Unruly sun emerges from solar space observatory SOHO

Abstract

In praise of the sun the Rig Veda states, "Sustainer of the Heaven, Lord of the cosmos, this sage puts on his golden coloured mail ... Maintaining his own laws he runs his course." And today in the space age, even with modern spacecraft technology, the sun remains a mysterious object from the scientific viewpoint. Nevertheless, proximity of the sun to the earth makes it the only star among the billions of stars in the sky available for detailed examination by telescopes. All other stars appear as blobs of light, or at best, with modern interferometry, as mottled discs. As a source of light and life to the entire solar system including our own planet, the sun serves as the 'Roset ta Stone' for astrophysics. In a major effort by ESA and NASA, a new solar space observatory SOHO (Solar and Heliospheric Observatory) was launched on December 2, 1995 to study the sun from its deep interior to its extensive outer atmosphere. Stationed in space, some 1.5 miIlion kilometers away from us where the gravitational pulls of the sun and earth are in balance, the spacecraft has been returning the sun's glare with a steady gaze of its own since early 1996. Like nearly all efforts to study the sun, the $ 1.2 billion spacecraft with twelve sophisticated instruments on board, has been providing unprecedented clues to the nature of the sun's milliondegree-temperature corona (the sun's halo of ionized gases), the solar wind and the conditions deep inside the sun (by making hi-fi recordings of the sun's strange song using the techniques of helioseismology). The sun is the dynamo, the energy generator, of the solar system. The sunlight, shining on our planet, provides heat for the atmosphere, drives the weather, and makes life possible. However, the sun in X-rays and EUV (extreme-ultraviolet) light appears completely different from the sun we see in the sky. Only very hot gases can emit X-rays and EUV. The sun's corona, at a million of degrees, is hot enough to emit X-rays or EUV, while the much cooler face of the sun (invisible light), at 6000 degrees, is not. In the corona, the shape and character of the hot gases are controlled by the solar magnetic fields, just as beads move with the string upon which they are threaded. In addition to electromagnetic radiation, the sun produces the solar wind. The solar wind is composed of charged particles, ions and electrons, which continuously stream out from the sun with velocities over 400 kilometers per second. Although the velocity is very high, the density of the wind is many millions of times lower than the atmospheric density at the earth's surface. If we were to go into space and put our hands in this wind, we would not even be able to feel it. Two main scientific goals of the SOHO mission are to address such unanswered questions as: what heats the corona to million