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Quantitative changes in the solar corona as seen by UVCS/SOHO from 1.5 to 3.5 so lar radii and by EIT/SOHO at lower heights during the rising phase of solar cycle 23. The five solar images from left to right are composites of UVCS/SOHO O VI 1032 Å (outer) and EIT/SOHO 284 Å (inner) synoptic images. The center of each solar image is positioned above the date of observation. The blue histogram represents the National Solar Observatory/Sacramento Peak sunspot numbers (monthly averages). The hatched blue region corresponds to predictions of future sunspot numbers from the Marshall Space Flight Center Solar Physics Branch. Both the UVCS and EIT instruments are calibrated, so the color scale represents the measured brightness of their respective sections of the composite image, but their color scales are much different with the same color representing a factor of about 10,000 greater brightness in the EIT images of the solar disk compared to the brightness of the UVCS images in the outer portion of the composite image. In each case, brightnesses increase from the black region through the red and yellow regions to the white regions. |
One way of tracking the solar activity is by observing sunspots. Sunspots are relatively cool areas that appear on the face of the Sun. During solar minimum there are only a few sunspots, and during solar maximum there are about 20 times more spots. Every 11 years the Sun undergoes a period of activity called solar maximum followed by a quiet period called solar minimum. The sunspot number is calculated by first counting the number of sunspot groups and then the number of individual sunspots. The ``sunspot number'' is then given by the sum of the number of individual sunspots and ten times the number of groups. Although sunspots themselves produce only minor effects on solar emissions, the magnetic activity that accompanies the sunspots can produce dramatic changes in the ultraviolet and soft X-ray emission levels. These changes over the solar cycle have important consequences for the Earth's magnetosphere and upper atmosphere.
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Picture Credit: Smithsonian Astrophysical Observatory; ESA & NASA