Читать книгу Exploring the Solar System - Peter Bond - Страница 32

The motion of superhot plasma (electrified gas) inside the Sun generates a powerful magnetic field. The Sun's atmosphere extends into interplanetary space through the motion of the electrically charged particles (mainly electrons and protons) of the solar wind, which streams outward in all directions at typical speeds of between 400 and 7,500 km/s (see Chapter 2).

As the particles spiral around the Sun, they carve out an invisible bubble which extends outward for many billions of kilometers. Although electrically neutral atoms, cosmic rays, and dust particles from interstellar space can penetrate this bubble, virtually all of the atomic particles in the heliosphere originate in the Sun itself.

The region of space in which the Sun's magnetic field and the wind of charged particles (solar wind) dominate the interstellar medium is known as the heliosphere (Figure 1.21). The shape of the heliosphere and the distance of the heliopause are determined by three main factors: the motion of the Sun as it plows through the interstellar medium, the density of the interstellar plasma, and the pressure exerted on its surroundings by the solar wind.

From theoretical studies and spacecraft observations of planetary magnetospheres and the solar wind, it is known that the density of the solar wind decreases as the inverse square of its distance from the Sun. In other words, solar wind density at 4 AU is only one quarter its density at 2 AU. The strength of the Sun's magnetic field also weakens with distance, although at a slower rate. Eventually, the density and magnetic influence of the solar wind decrease so much that its outward motion is impeded by the sparse plasma of the interstellar medium.

The heliosphere acts like an island in a stream, causing interstellar plasma to be diverted around it. At first it was thought that the heliosphere really was spherical, but the two Voyager spacecraft, which are currently heading out of the Solar System on different paths, observed what seemed to be a “squashed” heliosheath.

In this new model, the heliosphere resembled a huge windsock or tadpole – much like a comet's elongated tail – that is shaped by the motion of the Sun as it plows through a hot, tenuous cloud of interstellar gas and dust. Studies of the motion of nearby stars show that the Sun is traversing the cloud at a velocity of 25.5 km/s. The interstellar medium forces the solar wind to turn back and confines it within the heliosphere.

This picture had to be revised again in 2009, when data from the IBEX spacecraft and the Cassini spacecraft in orbit around Saturn showed that the heliosphere is roughly spherical – perhaps like an elongated balloon – after all. Instruments on the spacecraft were used to map the intensity of the energetic neutral atoms ejected from the heliosheath as the solar wind interacts with the interstellar medium. The data showed a belt of hot, high‐pressure particles where the interstellar wind flows by the heliosphere. Their distribution indicates that the heliosphere resembles a huge bubble which expands and contracts under the influence of the local interstellar magnetic field as it sweeps past.

The interaction of the heliosphere with the interstellar medium takes place in several stages. For a spacecraft traveling out of the Solar System, the first boundary to be reached is the termination shock. This is a standing shock wave where the supersonic solar wind slows dramatically from more than 100 km/s to about half that speed.

Beyond the termination shock is a region known as the heliosheath, where particles of the solar wind and interstellar gas mix. We are able to learn about conditions in this remote region by studying data from NASA's two Voyager spacecraft, which are heading out of the Solar System in different directions.

Voyager 1 crossed the termination shock on December 17, 2004, becoming the first spacecraft to enter the heliosheath. Voyager 2 crossed the termination shock on August 30, 2007, 30 years after it was launched from Florida. The Voyager 2 crossing took place almost 1.6 billion km closer to the Sun than Voyager 1's, confirming that the outer boundary of the Solar System is curved.

Observations by the Magnetospheric Imaging Instrument (MIMI) on board Cassini showed that the heliosheath is about 40 to 50 AU (6 billion to 7.5 billion km) thick. Further out is the heliopause, the boundary between the interstellar medium and the heliosphere.

The Voyager 1 spacecraft made history once more in August 2012 when it crossed the heliopause and entered interstellar space, leaving the solar wind behind. The crossing took place about 19 billion km from the Sun. The spacecraft entered a region where the density is 40 times greater because it is generated by material from other stars and stellar explosions.

On November 5, 2018, Voyager 2 became the second spacecraft to record crossing the heliopause, at a distance of more than 18 billion km from the Sun. The crossing was marked by a steep decline in the speed of the solar wind particles, followed by an absence of solar wind flow around the intrepid spacecraft.

Beyond the heliopause, the interstellar ions flow around the heliosphere, modifying its size and shape. Still further out, there is probably a bow shock, another shock surface where the supersonic flow of the interstellar medium is suddenly slowed as it approaches the heliosphere. All of these boundaries are thought to be moving back and forth at speeds of up to 100 km/s as the heliosphere is squeezed and released due to gusts in the solar wind and variations in the interstellar magnetic field.

Figure 1.21 The heliosphere is a bubble in space, filled with the particles and magnetic fields carried in the solar wind. The speed of the solar wind drops abruptly at the termination shock, as it begins to feel the effects of the interstellar wind. The heliosheath is the outer region of the heliosphere, where the solar wind piles up as it presses outward against the approaching wind in interstellar space. The boundary between solar wind and interstellar wind is the heliopause, where the pressure of the two winds is in balance. This causes the solar wind to turn back and flow down the tail of the heliosphere. As the heliosphere plows through interstellar space, a bow shock forms ahead of it. Also shown are the two Voyager spacecraft which have now crossed the heliopause.

(NASA/Goddard/Walt Feimer)