Sunstorm by Arthur C. Clarke Page A

cosmologists
don’t
know is that only the innermost heart of the sun is a fusion reactor. The rest of it is special effects . . .” Mikhail’s Russian accent was movie-actor thick, but quite compelling.
    During her training, Siobhan had of course studied the sun. She had learned that the sun, like all stars, is simple in principle, but as the nearest star the sun had been scrutinized in minute detail. The detail, it turned out, was rather overwhelmingly complex and still little understood, even after centuries of study. But it was that detailed behavior that now seemed to be endangering humankind.
    The sun is a ball of gas, mostly hydrogen, more than a million kilometers wide—that is, as wide as a hundred Earths strung side by side, and as massive as a
million
Earths. The source of its vast energy output is its core, a star within a star where, in complicated chains of reactions, swarming nuclei of hydrogen fuse to helium and other heavier elements.
    The fusion energy must pass out through the body of the sun from the hot core to the cold sink of space, driven by temperature differences as surely as a head of pressure drives water through a pipe. But the core is swaddled by a thick belt of turgid gas called the “radiative zone,” opaque as a brick wall, through which the inner heat passes in the form of X-rays. In the next layer out, the “convective zone,” the densities have lessened to the point where the sun’s material can boil, like a pan heated from below. Here the core heat continues its journey to space by powering huge convective spouts, each many times taller than Earth, ascending at not much more than walking pace. Above the convective zone lies the visible surface of the sun, the photosphere, the source of sunlight and sunspots. And just as the meniscus of a boiling pan of water will organize itself into cells, so the sun bubbles with granules, constantly changing, tiling the photosphere like a Roman mosaic.
    So immense and compressed are these layers that the sun is all but opaque to its own radiation; a given photon’s worth of energy takes millions of years to struggle from core to surface.
    Once released from its cage of gases, the core energy, in the form of light, races away at lightspeed as if with relief, spreading with distance as it travels. At the distance of the Earth, eight light-minutes from the photosphere, sunlight still delivers about a kilowatt of power per square meter—and even at a distance of light-years the sunlight is bright enough for any eyes there to see it.
    As well as the light it emits, the sun breathes a constant stream of hot plasma into the faces of its circling children. This “solar wind” is a complex, turbulent breeze. At certain frequencies of light can be seen dark patches on the sun’s surface—“coronal holes,” regions of magnetic anomaly, like flaws in the sun itself—from which pour higher-energy streams of solar wind. The turning sun sprays these streams around the solar system in spiral washes, like an immense lawn sprinkler.
    Mikhail said, “We watch out for those sprinkler streams. Every time the planet runs into one we get problems, as the Earth and its magnetosphere are battered by high-energy particles.”
    Still more problems are caused for the Earth by the sun’s occasional irregularities. Mikhail said, “You have coronal mass ejections—like the monster that hit us on June 9—large-scale outpourings of plasma flung at us from the sun’s surface. And then you have flares. These detonations on the sun’s surface, powered by magnetic flaws, are the largest explosions in the modern-day solar system, each amounting to the blast of billions of nuclear weapons. Flares bombard us with radiation from gammas to radio waves. Sometimes they are followed up by what we call ‘solar proton events’—cascades of charged particles.”
    The restless sun follows an eleven-year “solar cycle,” at the peak of which sunspots swarm and flares erupt