Title: On The Kinetic Parameters Of Interstellar Neutral Helium: An Update From ULYSSES/GAS Results

Authors: M. Witte, M. Banaszkiewicz, and H. Rosenbauer, D. McMullin

Reference: XXVII EGS General Assembly 2002, 22-26 April, Nice, France

While travelling out to its aphelion again, the velocity of the ULYSSES-S/C decreases such that the relative energy of the interstellar neutral He-atoms drops below their detection threshold. The signal of the He-atoms will vanish in the first half of 2002 and would not return before 2006, after the presently scheduled end of the ULYSSES mission. Final results on the velocity and temperature of the He-atoms will be presented obtained during the three mission phases when the particles‘ relative energy exceeded the detection threshold: after launch (1990/1991), the first (1994/1996), and the second (2001/2002) fast latitude scans around perihelion. The observations, covering a complete solar cycle, confirm the expectation that these parameters would not show significant temporal variations on that time scale. To infer the interstellar densities of the He-atoms from the local measurements, one must know the loss rates, these particles experience on their way to the observer. In case of He, the predominant loss process is photoionization by solar EUV-radiation. Significant progress was made recently when time-resolved data of the global solar EUV-irradiances became available from the SOHO/SEM instrument. However, because Ulysses travels over the solar poles, it became evident for the first time that in addition to temporal variations also latitudinal variations (decreases) of the solar irradiances must be taken into account. The progress made and present best estimates of the interstellar densities, based on simple model assumptions, will be reported.

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Last Update: 25 March 2003, Xuyu Wang