accelerating wind overcomes the ionisation/recombination time scale and therefore acts as a coronal thermometer. This ratio is one of the principal diagnostic tools for solar wind type and thus for source diversity. In combination with charge state ratios of other elements it is possible to infer a rough coronal temperature profile with altitude.
I will present observations of the Solar Wind Ion Composition Spectrometer (SWICS) sensor on the Ulysses mission accumulated over the past 12 years -- more than a complete solar cycle -- and summarise in what way they have contributed to our understanding of the Sun and the heliosphere.