Sun Spotting the Difference; April 1995; Scientific American Magazine; by Nemecek; 1 Page(s)

X-ray images of the sun offer a new view of the nearby star and its cycles--one that differs markedly from the more familiar images made using visible light. Recent xray pictures from the Japanese Yohkoh spacecraft reveal a striking dimming of the sun¿s corona, its hot outer atmosphere, between 1992 (left) and this year (right ). This change reflects the fluctuations of the 11-year solar cycle, as the star evolves from its period of maximum activity in 1991 to one of minimum action, which should occur by 1997. The cycle is characterized by shifts in the number of sunspots; these perturbations, which can be detected with standard imaging techniques, increase in number when solar activity is greatest. The x-ray data have shown that the corona changes as well: it is 30 times dimmer now than it was three years ago near the height of the solar cycle.