Heliosphere

The heliosphere is the vast, bubble-like region of space which surrounds and is created by the Sun. In plasma physics terms, this is the cavity formed by the Sun in the surrounding interstellar medium. The 'bubble' of the heliosphere is continuously 'inflated' by plasma originating from the Sun, known as the solar wind. Outside the heliosphere, this solar plasma gives way to the interstellar plasma permeating our galaxy. Radiation levels inside and outside the heliosphere differ; in particular, the galactic cosmic rays are less abundant inside the heliosphere, so that the planets inside (including Earth) are partly shielded from their impact. The word 'heliosphere' is said to have been coined by Alexander J. Dessler, who is credited with first use of the word in scientific literature in 1967. The scientific study of the heliosphere is heliophysics, which includes space weather and space climate. Flowing unimpeded through the Solar System for billions of kilometres, the solar wind extends far beyond even the region of Pluto, until it encounters the termination shock, where its motion slows abruptly due to the outside pressure of the interstellar medium. Beyond the shock lies the heliosheath, a broad transitional region between the inner heliosphere and the external environment. The outermost edge of the heliosphere is called the heliopause. The overall shape of the heliosphere resembles that of a comet – being approximately spherical on one side, with a long trailing tail opposite, known as the heliotail. The two Voyager spacecraft have explored the outer reaches of the heliosphere, passing through the termination shock and the heliosheath. NASA announced in 2013 that Voyager 1 had encountered the heliopause on 25 August 2012, when the spacecraft measured a sudden increase in plasma density of about forty times. In 2018, NASA announced that Voyager 2 had traversed the heliopause on 5 November of that year. Because the heliopause marks the boundary between matter originating from the Sun and matter originating from the rest of the galaxy, spacecraft such as the two Voyagers, which have departed the heliosphere, can be said to have reached interstellar space. The Heliosphere is the area under the influence of the Sun; the two major components to determining its edge are the magnetic field lines and the solar wind from the Sun. Three major sections from the beginning of the Heliosphere to its edge are the termination shock, the heliosheath, and the heliopause. Five spacecraft have returned much of the data about its furthest reaches, including Pioneer 10 (1972–1997; data to 67 AU), Pioneer 11 (1973–1995;44 AU), Voyager 1 and Voyager 2' (launched 1977, ongoing), and New Horizons (Launched 2006). A type of particle called an energetic neutral atom (ENA) has also been observed to have been produced from its edges. There are a host of spacecraft that observe the Sun and interplanetary space. One of the latest to explore the Sun, from nearer than ever before, is the Parker Solar Probe launched in 2018. Solar observations, such as during a Solar eclipse, allow observation of the Sun's Corona. While various types of space observatories also provide data on the Sun and its influences. The study of other Stars can also allow insights indirectly. Except for regions near obstacles such as planets or comets, the heliosphere is dominated by material emanating from the Sun, although cosmic rays, fast-moving neutral atoms, and cosmic dust can penetrate the heliosphere from the outside. Originating at the extremely hot surface of the corona, solar wind particles reach escape velocity, streaming outwards at 300 to 800 km/s (671 thousand to 1.79 million mph or 1 to 2.9 million km/h). As it begins to interact with the interstellar medium, its velocity slows to a stop. The point where the solar wind becomes slower than the speed of sound is called the termination shock; the solar wind continues to slow as it passes through the heliosheath leading to a boundary called the heliopause, where the interstellar medium and solar wind pressures balance. The termination shock was traversed by Voyager 1 in 2004, and Voyager 2 in 2007. It was thought that beyond the heliopause there was a bow shock, but data from Interstellar Boundary Explorer suggested the velocity of the Sun through the interstellar medium is too low for it to form. It may be a more gentle 'bow wave'. Starting in May 2012 at 120 au (1.8×1010 km; 1.1×1010 mi), Voyager 1 detected a sudden increase in cosmic rays, an apparent signature of approach to the heliopause. In the summer of 2013, NASA announced that Voyager 1 had reached interstellar space as of 25 August 2012.