First Results from SWAN Lyman-alpha Solar Wind
Mapper on SOHO
by J.L. Bertaux, E. Quémerais, R. Lallement, E. Kyrölä, W. Schmidt, T. Summanen, J.P. Goutail, M. Berthé, J. Costas, T. Holzer
- Movie 1:
This movie is made from 55 individual full sky observations made by the
SWAN/SOHO instrument between April 1996 and the end of May 1997. The
sequence of the movie is roughly one image per week although the time
between two images varies between 5 to 10 days. Each individual image
has been computed in ecliptic coordinates. One observation lasts 24
hours which is the time necessary for each sensor unit to map half of
the sky. The full sky image is made of two halves each one being
obtained by a different sensor unit. The cross calibration factor
between the two sensor units used here is 2.85, with the reference
being set by the North Sensor (+Z side of spacecraft). The color code
is defined between 100 and 800 counts per second which covers the range
of intensity of the Interplanetary Lyman alpha background (the
calibration factor is given in the text). There are two blind spots in
the images. The first one is almost square and is centered on the sun.
The second one is in the opposite direction from the sun and is due to
reflection from the spacecraft. Also, a dark region looking like a shark
fin is caused by a thruster shield. In one year, these artefacts make
a full rotation in ecliptic coordinates. During the same period, the
Lyman alpha intensity maximum clearly moves due to the parallax effect
(see text). The downwind cavity is smallest when SOHO is upwind (June
5) and largest six months later.
The huge hydrogen clouds of comet Hyakutake in April 1996 and comet
Hale-Bopp in 1997 are illuminated by solar Lyman alpha photons, and
their strong emission pattern may be seen moving in the sky from day to
day. The actual size of the Hydrogen cloud of comet Hale-Bopp was
measured by SWAN as more than 1OO million km, making it the largest
object in the solar system for several months.
- Large MPEG Versions:
- Small MPEG Versions:
- IDL SAVE Files:
More information on SWAN can be found at the SWAN Home Page.