# Bow shock

LL Ori bow shock in Orion nebula. The star's wind collides with the nebula flow.
Hubble, 1995

In astrophysics, a bow shock is the area between a magnetosphere and an ambient medium. For stars, this boundary is typically the edge of the astrosphere, where the stellar wind meets the interstellar medium. For a planetary magnetosphere, the bow shock is the boundary at which the speed of the stellar wind abruptly drops as a result of its approach to the magnetopause.[1] The best-studied example of a bow shock is that occurring where the Sun's wind encounters Earth's magnetopause, although bow shocks occur around all magnetized planets, such as Jupiter[2] or Saturn.[3] Earth's bow shock is about 17 kilometres (11 mi) thick[4] and located about 90,000 kilometres (56,000 mi) from the planet.[5]

For several decades, the solar wind has been thought to form a bow shock at the edge of the heliosphere, where it collides with the surrounding interstellar medium. This long-held belief was called into question in 2012 when data from the Interstellar Boundary Explorer (IBEX) found the Sun and heliosphere to be moving more slowly through the interstellar medium than previously believed.[6] This new finding suggests that beyond our Solar System's termination shock and heliopause, there may be no, or very little bow shock.[6]

## Description

The defining criterion is that the bulk velocity of the fluid (in this case, the plasma of the solar wind) drops from "supersonic" to "subsonic", where the speed of sound in a plasma is defined as

$c_s^2 = \gamma p/ \rho$

where cs is the speed of sound, $\gamma$ is the ratio of specific heats, p is the pressure, and $\rho$ is the density of the plasma.

The particles making up the solar wind follow spiral paths along magnetic field lines. The velocity of each particle as it gyrates around a field line can be treated similarly to a thermal velocity in an ordinary gas, and in an ordinary gas, the mean thermal velocity is roughly the speed of sound. At the bow shock, the bulk forward velocity of the wind (which is the component of the velocity parallel to the field lines about which the particles gyrate) drops below the speed at which the particles are corkscrewing.

Bow shocks are also a common feature in Herbig Haro objects, in which a much stronger collimated outflow of gas and dust from the star interacts with the interstellar medium, producing bright bow shocks that are visible at optical wavelengths.

The following images show further evidence of bowshock existence from dense gases and plasma in the Orion Nebula.

## The Sun

It was hypothesised that the heliosphere also forms a bow shock as it travels through the interstellar medium. This will occur if the interstellar medium is moving supersonically towards the Sun, since the solar wind is moving supersonically away from the Sun. The point where the flow of the interstellar medium becomes subsonic is the bow shock; the point where the interstellar medium and solar wind pressures balance is at the heliopause; the point where the solar wind flow becomes subsonic is the termination shock. According to Robert J. Nemiroff and Jerry Bonnell of NASA, the solar bow shock would lie at a distance around 230 AU[7] from the Sun - more than twice the distance of the termination shock as encountered by the Voyager spacecraft. However, data in 2012 from NASA's Interstellar Boundary Explorer (IBEX) and corroborated with results from the Voyager spacecraft, has found that due to refinements in the relative speed of the heliosphere and the local interstellar magnetic field strength it is believed the heliosphere is prevented from forming a bow shock in the region of our galaxy the Sun is currently passing through.

## In the infrared

In 2006, a far infrared bow shock was detected near the AGB star R Hydrae.[8]

The bow shock around R Hydrae[9]