Hole in the Sun
Coronal holes are areas where the Sun's corona is darker, and colder, and has lower-density plasma than average. In this case it looks like a giant hole in the middle of the sun. These were first found when X-ray telescopes in the Skylab mission were flown above the Earth's atmosphere to reveal the structure of the corona. An extensive coronal hole rotated towards Earth recently (May 28-31, 2013). The massive coronal area is one of the largest seen in a year or more. Coronal holes are the source of strong solar wind gusts that carry solar particles out to our magnetosphere and beyond. Solar wind streams take 2-3 days to travel from the Sun to Earth, and the coronal holes in which they originate are more likely to affect Earth after they have rotated more than halfway around the visible hemisphere of the Sun, which is the case here.
Watching the solar corona is like observing the patterns of clouds in the sky. They can form all sorts of shapes. In June 2012, for example, there was a big bird image. Snapped through three of NASA Solar Dynamics Observatory extreme ultraviolet filters, this current coronal hole is caused by a low density region of hot plasma.
The Sun's corona, or extended outer layer, is a region of plasma that is heated to over a million degrees Celsius. As a result of thermal collisions, the particles within the inner corona have a range and distribution of speeds described by a Maxwellian distribution. The mean velocity of these particles is about 145 km/s, which is well below the solar escape velocity of 618 km/s. However, a few of the particles achieve energies sufficient to reach the terminal velocity of 400 km/s, which allows them to feed the solar wind. At the same temperature, electrons, due to their much smaller mass, reach escape velocity and build up an electric field that further accelerates ions - charged atoms - away from the Sun. The corona is one of the sources of the solar wind.
Solar maximum or solar max is a normal period of greatest solar activity in the 11 year solar cycle of the Sun. During solar maximum, large numbers of sunspots appear and the sun's irradiance output grows by about 0.1%. The increased energy output of solar maxima can impact global climate and recent studies have shown some correlation with regional weather patterns.
At solar maximum, the Sun's magnetic field lines are the most distorted due to the magnetic field on the solar equator rotating at a slightly faster pace than at the solar poles. The solar cycle takes an average of about 11 years to go from one solar maximum to the next, with an observed variation in duration of 9 to 14 years for any given solar cycle. The cycle is peaking now and is causing increased solar activity such as coronal holes.
For further information see Sun Hole.
Corona image via NASA.