Heliosphere
The heliosphere is a vast region of space that surrounds the Sun and the solar system. It is filled with the solar magnetic field and the protons and electrons of the solar wind, which is a stream of electrically charged gas that blows outward from the Sun at high speeds.
The shape of the heliosphere is not a perfect sphere, but rather a bubble-like structure that is elongated in the direction of the Sun's motion through the interstellar medium (ISM), which is the gas and dust between the stars. The ISM exerts pressure on the heliosphere, causing it to have a blunt nose on one side and a long tail on the other.
The heliosphere has several distinct regions, each with different characteristics. The innermost region is the solar wind itself, which extends from the Sun to about 100 astronomical units (AU), or 100 times the distance from the Earth to the Sun. The solar wind travels at an average speed ranging from 300 to 700 kilometers per second (700,000 - 1,500,000 miles per hour) until it reaches the termination shock. This is where the solar wind slows down abruptly as it encounters the ISM.
Beyond the termination shock is the heliosheath, a turbulent and hot region where the solar wind piles up and becomes denser. The heliosheath extends to about 150 AU from the Sun. The outer boundary of the heliosheath is the heliopause, where the pressure of the solar wind and the ISM are in balance. This marks the edge of the heliosphere and the beginning of interstellar space.
As the heliosphere plows through the ISM, it creates a bow shock in front of it, similar to what happens when a boat moves through water. The bow shock is a region where the ISM is compressed and heated by the heliosphere's motion. However, some recent observations suggest that there may not be a bow shock at all, or that it may be very weak or variable.
The heliosphere is important for several reasons. It protects the solar system from most of the harmful cosmic rays that originate from outside sources, such as supernova explosions or active galactic nuclei. It also provides clues about the nature and history of the Sun and its interaction with the ISM. Moreover, it helps us understand how other stars and their planets may be affected by their own astrospheres, which are similar to heliospheres but around different stars.
The study of the heliosphere is called heliophysics, which is a branch of space science that also includes space weather and space climate. Space weather refers to how changes in the solar wind and magnetic field affect Earth's magnetosphere, ionosphere, atmosphere, and technological systems. Space climate refers to how long-term variations in solar activity influence Earth's climate and environment.
The main sources of information about the heliosphere are spacecraft that have explored its outer regions, such as Voyager 1 and Voyager 2. These two probes were launched in 1977 and have traveled beyond all the planets in our solar system. Voyager 1 crossed the termination shock in 2004 and entered interstellar space in 2012. Voyager 2 crossed the termination shock in 2007 and entered interstellar space in 2018. They are still sending back data about their surroundings as they continue their journey into deep space.
References:
- https://www.britannica.com/science/heliosphere
- https://www.nasa.gov/mission_pages/sunearth/science/Heliosphere.html
- https://en.wikipedia.org/wiki/Heliosphere
- https://science.nasa.gov/heliophysics/focus-areas/heliosphere/
- https://www.nasa.gov/feature/goddard/2019/nasa-s-voyager-2-probe-enters-interstellar-space
- https://www.nasa.gov/mission_pages/sunearth/spaceweather/index.html
- https://voyager.jpl.nasa.gov/
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