With less than a week to go until the Sun crosses northwards over the equator at the vernal equinox, it is showing real signs of rebirth in another respect. Solar activity, as measured by the number of sunspots, is at last picking up strongly after a prolonged period at near-record low levels.
Our image was obtained on 6 March and highlights two major sunspot groups, each much larger than the Earth, as they progressed from E to W (left to right) across the N hemisphere of the Sun's disc. They were not alone: another group was forming in the SW quadrant only a couple of days before it rotated beyond the Sun's limb, and yet more were following on in the E.
Spots form where lines of the Sun's magnetic field break through the surface and create cooler regions as they divert the convection of hot gas from below. Being more than 1,000C colder than the 5,500C of most of the visible Sun's surface, the photosphere, they are darker and stand out by contrast. A typical spot has a central dark umbra, surrounded by a lighter penumbra. Spots can last from a few hours to several weeks, appearing to drift across the disc as the Sun rotates every 26 days with respect to our view.
Their frequency, though, typically rises and falls in a cycle of 11 years. Since the last solar maximum came in 2000, we might expect another peak to be imminent. In fact, since 2006 solar experts have gradually pushed their predicted maximum year from 2010/2011 to 2013, while slashing its predicted level from the second highest on record to one of the lowest ever seen.
But there is no doubt that spot numbers are increasing, bringing with them more solar flares and the blasts of material from the Sun's atmosphere that we call coronal mass ejections. These can lead to communications blackouts around the Earth and produce aurorae; indeed, there have been several nice displays over recent weeks. ...
via Starwatch: The awakening Sun | Science | The Guardian.