As you can see, the Sun is keeping quiet, devoid of sunspots. As the world awaits an increase in solar activity to celebrate the onset of a new solar cycle, our closest star keeps a blank face and keeps us guessing. This most recent image was taken today by the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) instrument. MDI measures plasma velocity and magnetic field strength at the top of the convection zone, so it is an invaluable sunspot detector. Sunspots are a good indicator about how active the Sun is, as when the magnetic field becomes stressed and twisted, it is forced from the convection zone, through the photosphere, chromosphere and high into the corona. These protrusions then fill with plasma and glow as coronal loops. The more magnetic activity there is, more sunspots appear. But, it would seem, the Sun remains magnetically inactive seven months since the beginning of Solar Cycle 24…
There has been much debate about what this quiet period means for the Sun. Well, it might be having an extended sleep, but solar minimum is a natural period of time in the 11-year solar cycle. Some theories say that within the next few months, we’ll notice a massive increase in sunspot population, heralding a very violent solar maximum by 2011. Others are more cautious and say there will be nothing out of the ordinary during the next 11 years. At the other end of the spectrum, we have the theory that we are facing a debilitating lull in solar activity, possibly reducing solar heating on the Earth, plunging us into a mini-ice age. Personally, I reckon we’ll see more normal solar activity soon, possibly more, possibly less active than the last cycle, but certainly no extremes.
However, this is a superb time to compare the current Sun with a more active version six years ago. In the image (left), the contrast between solar maximum and solar minimum couldn’t be greater. On the left, the Sun has a highly dynamic surface, coronal loops and bright patches indicating rapid plasma heating. On the right is the Sun we see today, some minor activity but nothing to report home about. These images were taken again by SOHO, but looking through a different filter (and therefore different wavelength) with the Extreme ultraviolet Imaging Telescope (EIT). The false colour green means we are looking at a wavelength of 195A, which is plasma radiating at approximately 1 million Kelvin. The first thing that becomes obvious with the right-hand image is the lack of 1 million K plasma. Therefore there is little heating energy input to the coronal plasma from the solar interior. Weak plasma heating = low magnetic activity (as coronal heating mechanisms are dominated by magnetic energy). Boring really.
However, over at Space Weather Alan Friedman caught this spectacular image (above) of prominences hanging above the limb of the Sun. These eerie strands of cool plasma (as seen in the Hα line) are supported by a complex of tenuous magnetic fieldlines projecting a few hundred thousand kilometres into the lower corona that would otherwise be difficult to see if the Sun were more active.