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.