Media release
Invisible sunspots uncovered
A team of top scientists have discovered that 44 percent of new sunspots forming in
the West of the Sun are invisible to our best telescopes.
Their findings, which have just been published in the Astronomy and Astrophysics
Letters journal, were obtained using Virtual Observatory software tools developed by
The team of scientists analysed nearly 7,000 sunspots that were observed over a 25
year period and found some areas of the Sun's disk are missing new sunspots
compared to others.
Dr Silvia Dalla, from the Centre of Astrophysics at the University of Central
Lancashire, said: "This was very puzzling. It was even more surprising to find that
similar observations had been reported 100 years ago by the British astronomer
Annie Maunder, and had been forgotten since.
“Our analysis shows that this effect is very large and as many as 44 percent of new
sunspots in the West of the Sun are actually going undetected."
Sunspots appear dark because of their low temperature compared to the surrounding
regions. The Sun is constantly producing new spots and their identification and
tracking is essential for predicting Space Weather: the magnetic fields of sunspots
cause flares and huge eruptions of material called Coronal Mass Ejections that can
impact the Earth's atmosphere and disrupt satellite communications.
The reason why some sunspots remain invisible is that, just as they are growing, the
Sun's rotation is carrying them away from the centre of the disk, where visibility for
Earth telescopes is best.
In their work, the three strong team consisting of Dr Dalla, Dr Lyndsay Fletcher from
the University of Glasgow and Dr Nicholas Walton from University of Cambridge,
demonstrated that the sunspot visibility threshold varies strongly with solar longitude,
much more than previously thought.
-EndsNotes to Editors
Sunspots appear as dark patches on the Sun's photosphere. They were first observed by
Chinese astronomers and later extensively studied by Galileo with the aid of a telescope.
Their temperature is of approximately 4,000 degrees, lower than that of the surrounding
photosphere (6,000 degrees).
The birth of a new spot on the solar disk indicates the emergence of magnetic flux through the
photosphere. This process is key to the 11-year solar cycle, and the number of sunspots is
used as an indicator of solar activity. At peak solar activity (solar maximum) several spots are
typically visible on disk on any one day, while at solar minimum often there are no spots.
The NOAA Space Weather Prediction Centre assigns a number to
each new sunspot appearing on the Sun, and this is used to track the region and monitor its
production of flares and Coronal Mass Ejections. Coronal Mass Ejections that propagate
through space from the Sun to the Earth are the cause of geomagnetic storms and the
aurora. Space Weather monitoring aims to predict such events, because of their impact on
satellite communications, power grids and the safety of astronauts in space.
AstroGrid is the UK's Virtual Observatory project. AstroGrid is developing new software tools
allowing scientists to access a large number of astronomical datasets via the internet and
facilitating the analysis of large datasets.
More information can be found at:
Article reference:
The results have been published in Astronomy and Astrophysics Letters:
S. Dalla, L. Fletcher and N.A. Walton, “Invisible sunspots and rate of solar magnetic flux
emergence”, Astron. Astrophys. 479, L1-L4 (2008), DOI: 10.1051/0004-6361:20078800
For additional information please contact:
Dr Silvia Dalla
Centre for Astrophysics
University of Central Lancashire
Preston PR1 2HE
Tel. 01772 893527 / 07968 952859
Email: [email protected]
Dr Nicholas A. Walton
Institute of Astronomy
University of Cambridge
Madingley Road
Cambridge, CB3 0HA
Tel. 01223 337503
Email: [email protected]
Dr Lyndsay Fletcher
Dept of Physics and Astronomy
University of Glasgow
Glasgow G12 8QQ
Glasgow G12 8QQ
Tel. 0141 3305311
Email: [email protected]
Locations on the Sun's disk where new spots were
seen to emerge: on the right in these plots, the West
of the Sun, many new spots are missing as they are

here - University of Central Lancashire