Thermonuclear reactions that occur inside the Sun, emit huge amounts of energy.
Much of this energy is released in the near space in the form of electromagnetic radiation, mainly in the form of visible light. But also the Sun emits a stream of charged particles, known as solar wind.
The solar wind is bimodal, it is a mixture of several types of wind, fast (3 million km/h) at high latitudes of the Sun, above 40 °, and slow (1 million km/h) at latitudes between 22 ° S and 21 ° N. The solar wind is ionized, electrons and protons are separate, it is a plasma that journey to the outer solar system.
The heliopause is the last frontier of the solar system, it is at ≈130 AU (20 billion km), is the boundary where the solar wind begins fades and where interstellar space starts.
At this point the solar wind collides with the opposing winds from the interstellar medium, its thrust is no longer sufficient to repulse the rarefied hydrogen and helium of the Galaxy.
The termination shock is an intermediate border situated before the heliopause. The region between the termination shock and the heliopause is called the heliosheath.
The heliosheath is the turbulent region where the solar wind is slowed and compressed by the interstellar pressure. When particles emitted by the sun (a few particles per cubic centimeters) collide with interstellar particles, they slow down, heat up and emit energy. These highly energized particles accumulate in front of the heliopause, and creating a shock wave.
This shock wave is a trace left by the Sun during its journey through the Milky Way that it traverses in ≈220 million years.
The distance to the heliopause is not known precisely because it certainly varies with the speed of the solar wind and the temporal density of the interstellar medium.
In this region called "magnetic highway", instruments of Voyager 1 recorded the highest rate of cosmic rays from outer space and a sharp decrease of particles from the Sun.
« We saw a very sharp and sudden disappearance of particles from the sun whose intensity has decreased by more than a thousand times at the entrance of the magnetic Highway », says Stamatios Krimigis, an astrophysicist of the Applied Physics Laboratory of Johns Hopkins University (Maryland, est).
nota: The three units of measurement useful in astronomy to express distances:
- a light year is 63,242.17881 AU or exactly equal to 9,460,895,288,762,850 meters.
- one parsec is equal to 206,270.6904 AU or 3.2616 light years or 30,857,656,073,828,900 meters.
- an astronomical unit is worth since August 30, 2012, exactly 149,597,870,700 meters.
Table: equivalences between distance units.
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Image: The heliosphere protects the solar system of energy cosmic rays, beyond that boundary prevailing conditions of interstellar space extremely tenuous compared to Earth's atmosphere. The heliopause marks the boundary where the winds of particles emitted by the sun meet the other particles of the interstellar medium. The diffuse interstellar medium consists of material that fills the space between stars. Interstellar matter is mainly composed of ionized hydrogen (H+), atomic (H1) and molecular (H2), helium, grains whose size is of the order of several tens to several hundreds of nanometers and much dust bigger, from a few microns.