SOHO IS A WIND-GAUGE FOR THE INTERSTELLAR BREEZE

An unprecedented chart shows the true direction and apparent speed of a
breeze of atoms, mainly hydrogen, that comes from the stars and blows
right through the Solar System. In the solar spacecraft SOHO, built by
the European Space Agency, the SWAN instrument detects a characteristic
ultraviolet glow filling the sky, coming from the hydrogen atoms. Small
shifts in the ultraviolet wavelength reveal the speed of the breeze. 

On a map of the whole sky, the windspeed is shown in relation to the
direction from the Sun. A high negative velocity shown in purple depicts
the incoming breeze, while in the orange parts of the chart the
interstellar atoms are on their way out of the Solar System. In
intermediate sectors they are moving sideways in relation to the Sun, so
the radial speed is close to zero. White holes in the image are stars in
the Milky Way emitting the same ultraviolet wavelength. 

An analysis based on two years of observations with SWAN defines the
direction of the interstellar breeze more accurately than ever before.
The source lies in the direction of the constellation Ophiuchus, close
to Scorpius, and the night side of the Earth faces the breeze directly
on 3 June each year. 

The SWAN instrument was provided for SOHO by the CNRS Service
d'A[e/]ronomie at Verri[e\]res near Paris and the Finnish Meteorological
Institute in Helsinki. SWAN has already charted the destruction
(ionization) of interstellar hydrogen atoms by the impact of the solar
wind coming from the Sun. Downwind of the Sun, they are almost entirely
destroyed. That explains the difference in apparent windspeed, incoming
and outgoing. The outgoing atoms detected in this study are far away,
and have survived by giving the Sun a wide berth. They give a truer
measure of the windspeed than the incoming atoms, which are accelerated
by the Sun's gravity.  

Built in Europe for the European Space Agency, SOHO carries twelve sets
of instruments provided by European and American investigators and it
was despatched into space on 2 December 1995 by a NASA launcher. SOHO is
a project of international cooperation between ESA and NASA.  

Credits: 
Main chart: SOHO (ESA & NASA) and SWAN Consortium
Star image: DSS/STScI/NASA