![]() Some of the magnetic fields extend outward, within regions called coronal holes. Not all magnetic fields on the Sun are closed loops. Courtesy of Gregory L Slater, Gary A Linford, and Lawrence Shing, NASA, ISAS, Lockheed-Martin Solar and Astrophysics Laboratory, National Astronomical Observatory of Japan, and University of Tokyo. This image of the Sun’s corona was recorded by the Soft X-ray Telescope (SXT) aboard the Japanese Yohkoh satellite on 1 February 1992, near the maximum of the 11-year cycle of solar magnetic activity. The brightest features are called active regions and correspond to the sites of the most intense magnetic field strength. It shows magnetic coronal loops which thread the corona and hold the hot gases in place. The bright glow seen in this X-ray image of the Sun is produced by ionized gases at a temperature of a few million degrees kelvin. As a result, the million-degree corona can be seen all across the Sun's face, with high spatial and temporal resolution, in X-rays.įigure 1. Also, the photosphere is too cool to emit intense radiation at these wavelengths, so it appears dark under the hot gas. Very hot material-such as that within the corona-emits most of its energy at X-ray wavelengths. The solar corona has a temperature of millions of degrees kelvin, hundreds of times hotter than the underlying visible solar disk whose effective temperature is 5780 K. Modern solar satellites, such as the Solar and Heliospheric Observatory ( SOHO), use coronagraphs to get clear, edge-on views of the corona. Telescopes called coronagraphs allow us to see the corona by using occulting disks to mask the Sun's face and block out the photosphere's glare. They can both be seen during a total solar eclipse, when the Moon blocks the intense light of the photosphere. The thin chromosphere and extensive corona lie above the visible sharp edge of the photosphere. The visible photosphere, or sphere of light, is the level of the solar atmosphere from which we get our light and heat, and it is the part that we can see with our eyes. Lang, in Encyclopedia of Geology, 2005 The Outer Solar Atmosphere The Space Weather Prediction Center monitors solar activity, such as sunspots and solar flares, so we can prepare for potential disruptions.K.R. These charged solar particles can enter the atmosphere and create beautiful auroras, but they can also disrupt infrastructure, such as satellites, navigation, communications, and the electric power grid. Plasma emitted from the Sun shoots millions of electrons and protons toward Earth. During a solar maximum, when sunspot numbers are at their peak, the Sun emits more radiation than usual. Sunspots can cause geomagnetic storms in Earth’s magnetosphere. Notice how the sunspot count rises and falls in an 11-year cycle. This graph shows the number of sunspots counted each year for several decades. ![]() The 11-year sunspot cycle is actually half of a longer, 22-year cycle of solar activity During this period, sunspot counts reach maximum and minimum twice. A peak in the sunspot count is referred to as a time of solar maximum, whereas a period when few or no sunspots appear is called a solar minimum. The Sun is typically very active when sunspot counts are high. Historical records going back to the 1700s reveal that this sunspot cycle is roughly eleven years long - though sometimes it is longer or shorter. ![]() The number of sunspots varies over a multi-year cycle. Sunspots can last for weeks or even years before dissipating. The image with no spots (left) was taken near the solar minimum in December 2019, while the image with many sunspots (right) was taken near the solar maximum in April 2014. Sunspots have a lighter, outer section called the penumbra and a darker, central region named the umbra. If you could cut an average sunspot out of the Sun and place it elsewhere in the night sky, it would be about as bright as the full Moon. Sunspots are only dark in contrast to the bright face of the Sun. Large sunspots are thousands of degrees cooler than the areas that surround them (about 4,200 ☌ for a sunspot compared to 6,000 ☌ for the photosphere surrounding it). Sunspots appear dark because they are cooler than their surroundings. NASA/Royal Swedish Academy of Sciences Why Are Sunspots Dark? The darker umbra region is in the center, surrounded by the lighter penumbra region. Sunspots on the Sun’s surface (left) and a close-up view of a sunspot group (right). Because sunspots are associated with increased solar activity, space weather forecasters track these features to help predict solar storms that can impact Earth. These powerful magnetic disturbances produce active regions on the Sun, which can often create solar flares and coronal mass ejections (CMEs). They are caused by twisting, chaotic magnetic fields from within the Sun’s convective zone. Sunspots are dark, planet-sized regions that appear on the surface of the Sun.
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