日球層頂（Heliopause），也稱為太陽風層頂，是天文學中表示出自太陽的太陽風遭遇到星際介質而停滯的邊界。
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The **heliopause** is the boundary at the edge of our solar system where the solar wind, which is a stream of charged particles emitted by the sun, slows down and is stopped by the interstellar medium, the matter that exists in the space between star systems in our galaxy. It is essentially the point where the sun's influence ends and interstellar space begins.
### Characteristics of the Heliopause
1. **Solar Wind Termination Shock:** Before reaching the heliopause, the solar wind first encounters the termination shock, where it slows down abruptly due to interactions with the interstellar medium. This region is where the solar wind pressure decreases significantly.
2. **Heliosheath:** The region between the termination shock and the heliopause is called the heliosheath. In this area, the solar wind is turbulent and compressed as it slows down and mixes with the interstellar medium.
3. **Boundary of the Solar System:** The heliopause marks the boundary of the heliosphere, the bubble-like region of space dominated by the sun's solar wind and magnetic field.
### Distance from Earth
- **Approximate Distance:** The distance to the heliopause is not constant, as it fluctuates due to changes in the solar wind and the density of the interstellar medium. However, it is generally estimated to be about 120 astronomical units (AU) from the Earth. One astronomical unit is the average distance between the Earth and the sun, approximately 93 million miles (150 million kilometers).
- **Voyager Probes:** NASA's Voyager 1 and Voyager 2 spacecraft, launched in 1977, have both crossed the heliopause. Voyager 1 crossed the heliopause in August 2012 at a distance of about 121 AU from the sun, and Voyager 2 crossed it in November 2018 at about 119 AU.
### Significance
1. **End of Solar Influence:** The heliopause is significant because it represents the outermost boundary of the sun's influence. Beyond this boundary, the environment is dominated by the interstellar medium rather than the solar wind.
2. **Exploration and Study:** Understanding the heliopause and its characteristics provides valuable insights into the nature of the interstellar medium and the interactions between stellar winds and interstellar space. The data collected by the Voyager spacecraft have been crucial in expanding our knowledge of this region.
### Summary
The heliopause is the boundary where the solar wind from the sun is stopped by the interstellar medium, marking the outer edge of the heliosphere. It is located approximately 120 astronomical units (AU) from Earth, though this distance can vary. The heliopause represents the transition from the space influenced by our sun to the vast interstellar space, and its study helps scientists understand the nature of our solar system's boundary and its interactions with the galaxy.
