A spy mission into the Sun

SOHO examines the Sun from a vantage point 932,057 miles (1.5 million kilometers) out from Earth -- on its sunward side. Its instruments probe the Sun, from its nuclear core, through its turbulent interior and stormy atmosphere, all the way out to Earth's orbit and beyond, where a nonstop stream of atomic nuclei and electrons travels outward as the solar wind. To the naked eye the Sun looks calm and unchanging, but SOHO has infiltrated its hidden interior. Here are just 10 of the revelations based on data it has gathered:

• The Sun's surprising heartbeat. Currents of gas far beneath the visible surface speed up and slacken again every 16 months -- a wholly unexpected pulse rate. It was detected by combining data from SOHO and a U.S.-led network of ground stations called GONG.
• Brighter sunbeams. Watching minute by minute and year by year, SOHO has seen the Sun brighten, as expected, by 0.1 percent while the count of sunspots increased from 1996 to 2000. By studying the variations in detail, scientists estimate that high-energy ultraviolet rays from the Sun have become 3 percent stronger over the past 300 years.
• Eruptions coming our way. Most of the explosive outbursts of gas from the Sun, called coronal mass ejections, miss Earth. Only SOHO can reliably identify those heading in our direction, by linking expanding haloes around the Sun to shocks seen in the Earth-facing atmosphere. Engineers then have two to three days' warning of possible effects in the Earth's vicinity.
• Thousands of explosions every day. A reason why the Sun's atmosphere is far hotter than its visible surface is a nonstop succession of small explosions, observed by SOHO. They result from a continual rearrangement of tangled magnetic fields.
• The sources of the solar wind. SOHO sees gas leaking from the corners of a magnetic honeycomb of gas bubbles, mainly in polar regions, to supply a fast solar wind. Nearer the Sun's equator, a slow wind escapes from the edges of wedge-shaped features called helmets.
• Accelerating the solar wind. Charged atoms feeding the fast wind gain speed very rapidly -- evidently driven by strong magnetic waves in the Sun's outer atmosphere. Similar magnetic waves may accelerate the slow wind too, although many mass ejections also contribute to it.
• Elements in the solar wind. SOHO detected phosphorus, chlorine, potassium, titanium, chromium and nickel for the first time, and previously unseen isotopes of six more common elements. These give clues to conditions on the Sun, and also to the history of the solar system.
• Gigantic sunquakes. After a solar flare, SOHO sees waves rushing across the Sun's visible surface, like the ripples seen when a stone falls into a pond. One such event was judged to be 40 000 times more energetic than the San Francisco earthquake of 1906.
• Huge solar tornadoes. SOHO discovered tornadoes as wide as Africa, with hot gas spiraling outward from the polar regions of the Sun. Typical wind speeds of 31,068 miles per hour (50 000 kilometers per hour) can become 10 times faster in gusts.
• The alien breeze. A wind of gas from other stars blows through the solar system and the solar wind clashes with it. SOHO has more accurately fixed both its direction, from the Ophiuchus constellation, and its speed, 13 miles (21 kilometers) per second.