Читать книгу Exploring the Solar System - Peter Bond - Страница 57

The Solar and Heliospheric Observatory (SOHO) is the most important solar observatory ever sent into space. The European‐built spacecraft was launched on December 2, 1995, and transferred to the L1 Lagrangian point, approximately 1.5 million km from Earth on its sunward side. At this location, it is relatively easy to keep SOHO in a stable halo orbit from which its instruments can observe the Sun continuously, without being interrupted by eclipses.

SOHO carries 12 instruments that probe all aspects of the Sun. The spacecraft was designed to operate for two years, but it was still operating in 2019, despite some major technical problems which almost ended the mission.

SOHO has observed the Sun throughout the entire 11‐year solar cycle, and its observations have led to numerous discoveries. These include:

 Helioseismic data from SOHO and the GONG network of ground stations detected currents of gas beneath the visible surface, giving new insights into the layers of the Sun's interior, the behavior of the magnetic field, and the change in sunspot numbers during the solar cycle.

 A 0.1% increase in the Sun's luminosity as the count of sunspots increased 1996–2000. Scientists estimate that high‐energy ultraviolet rays from the Sun have become 3% stronger over the past 300 years.

 Until the launch of the STEREO mission, SOHO provided the only reliable way to identify coronal mass ejections that were heading towards Earth. This was done by linking expanding haloes around the Sun to shocks seen in the Earth‐facing atmosphere. This gave 2–3 days' warning of these potentially damaging storms.

 Thousands of nanoflares occur every day, due to continual rearrangement of tangled magnetic fields. This helps to explain why the corona is far hotter than its visible surface.

 SOHO helped to locate the sources of the fast and slow solar winds.

 Charged atoms that feed the fast solar wind gain speed very rapidly, apparently driven by strong magnetic waves in the corona. Similar magnetic waves may accelerate the slow wind.

 SOHO found many elements in the solar wind, including the first detections of phosphorus, chlorine, potassium, titanium, chromium, and nickel. These give clues to conditions on the Sun and to the history of the Solar System.

 After a solar flare, SOHO observed waves spreading outward across the Sun's visible surface.

 SOHO discovered large tornadoes, where hot gas was spiraling outwards from the Sun's polar regions.

 A wind of particles from distant stars blows through the Solar System, partially counteracting the solar wind. SOHO fixed its direction (from the Ophiuchus constellation) and speed (21 km/s) more accurately.

 Two instruments, SWAN and MDI, detected sound waves reflected from far‐side sunspots. This made it possible to “see” what was happening on the far side of the Sun, giving advance warning of active regions that had yet to appear on the Earth‐facing hemisphere.

 More than 3,000 sungrazing comets have been discovered in LASCO images. This makes SOHO by far the most successful comet discoverer in history.

In addition to dramatic changes in speed, spacecraft have recorded the presence of magnetic clouds – clumps of solar particles with embedded magnetic fields – and variations in the composition of the particle population which reflect conditions in their coronal source regions. For example, instruments have shown a higher abundance of magnesium ions compared to oxygen ions in the slow solar wind than in high‐speed streams.

Despite many years of observation, the precise mechanism of solar wind formation is still not fully understood. Although it is recognized that the fast solar wind originates from coronal holes, images of the outflowing material are still rare. Various source regions have been proposed for the slow solar wind. These include the boundary of coronal holes, helmet streamers located above closed loop structures in the corona, and the edges of active regions.

The suggestion that one of the sources of the slow solar wind is the boundary between coronal holes and active regions has been supported by high‐resolution images from Hinode. These show a continuous outflow of plasma along apparently open field lines that rise from the edge of an active region adjacent to a coronal hole.

The mechanism that drives the solar wind is also poorly understood. However, recent observations have indicated that Alfvén waves in X‐ray jets – fast‐moving eruptions of hot plasma that occur near the poles – may accelerate the solar wind to hundreds of kilometers per second. These magnetic oscillations, possibly associated with short‐lived spicules in the chromosphere (see Coronal Heating), appear to travel at very high speeds along open field lines that extend from the photosphere and into the corona. The wave energy that leaks into the corona may be sufficient to power the solar wind.