SOHO glimpses the far side of the Sun



Depicted here is a set of full-sky images taken by SOHO's SWAN instrument 10 days apart - 
on 20 July and 30 July, 1996.

The right disk in the first image shows the celestial hemisphere which is illuminated by 
the side of the Sun facing the Earth. The left disk shows the celestial hemisphere beyond the Sun as it is illuminated by
the ultraviolet light emitted by the Sun's far side.

In the 20 July image, a lighter shade is visible on the right portion of the left disk. 
This indicates the presence of an active region on the far side of the Sun as it emits strong 
beams of ultraviolet light. 

In the 30 July image, the lighter area that was on the left disk has spread to the right disk, 
i.e. to the opposite celestial hemisphere. This shift was concurrent with an active region 
appearing on the side of the Sun facing the Earth.

The green image on the right of each composite is the EIT image of the same day.

Observing the sky beyond the Sun in ultraviolet light provides a unique opportunity to detect 
the formation of new areas of solar activity on the far side of the Sun several days before its 
rotation brings them
into our view.

This could improve space weather forecasts and help limiting the impact of solar activity on 
space and ground-based activities.