Ulysses: the Sun around us
How the sun fights the cosmic rays
Ulysses: the Sun around us
Energetic particles that bear down upon us from distant parts of the Universe can damage living tissue and cause computer faults, and may affect the weather. The Sun helps to shield the Earth from these cosmic rays. Their intensity declines whenever the presence of many sunspots tells of high solar activity (see diagram.)
Figure 3. Sunspots up, cosmic rays down. Colorado neutron counts by Univ. Chicago (Ulysses/COSPIN leaders)
Ulysses confirmed and discovered various ways in which the Sun hampers and weakens cosmic rays before they reach us. Strong bursts of gas from the Sun repel cosmic rays. So do the shocks produced by colliding fast and slow solar wind-streams near the equator. The shocks are rhythmic because they rotate with the Sun. Ulysses found unexpectedly that the Sun's polar regions feel these rhythmic shocks coming from the equatorial zone.
Figure 4. Polar magnetic field lines are folded and kinked
In contrast to the solar wind, which flows much more steadily in the polar regions than it does near the equator, the magnetic field near the poles fluctuates erratically. The shifts are caused by large-amplitude waves travelling outward from the Sun. Polar magnetic field lines are folded and kinked. They shove cosmic rays away from the Sun and back into the outer heliosphere, just as a raging surf forces a would-be wader out of the ocean and onto the beach.
The intensity of cosmic rays is essentially the same over the Sun's poles as it is at the equator. Astrophysicists had reasoned that magnetic fields should converge toward the Sun's north and south magnetic poles, just as the field lines around a bar magnet do. From the point of view of cosmic rays, the field lines over the poles should act as magnetic funnels, concentrating the rays and increasing their intensity (diagram bottom, left). Ulysses found that this picture was wrong. The solar wind spreads the field lines more uniformly (diagram bottom, right).
"We went to the south magnetic pole of the Sun and it wasn't there!" was how André Balogh of Imperial College London expressed his surprise in 1994, when Ulysses first revealed the nature of the polar field.
Sensors on Ulysses identified atoms wafting into the Solar System from the thin gas that fills the space between the stars. From neutral helium atoms, Ulysses established the speed and direction of the Sun's motion relative to the gas. The proportions of various atoms in this local interstellar gas found by Ulysses are helping to reveal how the material from which the Solar System was made has aged.
From much farther away, Ulysses registered an intense burst of gamma rays from a new class of neutron star, a magnetar, 20 000 light-years away. Despite the immense distance, the five-minute radiation burst was strong enough to disturb the Earth's atmosphere.
Figure 5. Before Ulysses
Figure 6. After Ulysses
||Star-dust showers us from cosmic space
Last Update: 12 May 2003