Solar activity and sunspot numbers have probably increased to their maximum in the current solar cycle. We are in the middle of the current 11 yr. cycle. Sunspots have been associated with flares and Coronal Mass Ejections CME in recent months. Sometimes the CME are directed at Earth and cause auroral displays. Because of their potential for disruptions of power grids and communications, a large number of ground based and space observatories keep a close watch on them. This post will show some of the dynamics of sunspot activity.
Some of the highest resolution imagery of sunspots is from the Swedish 1-meter Solar Telescope (SST) on La Palma. Click on this sunspot image for a short 4 second movie of the dynamics visible in and around a sunspot. Replay often. Go full screen. The clock in the lower left of the movie runs for 80 minutes during the video. The temperature of the surrounding photosphere is about 5,500˚Celsius (10,000˚ F). The dark sunspot is cooler at about 2727–4227 °C. They are still hot. But, their relative coolness makes them appear dark. These structures are huge by earthly standards.
The next video will help give amore dynamic sense of scale for the image above. It will zoom in from a solar disc view at the left to closer and more detailed views on the right. For comparison, the Earth and Moon are superimposed at strategic times. A whirlpool of plasma about the size of a hurricane is eventually the smallest thing seen in the final part of the video (Courtesy of Bonet et al./IAC/UV/SST).
The boiling appearance surrounding the sunspots is called granulation. You might have seen it in a pot of hot liquid on the stove, or in a cup of hot cocoa. Heated cells of liquid rise to the surface, move horizontally, cool, and descend. On the Sun, the heated material is plasma. Rising columns, known as Barnard Cells, give the cellular appearance.
The time-lapse movie below is of Titanium Oxide TiO granulation recorded on 03 August 2010 from the Big Bear Solar Observatory in California. The distance between each tick mark around the perimeter is approximately 1000 km. Notice the clock record of time. Also, notice the white spots in the dark regions between the cells. They come and go rather quickly. The white spots are not well understood.
credit: Big Bear Solar Observatory / New Jersey Institute of Technology
Brief History of Sunspots
The earliest records of sunspots comes from before the time of Christ in 364 BC by a Chinese astronomer Gan De. Western literature also mentioned sunspots around 300 BC. Sunspots were first observed telescopically in late 1610. Galileo was one of the first to make careful drawings of his observations. The telescopic observations of that era showed that the Sun was not a perfect sphere. They showed that the Sun rotated, and their comings and goings showed that the Sun changed, contrary to Aristotle’s teachings. There is an excellent website called the Galileo Project at Rice University. They have digitized the drawings of Galileo and displayed them by date from the summer of 1612. Because these observations were made at approximately the same time of day, the motion of the spots across the Sun can easily be seen. Here is an animation of the Galileo’s drawings.
The cyclic variation of the number of sunspots was first observed by Heinrich Schwabe between 1826 and 1843. Their numbers peak on an 11 year cycle, but can be as short as 8 or a long as 14. We are currently experiencing what appears to be the peak of sunspot numbers for the current cycle. For reference, visit this url.
Current Solar Activity
Two of the space based solar observatories keeping constant watch are the Solar Dynamics Observatory and the STEREO Observatory. STEREO consists of two spacecraft A and B positioned on either side of the Sun. Their vantage points help to monitor CME and more accurately predict their impact upon the Earth.
SDO is positioned in Geosynchronous orbit above the Earth. It has several instruments for monitoring the Sun. One of the most important is AIA Atmospheric Imaging Assembly.
AIA images the outer layer of the Sun’s atmosphere, the corona, at all temperatures from 20 thousand to 20 million degrees. With high time resolution and a view that covers the entire visible hemisphere of the Sun, for the first time the evolution of all energetic solar events will be followed—from the original micro instabilities through the ejection of billions of tons of material into interplanetary space, to the bright flaring in the corona as the magnetic field is reconfigured in the biggest explosions in the solar system. Four telescope with two passbands each will provide eight full-Sun images every ten seconds, twenty four hours a day, seven days a week.
The AIA instruments provide publicly available imaging data. Users include the scientific community and private citizens. The image analysis can provide dramatic details of the solar activity in high resolution. For example, on March 5, 2012, this video showed the dynamics of sunspot group 1429 as it released flares and CME toward Earth. There is a gear shaped tool near the lower right of the video window to switch to HD. You can also view in full screen mode for more detail.
Because the Sun rotates, this same grouping 1429 is coming around toward the direction of Earth again. It still appears to be quite active and may send flares and CME toward us again. It deserves to be closely watched.
Another of the dynamics of the sunspot cycle is the appearance of sunspots in the southern hemisphere of the Sun. They have appeared in larger numbers now in the current solar cycle. The Sun is on the verge of reversing its magnetic field as it does at the middle of each cycle.
This image is from 1989 at solar maximum. Yellow represents positive, or north polarity pointing out of the Sun. Red is the strongest fields. Blue is negative, or south polarity that points into the Sun. Green is the strongest. In the northern hemisphere (top) positive fields lead, in the southern hemisphere (bottom) the polarities are exactly reversed and the negative fields lead. The Sun rotates east to west so that leading parts of active regions are to the right. [Courtesy of William C. Livingston, National Solar Observatory (NSO), National Optical Astronomy Observatories (NOAO).]
Once the Sun reverses the magnetic field, it will remain in that polarity for the next 11 or so years. Sunspot numbers will decrease, then increase again to a maximum. At the next maximum, the Sun will reverse again. The magnetic field cycle is 22 years. The sunspot cycle is then 11 years. Why and what triggers this cycle is not fully understood. It is a subject on ongoing research.