GAS - User GUIDE

GAS Contact:

M. Bruns
Max-Planck-Institut für Aeronomie
Max-Planck-Strasse 2
D-37191 Katlenburg-Lindau
Germany

Phone: (49) 5556 979 345
Telefax: (49) 5556 979 240

E-mail: bruns@linmpi.mpg.de

ULYSSES/GAS                      USER GUIDE                20.9.02 General: The GAS-Instrument on ULYSSES provides angular distributions of countrates from interstellar helium particles (in the NG-mode) or from celestial UV-intensities (UV-mode). Therefore the data are presented in the form of maps rather than as time series (as for most other ULYSSES experiments). The intensities are mapped in an ecliptic coordinate system centered in the spacecraft. In this system, the positions of the Sun and the planets vary with time and with the position of the spacecraft along its trajectory, however, the positions of stars remain fixed. The actual positions of Sun, Earth, and Jupiter are indicated by yellow, green and white dots, respectively. The area around the Sun is excluded from observation to avoid damaging intensities from direct sunlight. The intensities obtained in the UV-mode (the housekeeping parameter HV given in the bottom line of each map, has a positive value) are color coded in the range 10...1000 counts/s, in the NG-mode (HV has a negative value) in the range 0.1...10 counts/s. File Name Conventions: The maps are provided in three different formats, as indicated in the extensions of the filenames GAyydd_N_V.ext. 1.)   *.ps.gz :  postscript format (compressed) 2.)   *.png   :  ready-to-view PNG-format. 3.)   *.txt.gz:  ASCII-format to allow further computerized processing. This format is described in detail in the separate file FORMAT.TXT. These files are contained in a compressed format in the *.GZ-files. The further information encoded in the filename is GA           :   ID for ULYSSES/GAS data (fixed). yyddd        :   date of measurement(yy: year,ddd: day of the year). N            :   indicator of the telescope used, 1 =  narrow field of view (NFOV), 2 =  wide field of view (WFOV). V            :   Version indicator, 1,2, ... :  Running number of measurement during that day (mostly just 1). C        :  This map is composed from individual sub-images, named in the lower left corner of the map. This latter format -C- is used exclusively for Full-Sky-Maps, it is advised to start viewing the data in this format to get an immediate overview. For further information see the separate files REFERENCES.TXT and FORMAT.TXT . Interstellar Gas (NG-mode): Most of these maps are restricted to an area around the apparent flow direction of the interstellar helium particles. It is important to note, that the locally observed, apparent flow direction deviates from the flow direction at large distances (outside the solar system) for two reasons: 1.) The local flow direction results from a deflection of the asymptotic direction due to the solar gravity (hyperbolic orbits). 2.) The observed, apparent flow direction deviates from the local flow direction due to the abberation effect, resulting from a combination of the particle velocity vector and the spacecraft velocity vector. This effect is significant (of the order of 10 degrees) as the spacecrafts velocity is of the same order as the particle velocity (several 10 km/s). The systematic variation in the flow intensity which can be observed during over extended periodes in the ULYSSES mission is a result of the variation of the relative energy of the particles in the instrument's system. The detection efficiency of helium particles is strongly energy dependent with a sharp cut-off at energy around 30 eV (equivalent to a velocity of about 30 km/s for helium particles). This threshold is only exceeded when the spacecraft velocity (in magnitude and direction) adds in a favourable way to the local particle velocity. This is the case during extended periodes of the ULYSSES mission, from launch to Jupiter fly-by (Nov. 1990 to Feb. 1992) and in periods before and after perihelion (July 1994 to July 96 and again in 1999 to 2001). Due to a residual sensitivity of the instrument to UV-photons, even in the NG-mode, also the signal from UV-stars is registered. This has been used to verify the pointing accuracy of the instrument to be of the order to a few tenths of a degree. Neutral particles from Jupiter: Close to Jupiter, in January to April 1992, the integral fluxes of low energy neutral particles can be derived from the observation of their angular distributions. References Banaszkiewicz M., Witte M., Rosenbauer H., Determination of interstellar helium parameters from the ULYSSES-NEUTRALGAS experiment: Method of data analysis, Astron. Astrophys. Supp. Ser., 120, 1-16, 1996. Witte M., Rosenbauer H., Keppler E., Fahr H., Hemmerich P., Lauche H., Loidl A., Zwick R., The interstellar neutral-gas experiment on ULYSSES, Astron. Astrophys. Suppl. Ser., 92, 333-348, 1992. Witte M., Rosenbauer H., Banaszkiewicz M., Fahr H., The Ulysses Neutral Gas experiment: Determination of the velocity and temperature of the interstellar neutral helium, Adv. Space Res., 13, (6)121-(6)130, 1993. Witte M., Banaszkiewicz M., Rosenbauer H., Recent results on the parameters of the interstellar helium from the ULYSSES/GAS experiment, Space Sci. Rev., 78, 289-296, 1996.