| Throughout the few decades of the Space Age, various nations, led by the United States, have sent spacecraft to all but two of the major bodies in the solar system. One of these as-yet-unexplored bodies is Pluto, the most distant planet from the Sun, which has never been visited by a robot orbiter or even a flyby craft—and such a mission is not likely anytime soon. The other unexplored body is the Sun.
Currently, the next best thing to a dedicated reconnoitering spacecraft is the Solar and Heliospheric Observatory (SOHO), which since its launch in late 1995 has been radioing back to Earth volumes of new data—and a few new puzzles—about our parent star.
SOHO is a billion-dollar mission operated primarily by the European Space Agency. The 2-ton robot is now on-station about 1.5 million km sunward of Earth—about 1 percent of the distance from Earth to the Sun. This is the so-called L1 Lagrange point, where the gravitational pull of the Sun and Earth are precisely equal—a good place to park a monitoring platform.  (Sec. 14.1) There SOHO looks unblinkingly at our star 24 hours a day, eavesdropping on the Sun's surface, atmosphere, and interior. This automated vehicle carries a dozen instruments, capable of measuring almost everything from the Sun's corona and magnetic field to its solar wind and internal vibrations.
SOHO is positioned just beyond the region of space that is influenced by Earth's magnetic field, so its instruments can study cleanly the charged particles of the solar wind—the matter that escapes the Sun at high speed, flowing out into the corona and beyond. Coordinating these in situ measurements with SOHO images of the Sun itself, astronomers now think they can follow solar magnetic field loops expanding and breaking as the Sun prepares itself for mass ejections several days before they actually occur. Given that such coronal storms can play havoc with communications, power grids, satellite electronics, and other human activity, the prospect of having accurate forecasts of disruptive solar events is a welcome development. |
Section 16.2 discussed how astronomers can "take the pulse" of the Sun by measuring its complex rhythmic motions. SOHO also has the ability to study the weak sound waves that echo and resonate inside the Sun and can map these vibrations with much higher resolution than was previously possible. It does so not by sensing sound itself but by watching the Sun's surface move up and down ever so slightly. The Sun's "loudest" vibrations are extremely low pitched (0.003 Hz, or one oscillation every 5 minutes), more like rolling rumbles, just as we might expect from such a huge and massive object.
The accompanying figure is not an image but a radial-velocity map of the Sun's surface (at much higher resolution than the diagram in Figure 16.4), obtained by measuring the Doppler shift of different portions of the solar photosphere. Bright areas are moving toward us, dark areas away. Observations of this sort enable researchers to refine models of the solar interior. For example, they have pinpointed the exact boundary between the convection zone and the radiation zone at 71.3 percent of the radius of the photosphere. The hope is to extend this work to discover how sharply defined the core is and just where its outer boundary lies.
Perhaps the most puzzling solar problem regards the temperature of the corona. How can it be so hot compared to the Sun's surface—as though the air above a candle's flame were much hotter than the flame itself? It doesn't make sense that the corona is a million degrees or so, when the solar photosphere, which is much closer than the corona to the vastly hotter core, is a mere 5800 K. If anything, SOHO has deepened this 50-year-old mystery. By examining the Sun at ultraviolet wavelengths, it has found that the very hottest matter in the upper solar atmosphere is even hotter than was previously thought. Ultraviolet spectral lines emitted by some ions are very sensitive to their surroundings, and SOHO's spectroscopic observations of highly excited oxygen ions have demonstrated that at least some parts of the corona have temperatures closer to 100 million kelvins, nearly a hundred times higher than earlier estimates. |
