The spherical region around the sun filled with solar winds consists of electrons and protons and solar magnetic fields that protect us from the cosmic rays coming from outer space, just like our earth’s magnetic field protects us from solar wind and energetic particles and ions. It all starts from the corona in the sun itself. As we know, even our sun travels around the galactic core at a speed of 250 km/s, and its magnetic field creates this cavity, called the heliosphere, in the interstellar medium, which is beyond the orbits of the planets in our solar system. Basically, it is the volume of space within which our sun’s influence predominates. The size and shape of the heliosphere vary as per the strength and level of the solar wind. However, the shape of the heliosphere is not the same in all directions around the sun like an egg-shaped, as the sun, while moving, drags the solar wind into the medium just like a boat dragging the water along with it in the ocean.
As we go further deep into the heliosphere, gradually we will realize that the supersonic solar wind starts to lose its speed due to the interstellar medium penetrating within the heliosphere. This region where the solar wind starts to decrease its speed is called a termination shock, where the solar wind for the first time encounters the interstellar medium. A termination shock can be called the beginning of the end of the heliosphere.
Further ahead, the solar wind continues to drop its speed due to the interstellar medium acting as a barrier. The highly charged particles of the solar wind are compressed together here to form the Heliosheath, a region where solar winds still dominate over interstellar winds. However, the neutral atoms in the heliosheath form a “ribbon” as they get deflected back into the solar system due to the magnetic field of the interstellar medium.
The outermost edge of the heliosheath and end of the heliosphere is what we call the “heliopause”, which can also be called the outermost edge or the boundary of our solar system. Here, the solar wind pressure almost balances the outward pressure of interstellar winds, which means they are in equilibrium. However, it is a theoretical boundary and crossing it signals a sharp drop in the temperature of charged particles. The heliopause’s size is constantly changing as the heliosphere contracts and expands, followed by the solar cycle (periodic flipping of magnetic poles). However, not only does the solar cycle affect the shape of the heliosphere but also the interstellar medium that it drags along with it, which also has dramatic effects on the heliosphere. Still, for the idea, it is about 123 AU (or 18 billion km) away from the sun. Voyager 1 discovered the location of the Heliopause when it observed an increase in cosmic-ray particles and detected that radio emissions are generated when material thrown off by the Sun crosses it.
Where are Voyagers now?
Both Voyager 1 and Voyager 2 have reached “ interstellar space”. They are currently in the constellation of Ophiuchus and will continue their journey through the universe.