The Sun Through Different Filters

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Slide 6

The 11 Year Sunspot Cycle

The polarity of the Sun’s global magnetic field reverses every ~11 years, forming a complete cycle every ~22 years. The amount of magnetic field present on the Sun varies with time, increasing and decreasing with a fairly regular period of 11 years. The number of sunspots visible on the Sun at any one time reflects this cycle, as seen in the central plot of the above figure, but that is not the only change which occurs. The whole solar atmosphere adjusts to the changes in surface magnetic field distribution. The top row of images are X-ray images of the Sun taken with the Yohkoh spacecraft. They show the decrease in X-ray output of the Sun as the magnetic activity decreases at the end of one particular solar cycle. The bottom row shows how the outer solar atmosphere, called the corona, changes with solar cycle. These are eclipse photographs spanning the period between 1966 and 1988, with the exception of the image for 1985 which was constructed using data from the Solar Maximum Mission spacecraft. When the few magnetic sunspots are present on the Sun, called solar minimum, the closed (bright) regions of the corona are confined to lower latitudes centered around the solar equator. At solar minimum, the global magnetic dield on the sun resembles a simple dipole field, like that produced by a bar magnet. When many magnetic regions are present, the corona appears more symmetric, with closed (bright) structures located at all latitudes. These differences in the appearance of the corona are caused by changes in the magnetic field structures which thread it.

solar max

near minimum

near max

near minimum

Slide 7

The Coronal Mass Ejections

In general, the magnetic field, like gravity, acts to hold down the solar atmosphere. However, the temperature of the solar corona is about a million degrees (100 times greater than the solar surface). These high temperatures and the presence of stresses on the coronal magnetic field, act to drive the atmosphere outward. Previously closed magnetic regions can become unstable resulting in a sudden ejection of coronal plasma and magnetic field into the solar wind, known as Coronal Mass Ejections (CMEs).

This set of images shows a CME from 10:04 to 13:34 on August 28, 1980. Large ejections, such as the one pictured here, send upwards of 1013 kg of coronal material into space at rates of 100 km/s or more (although average values are closer to ~ 1012 kg and 400 km/s). The CME originates from a helmet streamer -- the bright, pointed feature seen in the lower half of the first two images of the sequence -- which is blown out by the passage of the CME. When CMEs are Earthward directed, they can affect communications, navigation systems, even power grids, while also producing auroras visible in the night skies