SWAN Observes Fading Sunspots On The Back Side Of The Sun

http://sci.esa.int/science-e/www/obj...objectid=34215

SWAN observes fading sunspots on the back side of the Sun
European Space Agency
19 Nov 2003

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 affected 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 in Lyman-alpha emission when it is hit by UV radiation from the
Sun. 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 in this way 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.


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.


Using SWAN data scientists can measure what the evolution of this MgII 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.

Credits:

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

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