Читать книгу Exploring the Solar System - Peter Bond - Страница 59

Nearly all scientific spacecraft that observe the Sun or are dispatched across the Solar System follow paths that lie close to the ecliptic. However, the European Space Agency's Ulysses spacecraft followed a highly elliptical orbit that carried it above and below the solar poles, regions that are usually extremely difficult to observe.

Figure 2.35 The Ulysses spacecraft was the first to leave the ecliptic and explore the Sun's polar regions. Launched in 1990, it was sent to Jupiter for a gravity assist that bent its orbit southward so that it could fly over the star's poles. Ulysses completed almost three orbits before it was shut down in 2009. The final orbit is shown here.

(ESA)

Launched on the Space Shuttle Discovery in October 1990, Ulysses received a gravity assist from Jupiter in 1992. This bent its path southward, so that it would fly over the solar poles. Before its mission ended in 2009, the nuclear‐powered spacecraft made nearly three complete orbits of the Sun, enabling it to observe the cyclical changes in solar activity.

Prior to Ulysses, it had only been possible to measure the solar wind near the ecliptic, giving the impression that it typically swept across the Solar System at about 400 km/s, with occasional faster gusts. Ulysses showed that this perception was incorrect. For much of the sunspot cycle, the dominant component is a fast wind from the cooler regions close to the poles that fans out to fill two‐thirds of the heliosphere. Blowing at a fairly uniform speed of 750 km/s, this far outstrips the slower wind that emerges from the equatorial zone. Rather than being typical, the slow wind is a relatively minor player.

When Ulysses first flew over the polar regions in 1994 and 1995, solar activity was close to minimum, providing a view of the three‐dimensional heliosphere at its simplest. The fast solar wind escaping from high latitudes flowed uniformly to fill a large fraction of the heliosphere, and solar wind variability was confined to a narrow region around the equator.

When Ulysses returned to high latitudes in 2000 and 2001, during solar maximum, the Sun displayed many active regions and solar storms were common. Solar wind flows from the poles appeared indistinguishable from flows at lower latitudes.

In 2007, Ulysses made its third polar passage. Compared with observations from the previous solar minimum, the strength of the solar wind pressure had decreased by 20%, and the field strength had decreased by 36%. Although the wind speed was almost the same, the density and pressure were significantly lower.

Of particular interest were Ulysses' observations of the Sun's magnetic field reversal at the change‐over between solar cycles. Despite the apparent chaos and complexity of the magnetic field at the surface, the Sun's magnetic equator was quite well defined and stable, clearly separating the negative and positive magnetic hemispheres. However, during the solar minimum of 1994–1995, the magnetic equator was pushed 10° southward with respect to the Sun's rotational equator. The reason for this offset is still not fully understood.

Ulysses' measurements also showed that energetic particles originating in storms close to the Sun's equator are much more mobile than previously thought, and are even able to reach the polar regions. Since these particles tend to move along the magnetic field lines in the solar wind (much like beads on a wire), this indicated that the structure of the magnetic field is more complex than previously thought.

In addition to studying the Sun, instruments on Ulysses detected small dust particles, hydrogen ions, cosmic rays, and neutral helium atoms that entered the heliosphere from interstellar space. The dust flowing into our Solar System was 30 times more abundant than predicted.

Figure 2.36 Polar plots of solar wind speed over each of Ulysses' orbits. The first orbit occurred during solar minimum, when there was a slow wind over the equator and a fast wind over the poles. The second orbit showed fast and slow winds at all latitudes, consistent with solar maximum. Data from its third orbit, during another solar minimum, indicated that the solar wind speed was similar, even though the density and pressure were significantly lower than in the previous solar minimum. The plot below shows the number of sunspots.

(ESA/NASA, SOHO and High‐Altitude Observatory, Mauna Loa, from McComas et al., 2008)

Figure 2.37 SOHO's Extreme Ultraviolet Imager took this image of the most powerful flare ever recorded by spacecraft, which erupted on November 4, 2003. Since the X‐ray detector on board NOAA's GOES satellite was saturated by the radiation outburst, there is some uncertainty over its magnitude, with estimates ranging from X28 to X45. The record‐breaking series of solar storms in October and November 2003 affected radio communications across the globe and caused temporary power outages in Europe due to fluctuations in Earth's magnetic field.

(ESA/NASA)

Figure 2.38 A flare is created when a magnetic loop in the corona rises to great height, becoming stretched and distorted. When the two sides of the loop get close enough, magnetic reconnection takes place. The loop splits in two, forming a smaller arch at the surface and a separate loop in the corona. The excess energy is released in explosive events like flares and coronal mass ejections (CMEs). The white flash represents a flare generated in the small arch as electrons are accelerated down its magnetic field and slam into the denser plasma near the solar surface, releasing high energy radiation. Plasma trapped in the coronal loop quickly rises and expands, propelling the CME plasma away from the Sun.

(NASA)

Figure 2.39 Seismic waves ripple away from the site of a moderate‐sized solar flare. The image was taken by SOHO's Michelson Doppler Imager on July 9, 1996. Over the course of an hour, the waves traveled for a distance equal to 10 Earth diameters before fading into the photosphere. Unlike water ripples that travel outward at a constant velocity, the solar waves accelerated from an initial speed of 35,200 km/h to a maximum of 250,000 km/h before disappearing.

(Alexander Kosovichev, Valentina Zharkova, ESA/NASA)

Over the course of an hour, the seismic waves may travel more than 100,000 km before they fade into the fiery background of the Sun's photosphere. The waves can accelerate from an initial speed of 35,200 km/h to a peak of 400,000 km/h before they disappear.

Flares may often erupt one after the other when a particularly active sunspot region appears. One of the most extraordinary

sequences of solar storms took place between October 18 and November 5, 2003, when more than 140 flares were observed, primarily associated with two large sunspot groups. Among them were 11 major X‐class flares, including an X17 event on October 28, and an even bigger one on November 4.

The strength of the latter event was difficult to determine, since it saturated the spacecraft detectors, but, based on radio wave‐based measurements of the X‐rays' effects on Earth's upper atmosphere, it was later uprated to X45, making it by far the largest flare detected since the GOES satellites began their solar X‐ray measurements in 1976.

Figure 2.40 On December 13, 2006, Hinode's Solar Optical Telescope imaged at different wavelengths a new, developing sunspot colliding with an existing spot and then exploding into an X‐class solar flare. The bright area in the upper image marks the footprint of a magnetic field that channeled suddenly released energy from the inner corona down to the surface. The lower image shows that the two sunspots had opposite magnetic polarities. The flare produced high‐energy protons that reached Earth.

(Hinode, JAXA‐NASA)