SWAN observes fading sunspots on the back side of the Sun (Forwarded)

ESA Science News
http://sci.esa.int

19 Nov 2003

SWAN observes fading sunspots on the back side of the Sun

The new Lyman-alpha method to monitor the activity of sunspots when they are on
the far side of the Sun has been recently applied to a duo of giant sunspots
number 486 and 488. The SWAN team (Jean-Loup Bertaux and Eric Quemerais) has
found that their activity has decreased significantly in the last few days.

These giant active regions were facing the Earth around October 28, when an
intense Coronal Mass Ejection (CME) sent giant clouds of energetic particles
(protons) out into the solar system. This effected many spacecraft, including
the Mars Express ESA mission currently cruising towards Mars, for an encounter
on 25 December.

Star trackers on the Mars Express spacecraft could not be used for a period of
time because of the effect of these high energy protons, accelerated in the
shock front produced when the fast solar wind plasma of the CME bumps into the
standard slower solar wind. Instead of identifying some stars in the field of
view of the star tracker (which can be viewed as quite similar to a standard
digital camera with a so-called CCD detector), the images are looking more like
a heavy snowstorm.

Mars Express survived this intense episode of solar activity easily, but there
was some concern about the possible repetition of such an event, when the same
sunspots 486 and 488 would come around on the Sun's east limb to face the Earth
and Mars Express again. Due to the 27 days solar rotation, the two sunspots
disappeared from direct view around November 4, and are reappearing today on the
east limb of the Sun.

Observing the far side of the Sun with SWAN

SWAN -- short for Solar Wind Anisotropies -- can indirectly monitor the activity
on the far side of the Sun by mapping the whole sky in ultraviolet light. A
huge cloud of interstellar hydrogen that bathes the entire Solar System
interacts with the solar wind, and lights up when it is hit by UV radiation from
the Sun. The cloud can then be observed by its Lyman-alpha emission. Since
active regions on the Sun are brighter in Lyman-alpha light, the sky displays
more intensity in the region facing them, like a rotating lighthouse beam. Any
change in the solar activity is directly reflected in the amount of Lyman-alpha
emission that is observed by SWAN.

SWAN sees decreasing activity

During the whole period after 28 October, SWAN was put in a more than usual
intensive mode of monitoring solar activity on the far side of the Sun. A series
of maps of the full sky were obtained in the UV. A quantitative analysis of the
Lyman-alpha brightness of the active region is given in relation to the
classical magnesium index (MgII index), which can be measured only from Earth.
The MgII index has a typical value of 0.265 for low solar activity, reaching
0.288 for highest solar activity. On October 28, it had a value of 0.280.

Using SWAN data scientists can measure what the evolution of this Mg II index
would be for an observer rotating with the Sun, who would always face the active
region during the solar rotation. The MgII index found by SWAN was still
increasing after October 28 up to the peak value of 0.283, which was reached on
November 7. Since then it has been rapidly decreasing down to 0.273 on November 15.

The corresponding decrease of the solar Lyman-alpha brightness found by SWAN was
20%. This is the sign that the activity of the two sunspots 486 and 488 has
significantly decreased since their maximum value, reducing also (but not to
zero) the probability of a new episode of solar storm as the one experienced on
October 28.

More generally, the SWAN method of monitoring the far side of the Sun allows a
better forecast of the upcoming solar activity and associated Space Weather,
which could present a danger to many space activities, and several ground based
activities as well.

SOHO (Solar and Heliospheric Observatory) is a joint ESA/NASA mission to study
the Sun and the heliosphere.

Contact points:

Jean-Loup Bertaux
Service d'Aeronomie du CNRS BP 3 91371
Verrieres le Buisson Cedex, France
Tel : 33-(0)1-64 47 42 51
Fax : 33-(0)1-69 20 29 99
Cell-phone: 33-(0)6 80 73 08 70
e-Mail:bertauxaerov.jussieu.fr

Eric Quemerais
Service d'Aeronomie du CNRS BP 3 91371
Verrieres le Buisson Cedex, France
Tel : 33-(0)1-64 47 43 17
Fax : 33-(0)1-69 20 29 99
e-Mail:quemeraisaerov.jussieu.fr

For further information please contact:


SEE ALSO

* Ongoing Solar Activity
http://sci.esa.int/science-e/www/obj...objectid=34058

RELATED LINKS

* SOHO Spaceweather
http://sohowww.estec.esa.nl/spaceweather/
* SWAN instrument
http://sohowww.nascom.nasa.gov/instruments.html

IMAGE CAPTION:
[http://sci.esa.int/science-e/www/obj...bjectid=34218]
Solid line: Time variation of the MgII solar index as seen by a virtual
observer, rotating with the Sun, and always facing the region of the Sun which
was facing the Earth on 28 October 2003, when a very active region including two
sunspots was visible. This variation was deduced from the SWAN full sky images
in the light of Lyman-alpha, produced by the illumination by the Sun of hydrogen
atoms in the solar system.

The MgII index is a direct index of the solar activity, and the strong decrease
observed between November 7 and November 17 is a sign of the fading activity of
the two sunspots.

Dotted line: Time variation of the MgII solar index as seen from the Earth for
the same time period.

Credits: Jean-Loup Bertaux and Eric Quemerais at Service d’Aéronomie du CNRS,
Verrieres-le-Buisson, France

Copyright: ESA