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 invisible.
Invisible spots on the Sun
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.’ But a team of UK scientists have discovered a surprising problem. Nearly half of the new sunspots forming in the west of the Sun are invisible, even to our best instruments.
The Sun rotates from east to west. As newly-formed sunspots are created on the eastern half of the Sun, its rotation carries them inexorably across the visible disc, where our instruments can get a clear look at them. However, the story is very different for new sunspots forming in the west. Even before those sunspots drift completely around the far side of the Sun, they begin to disappear. Many of them are not seen at all, and remain invisible.
As it edges closer to the western limb, the area of a sunspot appears smaller to an observer on the Earth, because we look at it at an angle, as opposed to head-on. As a result, some small newly-formed spots will be missed. That might seem a straightforward explanation, but many mysteries remain in this scenario, because the apparent ‘scarcity’ of new sunspots in the west becomes evident well inside the Sun’s main disc, long before the extremely oblique viewing angles at the edge of the disc should become a factor.
Dr Silvia Dalla, from the Centre of Astrophysics at the University of Central Lancashire, says, “Our analysis shows that this effect is very large. As many as 44 percent of new sunspots in the west of the Sun are actually going undetected. This is very puzzling. It was even more surprising to find that similar observations were reported 100 years ago by the British astronomer Annie Maunder, and have been forgotten since.”
The phenomena that are blocking many new sunspots from our view as they head west are still uncertain. It is likely that the effect is related to the journey that photons of light from sunspots—and from the surrounding regions—need to make through the solar atmosphere before they can reach our telescopes. As a spot edges towards the western limb, the photon's path through the Sun's atmosphere becomes longer and longer, compared to when they are near the centre of the disk.
The team’s findings, which have just been published in the Astronomy and Astrophysics Letters journal, were obtained using Virtual Observatory software tools developed by AstroGrid. The three-strong team consisting of Dr Dalla, Dr Lyndsay Fletcher from the University of Glasgow and Dr Nicholas Walton from University of Cambridge, analysed the observational history of nearly 7000 sunspots using data collated over a 25-year period.
AstroGrid: 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 their analysis. For more information on AstroGrid, click on the "Find out more" link below.
